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 > mW 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.