Terahertz Emitters, Receivers, and Applications XV

Frequency-modulated (FM) combs based on active cavities like quantum cascade lasers have recently emerged as promising light sources in many spectral regions. Unlike passive modelocking, which uses amplitude modulation to generate amplitude modulation, FM combs use phase modulation to generate phase modulation. They can therefore be regarded as a phase-domain version of passive modelocking. However, while the ultimate scaling laws of passive modelocking have long been known—Haus showed in 1975 that pulses have a bandwidth proportional to effective gain bandwidth—the limits of FM combs have been much less clear. Here, we show that FM combs are governed by the same fundamental limits, producing combs whose bandwidths are linear in the effective gain bandwidth. Not only do we show theoretically that the diffusive effect of gain curvature limits comb bandwidth, we also show experimentally how this limit can be increased. By adding carefully designed resonant-loss structures that are evanescently coupled to the cavity of a terahertz laser, we reduce the curvature and increase the effective gain bandwidth of the laser, demonstrating bandwidth enhancement.

Highly compact laser sources with low threshold, exceptional directivity, and single-mode operation are in great demand for on-chip integrated photonics. Photonic bound states in the continuum (BIC) are peculiar nonradiative localized modes that have theoretically infinite lifetime within the radiation continuum, making it a favorable candidate for pursuing single-mode, low-threshold, and surface-emitting lasers. In this presentation, I will introduce several electrically pumped Terahertz semiconductor lasers we have developed based on the BIC concepts for achieving high Q and low laser thresholds in compact cavities while achieving single mode operations, which would be promising as monolithically integrated laser sources.

The quantum cascade laser pumped molecular laser (QPML) is a polyvalent source spanning the THz gap, with demonstrated operation from 200 GHz up to more than 5 THz. Using various models for molecular relaxation permits the derivation of a laser model that describes the pressure dependent behavior of the QPML. Here, we will discuss new designs that enable high performance operation.

By counteracting the diffusive effect of the gain with the addition of carefully designed resonant-loss structures coupled to a quantum cascade laser cavity, we present a new method for broadening FM comb bandwidths, approaching the gain bandwidth limit of the medium.

This study utilized terahertz time-domain spectroscopy and a dual-path QCL to conduct self-referenced gain characterization at various temperatures and biases. By employing dispersion correction and analyzing path length differences, a more accurate gain profile was achieved, overcoming the limitations of zero-bias-reference methods. Key findings include an absorption peak shift with bias and a consistent gain clamping, highlighting the method's efficacy in detailed QCL performance analysis.

In the quest towards room temperature THz quantum cascade laser (QCL), reducing the loss due to electron-phonon scattering is a key strategy. With that goal, we present here a novel THz QCL design based on direct bandgap GeSn/SiGeSn semiconductor alloys. In contrast to the previously proposed THz QCL based on Ge/SiGe, these direct bandgap materials with low electron effective masses combine two major advantages for room temperature THz QCL. They offer not only weak electron-phonon scattering presented in non-polar group-IV semiconductors but also a high optical gain. Nextnano NEGF simulations predict the presence of gain and thus a possibility of lasing in the proposed GeSn/SiGeSn QCL up to room temperature using a conventional metal-metal waveguide.

[Invited] This paper reviews recent advances in the research and development of graphene-layer (GL) based van der Waals (vdW) two-dimensional (2D) heterostructures for fast, sensitive terahertz (THz) detection. 2D plasmonic nonlinearity as well as photothermoelectric effects in GL and other Dirac semimetals/semiconductors are promising mechanisms for highly sensitive, fast-response, room-temperature THz detection. The vertical GL and b-AsxP1-x heterostructures enable a new ultrafast bolometric mechanism enhancing the GL-based THz photodetector performance. We also introduce our recently developed GL- and other Dirac-semimetal/semiconductor-based rectenna FET structures, supporting a so-called 3D rectification mechanism. This mechanism supports fast and highly sensitive zero-power consumption extremely low-noise THz detection, which was experimentally verified, with further experiments in progress.

We review our recent studies on a newly discovered 3D rectification effect in grating-gate InGaAs-channel high-electron-mobility transistor (HEMT) THz detectors. The 3D rectification effect is a multiplication of horizontal hydrodynamic nonlinearities of 2D plasmons with a vertical diode-like current nonlinearity at the heterobarrier between the InGaAs channel and the InAlAs barrier layer, which takes place in the gate-readout configuration under a forward gate bias voltage application. The detector internal responsivity of 1.2 A/W at 0.8 THz was achieved by the 3D rectification effect in the gate-readout configuration, which is two-orders-of-magnitude higher than a typical responsivity in the conventional drain-readout configuration. Furthermore, we numerically demonstrated that the degradation of the detector external responsivity due to the dc potential rise in the channel can be resolved by a new grounding structure for the channel. These results pave the way towards applications of the grating-gate InGaAs-channel HEMT THz plasmonic detectors to beyond-5G THz wireless communication systems.

This study presents advancements in the synthesis and application of Bi2Se3, a topological van der Waals crystal, for terahertz (THz) detection. High-quality β-Bi2Se3 crystals were synthesized via the selenium vapor-induced supersaturated solution method (SVI-SSM), ensuring stoichiometric integrity. Heterostructures of Bi2Se3 were constructed using hot transfer methods, culminating in the fabrication of a rectenna THz detector. Utilizing a 0.95-THz injection-seeded THz parametric generator (is-TPG) as the light source for pulsed-continuous wave (CW) THz waves, THz detection experiments were conducted. The results demonstrate the achievement of a 200 ps fast response time and a sensitivity of 40 mV/W with the THz detector. Remarkably, this performance was maintained even under zero bias conditions. These findings establish a foundation for the development of passive THz detection systems that consume minimal energy, holding significant promise for advancing THz communication technology.

We investigated a new generation of terahertz optoelectronics that can be monolithically integrated using quantum well structures. A first-generation terahertz transceiver based on GaAs/AlGaAs quantum well structures was designed, fabricated and characterized. The transceiver chip includes a semiconductor optical amplifier (SOA) integrated with a p-i-n diode based on the same quantum well structure, enabling both generation and detection of frequency-tunable terahertz radiation in response to a heterodyning optical pump beam with a terahertz beat frequency. This structure can be directly transferred to InP based material system working at telecom wavelengths. The optical source can be integrated on the same substrate along with optical intensity/phase modulators, paving the way for fully integrated terahertz imaging, spectroscopy, and communication systems.

A new quantum phenomenon, the in-plane photoelectric effect, has recently been discovered as a mechanism of far-infrared (FIR) photoresponse generation in a two-dimensional electron gas (2DEG). This effect has shown promise for terahertz (THz) detection due to its high photoconversion efficiency and a lack of an intrinsic response time limit. Initial detectors utilising the in-plane photoelectric effect, known as photoelectric tunable-step (PETS) detectors, have been developed and demonstrated to work as high-sensitivity FIR detectors. Here, we propose a PETS detector utilising a novel, broadband antenna adopted from a wide bow-tie geometry that minimises the area of 2DEG covered by the antenna. We demonstrate experimentally a large photoresponse to 2 THz radiation of an AlGaAs/GaAs heterojunction-based PETS detector with our novel antenna design. Furthermore, we study the influence of a magnetic field on the PETS detector operation. Our findings help facilitate the development of future high-speed, low-noise, ultra-sensitive FIR detector arrays.

Utilizing plasmonic nanoantennas capable of concentrating light in the infrared spectrum and serving as terahertz antennas, we have engineered a terahertz focal-plane array (THz-FPA) suitable for integration into terahertz pulsed imaging systems. Comprising 64 pixels, this detector array enables scanning of a 5 cm line width. With its rapid scanning capability and expansive field-of-view, the THz-FPA has the potential to elevate terahertz pulsed imaging systems beyond mere metrology tools, rendering them high-throughput instruments applicable in industrial environments for diverse non-destructive evaluation purposes.

High-frequency on-chip communication necessitates compact plasmonic network designs, shrinking electromagnetic waves to subwavelength scales. While metallic metasurface pathways handle spoof surface plasmon polaritons (SSPPs) at microwave and terahertz frequencies, real-time reconfigurability remains challenging. We introduce a dynamically tunable metasurface employing a fishbone structure with electrostatically deflectable microbridges. Voltage application lowers the cut-off frequency and shifts the dispersion curve. Being scalable beyond 120 GHz, this leverages micro-electromechanical systems for active manipulation. We developed a tunable SSPP low-pass filter that effectively blocks SSPP transmission upon activation at a SSPP frequency of 106 GHz.

The development of planar metamaterials, specifically metalenses, has gained attention for creating ultra-thin optical components by manipulating subwavelength structures. This is crucial for reducing the size of traditional bulky high-NA lenses, especially in the terahertz range. Current metasurface fabrication relies on complex lithography techniques in clean rooms, but we are exploring ultrashort pulsed laser processing as a simpler alternative. We have successfully utilized femtosecond laser processing technology to fabricate terahertz membrane metalenses at 0.8 THz by forming controlled-diameter through-holes on a high-resistance silicon substrate. The focusing performance of the fabricated terahertz metalenses was found to be the same as that of meta-lenses fabricated by photolithography. This one-step process has a potential simplification in terahertz optics fabrication.

THz harmonic generation in integrated graphene/metamaterial arrays was demonstrated by using ultrafast table top powerful time domain spectroscopic systems. Harmonic resonant features have been recorded with incident E-field energy pulses between 1 and 1-150 kV/cm. The nonlinear interaction between the main resonance at 0.5 THz and the harmonic signal at approx. 1.5 THz shows an anticrossing trend driven by the incident E-field. The complex dispersive features presents a clear dependence from graphene’s carrier concentration, actively tuneable via electrostatic gating, and saturation effects for higher incident pump power levels.

Heterodyne receivers, based on plasmonic photomixers have demonstrated high-sensitivity and broadband terahertz detection with high spectral resolution. They consist of a terahertz antenna with plasmonic contact electrodes fabricated on a photoconductive substrate. When the photomixer is pumped by two continuous-wave (CW) lasers with a terahertz beat frequency, the received terahertz signal by the antenna is down-converted to an intermediate frequency (IF) equal to the difference between the received terahertz frequency and optical beat frequency. The IF signal power increases quadratically with the optical pump power at low optical pump powers. However, the dependence of the IF signal on the optical pump power deviated from a quadratic behavior at high optical powers and could eventually saturate, limiting the maximum responsivity and signal-to-noise ratio (SNR). We demonstrate that large-area nanoantenna arrays, based on plasmonic gratings expand the device active area, raising the ceiling on optical power and, therefore, sensitivity.

Vector beams, enabling spatially dependent polarization states in the radial and azimuthal directions, have been intensively studied for various applications such as imaging, communication, and optical manipulation of magnetic materials. While liquid crystal-based Q-plates have been routinely employed to convert linearly polarized light to vector beams efficiently, their utility diminishes in the terahertz (THz) spectrum due to excessive absorption and large wavelengths. Alternatively, resonant metasurface-based Q-plates have been employed to demonstrate THz-vector beam generation, however, these Q-plates suffer from narrow operational bandwidth. We propose a method to design and fabricate twisted effective media-based Q-plate generating broadband terahertz vector beams. The twisted media consisted of stacked multiple layers of 270-um thick Si substrates with rotated line and space patterns following a specific twisting power- the angle per unit length along the beam propagation direction. By calculating the effective media with Berreman 4x4 method, we obtained the operating bandwidth of 0.5-1.5 THz (0.5-2.5 THz) with the twisting power of 22.5°/mm (4.5°/mm).

A high-speed line imaging scanner for walk-through security gates has been developed by combining a THz radar technology by using 275-305 GHz single transceiver with a high-speed mechanical beam scanner. This body scanner successfully visualizes concealed objects carried by pedestrians walking at more than 4 km hr^-1 with a single transceiver. Our other recent topics will be also presented.

Carcinogenesis involves DNA methylation which is an epigenetic modification in which a methyl (CH3) group is attached to cytosine, a nucleobase in DNA. Terahertz (THz) radiation can manipulate the degree of DNA methylation, the spectral characteristics of which exist in the THz region. Although properly controlled DNA methylation leads to appropriate regulation of gene expression, abnormal gene expression that departs from controlled genetic transcription through aberrant DNA methylation may induce cancer or other diseases. The DNA methylation has been directly assessed by THz time-domain spectroscopy and this epigenetic chemical change could be modified to the state of demethylation in cells using resonant terahertz radiation. Using an enzyme-linked immunosorbent assay, we observed changes in the degree of global DNA methylation in the SK-MEL-3 melanoma cell line under irradiation with 1.6-THz radiation with limited spectral bandwidth. Resonant THz radiation demethylated living melanoma cells by 19%. Our results show that THz demethylation has the potential to be a gene expression modifier with promising applications in cancer treatment.

We propose a fusion terahertz deep learning computed tomography framework designed to precisely reconstruct object 3D geometric information from THz temporal-spatio-spectral signals acquired through a terahertz time-domain spectroscopy system. This Unet-based fusion framework utilizes multi-scale branches for extracting spatio-spectral features, which undergo processing through an element-wise filter adaptive convolutional layer, resulting in high-quality restoration of THz 3D images. Furthermore, the proposed framework offers high scalability and adjustability, allowing users to choose their processing signal domains and seamlessly integrate their own modified fusion network.

In response to the hardware assurance concerns arising from the global offshore fabrication of IC packaging, this research focuses on creating a unique THz-TDS 'fingerprint' for each IC sample. This approach shifts the emphasis from detecting anomalies in packaging to ensuring the hardware's integrity through fingerprinting. The study utilizes both supervised and unsupervised machine learning models to assess the effectiveness of THz-TDS 'fingerprinting' for IC sample identification. Additionally, it examines the tolerance levels in THz-TDS data collection across various IC packaging types. The research involves collecting THz-TDS data from diverse IC packaging samples at multiple locations, aiming to determine how these variables affect the accuracy of sample identification.

This talk will consider the future of metal halide perovskite (MHP) photovoltaic (PV) technologies as photovoltaic deployment reaches the terawatt scale. The requirements for significantly increasing PV deployment beyond current rates and what the implications are for technologies attempting to meet this challenge will be addressed. In particular how issues of CO2 impacts and sustainability inform near and longer-term research development and deployment goals for MHP enabled PV will be discussed. To facilitate this, an overview of current state of the art results for MHP based single junction, and multi-junctions in all-perovskite or hybrid configurations with other PV technologies will be presented. This will also include examination of performance of MHP-PVs along both efficiency and reliability axes for not only cells but also modules placed in context of the success of technologies that are currently widely deployed.

The recent advent of robust, refractory (having a high melting point and chemical stability at temperatures above 2000°C) photonic materials such as plasmonic ceramics, specifically, transition metal nitrides (TMNs), MXenes and transparent conducting oxides (TCOs) is currently driving the development of durable, compact, chip-compatible devices for sustainable energy, harsh-environment sensing, defense and intelligence, information technology, aerospace, chemical and oil & gas industries. These materials offer high-temperature and chemical stability, great tailorability of their optical properties, strong plasmonic behavior, optical nonlinearities, and high photothermal conversion efficiencies. This lecture will discuss advanced machine-learning-assisted photonic designs, materials optimization, and fabrication approaches for the development of efficient thermophotovoltaic (TPV) systems, lightsail spacecrafts, and high-T sensors utilizing TMN metasurfaces. We also explore the potential of TMNs (titanium nitride, zirconium nitride) and TCOs for switchable photonics, high-harmonic-based XUV generation, refractory metasurfaces for energy conversion, high-power applications, photodynamic therapy and photochemistry/photocatalysis. The development of environmentally-friendly, large-scale fabrication techniques will be discussed, and the emphasis will be put on novel machine-learning-driven design frameworks that leverage the emerging quantum solvers for meta-device optimization and bridge the areas of materials engineering, photonic design, and quantum technologies.

The ability to directly probe ultrafast phenomena on the nanoscale is essential to our understanding of excitation dynamics in materials. However, achieving this capability has been challenging and is the focus of research in many labs around the world. Terahertz scanning tunneling microscopy, or THz-STM, is a powerful new technique that enables direct imaging of sub-picosecond dynamics in materials down to the atomic scale. This talk will discuss how THz-STM works, how it can provide new insight into ultrafast nanoscale dynamics of materials and devices, some of the current challenges of THz-STM, and future directions.

We show how lightwave-driven terahertz scanning tunneling microscopy can be used as a platform for atomic-scale terahertz time-domain spectroscopy. We apply our new technique to silicon-vacancy centers at the surface of GaAs and discover a single-atom resonator with features reminiscent of the technologically important DX centers.

Photoconductive antennas are at the forefront of THz source technology, and large aperture photoconductive antennas (LAPCAs) can generate intense THz pulses with peak fields surpassing 100 kV/cm. Despite the unique properties of these generated THz pulses - such as high asymmetry, main frequency components around 0.1 THz, and a significant ponderomotive potential - the widespread adoption of LAPCAs has been hindered by limitations in peak intensity and fragility. In this talk, we discuss recent advancements in wide bandgap semiconductor LAPCAs featuring an interdigitated structure, facilitating the shaping of intense THz pulses with various waveforms, ranging from asymmetric quasi-half-cycle to symmetric single-cycle pulses and allowing for tunable polarization. Additionally, we explore the nonlinear interaction of these pulses with an n-doped InGaAs thin film, where we report THz high-frequency generation.

Non-linear optical phenomena, such as parametric detection and amplification, manifest themselves in materials such as lithium niobate (LN) under the influence of a powerful optical pump beam. These processes have facilitated the practical realization of femtosecond (fs) pulse sources in the visible (VIS) and near infrared (NIR) spectra. They are also central to quantum detection, promising extremely sensitive detection of low-energy photons, particularly in the terahertz (THz) frequency range. To explore this innovative detection approach, we used an intense and powerful THz source taking advantage of optical rectification in lithium niobate (LN) crystals with an inclined-pulse front-end pumping configuration. By taking advantage of the high brightness of this source, we can acquire NIR signals in real time by upconversion and broadband using a standard CCD camera. In this presentation, we will look at the technical intricacies of the source and detection methodologies, as well as our goal of achieving single THz photon detection capability in the near future, all in the context of using ytterbium lasers.

Up to 10% beam-to-radiation energy conversion efficiencies have been obtained at the UCLA THz FEL using a strongly tapered helical undulator at the zero-slippage resonant condition, where a circular waveguide is used to match the radiation group velocity to the electron beam longitudinal velocity. This results in short, broad bandwidth pulses with high peak power that are measured with single shot electro-optic sampling. This allows full reconstruction of the THz temporal field profile at varying beam energies, giving insight into the complex dynamics within the FEL.

Optical microscopy has long been pursued for its potential to bridge the gap between nanoscale dynamics and macroscopic application of condensed matter physics. The advent of super-resolution techniques demonstrated the possibility to overcome the diffraction limit, while near-field microscopy, exploiting evanescent light fields coupled to sharp metallic tips, has achieved even higher resolution, only limited by the tip’s radius of curvature. Here, we exploit extreme atomic nonlinearities in tip-confined evanescent fields to achieve atomic-scale spatial and true subcycle temporal resolution. We uncover a novel non-classical near-field response characterized by an optical phase delay of the scattered light of ~π/2, enabling direct monitoring of ultrafast tunneling dynamics. Our method reveals nanoscale defects and captures current transients on semiconducting van-der-Waals materials with subcycle sampling, for the first time. This approach offers unprecedented insights into quantum light-matter interaction and electronic dynamics in conductive and insulating quantum materials at ultimate spatiotemporal scales.

The unique properties of terahertz (THz) radiation, coupled with its non-ionizing nature, enable high-contrast imaging of soft materials with less stringent safety precautions than those required for X-ray systems. In this study, we introduce novel methods for THz imaging using a system that employs two photoconductive antennas (PCAs): one as a continuous-wave (CW) source and the other as a coherent detector. To enable precise multi-band imaging with high frequency resolution, a fiber stretcher for rapid phase modulation was included. This system allows for the registration of two types of imaging contrasts: light attenuation and phase shift. We outline methods for extracting these contrasts and evaluate the system's capabilities through experiments involving various materials and frequencies. Furthermore, its application in computed tomography is demonstrated. To the best of our knowledge, this is the first application of a CW setup based on PCAs for non-destructive inspection and imaging of concealed objects.

Terahertz (THz) imaging has emerged as a powerful technique in diverse fields, ranging from medical diagnostics to material characterization. Due to its lower frequency, compared to X-ray-based techniques, THz imaging provides an alternative view into the internal structures of samples composed of low-density materials. Notably, phase contrast imaging is advantageous as it generally exhibits reduced distortion from refraction and reflection losses compared to attenuation contrast. However, in the course of THz phase measurements, a fresh issue arises. Due to the relatively wide beam shape of THz imaging combined with rapid phase wrapping, a new type of artifact emerges, posing a significant hindrance to the accuracy and reliability of the imaging process. This paper explores the intricate interplay between beam shape dynamics and phase-wrapping mechanisms in THz imaging systems, unraveling the source of the artifacts that arise from this phenomenon.

In this work optimization of technology of QCLs operating at λ~14 µm wavelength is presented. The devices were grown by Molecular Beam Epitaxy and combined MBE and MOVPE overgrowth. Above- Room temperature operation of QCLs was achieved. The influence of various parameters was investigated, including the waveguide design in terms of thickness, growth conditions as well as doping. The work also covers study of optical coatings pn device performance. Detailed electro-optical and spectral characterization of LWIR QCL was performed and important device parameters were extracted. The analysis of laser parameters is presented. Additionally, results of single mode operation are presented. Acknowledgements: This work was supported by Polish National Science Centre (NCN) grant: SONATA BIS: UMO-2021/42/E/ST7/00263

In this work we focus on one of the possible laser chip to optical waveguide integration approaches: hybrid horizontal integration, in which the laser chip is aligned with PIC waveguides (face-to-face) and mounted on PIC chip. We have analyzed how modified geometry of active region influences the performance of QCLs and the efficiency of the optical coupling. Quantum Cascade Laser active region geometry was modified by introducing the cavity spacers to reduce the vertical beam divergence. This modification is introduced toward more efficient integration in mid-IR Photonic Integrated Circuits based on Si-based waveguides. Decreasing the vertical divergence of laser beam can be beneficial for coupling more light into Ge waveguides. Large Optical Cavity AlInAs/InGaAs/InP Quantum Cascade Lasers with different thicknesses of active region were designed, grown by Molecular Beam Epitaxy (MBE) and fabricated. The devices were characterized including far-field beam measurements, showing narrowing of the vertical beam divergence. We have also performed test of coupling the laser light to passive waveguide platform.