This PDF file contains the front matter associated with SPIE Proceedings Volume 12885, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

The voltage response of thermoelectrically coupled nanoantennas (TECNA) to infrared light (IR) is greatly enhanced by suspending them above quasi-spherical cavities. The cavities behave as optical elements that focus the IR radiation onto the nanoantennas. The wavelength response of a cavity depends on its geometrical profile, for which the resonant cavity mode must match the resonant wavelength of the antenna. In this work, TECNAs were designed to detect four different wavelengths in the mid- and long-wave IR and fabricated in a single chip, requiring different cavity profiles. Cavities were fabricated by isotopically etching Si using xenon difluoride (XeF₂). XeF₂ gas etches the Si substrate through circular windows patterned in resist. The antenna is patterned before the cavity is formed to avoid difficulties in filling and planarizing the supporting layer for antenna patterning and is left suspended over the cavity after etching. The different cavity profiles were formed in a single etch, since subsequent resist patterning and etching of unfilled cavities would destroy previous antennas. This work compares cavity profiles etched using both pulsed and continuous-flow XeF₂ etch methods using different etch window sizes to produce variously sized and shaped cavities in a single etch. The cavity profiles were characterized using a laser-scanning confocal microscope. Piecewise equations made up of a torus, a cone, and a cylinder were fitted to the profiles and used in COMSOL simulations of electromagnetic standing-wave patterns within the cavity. Antennas were then chosen to have peak responses at wavelengths that match a simulated cavity’s peak response.

We present a fiber-coupled terahertz (THz) time-domain spectrometer with a record bandwidth of 10 THz. This significant increase of 3.5 THz in bandwidth compared to state-of-the-art spectrometers is enabled by improved photoconductive antennas (PCAs). In particular, we show that THz absorption in the polar InP:Fe substrate of the standard InGaAs PCAs is the main limiting factor for the bandwidth. To overcome this issue, we have developed a wafer-scale substrate transfer-process for the thin photoconductive InGaAs layer from InP:Fe to low loss, high resistivity float-zone (HRFZ) silicon. The resulting photoconductive membrane (PCM) antennas combine the advantages of ultrafast iron-doped InGaAs photoconductors as efficient THz emitters and receivers with nearly absorption-free and spectrally flat transmission of HRFZ-Si up to 17 THz.

Recently, the combination of gas sensors and metamaterials has emerged as a popular research area. However, traditional gas sensors typically rely on electrochemical methods for detection, requiring operation in high-temperature environments and presenting potential explosion risks. To address these challenges, this study utilizes terahertz low-energy photons with high transmission and broad bandwidth as a detection source, enabling devices to operate under normal conditions at room temperature. This approach aims to enable comprehensive material information retrieval while preventing combustion or explosions during interaction with the tested substance. Zinc oxide (ZnO) has been widely utilized in gas sensors, with the technology reaching a certain level of advancement. Additionally, our focus lies on the study of metal-organic framework (MOF) materials, which offer significant advantages in gas sensing applications. Key characteristics of MOFs include a large surface area, high porosity, and unique surface properties. These materials can be fabricated using various metals to create corresponding gas sensing films, and their properties can be further tailored through modification with different polar molecules and the introduction of other catalysts. While there is extensive research on MOFs based on vanadium as a framework, there is limited literature on their gas sensing applications, underscoring their research value. Nitrogen dioxide (NO₂) is a volatile, pungent, odorous, and toxic gas that poses a fatal threat to both humans and the environment. Therefore, gas sensing technology for NO₂ is of utmost importance. In this regard, our research is dedicated to the development of optical gas sensors using novel composite materials. Nanostructures of ZnO and V-MOF are prepared respectively, and these composite materials are integrated with metamaterials for the detection of NO₂ gas.

We report on novel continuous wave (cw) photoconductive terahertz (THz) receivers based on rhodium (Rh) doped indium gallium arsenide grown by molecular beam epitaxy. The Rh-doped material exhibits outstanding charge carrier mobility up to 1800 cm²/Vs while maintaining ultrashort carrier lifetimes between 0.26 and 1.5 ps. The photoconductive antennas (PCAs) made from this material exhibit a THz responsivity significantly improved by a factor of 20 to 100, which overcompensates their slightly higher noise levels. In a homodyne coherent THz spectroscopy setup, these novel receivers enable us to measure THz spectra with a record peak dynamic range up to 125 dB and a spectral bandwidth of 4.5 THz. This is an improvement of 13 dB over the previous record with InGaAs:Fe-based PCAs and leads to improved accuracy and measurement speed in cw-THz spectroscopy.

In this work, we consider the governing limits of spatial resolution and spectral bandwidth in pursuing implementations of terahertz (THz) spectroscopy on the subwavelength scale. We discuss the need for effective focusing elements in driving sufficient levels of power from the (macroscopic) incident THz beam down to the (microscopic) focal spot. Such elements, when effective, enable large signal strengths and wide bandwidths, but this has proven to be challenging in contemporary implementations of near-field THz imaging and spectroscopy. To this end, we show theoretical and experimental results for focusing via parabolic mirrors, high-resolution lenses, and engineered dielectric spheres, with the latter yielding THz microjets with especially intense and small focal spots. We then discuss the need for near-field spatial constriction, to drive the spatial resolution down to an even smaller scale, and show that this constriction can lead to dispersive (i.e., frequency-dependent) characteristics. In this work, we demonstrate spatial constriction via simple circular apertures, which function as high-pass filters. Ultimately, our theoretical and experimental results reveal that implementations of THz spectroscopy on the subwavelength scale are governed by a spatial-spectral product—whereby reductions in the aperture's diameter (to improve the spatial resolution) raise the aperture's cutoff frequency (at the expense of spectral bandwidth).

The terahertz spectral regime finds applications in terahertz spectroscopy and next-generation wireless communications, necessitating improved frequency stability and accuracy. This study presents a novel approach to achieve ultra-stable terahertz continuous-wave generation by referencing an optical cavity using an optical frequency comb. The results demonstrate an extended operating frequency range of 0.1-1.1 THz with frequency stability of about 10^-15 at 1 s. This exceptional level of stability has significant implications for terahertz wave research, holding the potential for groundbreaking advancements in both fundamental and industrial applications.

The strong absorption of water in the mid-infrared (MIR) causes difficulties in performing combined near-infrared (NIR) and MIR spectroscopy on aqueous samples using a single instrument. Combining spectra from different instruments can result in unwanted spectral variations, which can influence the prediction models and mitigate the advantages of the combination approaches. In this work, absorption spectra were collected in the NIR (1300nm-2500nm) and MIR (2500nm-3800nm) region by combining a single high-brightness broadband supercontinuum (SC) laser spanning from ~1-4.0 μm with attenuated total reflectance (ATR) and a transmission cuvette in a single-path configuration to provide a uniform spectral response across the NIR and MIR regions. The measured NIR- and MIR- spectra were assessed based on their ability to predict varying concentrations of ethanol, sucrose, and L-proline in aqueous solutions. The NIR-based partial least square regression (PLSR) model gave higher prediction accuracy for sucrose (R² = 0.95) as compared to both ethanol and L-proline (R² = 0.75 and R² = 0.57 respectively). On the other hand, the MIR-based model enhances the prediction accuracy of ethanol (R² = 1.00) and L-proline (R² = 0.62) while demonstrating no significant change in prediction accuracy for sucrose (R² = 0.96). The prediction models based on the combined NIR-MIR spectra performed similar but slightly worse than the MIR-only models for ethanol and L-proline (R² = 0.97 and R² = 0.54 respectively), while for sucrose, it was slightly improved (R² = 0.99).

We present THz time-domain spectroscopy and imaging covering a wide frequency range up to 20 THz. Our approach uses highly efficient organic nonlinear optical crystals (DSTMS, OH1) and a compact femtosecond pump laser (1560 nm) with MHz repetition rate and ultrashort pulse width (<40 fs). By optimizing the optical geometry, we achieve high SNR (>80 dB at 4 THz, >70 dB at 10 THz). Our theoretical model agrees well with experiments. We validate our system by obtaining high-resolution THz spectra of water vapor and various solid substances. Additionally, we present THz imaging with sub-50 μm spatial and sub-10 fs temporal resolution.

Nowadays, a terahertz time-domain spectroscopy (THz TDS) is a well-applicable method for both basic research, industrial applications and security screening. As a rule, the detection and identification of a substance is carried out by comparing the absorption frequencies of the substance under study with the absorption frequencies belonging to the database. To find these frequencies, the substance is irradiated by the THz broadband pulse. Then the pulse, reflected from or transmitted through the substance, is measured. Real conditions and the substance opaque may perturb and attenuate the signal under measurement. Therefore, finding a way to increase the distance at which it is possible to provide reliable measurements and, consequently, enhance a distance of the remote sensing, is an actual problem. In the current study, we discuss a possibility of the broadband THz pulse’s spectrum finding based on measurements in time for a substance response at some frequencies belonging to the GHz range of the frequencies, under irradiating the substance by a broadband THz pulse. For simplicity, we consider the transmission mode of the THz spectrometer and illustrate the feasibility of the method using an artificial signal and a real measured signal.

Hardware security has been a significant challenge in semiconductor devices, leading to serious issues such as denial of service, data leakage etc at both transistor as well as package level. Detecting the usage state of a target semiconductor sample or generating a unique fingerprint can provide critical information about the sample, aiding in the prevention of counterfeiting to avoid hardware security risks. Terahertz Time-Domain Spectroscopy (THz-TDS) stands out as a powerful and non-destructive tool, capable of probing and analyzing the polymer materials. Its sensitivity to molecular-level changes makes THz-TDS suitable for studying the material, structural, and stress properties of IC packaging under real-world conditions. This research begins by reviewing the composition of Electronic Material Compatibility (EMC) materials and their variance under real-world conditions. Furthermore, the application of THz-TDS for polymer material characterization, which incloudes material, structural, and internal stress characterization, is also thoroughly reviewed. By combining these factors, we explore the potential and capability of using THz-TDS to detect real-world loaded samples and the feasibility of generating fingerprints for identifying counterfeit samples, including reused, recycled, or tampered ones.

Due to the global offshore fabrication of semiconductors, hardware security problems such as counterfeit ICs and Hardware Trojans (HTs) have affected semiconductor device trustworthiness in critical applications. Previous research has proven the encapsulant material difference exists in the counterfeit ICs. What’s more by using a pulsed THz signal, Terahertz Time-Domain Spectroscopy (THz-TDS) is able to detect the effective refractive index difference between authentic and counterfeit IC packaging. This research has also successfully observed the reflective index difference between authentic and counterfeit by measuring the layer thickness and THz-TDS time delay. However, the accuracy of calculating the effective refractive index depends on the accuracy of the layer thickness and the time delay measurements. Consequently, reflective index difference may arise from noise encountered during data collection, which can affect the accuracy and repeatability of counterfeit detection tasks. In this paper, we utilize an unsupervised machine learning model to further demonstrate the capabilities of THz- TDS in counterfeit IC detection. Additionally, the potential of using THz-TDS to generate a unique fingerprint is also discussed.

The channel lengths of deep submicron transistors and photodetectors reached dimensions smaller than the mean free path. These devices could operate at frequencies comparable to or higher than the electron momentum collision frequency. Their ballistic and quasi-ballistic operation regimes are dramatically different from a more conventional collision-dominated transport. In these regimes, the measured apparent electron mobility becomes proportional to the channel length (so-called “ballistic mobility”). At high mobility values, the electron transport becomes viscous, and the electron density oscillations in ultra short channels are damped by the electronic viscosity rather than by the collisions with impurities or lattice vibrations. In this paper, we show that viscosity dispersion is an important effect in determining the effective viscosity at high frequencies and/or for very short excitation pulses. The characteristic frequency determining the viscosity dispersion is the electron-electron collision frequency, which depends on the electron concentration and temperature. Using a Drude-like equation for the viscosity, we evaluate the viscosity dispersion effect for deep submicron devices implemented in Si, AlGaN/ GaN, p-diamond, and AlGaAs/InGaAs. The applications of the viscosity dispersion effect include terahertz and IR photodetectors. This effect is especially important for the detection of femtosecond and terahertz pulses.

We report an optical–electrical (OE) oscillator using two new key components (a high-power integrated photoreceiver module and a low-driving-voltage lithium niobite (LN) Mach–Zehnder modulator (MZM)) to configure key parts of the entire OE oscillator system. A 60-GHz narrowband photoreceiver module integrated with a 60-GHz photodetector chip with a gain emphasis circuit and a 60-GHz high-power amplifier chip was designed and fabricated. By increasing the input photocurrent to 5 mA at 60 GHz, we successfully achieved a 1-dB power compression level of +16 dBm with good output linearity. The LN-based bulk MZM was specially designed to decrease the driving voltage to 1.5 V by optimizing both the waveguide and electrode length at high frequencies. Excellent performance with a -5 dB insertion loss at 60 GHz was achieved. The OE oscillator system was configured via a feedback loop using optical and electrical components, which primarily consisted of the newly developed photoreceiver and MZM through a 100 m–1 km long optical fiber. We successfully demonstrated -106 dBc/Hz at a 10-kHz offset near 60 GHz. The design and fabrication of the OE system are discussed in detail herein.

A simple method compatible with all geometrical structures of terahertz metamaterials for enhancing the tunability, sensitivity, Q-factor, and figure of merit (FOM) is proposed. This method uses a multi-layer SiO₂:Ag to approach the floating-like double split ring resonators. The terahertz metamaterial consists of five layers of SiO₂:Ag, in which each layer of SiO₂:Ag comprises a 250-nm-thick SiO₂ layer and a 50-nm-thick Ag layer. The tunability, sensitivity, Q-factor, and figure of merit (FOM) of metamaterial can be increased with increasing the number of layers of SiO₂:Ag. Metamaterial with multi-layer SiO₂-Ag has a larger tunability, sensitivity, Q-factor, and figure of merit (FOM) than the common metamaterial based on a single metal layer. The multi-layer metamaterial is an excellent achievement in developing biomolecule biosensors and can potentially sense dangerous viruses.

So far, the instantaneous bandwidth of conventional digital real-time spectrum analyzers is limited to a few GHz, a limitation set by the clock jitter of the analog-to-digital converters, and by the processing capabilities in real time of digital electronics. Here, we propose a simple analog architecture of RF spectrum analyzer based on photonic frequency-shifting loops. It utilizes only compact commercial telecom components, a single CW laser, and slow electronic resources (10 MSa/s). The probability of intercept is 100%, the instantaneous bandwidth reaches 16 GHz, and the spectral and temporal resolutions are respectively equal to 160 MHz and 50 μs.

Sources for high frequencies in the THz range are of interest in both precision measurements in spectroscopy as well as high carrier frequencies and bandwidth in next-generation communication. Optically generated THz frequencies allow for broad tunability based on telecom technologies. Optical frequency division via an optical frequency comb has the potential to generate ultralow phase-noise THz sources. We present a system that is capable of generating tunable THz frequencies of up to 10 THz at 0.5 THz/s scan rates, phase-locked to a common comb spectrum providing absolute frequency calibration. We show results of combining the photonic source with an electronic receiving system in the waveguide band WR 2.2 (330-500 GHz).

Standard digital correlators, which rely on the acquisition of signals and their processing, are intrinsically limited by the clock jitter of the analog-to-digital converters, and by the processing capabilities in real time of digital electronics. Here, we demonstrate a novel analog correlator architecture based on a simple photonic platform, suitable for wideband RF signal processing. The system gives access in real time to the entire correlation function of two signals by computing the cross correlation coefficients for 200 values of their relative delay simultaneously. The time-delay step can be adjusted from a few ns down to a few ps, enabling us to process signals with MHz to multi-GHz bandwidth. The system, based on a frequency shifting loop, can find applications in radar, electronic warfare, imaging, and radio-astronomy.

In this study, a phase-change metamaterial (PCM) is proposed to be applied as tunable terahertz (THz) polarization converter. The structure of PCM consists of 4 layers such as hyperbolic medium (HM) layer, vanadium dioxide (VO₂) layer, the polyimide (PI) layer, and gold (Au) gratings, respectively. The HM layer is made by two-dimensional arrangement of gold (Au) nanowires in x-y plane immerged in PI host slab. The polarization converting characteristics of PCM structure is investigated numerically by using anisotropic transfer-matrix method (ATMM). The numerical results show that, when the VO₂ is in insulating state, the PCM structure provides transmitted cross-linear polarization conversion and when the VO₂ is changed to conducting state, the PCM structure gives both reflective cross-linear polarization and linear-to-circular polarization conversions.

We propose a novel approach to generate Nyquist pulse sequences utilizing a dual parallel Mach-Zehnder modulator (DPMZM) and a phase modulator (PM). The DPMZM, driven by a single RF signal of 10 GHz, produces flat-topped optical pulses. Leveraging the time-lensing effect, the PM is employed to map these optical pulses into the frequency domain, resulting in a flat optical frequency comb (OFC) with 21 comb teeth and a frequency interval of 10 GHz. After chirp compensation and spectral shaping, Nyquist pulse sequences are obtained with a duty cycle of 0.0422.

Communication technology has shown trends towards wireless systems (for improved mobility) and broader bandwidths (for high data rates). This has led to growing interest in terahertz (THz) wireless communication systems—for which there are great benefits and equally great challenges. Arguably, the single greatest challenge for THz wireless communication systems is the susceptibility of the THz spectrum to water vapour absorption in the free-space/air environment. Our study recognizes this challenge and introduces a physical model through which the water vapour characteristics can be fit and then removed from measured signal characteristics. We show the physical foundations of our model and demonstrate its effectiveness in fitting the water vapour characteristics in measured signals from a THz time-domain spectroscopy system. The theoretical and experimental results show strong agreement, suggesting that the model can be an effective tool for characterizing and mitigating the effects of water vapour absorption in future THz wireless communication systems.

This work has proposed and compared several millimeter wave (mmWave) radio-over-fiber (RoF) frequency multiplexing techniques based on InAs/InP quantum dash (QD) mode locked laser (MLL). QD-MLL is capable of generating tens of highly coherent optical carriers simultaneously with same frequency difference. After heterodyne detection, the RF phase noise of QD-MLL can be as low as 15kHz. In this paper, four different RoF based mmWave frequency multiplexing architecture is proposed and compared in experimentation. The EVM of two 2GBaud/s 16-QAM mmWave signal can be as low as 7.1% after demultiplexing in the experimentation.

The terahertz speed CMOS microprocessor that has been designed by Averoses Incorporated (US11063118B1) utilizes nano-vacuum tube elements with plasma interconnect of those elements, and has the capability to emit, detect, conduct, and analyze electromagnetic signals in the terahertz range. Nano-vacuum tube systems are resistant to ionizing radiation and to high temperatures, and there are emergent potentials of such systems beyond the obvious speed-up of data processing. Such a microprocessor can provide a platform for compact terahertz spectroscopy, especially for organic molecules, and this can also include DNA sequencing and DNA fingerprinting. Another emergent quality of such a system is that, for the first time, a complete operating electromagnetic wavelength (0.3 mm for a 1 THz wave) will fit within the geometric boundaries of the microprocessor, allowing picosecond comparisons of waves and wavelet Fourier transform functions.

A directly modulated quantum dash mode-locked laser is proposed for a 50-GHz millimeter-wave over fiber architecture. This advancement offers enhanced fiber-wireless communication capabilities, facilitating coverage of 15-km SMF and 2-m wireless QPSK transmissions. By utilizing the directly modulated laser (DML), we have successfully achieved an optical heterodyne mmW frequency of 50 GHz with exceptional stability. Notably, the measured RF beat-note 3-dB linewidth of the QDash DML is significantly reduced to 2.4 kHz, without the need for any optical feedback scheme. This DML-based hybrid architecture demonstrates the potential to seamlessly integrate with existing optical and wireless networks for next-generation indoor and short-reach mobile communications.

We measured microwave-power space distribution using an electric-field sensor utilizing the electro-optic effect. The sensor is a small optical interferometer on an electrooptic crystal with a dipole antenna. Since the sensor does not include electrical parts except for the dipole antenna, the sensing with less disturbance is possible. The sensing system is optical fiber connected system. So the system can be used in an electromagnetic wave shielded space such as in the underground space.

In THz homodyne systems, optical delay lines are the key to time-resolved measurements but they come with a high cost and complexity. They also limit the application of the system in industrial environment, due to their sensitivity to vibrations. Another important point is the scanning speed, for which the mechanical delay line sets severe limitations. Frequency scanning-based systems need a change in THz frequency to recover phase information. Furthermore, there is a tradeoff between phase sensitivity and necessary tuning range. This tradeoff is based on the difference in the length of transmitter and receiver arm in the setup. With our approach, we can introduce a controllable phase shift at 280 GHz by frequency tuning of both lasers. For that purpose, chirped mirrors were designed and introduced into a standard continuous wave Terahertz homodyne system, in order to induce a variable phase shift. In our chirped mirror-based configuration, the phase shift between both optical modes depends on the center frequency of the lasers. Thus, moving the delay stage can be replaced by variation of the center frequency in order to record a THz trace. This means that the measurements are no longer limited by the speed of the delay line. This phase shift is independent of the path length difference in the setup and does not need phase modulators. Simulations show, that these mirrors may achieve a phase shift up to π inside the C-Band for a difference frequency of 280 GHz. To confirm the calculated behavior of the chirped mirrors, initial characterization measurements were performed. We modified an existing delay stage-based THz system to include the chirped mirrors in front of the receiver. This enables the direct comparison while keeping all other parameters constant.

Planar metamaterials, or metasurfaces, have recently gained attention to realize ultra-thin, functional optical components. Metalenses, or metasurfaces with light-focusing capabilities, have gained attention to greatly reduce the size of conventional optics, including in the terahertz regime. The fabrication of metalenses currently relies on lithography techniques utilized in a clean room environment, and a simpler fabrication method is needed for more extensive use of metalenses. In this study, we report the fabrication of a THz metalens by using femtosecond laser ablation in a standard laboratory environment. The metalens, targeted at 0.8 THz, was fabricated by drilling through-holes with a 343-nm ultraviolet femtosecond laser on a 75-μm-thick high-resistivity silicon substrate. We evaluate the performance of the fabricated metalens with a THz imaging setup, and measure a focused spot full width at half maximum (FWHM) of 0.9 mm and a focusing efficiency of 67.8% when compared to a commercially available lens, results comparable to lithography fabricated metalenses. These results demonstrate the feasibility of the combination of laser processing with metasurface fabrication for the THz region, and represent a new, more versatile alternative to lithography for THz metalens fabrication.

Frequency-modulated continuous-wave (FMCW) light source is a key component in light detection and ranging (LiDAR) systems with high sensitivity and precision. In this paper, we report a triangular-waveform broadband FMCW light source based on pre-distorted injecting current of a distributed feedback (DFB) laser diode and a feedback control loop for keeping optical intensity constant during the frequency sweeping. A pre-distorted injecting current waveform generated by adaptive pre-distortion method is used to drive a DFB laser to generate FMCW light source. After 100 iterations, we achieved the effective frequency sweeping excursion about 31 GHz within 90% region of interest. The frequency sweeping nonlinearities of the down- and up-ramp were 3.10×10⁻⁹ and 5.32×10⁻⁹, respectively. The corresponding relative nonlinearities for down- and up-ramp were 0.00157% and 0.00206%. Due to the modulation of injecting current, the lasing optical intensity of DFB underwent undesired intensity modulation. To achieve constant optical intensity, we utilized a feedback control loop in combination with a Mach-Zehnder interferometer. By comparing the beat-note with and without feedback control loop, the intensity of waveform in the time domain was obviously corrected, which could further improve the ranging resolution from 4.59 mm to 4.30 mm.

First, the concept of 4D imaging is introduced, and for imaging application LiDAR would replace the role of Radar because of broad bandwidth in optical domain. Then, two different types of LiDARs, i.e., ToF LiDAR and FMCW LiDAR, are compared in the view of aspect in creating high quality 4D imaging data. At last, the FMCW LiDAR is selected for the imaging applications and recent experimental results are presented.

Despite its low index of refraction at visible wavelengths, water exhibits a large complex index of refraction at 2.5 GHz. As such, cm-scale aqueous objects like grapes and hydrogel beads are resonant in microwave radiation, showing the expected sequence of spherical Mie scattering modes. We expand our study to aqueous spheroids, where analytical prediction of resonant sizes and shapes are considerably more difficult to make. We show that 3D standing-wave solutions accurately predict certain fundamental microwave Mie resonances in both oblate and prolate spheroids, and we compare this conceptually and mathematically-simplified approach with results obtained using FEM simulations.

High speed photonic links require optical filter with high quality factor and free spectral range in GHz range. Such requirements motivate us to explore the characteristics of new configurations with various types of optical components and tuning mechanisms. In this work, different structures of dispersive elements are investigated to monitor the filtering scope of a microwave photonic link. The spectral characteristic of the optical filter is demonstrated in terms of normalized intensity and free spectral range. This study focusses on the various configurations with multiple number of optical components under test to reduce the ripple in the pass band of optical filter and to get linear phase response for distortion less transmission. The possible power tuning mechanism of the proposed structure is explained through various coupling coefficient and Mach-Zehnder interferometer structure. This helps in the design of tunable optical filters. Furthermore, the investigated elements are implemented in the design of optoelectronic oscillator for the generation of arbitrary shaped microwave signal. The chirping capability in the generated signal is introduced for energy efficient transmission of the signal in modern radar systems. The overall model is theoretically analyzed, and results are demonstrated through MATLAB simulations. The performance analysis of proposed optical components is demonstrated through microwave photonic link in Opti-System.

Microwave waves play a crucial role in various fields, including data transmission, sensing applications, and seismology. The advancement of technology has led to the increased use of microwave waves due to their wider bandwidth and higher frequency capabilities. However, conventional electrical systems often produce microwave waveforms with limited bandwidth and predefined frequency ranges, which may not be suitable for all applications. In this study, we focus on producing an arbitrary dual linear chirped microwave signal that offers configurable bandwidth within a predetermined frequency range. Our approach relies on optical external modulation as the foundation of the photonic method. By cascading two Mach-Zehnder Modulators (MZMs) functioning at the Minimum Transmission Point (MITP), we effectively suppress carrier signals in the modulated output. Through this technique, we successfully generate dual linear chirp microwave signals by superimposing an electrical baseband chirp signal onto the externally modulated signal in the optical domain. This approach demonstrates the potential to create microwave waveforms with wide bandwidth and linear chirping, opening up new possibilities in satellite communication, radar systems, and other advanced applications.

Terahertz antennas have garnered significant attention, particularly in the fields of photonics and wireless communication. Graphene, renowned for its exceptional strength and thinness, exhibits remarkable properties when employed as an antenna element for radiation. Its conductivity varies with thickness and can be controlled through biasing voltage. To mitigate surface wave effects, photonic band gap crystals serve as suitable substrates. This study proposes an antenna structure comprising a miniaturized patch fabricated from graphene material on a photonic band gap (PBG) substrate of Arlon, operating in the terahertz frequency range. A comparative analysis of the antenna structure with and without PBG is conducted, and the obtained results are discussed. The proposed antenna demonstrates optimal performance in terms of return loss, gain, and voltage standing wave ratio (VSWR) across multiple frequency bands within the terahertz spectrum. At 1 THz and 1.18 THz, it achieves an effective gain of 4.53 dB and a return loss of 22 dB. Additionally, various electrical properties, such as surface current, current density, and VSWR, are analysed. The simulations are conducted using the High-Frequency Structure Simulator (HFSS). With a compact volume of 500 μm x 500 μm x 200 μm, the proposed antenna structure is suitable for various optical applications, including nano-sensor networks and imaging. Encouraging results are demonstrated at higher optical ranges within the terahertz band.

This paper provides a comprehensive characterization of a graphene based microstrip antenna which shows plasmonic behaviour at terahertz (THz) frequency spectrum. The study focuses on analysing the behaviour of Surface Plasmon Polariton (SPP) waves in graphene sheets through the design and analysis of a rectangular patch antenna. The unique properties of graphene are analysed, and a thorough numerical and analytical investigation of SPP wave propagation in graphene is conducted. The dispersion equation, as well as the intraband and interband conductivity of graphene, are explained using Kubo's formula. The patch antenna is designed to operate at a resonant frequency of 0.55 THz, and its electrical characterization is performed using the High Frequency Structure Simulator.

We develop H₂ gas sensors based on CMOS compatible 20% ScAlN-based pyroelectric detectors fabricated in-house. Leveraging on the high thermal conductivity of H₂, ScAlN-based pyroelectric detector is used in the H₂ sensor for H₂ to conduct away thermal energy received by the detector, resulting in a drop in signal received by the detector, thereby leading to different voltage signals measured for different H₂ gas concentrations. The higher the H₂ gas concentration, the lower the voltage measured as more thermal energy is conducted away from the detector. We successfully demonstrate H₂ gas sensing with the signal received by the pyroelectric detector at concentration ranging from 400 ppm to 1% H₂ concentration. The gases are cycled at 2-minute intervals between different concentrations of H₂, using N₂ as the reference gas. Our measurements show H₂ sensing down to 400 ppm gas concentration with response time ranging from ~3-7 s. In addition, a linear relationship is also observed between the measured output signal from the H₂ gas sensor and the H2 gas concentration flowing across the pyroelectric detector. The results show promise in using CMOS compatible 20% ScAlN-based pyroelectric detectors for development of thermal conductivity H₂ gas sensor in H₂ leakage sensing to increase confidence towards adoption of H₂ as a clean energy as we move towards a sustainable society.

A stable single-mode multiwavelength fiber laser based on a semiconductor optical amplifier (SOA) and a Fabry-Perot (F-P) Etalon with high fineness is proposed and demonstrated. The cavity is compact and the fineness of F-P Etalon is high to suppress mode competition. Stable multiwavelength laser output with 23 wavelengths is achieved, and the wavelength spacing is 0.4 nm. Over a period of one hour, the power fluctuation is measured to be less than 1.89 dB.

A scheme of frequency linearly swept light source with a feedback phase-locked loop is proposed and demonstrated, which the stability can be significantly improved. By applying the pre-distortion method, the offset phase lock servo generates a control signal to feedback the loop and adjust the frequency of semiconductor laser source, and the frequency-swept signal can be produced. By adjusting the parameters of the offset phase lock servo, a linearly swept light source is achieved with low-frequency noise and improved robustness. In the experiment, a microwave signals with a sweep excursion of 4 GHz and phase noise of 4.6557*10^-7 rad²/Hz at 1 kHz have been successfully generated.

We report on THz emission in single-crystalline SnS₂ in response to above bandgap excitation. Symmetry properties of THz generation suggest that its origin is an ultrafast surface shift current, a 2nd order nonlinear effect that can occur as a result of above-gap photoexcitation of a non-centrosymmetric semiconductor. Multilayer SnS₂ can exist in several polytypes that differ in the layer stacking. Of those polytypes, 2H and 18R are centrosymmetric while 4H is not. While Raman spectroscopy suggests that the single crystalline SnS₂ in our experiments is 2H, its THz emission has symmetry that are fully consistent with the P3m1 phase of 4H polytype. We hypothesize that the stacking disorder, where strain-free stacking faults that interrupt regions of 2H polytype, can break inversion symmetry and result in THz emission. These results lay the foundations for application of SnS₂ as an efficient, stable, flexible THz source material, and highlight the use of THz spectroscopy as a sensitive tool for establishing symmetry properties of materials.

In the field of high-frequency signal transmission, reducing signal absorption is crucial for efficient long-distance communication. The traditional metamaterial manufacturing technology has disadvantages such as long processing time and expensive consumables in the terahertz (THz) frequency band. To overcome these challenges, we propose a solution based on electromagnetically induced reflection (EIR) and plasma simulation using 3D printing techniques. In traditional metamaterial fabrication, gold is often used as a thin-film material. However, the high cost of the gold manufacturing process has prompted us to explore alternative materials. Compared with traditional metals, we chose a high-entropy alloy film composed of niobium, molybdenum, tantalum, and tungsten in a certain proportion, and then combined with silver. At 0.375 THz, the absorption rate is 15% higher than that of the gold film, highlighting the superiority of the silver-HEA combination. To simplify the manufacturing process, we use 3D printing technology with the aim of reducing processing time, increasing design freedom and reducing manufacturing costs. In the terahertz frequency range, the silver-high-entropy alloy combination has excellent performance for the structures designed in this paper. Exhibits strong resonances at 0.375 THz and 0.64 THz. The absorption rates are about 99% and 90%, respectively. In addition, the frequency band between the absorption peaks exhibits an excellent reflectivity of about 99%.Using EIR-based plasma simulation and 3D printing techniques, we developed a solution to mitigate high-frequency signal absorption. Our thin films exhibit strong resonance, high absorptivity, and excellent reflectivity in this structure, surpassing the performance of gold-based conventional metamaterials. This advance holds great promise for optimizing signal transmission and facilitating efficient long-distance communications in the future.

Because of terahertz waves high frequency, low photon energy and high non-polar penetration, terahertz waves have become the focus of worldwide attention and research in recent years. Terahertz wave imaging technology has become an important aspect of the research of terahertz wave technology because of its special uses and advantages compared with x-ray, ultrasonic and infrared imaging methods. This paper studies the terahertz detection platform which can be used for non-destructive testing. Based on the analysis of different types of terahertz imaging technology in different application fields, the basic idea of using frequency-modulated continuous terahertz wave source to carry out reflective three-dimensional point-by-point scanning imaging of objects is determined by combining the characteristics of imaging samples and the requirements of system imaging indicators. Through the detection platform to detect the defects of samples with different refractive indices, and through the new quantum image algorithm to achieve image reconstruction, the corresponding defects of the samples can be clearly seen from the results, and the resolution can be further improved, laying the foundation for filling the vacancy in the field of nondestructive testing.

Terahertz (THz) waves possess unique abilities which drive applications in non-destructive characterization, spectroscopy, non-contact conductivity measurement, and through-packaging detection, to name a few. However, its relatively large wavelength restricts its applicability to image the subwavelength features of a material. In recent years, traditional near field imaging techniques, aperture-based, tip-assisted, etc., are being employed where the scattered near-field terahertz radiation is collected, which is not diffraction-limited, and, therefore, is able to resolve subwavelength features. In this work, we have investigated hollow-core metal waveguides (HCMWGs) made of copper and silver for their application in terahertz near field imaging. It is vital to understand the transmission characteristics of these guiding structures to deliver the terahertz power close to the sample and then collect the near-field scattered radiation. Therefore, we recorded the transmission efficiency of the HCMWGs of inner diameters 3.0 mm, 2.0 mm, and 1.5 mm using a time domain spectroscopic setup. We noted that the transmitted electric field profile was highly dependent on the positioning of the launching-lens and collecting-lens along the optical axis, and the transmission maxima showed two peaks. Furthermore, it was noted that the smaller the diameter of the HCMWG, the lesser the working distance of the collecting-lens. Using these profiles, we could design an optical arrangement for transmission-based terahertz near field imaging by employing 1.5 mm inner diameter silver hollow-core metal waveguide.