When two resonant modes in a system with gain or loss coalesce in both their resonance position and their width, a so-called exceptional point occurs, which acts as a source of non-trivial physics in ...a diverse range of systems. Lasers provide a natural setting to study such non-Hermitian degeneracies, as they feature resonant modes and a gain material as their basic constituents. Here we show that exceptional points can be conveniently induced in a photonic molecule laser by a suitable variation of the applied pump. Using a pair of coupled microdisk quantum cascade lasers, we demonstrate that in the vicinity of these exceptional points the coupled laser shows a characteristic reversal of its pump dependence, including a strongly decreasing intensity of the emitted laser light for increasing pump power.
Abstract
Spectral fingerprints of molecules are mostly accessible in the terahertz (THz) and mid-infrared ranges, such that efficient molecular-detection technologies rely on broadband coherent light ...sources at such frequencies. If THz Quantum Cascade Lasers can achieve octave-spanning bandwidth, their tunability and wavelength selectivity are often constrained by the geometry of their cavity. Here we introduce an adaptive control scheme for the generation of THz light in Quantum Cascade Random Lasers, whose emission spectra are reshaped by applying an optical field that restructures the permittivity of the active medium. Using a spatial light modulator combined with an optimization procedure, a beam in the near infrared (NIR) is spatially patterned to transform an initially multi-mode THz random laser into a tunable single-mode source. Moreover, we show that local NIR illumination can be used to spatially sense complex near-field interactions amongst modes. Our approach provides access to new degrees of freedom that can be harnessed to create broadly-tunable sources with interesting potential for applications like self-referenced spectroscopy.
We present a method of coupling free-space terahertz radiation to intersubband transitions in semiconductor quantum wells using an array of meta-atoms. Owing to the resonant nature of the interaction ...between metamaterial and incident light and the field enhancement in the vicinity of the metal structure, the coupling efficiency of this method is very high and the energy conversion ratio from in-plane to z field reaches values on the order of 50%. To identify the role of different aspects of this coupling, we have used a custom-made finite-difference time-domain code. The simulation results are supplemented by transmission measurements on modulation-doped GaAs/AlGaAs parabolic quantum wells which demonstrate efficient strong light-matter coupling between meta-atoms and intersubband transitions for normal incident electromagnetic waves.
We study THz-emission from a plasma driven by an incommensurate-frequency two-colour laser field. A semi-classical transient electron current model is derived from a fully quantum-mechanical ...description of the emission process in terms of sub-cycle field-ionization followed by continuum-continuum electron transitions. For the experiment, a CEP-locked laser and a near-degenerate optical parametric amplifier are used to produce two-colour pulses that consist of the fundamental and its near-half frequency. By choosing two incommensurate frequencies, the frequency of the CEP-stable THz-emission can be continuously tuned into the mid-IR range. This measured frequency dependence of the THz-emission is found to be consistent with the semi-classical transient electron current model, similar to the Brunel mechanism of harmonic generation.
In this work, we demonstrate the direct observation of non-equilibrium intersubband dynamics in a modulation-doped multiple quantum well sample induced by intense terahertz pulses. The transmission ...spectra show a clear dependence on the incident THz field strength, which gives rise to a multitude of nonlinear optical effects that go beyond the standard textbook two-level description of light-matter interaction. Of special interest is thereby the multiple octave spanning bandwidth of the used single-cycle THz pulses, which allows the phase-locked coupling of adjacent intersubband transitions. Examples of this interaction include the efficient, coherent population transfer, the THz induced undressing of collective excitations, and the THz Stark effect.
We present the design, fabrication and characterisation of an intersubband detector employing a resonant metamaterial coupling structure. The semiconductor heterostructure relies on a conventional ...THz quantum-cascade laser design and is operated at zero bias for the detector operation. The same active region can be used to generate or detect light depending on the bias conditions and the vertical confinement. The metamaterial is processed directly into the top metal contact and is used to couple normal incidence radiation resonantly to the intersubband transitions. The device is capable of detecting light below and above the reststrahlenband of gallium-arsenide corresponding to the mid-infrared and THz spectral region.
We demonstrate an optical machine learning method in the terahertz domain, which allows the recognition of objects within a single measurement. As many materials are transparent in the terahertz ...spectral region, objects hidden within such materials can be identified. In contrast to typical object recognition methods, our method only requires a single pixel detector instead of a focal plane array. The core of the calculation is performed by a quantum cascade laser generated terahertz beam, which is spatially modulated at a near-infrared encoded silicon wafer. We show that this method is robust against displacements of the objects and noise. Additionally, the method is flexible and, due to the optically performed recognition task, inherently fast.
We present the design and the realization of active photonic crystal (PhC) semiconductor lasers. The PhC consists of semiconductor nanostructure pillars which provide gain at a quantized transition ...energy. The vertical layer sequence is that of a terahertz quantum cascade laser. Thereby, the artificial crystal itself provides the optical gain and the lateral confinement. The cavities do not rely on a central defect, the lasing is observed in flat-band regions at high symmetry points. The experimental results are in excellent agreement with the finite-difference time-domain simulations. For the vertical confinement a double-metal waveguide is used. The lasers are showing a stable single-mode emission under all driving conditions. Varying the period of the PhC allows to tune the frequency by 400 GHz.
Using millimeter wave spectroscopy and numerical simulations we extract the effective parameters of a layered metamaterial made from split ring resonators. In agreement with recent theoretical ...considerations the effective model for a bulk metamaterial strongly contradicts the experimental spectra. In contrast, the description within the concept of a layered metamaterial well reproduces the transmittance and reflectance data. We demonstrate that the thickness of the metamaterial layer is not a well-defined parameter of the model. Instead, the product of the layer thickness and the dielectric susceptibility can be used to account for the experiments and numerical simulations.
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