Self‐mixing interferometry and near‐field nanoscopy in quantum cascade random lasers at terahertz frequencies
Kimberly S. Reichel, Eva A. A. Pogna, Simone Biasco, Leonardo Viti, Alessandra Di Gaspare, Harvey E. Beere, D. A. Ritchie, Miriam S. Vitiello
Abstract
Abstract 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.