This PDF file contains the front matter associated with SPIE Proceedings Volume 12776, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Two-color laser induced filament can emit coherent broadband terahertz (THz) radiation. Surprisingly, when using a relative long focal length of lens to generate a long plasma filament, the THz wave frequency of the radiation will peak at around 1 THz. This can be found in the measurements by using different methods, such as the electro-optic sampling, fluorescence, air-biased coherent detection and interferometer method, however this phenomenon and its underlying reason have never been discussed before. In this paper, the observed THz peak frequency from the long filament can be interpreted by the phase match between the femtosecond laser and the generated THz emissions. These findings gain more insights into the generation mechanism of THz wave, and can help to boost the THz emission yield and tune the central frequency of THz emissions by manipulating the plasma.

We expect an increase in the efficiency of terahertz radiation generation both due to the short period of the optical pulse containing 2–3 oscillations, and due to the fact that the process is modeled in a gradient waveguide. The generation process can be accompanied by the formation of spatiotemporal optical-terahertz solitons (optical-terahertz bullets). In the case of an intense and short optical signal, for the optical component, it is necessary to take into account the group dispersion of the third order, the dispersion of the quadratic optical-terahertz nonlinearity. For the terahertz component, one should take into account its dispersion, intrinsic quadratic nonlinearity, and quadratic nonlinearity, which carries information about the phase of the optical pulse.

Sum-frequency generation, which can convert low frequency signal to the higher frequency range based on parametric process, is a successful and effective method for infrared detection. Here we report a whispering gallery mode (WGM) microresonator with a double-ring coupled structure for efficient parametric upconversion of long-wave mid-infrared signal. Due to the large dispersion between signal and pump/upconversion light, it is difficult to meet the phase matching condition in micoresonators. In the double-ring coupled scheme, the phase matching is achieved through energy distribution of optical modes in different rings. Specifically, the inner ring is regarded as an effective refractive index regulator of the signal mode. Through theoretical calculation and finite element simulation, the feasibility and adjustability of this scheme are verified. Compared with previous schemes which usually use nonlinear crystal as platforms, WGM microresonators with ultrahigh Q and small mode volume have the potential to obtain high efficiency frequency conversion and ultra-low power requirement with a small physical footprint.

Currently, in photonic integrated circuits, the circuit size is limited by diffraction, which acts as a barrier to further development in the field of optical communications. Plasmonics could potentially address this size mismatch between electronic and photonic components. Photonics and plasmonics can complement each other, given the appropriate conditions, allowing optical signals to be converted into surface plasmon-polaritons (SPPs) and vice versa. Therefore, plasmonic integrated planar circuits for wireless communication devices are actively being developed. Transitioning to the terahertz (THz) frequency range will enhance data transmission and processing speed. In the development of plasmonic integrated circuits, understanding the optical properties of surfaces is crucial. Current methods for determining the optical properties of surfaces in the THz frequency range lack sufficient sensitivity to transitional surface layers of metal and films that are much thinner than the optical wavelength. This work will demonstrate experimental methods for measuring the penetration depth of the SPP field into the air. Through these methods, and utilizing other plasmonic refractometric characteristics, it is possible to reconstruct the effective surface dielectric permittivity of the metal.

Microwave kinetic inductance detectors (MKIDs) can detect photons ranging from terahertz, infrared, optical to X-ray frequencies. And it has the advantages of high sensitivity, easy frequency-domain multiplexing (FDM) readout, and simple structure for large-format arrays. With these advantages MKIDs are becoming the choice of incoherent detectors in astronomy such as imaging cameras and broadband imaging spectrometers. In this paper, we introduce the design and fabrication of a 5-THz 10×10 MKIDs array on a single chip from an 80-nm-thick aluminum (Al) superconducting film based on a high resistance silicon substrate. Each pixel is a quarter-wavelength resonator with its short-circuited end integrated with a 5-THz dipole antenna. The noise performance of the MKIDs is measured at 200-mK using both phase readout and amplitude readout, giving an electrical NEP ~10⁻¹⁷ W/Hz^0.5. Detailed results will be presented.

Metasurfaces have been extensively studied in the field of THz wavefront manipulation as an effective means for light field manipulation. Among them, the metasurface with geometric phase shows its natural and robust phase control ability for spin wave beams. However, the phase responses of opposite spins are locked to be opposite to each other, resulting in symmetric transmission of light. The symmetric transmission is not conducive to the multifunctional integration of optical systems, resulting in decreased efficiency of the metasurface. As a phase component of the metasurface, the transmission phase has spin-independent properties. Moreover, the research discovered that combining transmission phase gradients with geometric phase can achieve spin-decoupled manipulation of light. Based on this, we designed three spin-decoupled bifunctional THz metasurface, and spin-selective focusing, asymmetric hologram and vortex beam generation are realized. The proposed metasurface consists of an arrangement of 16 all-dielectric elliptical silicon pillar meta-atoms with different electromagnetic responses. The co-simulation method of MATLAB and CST realizes the efficient design of these meta-atoms. The average cross-polarization transmissivity of these meta-atoms exceeds 87%, and the transmission phase covers 0~2π. Utilizing these meta-atoms allows free design of spin-decoupled bifunctional THz metadevice to independently manipulate opposite spins. This work is expected to promote the development of THz metasurface devices in practical applications.

In this paper, we present a wideband antireflection coating designed for high-resistivity silicon and alumina lenses used in cryogenic terahertz detectors. A dual-layer coating structure based on diamond and polytetrafluoroethylene (PTFE) films is employed to achieve high transmittance in a wide frequency range, the film thickness of diamond and PTFE has been precisely controlled by mature microwave plasma chemical vapor deposition (MPCVD) and thermal spraying technology. The transmittances of coated silicon sample was measured within the frequency ranges of 0.3-0.5 THz using the Quasi Optical Vector Network Analyzers (QO-VNA). The measured transmittance of the one side coated silicon sample accords well with the simulation results, which demonstrates the accuracy of the coating process. A remarkable transmittance level of up to 99% can be achieved by applying the AR coating to silicon lens according to the simulations. This wideband antireflection coating can be applied to cryogenic terahertz detectors, such as superconducting hot electron bolometer (HEB) detectors and superconducting kinetic inductance (KIDs) detectors.

Surface plasmon interferometry has found wide application in optical sensor devices in the visible range. In the THz region, where the surface plasmon polaritons (SPPs) propagation length extends to tens of centimeters, surface plasmon interferometers can be effectively used to control quality of the metal surface, to determine the effective permittivity of thin metal and dielectric coatings used in THz plasmonic integrated circuits, and for various sensor applications. The first THz SPP interferometer design and experimental results obtained with them in 130 to 358μm wavelength region for testing of metal surfaces and thin dielectric coatings deposited on it will be presented in this paper. The SPPs were generated by the Novosibirsk free electron laser coherent radiation on flat surfaces with gold sputtering, coated with ZnS layers of thickness from 0 to 3μm. Based on the measurement results, the value of the effective permittivity of the deposited gold was found; it turned out to be an order of magnitude smaller than that of bulk crystalline gold, which show the practical relevance of this method.

Due to the limitation of Abbe diffraction limit, the traditional terahertz (THz) continuous wave imaging methods based on lenses or mirrors are difficult to achieve super-resolution. Here we present a confocal waveguide scanning method for THz reflective super-resolution imaging. In the method, a low loss THz hollow waveguide is used to replace the traditional terahertz lens or parabolic mirror. Through optimizing the size of the waveguide, we can achieve far field subwavelength focusing at 0.1THz and achieve super-resolution terahertz imaging. In addition, a slider-crank high-speed scanning mechanism is used in the scanning system, and the imaging speed is more than 10 times faster than the traditional step scanning system based on linear guides. In addition, its application in the field of nondestructive testing is introduced.

Photodetectors based on the photothermoelectric effect (PTE) have garnered significant attention due to their high-speed photoresponse and other notable characteristics. The PTE photodetector possesses inherent advantages in terms of a wide spectral range. Studies have demonstrated that infrared photodetectors based on the photothermoelectric effect are not limited by the operating wavelength. The photothermoelectric infrared photodetectors based on semimetallic ZrTe₃ have not yet been reported. We employed first-principles calculations to demonstrate the significant Seebeck coefficient of ZrTe₃, providing a crucial foundation for achieving photo-thermoelectric response. Moreover, we fabricated a metal-ZrTe₃-metal photothermal infrared photodetector using the dry transfer process. The photodetector achieves a responsivity of up to 45 mA/W at the 1177 nm. Additionally, it demonstrated an ultrafast response time of 316 μs, highlighting the exceptional functionality of semi-metallic ZrTe₃ as a detection layer.

Skin disease is one of the most common diseases affecting human health, and it is vital to detect and diagnose it rapidly and accurately through skin tissue. Terahertz metasurface sensors (THz MSs) offer the advantages of label-free and sensitive detection by virtue of the capacity to perceptively sense changes in the refractive index of substances and the dielectric environment on their surfaces. In this paper, an array of quadrupole metal split-ring resonators (SRRs) was proposed. For the purpose of achieving a high Q-factor and sensitivity at the same time, parameters including cell size and opening gap are tuned. On account of the centrosymmetric cell structure of the sensor, angle sensitivity-related experimental errors could be essentially eliminated. Resonant cavities and connecting arms excite LC resonance on metasurfaces upon interaction with THz waves, resulting in two resonant dips in the transmission spectrum. Using photolithographic processing techniques, periodic metallic patterns were formed on a thin quartz substrate to create the metasurface. In the experiments, skin disease tissue samples were taken from patients, and the transmission spectra of the metasurface sensor covered with skin tissue were obtained by a terahertz frequency domain spectroscopy system. The result shows that resonant frequencies of the sensors covered by skin tissues of disease are shifted towards the lower frequency region compared with normal tissues. This dual resonance frequency sensor made it possible to visually distinguish between diseased and normal tissues by analyzing the resonance frequency shift with superior sensing performance and accuracy without the necessity of analyzing the complex terahertz intrinsic spectrum and refractive index of the samples.

Approaches based on antialiasing filtration and decimation (subsampling or undersampling) to improve the accuracy and acquisition time of terahertz time-domain spectrometers are proposed. Both rely on preliminary THz-signal filtering and frequency band reduction, followed by sampling interval increasing in accordance with the sampling theorem. The key elements for the task are quasi-optical band-pass (BPFs) and low-pass filters (LPFs). The carefully modeled metasurface design of the multi-layered interference-based microstructures permits the high performance of the filters. They provide a bandwidth of 5% for BPFs and a sharp cut-off edge for LPFs, as well as more than 40 dB out-of-band suppression, although metasurface geometries are based on trivial capacitive and inductive square elements: slits for BPFs and patches for LPFs.

In free space optical communication (FSO) communication, it is assumed that the fading due to atmosphere turbulence (AT) is uncorrelated at the receiver. But, in practice the spacing between the receivers is less than the fading correlation length. So, we have to consider a correlated statistical model for FSO AT model. In this paper, we have analyzed space shift keying (SSK) modulation scheme over correlated Gamma-Gamma (GG) fading model. We have derived an average bit error rate (ABER) using moment generating function (MGF) for space shift keying modulation scheme over correlated Gamma-Gamma (GG) fading model. The system is analyzed for different atmospheric conditions, correlation values and different number of receivers. It can be observed that as we increase the correlation value, the ABER value deteriorates. It can be observed that the ABER does not depend on turbulence at low SNR, but for high SNR the ABER performance improves and the ABER performance is approximate same in weak and moderate turbulence. However, the difference of SNR between moderate and strong turbulence is approximate 5 dB for ABER value of 10⁻⁴. We note that at a given SNR, the ABER improves as we increase the number of receivers. Also, it can be observed that the ABER deteriorates as correlation increases for same number of receivers. We have also analyzed the system both uncoded and LDPC coded SSK transmitter systems. It is observed that for ABER value of 10⁻⁴, the SNR difference between LDPC coding and without encoding is approximate 3 dB.

The push to improve data capacity in wireless networks has led to an increase in carrier frequency in the terahertz band. This has enabled the use of direct beams transmission due to the shorter wavelength. One method of further densification of transmitted communication channels is through the use of vortex Bessel beams, which allow for signal combination in a single channel. In addition, these beams offer improved beam stability through their unique features of non-diffraction and self-healing, which is particularly important in wireless transmission where atmospheric turbulence is a factor. Vortex Bessel beams are characterized by a topological charge, l, which cannot be determined from their intensity profile alone. Therefore, spatial filtering is necessary at the receiver stage to identify the desired mode carried by a distorted or combined beam. A diffractive optical element with a specially designed complex transmission can serve as a filter matched to the desired mode. This study presents the experimental results of Bessel mode identification using beams formed by binary spiral binary phase axicons that transform the Gaussian mode of high-power terahertz radiation from the Novosibirsk Free Electron Laser. The specified mode is passed through a filter (similar spiral binary axicon with |l| = 1, 2, 3, or 4, and a lens) and then detected by a pyroelectric camera in the lens focal plane. A positive response, indicated by a narrow peak, confirms the compatibility of the specified and filtering modes. Experiments were conducted on single-mode and combined beams (l = -1 and -2), and mode identification was also demonstrated for beams passed through an inhomogeneous medium.

Optical Wireless Communication (OWC) has the potential to offer a transmission link that is both dynamic and flexible in comparison to fiber optics communication. The optical spectrum exhibits a significantly greater range than the conventional Radio Frequency (RF) communication spectrum. Optical Orthogonal Frequency Division Multiplexing (OOFDM) can be treated as an integral part of OWC Systems. It is a modulation technique which refers to the implementation of OFDM in the Optical Domain. For the OFDM signal to be transmitted in the Optical domain, we perform Intensity Modulation/Direct Detection and for that the signal needs to be real and positive. In this paper, we have made the signal Hermitian symmetric along with IFFT and performed adaptive biasing to make the transmitting signal real and positive respectively. One of the major issues which still persists is the high Peak-to-Average Power Ratio (PAPR). To overcome this, we have implemented Selected Mapping (SLM), using Shapiro-Rudin Phase Sequences (SRPS). We have used SRPS as they yield a low crest factor, have good randomness, strong correlation, and help in reducing the average transmit power, which will eventually help in decreasing the complexity of the system. By performing Adaptively Biased Optical OFDM (ABO-OFDM) in conjugation with SLM we have been able to achieve a reduction of about 3 dB in the PAPR graph, without affecting the BER curve. Moreover, the increase in the system complexity is comparatively lower, or it can be stated that the increase is approximately halved.

With the rapid advancement of the Internet of Things, terahertz band has emerged as a prominent area of research for high-speed wireless communication over the past decade to satisfy the growing demand for network traffic and is extensively employed in indoor wireless communication systems. This study experimentally examines the transmission efficacy of MSK signals in terahertz channels via upconversion. In the LabVIEW development environment, the PN sequence serves as the baseband signal. The MSK signal, with a bandwidth of 1GHz and a transmission rate of 1Gbit/s, is modulated and output using the high-speed vector signal transceiver platform, PXIe-5841. This signal is then upconverted to 230~300GHz for wireless transmission. The findings from the constellation and grid diagrams reveal the stable and reliable transmission of the MSK signal in the terahertz communication system, underscoring its viability and potential for future research and application.

Laser induced air-plasma is a simple and convenient source to generate intense terahertz (THz) waves through the ionization of gaseous molecules. However, the lack of enhancement and efficient frequency modulation methods for this source hinders its widespread application. Recently, we have shown that the two crossed two-color air-plasmas can significantly increase the THz wave generation or select the required THz emissions under the suitable time delay between the two identical ones (the same intensities of the two-color lasers and the same relative phase between them). In the latest research, we change one of the input laser pulses to generate the two crossed air-plasmas from the twocolors (800 nm + 400 nm) to only the single 800 nm. Interestingly, unlike the results we reported in our previous paper, no spectral interference fringe is observed in the THz spectrum, suggesting that it exhibits no interference between THz waves radiated from single-color and two-color laser generated filaments. For THz waves radiated by single-color laser generated plasma, both the absorption and radiation are related to the plasma frequency. However, the THz waves generated from the two-color laser fields appear to be quite different from the single-color one, thus leading to the absence of spectral interference. Research into the underlying mechanisms will reveal more on the plasma-based THz technology, promising to increase the THz intensity yield and deepening our understanding of the ultrafast dynamics.

Bound states in the continuum (BICs) are waves that lie inside the radiation continuum but are completely bound without any leakage of energy. Theoretically, they have infinite lifetimes and radiation quality factors, which have non-radiative characteristics with zero linewidth. By introducing structural perturbation, BICs suffer limited leakage and transform into observable quasi-BICs with a finite quality factor. Metasurfaces based on BICs have become an emerging tool in the field of high-sensitivity sensing because they possess high-quality factors and can enhance the interaction between light and matter. Here, we design an all-dielectric metasurface based on symmetry-protected quasi-BICs, consisting of an asymmetric elliptic pillar. By breaking the C2 symmetry of the structure, the BIC state transforms into quasi-BIC, and a sharp Fano resonance is obtained in the near-infrared band. The quality factor and working frequency can be adjusted by fine-tuning the asymmetric parameters. The eigenmode analysis of the structure is performed. Besides, the refractive index sensing performance of the proposed metasurface is investigated. With the increase in the environmental refractive index, the resonant wavelengths of the transmission spectra have a redshift, which indicates that the designed metasurface is sensitive to the changes in the environmental refractive index. The sensing sensitivity is calculated as 277 nm/RIU, and the FOM (figure of merit) is 396. This design provides a new way for the design of high-sensitivity and microchannel integrated sensors, which has potential applications in the fields of biological and chemical sensing.

Space-based infrared technology has the potential to detect aerial targets globally, for which strong atmospheric absorption of the background radiometric signal provides spectral detection windows. However, the atmospheric content varies in time and space, affecting its attenuation of the background. It is therefore necessary to assess the impact of the variation in the atmospheric content of absorption gas on the target detection. Based on the aerial target space-based observation characteristics model, the target radiation contribution ratio and the target-background contrast were calculated for different atmospheric contents with reference to the typical atmospheric profiles. The variation of the target-background relative differences due to changes in gas content was discussed. Furthermore, the range of change in the target-background contrast and signal-to-noise ratio(LSNR) for different atmospheric contents was calculated, considering instrument response characteristics. The effect of atmospheric content and full width at half maximum (FWHM) variation on the stability of the target-background relative relationship was analyzed. The findings highlight that the target radiation contribution ratio and the target-background contrast are more significantly affected by changes in water vapor, while changes in carbon dioxide concentration have little effect due to its large content base. Spectral bands where water vapor and carbon dioxide interact can reduce the effects of water vapor changes, but require narrow FWHM. But the too narrow FWHM can lead to changes in the target-background relative relationship and reduce its stability due to the influence of instrument noise. This research helps to understand the impact of changes in atmospheric condition on space-based infrared detection of aerial targets and the challenges faced in developing detection algorithms.

Superconductor-Insulator-Superconductor (SIS) mixers are currently the most widely used detectors in astronomical observations in the terahertz band. To meet the demands of future large-aperture terahertz radio telescopes for multipixel applications, this work focused on the integration technology of a 660 GHz 1×4 one-dimensional linear array SIS mixers. The SIS mixers based on Nb/AlN/NbN twin tunnel junctions. We developed the uniform distribution of local oscillator power by using an integrated waveguide and multi-path local oscillator power distribution network. We also integrated the passive frequency doubler and tripler at low temperature, and integrated four low-noise amplifiers with the mixer in the same block, resulting in a more compact structure. As a result, we successfully realized the simultaneous operation of four mixers in a cryogenic Dewar. This paper will provide a detailed description of the relevant design and testing results.

In this thesis, firstly, based on the MATLAB/Simulink platform, the simulation channel module for terahertz wave (300GHz) communication is established based on the MPM modeled clear sky atmospheric channel,and Hamming (7,4) and RS (188,204) channel compilation code and 2FSK, QPSK, 16QAM signal modulation and demodulation, which are commonly used in the traditional frequency bands, are selected to form a simulation model of terahertz communication system. By comparing the BER analysis, it is found that Hamming (7,4) coding and QPSK modulation technology can achieve relatively low BER, but the traditional modulation and coding method has poor performance in terahertz communication at low SNR.Then,the experiments conducted on the wireless communication system controlled by LabVIEW on the PXIe platform confirmed and validated the conclusions derived from the simulations.

With the advancement of terahertz technology, there is growing anticipation for 6G communication in the international community, and terahertz communication is gaining increasing attention. Nonetheless, terahertz waves possess strong directionality, leading to the obstruction of transmission signals. Consequently, achieving largeangle, directed transmission poses a technical obstacle for terahertz communication. Metasurfaces technology has great potential to improve the coverage performance of 6G networks, making it one of the most promising technologies in this field. This paper designed a terahertz directional reflector that is insensitive to polarization in the terahertz communication band (0.25 THz). The unit structure is selected using the resonant phase modulation principle, utilizing a metal-insulator-metal structure. Following the phase compensation principle, the structure of the unit is arranged and simulation experiments are conducted to achieve a 30-degree abnormal deflection of the reflected beam. This scheme will hold some application value for terahertz 6G channel control in future research.

Recently, research on 6G wireless communication technology has become a hot topic. In 6G technology, terahertz communication is considered the most promising part. The research and application of the terahertz communication frequency range (0.1-10 THz) will bring revolutionary changes to the field of communication. It has the potential to bridge the transmission gap between the infrared and microwave bands, and offers highspeed data transmission, low latency, and high-capacity communication. Therefore, the development of terahertz communication is highly anticipated as a significant driving force for 6G technology. At present, research on outdoor terahertz channels is far less extensive compared to indoor channels, requiring more efforts to explore the characteristics of outdoor terahertz channels. In this paper, we focus on the three-dimensional scenario of street canyon and model the terahertz channel using ray tracing. The carrier frequencies used for simulation range from 220 to 330 GHz. By calculating the power loss and required time for each path from the transmitter to the receiver, we obtain parameters such as power delay spectrum and power angle spectrum. Next, we analyze the relationships between path loss, delay spread, angle spread, and distance, gaining further understanding of the outdoor terahertz multipath channel characteristics.

In high-voltage substations, flashover will occur along the surface of wet insulators under bad weather conditions, resulting in pollution flashover accidents of power system, which will seriously threaten the safe and stable operation of power system. Room Temperature Silicone Rubber (RTV), as a new technology and material, has been rapidly developed and widely applied due to its outstanding characteristics such as long lasting and maintenance-free. However, its surface uniformity and defects in the curing process will affect the service life of the coating. In this paper, a terahertz time-domain spectroscopy system was used to analyze the defects, material parameters, thickness, and curing process of RTV coating. Firstly, RTV coating samples with different absences and different thicknesses were produced, and the imaging results of RTV coating prefabrication defects, the optical parameters of cured RTV in the terahertz band and the thickness of the samples were obtained by the terahertz spectroscopy system. In addition, the results of the optical parameters of the RTV material over time (experimental conditions: 20°C, 15%RH) were verified by the experiment over a 3 h period. Different methods were used to extract the optical parameters of the materials for comparison. The results obtained in this paper provide a theoretical model for the application of terahertz time-domain spectroscopy in the nondestructive testing of RTV coating in power systems, and can realize the lifetime analysis of RTV coating.

In terahertz (THz) range, ultra-long non-diffraction Bessel beams have been theoretically and experimentally developed and consequently achieved significant advancements. Nevertheless, the balance between non-diffraction length and optical intensity is restricted and their applications have been limited in part due to their complication in access and operation, loss of energy and spectral bandwidth. In this work, a composite system composed of multiple lenses and axicon is presented to engender ultra-long terahertz Bessel beams. By manipulating the spatial relationships between various THz components, the delivery distance and non-diffraction length surpass 40cm and 2m respectively.

In this paper, pulsed infrared thermography is applied to the study of an ancient Chinese bronze sword. A clay core, as a piece-mold casting remains, is found inside the sword handle, the length of which is obtained by analyzing the thermographic images. Meanwhile, corrosion and cracks are also located on the body of the bronze sword. To obtain the thickness of the bronze handle around the clay core, thermal quadrupole simulation is used. By analyzing the characteristic time from the thermographic experiment and the simulation, the thickness of the materials is determined. These results provide a deeper understanding of the manufacturing of the bronze sword and also contribute to the later conservation studies.

Gallium selenide nonlinear optical crystals are effective up- and down-converters of terahertz and infrared frequencies. Their nonlinear characteristics at the telecommunication wavelength have not received enough attention, particularly for sulfur-doped crystals. In order to close this gap, we put forward data on the optical and electro-optical characteristics of GaSe_(1-x)S_x crystals (where x = 0, 0.03, 0.12, 0.16, and 0.22) at the wavelength of 1.55 micrometers and in the terahertz range. This study provides an estimate of the GaSe:S crystal’s nonlinear coefficient d₂₂(ω₁, ω₂, ω₃) responsible for the terahertz wave generation by means of optical rectification and difference frequency generation, where ω₁ ≈ ω₂ – corresponds to the near infrared range, and ω₃ belongs to the subterahertz frequencies. We assume that the value of d₂₂ lies within the range from 13.9 to 20.5 pm/V for the wavelengths of 0.63 to 1.55μm. The potential of S-doped GaSe crystals to be used as nonlinear optical converters for photonic devices operating at telecom wavelengths is discussed.

The Leighton Chajnantor Telescope (LCT) is an international cooperation project among Caltech, University of Concepcion (UdeC), and Shanghai Normal University (ShNU), which aims to relocate the Caltech Submillimeter Observatory (CSO) telescope from Mauna Kea, Hawaii to the Chajnantor plateau in northern Chile. ShNU has the opportunity to upgrade a 345/650 GHz dual-color SIS receiver, which was made by Caltech, for the LCT. In order to technically support this dual-color receiver, 345/650 GHz SIS mixers were designed and optimized based on the existing superconducting technology in the lab of ShNU. Within the proposed frequency range, both mixers show good sensitivity and gain characteristics. The optimized mixers show a DSB noise temperature of 24K and a conversion gain of 4.2dB at 345GHz, and a DSB noise temperature of 61K and a conversion gain of -2.9dB at 650GHz, respectively. Detailed design and results will be provided.

The novel time-resolved terahertz (THz) Kerr effect (TKE) spectroscopy is a powerful tool for the study of the intermolecular structure and low-frequency molecular dynamics of liquid water. Here, we further observe the Kerr effect response in free-flowing aqueous membranes induced by strong ultra-broadband THz pulses. The intermolecular hydrogen bonding dynamics of a series of ionic aqueous solutions were investigated and a significantly enhanced Kerr effect response associated with polarization anisotropy was obtained. We analyze the spectra of the obtained Kerr effect responses of water and aqueous solutions and focus on the vibrational features of multiple molecular modes on the spectra. We decompose the molecular responses of ionic aqueous solutions into Debye relaxation related to molecular reorientation motions, intermolecular hydrogen bond bending and stretching modes associated with restricted translational motions, and specific ion-water hydrogen bond vibrational modes. The effects of different ions and vibrational modes on the hydrogen bonding network excited by the THz electric field and their corresponding manifestations in the spectrum are analyzed and compared. Our measurements positively affect the deeper understanding of intermolecular dynamics in liquid media.

Terahertz (THz) sensing of ultrathin layers has been a longstanding challenge due to limitations in conventional detection techniques. In this study, we present a novel approach for sensing sub-1 nm thin dielectric layers based on Fowler-Nordheim (FN) tunneling. Our method exploits the FN tunneling effect at a metal-dielectric interface, enabling sensitive detection of changes in dielectric layer thickness within the THz frequency range. To validate our FN tunneling-based THz sensing technique, we carried out a comprehensive analysis of experimental and simulated data. Our findings demonstrate that this approach exhibits exceptional sensitivity, capable of detecting dielectric layers with thicknesses down to the sub-nanometer scale. Such sensitivity has significant implications for various applications, including nanoscale dielectric characterization, advanced material development, and quality control in microelectronics manufacturing. The FN tunneling-based THz sensing methodology not only overcomes the limitations of traditional detection techniques but also paves the way for novel ultrathin layer sensing capabilities in the rapidly advancing field of terahertz technology. Our study showcases the potential of this groundbreaking technique to revolutionize the THz sensing landscape, offering new opportunities for research and development in various fields.

Accurate measurement of the permittivity of materials is important in the imaging, communication, radar, material analysis and other fields. The method of extracting material parameters under different model conditions of frequency domain continuous wave signal based on quasi-optics vector network analyzer system is realized. It provides reference conditions and practical basis for evaluating the reliability of dielectric parameter measurement and defect detection of terahertz band materials. The process of terahertz wave interaction with the tested sample produces intractable spike noise due to interfacial echo resonance. In the signal transmission, the singularity is due to the wavelength and the sample optical path parameters forming the resonant mode, which is different from other noise signals.We combine wavelet denoising and variational mode decomposition, and introduce frequency expansion, which not only removes frequency-dependent noise, but also avoids frequency curling, which improve the accuracy of the measurement results.

Graphene-based terahertz (THz) negative-conductivity metasurfaces based on two types of unit cell structures are investigated under the control of an external bias voltage. Electrical characterization is conducted and verification is performed using the finite-difference time-domain simulation and an optical-pump THz-probe system in terms of simulation and transient response analysis. Owing to the metal-like properties of graphene, the strong interaction between the metasurface and monolayer graphene yields a short-circuit effect, which considerably weakens the intensity of the resonance mode under passive conditions. Under active conditions, graphene, as an active load, actively induces a negative-conductivity effect, which enhances the THz transmission and recovers the resonance intensity gradually because of the weakening of short-circuit effect. The resulting resonance frequency shows a blue shift. This work provides a reference value for combining the bias-driven negative-conductivity graphene with metasurfaces and points the corresponding applications in the future.

Recently, the semiorganic crystal of guanylurea hydrogen phosphite was revealed as an extremely efficient narrow-band terahertz radiation source. It is proposed that the emission is closely connected with the excitement of high-quality phonon oscillations, which in turn necessitates a detailed characterization of the optical properties of the crystal in this range. The extreme dispersion of the angle between dielectric and crystallographic coordinates, permitted by the monoclinic syngony of the crystal, was estimated to be 2.3° per 100 GHz. The dielectric susceptibility of the crystal measured by means of time-domain spectroscopy is determined by the high-Q phonon at 1.45 THz for E_THz||x, which is responsible for the efficient terahertz generation. Also, two modes were found at 1.02 and 0.92 THz for E_THz||z and E_THz||y, respectively. All three components of the refractive index were measured in the range of 0.4–2 THz. We believe that modification of the crystal structure will allow tuning the characteristics of the narrowband terahertz source based on it.

Superconducting transition-edge sensors (TESs) are highly sensitive detectors and can detect electromagnetic wave radiations from millimeter/submillimeter, optical to 𝑥/γrays, suitable for cosmology, astrophysics, quantum information, and biosensing. In principle, thousands of TESs even more are required to enhance the detection efficiency for large-scale survey. Among other multiplexing schemes, microwave SQUID multiplexer (μMUX), consisting of resonators and RF SQUIDs, has a bandwidth of several GHz, thus multiplexing factor on the order of thousands, more suitable for readout of large TES arrays. We designed and fabricated superconducting coplanar waveguide (CPW) resonators with a high qualityfactor and second-order gradient RF SQUID with two inductive coupling structures respectively. Then, we optimized the critical current density of the Josephson junction and measured the mutual inductance parameters of the second-order gradient structure SQUID, which are consistent with the simulation results. Finally, we fabricated a cryogenic μMUX chip based on RF SQUID and resonator. We discussed the results of the development of μMUX in more detail.

Lens integrated twin slot antennas have been widely used in superconducting transition edge sensor (TES) detectors due to its high directivity and low cross-polarization. In this paper, we present the design and simulation of a 210 GHz dual-polarized twin slot antenna for TES detectors. We used Ansoft HFSS to simulate the return loss and isolation of the dual-polarized twin slot antenna. The results show that the return loss of the dual-polarized twin slot antenna is less than -15 dB and the isolation is great than 10 dB in the frequency range from 170 GHz to 230 GHz. We also used FEKO to simulate the beam pattern of the dual-polarized twin slot antenna integrated with a silicon lens with a diameter of 5 mm. After optimizing the extended length of the silicon lens, a near Gaussian beam with a half power beam width (HPBW) of 19.4 degrees and a side lobe level of 17.3 dB was obtained. In addition, we designed and simulated an air bridge that is used to transmit the signals received by the twin slot antennas in the orthogonal directions. We find that the transmission coefficient of the air bridge is close to 0 dB and the isolation in the orthogonal directions exceeds 35 dB.

Vector vortex beams (VVBs) possess a variety of application scenarios, ranging from particle capture to quantum information. Recently, applied metasurfaces have simplified traditional bulk-complex systems for evaluating VVBs in the terahertz (THz) band and have successively demonstrated the evolution of their coaxial polarization states. Nevertheless, it is quite challenging to obtain the polarization singularities when non-coaxial superposition is performed by two ondemand multiplexed VVBs. Here, an all-dielectric metasurface platform working in the THz band is evaluated utilizing polarization-multiplexing coding techniques, and the platform can be used to explore the non-coaxial superposition state of two identical VVBs. The employment of spin-polarization multiplexing techniques gives higher encoding degrees of freedom within the orthogonal circular polarization channel, enabling the superposition behavior of inhomogeneous polarization states to be performed at predefined focal planes. A systematic analysis of the newly generated polarization singularities in the interaction region is performed by applying the full Stokes polarization vector. With the ultra-compact configuration, the experimental results obtained by the proposed strategy under THz wave illumination exhibit good agreement with the numerical simulations. Thus, such mechanisms offer potential applications for multiple particle capture based on non-coaxial superimposition of VVB and provide efficient meta-platform in data encryption, high-resolution imaging, and multiplex communication.

When using the terahertz pulse signal for thickness detection, the system echo response directly affects the extraction of effective information in the signal. The double Gaussian inverse filtering (DGIF) method can effectively separate the sample reflection signal and the system echos. However, DGIF method will introduce high-frequency noise, seriously degrade the signal quality, and affect the detection result. In this paper, variational mode decomposition (VMD) combining DGIF algorithm and sparrow search algorithm (SSA) is used to denoise the signals of terahertz time-domain spectroscopy system. Firstly, the DGIF method is used to reconstruct the sample signal. Secondly, using the minimum envelope entropy as fitness value, the SSA was used to optimize the key parameters of VMD. Thirdly, the reconstructed terahertz time-domain spectral signal is decomposed by the optimization parameters to obtain intrinsic mode function. The kurtosis value and energy entropy of intrinsic mode function (IMF) is further extracted as the comprehensive evaluation index. The signal synthesized by the main mode functions is the reconstructed time domain signal after noise reduction. A quartz standard specimen was measured and compared with the original signal, DGIF signal, VMD signal and DG-SSA-VMD signal. Combined with the SNR and RMSE evaluation indexes, it is verified that the DG-SSA-VMD method can avoid the inconvenience of manually selecting key parameters, reduce the thickness extraction error, and has the best comprehensive denoising effect, which is conducive to further detection and imaging applications.

In order to realize multi-function, the radar has wide instantaneous bandwidth and working bandwidth. Unfortunately, the traditional phased array radar antenna by microwave phase shifter in the way of phasing. This paper presents a kind of optical beamforming network based on cascaded micro-ring resonator. The simulation results show that the proposed antenna system can be more effectively satisfied with requirement of wideband and wide-angle scanning of phased array radar.

The paper proposes a method of inter-frame data alignment, built on the operations of pre-processing and selection of object contours and based on the following basic operations: reading data from cameras, determining parameters and dimensions; solving the problem of contouring objects and simplifying them to form a mask for subsequent processing; filtering, using a multi-criteria method based on a combination of criteria, the standard deviation of the final differences of the generated estimates and the first difference of the deviation from the input implementation; the use of NS to search for animals or/and humans; parallel detection of objects located in the range of 34°C-42°C degrees Celsius (95°F-107.6°F); the use of NN to determine the type of object by analyzing the thermal portrait; use of a mask of live objects (visual data) and thermal imaging parameters as alignment centers; determining a thermal image transformation matrix; stitching a pair of images. On a set of test sequences, studies have been carried out to increase the degree of detection of living objects when a thermal imaging camera is included in the data detector. The studies were carried out on a pair of cameras of the visible spectrum (1920x1080, RGB, 8 bit) and thermal (240x180, RGB, 8 bit).

Terahertz wave can be produced from two-color laser induced filaments, whose length is one of the critical factors that determine the output yield. Here, we propose a new method to generate the axial-length controllable filament using a liquid crystal spatial light modulator (LC-SLM). Previous studies have employed optical elements such as phase plates and deformable mirrors to optimize and control the filamentation process, but these methods lack flexibility. Our approach enables the programmable modulation of the phase distribution and spatial distribution using SLM. By loading a diffraction phase pattern, it enables convenient manipulation of the length, diameter, electron density of the plasma filament, while also providing advantages in flexibility and feedback optimization for improving terahertz output. Moreover, we demonstrate how the fan-shaped segmentation phase method can be applied to generate air plasma and control the length of the filament to enhance terahertz output. Our method can overcome the saturation effect induced by plasma clamping, therefore improve the terahertz output. To verify its feasibility, we establish a far-field distribution model for terahertz yield based on the photo-current model and investigate the effect of filament length on terahertz output under different conditions. The simulation results show that our method can significantly improve terahertz output at different frequencies and initial phase of the two-color field. Overall, our approach offers a relative simple and effective way to control the length of plasma filaments and enhance terahertz output using SLM technology, it has great potential applications in terahertz biophysics.