This PDF file contains the front matter associated with SPIE Proceedings Volume 11725, including the Title Page, Copyright information and Table of Contents

Introduction to SPIE Defense and Commercial Sensing conference 11725: Next-Generation Spectroscopic Technologies XIV

We demonstrate passive optical-to-terahertz conversion through plasmon-coupled surface states. When excited with an optical pump beam, photogenerated carriers in a photo-absorbing substrate are swept to an array of terahertz radiating nanoantennas by a surface-state-induced built-in electric field formed between the nanoantennas and substrate. The nanoantennas are used to couple optically-excited surface waves to the interface region where the built-in electric field is maximized to provide high optical-to-terahertz conversion efficiencies. We have used this scheme to develop a fiber-coupled bias-free terahertz source that provides more than a 110 dB dynamic range over a 5 THz bandwidth.

We discuss the design of gratings that extract THz waves from the surface of quantum cascade lasers, in which the THz wave is generated by difference-frequency generation with Cerenkov phase-matching. The grating is a type of highcontrast- grating (HCG), which has been shown to be a good element of meta-surfaces. Although bare HCGs monolithically fabricated on the semiconductor do not work well, we found that the efficiency is increased to 34% in 1st order grating, 43% in 2nd order grating, and 75% in sub-1st order grating, by capping the ridge of the grating and the bottom of the groove by a metal layer.

To support the massive data rates demanded by next-generation digital technology, wireless communication systems will need to operate in the terahertz frequency bands, where bandwidth is large and atmospheric temporal dispersion can alter the received value of communication symbols. We present a method, based on prior spectroscopic characterizations of the atmosphere, of calculating the deterministic effects of atmospheric dispersion in terahertz communication, and explore the interaction of dispersion with thermal noise. We also present findings indicating that, due to atmospheric dispersion, traditional narrowband models cannot be used to accurately describe the performance of wireless communication systems at terahertz frequencies.

The terahertz spectrum is often seen as the potential future for communication systems due to its large available bandwidth and spectrum availability. Line of sight communications will prove challenging due to a variety of reasons for indoor scenarios including signal loss at walls. Textured, reflective paints were made and measured to test their reflectivity improvement using terahertz time domain spectroscopy. For textured surfaces, the measured results indicated multipath fading and temporal dispersion. This effect was then modeled and investigated in simulations to determine the overall paint/texture effects on wireless communication performance.

Low energy excitations can shed light on the interplay between different degrees of freedom in complex materials. Ultrashort terahertz (THz) pulses can be used to both drive and probe these excitations. This is particularly useful in quantum materials, since these materials exhibit novel magnetic and lattice excitations that can potentially be used to control their properties. Here, the use of ultrafast THz spectroscopy to understand the dynamic properties of Weyl semimetals, probe a graphene nanoribbon-based metasurface, and study conductivity dynamics in superconducting heterostructures will be discussed.

In the last few years, there has been significant advance in the design and modeling of plasmonic terahertz semiconductor devices for applications in detection, generation and modulation of high frequency signals. Plasmonic Terahertz field effect transistors (TeraFET) have been implemented in silicon, InGaAs, GaN], and graphene. Recently, the p-diamond plasmonic TeraFET has been proposed and demonstrated to be a promising candidate for THz and sub-THz applications. To explore more features of TeraFETs, we simulated the response using the hydrodynamic model under various incoming signal conditions, for Si, GaN, InGaAs, and p-diamond FETs. Under the small-signal detection mode, the p-diamond TeraFET has a higher detection sensitivity compared to other TeraFETs. When a strong signal is received, a shock wave develops along the channel. The measurements of the pulse response time could enable the parameter extraction of TeraFET materials.

Spatial symmetries determine how natural and artificial materials interact with light. For example, chiral molecules can cause the rotation of the polarization plane of linearly polarized light, an effect known as optical activity. Magnetic materials feature broken time-reversal symmetry, which can also lead to polarization rotation vie the Faraday effect. Magneto-chiral materials combine chirality with broken time-reversal symmetry. A new effect known as nonreciprocal directional anisotropy arises under such symmetry conditions, which results in the difference of transmitted light intensity in the forward and backward directions as measured with unpolarized light. We will present several magneto-chiral metamaterial architectures for the THz frequency range that exhibit this emerging phenomenology and also allow new ways of polarization control.

Artificial dielectrics are man-made media that mimic the properties of naturally occurring dielectric media, or even manifest properties that cannot generally occur in nature. The concept of an artificial dielectric dates back to the 1940s when it was introduced by the microwave community, although the idea never gained popularity. However, the wavelength scaling that results when transitioning from microwaves to THz waves gives new meaning to this concept, as evidenced by the recent demonstrations of high-performance, energy-efficient, low-cost, and versatile THz devices. Here, we will review some of these devices, expanding on both the homogeneous and inhomogeneous versions.

We propose an antireflection metasurface and a transmissive half-wave plate that are designed based on an extended semi-analytical approach. Simulations suggest that the antireflection metasurface can effectively suppress the reflection at an air-silicon interface to a level lower than −20 dB, while the transmission magnitude is higher than −0.2 dB from 203 to 357 GHz. Besides, the half-wave plate can maintain a simulated output wave extinction ratio above 15 dB from 219 to 334 GHz with a cross-polarization transmission higher than −1.4 dB. Importantly, the employed approach is capable of designing various metasurfaces to further explore the potential of terahertz waves.

We present the development of an eye-safe, invisible, stand-off technique designed for the detection of target chemicals (such as explosives) in a single “snapshot” frame. Broadband Fabry-Perot quantum cascade lasers (FP-QCLs) in the wavelength range of 7 to 12 microns, are directed to a target to interrogate its spectral features. The “backscatter” return signals from target chemicals are spectrally discriminated by an LWIR spatial heterodyne spectrometer (SHS). The SHS offers high throughput and full spectral coverage in each single frame from an IR imaging array. This presentation will cover the performance and optimization of FP-QCLs for this broadband spectroscopic application. We will also discuss the operation and processing of SHS images to extract spectral information. Finally, we will present results of measurements using specific analytes to demonstrate the application of the method to stand-off detection of targets such as explosives and other chemical threats.

Rapid scanning quantum cascade lasers are utilized in the detection of trace amounts of explosive materials. Infrared backscatter imaging spectroscopy employs a quick tuning infrared quantum cascade laser system to illuminate targets with mid-IR light, 6 – 11 μm in wavelength, and to perform spectroscopic measurements in less than one second. A narrow cone of the signal backscattered from targets at standoff distance is collected and imaged onto a liquid nitrogen cooled MCT focal plane array. This backscattered signal is processed into a hyperspectral image cube containing spectral and spatial information. The analysis of the experimental data measured with the system is discussed. This includes the processing of the raw camera frames (using signals from individual components of the system) into discrete wavelength bins, typically 0.01 μm in width. Spectra are generated by plotting the signal from regions of interest, typically clusters of adjacent pixels within the frames, as a function of the wavelength associated with the binned frames. These spectra are compared against the FTIR diffuse reflectance of the analytes on an equivalent substrate for identification. Methods to optimize signal to noise and produce identifications with high confidence are presented. For a single experiment, taking less than 1 second, with the camera running at full frame over 16,000 individual spectra are generated. Targets are prepared by sieving and also dry transfer to mimic real world threats, in trace amounts and on relevant substrates. Traces of explosives, as well as illicit drugs are investigated.

Copper Indium Gallium Selenide (CIGS) thin-film solar cells are a promising technology, but inline quality inspection systems are required for efficient high-volume production. Tests with two candidate methods: Raman spectroscopy and photo-luminescence imaging, are reported in this paper. The methods were used to estimate material compositions of CIGS samples that were varied in absorber thickness and the composition of the CIGS absorber layer. Our results indicate that both methods can be valuable for contact-free inline inspection during the manufacture of CIGS solar cells, both individually and in combination.