Frequency-tunable continuous-wave random lasers at terahertz frequencies

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.

Unveiling the detection dynamics of semiconductor nanowire photodetectors by terahertz near-field nanoscopy

Semiconductor nanowire field-effect transistors represent a promising platform for the development of roomtemperature(RT)terahertz(THz)frequency light detectors due to the strong nonlinearity of their transfer characteristics and their remarkable combination of low noise-equivalent powers(<1 nW Hz^(−1/2))and high responsivities(>100 V/W).Nano-engineering an NW photodetector combining high sensitivity with high speed(subns)in the THz regime at RT is highly desirable for many frontier applications in quantum optics and nanophotonics,but this requires a clear understanding of the origin of the photo-response.Conventional electrical and optical measurements,however,cannot unambiguously determine the dominant detection mechanism due to inherent device asymmetry that allows different processes to be simultaneously activated.Here,we innovatively capture snapshots of the photo-response of individual InAs nanowires via high spatial resolution(35 nm)THz photocurrent nanoscopy.By coupling a THz quantum cascade laser to scattering-type scanning near-field optical microscopy(s-SNOM)and monitoring both electrical and optical readouts,we simultaneously measure transport and scattering properties.The spatially resolved electric response provides unambiguous signatures of photo-thermoelectric and bolometric currents whose interplay is discussed as a function of photon density and material doping,therefore providing a route to engineer photo-responses by design.

Highly efficient surface-emitting semiconductor lasers exploiting quasi-crystalline distributed feedback photonic patterns

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 onedimensional 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 240mW(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%.

Spatial coherence of electrically pumped random terahertz lasers

Light sources with high radiance and tailored coherence properties are highly desirable for imaging applications in the mid-infrared and terahertz(THz) spectral regions, which host a large variety of molecular absorptions and distinctive fingerprints to be exploited for sensing and tomography. Here, we characterize the spatial coherence of random multimode THz quantum cascade lasers(QCLs) emitting > m W optical power per mode and showing low divergence(10°–30°), performing a modified Young’s double-slit experiment. Partial spatial coherence values ranging between 0.16 and 0.34 are retrieved, depending on the specific degree of disorder. These values are significantly lower than those(0.82) of conventional Fabry–Perot THz QCLs exploiting an identical active region quantum design. We then incorporate the devised low spatial coherence random lasers into a confocal imaging system with micrometer spatial resolution and demonstrate notable imaging performances, at THz frequencies,against spatial cross talk and speckles.