Recent works have suggested that transient suppression of a charge density wave (CDW) by an ultra-short excitation can lead to an inversion of the CDW phase. We experimentally investigate the dynamics of the CDW in K_0.3MoO_3 by time resolved x-ray diffraction after excitation with optical pulses. Our results indicate a transient inversion of the CDW phase close to the surface that evolves into a highly disordered state in less than one picosecond. Numerical simulations solving the Ginzburg-Landau equation including disorder from strong pinning defects reproduce our main observations. Our findings highlight the critical role of disorder in schemes for coherent control in condensed matter systems.

Topological Dirac and Weyl semimetals (D&W SM) have attracted a lot of attention due to their unusual electronic properties, which are strongly dependent on the symmetry of the system. By altering the symmetry of the lattice, using for example a pulse of light, one can expect to modify their topological nature and induce/suppress the related properties “on demand”. Highfield Terahertz (THz) radiation that interacts with vibrational states under near-resonant conditions is a promising pump candidate to induce structural changes in D&W SM compounds, possibly leading to topological transient or permanent changes. There are, however, some challenges to achieving such control. D&W SM have a specific band structure showing a 3D-linear dispersion around nodes, which makes them sensitive to THz-induced carrier multiplication. These carrier multiplication processes can compete with and ultimately screen the interaction of the THz light with vibrational modes. Here, we present recent results of 2D-THz spectroscopy of a bulk Weyl semimetal TaAs in reflection. We highlight THz-induced carrier multiplication effects and study the field and temperature dependence of the number of carriers induced by the THz field. We examine possible carrier multiplication mechanisms which could be involved.

Using two-dimensional THz spectroscopy in combination with numerical models, we investigate the dynamics linked to carrier multiplication caused by high-field THz excitation of the low-gap semiconductor InSb. In addition to previously observed dynamics connected with quasi-ballistic carrier dynamics, we observe other spectral and temporal features that we attribute to impact ionization for peak fields above 60 kV/cm, which continue up to the maximum investigated peak field of 430 kV/cm. At the highest fields we estimate a carrier multiplication factor greater than 10 due to impact ionization, which is well-reproduced by a numerical simulation of the impact ionization process which we have developed.

We demonstrate that electrically pumped random laser resonators, operating at terahertz (THz) frequencies, and comprising a quantum cascade laser heterostructure, can operate as sensitive photodetectors through the self-mixing effect. We devise two-dimensional cavities exploiting a disordered arrangement of surface holes that simultaneously provide optical feedback and allow light out-coupling. By reflecting the emitted light back onto the surface with random holes pattern, and by varying the external cavity length, we capture the temporal dependence of the laser voltage, collecting a rich sequence of interference fringes that follow the bias-dependent spectral emission of the laser structure. This provides a visible signature of the random laser sensitivity to the self-mixing effect, under different feedback regimes. The latter effect is then exploited, in the near-field, to demonstrate detectorless scattering near-field optical microscopy with nanoscale (120 nm) spatial resolution. The achieved results open up possibilities of detectorless speckle-free nano-imaging and quantum sensing applications across the far-infrared.

Near-field imaging techniques at terahertz frequencies (0.5-10 THz), conventionally rely on bulky laser sources and detectors. Here, we devise a compact configuration for scattering near-field nanoscopy based on quantum cascade lasers (QCL) that can simultaneously act as powerful THz source and phase-sensitive detector, exploiting optical feedback interferometry [1] , (see Fig 1a ). Self-detection is based on the reinjection of the field scattered by the AFM tip into the laser cavity causing coherent interference. The near-field scattering is measured through the induced changes in the contact voltage of the QCL. By changing the path length with a movable mirror, self-mixing interference fringes are acquired and allow to retrieve both the amplitude and phase of the scattered field giving access to the complex-valued dielectric response of the sample [2] . Interestingly for imaging applications, this detection approach is fundamentally limited only by electron transport in the QCL allowing for fast image acquisition.

Quasi-crystal distributed feedback lasers do not require any form of mirror cavity to amplify and extract radiation. Once implemented on the top surface of a semiconductor laser, a quasi-crystal pattern can be used to tune both the radiation feedback and the extraction of highly radiative and high-quality-factor optical modes that do not have a defined symmetric or anti-symmetric nature. Therefore, this methodology offers the possibility to achieve efficient emission, combined with tailored spectra and controlled beam divergence. Here, we apply this concept to a one-dimensional quantum cascade wire laser. By lithographically patterning a series of air slits with different widths, following the Octonacci sequence, on the top metal layer of a double-metal quantum cascade laser operating at THz frequencies, we can vary the emission from single-frequency-mode to multimode over a 530-GHz bandwidth, achieving a maximum peak optical power of 240 mW (190 mW) in multimode (single-frequency-mode) lasers, with record slope efficiencies for multimode surface-emitting disordered THz lasers up to ≈570 mW/A at 78 K and ≈720 mW/A at 20 K and wall-plug efficiencies of η ≈ 1%.

By lithographically patterning a sequence of air slits with different widths, following the Octonacci sequence, on the top metal layer of a double-metal quantum cascade laser, we demonstrate highly efficient surface emission at THz frequencies. The proposed versatile design allows either single-frequency-mode or multimode emission over a 530 GHz bandwidth, with peak optical powers of 240 mW (190 mW) in multimode (single-frequency-mode), record slope efficiencies up to ≈ 570 mW/A at 78 K and ≈ 720 mW/A at 20 K and wall-plug efficiencies η ≈ 1%.

Quantum cascade lasers are, by far, the most compact, powerful, and spectrally pure sources of radiation at terahertz frequencies, and, as such, they are of crucial importance for applications in metrology, spectroscopy, imaging, and astronomy, among many others. However, for many of those applications, particularly imaging, tomography, and near-field microscopy, undesired artifacts, resulting from the use of a coherent radiation source, can be detrimental. Random lasers can offer a concrete technological solution to the above issue. They, indeed, maintain a high degree of temporal coherence, as traditional lasers, while only exhibiting low spatial coherence, which can allow for the prevention of coherent artifacts, such as speckles. In this study, we report on the development of one-dimensional THz-frequency random wire lasers, patterned on the top surface of a double-metal quantum cascade laser with fully randomly arranged apertures, not arising from the perturbation of a regular photonic structure. By performing finite element method simulations, we engineer photonic patterns supporting strongly localized random modes in the 3.05–3.5 THz range. Multimode laser emission over a tunable-by-design band of about 400 GHz and with ∼2 mW of peak power has been achieved, associated with 10° divergent optical beam patterns. The achieved performances were then compared with those of perturbed Fabry–Perot disordered lasers, showing continuous-wave operation in the 3.5–3.8 THz range with an order of magnitude larger average power output than their random counterpart, and an irregular far field emission profile.

The increasing interest in manipulating electromagnetic waves in the terahertz (THz) spectral range has motivated, in the last decade, a great research effort in the development of miniaturized technologies for THz emission and detection. Recently, field effect transistors (FETs), exploiting semiconductor nanowires (NWs) as active element, have been proven successful in highly-sensitive room-temperature detection of THz frequency beams. The FETs act as rectifiers converting the impinging THz radiation into a DC signal between source (S) and drain (D), proportional to the absorbed power and controlled by the gate (G) bias. The THz-detection with the NW either exploits the photo-excitation of plasma waves [1], or, thermoelectric phenomena [2].

Random lasers are a class of devices in which feedback arises from multiple elastic scattering in a highly disordered structure, providing an almost ideal light source for artefact-free imaging due to achievable low spatial coherence. However, for many applications ranging from sensing and spectroscopy to speckle-free imaging, it is essential to have high-radiance sources operating in continuous-wave (CW). In this paper, we demonstrate CW operation of a random laser using an electrically pumped quantum-cascade laser gain medium in which a bi-dimensional (2D) random distribution of air holes is patterned into the top metal waveguide. We obtain a highly collimated vertical emission at ~3 THz, with a 430 GHz bandwidth, device operation up to 110 K, peak (pulsed) power of 21 mW, and CW emission of 1.7 mW. Furthermore, we show that an external cavity formed with a movable mirror can be used to tune a random laser, obtaining continuous frequency tuning over 11 GHz.

The quantum cascade laser (QCL) has been central to the development of terahertz (THz) science and technology in the last decade, thanks to its electronic and optical properties, which can be accurately tailored with quantum and photonic engineering. Control over the spatial and spectral features of THz QCL radiation has been addressed with different solutions, exploiting periodic structures such as edge-emitting 3rd-order distributed feedback lasers, plasmonic lasers, and one dimensional (1D) or two-dimensional (2D) photonic crystals. Novel resonators exploiting aperiodic photonic architectures recently emerged as a versatile approach to engineer the emission of THz QCLs, circumventing the power extraction issues of periodic resonators. Here, we report on the development of 1D and 2D THz QCL resonators providing a combination of high output power, controlled beam shaping and stable continuous wave (CW) operation. We devised a laterally corrugated wire laser with a top extraction hole array. The distinct periodicities of the lateral corrugation and the top hole array allow separate control of light feedback and extraction, optimizing the power output (42 mW in pulsed regime), slope efficiency (250 mW/A) and beam divergence (10°). Stable single-mode emission was achieved with a maximum CW power output of 6 mW. We furthermore demonstrate the first electrically pumped CW random THz lasers reaching a maximum peak power of 21 mW and a CW power of 1.7 mW with multimode emission over a 430 GHz-bandwidth. Using an external coupled cavity, a continuous tuning of 11 GHz and of 20 GHz with modehopping were demonstrated.

We observe the first evidence of self-mixing in THz frequency quantum cascade random lasers fabricated in both wire and irregularly squared resonator architectures with surface photonic patterns. By reflecting the emitted light back into the laser cavity and changing the external cavity length, we observe the interference fringes within the laser cavity, thereby proving evidence of self-mixing. This paves the way to detectorless speckle-free imaging applications in the far-infrared.

Terahertz (THz) quantum cascade lasers (QCLs) have undergone rapid development since their demonstration, showing high power, broad-tunability, quantum-limited linewidth, and ultra-broadband gain. Typically, to address applications needs, continuous-wave (CW) operation, low-divergent beam profiles and fine spectral control of the emitted radiation, are required. This, however, is very difficult to achieve in practice. Lithographic patterning has been extensively used to this purpose (via distributed feedback (DFB), photonic crystals or microcavities), to optimize either the beam divergence or the emission frequency, or, both of them simultaneously, in third-order DFBs, via a demanding fabrication procedure that precisely constrains the mode index to 3. Here, we demonstrate wire DFB THz QCLs, in which feedback is provided by a sinusoidal corrugation of the cavity, defining the frequency, while light extraction is ensured by an array of surface holes. This new architecture, extendable to a broad range of far-infrared frequencies, has led to the achievement of low-divergent beams (10°), single-mode emission, high slope efficiencies (250 mW/A), and stable CW operation.

Quasi-crystal structures are conventionally built following deterministic generation rules although they do not present a full spatial periodicity. If used as laser resonators, they open up intriguing design possibilities that are simply not possible in conventional periodic photonic crystals: the distinction between symmetric (vertically radiative but low quality factor Q) and anti-symmetric (non-radiative, high Q) modes is indeed here fully overcome, offering a concrete perspective of highly efficient vertical emitting resonators. We here exploit electrically pumped terahertz quantum cascade heterostructures to devise two-dimensional seven-fold quasi-crystal resonators, exploiting rotational order or irregularly distributed defects. By lithographically tuning the lattice quasi-periodicity and/or the hole radius of the imprinted patterns, efficient multimode surface emission with a rich sequence of spectral lines distributed over a 2.9–3.4 THz bandwidth was reached. We demonstrated multicolor emission with 67 mW of peak optical power, slope efficiencies up to ≈70 mW/A, 0.14% wall plug efficiencies and beam profile results of the rich quasi-crystal Fourier spectrum that, in the case of larger rotational order, can reach very low divergence.