This PDF file contains the front matter associated with SPIE Proceedings Volume 12134, including the Title Page, Copyright information, Table of Contents, and Committee Page.

We present a new technique for single shot Terahertz detection in electro-optics sampling (EOS) with a narrowband probe pulse shaped using a Fabry-Pérot etalon. The technique allows tdetection in the frequency domain using a high-resolution CCD spectrometer. The technique is simple and sensitive. It has a high time resolution and can be simply implemented in a standard EOS scanning experiment

The paper gives a short overview on the present status of the resonant-tunneling diodes (RTDs) in the area of THz electronics. In particular, the RTD oscillators are discussed: the achieved level of their output power and operating frequencies; an overview of the different types of RTD oscillators, their advantages and disadvantages; tunability and frequency stability of RTD oscillators. The use of RTDs as THz detectors is also shortly discussed, in particular: operating principles, limitations, application examples. Further on, an overview on the application examples of RTDs is given: RTDs oscillators in high-data-rate wireless transmission systems, imaging applications, spectroscopy, etc.

This paper presents a broadband THz generation and detection technique based on high-speed bipolar transistors and PIN diodes that act as fast switches. Traditionally, fsec lasers and photo-conductive antennas are used to produce psec pulses in the time domain. These techniques require an expensive and bulky laser, optical alignment, and moving parts that add to the complexity of the system. To alleviate these issues, laser-free fully electronic THz sources and detectors are designed and fabricated. The source technology can produce THz signals up to 1.1THz with linewidth of 2Hz. The detector technology can capture THz signals up to 500GHz. To produce and radiate THz pulses, a DC current is passed through an on-chip antenna that acts as an inductor in low frequencies and stores magnetic energy. The DC current is then disconnected with a bipolar transistor in a few psec. This event causes a large voltage transient on the antenna resulting in radiation of pulses with duration of less than 2psec. The produced THz pulse is then radiated with an on-chip antenna. With this method, THz pulses with repetition rate up to 15GHz are generated and radiated. To detect the THz pulses, a silicon-based detector is designed and fabricated. The detector captures THz pulses with an on-chip antenna. The received THz pulse is then sampled by a fast bipolar transistor. In addition to the transistor-based approaches, this presentation will also introduce silicon-based PIN diodes as alternative solutions for THz pulse generation and detection. Using the source and detector technology developed in this work, several systems are implemented. These systems include a THz gas spectrometer, a hyper-spectral imaging system, a micro-doppler radar, and a wireless communication link. In this presentation, the design, implementation, and performance of these systems will be reported as well.

Terahertz spectroscopy provides information on the motion of the charges in a sample at a picosecond scale. To recover this information from Terahertz time-domain spectroscopy (THz-TDS), one usually extracts the experimental refractive index then fits these curves. This approach suffers from several limitations, among them the difficulty to compare models of motions, provide the error bar associated with the extracted magnitude and a resolution limitation coming from the Fourier criteria of the fast Fourier transform. By adopting a Bayesian framework taking into account the experimental uncertainties and directly fitting the time-domain trace, we overcame these limitations. When correlated and epistemic uncertainties/noise are present, the algorithm considers its distribution as part of the data to fit and can mistake it for real physical features. Hence, it offers poor discrimination between good models and bad ones. After a thorough analysis of the experimental noise, we developed a preprocessing software removing epistemic noise on the time traces and providing an estimate of the noise correlation matrix (generalization of the standard deviation). It allows the proper weighting of the error function of the fit using these uncertainties and therefore the derivation of the Akaike information criteria, a metric enabling to calculate the most probable model from a set of models one wants to compare. In addition, by being in the time domain we avoid the Fourier criteria for the resolution and thus could get information on experimental lines down to 30 MHz with a commercial THz-TDS system.

We explored two ways to enhance light matter interaction in the THz range through spatial confinement of the electric field. Firstly, a broadband metallic waveguide with low losses and low dispersion used in a TDS setup to measure samples with volume as low as 200pL. In this proceeding, we explore a resonant structure allowing for tighter confinement at the price of narrower bandwidth. Split ring resonators are resonant structures analogous to LC circuit, where the electric field is confined in the capacitive part of the device. We fabricated SRRs with capacitive gaps as small as 30nm for measurements on extremely low volume sample such as macromolecules or viruses.

The state-of-the-art terahertz systems employ conventional, bulky, optical system design approach that lags the miniaturization, high-density integration, and mobility of the terahertz imaging systems. On the other hand, the motivation for miniaturization of the terahertz systems using integrated circuits (ICs) is limited by the conventional terahertz waveguide performance that requires utilization of a novel waveguiding technology. The spoof surface plasmon polariton (SSPP) waveguide (WG) measurements have recently been reached the record low insertion loss per unit length performance among all planar terahertz WGs at 0.3 THz suggesting tremendous potential for demonstration of high-performance terahertz ICs. Nevertheless, the real potential of the terahertz imaging systems requires demonstration of an imaging system that can provide high-resolution feature extraction of the targets covered by obstacles at real-terahertz frequencies. We present the design and simulation of 135° spoof surface plasmon polariton (SSPP) bending circuits at 1 THz that are one of the most fundamental building blocks in novel IC technologies that will enable development of high-performance, high-resolution terahertz imaging systems along with the investigation of the coupling mechanism of the SSPP waves through non-aligned waveguide geometries that is mandatory for implementing standalone terahertz ICs.

We use a slotted Y-branch Laser for Terahertz thickness measurements of high resistive float zone silicon wafers of different thicknesses. The laser provides two-color emission in the 1550 nm region with an optical beat frequency of 1 THz. It is used as a photonic source for thickness measurements of high resistive silicon wafers with continuous wave Terahertz radiation. Frequency tuning is obtained through segment current tuning of the individual branches. We determine the sample´s refractive index and thickness by MSE fitting of the theoretical etalon transmission to the experimental results without additional knowledge.

Recent advances in the field of photonics and topological physics can be combined to offer a solution to planar 6G, above 100 GHz, communication devices. As specific examples, we demonstrate that a hybrid photonic crystal waveguide can support a single-mode transmission covering 0.367–0.411 THz (over twice as wide as that of all-silicon photonic crystal waveguides). By breaking the photonic crystal symmetry, topologically protected modes can be introduced with a single mode linear-dispersion transmission window (over 0.143–0.162 THz) and robust transmission around sharp corners without any deterioration in the bandwidth. Such topologically protected waveguides, here produced using simple 3D printing techniques, offer a unique simplification in design. The absence of coupling to back-propagating modes removes the requirement to carefully design away spurious resonances, offering a pathway to a truly versatile planar platform for integrated 6G devices with low loss and wide bandwidth.

This work aims at illuminating an UTC-PD array with a multicore fiber towards multiple THz carrier generation. This photonics-based transmitter is expected to increase either the data rate via spatial multiplexing on different carriers or the emitted THz-power and therefore the transmission reach of THz systems operating in 300 GHz band. Preliminary results of the characterization of the sub-systems of the transmitter that is under development are presented here.

We propose a new solution for sensing a terahertz (THz) wavefront based on a THz reference-less shear interferometer. The key component of the experimental configuration of the proposed interferometer is a THz Ronchi phase grating (RPG). The RPG is custom designed and fabricated for a 0.28 THz source using mechanical milling on a block of high-density polyethylene (HDPE) with a computer numerical control (CNC) machine. It acts as a shearing element that generates two diffraction orders, thereby creating two laterally shifted copies of the investigated wavefront in the sensor plane where a THz camera is placed. The direction of the shear can be varied by rotating the grating. Since the grating is a phase grating, the diffraction efficiency is very high. The approach is verified experimentally by demonstrating interferograms of a spherical wave and wavefront reconstruction from five different shears using a gradient-based iterative process.

We have demonstrated injection-seeded backward terahertz (THz)-wave parametric oscillators (BW-TPOs) based on a slant-strip-type periodically poled lithium niobate (PPLN) crystal with two different poling periods. The BW-TPOs were pumped by sub-nanosecond pump pulses at 1064 nm and designed for generating backward-propagating THz waves around 0.30 and 0.46 THz with PPLN poling periods of 53 and 35 μm, respectively. As a result of an optical injection seeding for the forward-propagating idler wavelength in the BW-TPO process, we achieved over a 1000-fold enhancement in backward-propagating THz-wave output energy, a 63% reduction of the oscillation threshold, and long-term stable operation compared to the unseeded case. Furthermore, we demonstrated that the oscillation frequency of backward-propagating THz waves is continuously tunable in the range of 0.27–0.35 and 0.41–0.52 THz for the poling periods of 53 and 35 μm, respectively, by angle tuning of the PPLN crystal in parallel with seed wavelength tuning. Using the developed injection-seeded BW-TPOs, we also performed the THz-wave imaging test in transmission geometry for various materials, including glass, wood, and liquids.

The competition to suggest high performance solutions for terahertz communication targets 0.22-0.32 THz band because of its bandwidth and attenuation advantages over other terahertz frequencies. However, the state-of-the-art suffers from conventional terahertz waveguide performance. Alternatively, the spoof surface plasmon polariton waveguides (SSPP WGs) measurements achieve the record-low insertion loss per unit length at 0.3 THz. On the other hand, the SSPP WGs require high performance transitions to interface with terahertz active devices such as transistors and diodes. In this paper, we present design, optimization, and experimental verification of high-performance coplanar waveguide-to-SSPP WG (CPW-to-SSPP WG) transitions at 0.25-0.3 THz band. The measurements show that the insertion loss of a CPW to SSPP WG transition can be suppressed up to -0.5 dB at the proposed frequency band.

Agarose is a gel-forming polysaccharide extracted from marine red algae. This biopolymer is an important analytical separation medium, stabilizer and thickener in the food and pharmaceutical industries, and phantom material for biomedical research. The distribution of water to the different hydration zones in the gel can significantly influence the functional properties of agarose-based hydrogels. In this work, the application of terahertz spectroscopy in assessing the hydration state of biomolecules was extended from aqueous solutions and lipid-water emulsions to polysaccharide gels. The agarose gel system, whose structure is constituted by water confined in a complex network of polymer assemblies, was investigated. THz time-domain spectroscopy (THz-TDS) measurements were performed in transmission mode. The behavior of the absorption and dielectric loss spectra of the gels relative to the polymer concentration indicate that the agarose network acts as a kosmotropic agent that favors the formation of the regular hydrogen bond structure. The hydration number equation of Hishida and Tanaka was modified to account for heterogeneity and confinement effects using effective medium theory. The hydration number of the agarose monomers was found to decrease with increasing polymer concentration. This trend in hydration number can be attributed to the increase in fiber density as a result of increasing agarose concentration which causes the thinning of the hydration shells and the sequestration of water molecules to the inside cavity of the agarose double helices.

Rapid changes in the agricultural sector in the past two decades have given rise to several new technologies and superior products including genetically modified crops; the identification of which still requires robustness and rapidity. In this work, we report the use of continuous wave terahertz (CW THz) spectroscopy as a means to identify biomechanical changes at the tissue level based on systemic dehydration. We have also identified the factors affecting this progression and propose a biomechanical model towards genetic discrimination in plants. Our results indicate that within the same family, factors such as cell size and age, tissue composition, hydration retention capacity and water percentage to cell volume ratio affect the systemic dehydration in the plant and thereby show unique biomechanical profiles.

The terahertz imaging systems bring the advantage of both optical and microwave frequency spectrums, thanks to the invasion capability of the terahertz waves through different media providing high-resolution imaging at real terahertz frequencies such as 1 THz. Nevertheless, the state-of-the-art terahertz technologies employ bulky optical system design approach. In consequence, the state-of-the-art terahertz systems are not suitable for high mobility terahertz imaging applications. On the other hand, the state-of-the-art terahertz integrated circuits (TICs) suffer from high attenuation due to conventional terahertz waveguides, and hence, a novel high-performance terahertz waveguide is needed. In this paper, we present the investigation of loss performance of spoof surface plasmon polariton (SSPP) waveguides (WGs) that operate at 1 THz, which will enable the demonstration of compact and high-performance TICs. We present a relationship between the corrugation dimensions, radiation, and metallic losses and guided wavenumber for the first time. The proposed SSPP WGs are able to transmit the terahertz wave in expense of an insertion loss of -4.93 dB through 250 µm at 1 THz.

The aim of this study is to investigate and design broadband, terahertz antennas for the time-domain, pulsed operation of the photoconductive sources and detectors. Different antenna types, i.e., thick dipole, bowtie, and spiral antenna, are designed, and their performances are analyzed both analytically and numerically for radiation in 0.8-2 THz band. A very effective ultra-wideband antenna configuration, Archimedean spiral antenna, which has promising time-domain radiation results according to analytical studies, is proposed for terahertz radiation. To the best of the authors’ knowledge, this is the first study on fully time-domain analysis of terahertz antennas operating in 0.8-2 THz band.