This PDF file contains the front matter associated with SPIE Proceedings Volume 11827, including the Title Page, Copyright Information, and Table of Contents.

The metrology of terahertz waves is a prerequisite for future applications in communications, sensing or spectroscopy. Electro-optic transduction, the technique by which a terahertz signal is mapped onto an optical or near-infrared signal, has emerged as one of the most sensitive metrology techniques that also provides sub-picosecond temporal resolution. As such, it has been employed to detect quantum terahertz fields for the first time. Recent advances in the miniaturization of these transducers target increased sensitivities, a small footprint and compatibility with large photonic architectures. In this talk, I will outline the basics of electric field metrology at the quantum limit and discuss how integrated photonics could open entirely new avenues in the area of spatially and temporally multiplexed terahertz detection.

We introduce a physical mechanism to perform machine learning by demonstrating a Diffractive Deep Neural Network (D2NN) architecture that can all-optically implement various functions following the deep learning-based design of passive layers that work collectively. We created 3D-printed diffractive networks that implement all-optical classification of images of handwritten digits and fashion products as well as the function of an imaging lens, spectral filters and wavelength demultiplexers at terahertz part of the spectrum. This passive diffractive network framework is broadly applicable to different parts of the electromagnetic spectrum, and can perform at the speed of light various complex functions that computer-based neural networks can implement, and will find applications in all-optical image analysis, feature detection and object classification, also enabling new camera designs and optical components that perform unique tasks using diffractive optical networks.

Paper, as a hygroscopic dielectric material, does not have specific spectral signatures in the Terahertz (THz) range from 0.2-6 THz. However, because of its constituent materials, including dry matter, moisture, and air pockets, it absorbs THz radiation, similar to biological tissues and green leaf. Though the absorption loss is not significant, varying levels of dampness in wet paper are observed over time using continuous wave (CW) based THz Spectroscopic system to quantify the moisture content of wet paper relative to paper at ambient environment. For this purpose, effective medium theory (EMT) approaches including Bruggeman (BM), Landau–Lifshitz–Looyenga (LLL), and Complex Refractive Index (CRI) models are analysed. However, EMT models are dependent on physical and optical properties of paper and water, which are not well-defined and are dependent on assumptions, approximations and rigorous calculations. To remove such dependencies, supervised machine learning regression (SMLR) algorithms in the form of decision tree (DT), random forest (RF), and support vector regression (SVR) are investigated. The conditioning of the training parameters is dependent on spectroscopic data which reduces the processing time and improves efficiency due to elimination of approximations. Prediction efficiency of SMLR models is observed to be better than that of EMT models. RF shows the best results in terms of coefficient of determination, 𝑅² but the time required for training is more when compared to DT and SVR models. DT models show consistent performance, while predictions using different SVR models show variance with 𝑅² ranging from 0.42 to 0.98.

In terahertz (THz) communications, most of the research work is focused on wireless systems, while waveguide/fiber-based links have been less explored. Although wireless communications have several advantages, the fiber-based communications provide superior performance in certain short-range communication applications with complex geometrical environments. In this work, we present an in-depth experimental and numerical study of the short-range THz communications links (carrier frequency:128 GHz) that use subwavelength dielectric fibers of varying diameters (0.57-1.75 mm) and up to 10 m for information transmission (up to 6 Gbps) and define main challenges and tradeoffs in the link implementation.

The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. Here, using time-resolved THz techniques, we show the first of demonstration harmonic injection and mode- locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. This generates multiple THz pulses per round trip in both active and self-starting harmonic regimes. This behaviour is supported by time-resolved Maxwell-Bloch simulations of induced gain and loss in the system. This work exploits the inherent ultrafast dynamics of QCLs and opens up new avenues in THz pulse generation.

In this work, we numerically study the losses and quality factors of arrays of square patch-antenna microcavities interconnected with subwavelength wires designed for terahertz (THz) emission. In general, the array geometry provides larger quality factors than the single resonator. Moreover, when the patch-antennas are coupled to subwavelength wires, the intracavity field distribution of the fundamental mode TM₀₁ extends spatially into the direction of the wires, enhancing significantly the quality factors. By replacing the lossy materials by perfect lossless elements in our model, we extract the radiative and non-radiative contributions to the total losses in the system, allowing the estimation of the photon extraction efficiencies, found to be of as high as 90%.

In certain lasers, including terahertz quantum cascade lasers, frequency combs can form whose output is frequency- modulated (FM) linearly in time. While this result had been replicated experimentally and numerically, an analytical description had been elusive. We will discuss our recent work showing how this can be achieved. By deriving a mean-field theory for lasers analogous to the Lugiato-Lefever equation and solving it analytically with some weak assumptions, we show that it simplifies to the nonlinear Schrodinger equation with a phase potential. The phase of its solution is piecewise quadratic in time—an FM comb. Our results apply to many lasers and explains the diverse array of experimental observations. We will discuss prospects and opportunities for improving the performance of these combs.

In this work, we perform Terahertz (THz) Kerr Effect (TKE) experiments on several types of molecular liquids with dissolved iodide anions. We couple high intensity single-cycle THz pulses to the re-orientational motions of the aforementioned liquids and observe a subsequent energy transfer to the translational degrees of freedom of the ionic solutes manifesting as an increase of their translational kinetic energy. Surprisingly, this solvent-to-solute energy transfer is found to scale significantly with the particular intermolecular interactions of the liquids, being more dramatic when a hydrogen-bonding network is present. Our observations set the basis for future coherent control of chemical reactions in the liquid state, by means of THz radiation.

The terahertz absorption spectra of sodium magnesium chlorophyllin (Chl-Mg-Na) and sodium copper chlorophyllin (Cu-Chl), two major members of the chlorophyll derivative family, have been measured in the range 0.2−3.0 THz (6.6−100 cm^-1), at room temperature. Additionally, surface-enhanced Raman scattering spectroscopy was used to supplement data in the higher frequency range. The capability of terahertz spectroscopy for quantitative characterization of Chl-Mg-Na intermolecular vibrations was investigated and the sensitivity of the 1.82-THz feature with degree of hydration by changes in the molecular environment was examined. For Cu-Chl derivative, a broad feature was observed around 1.8 THz which currently hinders clear Cu-Chl identification and quantification.

In the paper, the double-Debye and double-overdamped-oscillator models are considered for parametrization of the terahertz (THz) dielectric response of human brain tissues. Experimental data can be accurately reproduced for the intact tissues and gliomas using both models. While the double-Debye dielectric model is widely used in THz biophotonics, the double-overdamped-oscillator model appears to be more physically rigorous, since it satisfies the sum rule. In our opinion, the described double-overdamped-oscillator model is important for further research and development in THz biophotonics and neurodiagnosis of tumors.

A pair of parallel metallic plates with nanometer-scale separations, or a ‘metallic nanotrench’, creates strongly enhanced electric field with uniform spatial distribution when a long wavelength radiation is incident. This property is not only useful for quantitative analysis of light-matter interactions, but also for potential electrochemical studies on nanoconfined molecules. Here, we show our progress on realizing sub-10 nm-wide metallic nanotrenches filled with various liquids to study interaction of nano-confined molecules with terahertz radiation. Large height-to-width aspect ratio and strong field enhancement of the nanotrenches enable sensitive detection of the nano-confined molecules, from which optical properties of the molecules can be determined. We demonstrate fabrication of the nanotrenches with widths as small as 2 nm, and study changes in their terahertz optical properties upon integration with various liquids. Also, we discuss anomalous optical properties of water molecules confined within sub-10 nm-wide metallic nanotrenches.

Spatial symmetries and the time-reversal symmetry determine how natural and artificial materials interact with light. The time-reversal symmetry can be broken in magnetic materials, which leads to polarization rotation via the Faraday effect. A new effect known as nonreciprocal directional anisotropy emerges when the magnetic material also lacks the spatial inversion symmetry, which results in the difference of transmitted light intensity in the forward and backward directions as measured with unpolarized light. We will consider several cases studies, including a polar magnet and artificial magneto-chiral metamaterials for the THz frequency range that exhibit this emerging phenomenology and also allow new ways of polarization control.

The microbolometer technology has proved its potential in the Infrared (IR) region due to its low fabrication costs, and room temperature operation, making this technology desirable to be used in various applications, and this interest has recently expanded into the Terahertz (THz) region as well. The detection in microbolometers is achieved through the absorption of THz radiation which subsequently heats up and is sensed by the temperature sensitive material at the core of the device. This temperature sensitive material is typically based on VO_x, which exhibits a sufficient change in resistance with temperature. While this temperature sensitive material is useful in the IR, the low energy of the THz wave compared to the background radiation makes it a challenge to operate the device at room temperature and show a large change in resistance with respect to the slight change in temperature. Metal doped VO_x films can show a better performance however these effects are not well understood in the THz region. In this study, Tungsten (W) doped and undoped VO_x films are fabricated and then analyzed using Time Domain THz Spectroscopy. The DC electrical properties of the films as well as their optical behaviors in the region of 0.2-2.0 THz are analyzed as a function of temperature. The metal doping is seen to affect the overall electrical and optical response of the film. Understanding this dependence is key to achieving a better film for applications in the THz region.

This work presents the generation of pulsed THz radiation from a quantum dot photoconductive antenna (PCA) pumped at 800nm. The work investigates the output and characteristics of the generated THz from the QD PCA alongside a comparison with a commercial antenna from Teravil. The QD PCA outputs significantly higher THz power at low pump powers than the commercial PCA and would therefore be suitable for any application that would require a low-pump power such as the use of semiconductor lasers as pump sources for THz generation.

We study a new method in fabrication of THz optical components from previously studied porous three-dimensional nanostructures based on SiO2 [Optical Materials Express 10, 2100-2113 (2020)]. Porous SiO2 is represented by artificial opals [Optical Materials 49, 208–212 (2015)]. By the thermal treatment, it is able to achieve materials with different stoichiometric composition and porosity, and obtaining pre-determined physical and optical properties of thematerial. In this paper we produced the THz conical lens (axicone) by direct sedimentation of aqueous colloidal suspension of the SiO2 nanoparticles onto a shaped mold, followed by annealing and finished machining. The THz field transformation behind the axicone was studied by THz imaging system. The observed results were then compared with numerical predictions obtained by using the methods of computational electrodynamics. Finally, we discuss the potential of the described fabrication method in the THz optics.

Millimeter (MM) and Terahertz (THz) waves are transparent to many non-metallic materials and is expected to be applicable for non-destructive inspections. Especially, the radar 3D imaging technology using the semiconductor continuous wave (CW) MMW and THz sources is important because of its compactness, stability and high signal-to-noise ratio (SNR). In this presentation, we will introduce the development of a prototype of a 300 GHz walk-through body scanner for the security gates. For the actual system, we are developing the frequency-modulated continuous wave (FMCW) imaging method around 300 GHz. We have demonstrated the 3-dimentional real-time imaging of a material hidden underneath cloths. We will demonstrate the system configuration and the current status as well as some results of the actual imaging.

Imaging in the Terahertz (THz) region has drawn attention in recent years, but the nature of the THz frequency regime causes some drawbacks in imaging such as long wavelength, high cost, and low emission levels at room temperature. Because of the high atmospheric absorption of THz waves, fabrication of a microbolometer pixel that works in the sub-1 THz frequency regime is necessary. Large pixel pitches due to longer wavelengths and the resulting higher thermal mass pose a difficult challenge. Due to those limitations, a unique design of an absorber is essential for THz microbolometers. This study investigates the use of absorbers based on novel materials and alloys with the goal of developing efficient absorbers in small pixel pitches. First, thin layer metal absorbers typically used in commercial IR microbolometers are characterized in terms of absorption performance in the sub-1 THz region. Thin films based on metal alloys such as TiAlV show a markedly lower absorption in this region than in the IR. To improve the performance of these absorbing layers and reduce pixel pitches. Use of effective media based on the mixture of dielectric materials and metals with patterned thin films are investigated to develop unique absorbing thin layers. It is seen that with the use of an effective medium whose complex dielectric constant is tailored appropriately, efficient absorption of sub-1 THz radiation can be achieved.

Here we present a photoconductive antenna (PCA) detector based on a novel perfectly absorbing, all-dielectric metasurface design. The metasurface consists of interconnected LT-GaAs channels, and supports degenerate electric and magnetic dipole modes that are critically coupled to the incident field. By replacing the photoconductive region in a THz PCA with this metasurface, full absorption of the optical pump beam is achieved in an ultra-thin photoconductive layer of only 160 nm. By maximizing absorption, high signal-to-noise is achieved at extremely low pump powers (below 100 μW). The saturation power is measured to be only 150 μW – an order of magnitude lower than typical PCA detectors. These highly efficient PCA detectors are applicable to a wide range of THz spectroscopy and imaging systems, particularly those where low power operation is required such as detector arrays and cryogenic environments.

We present recent strategies allowing THz pulses to be recorded in single-shot, using chirped pulse electro-optic sampling. We show in particular that high repetition rates (typically 1-100e6 TDS traces per second) can be obtained using an association of electro-optic sampling with the so-called time-stretch technique. We also present recent results demonstrating that reaching simultaneously high bandwidth and high frequency resolution (i.e., long recordings) is possible with these single-shot systems, using a variant called called Diversity Electro-Optic Sampling (DEOS). Applications to studies of accelerator physics, THz sources, as well as single-shot Time-Domain spectroscopy will also be discussed.

Using attenuated total reflection (ATR) in the terahertz domain, we demonstrate non-invasive, non-staining real time measurements of cytoplasm leakage during permeabilization of live epithelial cells by saponin detergent and after electropermeabilization. The origin of the contrast observed between cells and culture medium is addressed by both experimental and theoretical approaches, and gives access to permeabilization dynamics of live cells in real time. We show that terahertz modalities are more sensitive than fluorescence microscopy which is the reference optical technique for electropermeabilization. We propose analytical models for the influx and efflux of non-permeant molecules through permeabilized cell membranes.

The realisation of hyperspectral terahertz imaging is a significant step towards understanding of the life sciences on all scales. A key to this understanding is the retrieval of dielectric properties from such images, a task which is plagued by experimental limitations, challenging the terahertz community for more than two decades. In this contribution, we propose a new combined retrieval methodology to overcome misalignments and Fabry-Pérot effects on the extraction of the dielectric properties of human bone samples through the combination of the Kramers-Kronig relations and Fabry-Pérot reflection modelling. Results extracted from ∼100 µm human bone slices composed largely of collagen are consistent with those measured for pristine collagen samples. This represents another stepping-stone towards the adoption of terahertz imaging into pre- and clinical practice.