Low intensity saturation of an ISB transition by a mid-IR quantum cascade laser

TL;DR

This paper demonstrates room-temperature, moderate-intensity saturation of a mid-infrared intersubband transition using a specially designed GaAs/AlGaAs heterostructure, enabling potential mid-IR saturable absorber applications.

Contribution

It introduces a novel mid-IR saturable absorption system operating at moderate intensities, with a detailed theoretical model explaining the saturation behavior.

Findings

Saturation occurs at 10-20 kW/cm² at room temperature.

The system transitions from strong to weak coupling regime with increasing intensity.

Reflectivity behavior varies depending on laser tuning relative to polariton frequencies.

Abstract

We demonstrate that absorption saturation of a mid-infrared intersubband transition can be engineered to occur at moderate light intensities of the order of 10-20 kW$.$cm$^{-2}$ and at room temperature. The structure consists of an array of metal-semiconductor-metal patches hosting a judiciously designed 253 nm thick GaAs/AlGaAs semiconductor heterostructure. At low incident intensity the structure operates in the strong light-matter coupling regime and exhibits two absorption peaks at wavelengths close to 8.9 $\mu$m. Saturation appears as a transition to the weak coupling regime - and therefore to a single-peaked absorption - when increasing the incident intensity. Comparison with a coupled mode theory model explains the data and permits to infer the relevant system parameters. When the pump laser is tuned at the cavity frequency, the reflectivity decreases with increasing incident intensity. When instead the laser is tuned at the polariton frequencies, the reflectivity non-linearly increases with increasing incident intensity. At those wavelengths the system therefore mimics the behavior of a saturable absorption mirror (SESAM) in the mid-IR range, a technology that is currently missing.