Coherent imaging and sensing using the self-mixing effect in THz quantum cascade lasers

We present recent advancements in the development of coherent THz imaging and sensing systems that exploit the self-mixing (SM) effect in quantum cascade lasers (QCLs). SM occurs when radiation from a laser is partially reflected from an external object and injected back into the laser cavity. The reflected radiation interferes („mixes‟) with the inter-cavity field, producing variations in the emitted power and terminal voltage. Thus, by combining the local oscillator, mixer, and the detector all in a single laser, this technique allows the development of simple, self-aligned systems that can sense both the phase and amplitude of the THz field reflected from samples. We demonstrate the coherent nature of this sensing technique for depth-resolved reflection imaging, whereby the phase-shift induced upon reflection is interpreted in terms of surface morphology of the sample. We will also present an alternative, novel sensing modality based on this self-mixing approach.