This PDF file contains the front matter associated with SPIE Proceedings Volume 13141, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Frequency-Modulated (FM) combs based on active cavities, such as quantum cascade lasers, have recently shown potential as light sources across various spectral regions. Unlike passive mode-locking, which generates amplitude modulation from the field’s amplitude, FM comb formation relies on phase modulation from the field’s phase, essentially acting as a phase-domain version of passive mode-locking. While the fundamental scaling laws of passive mode-locking have been well-established since Haus’s 1975 work showing that the bandwidth of pulses mode-locked by a fast saturable absorber is proportional to the effective gain bandwidth, the limits of FM combs have been less clear. This talk will discuss our recent findings, demonstrating that FM combs based on fast gain media adhere to the same fundamental limits, resulting in combs with bandwidths linear in the effective gain bandwidth. Theoretically, I will show that the diffusive effect of gain curvature constrains comb bandwidth and experimentally how this limit can be increased, particularly focusing on terahertz quantum cascade lasers.

Integrated optical frequency combs based on active cavities like Quantum Cascade Lasers (QCLs) have emerged as promising light sources in the mid-infrared and terahertz (THz) spectral regions. Their bandwidths are limited by two fundamental parameters: dispersion, which originates from variation in the group velocity, and diffusion, which originates from variation in the gain. However, while dispersion has been extensively engineered, diffusion shaping has been elusive. In this work, we show that the addition of carefully engineered anti-diffusive loss can enhance the bandwidth of QCL combs. We demonstrate theoretically and experimentally that adding resonant loss to the cavity of a THz QCL can counteract the diffusive effect of the gain and allow broader bandwidth combs to form, fully exploiting the bandwidth and dynamic range of the gain medium. Our results give a new degree of freedom for creating active chip-scale combs and can be applied to a wide array of cavity geometries and comb systems.

Terahertz Quantum Cascade Lasers (QCLs) are crucial for advancing research due to their high-power output, compact size, and efficiency. These lasers, designed through precise intersubband structure engineering, often show experimental outcomes that diverge from theoretical prediction. This discrepancy highlights the need for experimental gain characterization of QCLs. Using terahertz time-domain spectroscopy and a uniquely structured QCL with two beam paths of identical curvature but different lengths, self-referenced gain measurements were conducted across various temperatures and biasing voltages. A dispersion correction technique was used to distinguish signals from the two paths. The gain profile was extracted by analyzing the spectra of pulses through these different paths, providing an accurate gain profile by negating the zero-bias loss profile’s impact. At 23 K, an absorption peak was identified at 2.37 THz under low bias, shifting to lower frequencies with increased bias, aligning with density matrix simulations. Beyond the lasing threshold, the peak gain at 3.2 THz was consistently around 0 cm⁻¹ for all bias levels. These findings underscore the self-referenced method’s significance in extracting absolute gain and dispersion information, enhancing device performance understanding.

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-As_xP_1-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 three-dimensional (3D) rectification effect in grating-gate InGaAs-channel High-Electron-Mobility Transistor (HEMT) THz plasmonic detectors. We demonstrate that the detector internal responsivity can be exponentially enhanced under a forward (positive) bias voltage application in the gate-readout configuration. Also, we demonstrate that the dc potential rise in the channel due to a strong dc current flow causes the degradation of the internal responsivity and can be suppressed by an appropriate grounding structure for the channel. A record internal responsivity of 1.2 A/W at 0.8 THz for grating-gate plasmonic detectors was achieved.

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 using the Selenium Vapor-Induced Supersaturated Solution Method (SVI-SSM), ensuring stoichiometric integrity. Heterostructures of Bi₂Se₃ were constructed using hot transfer methods, leading to 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 demonstrated a fast response time of 200 ps and a sensitivity of 40 mV/W with the THz detector, which was maintained even under zero bias conditions. These findings lay the foundation for developing passive THz detection systems with minimal energy consufmption, holding significant promise for advancing THz communication technology.

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 utilizing 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 minimizes the area of 2DEG covered by the antenna. We demonstrate experimentally a large photoresponse to 2.0 THz radiation of an AlGaAs/GaAs heterojunction-based PETS detector with our novel antenna design. Under the same operating conditions, this detector shows much larger photocurrent and two-times improvement in rise time compared to an identical PETS detector fabricated simultaneously on the same chip but instead incorporating a bow-tie antenna. Our findings help facilitate the development of future high-speed, low-noise, ultra-sensitive FIR detector arrays.

THz nonlinearity is reported in actively tunable integrated graphene/metamaterial arrays by using an ultrafast tabletop powerful time domain spectroscopic system with incident E-field energy pulses between 1 kV/cm and 100 kV/cm. The nonlinear interaction between the main metamaterial resonance and the sub-ps THz pulses produced a few distinct nonlinear phenomena, such as harmonic generation, optical Kerr effect. The complex dispersive properties of these devices pave the way for a plethora of novel functionalities, ranging from frequency generation to saturable absorbers.

Carcinogenesis involves DNA methylation which is a primary alteration in DNA in the development of cancer before genetic mutation. Because the abnormal DNA methylation is found in most cancer cells, the assessment of DNA methylation using terahertz radiation can be a novel optical method to detect and control cancer. The methylation has been directly observed by terahertz time-domain spectroscopy and this epigenetic chemical change could be manipulated to the state of demethylation using resonant terahertz radiation. Demethylation of cancer cells is a key issue in epigenetic cancer therapy and our results demonstrate the feasibility of the cancer treatment using optical technique.

The global outsourcing of semiconductor fabrication has led to hardware security concerns such as counterfeit Integrated Circuits (ICs) and Hardware Trojans (HTs), compromising the trustworthiness of semiconductor devices in critical applications. To address the issue of counterfeit ICs and HTs, various physical inspection methods have been developed. These methods, which include x-ray imaging, Scanning Acoustic Microscopy (SAM), and Scanning Electron Microscopy (SEM), are employed to detect irregularities within the packaging of ICs, aiding in the identification of counterfeit samples and the detection of HTs. Previous studies have shown that encapsulant material differences in counterfeit ICs can be detected by observing the refractive index variance between genuine and counterfeit products. This is achieved by measuring layer thickness and time delay in THz-TDS. THz-TDS employs a pulsed Terahertz signal to discern the effective refractive index differences between authentic and counterfeit IC packaging. However, anomaly detection often requires high resolution, which is time-consuming and necessitates standard samples for comparison, which are challenging to obtain. In this research, we focus on generating a THz-TDS ’fingerprint’ for each IC sample for hardware assurance, rather than detecting packaging anomalies. This paper explores using both supervised and unsupervised machine learning models to demonstrate the effectiveness of THz-TDS ’fingerprinting’ in IC sample identification. We also investigate the tolerance of THz-TDS data collection locations to identify various types of IC packaging. This involves collecting THz-TDS data from different IC packaging samples at multiple locations to assess the impact on accuracy in sample identification.

Photoconductive antennas are at the forefront of THz source technology, and the Large-Aperture Photoconductive Antenna (LAPCA) can generate intense THz pulses with peak fields surpassing 100 kV/cm. Despite the unique properties of these generated THz pulses—such as high THz field asymmetry, low central frequency around 100 GHz, and a significant ponderomotive potential—the widespread adoption of LAPCAs has been hindered by limitations in peak intensity and their fragility. In this paper, 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, for the first time, high-frequency generation.

We introduce a novel contrast mechanism in near-field microscopy which allows for all-optical atomic-scale microscopy with subcycle temporal resolution. To this end, we combine near-field microscopy with ultrahigh vacuum, low temperatures and sub-nanometer tip tapping amplitudes. On these scales, a surprisingly efficient non-classical near-field response occurs, which follows the vector potential of light and is strictly confined to atomic length scales. This ultrafast signal features an optical phase delay of ~π/2 and facilitates tracking of tunnelling dynamics. Our method reveals nanoscale defects and captures current transients on semiconducting van-der-Waals materials with subcycle sampling, allowing us to record the quantum flow of electrons 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 multiband 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.