This PDF file contains the front matter associated with SPIE Proceedings Volume 12000, including the Title Page, Copyright information, Table of Contents and Conference Committee listings.

Phased array antennas are used in applications where beam steering is required. However, especially for large tilt angles, the polarization purity of such antennas can be compromised, which leads to a significant degradation of the signal-to-noise ratio. To mitigate this effect, we consider a link working with circularly polarized waves in the Ka band. The transmission side has beam steering functionality. The receiver antenna elements have right- and left-hand circular polarization ports, and each port is equipped with a variable gain and phase shift elements, whose values are selected to provide optimum cross-polarization discrimination.

To date, there have been very few research on electromagnetic (EM) wave absorption by such antennas in the infrared (IR) region. Therefore, in this article we have proposed a new type of gradient log-spiral antenna to enhance the absorption bandwidth. The unit cell antenna is designed by considering the special arrangements of different log-spiral arms. The unit cell antenna can achieve an average absorption ˃88.5% from 4.5 to 100 THz under normally incident plane waves. The absorption of the antenna is incident angle insensitive up to 45° and polarization independent up to 75° for both the transverse electric (TE) and transverse magnetic (TM) modes. The antenna shows high directivity of 6.68 dBi and high gain of 3.55 dB with 3dB beamwidth 59.2°. Moreover, omnidirectional directivity is observed in both low, mid and high frequencies. This innovative design approach opens a new path for thermal detection, bolometer, and energy harvesting research.

Polarization information regarding solar radiation is not readily available in the mid- to far-infrared regimes. Conventional thermal IR detectors capture intensity with a loss of specific spectral and polarization information. Thermoelectrically coupled nanoantennas (TECNAs) capture infrared radiation by using an antenna that provides the capability for spectral, polarization, and angle-of-incidence selectivity. The nanoantenna resonantly absorbs the incident IR radiation and heats the hot junction of a nanothermocouple, which provides an output voltage that is proportional to the intensity. This is accomplished with minimal thermal mass, and provides μs response times. Here we present TECNAs with log-spiral antennas that are capable of distinguishing left- and right-handed circular polarization (LHCP/RHCP) in the long-waveinfrared. The log-spiral TECNAs are suspended above quasi-hemispherical cavities etched into a Si substrate. The cavity thermally isolates the nanoantenna from the substrate and focuses the incident radiation onto it. Simulations show electromagnetic (EM) fields and resulting thermal distributions along the antennas for different polarizations. When the handedness of the EM polarization matches that of the antenna, the EM field is concentrated at the center of the antenna, while for opposite polarization it is concentrated toward the antenna leads. As a result, the temperature increase at the center of the nanoantenna for the two polarization directions is different. This provides an extinction ratio V_RHCP/V_LHCP ~ 4.

We present the material characteristics, device design, and device manufacturing process of high performance, low loss 3D printed GRIN lenses like a Luneburg-style lens. To validate the technology, a simple spherical, radially symmetric GRIN lens was designed and printed using a commercial DLP printer and low loss polymer dielectric material. The lens was mounted to a co-axe to waveguide transition and served to increase gain of the resulting antenna system. The device was tested in the Ka band, and the practical test results are compared against HFSS simulated results. The result is a passive lens design that has the capability form and steer beams in the Ka-band.

Short channel Si CMOS have been used as the THz detectors and have found applications in the THz imaging arrays. Sub-terahertz excitation of phase-shifted resonant or overdamped plasma waves in short CMOS channels enables the operation of such TeraFETs as THz spectrometers. Further developments in Si CMOS sub-THz and THz applications will use Si CMOS integrated circuits. Examples of such circuits include the line-of-sight detectors (very important for future 300 GHz band 6G communications), traveling wave sub-THz amplifiers, and frequency-to-digital converters.

6G/beyond 5G technologies should meet the requirement for both high speed and high security. We have focused on terahertz (THz) waves for high-speed transmission, and by photomixing, we have demonstrated power enhancement and beam steering. In terms of the security of wireless transmission, physical-layer security becomes an important issue. We have proposed a novel wireless communication system based on beam steering and coherent detection. In this system, two different data encoded from original data on two THz beams are radiated from two different locations and are overlapped at a target area by beam steering. Then, a receiver at the area implements logical AND operation between the two data as a result of coherent detection, which deduces the original data. This time, as a feasibility confirmation, we performed logical AND operation between two data on different lightwaves and measured the bit error rate of the deduced original data. Experimental results showed that the original data is deduced exactly from the two encoded data when the amplitude ratio between the two data is smaller than 5. This indicates that the acceptable ratio between the distances from two different transmitters would be up to 5. Experimental results also showed that the bit error rate suddenly deteriorates when the time slots of the two data are shifted by 0.7 bits This indicates that the receivable area is limited in several centimeters square for a 1-Gbit/s data.

Terahertz sources that offer broadband radiation, compact footprint, and relatively low cost are mostly realized with photoconductive antennas built on a photo-absorbing substrate, where incident laser pulses generate electron-hole pairs and induce radiating ultrafast photocurrents. To create a strong field within the photoconductive active region and accelerate the photo-generated carriers, an external bias voltage is typically applied to the terahertz source to achieve high enough radiation power. However, with rapidly increasing interest for terahertz sources operating at 1550 nm - to take advantage of low-cost and compact fiber-based optics - it has become challenging to obtain high efficiency and at the same time overcome the degraded reliability due to the low resistivity and high dark current of photo-absorbing substrates at 1550 nm. To mitigate this challenge, we demonstrate a bias-free photoconductive terahertz source based on a graded composition InGaAs photoconductive layer, leading to completely passive optical-to-terahertz conversion with zero dark current. Using a linear gradient of Indium composition in a highly Be-doped InGaAs layer, a built-in electric field that extends deep into the substrate is created, leading to efficient collection of almost all optically generated electrons. Additionally, a large area plasmonic nanoantenna array is utilized to enhance the optical absorption at 1550 nm near each nanoantenna to reduce the average electron transit time to the radiating elements. With the graded InGaAs-based terahertz source, we experimentally demonstrate a 4-fold increase in radiation power compared to previously demonstrated passive plasmonic terahertz sources.

Topological crystalline insulators—topological insulators whose properties are guaranteed by crystalline symmetry— can potentially provide a promising platform for terahertz optoelectronic devices, as their properties can be tuned on demand when layered in heterostructures. We perform the first optical-pump terahertz-probe spectroscopy of topological crystalline insulators, using them to study the dynamics of Pb_1−xSn_xSe as a function of temperature. At low temperatures, excitation of Dirac fermions leads to an increase in terahertz transmission; from this negative photoconductivity, the intrasubband relaxation rate of 6 ps is extracted. At high temperatures where only massive fermions exist, the free-carrier losses induced by the pump reduce the terahertz transmission for the duration of the 27 ps interband lifetime. Both effects are present at temperatures near the topological-to-trivial transition. Our experimental observations provide critical details for potential applications of Pb_1−xSn_xSe and provide a direct measurement of the topological character of Pb_1−xSn_xSe heterostructures.

Photoconductive emitters and receivers are widely accepted as the best combination for applications requiring broadband and high dynamic range and are nowadays deployed in most commercially available systems. Novel laser sources with higher repetition rate and power levels are a promising route towards further improvements in this area. We present our first steps in this direction by combining state-of-the-art emitters and receivers with an ultra-stable commercial fs laser (MENHIR-1550 SERIES) at 1 GHz repetition rate as the optical source. The output of the laser is amplified and compressed by a commercial fiber amplifier setup. In this experiment, we use 17 mW as the probe beam and 30 mW as the pump beam with a pulse duration of 150 fs, as these are the best operation points for the emitter and receiver available. The emitter is based on iron doped InGaAs in a strip line geometry with an active region of 50 μm x 50 μm while a fiber coupled dipole antenna with a 10 μm gap is used as the receiver. We demonstrate a 1 GHz repetition rate terahertz time-domain spectroscopy (THz-TDS) system with a dynamic range of 73 dB and a bandwidth of 3.5 THz using state-of-the-art THz photoconductive emitter and receiver with a measurement time of 60 s. This result is part of a larger effort to understand the compromises to be realized in terms of repetition rate and average power to take photoconductive emitters and receivers to the next step in dynamic range enhancement.

With emergent fifth-generation (5G) communications systems harnessing ever-higher carrier frequencies and wider instantaneous bandwidths to enable denser user-space and higher data throughput, the ability to address high numbers of users becomes paramount. Existing digital beam-forming technology does not break down gracefully in the presence of a large number of data channels, and high-speed digital-to-analog converters (DACs) are both expensive and power-hungry. The RF-Photonic approach to millimeter-wave (mmW) imaging offers alleviations to this problem by relying on a low-speed phase-locked loop to mitigate phase variations incurred through mechanical means. We present a photonic mmW imager with a fiber arrayed waveguide grating (FAWG), enabling quasi-instantaneous spatial-spectral localization of all incident signals in the array field of regard through optical processing of the up-converted RF field. This configuration additionally enables recovery of the data encoded upon the received carriers through the use of a tunable optical local oscillator, with high-fidelity detection of quadrature amplitude modulation (QAM) signals demonstrated up to a 16-point constellations, and the recovery of multiple signals at once over a wide spectral separation. Additional techniques are presented, using multiple fiber projections to remove the ambiguity between spatial and spectral locations of multiple sources in an RF scene, providing quasi-instantaneous detection of the spatial-spectral location in frequency- and angle-space for each source incident upon the array. In a complete implementation, we expect such a system to possess the capability of simultaneous signal identification and data recovery, contained in a portable form factor for rapid deployment.

We demonstrate a radio frequency (RF) phase-encoded signal generator as well as a user-defined RF arbitrary waveform generator (AWG) based on a soliton crystal micro-comb generated by an integrated MRR with a free spectral range of ~49 GHz. Owing to the soliton crystal’s robust and stable generation as well as the high intrinsic efficiency, RF phase-encoded signal generators and AWGs with simple operation and fast reconfiguration are realized. The soliton crystal micro-comb provides 60 wavelengths for RF phase-encoded signal generators, achieving a phase encoding speed of 5.95 Gb/s and a high pulse compression ratio of 29.6. Over 80 wavelengths are employed for the AWGs, achieving tunable square waveforms with a duty cycle ratio ranging from 10% to 90%, sawtooth waveforms with tunable slope ratios from 0.2 to 1, and symmetric concave quadratic chirp waveforms. Our system has great potential to achieve RF and microwave photonic signal generation and processing with low cost and footprint.

We report on pulse amplitude modulation (PAM) communication in the terahertz (THz)-band using an integrated-optic PAM signal emulator, which consists of asymmetric Mach-Zehnder interferometer (MZI). The asymmetric MZI, whose edge couplers are composed of symmetric MZIs (coupling ratio tunable couplers), produces an optical PAM signals from an input optical on-off keying signal by adjusting each phase shift at each interferometer arm. We generated an optical 25 Gsymbol/s three-level PAM (PAM3) signal with this emulator and converted the generated optical signal into a PAM3 signal in the 300 GHz band with high-speed photo-mixing. We show obtained results on the THz-wave 25 Gsymbol/s PAM3 communication.

In the last decades, highly integrated electronic circuits have paved the way towards compact, smart electronic devices with excellent computing power. Considering the benefits of electronic integration, it is compelling to apply similar integration methods to shrink the size of photonic on-chip devices in the terahertz and optical regime. Here, we numerically and experimentally investigate the guiding, routing and manipulation of strongly confined spoof terahertz surface plasmon polaritons (terahertz SSPPs) on metasurface pathways. The pathways are composed of single-, two- or three-cut wires that define the subwavelength width with respect to the SSPP wavelength. We measured the spatio- and spectro-temporal dynamics of the electric field of the SSPPs by electro-optic imaging. We observed that the terahertz SSPPs exhibit a strong out-of-plane and in-plane confinement, even when they propagate on curves of subwavelength path width. The spatio- and spectro-temporal behavior of the terahertz SSPPs evidences that they can be tightly guided within subwavelength space on metasurfaces without loss of the out-of-plane confinement. Due to these beneficial electromagnetic properties, metasurface pathways of subwavelength width seem to be ideally suited for the implementation of on-chip terahertz networks and sensor systems.

In developing a tunable fishnet metamaterial (TFMM) for interacting with terahertz radiation, various geometries of the basic unit cell are investigated for unique performance characteristics and optimal phase shift effect. Being able to generate a large phase shift is critical in creating a device for wave manipulation. In this paper, the study of two different unit cell geometry designs are presented. Simulation studies are performed on the designs prior to device fabrication to identify the characteristics of the devices. Each of the designs shows different characteristic in the study, such as displaying frequency response that is independent of polarization and creating large phase shift to the incident beam when the device is tuned. Experiments are performed on the fabricated unit cell arrays and the characteristics of the devices are verified.

In imaging systems, a lens performs spatial Fourier transformation of the incoming waves, thereby mapping each plane wave into a point, or, equivalently, the lens focuses each set of parallel rays to a corresponding spot at the sensor array. Accordingly, two-dimensional (2D) vision requires a conventional 2D lens. This has been the common assumption since the dawn of free-space optics as a discipline. In this presentation, we demonstrate 2D image formation using a 1D lens. Such imaging is obtained in the context of RF phased arrays where image forming means beam forming of multiple beams simultaneously for receiving (transmitting) independent radio waves from (to) many different directions at the same time. The method is based on spatially coherent electro-optic up-conversion of the incoming radio signals to the optical domain and it exploits the arrangement of antennas in a regular array to map between different imaging topologies. Space-division multiple access (SDMA) enabled by this imaging modality is instrumental to ushering the age of virtually unlimited information bandwidth in wireless communication. The approach overcomes topological constraints of imaging systems in general, and of phased arrays in particular. We explain the principles of method and present experimental results illustrating its viability in a practical setting.

Microwave photonics can be used to conduct instantaneous spectral imaging of incoming microwave signals. Here, we demonstrate the use of a fiber-based arrayed waveguide grating (FAWG) to detect the microwave spectrum over a 25 GHz bandwidth with sub-2-GHz resolution. The received microwave signal, amplified by the device radio-frequency (RF) front end, is upconverted to the optical domain using an electro-optic modulator and sent through an array of optical fibers of different lengths. Like an arrayed waveguide grating (AWG), the different fiber lengths produce observable frequency dispersion. Fine frequency resolution is enabled by large path length differences among the optical fibers. Since fibers are susceptible to environmental fluctuations such as mechanical vibrations or temperature changes, active phase control is utilized to compensate for phase variations in real time. After optical filtering, the spectrum is captured by a linear IR camera that is placed at the Fourier-transform plane of the free-space optical-detection system. The FAWG has possible applications for microwave sensing as well as optical communications and can be adapted for different frequency ranges and resolutions. Here, it is discussed in the context of atmospheric sounding to measure remotely the temperature and water-vapor content.

Hyperbolic Metamaterials, as a non-magnetic anisotropic artificial structure, show metal properties in one direction and dielectric behavior in orthogonal directions. The proposed hyperbolic metamaterial filter in this project is designed with the metal wire mesh perpendicular to the alternative layers of dielectric materials, keeps TM center wavelength unchanged for the different angle of incident light in MDIR regime. The geometric size of this nanostructure is smaller than the working wavelength and supports big wavevectors due to hyperbolic dispersion. In contrast with conventional Bragg stack, the copper fakir bed makes the transmission properties of the filter the same. For this purpose, the state-of-the-art fabrication methods are required to make such small dimensions in alternative layers of amorphous silicon and silicon dioxide. In this work, first we demonstrate the simulation of Bragg stack with RCWA and finite element methods. Then we focus on our first-time multistep lithography method used to fabricate the filter at Cornell University’s Nanoscale Science and Technology Center. Finally, we experimentally verify the optical characteristic of the fabricated filter using Fourier-transform infrared spectroscopy. The experimental and spectrometry data shows that transmission properties of the hyperbolic metamaterial filter remain the same for oblique TM polarized incident light.

In this paper, we discuss the changes in the electrical performance induced by operating time in hydrogen-terminated diamond MESFETs for high power and high frequency applications.

During the single stress step an increase of the current flowing in the sample is visible, possibly caused by the self-heating of the sample, as supported by temperature-dependent measurements, and by charge detrapping processes. In the full experiment the drain current was found to decrease, whereas the gate current remains below the detection limit.

In the characterization phase, we detected an increase in on-resistance, a decrease in the saturation current, a shift in the threshold voltage and a decrease in the transconductance peak. We found a time-dependent behavior for all these parameters, showing a further worsening up to 10 minutes after the end of the stress step. The time-dependent behavior is related to the creation of defects inside the structure and not to the self-heating, since the dynamic variation was found to increase as a consequence of stress, whereas the power dissipation decreases. The increase in the concentration of defects with activation energy of 0.30 eV was confirmed by ON-resistance and threshold voltage transient spectroscopy.

The variations in on-resistance and threshold voltage are not correlated in the full duration of the stress, suggesting that the generation of defects has (i) a different impact or (ii) a different generation rate in different parts of the device, with (iii) a possible role of the worsening of the contacts. Furthermore, a decrease in electroluminescence with higher magnitude than the decrease in drain current was found, compatible with an increased carrier-defect scattering.

Tunable dielectric meta-surface nanostructures offer incredible performance in optical application due to their extraordinary tunability of the polarization and engineering the dispersion of light with low loss in infrared range. In this article, we designed and experimentally measured the tunability of all-dielectric subwavelength silicon nanoparticles with the help of the temperature-based refractive index of the liquid crystal in the telecom regime. The proposed structure composed of high dielectric nanodisk surrounded by nematic liquid crystal (NLC) is simulated with numerical software, assembled with pre-alignment material, and optically measured by Fourier-transform infrared (FTIR) spectroscopy. The simulated result is compatible with the practical measurements, shows that the tunability of 30nm is achieved. Electric and magnetic resonance modes of the high dielectric nanodisks are tailored in different rates by anisotropic temperature dependent NLC. The phase switching of anisotropic to isotropic nematic liquid crystal enables spectral tunning of the two modes of all dielectric metasurface and modifies the symmetry of the optical response of the metamaterial structure.

The THz/Far-IR Beamline at the Australian Synchrotron was used to demonstrate a novel method of estimating the dielectric properties of homogenous substances in the 1.0 THz to 4.0 THz region. Attenuated total reflection (ATR) coupled with synchrotron sources allow rapid evaluation of samples. The source is incoherent, thus normally only reflectance can be derived, as phase shift data cannot be obtained with this arrangement. A method is presented of deriving the full complex dielectric parameters by a modified frustrated internal reflection technique which reflects the evanescent wave using a gold plated mirror. Oil and alcohol samples were used in the study. With the mirror in situ, the reflectance changed from being enhanced at some frequencies to undergoing a frequency dependent destructive interference at other frequencies. The change in different samples was noted to vary according to the refractive index (but not the absorption coefficient) of the sample.

Low-temperature photoluminescence spectroscopy (PL) and excitation spectroscopy(PLE) are used to characterize and compare high current density resonant tunnelling diodes (RTD) structures. RTD structural characteristics are detected using X-Ray diffraction (XRD) while the electrical characteristics are detected by PL and PLE. Results are used to link the structure electrical properties to the RTD device IV characteristic. We started focusing the attention on the first quasi bound state (e₁) energy, fundamental for the RTD operation. PL is used to detect the TypeI and Type II QW radiative transitions. The e₁ state is obtained by the difference between the Type I (e₁–hh₁) and type II (conduction band–hh₁) transitions. PLE is consequently used to detect the e2-hh2 transition from which we characterize the energy of the e2 state and its position with respect to the e1 state. Experimental data are confirmed by the RTDs device IV characteristics. We highlight the combination PL and PLE as a powerful, fast, and non-destructive characterization method to link wafer properties and device performance in RTD structures.

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

We propose and report on a method that demultiplexes multiple carrier channels directly in the terahertz (THz)-domain by utilizing a THz-wave asymmetric Mach-Zehnder interferometer (AMZI). The frequency division multiplexing (FDM) channels can be demultiplexed with the AMZI. As preliminary investigation, we fabricated the AMZI by employing THz-wave bulk components including beam splitters, mirrors, and lenses. We show demultiplexed results of a two channel FDM on-off keying signal in the 300 GHz band, which was produced by utilizing a high-speed photo-mixing, with the AMZI.

In this work, we investigate the fabrication and the application of ultra-thin terahertz metasurfaces as thermal converters for indirect terahertz imaging. The microstructures fabricated by an ultrasonically driven printing process (Microplotter) are used to improve the THz to IR conversion efficiency and tune the spectral or polarisation selectivity. The resulting conversion membranes show optical and thermal responses which are consistent with numerical simulations establishing reliable rules to design such membrane at various wavelengths. This work paves the way for a low-cost solution of multispectral terahertz imaging with a standard infrared camera.

The alleged observation of traces of inflation of the Universe in CMB spectra was later ascribed to the scattering from the space dust (BICEP2, 2018). Hence, the development of experimental methods, which distinguish between light from objects with a similar color temperature and polarization presents an important practical problem. In this paper, we discuss the proposal to discriminate between objects with the same color temperature but having different angular spectrums by intensity interferometry. The two-point correlation function of the black body image with extended angular spectrum has significant differences with a correlation function of a black body with a narrow angular spectrum.

This work demonstrates the high-sensitive terahertz (THz) detector based on topological semimetal platinum telluride (PtTe₂). The device shows broad THz frequency (0.1-0.5 THz) detection capability even under the self-driven mode. The thin PtTe₂ film is grown by direct tellurization of the sputtered platinum film on the high-resistivity silicon substrate using the chemical vapor deposition (CVD) method. Furthermore, the device exhibits responsivity of 29.2 and 63.1 mA/W at 0.1 and 0.4 THz, respectively, at zero bias voltage. These responsivity values increase to 47 and 82.8 mA/W, respectively, under 200 mV bias voltage. The significant attributes of these devices are the high responsivity, self-driven operation mode, easy fabrication process, and broadband response incurred in the simple device structure.