This PDF file contains the front matter associated with SPIE Proceedings Volume 11685

Most state-of-the-art terahertz time-domain imaging technologies are based on single-pixel systems, which mechanically scan either the imaging object or the terahertz system, limiting the imaging speed. We present a new terahertz time-domain imaging modality using a terahertz photoconductive focal-plane array. The focal-plane array consists of plasmonic nano-antenna arrays on an LT-GaAs substrate. The dynamic range of a single pixel can reach up to 75 dB with more than a 4 THz bandwidth. We demonstrate clear terahertz images up to 2.5 THz. We also demonstrate that the focal-plane array can operate at video-rate imaging speeds.

We developed a stable, compact, terahertz attenuated total reflected (ATR) spectrometer using an integrated prism, which unifies a terahertz emitter, a terahertz detector, and an ATR prism. Because the prism confines terahertz wave propagation within it and shortens the terahertz path length, our spectrometer has the following two advantages: 1) because water vapor does not disturb the terahertz wave propagation, a high-quality terahertz spectrum is obtained even without a nitrogen purge, and 2) the shorter propagation length provides distinct stability and compact features. To verify our spectrometer's stability, we successively measured water absorption for 9 days and found the relative error to be ±3%. We also provide three distinctive examples by adapting our spectrometer. The first is the quantitative measurement of a solute. We determined the detection limit of H₂SO₄ to be 0.21% using the calibration curve of the refractive index. The second example is monitoring fermentation. Because terahertz absorption is a useful, new indicator against the potential of hydrogen (pH) as a standard indicator, we confirmed that the absorption reduction agreed with the pH during the fermentation of milk to yogurt by our prism for 1 day. The third is the observation of a crystal transition. The transition process of theophylline anhydrate to hydrate was measured on the prism because of the sensitivity of terahertz to the crystal and our spectrometer's stability. Therefore, our apparatus has potential application to different types of new quantitative measurements and real-time monitoring.

Terahertz time-domain spectroscopy has the potential to revolutionize quality inspection by allowing contactfree and non-destructive measurements. However, since most currently available spectrometers only measure one location point per measurement, a raster scan procedure is necessary to obtain an image. Depending on the size of the object and the desired density of measurement points, even fully automated raster scans are timeconsuming. Often, a defect on the surface can be seen with the bare eye. In such a case, the goal of a terahertz analysis is to identify the type of defect below the surface. To fill this gap, we have developed a demonstrator with a collaborative robot (cobot) that allows interactive measurements to be performed where the measurement system can be guided by hand. In this case, however, it is no longer possible to stop to perform a measurement, so measurements must be taken during the movement. This places special demands on the simultaneous acquisition of measurement data and the robot position from the two subsystems: robot and terahertz spectrometer. In this paper, we describe a way to synchronize and combine the data from the two subsystems to form a 2D image. These imaging results are displayed in real time in a web-based user interface.

Material classification with THz radiation is typically done in transmission geometry.¹ However, in many situations a reflection based classification is highly desirable. For a reflection based classification scheme, it is necessary to compensate the impact of the surface morphology on the reflection signal.² As the surface morphology will mainly change the frequency dependent reflection pattern of the beam, we use different observation angles to improve classification based on THz reflection data. We use a THz TDS reflection setup measuring at several input and output angles. While the sample can be rotated, the transmitter can be moved on a semi-arc (see Fig. 1). We measure the reflection spectrum at different input-/output- angle configurations, which can be retrieved by an Euler transform of transmitter angle and sample angle. A measurement of a grating structure can be seen in Fig. 1. To reduce measuring time while maintaining a sufficient signal to noise ratio, we measure small angle variations around the main specular reflection. For classification we use a supervised machine learning approach based on principal component analysis for feature reduction and a support vector machine for classification.³ In this paper we present the impact of different observation angles on the classification accuracy in contrast to single-observation-angle classification, to check on the hypothesis that an increase in observation angle helps to classify a set of known materials by THz TDS reflection spectroscopy. In consequence we can estimate requirements on the observation angle and identify surface structures which will prevent classification.

Oscillations of the electron density (plasma waves) in the channels of field effect transistors (FETs) propagate with velocities much larger than typical electron drift velocities. FETs operating in the plasmonic regime (TeraFETs) could detect and emit terahertz (THz) and sub-THz radiation. The TeraFET plasmonic detectors have demonstrated excellent performance and are now being commercialized and used as pixels of THz cameras. New ideas of TeraFET THz detection breaking symmetry by phase enable TeraFET THz spectrometers and THz line-of-sight detectors. Heterodyne modes of operation and operation under bias increase the TeraFET sensitivity by orders of magnitude. The challenge now is to develop compatible TeraFET plasmonic sources, which, in combination with TeraFET plasmonic detectors, will support THz and sub-THz communications, including beyond 5G – 6G WiFi in the 240 GHz to 320 GHz range. Many other applications of such sources range from radio astronomy to industrial controls, security, biomedical, pharmaceutical, compact radar, drone, VLSI testing, and IoT applications. Silicon ultra-short channel MOS, AlGaN/GaN and AlGaAs/InGaAs HEMTs, p-diamond and graphene transistors are all candidates for the TeraFET plasmonic sources. Such sources are driven by a ballistic or quasi-ballistic current leading to a plasma wave instability. The instability mechanisms range from the Dyakonov-Shur instability involving the reflection of the plasma waves carried by drift, the instability using the transit time delays, and the “plasmonic boom” instability arising when the electron drift velocity crosses the plasma velocity. Even though the instability increments are reduced by the electron (or hole) scattering and by the electron viscosity the analysis of the TeraFET performance for different materials systems show that the room temperature operation is possible. The grating gate structures should allow to obtain sufficient powers and efficiencies. Our estimates show that it is possible to achieve 100 mW power at 1 THz. I estimate that it would take from 3 to 5 years for the THz plasmonic sources to reach practical Technology Readiness Levels for commercial applications.

We present a telecommunication-compatible terahertz source based on passive transponder nanoantennas. 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 built-in electric field formed between the nanoantennas and substrate. The photoconductive substrate is specifically grown to maximize the strength and overlap of the built-in electric field with the photogenerated carriers to provide high optical-to-terahertz conversion efficiencies. We have used this terahertz generation scheme to develop a fiber-coupled passive transponder that provides more than a 110 dB dynamic range over a 5 THz bandwidth.

We report on the investigations of the fin-shaped GaN/AlGaN field effect transistors with two lateral Schottky barrier gates exactly placed on the edges of the fin-shaped transistor channel. This kind FinFET modification (EdgeFET) allowed us to efficiently control the current flow in two-dimensional electron gas conduction channel. We present experimental data of sub THz detection by EdgeFETs. We describe also how it is beneficial for observation of resonant plasma wave THz detection and emission.

Thermoelectrically coupled nanoantennas (TECNAs) for mid- to far-infrared radiation are developed to study the Sun. TECNAs are resonantly absorbing the IR radiation using a nanoantenna that provides wavelength and polarization selectivity, as well as providing uncooled, and fast detection. This technology will enable transient solar measurements in the mid- to far- and wavelength ranges.

We present a multi-species trace gas sensor based on a broadband mid-infrared supercontinuum source and a compact home-built Fourier Transform Spectrometer with a balanced detection scheme. The gas sensor provides a spectral resolution of 1 GHz in the wavelength range of 2.4-4.2 µm. Using a global fitting routine, we are able to retrieve the concentrations of different species in complex gas mixtures, achieving detection sensitivities in the order of 100 ppbv.Hz-1/2 (4s measurement time). We used the sensor to measure volatiles from fruit (apples, pears) under storage conditions, demonstrating its potential for monitoring fruit in commercial storage facilities.

We propose a gas sensor using quantum cascade lasers which achieves high sensitivity and gas selectivity using a compact configuration and simple signal processing. This sensor features a small-volume Herriott cell and a simple concentration quantification algorithm. Our invented Herriott cell offers 5 m optical path length with only 40 mL internal volume. To preform concentration determination, feature quantities are extracted from an absorption signal by correlating the measured absorption signal with predetermined reference signals. The extracted feature quantities are used for the determination of target and interfering gas concentrations with simple simultaneous linear equations. The method is also capable of correcting for various disturbances, such as spectral shift due to laser wavelength drift and spectral broadening due to partial pressure changes of coexisting gases, by extracting additional, appropriate, feature quantities. This concentration quantification algorithm dramatically reduces calculation time compared to conventional spectral curve fitting methods because hundreds of spectral data points are replaced with a small number of feature quantities used during calculation. This approach allows the construction of a compact, fast-responding and robust gas sensor. We demonstrate a multicomponent and real-time measurement with a prototype of the proposed gas sensor. The prototype showed sub-ppm detection limits for NO, NO₂ and N₂O with 0.1 s integration time, and the interferences from high concentrations of H₂O and CO₂ could be removed completely. Furthermore, it was also shown that the prototype demonstrates excellent robustness against ambient temperature changes and spectral broadening effects caused by coexisting gases.

In this paper, we review the operating principles of hydrogen-terminated diamond MESFETs for high power and high frequency applications. Thanks to the transfer of electrons from the diamond valence band to the hydrogen termination a mobile hole channel is formed, leading to p-MOS operation. The resulting transistor shows good static performance, when the low technology readiness level of these devices is taken into account, owing to the excellent properties of diamond as a semiconductor. The dynamic performance is analyzed by means of quick ID-VD and ID-VG measurements carried out during a filling phase in off-state and on-state, and the corresponding recovery is monitored by the same measurements at different ambient temperatures. This measurement procedure allows for the extraction of the emission time constant and for the creation of the Arrhenius plot of the deep level involved, showing an activation energy of 0.30 eV. The short-term reliability of the devices is investigated by means of step-stress experiments in on-state condition. The degradation causes an increase in the on-resistance, a negative shift in the threshold voltage and a decrease in the peak transconductance value. The correlation between the variation in the electrical parameters and in the results of the dynamic characterization suggests that the degradation is caused by an increase in concentration of the aforementioned deep level, taking place both in the region below the gate and in the access region. A reduction in hole transfer efficiency can also be present, taking place at very high stress voltage.

High frequency analog RF photonic links are desirable to reduce the size, weight and power of RF systems by offering the replacement of lossy, bulky coaxial RF cabling for lightweight, low loss and broadband optical fiber. This talk presents an overview of high-performance photodiodes for analog photonic links, highlighting recent advances both from the perspective of products and technology in the commercial space, as well as a few notable demonstrations from research institutions over the past year.

Silicon optomechanical (OM) cavities have been presented as relevant elements in microwave photonics and optical RF processing, particularly in applications requiring low-weight and compactness. In this work, we introduce and demonstrate a new functionality by employing a silicon OM crystal cavity operated in the phonon lasing regime for optical upconversion of a radio-frequency data signal employing orthogonal frequency division multiplexing (OFDM) modulation. The OM crystal cavity is created on suspended silicon nano-beams with one-dimensional (1D) periodicity with <10μm² foot-print. The proposed OM crystal cavity operates as an optoelectronic oscillator at the GHz regime, with a low phase noise for the first harmonic at 3.9 GHz in the self-sustained oscillation regime. The OM crystal cavity characterization indicates that the optical resonance is centered at 1541.2±0.3 nm with a loaded optical quality factor Q_o ≈ 4×10³. Using such cavity we demonstrate successful upconversion of full-standard IEEE 802.16e WiMAX signals employing OFDM with QPSK modulation per-carrier over different bandwidths.

The monitoring and decommissioning of a nuclear power plant requires an image sensor resistant to gamma-ray irradiation that operates around the reactor. In this study, a single-pixel image sensor consisting of a photodiode (PD) and three-n-channel metal-oxide-semiconductor field-effect transistor (n-MOSFET) was initially prototyped using 4H-SiC complementary metal-oxide-semiconductor (CMOS) technology. The single-pixel circuit responded correctly to the UV lamp being switched on/off, even at 1 MGy. However, the output current (I_OUT) of the image sensor increased by 60% at 200 kGy and then remained constant until 1 MGy. This behavior is caused by a shift in the threshold voltage (▵V_th) of the n-MOSFET. A simulation (using LTspice) confirmed that the increased I_OUT can be suppressed with a differential circuit that reads out the reference voltage and signal voltage at different timing and outputs the difference in their voltage. Although the differential circuit can suppress the increase of I_OUT caused by gamma-ray irradiation, this circuit has its own problem vis-a-vis ▵V_th caused by bias temperature stress. To suppress ▵V_th caused by positive bias temperature instability (PBTI), we proposed a drive method wherein before reading the two voltages, a bias stress is applied for n-MOSFET to saturate ▵V_th. As a result, ▵V_th and its variation can be suppressed. From these results, we expect the single-pixel circuit to function without problems between 0 kGy and 1 MGy using the differential circuit and proposed drive method. The 4H-SiC image sensor could be a promising candidate for a camera with gamma-ray irradiation resistance.

Optoelectronic oscillators (OEOs), realized on photonic integrated circuits (PICs), have the potential of producing millimeter-wave (mm-wave) clock signals with lower timing jitter and higher operating frequencies than their all-electronic counterparts. To have a proper design tool for these PIC-based OEOs, a novel, computationally efficient time-domain circuit simulation model is presented. It relies on describing the propagation, filtering and mixing of the spectral contents of the circulating optical and mm-waves. This work specifically targets OEOs consisting of building blocks offered by commercial PIC platforms, such as high-speed modulators, high-Q filters, semiconductor optical amplifiers (SOAs) and high-speed photodetectors (PDs). The model can simulate a wide range of OEO topologies, including OEOs that use an array of SOAs and PDs to boost the generated mm-wave signal power, or OEOs that employ modulator configurations other than the often-used Mach-Zehnder devices. The model also takes into account the saturation effects and noise of the SOAs and PDs, as well as all the propagation losses and delays experienced by the optical and mm-waves, which allows for investigating the effects of fabrication errors. As a test case, this model is applied to a proposed design of a hybridly integrated 20-GHz OEO, which relies on a combination of indium phosphide (InP) and silicon nitride (SiN) based PICs, using realistic parameters representative for these platforms. Timing jitters of almost 100 fs (10 kHz { 10 MHz) are demonstrated by optimising the SOA gain and the laser frequency detuning from the high-Q filter resonances.

We report on Nyquist wavelength division multiplexing (WDM) communication in the terahertz (THz)-band utilizing an integrated-optic spectrum synthesizer. The synthesizer is composed of two arrayed-waveguide gratings, and an array of phase shifters and variable optical attenuators, and it can individually control amplitude and phases of up to 64 optical discrete frequency components with 40 GHz spacing. Three optical frequency combs are produced by combining the synthesizer with a mode-locked laser diode. The produced combs are used to generate a THz-wave Nyquist WDM signal with photo-mixing. We demonstrate 2 x 40 Gbit/s Nyquist WDM communication in the 300 GHz band.

A powerful class of techniques utilizing frequency combs is that of multiheterodyne techniques. These techniques use each individual, evenly space, spectral line of a comb as a local oscillator to measure a source's spectrum. By mixing an unknown source with that of a comb one can convert an optical signal into an electrical signal where standard radio-frequency (RF) electronics can be used. However, these techniques have been limited to measuring coherent sources, such as lasers, due to their inability to disambiguate signals that overlap in the down-converted intermediate frequencies (IF). This excludes most natural sources from being measured. In this manuscript, we present a new dual-comb technique that allows for the measurement of any arbitrary spectrum, even incoherent ones that span multiple comb lines.^{1, 2} It is shown that using the same equipment required in a dual-comb experiment one can calculate a correlation function between the two channels that has all the information required for accurate reconstruction. We brie y present the theory followed by simulation and an RF comb experiment.

We present a terahertz quantum cascade laser array of coupled parallel double metal waveguides, spaced by narrow gaps. The entire array is pumped by a single electrical source, and at a relatively low bias, an equidistant mode spacing and a narrow single beatnote are observed, which are indicating frequency comb operation. The spectrum consists of subgroups of modes separated by an integer multiple of the cavity round-trip frequency. This is a sign that the individual array elements combine to form a global comb, and also suggests the possibility of a harmonic comb formation.

We review recent work on broadband RF channelizers based on integrated optical frequency Kerr microcombs combined with passive micro-ring resonator filters, with microcombs having channel spacings of 200GHz and 49GHz. This approach to realizing RF channelizers offers reduced complexity, size, and potential cost for a wide range of applications to microwave signal detection.

The degenerate mode interaction can possess a clear avoided mode crossing to manipulate the cavity mode spectra by a mode-splitting process under a strong mode-coupling condition in high-Q microresonators. Here, the mode splitting strength can be changed by controlling the pump-resonance effective detuning in a dispersion-managed Si3N4 microresonator through a differential thermo-optic effect. The splitting mode can locally facilitate the frequency matching in normal dispersion microresonators so that the tunable parametric oscillation can be observed by tuning pump-resonance detuning. A broadly tunable THz wave radiation is generated after injecting the tunable parametric oscillation into a bias-free photomixer at room temperature.

It has been shown that the quantum Zeno effect can be used to inhibit the evolution of a quantum state, for example the transition of a particle of a two-state system would be hindered if it was observed frequently enough. The observations can be realized by an optical pulse source at very high pulse repetition rate. The effect could be used to realize the power amplification in RF or THz domain without population reversal because the simulated absorption from lower state could be suppressed. So far, some applications of Zeno effect have been demonstrated in a few material systems which are mostly gaseous state or trapped atoms or ions. In this paper we designed an experiment to show the Zeno effect in a solid-state material, i.e. bilayer graphene.

The efficient stimulation of a graphene microstrip plasmoni splitter via higher-order mode propagation is proposed in the current work. Initially, graphene waveguiding systems are investigated thoroughly in terms of the supported propagating modes and their potential excitation. Specifically, the microstrip apparatus is examined focusing on the distribution of bulk modes. This analysis indicates that the transition of a higher-order mode to a lower one is potentially smooth if an appropriately selected microstrip width is utilized. Consequently, an effective graphene plasmonic device is designed and stimulated via a higher-order bulk mode that equally splits the propagating surface wave to separated microstrips. The latter supports a lower-order mode and it is evaluated that the undesired back-reflected waves are minimized. Moreover, a thorough performance analysis is numerically conducted by means of a flexible Finite-Difference Time-Domain algorithm validating the remarkable functionality at a wide frequency range.

We present a photoconductive terahertz detector, which offers high-sensitivity and broadband detection performance for terahertz time-domain spectroscopy at record-low optical pump power levels. The detector employs a plasmonic nanocavity designed to confine the optical pump photons in a thin photoconductive region. By providing an efficient optical absorption in this thin layer, the carrier transport time to the device contact electrodes is maintained in a sub-picosecond range for the majority of the photo-generated carriers. Therefore, ultrafast operation and high quantum efficiency is achieved simultaneously, which significantly increases the detector responsivity and dynamic range even at very low optical pump power levels. We experimentally demonstrate a 110 dB dynamic range over a 0.1-7 THz frequency range at only a 0.1 mW optical pump power level.

Terahertz (THz) spectroscopy provides a crucial view on the electrical properties of emerging materials complementary to that which can be provided by more standard conductivity methods. The high frequency properties acquired in the THz range can be used to predict low frequency and DC behavior. THz spectroscopy provides insight into grain boundaries, phonon modes, and the underlying mechanism of charge transfer in general, paving the way towards superior design of novel materials. In this paper, we present pump-probe and time-domain THz spectroscopy on metal organic frameworks (MOFs) providing a detailed insight into carrier lifetime and charge scattering in MOFs.

A method will be presented to shape terahertz spectra from different semiconductor-based sources by manipulating the phase and amplitude. The phase and amplitude are manipulated by a c-band programmable optical filter with 10 GHz resolution and a total bandwidth of 5 THz with a group delay range between -25 ps and 25 ps. As optical sources, a superluminescent diode (SLD) and different mode-locked laser diodes (MLLD) are employed. For the spectral shaping of the MLLD, a model-driven genetic algorithm is used to achieve different optimization goals such as spectral flatness and maximum bandwidth.

Recent studies using Laser THz Emission Nanoscopy have shown that a single gold nanorod was able to modify the emission from an InAs surface although the nanorod was not resonant with the incident laser pulses. Here, we present our studies on THz emission spectroscopy on gold nanorods with different geometrical orientations i.e. standing versus lying on an InAs surface. We show that gold nanorods resonant with the incident femtosecond laser gives an enhanced THz emission from the InAs surface, while the separation distance between each individual nanorod also plays a significant role.

UMass Lowell BTTC researchers are investigating the use of a high resolution 75 GHz radar to identify the internal defects of the fiberglass filaments in wind turbine blades. Developing the acquisition and image analysis software, the team is fabricating a laboratory based test radar and anechoic chamber for performing the preliminary measurements. Central to the team's challenge is the development of an implementable design for a manufacturing plant’s wide area inspection system. The measurements and system design will be presented.

High-resolution X-ray diffraction (HR-XRD), and low-temperature photoluminescence spectroscopy (LT-PL) are used to investigate the structural properties and inhomogeneities of high current density InGaAs/AlAs/InP resonant tunnelling diode (RTD) wafer structures. The non-destructive assessment of these structures is challenging, with structural variables: well and barriers thickness and the well indium molar fraction, in addition to electronic variables such as the band-offsets being functions of strain, growth sequence, etc.. Experimental PL data are compared with simulations allowing the deconvolution of the PL spectra, that includes Type I and Type II transitions broadened by interface fluctuations on length scales smaller and much larger than the exciton. This method provides details of the non-uniformity of the epitaxial material nondestructively.

We demonstrate, for the first time, a telecommunication-compatible terahertz frequency-domain spectroscopy system without requiring any short-carrier-lifetime photoconductors. The ultrafast response of the terahertz source and detector is achieved by incorporating plasmonic antenna electrodes and a thin layer of epitaxially-grown In0.53Ga0.47As to confine optical generation very close to the antenna electrodes. As a result, a short carrier transit time for most of the photocarriers is provided, inducing an ultrafast photocurrent for terahertz generation and detection. This design approach enables larger flexibility for the choice of photoconductors and operation wavelengths without being limited by the availability of short-carrier-lifetime photoconductors.

Terahertz (THz) technology has matured over the past few decades and recently innovative applications beyond spectral sensing and imaging, and industrial quality control are being pursued. Earlier we had proposed a high-pressure-hightemperature (high-PT) THz waveguide sensor for harsh environments using hollow-core-metal-waveguides (HCMWs). The modal response in terms of change in dispersion due to applied pressure and/or temperature is highly sensitive for these structures and could be calibrated directly in the time-domain. Moreover, for sensing purposes one requires very short lengths of HCMWs which could be replaced easily with minimal interference to the sensing system. Towards realizing this sensor, however, one requires systematic study of the loss and dispersion of these structures over a broad frequency range. Here we have characterized HCMWs of Copper and Silver claddings having lengths between 20 to 160 mm with inner (core) diameters of 1.5, 2 and 3 mm using photoconductive antenna-based pulsed THz system with bandwidths up to 5.5 THz. Employing hybrid THz-optics, careful cut-back measurements were performed to obtain a loss ~ 34 dB/m for 2-mm core Silver waveguide, and we found that the coupling efficiency is highest for TE₁₁ mode. Using Short-Time Fourier Transform Analysis, complete modal mapping revealed that this mode is the only principal supported mode over the range of 0.8 to 1.6 THz. The waveguide dispersion of this mode directly relates to the core diameter which in turn, is related to the application of external high-PT and could be calibrated in future over this frequency range for sensing purposes.

We provide a quantitative theory of discrimination between objects with the same color temperature but having different angular spectrum by intensity interferometry. The twopoint correlation function of the black body image with extended angular spectrum has significant differences with a correlation function of a black body with a narrow angular spectrum.

Moisture measurement is one of the key components of food quality control sector as it influences the shelf life and storage condition of the product. Milk powder is hydroscopic in nature and typically contains 4-5% moisture content. Excess amount of moisture can lead to lumping of milk powder, which in turn changes its flavor, quality, and shelf life. However, over prolonged shelf time, this degraded product might lead to formation of microbes and bacteria. In this work, we have employed terahertz time-domain spectroscopy (THz-TDS) to determine the moisture content of milk powder through its response to the increased humidity in the sample. Moisture level in the probing sample was increased in a controlled manner from 45% to 95%. To obtain optimum information regarding changes in the sample, different parameters of THz electric field were investigated. Observed changes in THz pulse were also correlated further with the change in weight of the sample. Results suggest that THz-TDS is an effective tool to estimate the water content and resulting change in structural composition of milk powder.

We report electroluminescence at 14meV and 20meV from a n-type Ge/Si0.15Ge0.85 quantum cascade heterostructure on Si substrate grown by ultra-high vacuum chemical vapour deposition. The electroluminescence signal of the single quantum well active region design, extracted through diffraction gratings from mesa structures, is compared with its GaAs counterpart.The spectral features agree well with modeling based on Non-equilibrium Green's function calculations. The observed electroluminescence peaks show a full width at half maximum of 3meV and 4meV. These results are an important step towards the realization of an n-type THz quantum cascade laser on a non-polar material system.

Frequency red tuning of GHz-level is shown in THz Quantum Cascade Lasers, by post-processing alteration of the device. It involves changing the height difference between the active region and surrounding medium, and reapplying a material of known refractive index with an appropriate thickness according to simulations. Thus the mode effective refractive index is increased, causing the device to lase at a lower frequency. With the correct material and thickness, shifts down to 1 GHz can be observed.

Stability of optical beats in a chaotically oscillating laser is compared to that of a free-running continuous-wave laser using a highly efficient plasmonic photomixer (Anttena). The high stability of optical beats in chaotically oscillating lasers is verified.

We apply the single-pixel imaging technique to retrieve multi-dimensional (space, time/frequency) images at terahertz frequencies by indirectly reconstructing the temporal waveform in each pixel. Moreover, we exploit compressed sensing algorithms to reduce the acquisition time.

We report room temperature terahertz detection in hBN/graphene/hBN heterostructures. The obtained record combination of high-speed (response time < 1 ns) and high sensitivity (noise equivalent power ~ 100 pWHz-1/2) is enabled by the photo-thermoelectric effect.

This paper presents the necessary building blocks towards the realization of on-chip, lens-free, spectrally selective, THz beam steering. We demonstrate continuous wave (CW) THz generation up to 2.2 THz by photomixing using antenna-coupled silicon-integrated germanium photodiodes, which reach an optical-to-THz conversion efficiency of about 1% at 100 GHz. We show THz beam forming within a small antenna array and address key challenges towards the realization of large quasi-optical THz phased arrays, by demonstrating low-loss (<0.2 dB/cm), low phase error routing and optical beam steering within hybrid Si/SiN optical phased arrays. Finally, we present an anti-reflection structure enabling lensfree THz beam steering.

Terahertz time-domain spectroscopy has emerged as a competent technique for real-world applications such as quality control in pharmaceutical and food industry, biomedical and precision-agriculture. However, investigating the optical response of the practical composite samples such as pharmaceutical tablets, biological composites and nano-composites using Terahertz (THz) time-domain spectroscopy (TDS) is quite challenging. In the present work, we have prepared two series of binary composites, with different volume fraction of constituent materials. These composites were probed using pulsed THz TDS and their dielectric response was modeled using the effective medium theory (EMT). Based on the volume fraction of the constituent material, three EMT models, including Maxwell-Garnet (MG), Polder-van-Santen (PvS) and Landau-Lifshitz-Looyenga (LLL), were compared empirically with the conventional analysis at different frequencies; and the accuracy and ambiguity of the extracted parameters using these quasi-static models were determined. Hence, the feasibility study of these theoretical models was discussed to assess their practicality.

The spectral components of a multifrequency THz-wave may be characterized using the nonlinear electrical response of a low temperature grown GaAs photomixer. The heterodyne detection with the photomixer is exploited to down-convert a radiofrequency-modulated THz-wave to the microwave domain, by using a suitably tuned optical beat that acts as local oscillator. The photomixer is electrically driven with an alternative voltage oscillating at the modulation frequency and provides a phase-sensitive DC response signal indicating the demodulation of the THz-wave.

We present a tunable multi-channel optical true time delay (OTTD) using frequency interval tunable multiwavelength light source (MWLS). The frequency interval of MWLS can be flexibly tuned by programming a driving radio frequency (RF) signal. By using this tunable MWLS, the time delay difference (TDD) of multi-channel OTTD can be flexibly tuned with time in different function forms. The TDD of linear tuning with variation range of 61.84-119.71 ps and sinusoidal tuning with variation range of 60.97-87.76 ps with time are experimentally demonstrated.

We present novel photoconductive antennas (PCAs) compatible with 1550 nm excitation for terahertz (THz) time-domain spectroscopy (TDS). Rhodium (Rh) doped InGaAs grown by molecular beam epitaxy is used as the underlying photoconductor. Due to the advantageous combination of sub-picosecond carrier lifetime and excellent electronic properties, InGaAs:Rh based emitters feature an unprecedented emitted THz power of 637 µW. A record peak dynamic range of 110 dB is demonstrated with a THz TDS system using InGaAs:Rh based PCAs only. This sets a new benchmark for THz TDS systems operating at 1550 nm.

Investigating layer thicknesses with terahertz time-domain spectroscopy usually requires the refractive index of the layer of interest. The refractive index is either investigated beforehand by independent measurements or it is measured simultaneously. The challenge of a simultaneous determination is that measuring two unknown values requires the use of two linearly independent equations. To address this challenge, our approach is to combine transmission and re ection time-of-flight measurements. By combining these measurements, we obtain a system of two linearly independent equations for the two unknown values: thickness and average refractive index. We experimentally validate our simultaneous estimation approach by measuring calibration foils of different thicknesses.

Photoconductive Antennae have established itself as a cornerstone in the development of portable THz technology. The understanding of the complex dynamics of the carriers during the operation of the PCA is of utmost importance not only during the operation but also in further design evolution of the same. However practical design considerations are rarely incorporated into the theoretical simulations of Photoconductive Antennae. In this article, we propose the use of a Spatio- Temporal Finite Difference Time Domain modification of the existing simulation techniques to estimate THz pulse profiles. The article shows that the treatment of the instantaneous electrostatic forces and electron dynamics in a hypothetical 1-Dimensional semiconductor pathway used for the operation of a PCA can yield similar types of pulse profiles as obtained in experimental conditions. Also, the evolution of such a system in terms of dimensions (i.e. upto 3D) and incorporating other forces / physical effects mentioned may also lead to a more accurate simulation of a proposed structure.

For anti-resonant fibers (ARF), when the diameter of cladding tube approaches the effective diameter of fiber core, the fundamental mode (FM) in core appears an index-induced mode coupling with the FM in cladding tubes. The FMs mode coupling between core and cladding forms a super FM in ARF, and simultaneously introduces an obvious suppression on the FM in core, which provides a novel theory for polarization-maintaining ARF. In this paper, we proposed a polarization-maintaining ARF (PM-ARF) with four asymmetrical cladding tubes for 2.5 THz wave. In PM-ARF, the horizontal cladding tubes are designed to generate the suppression on modes, and the vertical tubes are designed to generate the guidance for modes. Profiting from the mode coupling between core and cladding, a birefringence index of 9×10^-6 is obtained by the compact PM-ARF. The mode coupling was confirmed can be combined with methods of bi-thickening the core boundary and elliptical core boundary respectively to improve the polarization-maintaining ability. Optimizing the shells of horizontal and vertical cladding tubes into different thickness respectively, and simultaneously working with the mode coupling between core and cladding, the birefringence index of ARF was improved to larger than 1×10^-4. Besides, the mode coupling worked with the method of elliptical core boundary, the birefringence index was further improved to higher than 2×10^-4.

Magneto-optic Faraday effect in unaligned single-wall carbon nanotube thin films with different geometric parameters on transparent float glass substrates was experimental studied in a frequency range 0:2–0:8 THz (corresponding to a range from ~1:50mm to ~0:37mm) at a controlled room temperature of 291–293K, and a relative humidity of 40–45%. A change of 15° in an azimuth angle, and of 10° in an ellipticity angle was achieved. The results show that by using carbon nanomaterials-based structures it is possible to devise efficient tunable polarizers that can be used in the advanced areas of terahertz nanophotonics.

Since nanoporous three-dimensional structure based on SiO2 has been studied as a THz optical material [Optical Materials Express 10(9), 2100-2113 (2020)], in this work we study its fabrication properties. Porous SiO2 is represented by porous opal matrixes [Optical Materials 49, 208–212 (2015)], which are based on globules of amorphous SiO2 [Nano 8(4), 1350036 (2013)]. By the thermal treatment, the material is able to achieve materials with different stoichiometric composition and porosity, minimizing an amount of residual water, and obtaining pre-determined physics properties of material. We fabricated couple of cylindrical lenses and flat plates made of abovementioned nanoporous SiO2. We showed that simple-form components could be easily fabricated by grinding, while mechanical processing strategy depends on the annealing temperature used and the material strength.

In this paper, an experimental study was conducted using terahertz (THz) spectroscopy to assess the electromagnetic (EM) properties in the THz range of various materials that can be found in oil and gas fields. The first set of experiments were conducted to assess the effects of temperature on THz absorbance. This revealed that the temperature does not have a substantial effect on the absorbance coefficient. On the other hand, other experiments indicated that the absorbance coefficient is much higher for solid contaminants particles than for either crude oil or water. In addition, when solid contaminants are mixed with the crude oil, the absorbance coefficient varies with respect to the way the solid contaminants are distributed within the sample and increase proportionally to the amount of solid contaminants. In case of crude oil-water mixture, it is found that the absorbance coefficient increases as the water content increases. However, compared to solid contaminants, its absorbance value is much lower.

In this paper, a new FPGA-based THz imaging device for real-time multiphase flow metering is proposed. The probe comprises a THz source and a 64 x 64 THz camera within which a stainless steel-flanged teflon-made cylindrical probe is used. A dedicated digital video bus is used to transfer the output frames of the camera to a Startix V-based FPGA board for bit-level real-time video processing and display. The algorithm consists of a cascade of consecutive tasks which include image filtering and histogram, feature extraction (for phase fraction measurement), in addition to block-based motion estimation (for flow rate measurement). Extensive experiments were successfully carried out on the developed device using various two phase samples with different concentrations of water, or solid particles added with air. Hence, an overall accuracy of 98.65 %, with a total processing time of less than 30 ms/frame was achieved. While one Stratix V FPGA is enough to run the motion estimation algorithm at one pixel precision for a 64 x 64 THz image with a search area of 24 x 24 pixels in less than 75 μs, larger image sizes of up to 512 x 512 pixels can still be processed in real-time using a reasonable time sharing of resources. The suggested system may constitute a breakthrough in the field of multiphase flow metering since it allows simultaneous visualization of the flow, in addition to accurately determine the flow rate of individual phases even in challenging situations such as the case for high gas void fraction (GVF) multiphase flow (i.e. GVF exceeding 95%).

A terahertz imaging method based on aperture coding is proposed to solve the problems of large pixel size and low resolution of the terahertz imaging detector. The forward model of the terahertz coded incoherent imaging system is established, and the optimal coding imaging strategy is discussed. By adding coded modulation to the aperture, the image detected by the imaging detector can generate pixel-level light intensity conversion. Through the multi-frame aperture coding simulation experiment, the pixel aliasing problem caused by the detector pixel size is effectively solved, and the imaging resolution is improved to the diffraction limit of the approximate lens, so as to guide the future experiments.