Semiconductor Saturable Absorber Mirrors in the mid-IR

Saturation of the light-matter interaction is a general non-linear feature of material systems, be they atoms or semiconductors. A saturable absorber exhibits an absorption coefficient that depends on the incident intensity. In semiconductors, the possibility of judiciously controlling saturation phenomena is of great importance for fundamental physics as well as applications. A seminal example is the development of the semiconductor saturable absorber mirror (SESAM) based on interband transitions in quantum wells, that revolutionized the field of ultra-fast lasers in the vis/near-IR spectral range, allowing ultra-fast lasers pulses. Ultra-fast lasers based on SESAMs find applications in several domains, and even in quantum phenomena. In the mid-IR (around 10 micrometers in wavelength), the intensity required to reach saturation is very high, about 1 MW/cm2. This very high value explains why saturable absorbers, SESAM mirrors, bistable systems are missing from the current toolbox of mid-IR opto-electronic devices: they could only be used with extremely high-power laser sources and are incompatible with the output power levels of typical mid-IR semiconductor lasers such as quantum cascade lasers (QCLs). The absorption saturation can be engineered if the system operates in the so-called strong light-matter coupling regime. In this regime, the response is governed by coupled light-matter states called polaritons. The intersubband polaritonic SESAM resulting from this design shows ultra-low saturation intensities, that are compatible – for the first time – with commercial table-top QCLs. The results shown in this thesis report clear evidence of the non-linear optical response occurring in the device, witnessing a strong spectral feature in terms of change in reflectance by moving from low to high intensity condition. The experiments were performed by optical pumping with a tunable, commercial QCL to address the response of the system both under low and high intensity condition, showing the manifestation of saturation. This project opens exciting perspective in the realization of ultrafast, mode-locked mid-IR semiconductor lasers.