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Antao Chen

Principal Engineer

Affiliate Associate Professor, Electrical Engineering

Email

antaochen@apl.washington.edu

Phone

206-543-1274

Research Interests

Photonics, Nonlinear Optical Polymer Materials, Optical Waveguide Devices, Optical Sensors, and Nanometer-scale Photonic Structures

Biosketch

Dr. Chen's expertise is in photonics-the science and technology of the application of light, which incorporates optics, laser technology, electrical engineering, materials science, and information storage and processing. Dr. Chen worked previously for Lucent Bell Labs Optoelectronics Center on the DARPA MONET program making lithium niobate polarization independent, strictly non-blocking optical switches. The work won the 1999 Bell Labs President Gold Award. His design and development of a lithium niobate modulator and polarization controller was honored by Photonics Spectra the Photonics Circle of Excellence Award. Dr. Chen joined the Applied Physics Laboratory in early 2004; he supervises the research of several graduate students in his lab and is an Affiliate Associate Professor in the UW's Department of Electrical Engineering.

Education

B.S., Beijing Institute of Technology, 1983

M.Eng., Beijing Institute of Technology, 1989

M.S., University of Southern California, 1995

Ph.D., University of Southern California, 1998

Publications

2000-present and while at APL-UW

Electro-optical materials: Efficient poling of electro-optic polymers in thin films and silicon slot waveguides by detachable pyroelectric crystals

Huang, S., J. Luo, H-L. Yip, A. Ayazi, X-H. Zhou, M. Gould, A. Chen, T. Baeher-Jones, M. Hochberg, and A.K.-Y. Jen, "Electro-optical materials: Efficient poling of electro-optic polymers in thin films and silicon slot waveguides by detachable pyroelectric crystals," Adv. Mater., 24, OP1, doi:10.1002/adma.201290057, 2012.

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8 Mar 2012

Pyroelectric crystals are used as a conformal and detachable electric field source to efficiently pole electro-optic (E-O) polymers in both parallel-plate (transverse) and in-plane (quasi-longitudinal) configurations. Large Pockels coefficients in poled thin films and high tunability of resonance wavelength shift in hybrid polymer silicon slot waveguide ring-resonator modulators have been achieved using this method.

Chemiresistive response of silicon nanowires to trace vapor of nitro explosives

Wang, D., H. Sun, A. Chen, S.-H. Jang, A. Jen, and A. Szep, "Chemiresistive response of silicon nanowires to trace vapor of nitro explosives," Nanoscale, 4, 2628-2632, doi:10.1039/C2NR30107E, 2012.

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24 Feb 2012

Silicon nanowires are observed to behave as chemically modulated resistors and exhibit sensitive and fast electrical responses to vapors of common nitro explosives and their degradation by-products. The nanowires were prepared with a top-down nano-fabrication process on a silicon-on-insulator wafer. Surface of the silicon nanowires was modified by plasma treatments. Both hydrogen and oxygen plasma treatments can significantly improve the responses, and oxygen plasma changes the majority carrier from p- to n-type on the surface of silicion nanowires thin film. The sensitivity is found to increase when the cross section of the nanowires decreases.

Fabrication and characterization of a hybrid SOI 1x4 silicon-slot optical modulator array incorporating EO polymers for optical phased-array antenna applications

Kim, R.S., A. Szep, N.G. Usechak, A. Chen, H. Sun, S. Shi, D. Abeysinghe, Y.-H. You, and L.R. Dalton, "Fabrication and characterization of a hybrid SOI 1x4 silicon-slot optical modulator array incorporating EO polymers for optical phased-array antenna applications," Proc. SPIE, 8259 and RF and Millimeter-Wave Photonics II, 82590B, doi:10.1117/12.913652, 2012.

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23 Feb 2012

Optical phased arrays are promising candidates for both RF signal processing and optical beam forming and steering. These platforms not only enable accurate electrically controlled beam steering at high frequencies but also have the potential to significantly improve the performance of future free-space optical communications systems. In this work we exploit recent advancements in both nano-scale hybrid silicon-slot waveguides and electro-optic (EO) polymers to demonstrate an integrated optical phased-array antenna. Specifically, we create a hybrid integrated "photonic circuit" that connects an array of optical phase modulators, fed by a common optical signal and a 1x4 splitter, to a compact optical waveguide diffraction array for optical beam steering applications. The fundamental characteristics of the resulting integrated optical beam former, including the optical insertion loss, driving voltage, and phase control from the waveguide aperture are summarized in this letter.

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Reduction of scattering loss of silicon slot waveguides by RCA smoothing

Sun, H., A. Chen, D. Abeysinghe, A. Szep, and R.S. Kim, "Reduction of scattering loss of silicon slot waveguides by RCA smoothing," Opt. Lett., 37, 13-15, doi:10.1364/OL.37.000013, 2011.

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22 Dec 2011

Because of stronger optical confinement density, silicon slot waveguides tend to have higher scattering loss than normal ridge waveguides with same sidewall roughness. A wet chemical process is found to be highly effective in reducing the surface roughness and scattering loss. A reduction in scattering loss by 10.2 dB/cm for TE and 8.5 dB/cm for TM polarizations has been achieved.

Broadband Optical Modulators: Science, Technology, and Applications.

Chen, A., and E. Murphy, "Broadband Optical Modulators: Science, Technology, and Applications." CRC Press, Boca Raton, FL, 2011, 568 pp.

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16 Nov 2011

Considered the most comprehensive book yet published on this critical subject, Broadband Optical Modulators: Science, Technology, and Applications offers an incredibly wide-ranging yet in-depth overview of the state of the art in the design and use of optical modulators. A compilation of expert insights, this book covers fundamental and practical aspects, from materials to systems, addressing historical and more recent developments.

Considered the most comprehensive book yet published on this critical subject, Broadband Optical Modulators: Science, Technology, and Applications offers an incredibly wide-ranging yet in-depth overview of the state of the art in the design and use of optical modulators. A compilation of expert insights, this book covers fundamental and practical aspects, from materials to systems, addressing historical and more recent developments.

Achieving higher modulation efficiency in electrooptic polymer modulator with slotted silicon waveguide

Chen, A., H. Sun, A. Szep, S. Shi, D. Prather, Z. Lin, R. Kim, and D. Abeysinghe, "Achieving higher modulation efficiency in electrooptic polymer modulator with slotted silicon waveguide," J. Lightwave Technol., 29, 3310-3318, doi:10.1109/JLT.2011.2168385 , 2011.

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1 Nov 2011

Silicon slot waveguide based Mach-Zehnder interferometric modulators with electrooptic polymers in the slot have the advantage of low half-wave voltage-length product. Several key aspects of this unconventional electrooptic polymer modulator design to optimize the modulator performance are studied in this work. Both computer simulation and experiments have been conducted to understand the relationship between modulator performance such as modulation efficiency, optical loss and the waveguide design parameters. Techniques to achieve efficient poling of electrooptic polymers in the silicon slot waveguide have been developed. The doping of the silicon to enhance conductivity for efficient poling and the trade-off between conductivity and optical loss are experimentally investigated. Surface passivation of silicon nanophotonic structures has been found to be effective in improving poling efficiency. By applying these techniques to a silicon slot waveguide Mach-Zehnder modulator, a low voltage length product of 0.52 V cm has been achieved. Finally travelling wave electrode designs have been evaluated and the results show that the bandwidth is mainly limited by the attenuation of the radio frequency signal in the electrode and a standard coplanar waveguide electrode design is able to reach 20 GHz in modulators of silicon slot waveguide embedded in electrooptic polymer.

Electro-optic modulation in horizontally slotted silicon/organic crystal hybrid devices

Figi, H., D.H. Bale, A. Szep, L.R. Dalton, and A. Chen, "Electro-optic modulation in horizontally slotted silicon/organic crystal hybrid devices," J. Opt. Soc. Am. B, 28, 2291-2300, doi:10.1364/JOSAB.28.002291, 2011.

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1 Sep 2011

Electro-optic modulation is achieved in devices consisting of single-crystalline thin films of N-benzyl-2-methyl-4-nitroaniline grown from the melt in the slots of phase modulators based on horizontally slotted silicon waveguides. To the best of our knowledge, this is the first experimental realization of an electro-optically active horizontally slotted silicon waveguide and also the first demonstration of organic crystalline materials being implemented into the slotted silicon photonics technology. The experimentally determined half-wave voltage times length product and the losses are estimated to be 14.7 plus/minus 2 V cm and 10 plus/minus 2.4 dB/cm, respectively. The fabrication concept employed here circumvents technological issues present in the context of conventional vertically slotted waveguide structures, since the slots with cross-sectional dimensions of about 1000 nm x 160 nm have been patterned with standard optical photolithography into thermally grown oxide sandwiched between two fusion bonded device silicon layers. In contrast to previously reported vertically slotted silicon waveguides with polymeric slot materials, organic crystalline based devices do not require high-electric-field poling prior to operation and feature an excellent long-term stability of dipole orientation in addition to superior photochemical stability.

The effect of dipole moment and electron deficiency of analytes on the chemiresistive response of TiO2(B) nanowires

Wang, D., A. Chen, S.H., Jang, J. Davies, and A.K.-Y. Jen, "The effect of dipole moment and electron deficiency of analytes on the chemiresistive response of TiO2(B) nanowires," Analyst, 136, 4179-4182, doi:10.1039/C1An16531A, 2011.

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22 Aug 2011

Nanostructured TiO2(B) thin films were found to have strong and fast chemiresistive response to nitro-aromatic and nitro-amino explosives recently. In this study, the effects of dipole moment and electron deficiency of the analyte molecules on the chemiresistive response are explored to understand the details of molecular interactions of analytes with the sensor surface which lead to charge depletion and the chemiresistive effect. It was found that the speed of the response is dominated by the polarity of the analytes and molecules with larger dipole moments produce faster responses. The degree of the response was found to be dominated by the electron deficiency of the analytes and molecules with greater electron deficiency produce stronger chemiresistive responses.

Induction detection of concealed bulk banknotes

Fuller, C., and A. Chen, "Induction detection of concealed bulk banknotes," Meas. Sci. Technol., 22, doi:10.1088/0957-0233/22/10/105703, 2011.

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19 Aug 2011

Bulk cash smuggling is a serious issue that has grown in volume in recent years. By building on the magnetic characteristics of paper currency, induction sensing is found to be capable of quickly detecting large masses of banknotes. The results show that this method is effective in detecting bulk cash through concealing materials such as plastics, cardboards, fabrics and aluminum foil. The significant difference in the observed phase between the received signals caused by conducting materials and ferrite compounds, found in banknotes, provides a good indication that this process can overcome the interference by metal objects in a real sensing application. This identification strategy has the potential to not only detect the presence of banknotes, but also the number, while still eliminating false positives caused by metal objects.

Organic electro-optic thin films by simultaneous vacuum deposition and laser-assisted poling

Wang, Z., W. Sun, A. Chen, I. Kosilkin, D. Bale, and L.R. Dalton, "Organic electro-optic thin films by simultaneous vacuum deposition and laser-assisted poling," Opt. Lett., 36, 2853-2855, doi:10.1364/OL. 36.002853, 2011.

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22 Jul 2011

Organic materials with a high second-order optical nonlinearity have an important application for the next generation of computing, telecommunications, and other industries. Because of a high electro-optic coefficient and low dielectric constant, N-benzyl-2-methyl-4-nitroaniline (BNA) single crystals have been grown and their optical properties have been extensively studied. In this Letter, a poled BNA thin film material was prepared through simultaneous vacuum evaporation and laser-assisted electrical poling. The linear electro-optic coefficients of BNA were measured by a Young's two-slit interference electro-optic measurement method. The dependence of the resulting electro-optic properties on optical power was explored. Enhancement induced by laser illumination was demonstrated. The BNA thin film can exhibit an r33 value comparable to that of BNA single crystals.

Terahertz reflectometry of burn wounds in a rat model

Arbab, M.H., T.C. Dickey, D.P. Winebrenner, A. Chen, M.B. Klein, and P.D. Mourad, "Terahertz reflectometry of burn wounds in a rat model," Biomed. Opt. Express, 2, 2339-2347, doi:10.1364/BOE.2.002339, 2011.

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21 Jul 2011

We present sub-millimeter wave reflectometry of an experimental rat skin burn model obtained by the Terahertz Time-Domain Spectroscopy (THz-TDS) technique. Full thickness burns, as confirmed by histology, were created on rats (n = 4) euthanized immediately prior to the experiments. Statistical analysis shows that the burned tissue exhibits higher reflectivity compared to normal skin over a frequency range between 0.5 and 0.7 THz (p < 0.05), likely due to post-burn formation of interstitial edema. Furthermore, we demonstrate that a double Debye dielectric relaxation model can be used to explain the terahertz response of both normal and less severely burned rat skin. Finally, our data suggest that the degree of conformation between the experimental burn measurements and the model for normal skin can potentially be used to infer the extent of burn severity.

Room-temperature chemiresistive effect of TiO2!-!B nanowires to nitroaromatic and nitroamine explosives

Wang, D.L., A.T. Chen, Q.F. Zhang, and G.Z. Cao, "Room-temperature chemiresistive effect of TiO2!-!B nanowires to nitroaromatic and nitroamine explosives," IEEE Sensor. J., 11, 1352-1358, doi:10.1109/JSEN.2010.2089618, 2011.

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1 Jun 2011

Semiconducting TiO2-B nanowires were synthesized and their chemical sensor performance was studied. It was found that the TiO2-B nanowires exhibited a large and reversible change in electrical resistivity when exposed to trace vapor of nitroaromatic and nitroamine explosives. The sensor showed high sensitivity and fast response at room temperature. The chemiresistive effect was attributed to the adsorption of high electronegativity explosives on the TiO2-B nanowires and a consequent depletion of charge carriers in the nanowires by surface states created by the explosive molecules. The role of the TiO2-B nanowires is proposed to impart both an extremely large surface area for the adsorption of gas molecules and a surface enriched in hydroxyl groups which connect to nitro groups of the explosive molecules. Chemiresistive sensors made of TiO2-B semiconducting nanowires promised to achieve micro-sized devices with several orders of magnitude reduction in dimensions, weight, and power consumption over current explosive detectors.

Electro-optic thin films of organic nonlinear optic molecules aligned through vacuum deposition

Sun, W., Z. Wang, A. Chen, I. Kosilkin, D. Bale, and L.R. Dalton, "Electro-optic thin films of organic nonlinear optic molecules aligned through vacuum deposition," Opt. Express, 19, 11189-11195, doi:10.1364/OE.19.011189, 2011.

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24 May 2011

Nonlinear optical molecules can be vacuum deposited into uniform thin films using thermal evaporation. Alignment order can be achieved during thin film deposition by an in-plane electrical field poling using electrodes patterned on the substrate. Electro-optic (EO) coefficients, r_33 and r_13 are independently measured using Young's interferometry technique. Thin-films of N-benzyl-2-methyl-4-nitroaniline (BNA) can exhibit an EO coefficient, r_33, comparable to that of BNA single crystals. EO coefficients of BNA at different poling fields, wavelengths, and frequencies are investigated.

Horizontally slotted electro-optic silicon waveguides featuring an organic crystalline slot material

Figi, H., D.H. Bale, A. Szep, L.R. Dalton, and A. Chen, "Horizontally slotted electro-optic silicon waveguides featuring an organic crystalline slot material," Proceedings, European Conference on Lasers and Electro-Optics (CLEO/Europe), 22 May, Munich, Germany, paper #CD6_4 (Optical Society of America, 2011).

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22 May 2011

We show the first demonstration of electro-optic modulation in horizontally slotted silicon/organic hybrid waveguides. The slots are patterned into thermal oxide sandwiched between fusion bonded silicon layers and filled with an organic single-crystalline electro-optic material.

Sensitivity of titania(B) nanowires to nitroaromatic and nitroamino explosives at room temperature via surface hydroxyl groups

Wang, D.L., A. Chen, S.H. Jang, H.-L. Yip, and A.K.-Y. Jen, "Sensitivity of titania(B) nanowires to nitroaromatic and nitroamino explosives at room temperature via surface hydroxyl groups," J. Mater. Chem., 21, 7269-7273, doi: 10.1039/C1JM10124B, 2011.

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15 Apr 2011

Thin films of TiO2(B) nanowires are known to have sensitive and fast response to vapors of nitro-explosives under ambient conditions. The sensing response is believed to be affected by the humidity of ambient air that changes the density of hydroxyl groups on the TiO2(B) surfaces. To verify this mechanism, the role of surface hydroxyl groups on TiO2(B) nanowires was investigated via various surface modifications. It was found that a higher density of surface hydroxyl groups will constantly enhance the chemiresistive response of TiO2(B) nanowires to the nitro-explosives vapors. These surface hydroxyl groups serve as a pathway for effective charge transfer between the nitro groups on the explosive molecules and the TiO2(B). The evidence of charge transfer complex formation between nitro groups and titanium dioxide is also confirmed by Fourier transform infrared spectroscopy.

Characterization of burn injuries using terahertz time-domain spectroscopy

Arbab, M.H., T.C. Dickey, D.P. Winebrenner, A. Chen, and P.D. Mourad, "Characterization of burn injuries using terahertz time-domain spectroscopy," Proc. SPIE, 7890, 78900Q, doi:10.1117/12.876229, 2011.

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10 Feb 2011

The accuracy rates of the clinical assessment techniques used in grading burn injuries remain significantly low for partial thickness burns. In this paper, we present experimental results from terahertz characterization of 2nd and 3rd degree burn wounds induced on a rat model. Reflection measurements were obtained from the surface of both burned and normal skin using pulsed terahertz spectroscopy. Signal processing techniques are described for interpretation of the acquired terahertz waveform and differentiation of burn wounds. Furthermore, the progression of burn injuries is shown by comparison between acute characterization and 72-hours survival studies. While the water content of healthy and desiccated skin has been considered as a source of terahertz signal contrast, it is demonstrated that other biological effects such as formation of post-burn interstitial edema as well as the density of the discrete scattering structures in the skin (such as hair follicles, sweat glands, etc.) play a significant role in the terahertz response of the burn wounds.

Measuring order in contact-poled organic electrooptic materials with variable-angle polarization-referenced absorption spectroscopy (VAPRAS)

Olbricht, B.C., P.A. Sullivan, P.C. Dennis, J.T. Hurst, L.E. Johnson, S.J. Benight, J.A. Davies, A. Chen, B.E. Eichinger, P.J. Reid, L.R. Dalton, and B.H. Robinson, "Measuring order in contact-poled organic electrooptic materials with variable-angle polarization-referenced absorption spectroscopy (VAPRAS)," J. Phys. Chem. B, 115, 231-241, doi:10.1021/jp107995t, 2011.

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20 Jan 2011

Organic nonlinear electrooptical (ONLO) chromophores must be acentrically ordered for the ONLO material to have electrooptic (EO) activity. The magnitude of the order is characterized by the acentric order parameter, cos3 Beta, where Beta is the major Euler angle between the main axis of the chromophore and the poling field which imposes the acentric order. The acentric order parameter, which is difficult to measure directly, is related to the centrosymmetric order parameter through the underlying statistical distribution. We have developed a method to determine centrosymmetric order of the ONLO chromophores when the order is low. We have extended the method based on the absorption of light to determine the centrosymmetric order parameter induced by a poling field on a thin film sample of ONLO material. We find that the order parameters, analyzed by two different methods, are similar and also consistent with theoretical estimates from modeling of the system using coarse-grained Monte Carlo statistical mechanical methods.

Retrieval of terahertz spectroscopic signatures in the presence of rough surface scattering using wavelet methods

Arbab, M.H., D.P. Winebrenner, E.I. Thorsos, and A. Chen, "Retrieval of terahertz spectroscopic signatures in the presence of rough surface scattering using wavelet methods," Appl. Phys. Lett., 97, doi:10.1063/1.3507384, 2010.

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1 Nov 2010

Scattering of terahertz waves by surface roughness can obscure spectral signatures of chemicals at these frequencies. We demonstrate this effect using controlled levels of surface scattering on alpha-lactose monohydrate pellets. Furthermore, we show an implementation of wavelet methods that can retrieve terahertz spectral information from rough surface targets. We use a multiresolution analysis of the rough-surface-scattered signal utilizing the maximal overlap discrete wavelet transform (MODWT) to extract the resonant signature of lactose. We present a periodic extension technique to circumvent the circular boundary conditions of MODWT, which can be robustly used in an automated terahertz stand-off detection device.

Application of wavelet transforms in terahertz spectroscopy of rough surface targets

Arbab, M.H., D.P. Winebrenner, E.I. Thorsos, and A. Chen, "Application of wavelet transforms in terahertz spectroscopy of rough surface targets," Proc. of SPIE, 7601, Terahertz Technology and Applications III, L.P. Sadwick and C.M.M. O'Sullivan, eds., doi:10.1117/12.845944, 2010.

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18 Feb 2010

Previously, it has been shown that scattering of terahertz waves by surface roughness of a target can alter the terahertz absorption spectrum and thus obscure the detection of some chemicals in both transmission and reflection geometries. In this paper it is demonstrated that by employing Maximal Overlap Discrete Wavelet Transform (MODWT) coefficients, wavelet-based methods can be used to retrieve spectroscopic information from a broadband terahertz signal reflected from a rough surface target. It is concluded that while the commonly used direct frequency domain deconvolution method fails to accurately characterize and detect the resonance in the dielectric constant of rough surface lactose pellets, wavelet techniques were able to successfully identify such features.

Field-induced guiding optical devices made from electro-optic polymers

Pyayt, A., J. Luo, A.K.-Y. Jen, L. Dalton, and A. Chen, "Field-induced guiding optical devices made from electro-optic polymers," Appl. Optics, 49, 892-896, doi:10.1364/AO.49.000892, 2010.

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4 Feb 2010

Field-induced guiding (FIG) has been known for several decades. Many advanced optical FIG devices (light modulators, splitters, scanners, etc.) were proposed, but due to high fabrication cost and/or high operation voltages they were considered impractical. For the first time to our knowledge we propose to use electro-optic (EO) polymers for the fabrication of FIG devices. This alleviates these limitations and makes this powerful device fabrication paradigm applicable in practice. We theoretically predict and experimentally demonstrate FIG in EO polymers. We also propose the idea of new class of devices--multifunctional optical devices that can completely change their functionality on the fly.

Measurement and modeling of rough surface effects on terahertz spectroscopy

Henry, S.C., S. Schecklman, G.P. Kniffin, L.M. Zurk, and A. Chen, "Measurement and modeling of rough surface effects on terahertz spectroscopy," In Proceedings, SPIE - Terahertz Technology and Applications III, San Francisco, 27 January, L.P. Sadwick and C.M.M. O'Sullivan, eds., paper no. 760108 (2010).

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27 Jan 2010

Recent improvements in sensing technology have driven new research areas within the terahertz (THz) portion of the electromagnetic (EM) spectrum. While there are several promising THz applications, several outstanding technical challenges need to be addressed before robust systems can be deployed. A particularly compelling application is the potential use of THz reflection spectroscopy for stand-off detection of drugs and explosives.

A primary challenge for this application is to have sufficient signal-to-noise ratio (SNR) to allow spectroscopic identification of the target material, and surface roughness can have an impact on identification. However, scattering from a rough surface may be observed at all angles, suggesting diffuse returns can be used in robust imaging of non-cooperative targets. Furthermore, the scattering physics can also distort the reflection spectra, complicating classification algorithms.

In this work, rough surface scattering effects were first isolated by measuring diffuse scattering for gold-coated sandpaper of varying roughness. Secondly, we measured scattering returns from a rough sample with a spectral signature, namely a-lactose monohydrate mixed with Teflon and pressed with sandpaper to introduce controlled roughness. For both the specular and diffuse reflection measurements, the application of traditional spectroscopy techniques provided the ability to resolve the 0.54 THz absorption peak. These results are compared with results from a smooth surface. Implications of the results on the ability to detect explosives with THz reflection spectroscopy are presented and discussed. In addition, the Small Perturbation Method (SPM) is employed to predict backscatter from lactose with a small amount of roughness.

Efficient fiber coupler for vertical silicon slot waveguides

Sun, H., A. Chen, A. Szep, and L.R. Dalton, "Efficient fiber coupler for vertical silicon slot waveguides," Opt. Express, 17, 22571-22577, 2009.

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24 Nov 2009

A mode size converter for efficient fiber coupling to silicon slot waveguides was proposed and demonstrated. It consists of two inverted lateral tapers that extend from the two strips of the silicon slot waveguide, and an overlaid low index waveguide with expanded mode size. Parameters including taper length and taper tip width were optimized with computer simulations. Samples were fabricated with a combined electron beam and photolithography process on a silicon-on-insulator wafer. The measured coupling loss to a standard single mode optical fiber was reduced by 8 dB for TE mode and 5.2 dB for TM mode with the converter.

Multiple slot waveguides for enhanced biochemical sensing

Sun, H., A. Chen, and L.R. Dalton, "Multiple slot waveguides for enhanced biochemical sensing," Proceedings, IEEE/LEOS International Conference on Optical MEMS and Nanophotonics, Clearwater, FL, 17-20 August, 21-22, doi:10.1109/OMEMS.2009.5338626 (IEEE, 2009).

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20 Nov 2009

Multiple slots in optical waveguides greatly increase evanescent field and surface area for interaction with surrounding media, leading to more than 5 times improvement of sensitivities for biochemical detections as confirmed by simulations and experiments.

Terahertz generation and detection setup based on pump-probe scheme

Zhou, Z., A. Chen, L.-S. Feng, X.-J. Xin, and C.-X. Yu, "Terahertz generation and detection setup based on pump-probe scheme," Microw. Opt. Technol. Lett., 51, 1617-1619, doi:10.1002/mop.24413, 2009.

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1 Jul 2009

A terahertz (THz) generation and detection setup based on pump-probe scheme is presented. THz pulses are generated and detected successfully using nonlinear optical rectification of high power femtosecond laser pulses and electro-optic sampling. 3 THz available spectrum range is obtained for the setup. The use of THz pulses for spectroscopy is tested by measuring the absorption spectrums of water in air and dry nitrogen, respectively. The measurement results show the setup has a high potential of THz spectroscopy.

Enhanced evanescent confinement in multiple-slot waveguides and its application in biochemical sensing

Sun, H., A. Chen, and L.R. Dalton, "Enhanced evanescent confinement in multiple-slot waveguides and its application in biochemical sensing," IEEE Photonics J., 1, 48-57, doi:10.1109/JPHOT.2009.2025602, 2009.

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16 Jun 2009

The interaction of light propagating in the wave-guiding structures with the surrounding media through its evanescent field has been used to detect biochemical activities without labeling the target molecules. By introducing multiple slots in a dielectric waveguide, we found that a much stronger evanescent field could be achieved in the surrounding media while still maintaining good optical confinement. The experimental demonstration of microring resonator sensors using single-mode waveguides having three slots indicates a 5-fold increase in sensitivity for homogeneous sensing and more than 3-fold increase in sensitivity for surface sensing in comparison to those of a waveguide without slots. Numerical simulations suggest an even higher sensitivity enhancement (as high as 100 times) as more slots and larger waveguide widths are used.

A reflective microring notch filter and sensor

Sun, H., A. Chen, and L.R. Dalton, "A reflective microring notch filter and sensor," Opt. Express, 17, 10731-10737, 2009.

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11 Jun 2009

We present a new design of wavelength selective reflector composed of a Y junction and a singly coupled microring resonator, and demonstrate its biochemical sensing applications with a prototype device. In contrast with other reflectors like distributed Bragg reflectors, this device acts as notch filter at its reflection port. One promising application of the device is for remote sensing of harsh or inaccessible site, where only one optical fiber is required to transmit the input and reflected light signal over a long distance. The design can also be used to make microring cavity lasers.

Micro-ring resonators made in poled and un-poled chromophore-containing polymers for optical communication and sensors

Chen, A., H. Sun, A. Pyayt, L.R. Dalton, J. Luo, and A.K.-Y. Jen, "Micro-ring resonators made in poled and un-poled chromophore-containing polymers for optical communication and sensors," IEEE J. Selected Topics Quantum Elec., 14, 1281-1288, doi:10.1109/JSTQE.2008.2001474, 2008.

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30 Sep 2008

Poled and unpoled chromophore-containing polymers offer some unique advantages in device functionality and fabrication. UV light and electron beam (e-beam) can bleach out the color of chromophores and reduce the index of refraction of the polymer. The photobleaching and e-beam bleaching methods form optical waveguides in a single step and do not involve solvents or wet chemicals, and can be applied to polymers that are not compatible with other waveguide fabrication techniques. A variety of microring resonator devices for fiber-optic telecommunication and sensors have been realized with chromophore-containing polymers. A novel broadband fiber-optic electric field sensor is presented as an example. The sensor uses a polymer with chromophores preferentially aligned after electric poling, and the microring resonator is directly coupled to the core of optical fiber. The feasibility of vertical integration of a poled electrooptic polymer waveguide device interfaced with silicon microelectronic circuits is also demonstrated.

Measurement and application of incoherent terahertz scattering using time-domain spectroscopy

Arbab, M.H., D.P. Winebrenner, A. Chen, D. Wang, E.I. Thorsos, and L.M. Zurk, "Measurement and application of incoherent terahertz scattering using time-domain spectroscopy," Proceedings, 33rd International Conference on Infrared and Millimeter Waves and 16th International Conference on Terahertz Electronics, 15-29 September, Pasadena, CA (IEEE, 2008).

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15 Sep 2008

Measurement of incoherent rough surface scattering powers in terahertz frequency regime by means of time-domain spectroscopy has been for the first time demonstrated. Furthermore, applications of such incoherent measurements in spectroscopy and detection of chemicals are presented.

Poled and unpoled polymers with chromophore dopants for optical sensors

Chen, A., "Poled and unpoled polymers with chromophore dopants for optical sensors," Proceedings, Linear and Nonlinear Optics of Organic Materials VIII, 12-14 August, San Diego, CA, doi:10.1117/12.792379 (SPIE, 2008).

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2 Sep 2008

Optical properties such as index of refraction and optical absorption of many chromophore-doped polymers are sensitive to the physical and chemical environment to which the polymers are exposed. Detection sensitivity is further enhanced by optical micro-resonator structures such as waveguide micro-ring resonator and fiber Bragg gratings. Chromophore-doped polymers also offer some desirable flexibility in device fabrication. Ultraviolet light and electron beam can reduce the index of refraction of the polymer. The photobleaching and electron beam bleaching methods form optical waveguides in a single fabrication step and do not involve solvents or wet chemicals, and can be applied to polymers that are not compatible with other waveguide fabrication techniques.

This paper provides an overview of the basic principles and designs of such sensors. A chemical sensor to detect trace explosives and a broadband fiber optic electric-field sensor are presented as practical examples.

Polarization selective electro-optic polymer waveguide devices by direct electron beam writing

Sun, H., A. Chen, B.C. Olbricht, J.A. Davies, P.A. Sullivan, Y. Liao, Z. Shi, J. Luo, A.K.-Y. Jen, and L.R. Dalton, "Polarization selective electro-optic polymer waveguide devices by direct electron beam writing," Opt. Express, 16, 8472-8479, 2008

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27 May 2008

A novel technique for the fabrication of polarization selective electro-optic polymer waveguide devices with direct electron beam writing was described. Birefringence induced by the electric field poling in the electro-optic polymer film was erased in the electron beam exposed regions. The formed waveguides had stronger confinement for the light polarized along the poling direction. High fabrication resolution on the 100 nm scale or smaller could be achieved. Fabrication of polymer polarizer and polarization selective microring resonators with this technique was reported. The highest polarization extinction ratio was measured to be 21.4 dB.

Electro optic polymer microring resonators patterned with electron beam bleaching

Sun, H., B.C. Olbricht, J.A. Davies, P.A. Sullivan, L.R. Dalton, and A. Chen, "Electro optic polymer microring resonators patterned with electron beam bleaching," Appl. Phys. Lett., 92, 193305-1 - 2013193305-3, doi:10.1063/1.2926680, 2008.

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13 May 2008

Decomposition of chromophore molecules under direct electron beam irradiation reduces the refractive index of chromophore containing polymers. The induced refractive index contrast between the exposed and unexposed regions is high enough for waveguide bends of small radius and thus microring resonator devices. This electron beam bleaching of chromophore-containing polymers provides a fabrication approach for nonlinear polymer optical waveguide devices. Fabrication of high quality microring resonators with critical feature size on the order of 100 nm was demonstrated with this technique in an electro-optic polymer that contains YL124 chromophores.

Chromophore-containing polymers for trace explosive sensors

Chen, A., H. Sun, A. Pyayt, X. Zhang, J. Luo, A. Jen, P.A. Sullivan, S. Elangovan, L.R. Dalton, R. Dinu, D. Jin, and D. Huang, "Chromophore-containing polymers for trace explosive sensors," J. Phys. Chem. C, 112, 8072-8078, doi:10.1021/jp7118372, 2008.

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29 Apr 2008

The optical properties such as index of refraction and optical absorption of many chromophore-containing polymers are sensitive to the physical and chemical environment to which the polymers are exposed. To demonstrate such applications, chemical sensors to detect trace explosives are presented. The sensors use polymers that contain chromophores whose molecular structure consists of an electron donor and an electron acceptor connected by a charge-transfer bridge of conjugated π-orbital electrons. The polymers used for the trace explosives sensor are not poled, and the chromophores are randomly oriented in the polymer host. Waveguide microring resonator and fiber Bragg grating structures were used in these sensors to enhance the detection sensitivity. Because chromophores undergo photodecomposition under intense ultraviolet radiation, chromophore-containing polymers can be patterned with ultraviolet light to create optical resonator structures in a single photobleaching step. The chemical sensor has shown part-per-billion level sensitivity and good specificity to the vapor, in air, of an explosive simulant 2,4-dinitrotoluene.

Direct electron beam writing of electro-optic polymer microring resonators

Sun, H., A. Chen, B.C. Olbricht, J.A. Davies, P.A. Sullivan, Y. Liao, and L.R. Dalton, "Direct electron beam writing of electro-optic polymer microring resonators," Opt. Express, 16, 6592-6599, doi:10.1364/OE.16.006592, 2008.

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24 Apr 2008

Electro-optic polymer waveguides in electron beam sensitive polymethyl methacrylate (PMMA) polymer matrix doped with organic nonlinear chromophores could be directly patterned by electron beam exposure with high resolution and smoothness. The polymer in the exposed regions was removed with standard electron beam resist developer and without damaging the chromophore containing polymer waveguides. Feature sizes on the order of 100 nm could be clearly resolved. High quality microring resonators made of YL124/PMMA electro-optic polymer were successfully fabricated with this technique. The measured resonance extinction ratios were more than 16 dB and quality factors were in the range of 103~104.

Scattering effects in terahertz reflection spectroscopy

Zurk, L.M., G. Sundberg, S. Schecklman, Z. Zhou, A. Chen, and E.I. Thorsos, "Scattering effects in terahertz reflection spectroscopy," Proc. SPIE, 6949, 694907-1 - 694907-8, doi:10.1117/12.784222, 2008.

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15 Apr 2008

Recent advances in ultrafast optical laser technology have improved generation and detection of energy within the terahertz (THz) portion of the electromagnetic (EM) spectrum. One promising application of THz spectroscopy is the detection of explosive materials and chemical or biological agents. This application has been motivated by initial measurements that indicate that explosives may have unique spectral characteristics in the THz region thus providing a discernible fingerprint. However, since THz wavelengths are 10's to 100's of microns in scale, rough interfaces between materials as well as the granular nature of explosives can cause frequency-dependent scattering that has the potential to alter or obscure these signatures. For reflection spectroscopy in particular the measured response may be dominated by rough surface scattering, which is in turn influenced by a number of factors including the dielectric contrast, the angle of incidence and scattering, and the operating frequency. In this paper, we present measurements of THz scattering from rough surfaces and compare these measurements with analytical and numerical scattering models. These models are then used to predict the distortion of explosive signatures due to rough surface interfaces with varying surface height deviations and correlation lengths. Implications of scattering effects on the performance of THz sensing of explosive materials are presented and discussed.

Effect of surface scattering on terahertz time domain spectroscopy of chemicals

Arbab, M.H., A. Chen, E.I. Thorsos, D.P. Winebrenner and L.M. Zurk, "Effect of surface scattering on terahertz time domain spectroscopy of chemicals," Proc. SPIE, 6893, Terahertz Technology and Applications, K.J. Linden and L.P. Sadwick, eds., doi:10.1117/12.769015, 2008.

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14 Feb 2008

The existence of unique absorption spectrum patterns for many chemicals at terahertz frequencies has opened an exciting avenue in non-contact safe detection of such materials by terahertz spectroscopy. However, scattering of THz waves, which have wavelengths on the order of material grain sizes, by surface roughness challenges the sensitivity of this detection scheme in practice. In this work, we present terahertz time domain spectroscopy results for materials with rough surfaces. Both reflection from and transmission through lactose, which has sharp absorption peaks in the terahertz regime, are studied and the effect of increasing scattering through controlled surface roughness is investigated. Such electromagnetic scattering can alter the terahertz absorption spectrum and thus obscure the detection of chemicals. Furthermore we examine electro-optic detection of terahertz signals reflected from randomly rough targets with a theoretical electromagnetic system perspective and provide a method to retrieve coherent reflection responses from rough surface targets.

Impacts of terahertz scattering on the reflection spectrum for explosive detection

Zhou, Z., A. Chen, J. Zhang, L.M. Zurk, B. Orlowski, E. Thorsos, D. Winebrenner, and L.R. Dalton, "Impacts of terahertz scattering on the reflection spectrum for explosive detection," Proc. SPIE, 6772, 67720T-1 - 67720T-7, doi:10.1117/12.733305, 2007.

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11 Sep 2007

The effects of surface scattering on terahertz reflection spectrum for explosive detection are studied by measuring terahertz reflection pulses from sandpapers with different roughness coated with gold. The experimental results show that the amplitude decrease and pulse broadening of the detected signal caused by the surface scattering result in the width reduction of Gaussian distribution of the specular scattering coefficient spectrum. A simple analytical model is applied to the analysis of experimental results and good agreements are obtained.

Highly efficient Diels-Alder crosslinkable electro-optic dendrimers for electric-field sensors

Shi, Z., S. Hau, J. Luo, T.-D. Kim, N.M. Tucker, J.-W. Ka, H. Sun, A. Pyajt, L. Dalton, A. Chen, and A.K.-Y. Jen, "Highly efficient Diels-Alder crosslinkable electro-optic dendrimers for electric-field sensors," Adv. Function. Mater., 17, 2557-2563, doi:10.1002/adfm.200600778, 2007.

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1 Sep 2007

One of the most challenging tasks encountered in developing highly efficient electro-optic (EO) devices is to find a material system that possesses all desirable properties such as large EO coefficients, good thermal and mechanical stability, and low optical loss. In order to meet this stringent requirement, we have developed a series of crosslinkable EO dendrimers using the standardized AJL8-type chromophore as the center core and the furyl- and anthryl-containing dendrons as the periphery. Upon adding a trismaleimide (TMI) dienophile, these dendrimers could be in-situ crosslinked via the Diels–Alder cycloaddition and efficiently poled under a high electric field. Through this dynamic process, the spatially voided and π-electron-rich surrounding of the chromophore core changes into a dense and more aliphatic network, with the dipolar chromophore embedded and aligned inside. The resultant materials exhibit large EO coefficients (63–99 pm V-1 at 1.31 µm), excellent temporal stability (the original r33 values remain unchanged at 100°C for more than 500 h), and blue-shifted near-IR absorption. With these combined desirable properties, a poled EOD2/TMI film could be processed through multiple lithographic and etching steps to fabricate a racetrack-shaped micro-ring resonator. By coupling this ring resonator with a side-polished optical fiber, a novel broadband electric-field sensor with high sensitivity of 100 mV-1 at 550 MHz was successfully demonstrated.

Novel hybrid electro-optic modulators with horizontal taper structure

Yu, G., B. Li, D. Jin, L. Zheng, R. Dinu, and A. Chen, "Novel hybrid electro-optic modulators with horizontal taper structure," Proc. SPIE, 6713, 67130B-1 - 67130B-8, 2007.

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27 Aug 2007

Hybrid sol-gel/polymer electro-optic modulators with horizontal taper structure have been designed and fabricated. Optical transition between sol-gel passive waveguides and electro-optic polymer waveguides via horizontal tapers has been realized in the electro-optic modulators. With 1 cm interaction length these hybrid electro-optic modulators have been measured to have a half-wave voltage of 8.91 V (dual drive 4.45 V), an extinction ration of 21.2 dB and an optical insertion loss of 11.8 dB.

Systematic design and simulation of polymer microring resonators with the combination of beam propagation method and matrix model

Sun, H., L. Dalton, and A. Chen, "Systematic design and simulation of polymer microring resonators with the combination of beam propagation method and matrix model," 2007 Digest of the IEEE LEOS Summer Topical Meetings, 217-218, doi:10.1109/LEOSST.2007.4288411, 2007.

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23 Jul 2007

In this paper, a basic microring resonator structure is divided into two separate regions, i.e., a 4-port coupler and a 2-port ring/curved waveguide, thus can be regarded as an optical system composed of two components.

Theory-guided design and synthesis of multichromophore dendrimers: An analysis of the electro-optic effect

Sullivan, P.A., H. Rommel, Y. Liao, B.C. Olbricht, A.J.P. Akelaitis, K.A. Firestone, J.-W. Wang, J. Luo, J.A. Davies, H.C. Dong, B.E. Eichinger, P.J. Reid, A. Chen, A.K.-Y. Jen, B.H. Robinson, and L.R. Dalton, "Theory-guided design and synthesis of multichromophore dendrimers: An analysis of the electro-optic effect," J. Am. Chem. Soc., 129, 7523-7530, doi:10.1021/ja068322b, 2007.

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25 May 2007

Extensive experimental and theoretical study suggests that interchromophore electrostatic interactions are among the most severe impediments to the induction and stability of large electro-optic coefficients in electric-field-poled organic materials. In this report, multichromophore-containing dendritic materials have been investigated as a means to minimize unwanted attenuation of nonlinear optical (electro-optic) activity at high chromophore loading. The dendritic molecular architectures employed were designed to provide optimized molecular scaffolding for electric-field-induced molecular reorientation. Design parameters were based upon past experimental results in conjunction with statistical and quantum mechanical modeling. The electro-optic behavior of these materials was evaluated through experimental and theoretical analysis. Experimental data collected from the dendrimer structures depict a reasonably linear relationship between chromophore number density (N) and electro-optic activity (r33) demonstrating a deviation from the dipolar frustration that typically limits r33 in conventional chromophore/polymer composite materials. The observed linear dependence holds at higher chromophore densities than those that have been found to be practical in systems of organic NLO chromophores dispersed in polymer hosts. Theoretical analysis of these results using Monte Carlo modeling reproduces the experimentally observed trends confirming linear dependence of electro-optic activity on N in the dendrimer materials. These results provide new insight into the ordering behavior of EO dendrimers and demonstrate that the frequently observed asymptotic dependence of electro-optic activity on chromophore number density may be overcome through rational design.

Optical micro-resonator chemical sensor

Pyayt, A., X. Zhang, J. Luo, A. Jen, L. Dalton, and A. Chen, "Optical micro-resonator chemical sensor," Proc. SPIE, 6556, 65561D1-65561D6, doi:10.1117/12.720329, 2007.

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10 Apr 2007

Polymers that contain conjugated molecules can change their index of refraction upon bonding with high explosive molecules. These polymers can be incorporated into micro-ring resonators as trace explosive sensor. Since the resonator cavity itself is made of sensing material, the detection is intrinsic, which may lead to higher sensitivity and faster response than other fiber optic chemical sensors. Photobleaching was used for the fabrication of the microrings resonators. The sensor has shown ppb level of sensitivity to the vapor of an explosive stimulant 2,4- dinitrotoluene and is insensitive to common chemical pollutants including nitrates, sulfates and phosphates.

All-dielectric electro-optic sensor based on polymer micro-resonator coupled side-polished optical fiber

Sun, H., A. Pyayt, J. Luo, Z. Shi, S. Hau, A. Jen, L. Dalton, and A. Chen, "All-dielectric electro-optic sensor based on polymer micro-resonator coupled side-polished optical fiber," IEEE Sens. J., 7, 515-524, doi:10.1109/JSEN.2006.886901, 2007.

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1 Apr 2007

A novel electrooptic (EO) electric field (E-field) sensor based on side-polished fiber coupled with an EO polymer microring resonator is proposed and demonstrated. An EO polymer waveguide with a ring shape is fabricated on the polished flat of an optical fiber. Light in the fiber evanescently couples into the resonator and forms resonant modes for certain wavelengths and produces notches in the output intensity of the fiber. External electric fields change the index of refraction of the ring waveguide and therefore dither its resonant wavelengths. For light of wavelength on the slope of a resonance notch, a change in the output intensity can be detected. The sensor is all dielectric without metal layers to distort the measured E-field. The resonant structure allows the sensor to potentially have much higher sensitivity than other electrooptic sensors based on Mach-Zehnder or polarization modulation. Since electrooptic polymers have higher electrooptic coefficients, lower dielectric constants and faster electrooptic responses than inorganic crystals, higher sensitivity, lower invasiveness, and higher bandwidth of E-field sensing can be expected. This sensor eliminates unreliable fiber-to-waveguide butt coupling as well as the high propagation loss encountered in the long straight EO polymer waveguides of sensors based on Mach-Zehnder structures. By using the fiber itself as the supporting substrate of the ring waveguide, the sensor can have small size and low disturbance to the measured electric field. The concept is demonstrated using AJLS103 EO polymer. A sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz (limited by the measurement system).

Electro-optic polymer microring resonators made by photobleaching

Pyayt, A., J. Zhou, A. Chen, J. Luo, S. Hau, A. Jen, and L. Dalton, "Electro-optic polymer microring resonators made by photobleaching," Proc. SPIE, 6470, 64700Y1-64700Y7, 2007.

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21 Jan 2007

Photobleaching was used for the fabrication of electro-optic polymer microrings resonators. All the device parameters were theoretically optimized. The photobleaching mask was made using e-beam lithography. UV exposure through the mask defined the pattern in the polymer by an irreversible change of the material properties and decrease of the refractive index. Waveguide coupled microring resonators have shown 15 dB resonance contrast and low total insertion loss of 9 dB, most of which was mainly due to fiber coupling in and out of the chip.

Electromagnetic scattering calculations for terahertz sensing

Zurk, L.M., B. Orlowski, G. Sundberg, D.P. Winebrenner, E.I. Thorsos, and A. Chen, "Electromagnetic scattering calculations for terahertz sensing," Proceedings of SPIE, Terahertz and Gigaherz Electronics and Photonics VI, 64720A, doi:10.1117/12.698720, 2007.

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21 Jan 2007

This paper presents predictions of classical electromagnetic scattering from granular material illuminated by a terahertz (THz) source. Random media models are created to represent the explosive grains, air voids and filler material commonly found in explosive devices. These constituents can cause significant volume scattering that may alter or obscure the chemical response of the explosive, thus impacting THz detection of explosives. Furthermore, the air-explosive interface may have significant roughness, and scattering from this interface may be a dominant factor - particularly in reflection spectroscopy. The volume scattering is calculated using the Quasi-Crystalline Approximation (QCA) and a Finite Difference Time Domain (FDTD) calculation; the FDTD method is also used to estimate the rough surface scattering. Results from these calculations are provided for mixtures that are representative of common classes of explosives.

Inventions

Induction Detection of Concealed Bulk Banknotes

Record of Invention Number: 45577

Antao Chen, Michael Yost, Christopher Fuller

Disclosure

11 Apr 2011

Method for Characterization of Superficial Wounds

Record of Invention Number: 45573

Hassan Arbab, Antao Chen, Dale Winebrenner, Trevor Dickey, Pierre Mourad, Matthew Klein

Disclosure

1 Apr 2011

Retrieval of Terahertz Spectroscopic Information in the Presence of the Rough Surface Scattering Effects

Record of Invention Number: 8575D

Hassan Arbab, Antao Chen, Eric Thorsos, Dale Winebrenner

Disclosure

20 Jan 2010

More Inventions

Using the Electrostatic Field Generated by Ferroelectrics to Process Organic and Polymeric Nonlinear Optical Materials

Record of Invention Number: 8321D

Antao Chen, Su Huang, Alex Kwan-yue Jen, Jingdong Luo

Disclosure

21 Mar 2009

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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