Jun Cheng Cao

Superconducting quantum capacitance detectors (QCDs) have emerged as among the most sensitive terahertz (THz) photon detectors, positioning them as promising detector components for next-generation cryogenic telescopes. However, their limited dynamic range (DR, ∼32 dB) and low saturation power (∼0.1 fW) have hindered their broader applications, such as in THz astronomy, high-resolution spectroscopy, and microscopic imaging. Here, we propose a mechanism that enables controllable adjustment of the sensitivity and DR of THz QCDs, supported by both theoretical modeling and experimental validation. We developed a QCD design that features a fork-shaped gate capacitor and a high-efficiency grid absorber. The resulting devices exhibit a noise equivalent power of 5.0 × 10−20 W/Hz1/2 and a DR of 40 dB, providing a favorable balance between sensitivity and DR and advancing the practical application of QCDs in future THz applications.

Terahertz metamaterials, composed of subwavelength artificial structures, exhibit strongly nonlinear and tightly coupled electromagnetic responses governed by geometry, material properties, and resonance modes. Driven by growing demands in high-sensitivity sensing, terahertz communication, and functional imaging, the design space of these devices has rapidly expanded, rendering conventional empirical and parameter-sweeping approaches inefficient for global optimization. Algorithm-driven strategies, particularly those leveraging machine learning and deep learning, have emerged as powerful surrogates for electromagnetic simulation, enabling automated multi-objective optimization and rapid inverse design. Beyond predictive capabilities, these algorithms uncover latent physical mechanisms and support adaptive, closed-loop experimental implementations. Within a unified framework, this review systematically categorizes and summarizes advances in algorithm-driven design of terahertz metamaterials, including traditional optimization methods, machine learning, deep learning, and reinforcement learning, highlighting their roles in multi-layer structural design, multimodal coupling control, and dynamic multi-target detection. Finally, we conducted a detailed analysis of current challenges, including data quality, model generalization, and physical interpretability, and clearly identified future research directions. These directions are expected to lead to practical applications in high-sensitivity chemical and biological detection, terahertz wireless communication, and wave-based functional imaging.

A mode-selective detection method utilizing self-mixing effects and a terahertz reflective blazed grating is presented. It enables mode selection in multimode terahertz quantum cascade lasers, based on the principle of single laser mode feedback. By adjusting the incident angle of grating and the current of terahertz quantum cascade laser, six distinct modes were gradually resolved across a grating rotational angle range of 0.8° and a side mode suppression ratio of approximately 5 dB was demonstrated with mode separation of 9.35 GHz. This method has potential applications in THz imaging and rapid gas sensing.

In this study, the transmission properties of flexible dielectric coated metallic hollow waveguides at 4.28 THz were studied. Waveguide samples was fabricated, characterized, and quantitatively analyzed including loss and mode structure. High-reflective silver and low-absorptive cyclic olefin polymer (COP) were selected and plated on the inner wall of flexible manufactured polycarbonate (PC) base tubes through dynamic liquid-phase coating method. Waveguides with a theoretically optimized dielectric film thickness and different inner diameters were characterized by using terahertz quantum cascade laser. Their mode structure and energy coupling efficiency with fundamental mode were analyzed. And distinctive results were obtained. The waveguide with 2mm inner has a quasi-single-mode property with coupling efficiency of 90.2%, corresponding to a gaussian field distribution. Moreover, the efficiency is above 89% even as the waveguide is bent to 45°. The transmission loss are 2.5 and 1.7 dB/m for the waveguides with 1mm and 1.5mm inner radius, respectively.

This paper investigates the turn-on delay and jitter characteristics of terahertz quantum cascade lasers (THz QCLs) under external influence. The results demonstrate that both external optical feedback and injection current can significantly influence the turn-on delay of THz QCLs. The delay time decreases with the increase of optical feedback strength and injection current and exhibits an additive effect that higher currents combined with stronger feedback yield shorter delays. The impact of optical feedback on jitter displays nonlinear behavior, i.e., optical feedback introduces additional intensity and phase noise superimposed on spontaneous emission-induced intensity noise. At weak optical feedback level, the jitter increases markedly and decreases after arriving at a maximum. At a moderate optical feedback level, the jitter stabilizes as the feedback intensity increases. At strong optical feedback levels, the jitter exhibits complex behavior. The cavity length of optical feedback exhibits periodic modulation on THz QCLs performance. When the phase of optical feedback aligns or opposes to the emitted phase, phase noise diminishes and jitter is significantly reduced. While for other cases, phase noise dominates at intermediate positions, inducing periodic jitter variations.

Terahertz (THz) technology has been widely used in astronomical observation where blocked impurity band (BIB) terahertz detectors are key components for acquiring THz signals in space scenario. In this study we proposed the integration of a plasmonic micro-nano structure onto Ge:Ga BIB detector and successfully realized the enhancement of the THz response performance. The BIB detector integrated with plasmonic structure shows a 64.5% improvement in blackbody responsivity under 4 K and 300 mV bias. The peak detectivity and noise equivalent power (NEP) are also significantly improved, 7.0×10¹⁴ Jones and 7.0×10^-17 W/Hz^1/2, respectively. It is revealed that hot-electron effect arisen from the plasmonic excitation plays key role in the detector performance improvement. This work provides a new perspective for the development of high-sensitivity THz detectors.

In the terahertz (THz) frequency range, chip-scale quantum cascade lasers (QCLs) are ideal for frequency comb and dual-comb operation. At present, QCL-based dual-comb spectrometers were normally constructed by using open-cycle liquid helium cryostats to provide cryogenic and non-vibrational environment for stable comb operation. Although the open-cycle liquid helium cooler without a cryogenic compressor is practically feasible for THz QCL dual-comb spectroscopy, the large consumption of liquid helium is the big barrier for applications. Here, we propose a self-detected dual-comb spectrometer deployed with a closed-cycle liquid helium cooler. By designing and implementing vibration suppression components, we demonstrate that the stable dual-comb operation can be obtained in a closed-cycle cooler system. Experimental results show that the “maxhold” dual-comb linewidths measured from the closed-cycle system are even smaller than those measured from an open-cycle cooler system. Furthermore, employing the developed dual-comb spectrometer housed in a closed-cycle liquid helium cooler, we measure the long-term frequency stability of the dual-comb signal for a time duration of 30 minutes, which shows a frequency drift of 3.25 MHz. To prove its spectroscopic capability, the phase-locking is applied onto the dual-comb system and then water vapor absorption measurements are performed. The established THz dual-comb spectrometer deployed with a closedcycle liquid helium cooler can largely reduce the running expense and facilitate mass measurements for practical applications.

The terahertz (THz) band is of immense interest in astronomy as it encompasses significant energy generated following the Big Bang, offering critical insight into processes invisible in other bands, such as the earliest stages of planet, star, and galaxy formation. Kinetic inductance detectors (KIDs) have emerged as a formidable contender in the field of THz astronomy, attributed to their exceptional sensitivity and scalability. In this study, we introduce a kind of KIDs incorporating a lumped element (LE) resonator design, with inductors fabricated on β-Ta film and capacitors on α-Ta film. We characterize the noise of the hybrid α-Ta/β-Ta LEKIDs, achieving an optical noise equivalent power of 8.3 ± 5.7 × 10−19 W/Hz1/2, demonstrating high sensitivity. Additionally, the LEKIDs exhibited stability across multiple thermal cycles. The combination of high sensitivity and stability makes the hybrid LEKIDs promising for the stringent demands of THz astronomy.