Thermal modeling of terahertz quantum-cascade lasers: comparison of optical waveguides

We compare a set of experimental lattice temperature profiles measured in a surface-emitting terahertz (THz) quantum-cascade laser (QCL) with the results of a 2-D anisotropic heat diffusion model. We evaluate the temperature dependence of the cross-plane thermal conductivity (kappaperp) of the active region which is known to be strongly anisotropic due to its superlattice-like nature. Knowledge of kappaperp and its temperature dependence is crucial in order to improve the temperature performance of THz QCLs and this has been used to investigate the longitudinal lattice temperature distribution of the active region and to compare the thermal properties of metal-metal and semi-insulating surface-plasmon THz optical waveguides using a 3-D anisotropic heat diffusion model

Near-field detection of gate-tunable anisotropic plasmon polaritons in black phosphorus at terahertz frequencies

Polaritons in two-dimensional layered crystals offer an effective solution to confine, enhance and manipulate terahertz (THz) frequency electromagnetic waves at the nanoscale. Recently, strong THz field confinement has been achieved in a graphene-insulator-metal structure, exploiting THz plasmon polaritons (PPs) with strongly reduced wavelength (λp ≈ λ0/66) compared to the photon wavelength λ0. However, graphene PPs propagate isotropically, complicating the directional control of the THz field, which, on the contrary, can be achieved exploiting anisotropic layered crystals, such as orthorhombic black-phosphorus. Here, we detect PPs, at THz frequencies, in hBN-encapsulated black phosphorus field effect transistors through THz near-field photocurrent nanoscopy. The real-space mapping of the thermoelectrical near-field photocurrents reveals deeply sub-wavelength THz PPs (λp ≈ λ0/76), with dispersion tunable by electrostatic control of the carrier density. The in-plane anisotropy of the dielectric response results into anisotropic polariton propagation along the armchair and zigzag crystallographic axes of black-phosphorus. The achieved directional subwavelength light confinement makes this material system a versatile platform for sensing and quantum technology based on nonlinear optics

Sculpting harmonic comb states in terahertz quantum cascade lasers by controlled engineering

Optical frequency combs (OFCs), which establish a rigid phase-coherent link between the microwave and optical domains of the electromagnetic spectrum, are emerging as key high-precision tools for the development of quantum technology platforms. These include potential applications for communication, computation, information, sensing, and metrology and can extend from the near-infrared with micro-resonator combs, up to the technologically attractive terahertz (THz) frequency range, with powerful and miniaturized quantum cascade laser (QCL) FCs. The recently discovered ability of the QCLs to produce a harmonic frequency comb (HFC)—a FC with large intermodal spacings—has attracted new interest in these devices for both applications and fundamental physics, particularly for the generation of THz tones of high spectral purity for high data rate wireless communication networks, for radio frequency arbitrary waveform synthesis, and for the development of quantum key distributions. The controlled generation of harmonic states of a specific order remains, however, elusive in THz QCLs. Here, and by design, we devise a strategy to obtain broadband HFC emission of a pre-defined order in a QCL. By patterning n regularly spaced defects on the top surface of a double-metal Fabry–Perot QCL, we demonstrate harmonic comb emission with modes spaced by an (n+1) free spectral range and with an optical power/mode of ∼270µW.</jats:p

Terahertz near-field microscopy of metallic circular split ring resonators with graphene in the gap

Optical resonators are fundamental building blocks of photonic systems, enabling meta-surfaces, sensors, and transmission filters to be developed for a range of applications. Sub-wavelength size (< λ/10) resonators, including planar split-ring resonators, are at the forefront of research owing to their potential for light manipulation, sensing applications and for exploring fundamental light-matter coupling phenomena. Near-field microscopy has emerged as a valuable tool for mode imaging in sub-wavelength size terahertz (THz) frequency resonators, essential for emerging THz devices (e.g. negative index materials, magnetic mirrors, filters) and enhanced light-matter interaction phenomena. Here, we probe coherently the localized field supported by circular split ring resonators with single layer graphene (SLG) embedded in the resonator gap, by means of scattering-type scanning near-field optical microscopy (s-SNOM), using either a single-mode or a frequency comb THz quantum cascade laser (QCL), in a detectorless configuration, via self-mixing interferometry. We demonstrate deep sub-wavelength mapping of the field distribution associated with in-plane resonator modes resolving both amplitude and phase of the supported modes, and unveiling resonant electric field enhancement in SLG, key for high harmonic generation

Temperature dependence of hermal conductivity and boundary resistance in THz quantumcascade lasers

We measured the lattice temperature (TL) distribution, the cross-plane thermal conductivity (k⊥), and the thermal boundary resistance (TBR) of the GaAs/Al0.15Ga0.85As quantum cascade lasers (QCLs) operating at 2.83 THz in the heat sink temperature range 45–300 K. This information was extracted fromthe analysis of microprobe band-to-band photoluminescence in QCLs operating in continuous wave. Both k⊥ and TBR decrease monotonically at increasing temperature, the main influence on k⊥ arising from the high density of interfaces

Nanoscale heat transfer in quantum cascade lasers

We have measured the local lattice temperature distribution and modeled the heat transport in all classes of quantum cascade lasers operating both in the mid-infrared and terahertz ranges. All relevant active regions based on GaAs/AlGaAs, GaInAs/AlGaAsSb, GaInAs/AlInAs/InP material systems have been investigated. A common feature of such complex multiple heterostructures is the strong anisotropy of thermal conductivity, its cross-plane component being much smaller than the in-plane one. Bulk contributions to this phenomenon are negligible, whereas a dominant role is played by the presence of abrupt sub-nanometer sized interfaces. The presence of a high density of interfaces causes phonon interference effects, which inherently limit the heat extraction. The values of the thermal boundary resistance have been extracted from our experimental data and compared among several devices. The possibility of generating stimulated emission of phonons in terahertz quantum cascade lasers will be also discussed