Alexander Giles Davies

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.

The intersection of topological photonics and nonlinear optics provides new opportunities for compact, tunable light sources. Here, we demonstrate even and odd harmonic generation, in the 6–10 THz range, in electrically driven micro‐lasers by integrating a set of different topological insulator (TI)–metamaterials. The unique surface states of TI, characterized by robust symmetry properties, enable enhanced nonlinear interactions when coupled with the tailored dispersion and field confinement of metamaterials. By leveraging the nonlinear susceptibilities of the composite structure by electrical gating, we demonstrate controlled generation of second‐ or third‐harmonic signals of up to 33 µW or 6.4 µW power, respectively, dominated either by the topological surface states or by the bulk. Moreover, by selectively pumping resonant infrared (IR) phonons with the intracavity‐generated THz pulses, we induce dynamic lattice distortions that modulate the electronic band structure and further enhance the nonlinear optical susceptibilities. The interplay between phonon‐driven lattice anharmonicities and the topologically protected surface states, leads to a pronounced enhancement in nonlinear THz response, offering new mechanisms for nonperturbative light–matter interactions in quantum materials. Our findings establish a novel versatile framework for compact, tunable sources of coherent radiation across multiple frequency bands, so far inaccessible in a solid‐state laser, and pave the way for applications in integrated photonics, on‐chip frequency conversion, and quantum information processing.

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.

Population inversion in quantum heterostructures underpins light generation in a wide range of photonic devices, including quantum cascade lasers and intersubband emitters. It is commonly assumed that increasing carrier density enhances inversion and optical gain; however, this trend can reverse at elevated densities due to many‐body interactions. Here, we investigate this behavior using a controlled doping series in terahertz quantum cascade lasers combined with self‐consistent nonequilibrium Green's function simulations. We show that Coulomb‐induced quasiparticle broadening increases with carrier density and competes directly with the intrinsic energy separation of the active states, reducing the spectral selectivity of optical transitions. We introduce a dimensionless spectral‐overlap parameter, Γ _avg /Δ, where Γ _avg represents the interaction‐induced optical linewidth and Δ is the interlevel energy separation, and demonstrate that gain across distinct device architectures follows a common trend with a maximum near Γ _avg /Δ ≈ 1. Beyond this point, spectral overlap suppresses inversion and limits optical gain. These results reveal a density‐driven crossover in which many‐body broadening sets a fundamental spectral limit on optical gain in quantum‐confined photonic systems. The Γ _avg /Δ parameter provides a universal design metric for identifying inversion limits and optimizing performance in quantum heterostructure light‐emitting devices.

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.

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.