Proceedings Volume 10983

Next-Generation Spectroscopic Technologies XII

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Proceedings Volume 10983

Next-Generation Spectroscopic Technologies XII

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Volume Details

Date Published: 9 July 2019
Contents: 11 Sessions, 19 Papers, 21 Presentations
Conference: SPIE Defense + Commercial Sensing 2019
Volume Number: 10983

Table of Contents

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Table of Contents

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  • Front Matter: Volume 10983
  • MEMS in Spectroscopy
  • Smartphone Spectroscopy
  • New Technologies
  • Spectroscopy in CBRNE Applications
  • Advances in Raman Spectroscopy
  • Terahertz I
  • Terahertz II
  • Terahertz III
  • Terahertz IV
  • Poster Session
Front Matter: Volume 10983
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Front Matter: Volume 10983
This PDF file contains the front matter associated with SPIE Proceedings Volume 10983, including the Title Page, Copyright Information, Table of Contents, Author and Conference Committee lists.
MEMS in Spectroscopy
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MEMS based SpectroChip for healthcare, food safety and blockchain applications
The overall chip size is less than half of a SD card. The spectral resolution is 3~5 nm for the whole spectral range of 350~1100 nm. The first order diffraction efficiency reaches over 70% at the blaze wavelength, which is at 550 nm. The signal-to-noise ratio of the SpectroChip system is 1000:1 with 50 ms integration time. The stray light is about 0.04%. A total solution is ready to be incorporated into any smart phone, wearable devices, and handheld device systems. With the incorporation of our SpectroChip sensors, hundreds to thousands items related to personal healthcare can be added to the worldwide health analysis cloud platform.
Battery-operated microplasma coupled to a portable fiber-optic spectrometer and its application for the determination of nano-Silver leaching from garments during washing
Shannon Marie McBride, Jessica Remedios, Vassili Karanassios
Microplasmas are those with one critical dimension (e.g., depth, height, or radius) in the micrometer regime. Unlike their lab-scale counterparts that consume 20 Lit/min of expensive inert gas and require 1-2 kW of electrical power, microplasmas consume 0.250 mL/min of inert gas and are operated from a battery. We developed and evaluated a variety of microplasma designs using different substrates (including polymeric ones). Their analytical performance characteristics gradually improved after each design iteration. To demonstrate capabilities, we show that a microplsma coupled to a fiber-optic spectrometer can be used for the determination of Silver (Ag) leaching out from a nano-Ag impregnated garment during washing.
Chip-scale high-performance digital Fourier Transform (dFT) spectrometers
Derek M. Kita, Brando Miranda, Carlos Ríos, et al.
On-chip spectrometers have recently emerged as a promising alternative to conventional benchtop instruments with apparent Size, Weight, and Power (SWaP) advantages for applications including spectroscopic sensing, optical network performance monitoring, RF spectrum analysis, optical coherence tomography, and hyperspectral imaging. Existing onchip spectrometer designs, however, are limited in spectral channel count and signal-to-noise ratio (SNR). Here we demonstrate a transformative on-chip digital Fourier transform (dFT) spectrometer that can acquire high-resolution spectra via time-domain modulation of a reconfigurable Mach-Zehnder interferometer. The device, fabricated and packaged using industry-standard silicon photonics technology, claims the multiplex advantage to dramatically boost SNR and unprecedented scalability capable of addressing exponentially increasing numbers of spectral channels. We further explored and implemented machine learning regularization techniques to spectrum reconstruction. Using an ‘elastic-D1’ regularized regression method that we developed, we achieved significant noise suppression for both broad (> 600 GHz) and narrow (< 25 GHz) spectral features, as well as spectral resolution enhancement beyond the classical Rayleigh criterion. The dFT architecture and spectrum reconstruction techniques demonstrated in this work will drive future work in on-chip optical spectroscopy and enable practical realizations of high-performance chip-scale spectrometers with large (> 1,000) spectral channel counts.
Smartphone Spectroscopy
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A smart handheld Raman spectrometer with cloud and AI deep learning algorithm for mixture analysis
Lynn Chandler, Bill Huang, Tao Tao Mu
Raman spectrometry has proven to be by far the most powerful noninvasive analytical technique for direct material identification. In this paper we introduce the first smart Raman device with a Cloud data platform and AI deep learning algorithms- the CloudMinds XI™. This smart phone operated Raman features high performance, fully automated operation, and capability for mixtures analysis in real time. This novel Cloud AI Raman spectrometer is fully integrated with the Android-based CloudMinds A1 smart phone. The A1 phone provides the full functionality of a smartphone including voice calls, emails, GPS location, and image capture by camera, and maintains constant Wi-Fi/blue tooth and 4G LTE connections, letting you stay connected to Raman data constantly. The cloudbased data platform not only allows speedy analysis but also enables spectral library expansion with ensured security. In addition, CloudMinds has developed its proprietary Al algorithm using Google Brain's second-generation machine learning system, TensorFlow. This technology improves analysis accuracy and gets continually better results as it learns and trains data while connected to the cloud. A mixture of three substances has been successfully analyzed with ratios within seconds by this handheld Raman spectrometer for the first time, and this paper will present the results from the mixture analysis. This Cloud AI handheld Raman is the best solution for many field applications, especially when real time analysis and central cloud data platform support are essential.
Smartphone-enabled data acquisition and digital signal processing: From current-output or voltage-output sensors for use on-site, to their use in IoT, in Industry 4.0 and (potentially) in Society 5.0
Ryan Fitzgerald, Emily Wang, Vassili Karanassios
Smartphones are widely used in daily life. Can they also be used for wireless data acquisition from voltageoutput or current-output sensors in the lab or outside of a lab, for example, for taking part of the lab to the sample types of applications? Could they be used for digital signal processing (e.g., filtering) of noisy signals (e.g., using a Savitzky-Golay filter)? Could they also find applicability in IoT, in Industry 4.0 or in Society 5.0 applications? In this paper, these questions are addressed in some detail.
New Technologies
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A wavelength interrogator employing tapered hollow waveguides and a low-cost silicon board camera
Timothy R. Harrison, Graham J. Hornig, Jorge Marin, et al.
We report on a multi-channel wavelength interrogator for the 850 nm wavelength region, constructed by coupling an integrated array of multi-mode, tapered hollow waveguides to a low-cost silicon-based image sensor. The waveguides are clad by omnidirectional Bragg reflectors, such that guided light is radiated in an out-of-plane direction near and at cutoff. Wavelength shifts were extracted using a simple centroid detection algorithm applied to the terminal cutoff point. This concept combines the small size of a Fabry Perot filter with the dispersive property of a diffraction grating. By imaging multiple tapered waveguides onto a single image sensor with each waveguide coupled to a different fiber, simultaneous extraction of wavelength shifts from several wavelength-multiplexed sensors can be achieved in a very compact package. The prototype described provides resolution on the order of 5 pm and can accommodate ~ 20 sensors spaced by 5 nm on each of the 4 fiber input channels. Enhanced capacity and performance are anticipated through future improvements in waveguide materials and the use of more advanced image processing algorithms.
Non-destructive characterization of transparent armor layups
Filipp V. Ignatovich, Kyle J. Hadcock, Donald S. Gibson, et al.
We have developed a material identification instrument, based on measuring the group refractive index dispersion curve using a low-coherence interferometer. Non-destructive product verification testing is critical for multilayer structures used in commercial or military applications. The ability to identify the number of layers in multilayer structures, the material composition of each layer, as well as the thickness of each layer non-destructively is important to ensure product quality in many fields, such as aerospace, defense, automotive and semiconductor. Low-coherence interferometry offers a quick and reliable way of obtaining material dispersion properties by measuring the spectral dependence of the optical thickness of the material. Latest advancements in the supercontinuum light generation have opened new opportunities for these highly accurate spectroscopic measurements. We have successfully applied the developed system to several known and unknown transparent layups.
Optical early warning detection of aromatic hydrocarbons in drinking water sources with absorbance, transmission and fluorescence excitation-emission mapping (A-TEEM) instrument technology
Global drinking water sources remain prone to carcinogenic petroleum product contaminations due to lack of detection capacity at or before treatment plant intake. A major challenge to detect water-soluble petroleum products using optical techniques is discrimination of low contaminant quantities from the highly absorbing and fluorescent backgrounds of natural Dissolved Organic Matter (DOM) components. One key example is Benzene subject to a USEPA regulation maximum contaminant level in the distribution system of 5 μg/L. In contrast, typical surface water DOM concentrations range from 1 to 20 mg/L and many DOM components have much higher extinction and fluorescent quantum yields than Benzene, Toluene, Ethylbenzene and Xylene (collectively known as BTEX). We present a new optical method for rapid (3-5 min) reagent- and extraction-free detection of all BTEX compounds in typical raw surface water with respective Limits of Detection (LOD) and Quantification (LOQ) of 1 and 3 μg/L. The method uses patented simultaneous Absorbance, Transmission and fluorescence Excitation-Emission Mapping (A-TEEM) instrument technology with deep UV sensitivity. The rapid acquisition is supported by automated, targeted Partial Least Squares (PLS) library analysis. The instrument can be equipped with an automated surface water flow-sampling device at the plant intake or upstream and internet based communication to facilitate early warning reports.
Spectroscopy in CBRNE Applications
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Eye-safe standoff chemical threat detection using deep ultra-violet Raman spectroscopy and lidar imaging
Jeffrey B. Oleske, Justin T. Cooper, Bradley R. Arnold, et al.
Foreign and homegrown terrorism, illegal drug manufacture and environmental contamination, have lead to an increased need for rapid, portable and standoff chemical threat (explosives, narcotics, toxic industrial chemicals, etc.) detection technology. Analytical techniques; like High-Performance Liquid Chromatography (HPLC), Gas Chromatography-Mass Spectrometry (GC-MS), and Ion-Mobility Spectrometry (IMS), are industry standards due their unmatched combination of sensitivity and selectivity. However, samples must be handled by and handler and for analysis, putting the user at risk if samples are of a potentially threatening nature. Vibrational spectroscopic techniques are uniquely suited to this application as they offer chemical identification capabilities as well as the ability to be collected at standoff distances, as emitted or scattered photons are collected at some distance from the sample. Here we report on chemical threat detection instrumentation methods which employs deep ultra-violet (DUV) Raman spectroscopy and Light Detection And Ranging (LiDAR) imaging. UV Raman spectra were measured using a novel experimental configuration. This configuration allows many of the difficulties associated with UV excitation and high-power pulsed laser sources to be mitigated. Large sample areas are imaged into the detection system allowing high power excitation sources to be used while simultaneously avoiding sample degradation and multi-photon absorption effects. Such large detection areas allow large numbers of molecular scatters to be probed even with minimal penetration depth. Alignment issues between sample and collection optics are also simplified.
Control of quantum cascade laser sources in stand-off detection of trace explosives
We pursue the development of an eye-safe stand-off technique suitable for the detection of trace explosives. As the active illumination sources, tunable quantum cascade lasers (QCLs) are employed in Mid-LWIR (long-wave infrared) in the range of 6 to 11 μm, which contains many spectral features from analytes of interest. Any fluctuation of the laser beam direction and/or beam profile is amplified at the sample position, which would lead to diminished performance of the detection technique, both in sensitivity and selectivity. Several beam stabilization approaches were conducted to overcome this challenge: 1) Using a KBr/diamond pellet as a diffuser in combination with a multimode fiber 2) Feedback stabilization of quantum cascade laser beam steering. The purpose of the first method is to make a temporally and spatially incoherent laser beam source through the multimode fiber and KBr/diamond pellet. The second approach is to stabilize the beam position by using an active feedback loop. We have demonstrated that beam wander and speckle noise were successfully suppressed by these approaches. Independently, we have developed a custom-built broadband laser source in the Mid-LWIR range consisting of several high power Fabry Perot (FP)-QCLs. The FP-QCLs were operated in both CW and pulsed modes at different diode temperatures, and the emission spectra were collected by a FTIR. For our future work, the output beams will be collimated to spectrally combine multi-QCLs and aligned toward the same target. Also, a spatial heterodyne spectrometer (SHS) will be applied to discriminate spectral and spatial information from a single snapshot.
Deep-UV standoff Raman spectroscopy
The availability of high peak-power laser systems capable of delivering intense deep-UV pulses has brought renewed interest in using Raman spectroscopy as both a selective and sensitive analytical technique for stand-off detection. Our approach uses a high power pulsed-laser as the excitation source, specifically the fourth and fifth harmonics of a Nd:YAG laser. One of the hurdles to be overcome to allow deep-UV Raman spectroscopy to become accessible is a direct method of calibrating both the observation frequency and detector response of the spectrograph being used. This report outlines our efforts to understand the photochemical and photophysical consequences of high-peak power excitation of cyclohexane for potential use as a secondary Raman standard in the deep-UV. Evaluation of the photochemical stability, both from multi-photon absorption and in the presence or absence of dissolved oxygen as well as the possibility of (near) resonance enhancement of the C-H stretching region will be described.
Portable spectroscopy in 2019: smaller, cheaper and in consumer products?
In recent years, portable spectrometers (XRF, Raman, near-IR, FT-IR),produced by major analytical instrument companies, have cost in the range of $20,000 - $50,000, and sold for very specific applications. One of the most intriguing trends in the past year is the appearance of portable photonic devices and spectrometers in consumer products, for instance, washing machines and hair care. This paper surveys the technologies and emerging products.
Advances in Raman Spectroscopy
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Advancements in Raman technology for identifying real-world samples
Jun Zhao, Katherine A. Bakeev, Xin Jack Zhou
Handheld Raman spectroscopy’s value for rapid field screening by safety and security personnel is well understood as evidenced by its implementation across the globe. Though Raman spectroscopy has the ability to nondestructively identify samples through transparent packaging, in real world scenarios challenging samples are frequently encountered. Raman screening must be effective for samples in a variety of packaging and for samples with coloration or impurities that give a high fluoresce that can overwhelm the Raman signal depending on the excitation laser wavelength. See through Raman technology has been developed to enable measurement through opaque packaging, sampling a larger area and with a deeper penetration depth of the Raman signal. Using a design of collinear sample illumination and Raman scattering collection at a higher efficiency, and spread over a larger sampling area, there is a lower power density of illumination on the sample, reducing issues of sample heating that can be problematic for dark samples. The measurement of a larger sample area provides more reliable identification of solids that are often inhomogeneous. This see through Raman technology and the use of a longer laser wavelength excitation overcomes many of the difficulties encountered in use of Raman spectroscopy for field testing.
Spectroscopic gas identification using piezo tuned micro-cavity enhanced Raman scattering (Conference Presentation)
Alan Feinstein, Roman Yasinov, Nir Karasikov, et al.
Raman spectroscopy has been shown to be a powerful spectroscopic approach for detecting gases.  It is capable of detecting gases that possess no infrared absorption feature, such as diatomics, and it also does not suffer from signal saturation as can be the case for transmission measurements.  However, the Achilles heel of Raman based gas detection is the low scattering cross section.  Here we show that introducing gas into an optical open micro-cavity (OMC) can greatly enhance the Raman signal.  The enhancement comes from 2 factors: namely confining the light longer in the gas containing volume, hence strengthening the light-matter interaction, and also directing the Raman scattering in a more collimated fashion towards the detector.  The OMC has been designed such that the separation between the mirrors is piezo controlled, hence it is possible to scan the micro-cavity resonances and tune the OMC to a particular Raman transition.  Using the OMC, with laser excitation at 532 nm, we show detection of several gases simultaneously such as nitrogen and oxygen, and discuss prospects for boosting the limit of detection. The presentation will cover the design consideration of the Tip, Tilt, Z microcavity having a range of 40 micron and a positioning resolution of 10 nm. The design comprises an intrinsic position to allow close loop operation. Response time is in the msec range to facilitate fast scanning. The spectrometer set up will be presented as well as representative experimental results.
Terahertz I
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Terahertz (THz) direct detectors based on superconducting HEBs with thermal, THz and microwave biasing methods (Conference Presentation)
Superconducting hot electron bolometers (HEBs) are widely used as terahertz (THz) heterodyne detectors (mixers) and becoming the most advantageous candidate for low noise receiver at frequencies above 1.2 THz. However, there is still lack of accurate theoretical modeling which explains the complex mechanism of different aspects of superconducting HEBs. These difficulties do not prevent researchers around the world from making experimental progress. In the last few years, superconducting HEBs are also used as THz direct detectors to develop the array for the applications in biology, medicine and security systems. Recently, a noise equivalent power (NEP) of 0.2 pW/√Hz at 2.5 THz is reported for the superconducting HEBs with thermal biasing. Instead of thermal one, microwave (MW) which was first used to stabilize the HEB mixer is also applied to bias the device as a direct detector. Furthermore, THz can be used to bias the HEBs, if THz sources can be provided easy. Although MW biasing can improve sensitivity of the superconducting HEB direct detectors, little work has been done to compare the responsivity and NEP among the THz direct detectors with thermal, THz and MW biasing. To find a best biasing method, we have studied the similarities and differences of the direct detectors based on the superconducting HEBs with three different biasing methods. The fabricated NbN HEB detectors consist of a complementary logarithmic-spiral antenna made of gold and an NbN film (bridge) connecting across the antenna’s inner terminals. We have fabricated the niobium nitride (NbN) superconducting HEB mixers with the system double sideband (DSB) noise temperature better than 8 times of the quantum limit: hf/kB (where h is the Planck constant, kB is the Boltzmann constant and f is the operating frequency) at the frequencies higher than 1 THz. Here, the mixer chips with the system DSB noise temperature of about 400 K and the intermediate frequency (IF) gain bandwidth (GBW) of larger than 5 GHz at 4.2 K and 0.65 THz have been used. As the direct detectors, the current responsivity of 290 A/W and 105 A/W are obtained for MW and THz biasing, which about one order higher than that of thermal biasing. The NEP of 2.7 pW/√Hz is obtained with MW biasing, which can be expected to be improved in the future.
Terahertz II
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Interfacial electron transfer in dye-sensitized mixed metal oxides for water splitting
Interfacial electron transfer (IET) dynamics have been measured in a series of dye-sensitized metal oxides commonly using in water splitting dye-sensitized photoelectrochemical cells using optical-pump terahertz-probe spectroscopy. The IET dynamics were compared using a model that allows for the relative contributions of injection from the singlet and triplet excited states of the dye to be distinguished. In addition, we present a newly developed method for performing terahertz (THz) measurements under potentiostatic control (i.e., THz spectroelectrochemistry).
Broadband terahertz spectroscopy at high magnetic field using the 25 Tesla Split Florida-Helix magnet
David J. Hilton
We describe the development of a novel high magnetic field ultrafast spectrometer that uses the 25 Tesla Split Florida-Helix magnet. This system has a large numerical aperture that permits the free space propagation of ultrafast pulses to a sample in this large external magnetic field. The system has an operating bandwidth that spans 0.3 to 10 THz. We discuss the utility of this novel instrument for nonequilibrium dynamical measurements and stabilization of alternate orders in magnetic active materials.
Measurement of quadratic terahertz nonlinearities in time-domain spectroscopy (Conference Presentation)
Optical response of materials to intense terahertz electric fields has become a new frontier in optics in the recent decade. We focus on the nonlinear optical responses that are quadratic in terahertz electric field, which can arise in second and third orders of nonlinearity. The second order nonlinear polarizability can lead to terahertz second harmonic generation, a phenomenon that has not been experimentally observed yet. The difficulty in detecting terahertz second harmonic generation stems from vanishingly small conversion efficiencies in this region of spectrum. However, an additional experimental difficulty results from a significant overlap of the fundamental and second-harmonic terahertz pulses both in the time and frequency domain. This makes it hard to distinguish the second harmonic from the fundamental terahertz wave. The third order nonlinear polarizability results in terahertz Kerr effect, which has been observed as an induced gating-beam birefringence that is quadratic in terahertz electric field. In noncentrosymmetric materials, terahertz Kerr effect may coexist with terahertz Pockels effect that is commonly used for time-domain terahertz detection via electro-optic sampling. Distinguishing the terahertz Kerr effect from the Pockels effect can also be difficult if the latter is significantly stronger. In this paper, we will present a method for measuring quadratic terahertz nonlinearities based on the second-harmonic lock-in detection in a time-domain electro-optic sampling experiment. We illustrate our method using a measurement of terahertz Kerr effect in a zinc blende semiconductor in geometry where both terahertz Kerr and Pockels effects are present. We will also discuss the possibility of measuring terahertz second harmonic generation in metamaterials.
Terahertz III
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Terahertz transmittance properties of all-dielectric guided-mode resonance filter (Conference Presentation)
Tae-In Jeon
In this research, an all-dielectric guided-mode resonance (GMR) filter operating in the THz region is proposed. The GMR filter is a combination of a grating on the filter surface and slab waveguides of the filter substrate. Two strong transverse electric (TE) resonance and transverse magnetic (TM) resonance modes were detected. The limited resonance depth resulting from the finite grating periods can be overcome by using two identical GMR filters. We were able to obtain a Q-factor of up to 74.4 for the TM0,1 mode. When the grating structure of the GMR filter was 45° (rotation angle) to polarize the THz beam, the resonance amplitudes of the TE and TM resonances became 50% and 50%, respectively. Also, we obtained polarization efficiencies of up to 96.9% for the TE0,1 mode when the filter was rotated to 90°. Furthermore, if the GMR filter is inserted between Teflon plates, the TE1.1 mode can be perfectly removed. Meanwhile, a variable grating period made of quartz has been applied to fabricate a tunable guided mode resonance (TGMR) filter. With a metal slit spacing of 2 mm located in front of the TGMR filter, the resonance frequency could be shifted up to 87, 96, and 82 GHz, where the center frequencies of each resonance were 0.402, 0.579, and 0.460 THz, for the TE0,1, TE1.1, and TM0,1, respectively. Furthermore, because the TGMR and GMR filters are placed independently in the THz beam path, both tunable and fixed resonances can be obtained at the same time in the spectrum.
Enhancement of electro-optic sampling signal in detection of pulsed terahertz waves by polarization filtering (Conference Presentation)
Masahiko Tani, Hideaki Kitahara, Hiroyuki Kato, et al.
In this paper, we demonstrate significant enhancement of electro-optic (EO) sampling signal in the detection of pulsed terahertz (THz) waves by using a technique we call “polarization filtering”. In the EO sampling of pulsed THz waves, a linearly polarized probe optical pulse is phase-modulated by THz electric field through the linear EO effect and, as the result, it becomes slightly elliptically polarized after passing through the EO crystal. The phase-retardation of the probe optical pulse is then detected as an optical intensity modulation (EO signal), dI/I (the ratio of the intensity change, dI, and the original intensity, I) with an appropriate optical detection system. In “polarization filtering,” the EO sampling signal, dI/I, is enhanced by suppressing the main polarization component of the probe beam, resulting in a reduced probe beam intensity I’ = b^2*I, after the interaction with THz field in the EO crystal. Since the intensity modulation, dI, also reduces to dI’= b*dI, as the result of the polarization filtering, the THz EO sampling signal is enhanced by a factor of 1/b: dI’/I’ =(1/b)*dI/I. This “polarization filtering” is applicable not only to the conventional ellipsometric EO sampling but also to the heterodyne EO sampling. Firstly, we explain the principle of the polarization filtering, and then show the results of the proof-of-principle experiment for the standard and the heterodyne EO sampling, respectively.
Asymmetric gap resonator based near-field coupling in terahertz metamaterials (Conference Presentation)
S. Jagan Mohan Rao, Yogesh Kumar Srivastava, Gagan Kumar, et al.
The metamaterial is an arrangement of artificial structural elements designed to achieve advantageous and unusual electromagnetic properties. In the unit cell level, metamaterials are composed of an array of small structured elements called split ring resonators (SRRs). Recently, a lot of emphases has been given to the realization of terahertz metamaterials owing to its significance in the construction of terahertz photonic components. In this context, near-field coupling in terahertz metamaterials is extremely crucial. The short-range coupling in metamaterials occurs via the electric and magnetic fields due to the close proximity of the neighboring resonators. The electric field mainly couples through the gaps of SRRs, while the magnetic field couples through the circumference. In this work, we experimentally investigate near-field gap to gap capacitive coupling between a pair of single split gap ring resonators (SRRs) in a terahertz metamaterial. This has been achieved by manipulating the near field electric interactions via changing one resonator split gap with respect to the other resonator split gap for several inter resonator separations. Introducing asymmetry by changing the split gap in one resonator with respect to the other resonator, results in the split in the fundamental resonance mode when operated in the strong near-field coupled regime. The split occurs because of the strong near field capacitive/electric interactions between the resonators. We have further calculated Q factor for the lower and higher resonance modes for different inter resonator separations. The modulation of resonances in capacitive coupled planar terahertz metamaterial systems studied through this work has great potential in manipulating and controlling electromagnetic waves which can ultimately result in novel applications for terahertz frequency domain.
Tunable compact narrow band THz sources for frequency domain THz anisotropic spectroscopy
D. K. George, TJ LaFave, A. G. Markelz, et al.
We demonstrate frequency domain THz anisotropy signature detection for protein crystal models using newly developed compact tunable narrow band THz sources based on Orientation Patterned Gallium Phosphide for turn-key spectroscopic systems.
Actively modulated surface phonon polariton resonances within the long‑wave and far-infrared (Conference Presentation)
Chase T. Ellis, Adam D. Dunkelberger, Virginia D. Wheeler, et al.
The high optical losses of metal-based plasmonic materials have driven an extensive search for alternative lower-loss materials that can support plasmonic-like effects, such as sub-diffraction confinement of optical fields. One such alternative employs phonon-mediated collective-charge oscillations (surface phonon polaritons, SPhPs) that can be optically excited in nanostructured polar dielectric materials. Similar to plasmonics, tailoring the geometry of polar-dielectric resonators results in resonances that can be spectrally tuned throughout the spectral range between the LO and TO phonons. However, generally, the spectral position and amplitude of these resonances remain fixed after sample fabrication. In this presentation, we discuss recent advancements made by our group in achieving actively tunable localized SPhP resonances in the long-wave- and far-infrared spectral regimes. In particular, we focus on three experiments that demonstrate active modulation of resonances. The first and second experiments focus on tuning the spectral position of localized SPhP resonances in cylindrical nanopillars that are etched into indium phosphide and silicon carbide substrates. In both of these cases we are able to induce resonance shifts as large as 15 cm-1 by optically injecting free-carriers into the pillars. The optical injection introduces a reversible, free-carrier perturbation to the dielectric permittivity that results in a spectral shift of the resonances. While the effects investigated for both the InP and SiC systems are similar, each material allows us to explore a different aspect of the phenomena. For InP we investigate the effects in the far-infrared (303-344 cm-1) with steady-state carrier photoinjection, while for SiC we investigate the dynamics of frequency modulated resonances in the long-wave infrared (797-972 cm-1) via transient reflection spectroscopy. Lastly, in the third experiment we demonstrate the ability to modulate the amplitude of resonances by coating SiC nanopillars with vanadium dioxide, a well-known phase change material that undergoes a metal-to-insulator transition near a temperature of 70 C. As such, we show that by exploiting this phase change we are able to modulate the reflectance and thermal emission of nanopillar arrays. The results described in this work may open the door to tunable, narrow-band thermal sources that operate in the long-wave to far-infrared spectral regimes.
Terahertz IV
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Observation of the circular photogalvanic effect in the Weyl semimetal TaAs using THz emission spectroscopy
N. Sirica, R. I. Tobey, L. X. Zhao, et al.
We investigate polarization-dependent ultrafast photocurrents in theWeyl semimetal TaAs using terahertz (THz) emission spectroscopy. Our results reveal that highly directional, transient photocurrents are generated along the non-centrosymmetric c-axis regardless of incident light polarization, while helicity-dependent photocurrents are excited within the ab-plane. Such findings are consistent with earlier static photocurrent experiments, and demonstrate on the basis of both the physical constraints imposed by symmetry and the temporal dynamics intrinsic to current generation and decay that optically induced photocurrents in TaAs are inherent to the underlying crystal symmetry. Such generality in the microscopic origin of photocurrent generation in the transition metal monopnictide family of Weyl semimetals makes these materials promising candidates as next generation sources or detectors in the mid-IR and terahertz frequency ranges.
A Luneburg lens for the THz region (Conference Presentation)
Various Luneburg-lens geometries are used in the microwaves industry as radar reflectors and omnidirectional antennas. Here, we implement a two-dimensional Luneburg lens for the THz frequency region using a waveguide-based artificial-dielectric technology. The cylindrical device has a parabolic shaped top surface and a flat bottom surface. The substrate material of the lens is ultra-pure Teflon, with the top and bottom surfaces coated with high-conductivity silver paint to form a quasi-parallel-plate waveguide. Our experimental results show that the lens can focus an approximately 2-cm diameter input beam to a spot size of 3.4 mm at the diametrically opposite edge, at an operating frequency of 0.162 THz. This work demonstrates the versatility of this artificial-dielectric technology to design and fabricate inhomogeneous, gradient-index devices for the THz region.
Poster Session
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From nanoenergy harvesting to self-powering of micro- or nano-sensors for measurements on-site or for IoT applications
Jayme Alexandra Millar, Zhen Gao, Siva Sivoththaman, et al.
Operation on-site for Internet of Things (IoT) applications and (to a lesser extend) driven by the needs of Industry 4.0 and the requirement for “bringing part of the lab to the sample” (for on-site chemical analysis applications), are the main driving forces behind development of fieldable micro- or nano-sensors, and of micro- or nano-instruments. Such approaches typically require battery-operation (thus requiring regular battery-replacement). Would it not be ideal if field-operated systems were powered from nanoenergy harvested from ambient sources or even if they were self-powered (i.e., without needing an external power supply)? In this paper, two approaches are explored: One approach involves use of Tribo Electric Nanogenerators (TENGs) and the other a self-powered detector.