2018

K. Song, J. Kim, S. Cho, N. Kim, D. Jung, Hyuck Choo, J. Lee, "Flexible-device injector with a microflap array for subcutaneously implanting flexible medical electronics," accepted as a full paper, Advanced Health Care Materials

Vinayak Narasimhan, Radwanul Hasan Siddique, Jeong Oen Lee, Shailabh Kumar, Blaise Ndjamen, Juan Du, Natalie Hong, David Sretavan & Hyuck Choo, "Multifunctional biophotonic nanostructures inspired by the longtail glasswing butterfly for medical devices", Nature Nanotechnology, April 30, 2018

[Video] [Nature Nanotechnology] [Caltech Release]


Numerous living organisms possess biophotonic nanostructures that provide colouration and other diverse functions for survival. While such structures have been actively studied and replicated in the laboratory, it remains unclear whether they can be used for biomedical applications. Here, we show a transparent photonic nanostructure inspired by the longtail glasswing butterfly (Chorinea faunus) and demonstrate its use in intraocular pressure (IOP) sensors in vivo. We exploit the phase separation between two immiscible polymers (poly(methyl methacrylate) and polystyrene) to form nanostructured features on top of a Si3_N_4 substrate. The membrane thus formed shows good angle-independent white-light transmission, strong hydrophilicity and anti-biofouling properties, which prevent adhesion of proteins, bacteria and eukaryotic cells. We then developed a microscale implantable IOP sensor using our photonic membrane as an optomechanical sensing element. Finally, we performed in vivo testing on New Zealand white rabbits, which showed that our device reduces the mean IOP measurement variation compared with conventional rebound tonometry without signs of inflammation.

Park, Haeri, S. Han, and Hyuck Choo. “Influence of Corneal Optical Aberrations on the Accuracy of Intraocular Pressure Sensor with Optical Readout.” Journal of Biomedical Optics, 23 (4), 047002, April 12, 2018.

Elevated intraocular pressure (IOP) is the only modifiable major risk factor of glaucoma. Recently, accurate and continuous IOP monitoring has been demonstrated in vivo using an implantable sensor based on optical resonance with remote optical readout to improve patient outcomes. Here, we investigate the relationship between optical aberrations of ex vivo rabbit eyes and the performance of the IOP sensor using a custom-built setup integrated with a Shack–Hartmann sensor. The sensor readouts became less accurate as the aberrations increased in magnitude, but they remained within the clinically acceptable range. For root-mean-square wavefront errors of 0.10 to 0.94 μm, the accuracy and the signal-to-noise ratio were 0.58 +/- 0.32 mmHg and 15.57 +/- 4.85 dB, respectively.

D. Yang, S. Afroosheh, J. O. Lee., H. Cho, S. Vaidyanathan, K. Woo, A. Zayak, and H. Choo, “Glucose Sensing Using Raman Mode Constraining,” submitted and under review, March 2018

Cho, Hyunjun, Shailabh Kumar, Daejong Yang, Sagar R. Vaidyanathan, Kelly Woo, I Garcia, H.J. Shue, Y. Yoon, K. Ferrari, and Hyuck Choo. "SERS-Based Label-Free Insulin Detection at Physiological Concentrations Using 3D Gold Nanoparticle Pillar Arrays." ACS Sensors, 10.1021/acssensors.7b00864, January 11, 2018.

Label-free optical detection of insulin would allow in vitro assessment of pancreatic cell functions in their natural state and expedite diabetes-related clinical research and treatment; however, no existing method has met these criteria at physiological concentrations. Using spatially uniform 3D gold-nanoparticle sensors, we have demonstrated surface-enhanced Raman sensing of insulin in the secretions from human pancreatic islets under low and high glucose environments without the use of labels such as antibodies or aptamers. Label-free measurements of the islet secretions showed excellent correlation among the ambient glucose levels, secreted insulin concentrations, and measured Raman-emission intensities. When excited at 785 nm, plasmonic hotspots of the densely arranged 3D gold-nanoparticle pillars as well as strong interaction between sulfide linkages of the insulin molecules and the gold nanoparticles produced highly sensitive and reliable insulin measurements down to 100 pM. The sensors exhibited a dynamic range of 100 pM to 50 nM with an estimated detection limit of 35 pM, which covers the reported concentration range of insulin observed in pancreatic cell secretions. The sensitivity of this approach is approximately 4 orders of magnitude greater than previously reported results using label-free optical approaches, and it is much more cost-effective than immunoassay-based insulin detection widely used in clinics and laboratories. These promising results may open up new opportunities for insulin sensing in research and clinical applications.

2017

Lee, Jeong Oen, Juan Du, Ashwin Balakrishna, Oliver Chen, David W.Sretavan, and Hyuck Choo. "A microscale optical implant for continuous in vivo monitoring of intraocular pressure, "Microsystems & Nanoengineering (Nature Publishing Group), 3, 17057, December 2017.

Intraocular pressure (IOP) is a key clinical parameter in glaucoma management. However, despite the potential utility of daily measurements of IOP in the context of disease management, the necessary tools are currently lacking, and IOP is typically measured only a few times a year. Here we report on a microscale implantable sensor that could provide convenient, accurate, on-demand IOP monitoring in the home environment. When excited by broadband near-infrared (NIR) light from a tungsten bulb, the sensor’s optical cavity reflects a pressure-dependent resonance signature that can be converted to IOP. NIR light is minimally absorbed by tissue and is not perceived visually. The sensor’s nanodot-enhanced cavity allows for a 3- to 5-cm readout distance with an average accuracy of 0.29 mmHg over the range of 0–40 mmHg. Sensors were mounted onto intraocular lenses or silicone haptics and secured inside the anterior chamber in New Zealand White rabbits. Implanted sensors provided continuous in-vivo tracking of short-term transient IOP elevations and provided continuous measurements of IOP for up to 4.5 months.

Kim, K.H., Jeong Oen Lee, Juan Du, David W. Sretavan, and Hyuck Choo.  “Real-Time In Vivo Intraocular Pressure Monitoring using an Optomechanical Implant and an Artificial Neural Network,” IEEE Sensors Journal, vol.17 (22), pp. 7394-7404, November 2017.

Optimized glaucoma therapy requires frequent monitoring and timely lowering of elevated intraocular pressure (IOP). A recently developed microscale IOP-monitoring implant, when illuminated with broadband light, reflects a pressure-dependent optical spectrum that is captured and converted to measure IOP. However, its accuracy is limited by background noise and the difficulty of modeling non-linear shifts of the spectra with respect to pressure changes. Using an end-to-end calibration system to train an artificial neural network (ANN) for signal demodulation we improved the speed and accuracy of pressure measurements obtained with an optically probed IOP-monitoring implant and make it suitable for real-time in vivo IOP monitoring. The ANN converts captured optical spectra into corresponding IOP levels. We achieved an IOP-measurement accuracy of ±0.1 mmHg at a measurement rate of 100 Hz, which represents a ten-fold improvement from previously reported values. This technique allowed real-time tracking of artificially induced sub-1 s transient IOP elevations and minor fluctuations induced by the respiratory motion of the rabbits during in vivo monitoring. All in vivo sensor readings paralleled those obtained concurrently using a commercial tonometer and showed consistency within ±2 mmHg. Real-time processing is highly useful for IOP monitoring in clinical settings and home environments and improves the overall practicality of the optical IOP-monitoring approach.

Kwarsch, R., J. Janzen, Hyunjun Cho, Hyuck Cho, and C. Rembe. “Scanning confocal vibrometer microscope for vibration analysis of energy-harvesting MEMS in wearables,”
tm - Technisches Messen, 84 (S1): S131-S137, De Gruyter, September 2017.

We present a scanning confocal laser-Doppler vibrometer microscope for sensitive, contactless measurement of microelectromechanical systems (MEMS). This system enables the dynamic analysis up to 3.2 MHz with a lateral resolution of few micrometers. We show measurements on developed MEMS for vocal-energy harvesting in wearables and medical implants. For efficient harvesting a cantilever beam with a serpentine form was designed with a fundamental resonance at 200 Hz. We verified the simulated mode shapes with our vibration measurements. The observed deviations in resonance frequencies between simulation and measurement are due to modelling and manufacturing dissimilarities.

Koo, S. Radwanul H. Siddique, and Hyuck Choo, “Quantitative analysis of III-V horn-shaped metal-clad nano-cavity as an on-chip light source,” AIP Advances (Journal of Applied Physics), 7, 075017, July 2017.

A horn-shaped metal-clad InGaAsP nano-cavity with sloped sidewalls is proposed as a platform for nanoscale light sources. The nano-cavity’s physical dimensions are 350 × 350 × 350 nm3, and its mode volume is 0.5 (λ0/n)3. In our numerical simulations and quantitative analysis, we have shown that the sloped sidewalls reduce metallic absorption and improve resonant mode confinement; and adjusting their slope from 0 to 16° increased the Q factor from 150 to 900 and laser modulation 3dB bandwidth from 4.3 to 36 GHz. The lasing threshold current was expected to be 35 μA at 16°. In a simulated feasibility study, we demonstrate 60 Gbps modulated laser signal (5 fJ/bit), producing 20 μW output power at the 1.5 μm wavelength with injection current 100 μA, as an implementation of horn-shaped nano-cavity platform to the low power and ultra-fast on-chip nano-laser.

Brodie, Frank L., David A. Ramirez, Sundar Pandian, Kelly Woo, Ashwin Balakrishna, Eugene De Juan, Hyuck Choo, and Robert H. Grubbs. “Novel Positioning Sensor with Real-time Feedback for Improving Postoperative Positioning: Pilot Study in Control Subjects.” Journal of Clinical Ophthalmology 11 (2017): 939-944.

Repair of retinal detachment frequently requires use of intraocular gas. Patients are instructed to position themselves postoperatively to oppose the intraocular bubble to the retinal break(s). We developed a novel wearable wireless positioning sensor, which provides real-time audiovisual feedback on the accuracy of positioning. Eight healthy volunteers wore the wireless sensor for 3 hours while instructed to maintain their head tilted toward the 2 o’clock meridian with no audiovisual feedback. Positioning accuracy was recorded. The subjects repeated the experiment for 3 hours with the audiovisual feedback enabled. With no audiovisual feedback, the percentage of time greater than 10° out of position varied from 8.9% to 93.9%. With audiovisual feedback enabled, these percentages ranged from 9.4% to 65%. Three subjects showed significant improvement in their time out of position (P<0.01, Fisher’s exact test). Four subjects demonstrated a nonsignificant improvement, and one subject had a significant increase in time out of position with feedback (P<0.01). When pooled, all subjects demonstrated a statistically significant decrease in degrees out of position (P<0.001, Wilcoxon test) and a statistically significant improvement in total time out of position (P<0.001).

Yang, Daejong, Hyunjun Cho, Sukmo Koo, Sagar R. Vaidyanathan, Kelly Woo, Youngzoon Yoon, and Hyuck Choo. “, “Simple, Large-Scale Fabrication of Uniform Raman-Enhancing Substrate Based on Enhancement Saturation,” ACS Applied Materials and Interfaces, 9, (22), pp 19092 – 19101, April 2017

It is well-known that gold nanoparticle (AuNP) clusters generate strong surface-enhanced Raman scattering (SERS). In order to produce spatially uniform Raman-enhancing substrates at a large scale, we synthesized vertically perforated three-dimensional (3D) AuNP stacks. The 3D stacks were fabricated by first hydrothermally synthesizing ZnO nanowires perpendicular to silicon wafers followed by repetitively performing liquid-phase deposition of AuNPs on the tops and side surfaces of the nanowires. During the deposition process, the nanowires were shown to gradually dissolve away, leaving hollow vestiges or perforations surrounded by stacks of AuNPs. Simulation studies and experimental measurements reveal these nanoscale perforations serve as light paths that allow the excitation light to excite deeper regions of the 3D stacks for stronger overall Raman emission. Combined with properly sized nanoparticles, this feature maximizes and saturates the Raman enhancement at 1-pM sensitivity across the entire wafer-scale substrate, and the saturation improves the wafer-scale uniformity by a factor of 6 when compared to nanoparticle layers deposited directly on a silicon wafer substrate. Using the 3D-stacked substrates, quantitative sensing of adenine molecules yielded concentrations measurements within 10% of the known value. Understanding the enhancing mechanisms and engineering the 3D stacks have opened a new method of harnessing the intense SERS observed in nanoparticle clusters and realize practical SERS substrates with significantly improved uniformity suitable for quantitative chemical sensing.

Narasimhan, Vinayak, Jeong Oen Lee, Juan Du, Blaise Ndjamen, David Sretavan, and Hyuck Choo. "Black-Silicon as a Multifunctional Material for Medical Implants: First Demonstrated Use in In vivo Intraocular Pressure Sensing." Transducers 2017 - International Solid-State Sensors, Actuators and Microsystems Conference, Jun 18-22, 2017, Kaohsiung, Taiwan (Oral Presentation).

We report the first in vivo demonstrated use of multifunctional black silicon (b-Si) on medical implants. B-Si is integrated onto the surface of a highly miniaturized sub-mm implantable intraocular pressure (IOP) sensor. This integration has significantly improved sensor signal-to-noise ratio (SNR) through the suppression of background noise as well as durability through minimized device biofouling. The incorporation of b-Si has enabled the use of a slit-lamp, the most widely used clinical ophthalmic microscope, for real-time IOP measurements on fully awake rabbits at a world-record 12-cm readout distance. Furthermore, b-Si has shown remarkable antifouling properties during a 6-month in vivo study by minimizing tissue proliferation and encapsulation on the ocular implant, promising much improved long-term implant serviceability.

Lee, Jeong Oen, Vinayak Narasimhan, Juan Du, David W. Sretavan, and Hyuck Choo. "Black-Silicon as a Biocompatible Anti-Reflection Coating for Medical Implants: First Demonstrated Use in In vivo Optical Intraocular Pressure Sensing." Advanced Healthcare Materials 6, no. 4 (2017): 1601356 (Cover Article). Featured in Advanced Science News.


Multifunctional black‐silicon (b‐Si) integrated on the surface of an implantable intraocular pressure sensor significantly improves sensor performance and reliability in six‐month in vivo studies. The antireflective properties of b‐Si triple the signal‐to‐noise ratio and increases the optical readout distance to a clinically viable 12 cm. Tissue growth and inflammation response on the sensor is suppressed demonstrating desirable anti‐biofouling properties.

Magley, Daniel, Vinayak Narasimhan, and Hyuck Choo. “Hydro-Ionic Microthruster for Locomotion in Low-Reynold's Number Ionic Fluids.” IEEE MEMS, Jan 22–26, 2017, Las Vegas, NV.

We have demonstrated a fast, extremely power-efficient, hydro-ionic microthruster that utilizes electro-osmotic propulsion for operation in a low Reynolds-number ionic environment. Powered by an onboard power supply, the microthruster achieves speeds of up to 5.24 cm/s (131 body-lengths/s) while consuming less than 252 nW/(Bodylength/s). It is 31% faster and ∼10^3 times more energy-efficient than prior designs. Including the onboard Al-air battery consisting of an Al anode and an Au cathode, the microthruster measures 1400μm (H) × 1400μm (W) × 400μm (L) in dimension and contains a 1000μm-wide, 300μm-long cylindrical thruster channel in the center. With significantly improved speed and drastically higher energy-efficiency, the microthruster opens up new possibilities for various self-powered biomedical applications that can operate within the power budget of modern implantable and integrable microscale batteries and bio-fuel cells.

Cho, Hyunjun, Kyoo Hyun Noh, Tomohiro Ishikawa, Daejong Yang, Edgar Sanchez-Sinencio, and Hyuck Choo. “Powering Portable Electronics Using Vocal Fold Vibrations.” IEEE MEMS, Jan 22–26, 2017, Las Vegas, NV (Oral Presentation).

Using a multi-stacked array of vibration-driven energy harvesters and a custom-tailored energy-harvesting (EH) circuit, we have achieved stable 3.12-mW power generation at 5.5 Vdc from the acousto-mechanical vibrations of the human vocal folds at 75 dB and demonstrated its use as a practical on-demand power source for portable and wearable electronics. The voltage and power outputs over 3.7 Vdc and 1 mW necessary for charging lithium polymer (LiPo) batteries were accomplished using a 3D-printed packaging platform whose physical design and mechanical properties maximized the vibration transfer and effectively combined 10 or more individual energy harvesters into a compact unified stack. The custom-designed LC-resonant EH circuit efficiently converted raw AC output into DC. Using our EH device, we have successfully charged 100-mAh LiPo battery and operated a portable 2×16 LCD display (requiring 2.8 V, 10 mA).

2016

Brodie, Frank, Kelly Woo, Ashwin Balakrishna, Hyuck Choo, and Robert H. Grubbs. “Validation of Sensor for Postoperative Positioning with Intraocular Gas.” Journal of Clinical Ophthalmology 10 (2016): 955-960.

Surgical repair of retinal attachment or macular hole frequently requires intraocular gas. This necessitates specific postoperative positioning to improve outcomes and avoid complications. However, patients struggle with correct positioning. We have developed a novel sensor to detect the position of the gas bubble in the eye and provide feedback to patients in real time. In this paper, we determine the specificity and sensitivity of our sensor in vitro using a model eye. We assessed the reliability of our sensor to detect when a gas bubble has deviated off a model retinal break in a model eye. Various bubble sizes representing the intraocular kinetics of sulfur hexafluoride gas and varying degrees of deviation from the correct position were tested using the sensor attached to a mannequin head with a model eye. We recorded 36 data points. The sensor acted appropriately in 33 (91.7%) of them. The sensor triggered the alarm every time the bubble deviated off the break (n=15, sensitivity =100%). However, it triggered the alarm (falsely) 3/21 times when the bubble was correctly positioned over the retinal break (specificity =86%).

Siddique, Radwanul H., Shailabh Kumar, Jürgen Mertens, Hendrik Hölsher, Silvia Vignolini, and Hyuck Choo. “Bio-Inspired Aluminum-Based Random Plasmonic Metasurface for Biosensing Applications.” 2016 MRS Fall, Nov 27–Dec 2, 2016, Boston, MA (Oral Presentation).

Cho, Hyunjun, Daejong Yang, Shailabh Kumar, and Hyuck Choo. “Insulin Sensing Using Surface-Enhanced Raman Spectroscopy.” 2016 MRS Fall, Nov 27–Dec 2, 2016, Boston, MA.

Yang, Daejong, Hyunjun Cho, Madelyn I. Wang, Kelly Woo, Sagar R. Vaidyanathan, and Hyuck Choo. “Fabrication of Wafer-Scale Uniform Surface Enhanced Raman Scattering (SERS) Substrates for Quantitative Bio-Sensing. 2016 MRS Fall, Nov 27–Dec 2, 2016, Boston, MA.

Choo, Hyuck. “Remote Intraocular Pressure Monitoring Using an Implantable Compact Optomechanical Sensor.” ISOT 2016, Nov 7–9, 2016, Tokyo, Japan (Invited Oral Presentation).

Lee, Jeong Oen, Haeri Park, Du Juan, Vinayak Narasimhan, Ashwin Balakrishna, Oliver Chen, David Sretavan, and Hyuck Choo. “In vivo Intraocular Pressure Monitoring Using Implantable Optomechanical Sensor.” 2016 International Symposium on Optomechatronic Technology (ISOT 2016), Nov 7–9, 2016, Tokyo, Japan (Invited Oral Presentation).

Lee, Jeong Oen, Du Juan, Haeri Park, Mehmet Sencan, Max Kudisch, David W. Sretavan, and Hyuck Choo. “In vivo Intraocular Pressure Measurements Using a Miniaturized Nano-Photonic Sensor Implant.” PIERS 2016, Aug 8–11, 2016, Shanghai, China (Invited Oral Presentation).

We have been developing a nanophotonic pressure sensor whose optical resonance is directly related to intraocular pressure (IOP). Bench testing has demonstrated sensor near-infrared (NIR) reflectance to accurately track pressures from 0-50 mmHg. The current study examined sensor performance following implantation into rabbit eyes for up to one month.

Woo, Kelly, Daejong Yang, Hyunjun Cho, and Hyuck Choo. “ZnO-Nanowire Morphology Optimization for Glucose-SERS Sensing.” PIERS 2016, Aug 8–11, 2016, Shanghai, China (Oral Presentation).

Current technology requires diabetics to undergo painful, inconvenient, and discontinuous measurement processes several times a day. Thus, more convenient ways of measuring glucose by utilizing surface enhanced Raman spectroscopy (SERS) technique can be highly desirable. To accomplish commercially viable SERS technologies for glucose detection, an optimal substrate must be designed with higher electromagnetic (EM) enhancement and better spatial uniformity, and many parameters must be taken into consideration, such as nanoparticle sizes, shapes, dielectric environment, which all affect the EM enhancement and resonance properties of the designed structures. Our straightforward two-step substrate fabrication process involves (1) the hydrothermal synthesis to create ZnO-NWs perpendicularly standing on the substrate and (2) the liquid phase deposition (LPD) to create Au NPs on ZnO-NWs. In the presentation, we will share our most updated results on NW synthesis outcomes and Raman measurements. We will discuss (1) our extensively modified synthesis process, which includes the types of chemical compounds used, time-based synthesis temperature adjustments, and acidity levels; (2) resulting NW morphologies with different NW growth rates, lengths, diameters, and branching/tapering geometries; and (3) Raman enhancement and spatial uniformity on a large scale.

Balakrishna, Ashwin, Oliver Chen, Jeong Oen Lee, and Hyuck Choo. “A Neural Network Approach to Monitor Intraocular Pressure for Glaucoma Diagnosis.” PIERS 2016, Aug 8–11, 2016, Shanghai, China (Oral Presentation).

Yang, Daejong, Jeong Oen Lee, Hyunjun Cho, Sukmo Koo, Sagar R. Vaidyanathan, Kelly Woo, and Hyuck Choo. “Glucose Measurement Using Surface Enhanced Raman Scattering.” PIERS 2016, Aug 8–11, 2016, Shanghai, China (Invited Oral Presentation).

Surface Enhanced Raman Scattering (SERS) has a great potential to serve as a monitoring technology for biomolecules, but sensing biomolecules for practical purposes have remained challenging for two reasons. One of the challenges is securing SERS substrates with uniform spatial enhancement that is crucial for quantitative measurements, and the other is finding proper linker molecules that will promote the surface enhancement. To address these challenges, we have been developing a new approach of using highly sensitive surface enhanced Raman scattering (SERS) platform for glucose sensing. In the presentation, I will discuss the fabrication of high performance 3D SERS substrate based on straightforward, two successive wet chemical processes, with experimentally proven strong enhancement and excellent spatial uniformity as well as the use of new linker molecules for making glucose-specific SERS substrates and their use in performing quantitative glucose measurements. Glucose sensing results from different development stages will be discussed.

Yang, Daejong, Hyunjun Cho, Sukmo Koo, Sagar Vaidyanathan, Kelly Woo, and Hyuck Choo. “3 Dimensionally Stacked Surface Enhanced Raman Scattering (SERS) Substrates with pico-Molar Sensitivity: Experimental and Simulation Studies.” META 2016, Jul 27, 2016, Malaga, Spain (Invited Oral Presentation).

Lee, Jeong Oen, Juan Du, Haeri Park, Mehmet Sencan, Max Kudisch, David W. Sretavan, and Hyuck Choo. "In vivo Intraocular Pressure Measurements Using a Miniaturized Nano-Photonic Sensor Implant." 2016 ARVO Annual Meeting, May 1–5, 2016, Seattle, WA.

Brodie, Frank, Kelly Woo, Ashwin Balakrishna, Hyuck Choo, and Robert H. Grubbs. “Improving Post-Operative Positioning Compliance: A Novel Device.” 2016 ARVO Annual Meeting, May 1–5, 2016, Seattle, WA.

The use of intraocular gas for repair of retinal detachments and macular holes is a mainstay of vitreoretinal surgery. Post-operatively, the patient is positioned so that the gas bubble is in apposition to the retinal pigment epithelium. In order to ensure correct position of the gas bubble against these breaks, patients are instructed in specific postoperative positioning of their head. In spite of instructing patients on the importance of proper head positioning after use of intraocular gas, numerous studies have shown patients are not compliant with the prescribed regimen. Our novel positioning device provides real time alerts when the bubble is out of position in the eye and allows patients to correct their position.

Yang, Daejong, Hyunjun Cho, Sukmo Koo, Sagar Vaidyanathan, Kelly Woo, and Hyuck Choo. “3 Dimensionally Stacked Surface Enhanced Raman Scattering (SERS) Substrates with pico-Molar Sensitivity: Experimental and Simulation Studies.” 2016 MRS Spring, Mar 28–Apr 1, 2016, Phoenix, AZ (Oral Presentation).

Lee, Jeong Oen, Haeri Park, Du Juan, David Sretavan, and Hyuck Choo. “Achieving Clinically Viable 12-CM Readout Distance from Micromachined Implantable Intraocular Pressure Sensor Using a Standard Clinical Slit Lamp.” IEEE MEMS, Jan 24–28, 2016, Shanghai, China (Oral Presentation, nominated for the Best Paper Award).

Achieving a practical readout distance for implantable intraocular pressure (IOP) sensors is an essential step toward commercialization yet has remained as a major challenge. Using the Zeiss SL-30 slit lamp ??? a standard ophthalmic scope widely used by clinicians, we have demonstrated an optical readout distance of 12 cm from a micromachined IOP sensor implanted in an ex-vivo rabbit eye. We show that we have achieved this readout distance by (1) redesigning the sensing area of the IOP sensor and its fabrication steps to significantly improve the signal-to-noise ratio; and (2) incorporating a novel robust detection algorithm, which includes a much-improved opto-mechanical model, that allows us to remove the background noise and instantaneously map the sensor's optical signal to the corresponding IOP value. A significant increase in readout distance accomplished using a well established ophthalmic clinical scope makes our IOP system a more clinically viable choice.

Cho, Hyunjun, Ashwin Balakrishna, Yuan Ma, Jeong Oen Lee, and Hyuck Choo. “Efficient Power Generation from Vocal Folds Vibrations for Medical Electronic Implants.” IEEE MEMS, Jan 24–28, 2016, Shanghai, China.

The availability of practical, implantable, efficient power generators will proliferate the use of medical electronic implants that can be very useful for treating and managing various medical conditions. Using a vibration-driven power generator, we have successfully generated 0.3-mW/cm2 of electric power continuously from the acousto-mechanical vibrations that originate from the human vocal folds and propagate along the skeletal frame and air passage throughout the head and neck. Our energy harvesters are highly efficient because vocal vibrations excite them at their designed resonant frequencies at 100 and 200 Hz, which are the dominant vocal vibrations of men and women, respectively. In addition, we use laser micromachining to pattern single crystal lead-zirconate-titanate (PZT) sheets for better efficiency. Our harvesters are designed to fit into a square area (1??1 cm2 or smaller) so that they can form a flexible large array to generate more power.

2015

Choi, Seong Soo, Myoung Jin Park, Chul Hee Han, Sae Joong Oh, Sang Hun Han, Nam Kyou Park, Yong-Sang Kim, and Hyuck Choo. "Fabrication of Pyramidal Probes with Various Periodic Patterns and a Single Nanopore." Journal of Vacuum Science and Technology B33, no. 6 (2015): 06F203-1–7.

The nanometer-scale patterned pyramidal probe with an electron beam-induced nanopore on the pyramid apex is an excellent candidate for an optical biosensor. The nanoapertures surrounded with various periodic groove patterns on the pyramid sides were fabricated using a focused ion beam technique, where the optical characteristics of the fabricated apertures with rectangular, circular, and elliptical groove patterns were investigated. The elliptical groove patterns on the pyramid were designed to maintain an identical distance between the grooves and the apex for the surface waves and, among the three patterns, the authors observed the highest optical transmission from the elliptically patterned pyramidal probe. A 103-fold increase of the transmitted optical intensity was observed after patterning with elliptical grooves, even without an aperture on the pyramid apex. The nanopore on the apex of the pyramid was fabricated using electron beam irradiation and was optically characterized.

Park, Haeri, Jeong Oen Lee, and Hyuck Choo. Butterfly-Cornea-Inspired Nanoplasmonic Array with Angle-Insensitive Reflectance for Use in Intraocular Pressure Sensor with Remote Optical Readout. 2015 MRS Fall, Nov 29–Dec 4, 2015, Boston, MA (Oral Presentation).

Yang, Daejong, Hyunjun Cho, Sagar Vaidyanathan, Kelly Woo, and Hyuck Choo. Nanofabrication of Gold-Nanoparticle-Linked Sea-Sponges on ZnO Nanowires Using Hydrothermal Synthesis and Its Use as 3D Surface-Enhanced Raman Bio-Sensing Substrates. 2015 MRS Fall, Nov 29–Dec 4, 2015, Boston, MA.

Lee, Jeong Oen, Haeri Park, Juan Du, David Sretavan, and Hyuck Choo. “A Nanophotonic-Based Intraocular Pressure Sensor with Remote Optical Readout: In vitro and In vivo Study,” IEEE NanoMed 2015, November 15-18, 2015, Waikiki Beach, Hawaii (Invited Talk).

Choo, Hyuck, Stefano Cabrini, P. James Schuck, Xiaogan Liang, and Eli Yablonovitch, Eli. Nano-Fabricated Plasmonic Optical Transformer. US Patent: 9,052,450, issued 2015.

The present invention provides a plasmonic optical transformer to produce a highly focuses optical beam spot, where the transformer includes a first metal layer, a dielectric layer formed on the first metal layer, and a second metal layer formed on the dielectric layer, where the first metal layer, the dielectric layer, and the second layer are patterned to a shape including a first section having a first cross section, a second section following the first section having a cross-section tapering from the first section to a smaller cross-section, and a third section following the second section having a cross-section matching the tapered smaller cross-section of the second section.

Chen, Sophia, Jeff Rosenberg, Ashwin Balakrishna, Grace Ma, Hyunjun Cho, Joeng Oen Lee, and Hyuck Choo. "On-Demand Power Source for Medical Electronic Implants: Acousto-Mechanical Vibrations from Human Vocal Folds." NAPA Institute 2015 Workshop on Enabling Future Health Care: the Role of Micro and Nano Technologies Aug 24-26, 2015, Napa, CA.

For use in vibration-driven power generation, we have quantitatively characterized the acousto-mechanical vibrations that propagate from the human vocal folds through the neck and head along the skeletal frames. We have used five MEMS accelerometers to characterize the acousto-mechanical vibrations present in various situations. The acousto-mechanical vibrations excite vibration-driven energy harvesters at their resonance frequencies between 90-300 Hz and generate up to 0.15 mW/cm^3 on demand.

Kou, Junlong and Li, Zheng and Choo, Hyuck "Nanofabrication of 3 Dimensional Taper Structures for Nanofocusing Purposes." Progress In Electromagnetics Research Symposium (PIERS 2015), Jul 6-9, 2015, Prague, Czech Republic.

We have demonstrated experimentally a highly efficient on-chip three-dimensional (3D) linearly tapered metal-insulator-metal (MIM) nanoplasmonic photon compressor (3D NPC) with a final aperture size of 14 x 80nm^2. An optimized and linearly tapered MIM gap plasmon waveguide could theoretically reduce the excessive losses that would occur during nanofocusing processes. This nanofocusing concept has existed for some time, yet researchers had difficulty in realizing structures based on the concept because precisely fabricating the nanoscale waveguides that taper in three dimensions had been very challenging. In simulation study, this approach could enable nanofocusing into a 2 X 5nm^2 area with the coupling loss and maximum E^2 enhancement of 2.5 dB and 3.0x10^4, respectively. We fabricated the 3D NPC on a chip employing electron beam-induced deposition and demonstrated its highly localized light confinement using a two-photon photoluminescence (TPPL) technique. From the TPPL measurements, we experimentally estimated an intensity enhancement of 400 within a 14x80nm^2 crosssectional area and a coupling efficiency of -1.3dB (or 74% transmittance).

Kou, Junlong, Zheng Li, and Hyuck Choo. “Nanofabrication of 3 Dimensional Taper for Nanofocusing Purposes.” ICMAT 2015, MRS of Singapore, Jun 28-Jul 3, 2015, Singapore (Invited Oral Presentation).

We have demonstrated experimentally a highly efficient on-chip three-dimensional (3D) linearly tapered metal-insulator-metal (MIM) nanoplasmonic photon compressor (3D NPC) with a final aperture size of 14 x 80nm^2. An optimized and linearly tapered MIM gap plasmon waveguide could theoretically reduce the excessive losses that would occur during nanofocusing processes. This nanofocusing concept has existed for some time, yet researchers had difficulty in realizing structures based on the concept because precisely fabricating the nanoscale waveguides that taper in three dimensions had been very challenging. In simulation study, this approach could enable nanofocusing into a 2 X 5nm^2 area with the coupling loss and maximum E^2 enhancement of 2.5 dB and 3.0x10^4, respectively. We fabricated the 3D NPC on a chip employing electron beam-induced deposition and demonstrated its highly localized light confinement using a two-photon photoluminescence (TPPL) technique. From the TPPL measurements, we experimentally estimated an intensity enhancement of 400 within a 14x80nm^2 crosssectional area and a coupling efficiency of -1.3dB (or 74% transmittance).

2014

Li, Zheng, Jun-long Kou, Myungki Kim, Jeong Oen Lee, and Hyuck Choo. "Highly Efficient and Tailorable On-Chip Metal–Insulator–Metal Plasmonic Nanofocusing Cavity." ACS Photonics 1, no. 10 (2014): 944-953. ISSN 2330-4022.

Simulation techniques were used to investigate the properties of a deep subwavelength-scale on-chip optical cavity composed of a highly efficient metal–insulator–metal 3D-tapered plasmonic nanofocusing waveguide and easily tailorable metal–insulator–metal plasmonic crystals. The configuration described here significantly enhanced the highly efficient field localization in the plasmonic nanofocusing waveguide at the center of the cavity due to the impedance tuning capabilities of the plasmonic crystals. The plasmonic crystals served as nanoscale input and output couplers with designable reflectivities and a clear band-stop regime around the telecommunication wavelength, λ0 = 1.55 μm. Simulation studies indicated that this configuration could efficiently confine electromagnetic waves on the nanometer length scale through a field intensity enhancement of 7 × 103 and a Purcell enhancement of 8 × 103 within a volume of 1.4 × 10–5 λ03. To evaluate the performance of the highly efficient metal–insulator–metal 3D-tapered plasmonic nanofocusing waveguide structure itself, the overall focusing efficiency, that is, the transmission rate from the wavelength-scale input side to the deep subwavelength-scale focusing core in the tapered waveguide, was calculated to be around 85%.

Kou, Jun-long, Fei Xu, and Hyuck Choo. "Implementation of a High-Q, Small Mode Volume Cavity in Microfibers Using Lattice-Constant-Varying Nanohole Arrays." IEEE Journal of Selected Topics in Quantum Electronics 20, no. 5 (2014). Art. No. 0903104. ISSN 1077-260X.

We proposed a novel approach to confine light in a silica microfiber (MF) cavity using a lattice constant-varying nanohole array. In our simulation study, the MF cavity is integrated with the optimally designed nanostructure, and produces Q-factors as high as 7 × 10 5 in the near-infrared region. The mode volume of the cavity remains close to λe3, a much smaller value than those previously reported for MF-based high- Q resonant structures. Our MF cavity with enhanced performance should find many potential applications in fiber lasers, nonlinear fiber optics, cavity quantum electrodynamics (QED), and cavity optomechanics.

Choo, Hyuck. "3D Nanofocusing Plasmonic Waveguide." In 2014 International Conference on Optical MEMS and Nanophotonics (OMN). (Piscataway, NJ: IEEE, 2014), 203-204. ISBN 978-0-9928-4140-9.

Choo, Hyuck. "Engineered Highly Efficient Nanofocusing Plasmonic Waveguides." PIERS Proceedings (2014): 101. ISSN 1559-9450.

Scientific and commercial applications of optical nanofcousing devices have attracted a lot of research efforts in recent years. Sub-diffraction-limited microscopy, nano-photolithography, tipbased nanofabrication, and heat-assisted magnetic recording (HAMR) for achieving data density beyond 1 Tbits/in^2 are a few examples. To realize these applications, nanofocusing devices must be able to confine light into sub-50-nm space (sub-20nm for HAMR applications) at high efficiency. Previously, researchers have achieved nanofocusing using scanning-tunnelingmicroscope (STM) tips, tapered plasmonic pins, enhanced transmission apertures, and tapered fiber probes [1-4].

Lee, Jeong Oen, Trong-Tuong Nguyen, David Sretavan, and Hyuck Choo. "Nanoarray-enhanced Implantable Intraocular Pressure Sensor with Remote Optical Readout." PIERS Proceedings (2014): 826. ISSN 1559-9450.

An elevated intraocular pressure (IOP) is a major risk factor of glaucoma, which is the second leading cause of blindness [1]. Traditional IOP-detection techniques such as tonometry cannot provide direct, accurate, and continuous IOP measurements [2]. Recently explored LC-resonator IOP-sensing implants are limited by their large sizes and short readout distances [3]. Our compact, robust IOP-sensing implant can greatly improve clinical management of glaucoma and accelerate related drug discoveries.

Lee, Jeong Oen, Trong-Tuong Nguyen, David Sretavan, and Hyuck Choo. "Nanoarray-Enhanced Micromechanical Pressure Sensor with Remote Optical Readout." Advanced Photonics. (Washington, DC: Optical Society of America, 2014). No. SeTh2D.3.

We demonstrate a compact implantable intraocular pressure (IOP) sensor with remote optical readout for glaucoma research and patient management. Using non-invasive white light, we excite the sensor’s pressure-sensitive optomechanical cavity and detect the reflected light, whose optical signature changes as a function of IOP. The sensor has provided robust measurements of hydrostatic pressure between 10-60 mmHg with an accuracy of 0.15 mmHg.

Huang, Kun, Jeong Oen Lee, Christopher F. Divsalar, Trong T. Nguyen, David W. Sretavan, and Hyuck Choo. "Nanophotonics-based Intraocular Pressure (IOP) Sensor with Remote Optical Readout." AVRO Annual Meeting, May 4-8, 2014, Orlando, FL.

To develop a miniaturized nanophotnonics-based implantable device for frequent, automated remote monitoring of IOP.

Li, Zheng and Hyuck Choo. “On-chip Low-profile Nano-horn Metal-clad Optical Cavity with Much Improved Performance.” CLEO, Jun 8-13, 2014, San Jose, CA.

We propose an on-chip nano-horm-shaped metal-clad cavity. The proposed device is 0.8 µm in height-half the size of the previously reported devices— and achieves the quality factor of 1000 and effective volume of 0.31(λ/n)^3.

Lee, Jeong Oen, Kun Huang, Trong-Tuong Nguyen, David Sretavan, and Hyuck Choo. “Nanoarray-Enhanced Implantable Intraocular Pressure Sensor with Remote Optical Readout.” 2014 Hilton Head Workshop, Jun 8-12, 2014, Hilton Head Island, SC.

An elevated intraocular pressure (IOP) is a major risk factor of glaucoma, which is the second leading cause of blindness [1]. Traditional IOP-detection techniques such as tonometry cannot provide direct, accurate, and continuous IOP measurements [2]. Recently explored LC-resonator IOP-sensing implants are limited by their large sizes and short readout distances [3]. Our compact, robust IOP-sensing implant can greatly improve clinical management of glaucoma and accelerate related drug discoveries.

Choo, Hyuck. “Highly Efficient 3D Nanofocusing Plasmonic Waveguide." 2014 META, May 20-23, 2014, Singapore (Invited Oral Presentation).

Li, Zheng and Hyuck Choo. “Dielectric Constant Contrast-Enabled Field Enhancement in Plasmonic Nanofocusing.” 2014 MRS Spring, Apr 21-25, 2014, San Francisco, CA.

Choo, Hyuck. “On-Chip Nanophotonics.” 2014 SPIE (International Society for Optics and Photonics) Photonics West Conference, Feb 1-6, 2014, San Francisco, CA (Invited Oral Presentation).

Choo, Hyuck. "Highly Efficient Nanofocusing for Integrated On-chip Nanophotonics." SPIE Newsroom, 2014. ISSN 1818-2259.

A linearly tapered 3D metal–insulator–metal nanoplasmonic photon compressor reduces resistive and scattering losses, showing promise for future nanoscale photonic and plasmonic applications.

Kou, Jun-long and Hyuck Choo. "A High-Q and Small-Mode-Volume Cavity in Microfibers." Frontiers in Optics 2014. (Washington, DC: Optical Society of America, 2014). No. LTh1H.5.

We propose a novel approach to confine light in a silica microfiber (MF) cavity using a lattice-constant-varying nanohole array. The MF cavity integrated with an optimally designed nanohole array produces Q-factors ~ 7×10^5 in the near infrared region while its mode volume remains ~ (λ_e)^3. Our MF cavity of enhanced performance will find potential applications in fiber lasers, nonlinear fiber optics, cavity quantum electrodynamics (QED), and cavity optomechanics.

2013

Li, Zheng, Myung-Ki Kim, Kun Huang, and Hyuck Choo. “Mode-Matching in 3D Nanophotonic Compressor to Enhance Nanofocusing.” 2013 MRS Fall, Dec 1-6, 2013, Boston, MA (Oral Presentation).

Li, Zheng, Myung-Ki Kim, Kun Huang, and Hyuck Choo. “Computationally Efficient and Intuitive 1D Analytical Model for 3D Metal-Clad Nanocavities.” 2013 MRS Fall, Dec 1-6, 2013, Boston, MA.

Kim, Myung-Ki, Zheng Li, Kun Huang, Ryan Going, and Ming C. Wu, and Hyuck Choo. "Engineering of Metal-clad Optical Nanocavity to Optimize Coupling with Integrated Waveguides." Optics Express 21, no. 22 (2013): 25796-25804. ISSN 1094-4087.

We propose a cladding engineering method that flexibly modifies the radiation patterns and rates of metal-clad nanoscale optical cavity. Optimally adjusting the cladding symmetry of the metal-clad nanoscale optical cavity modifies the modal symmetry and produces highly directional radiation that leads to 90% coupling efficiency into an integrated waveguide. In addition, the radiation rate of the cavity mode can be matched to its absorption rate by adjusting the thickness of the bottom-cladding layer. This approach optimizes the energy-flow rate from the waveguide and maximizes the energy confined inside the nanoscale optical cavity.

Kim, Myung-Ki, Zheng Li, Ming C. Wu, Ryan Going, and Hyuck Choo. "Engineering Metallic Nanocavity Radiation for Efficient Uni-/Bi-directional Coupling into Integrated Waveguide." CLEO: 2013 Technical Digest. (Washington, DC: Optical Society of America, 2013). No. QW3N.2. ISBN 978-1-55752-972-5.

We propose a new, simple way to engineer the radiation patterns of subwavelength-scale metallic semiconductor cavities for coupling light from a nanoscale metal cavity into integrated waveguides uni-/bi-directionally with efficiency up to ~90%.

Kim, Myung-Ki and Hyuck Choo. "A Highly Efficient On-chip 3D Plasmonic Nanofocusing Structure." MRS Proceedings (2013), 1566. ISSN 1946-4274.

We demonstrate and analyze a highly efficient on-chip 3D metal-insulator-metal (MIM) nanofocusing structure. Here, we show the in-depth theoretical design, analysis and discussion to provide a detailed picture of the highly efficient, on-chip nanofocusing process which is linearly tapered in 3D.

2012

Bao, Wei, M. Melli, N. Caselli, F. Riboli, D. S. Wiersma, M. Staffaroni, H. Choo, D. F. Ogletree, S. Aloni, J. Bokor, S. Cabrini, F. Intonti, M. B. Salmeron, E. Yablonovitch, P. J. Schuck, and A. Weber-Bargioni. "Mapping Local Charge Recombination Heterogeneity by Multidimensional Nanospectroscopic Imaging." Science 338, no. 6112 (2012): 1317-1321. ISSN 0036-8075.

As materials functionality becomes more dependent on local physical and electronic properties, the importance of optically probing matter with true nanoscale spatial resolution has increased. In this work, we mapped the influence of local trap states within individual nanowires on carrier recombination with deeply subwavelength resolution. This is achieved using multidimensional nanospectroscopic imaging based on a nano-optical device. Placed at the end of a scan probe, the device delivers optimal near-field properties, including highly efficient far-field to near-field coupling, ultralarge field enhancement, nearly background-free imaging, independence from sample requirements, and broadband operation. We performed ~40-nanometer–resolution hyperspectral imaging of indium phosphide nanowires via excitation and collection through the probes, revealing optoelectronic structure along individual nanowires that is not accessible with other methods.

Choo, Hyuck, Myung-Ki Kim, Matteo Staffaroni, Tae Joon Seok, Jeffrey Bokor, Stefano Cabrini, P. James Schuck, Ming C. Wu, and Eli Yablonovitch. "Nanofocusing in a Metal–Insulator–Metal Gap Plasmon Waveguide with a Three-Dimensional Linear Taper." Nature Photonics 6, no. 12 (2012): 838-844. ISSN 1749-4885.


The development of techniques for efficiently confining photons on the deep sub-wavelength spatial scale will revolutionize scientific research and engineering practices. The efficient coupling of light into extremely small nanofocusing devices has been a major challenge in on-chip nanophotonics because of the need to overcome various loss mechanisms and the on-chip nanofabrication challenges. Here, we demonstrate experimentally the achievement of highly efficient nanofocusing in an Au–SiO2–Au gap plasmon waveguide using a carefully engineered three-dimensional taper. The dimensions of the SiO2 layer, perpendicular to the direction of wave propagation, taper linearly below 100 nm. Our simulations suggest that the three-dimensional linear-tapering approach could focus 830 nm light into a 2 × 5 nm2 area with ≤3 dB loss and an intensity enhancement of 3.0 × 104. In a two-photon luminescence measurement, our device achieved an intensity enhancement of 400 within a 14 × 80 nm2 area, and a transmittance of 74%.

Zayak, Alexey T., Hyuck Choo, Ying S. Hu, Daniel J. Gargas, Stefano Cabrini, Jeffrey Bokor, P. James Schuck, Jeffrey B. Neaton. "Harnessing Chemical Raman Enhancement for Understanding Organic Adsorbate Binding on Metal Surfaces." Journal of Physical Chemistry Letters 3, no. 10 (2012): 1357-1362. ISSN 1948-7185.

Surface-enhanced Raman spectroscopy (SERS) is a known approach for detecting trace amounts of molecular species. Whereas SERS measurements have focused on enhancing the signal for sensing trace amounts of a chemical moiety, understanding how the substrate alters molecular Raman spectra can enable optical probing of analyte binding chemistry. Here we examine binding of trans-1,2-two(4-pyridyl) ethylene (BPE) to Au surfaces and understand variations in experimental data that arise from differences in how the molecule binds to the substrate. Monitoring differences in the SERS as a function of incubation time, a period of several hours in our case, reveals that the number of BPE molecules that chemically binds with the Au substrate increases with time. In addition, we introduce a direct method of accessing relative chemical enhancement from experiments that is in quantitative agreement with theory. The ability to probe optically specific details of metal/molecule interfaces opens up possibilities for using SERS in chemical analysis.

2011

Garmire, David, Hyuck Choo, Richard S. Muller, James Demmel, and Sanjay Govindjee. Integrated MEMS Metrology Device Using Complementary Measuring Combs. US Patent: 8,079,246, issued 2011.

The present invention provides a device for in-situ monitoring of material, process and dynamic properties of a MEMS device. The monitoring device includes a pair of comb drives, a cantilever suspension comprising a translating shuttle operatively connected with the pair of comb drives, structures for applying an electrical potential to the comb drives to displace the shuttle, structures for measuring an electrical potential from the pair of comb drives; measuring combs configured to measure the displacement of the shuttle, and structures for measuring an electrical capacitance of the measuring combs. Each of the comb drives may have differently sized comb finger gaps and a different number of comb finger gaps. The shuttle may be formed on two cantilevers perpendicularly disposed with the shuttle, whereby the cantilevers act as springs to return the shuttle to its initial position after each displacement.

Seok, Tae Joon, Arash Jamshidi, Myungki Kim, Scott Dhuey, Amit Lakhani, Hyuck Choo, Peter James Schuck, Stefano Cabrini, Adam M. Schwartzberg, Jeffrey Bokor, Eli Yablonovitch, and Ming C. Wu. "Radiation Engineering of Optical Antennas for Maximum Field Enhancement." Nano Letters 11, no. 7 (2011): 2606-2610. ISSN 1530-6984.

Optical antennas have generated much interest in recent years due to their ability to focus optical energy beyond the diffraction limit, benefiting a broad range of applications such as sensitive photodetection, magnetic storage, and surface-enhanced Raman spectroscopy. To achieve the maximum field enhancement for an optical antenna, parameters such as the antenna dimensions, loading conditions, and coupling efficiency have been previously studied. Here, we present a framework, based on coupled-mode theory, to achieve maximum field enhancement in optical antennas through optimization of optical antennas’ radiation characteristics. We demonstrate that the optimum condition is achieved when the radiation quality factor (Qrad) of optical antennas is matched to their absorption quality factor (Qabs). We achieve this condition experimentally by fabricating the optical antennas on a dielectric (SiO2) coated ground plane (metal substrate) and controlling the antenna radiation through optimizing the dielectric thickness. The dielectric thickness at which the matching condition occurs is approximately half of the quarter-wavelength thickness, typically used to achieve constructive interference, and leads to ∼20% higher field enhancement relative to a quarter-wavelength thick dielectric layer.

Zayak, A. T., Y. S. Hu, H. Choo, J. Bokor, S. Cabrini, P. J. Schuck, and J. B. Neaton. "Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces." Physical Review Letters 106, no. 8 (2011). Article 083003. ISSN 0031-9007.

Using first-principles theory and experiments, chemical contributions to surface-enhanced Raman spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on planar Au(111) surfaces are explained and quantified. Density functional theory calculations of the static Raman tensor demonstrate a strong mode-dependent modification of benzene thiol Raman spectra by Au substrates. Raman active modes with the largest enhancements result from stronger contributions from Au to their electron-vibron coupling, as quantified through a deformation potential. A straightforward and general analysis is introduced to extract chemical enhancement from experiments for specific vibrational modes; measured values are in excellent agreement with our calculations.

2010

Hu, Ying S., Jaeseok Jeon, Tae J. Seok, Seunghyun Lee, Jason H. Hafner, Rebekah A. Drezek, Hyuck Choo. "Enhanced Raman Scattering from Nanoparticle-Decorated Nanocone Substrates: A Practical Approach to Harness In-Plane Excitation." ACS Nano 4, no 10 (2010): 5721-5730. ISSN 1936-0851.

We investigate surface-enhanced Raman scattering (SERS) from gold-coated silicon−germanium nanocone substrates that are decorated with 30-nm spherical gold nanoparticles (AuNPs). Finite-element simulations suggest that individual nanocones generate stronger electromagnetic enhancement with axial polarization (i.e., polarization parallel to the vertical axis of the nanocones) than with transverse polarization (i.e., polarization in the plane of the nanocone substrate), whereas the excitation in a typical Raman microscope is mainly polarized in the transverse plane. We introduce a practical approach to improve the SERS performance of the substrate by filling the valleys between nanocones with AuNPs. Simulations reveal an enhanced electric field at the nanoscale junctions formed between AuNPs and nanocones, and we explain this lateral coupling with a hybridization model for a particle-film system. We further experimentally verify the added enhancement by measuring SERS from trans-1,2-bi-(4-pyridyl) ethylene molecules absorbed onto the substrates. We report over one order-of-magnitude increase in SERS activities with the AuNP decoration (compared to the nanocone substrate without AuNPs) and achieve a spatially averaged enhancement factor of 1.78 × 108 at 785-nm excitation. Understanding and implementing the enhancing mechanism of structured metallic surfaces decorated with plasmonic nanoparticles open possibilities to substantially improve the SERS performance of the existing process-engineered substrates.

Seok, Tae Joon, Arash Jamshidi, Amit Lakhani, Kyoungsik Yu, Hyuck Choo, Owen Miller, Eli Yablonovitch, and Ming C. Wu. "Characterization of Extended Width Optical Dipole Antennas." In: Conference on Lasers and Electro-Optics 2010: OSA Technical Digest (CD). (Washington, DC: Optical Society of America, 2010). No. CF14.

Optical dipole antennas with varying length and width are fabricated using e-beam lithography. Antennas with wider width are shown to exhibit stronger scattering while preserving the same resonance frequency.

Choo, Hyuck, Matteo Stafarroni, Tae Joon Seok, Jeffrey Bokor, Ming Wu, P. J. Schuck, S. Cabrini, and Eli Yablonovitch. "Three-Dimensional Optical Transformer - Highly Efficient Nanofocusing Device." In: Conference on Lasers and Electro-Optics 2010: OSA Technical Digest (CD). (Washington, DC: Optical Society of America, 2010). No. CFI5.

Using electron-beam-induced deposition and focused-ion-beam milling, we have fabricated and demonstrated a nanofocusing optical transformer with a 3-dimensionally tapered tip. At the tip, the light is confined to 13-by-80-nm area with intensity enhancement exceeding 1500.

Choo, Hyuck and Richard S. Muller. Optical Switch Using Frequency-Based Addressing in a Microelectromechanical Systems Array. US Patent: 7,656,568, issued 2010.

Embodiments of the present invention provide structures for microelectromechanical systems (MEMS) that can be sensed, activated, controlled or otherwise addressed or made to respond by the application of forcing functions. In particular, an optical shutter structure suitable for use in an optical switch arrangement is disclosed. In one embodiment, an optical shutter or switch can be scaled and/or arranged to form arbitrary switch, multiplexer and/or demultiplexer configurations. In another embodiment of the present invention, an optical switch can include: a shutter; and a flexure coupled to the shutter, whereupon a vibration transmitted to the flexure when in the presence of a resonant frequency causes the shutter to move across an opening for the passage of an optical signal.

Zayak, Alexey T., Ying Hu, Hyuck Choo, Stefano Cabrini, P. James Schuck, and Jeffrey B. Neaton. "Selective Chemical Raman Enhancement for Organic Adsorbates at Metal Surfaces." Bulletin of the American Physical Society 55, no. 2 (2010). ISSN 0003-0503.

It has long been observed that in surface enhanced Raman spectroscopy (SERS) relative mode intensities differ from gas- and solution-phase data, which obscures understanding of SERS in general. Using first-principles methods, we examine how chemisorption affects Raman scattering of molecules on metal surfaces relative to gas-phase, and provide a quantitative description of this effect. Calculated Raman spectra for benzene thiol bound at different sites on Au(111) show that chemical enhancement arises from the mode dependent electron-phonon coupling of the metal-molecule interface. Site-dependent enhancements are explained correlated to interfacial electronic structure. Comparison to experiments suggests affinity of benzene thiol for bridge sites on Au(111) surfaces.

2009

Choo, Hyuck, David Garmire, Richard S. Muller, and James Demmel. Method for Fabricating Vertically-Offset Interdigitated Comb Actuator Device. US Patent: 7,573,022, issued 2009.

The present invention relates to systems and methods for fabricating microscanners. The fabrication processes employed pursuant to some embodiments are compatible with well known CMOS fabrication techniques, allowing devices for control, monitoring and/or sensing to be integrated onto a single chip. Both one- and two-dimensional microscanners are described. Applications including optical laser surgery, maskless photolithography, portable displays and large scale displays are described.

Choo, Hyuck, Richard S. Muller, David Garmire, James W. Demmel, and Rishi Kant. MEMS-based, Phase-Shifting Interferometer. US Patent: 7,564,559, issued 2009.

Provided herein are optical devices fabricated to include a reflective surface, actuators and stress-relieving structures. Systems containing such devices, and methods of manufacturing such devices, are also provided.

2008

Choo, Hyuck and Richard S. Muller. Optical System Applicable to Improving the Dynamic Range of Shack-Hartmann Sensors. US Patent: 7,355,793, issued 2008.

An addressable array of lenses is disclosed. Two electrical connections per row address specific lenses within that row. Carriages support individual lenses, thus forming resonant units with frequencies unique within each row. A voltage, having the same frequency as a selected resonant unit is applied. The selected lens produces a resonating image. Testing has verified proper resonance addressing within a 5-by-5 array of microlenses. The array can be applied to a Shack-Hartmann (SH) sensor. To compensate for errant images formed outside of their image area, resonating images are identified by a processor. The array thus improves the dynamic range of the wavefront aberration that can be measured by an SH sensor. The inventors currently estimate the improvement over conventional designs to be about a factor of 30.

2007

Choo, Hyuck and Richard S. Muller. "Devices, Structures, and Processes for Optical MEMS." IEEJ Transactions on Electrical and Electronic Engineering 2, no. 3 (2007): 216-231. ISSN 1931-4981.

New processes and devices in the area of optical microelectromechanical systems (MEMS) as researched in the Berkeley Sensor & Actuator Center (BASC) are described. A technique and fabrication procedure is presented to produce high-quality microlenses at selected locations in a micro-optical system. Polarization beam splitters are produced by another process, and their performance is measured and described. A new, much simplified process to fabricate vertically offset comb actuating structures is applied in the design of high-performance scanners, which are in turn used to control a laser ablation system. Very favorable performance comparisons are demonstrated between the researched system and a conventional commercial laser ablation system. A second system demonstration is a prototype Shack–Hartmann (SH) sensor, in which microlenses are mounted in carriages that can be individually addressed using the selectivity of their mechanically resonant mountings. This design is shown to increase markedly the dynamic range in wave aberration to which the SH sensor can be applied. A final application of optical MEMS design is to a reduced-size, enhanced-performance, phase shifting interferometer.

Choo, Hyuck, David Garmire, James Demmel, and Richard S. Muller. "Simple Fabrication Process for Self-Aligned, High-Performance Microscanners—Demonstrated Use to Generate a 2-D Ablation Pattern." Journal of Microelectromechanical Systems 16, no. 2 (2007):260-268. ISSN 1057-7157.

A new, straightforward, complementary metal–oxide–semiconductor (CMOS)-compatible, three-mask process is used to fabricate high-performance torsional microscanners driven by self-aligned, vertically offset comb drives. Both the moving and fixed combs are defined using the same photolithography mask and fabricated in the same device layer, a process allowing the minimum gap between comb fingers to be as small as twice the alignment accuracy of the photolithography process. Our fabricated microscanners have torsional resonant frequencies between 58 Hz and 24 kHz and maximum optical-scanning angles between 8° and 48° with actuation voltages ranging from 14.1 to 67.2 V_(ac-rms). The yields on two separate fabrication runs have been better than 70%. To demonstrate an application for these scanners, we used them to generate laser-ablation patterns suitable for ocular cornea surgery. We assembled a 2-D scanning system by orienting two identical microscanners at right angles to one another. When driven by two 90° out-of-phase 6.01-kHz sine waves, the cross-coupled scanners produce circular patterns having radii fixed by the amplitude of the driving voltage. Then, we emulated a small pattern from the surface topography found on a U.S. Roosevelt dime and built up an ablation pattern that compares favorably with similar emulations reported by earlier researchers who used larger, more complicated ablation systems.

Kant, Rishi, David Garmire, Hyuck Choo, and Richard S. Muller. "Characterization of an Improved, Real-Time MEMS-Based Phase-Shifting Interferometer." In: 2007 IEEE/LEOS International Conference on Optical MEMS and Nanophotonics (Piscataway, NJ: IEEE, 2007): 57-58. ISBN 978-1-4244-0641-8

We describe and present detailed performance characterizations for an enhanced version of our MEMS-Based Phase-Shifting Interferometer (MBPSI) that achieves 13 times denser motion reconstruction than our original system. We measure the noise level to be ≤±6 nm λ for a 660 nm laser), and the frequency-resolution to be ≤ 0.03Hz for 31Hz motion captured at 300Hz. We have successfully tracked a piezo-based actuator, driven with an arbitrary waveform composed of transients ≤10Hz.

Garmire, David, Hyuck Choo, Rishi Kant, Sanjay Govindjee, Carlo H. Séquin, Richard S. Muller, and James Demmel. "Diamagnetically Levitated MEMS Accelerometers." In: TRANSDUCERS 2007 - 2007 International Solid-State Sensors, Actuators and Microsystems Conference. (Piscataway, NJ: IEEE, 2007): 1203-1206. ISBN 1-4244-0842-3.

We introduce the theory and a proof-of-concept design for MEMS-based, diamagnetically-levitated accelerometers. The theory includes an equation for determining the diamagnetic force above a checkerboard configuration of magnets. We demonstrate both electronic probing and a rapid MEMS-based interferometer technique for position sensing of the proof mass. Through a proof-of-concept design, we show electrostatic-measurement sensitivity achieving 34 μg at a 0.1 V sense signal and interferometer-measurement sensitivity achieving 6 μg for in-plane vibrations at 5 Hz. We conclude by outlining batch-fabrication steps to produce levitated accelerometers.

2006

Choo, Hyuck and Richard S. Muller. "Addressable Microlens Array to Improve Dynamic Range of Shack–Hartmann Sensors." Journal of Microelectromechanical Systems 15, no. 6 (2006): 1555-1567. ISSN 1057-7157.

In this paper, we have demonstrated an addressable array (5-by-5) of high-quality microlenses suitable for application in a Shack–Hartmann (SH) sensor in a microoptical system. Specific lenses in the array can be addressed using a new selection scheme (that we have designed, built, and tested) in which the mechanical resonant frequencies of individual lens-support carriages are varied. Thus, by changing the frequency of the drive voltage, we require only two electrical connections per row in the lens system to identify the selected lens by its resonating focal image. We show that using this lens-identification method will allow us to improve the dynamic range of SH sensors by a factor of 12–46 above values reported for conventional SH designs.

Kim, Jongbaeg, Hyuck Choo, Liwei Lin, and Richard S. Muller. "Microfabricated Torsional Actuators Using Self-Aligned Plastic Deformation of Silicon." Journal of Microelectromechanical Systems 15, no. 3 (2006): 553-562. ISSN 1057-7157.

In this paper, we describe angular vertical-comb-drive torsional microactuators made in a new process that induces residual plastic deformation of single-crystal-silicon torsion bars. Critical dimensions of the vertically interdigitated moving-and fixed-comb actuators are self-aligned in the fabrication process and processed devices operate stably over a range of actuation voltages. We demonstrate MEMS scanning mirrors that resonate at 2.95kHz and achieve optical scan angles up to 19.2 degrees with driving voltages of 40V_(dc) plus 13V_(pp). After continuous testing of five billion cycles at the maximum scanning angle, we do not observe any signs of degradation in the plastically deformed silicon torsion bars.

2005

Choo, Hyuck, David Garmire, James Demmel, and Richard S. Muller. "A Simple Process to Fabricate Self-Aligned, High-Performance Torsional Microscanners; Demonstrated Use in a Two-Dimensional Scanner. In: IEEE/LEOS International Conference on Optical MEMS and Their Applications Conference, 2005. (Piscataway, NJ: IEEE, 2005): 21-22. ISBN 0-7803-9278-7

CMOS-compatible process carried out on SOI wafers, we have built high-performance torsional microscanners having vertically offset interdigitated-comb actuators. Our microscanner-fabrication process requires three photolithography masks: two to form the front-side microscanner structures and a third to define the backside openings. Both moving and fixed combs are fabricated in the same device layer (30 µm in thickness), and the offset combs are created by reducing the thickness of the fixed combs, but not that of the moving combs. Our process begins by removing the 1-µm thick thermal oxide selectively to open rectangular windows at locations where the fixed combs are to be defined. In the following step, both fixed- and moving-comb sets are defined simultaneously with a single photolithography mask; this is followed by deep-reactive ion etching (DRIE). We then perform a tuned-etch in the DRIE-etcher to obtain the desired vertical thickness for the fixed combs without affecting the moving combs. The minimum gap between comb fingers can be as small as twice the alignment accuracy of the photolithography process, which is ≤ 0.4 µm for state-of-the-art photolithography steppers. We consider these microscanners as especially well adapted for applications to refractive laser surgery of ocular corneas where small spot size and high scan speeds are important assets.

2004

Choo, Hyuck, David Garmire, James Demmel, and Richard S. Muller. "Addressable Microlens Array to Improve Dynamic Range of Shack-Hartmann Sensors." In: Solid-State Sensor, Actuator and Microsystems Workshop, Hilton Head Island, South Carolina, June 6-10, 2004: Technical Digest. (Cleveland, OH: Transducers Research Foundation, 2004). ISBN 0964002450

We demonstrate an addressable array (5-by-5) of high-quality microlenses applied to a Shack-Hartmann (SH) sensor in a microoptical system. Specific lenses in the array can be addressed using a new selection scheme (that we have designed, built, and tested) in which the mechanical resonant frequencies of individual lenssupport carriages are varied. Thus, by changing the frequency of the drive voltage, we require only two electrical connections per row in the lens system to identify the selected lens by its resonating focal image. We show that using this lensidentification method allows us to improve the dynamic range of Shack-Hartmann sensors by 2600-4600% over values attained in conventional SH designs.

2003

Kim, Jongbaeg, Hyuck Choo, Liwei Lin, and Richard S. Muller. Microfabricated Torsional Actuator Using Self-Aligned Plastic Deformation." In: 12th International Conference on TRANSDUCERS, Solid-State Sensors, Actuators and Microsystems 2003, Vol. 2. (Piscataway, NJ: IEEE, 2003): 1015-1018. ISBN 0-7803-7731-1.

We describe microfabricated torsional actuators that are made using self-aligned plastic deformation in a batch process. The microactuators are formed in single-crystal silicon and driven by vertical comb-drives. Structures have been built that resonate at frequencies between 1.90 and 5.33 kHz achieving scanning angles up to 19.2 degrees with driving voltages of 40 V_(dc) plus 13 V_(ac). After continuous testing of 5 billion cycles at the maximum scanning angle, there appears to be no observable degradation or fatigue of the plastically deformed silicon tors ion bars. We present measured results obtained with MEMS scanning mirrors; the actuators may be useful for many other MEMS applications.

Gupta, Kishnan, Hyuck Choo, Hanjun Kim, and Richard S. Muller. "Micromachined Polarization Beam Splitters for the Visible Spectrum." In: 2003 IEEE/LEOS International Conference on Optical MEMS. (Piscataway, NJ: IEEE, 2003): 171-172. ISBN 0-7803-7830-X.

We have built and characterized batch-processed polarization beam splitters (PBS), important optical components to separate the orthogonal TE and TM components of light. The devices were fabricated from thin-film, low-stress silicon nitride membranes and showed excellent performance. By stacking membranes, and a triple-layer PBS produced extinction ratios of 21 and 16dB for reflected and transmitted light rays, respectively.

Choo, Hyuck and Richard S. Muller. "Optical Properties of Microlenses Fabricated Using Hydrophobic Effects and Polymer-Jet-Printing Technology." In: 2003 IEEE/LEOS International Conference on Optical MEMS. (Piscataway, NJ: IEEE, 2003): 169-170. ISBN 0-7803-7830-X

We describe high-precision microlenses with excellent optical characteristics. The lenses are formed precisely at desired locations on a wafer using a polymer-jet system in which hydrophobic effects define the lens diameter and surface tension creates a high-quality optical surface. To make the lenses, we defined hydrophilic circular regions at desired locations using photolithography to pattern a 0.2-pm thick Teflon (hydrophobic) layer on a quartz substrate, as shown in Figures 1 and 2. Then, using a polymer-microjet printing system (Figure 3), we dispense an exact amount of UV-curable polymer within hydrophilic circles to obtain microlenses having desired optical properties [ 13. Figure 4 shows that adjusting the volume of the UV-curable optical epoxy within a hydrophilic circle of a given diameter changes the curvature of the microlens. The step resolution of the microlens volume is determined by the average droplet size (~25pL) of the polymer-jet print head. This hybrid method enables us to define the locations and diameters of microlenses with a ±1 μm precision as well as to control the curvatures of the microlenses accurately.