Lianhe Li

Reproducibility in terahertz-frequency quantum cascade laser (THz QCL) growth by molecular beam epitaxy is commonly discussed in terms of controlling growth parameters, including growth rate, alloy composition, and doping level. Although control of these parameters is essential, the role of long-term dynamical stability in preserving the structural continuity of the cascade remains less well understood. Here, we provide experimental support for a physical picture in which discrete transient growth events can partition the active region into portions with different effective periodicities, thereby disrupting cascade continuity in a manner distinct from monotonic drift. Using in situ pyrometry phase analysis combined with high-resolution x-ray diffraction (HRXRD) and device characterization, we show that nominally identical GaAs/AlGaAs THz QCL structures can exhibit markedly different dynamical evolution during growth. These differences are reflected in pyrometry phase departure, satellite peak splitting, and device behavior consistent with a narrowed common transport window. Representative post-operation images of Ga-source components show liquid-Ga accumulation and morphologies consistent with redistribution near the evaporation aperture after extended operation. These observations, together with the pyrometry and HRXRD results, are consistent with a possible Ga-source contribution to transient growth instability during long-duration molecular beam epitaxy growth. The work also highlights the sensitivity of THz QCL structures as probes of transient epitaxial dynamics.

Terahertz (THz) quantum cascade lasers (QCLs) provide compact sources of coherent radiation in a spectral region that remains difficult to access with conventional semiconductor lasers. Yet, despite extensive studies of their frequency‐fluctuations, the frequency‐dependent signatures and microscopic origins of their intensity‐fluctuations are largely unexplored. Here, we measure the intensity‐noise power spectral density of single‐mode, frequency‐comb and harmonic‐frequency‐comb THz QCLs using ultrafast graphene thermoelectric nanodetectors integrated on optical platforms. Beyond the pronounced low‐frequency (1/f) noise associated with carrier transport through the heterostructure, we uncover distinct high‐frequency power‐law regimes that depend strongly on both comb state and active‐region design. Harmonic frequency combs exhibit, on average, up to ∼10% lower relative intensity noise than conventional frequency combs, despite their enhanced nonlinear modal dynamics. The relative intensity noise (RIN) decreases with the optical power P ₀ following the power law , with , revealing a crossover from spontaneous‐emission‐dominated fluctuations to regimes increasingly governed by non‐radiative carrier relaxation and electron–phonon scattering. These results establish a unified picture of intensity‐noise dynamics in THz QCLs and show that active‐region engineering and comb‐state control offer direct routes to tailoring fluctuations in intersubband lasers. Our findings provide a foundation for low‐noise THz sources for precision metrology, coherent spectroscopy, and quantum photonics.

We demonstrate broadband wireless transmission at 2.54 Terahertz (THz) using directly modulated THz Quantum Cascade Laser (QCL) and Direct Current-biased optical orthogonal frequency-division multiplexing (DCO-OFDM). Fixed quadrature-amplitude modulation (QAM) transmission was investigated for modulation bandwidths of 2, 3, 4, 5 and 10 GHz, achieving gross data rates from 17.99 to 39.98 Gbit/s with modulation formats ranging from 16-QAM to 512-QAM. The measurements reveal a trade-off between modulation bandwidth, modulation order and bit-error rate arising from the limited usable bandwidth of the THz transmitter. Adaptive bit and power loading was subsequently implemented using measured subcarrier channel information, yielding gross data rates of 44.0 Gbit/s and 54.3 Gbit/s at the Hard-Decision Forward Error Correction (HD-FEC) and Soft-Decision Forward Error Correction (SD-FEC) thresholds, respectively. These results establish adaptive OFDM as an effective approach for efficiently utilizing the available bandwidth and increasing the throughput of THz wireless communication systems. To the best of the authors' knowledge, this represents the highest reported wireless transmission data rate at carrier frequencies above 1 THz.

We demonstrate a near-field terahertz (THz) imaging modality based on asynchronous optical sampling of on-chip THz sensors, and apply it to the sub-wavelength imaging of silicone soft-tissue phantoms. The sensor consists of a planar Goubau-line waveguide that incorporates a quarter-wavelength stub band-stop resonator, with photoconductive switches integrated into the waveguide for broadband THz signal generation and detection. By raster-scanning soft-tissue phantoms through the evanescent electric field above the resonator using a robotic manipulator, we reconstruct two-dimensional THz images with spatial resolution down to 50 μm - more than one order of magnitude below the free-space wavelength at the operating frequency (355 GHz) - and with pixel acquisition times down to 100 ms. Image contrast is obtained using both frequency shifts of the resonator and changes in power transmission through the device, enabled by the broadband nature of the on-chip system. These results simultaneously establish the potential of on-chip near-field THz sensors as a platform for high-resolution imaging of soft tissue analogues, while also highlighting their potential for future applications in biomedical imaging and real-time sensing.

In this article, we report the design and experimental demonstration of a passive beam-shaping module integrated into a terahertz (THz) imaging system. THz quantum-cascade lasers (QCLs) inherently produce highly divergent beams, posing challenges for beam shaping and system integration. The proposed module, comprising a feedhorn, a standard waveguide, and a transmitting horn antenna, was optimized using electromagnetic simulations to minimize coupling loss while achieving favorable far-field beam characteristics. Sensitivity analyses quantified the impact of fabrication and alignment tolerances. Far-field measurements confirm the generation of a low-divergence, single-lobe Gaussian beam, supporting high-resolution THz imaging, and extending the utility of QCL-based systems to applications including biomedical diagnostics, security screening, and nondestructive testing.

Quantum cascade lasers (QCLs) are unipolar semiconductor lasers first demonstrated in 1994. Since then, they have played a central role in advancing mid-infrared and terahertz photonics, becoming among the most reliable light sources in these regions of the electromagnetic spectrum. Their importance is further reinforced by their ability to generate self-starting optical frequency combs, whose investigation is motivated both by fundamental physics and by a wide range of applications, including molecular spectroscopy and free-space optical communications. This Roadmap provides a unified overview of current advances and emerging directions in QCL research. The chapters are organized into three main sections: device design and technology; frequency combs and pulse formation; and applications of QCLs. Each chapter reviews the relevant background, summarizes the current state of the art, and identifies key challenges and future directions within its specific research area.

We show that the sensitivity of antennacoupled field-effect transistors (FETs) to terahertz (THz) radiation improves continuously with decreasing temperature. The noise-equivalent power (NEP) of 540 GHz patch-antenna-coupled FETs decreases as temperature reduces to 20 K. We project NEP values approaching $1\rm{--}2\rm{ pW}/\sqrt{\rm{Hz}}$ under efficient power coupling conditions (e.g., using a superstrate Si-lens), which is comparable to superconducting niobium transitionedge sensors (TESs) at 4 K. Building on these findings, a compact, low-noise, liquid-nitrogen-cooled (77 K) FETbased direct (incoherent) THz-power sensing system for spectroscopy applications was realized. Here, an $8 \times 8$ pixel-binned detector array fabricated in a commercial 65-nm Si-CMOS process, was optimized for operation in the 2.85-3.4 THz band. Characterization was performed in the focal plane of a 2.85-THz quantumcascade laser delivering $\sim 2\rm{ mW}$ of THz power. A linear dynamic range exceeding 67 dB was achieved without saturation (for 1 Hz-detection bandwidth). The system provides a -3 dB readout bandwidth of 5 MHz, exceeding that of conventional thermal detectors (typically 1 kHz). Combined with its broad temperature operability (20-300 K) and compact design, the system is particularly well suited for space- and payload-constrained platforms such as balloon- and satellite-based missions, where deep cryogenic cooling is impractical.

Terahertz-frequency (THz) carrier waves in free-space optical (FSO) communications offer the potential for > 1 Tbit/s data rates and stable latency. They offer wider bandwidths than available in the microwave region, together with reduced scattering and relaxed pointing requirements compared with visible and near-infrared regions. However, 1–10 THz FSO communications systems have thus far been limited to data rates several orders of magnitude lower than those of infrared systems. This work describes an experimental demonstration of multi-gigabit-per-second FSO communication using a THz quantum cascade laser (QCL), opening a new frontier for next-generation wireless communications. The FSO communication system consists of a 2.4 THz QCL source as the transmitter and a room-temperature Schottky barrier diode detector as the receiver. By directly modulating the terahertz QCL, we achieved non-return-to-zero on-off keying (NRZ-OOK) with a transmission rate of up to 4 Gbit/s. We evaluated the performance of the communication link by analyzing the bit error rate (BER) of the demodulated signal at the receiver while examining its relation to received optical power, QCL modulation power, and various bias points. Our work establishes the foundation for high-speed optical wireless communication based on terahertz QCL technology systems.

Topological lasers, born from the fusion of topological physics and lasing theory, offer an unprecedented mechanism to achieve coherent lasing. By utilizing topologically protected boundary states, they exhibit robustness against defects and imperfections. However, this mechanism faces a fundamental limitation: while relying on the bulk of a photonic topological insulator for topological protection, the lasing modes, defined as boundary states, must be evanescently confined to the boundary, leading to severely limited mode volume. Here, using the platform of topological quantum cascade lasers previously demonstrated with valley edge states, we demonstrate a substantial expansion of the lasing mode volume under topological protection. Unlike conventional designs, we introduce an additional bulk region featuring gapless Dirac points at the interface between topologically distinct domains. This Dirac region enables the topological lasing mode to uniformly extend across the entire region, dramatically increasing the lasing mode volume. The scaled-up mode volume enhances the robustness of the laser against defects of comparable size, outperforming unscaled designs. Our results resolve a fundamental challenge in topological laser design and also contribute to the development of cutting-edge quantum cascade lasers with enhanced efficiency and robustness.

Photonic technologies that exploit surface plasmons in graphene can offer groundbreaking opportunities for the development of compact and inexpensive active photonic devices, owing to the unique combination of tight field localization, giant optical nonlinearities and electrostatic gating tuning. Here we take advantage of this unique combination of properties to engineer frequency up-converted, electrically driven, single-mode photonic sources in the 9.0–10.5 THz range, with an emission frequency entirely tunable by design. We excite plasmons confined in a multilayer graphene micro-ribbon grating within a distributed-feedback terahertz quantum cascade laser that incorporates a top supercapacitor to tune the graphene Fermi energy, demonstrating third harmonic generation. Our monolithic, electrically driven laser works in the inaccessible Reststrahlen band of its core III–V semiconductor heterostructure and shows a peak power of ~9 μW, laying the foundation of a new generation of plasmonic, nonlinear light-emitting sources.