Graphene-loaded metal wire grating for deep and broadband THz modulation in total internal reflection geometry

We employed a metallic wire grating loaded with graphene and operating in total internal reflection (TIR) geometry to realize deep and broadband THz modulation. The non-resonant field enhancement effect of the evanescent wave in TIR geometry and in the subwavelength wire grating was combined to demonstrate a ～77% modulation depth (MD) in the frequency range of 0.2–1.4 THz. This MD, achieved electrically with a SiO_2∕Si gated graphene device, was 4.5 times higher than that of the device without a metal grating in transmission geometry. By optimizing the parameters of the metallic wire grating, the required sheet conductivity of graphene for deep modulation was lowered to 0.87 mS. This work has potential applications in THz communication and real-time THz imaging.

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.

All dielectric metasurfaces for spin-dependent terahertz wavefront control

Metasurfaces consisting of artificial subwavelength structure arrays have shown unprecedented ability to manipulate the phase,amplitude,and polarization of light.Separate and complete control over different spin states,namely the orthogonal circular polarizations,has proven more challenging as compared to the control over orthogonal linear polarizations.Here,we present and experimentally demonstrate several spin-dependent wavefront control metasurfaces in the terahertz regime using all-silicon dielectric structures.Such spin-dependent allsilicon metasurfaces are easy to fabricate and have potential applications in spin-involved ultracompact and miniaturized terahertz optical systems as well as terahertz communication systems.

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.