Mid–infrared semiconductor saturable absorber mirrors (SESAMs) based on intersubband polaritons

Ultrafast lasers revolutionized photonics by providing high peak intensities and pulse durations in the picosecond to femtosecond ranges. Passive mode-locking is the leading technique for generating short pulses, relying on saturable absorbers—nonlinear optical elements with intensity-dependent absorption. While semiconductor saturable absorber mirrors (SESAMs) are well-established for visible and near-infrared frequencies, a compact mid-infrared equivalent is lacking at longer wavelengths (λ > 3 µm). This thesis work proposes a novel mid-infrared SESAM concept based on the strong light-matter coupling between intersubband transitions in III-V semiconductor quantum wells and the resonant mode of metal-metal microcavities. Leveraging the strong field confinement in these structures, the saturation intensity of the intersubband transition is lowered to 10–20 kW cm−2, a level accessible by quantum (or interband) cascade lasers. We implemented a predictive tool based on coupled-mode theory to design the main figures-of-merit of a SESAM, such as modulation depth, non-saturable losses, low-intensity reflectivity and saturation intensity, exploiting both the sample geometry and the active region material. Time-domain pump–probe measurements revealed ultrafast decay times for these SESAMs of approximately 2 ps. These findings open the route towards applications with broad-gain laser sources such as mid-infrared fiber lasers, aiming to demonstrate self-starting passive mode-locking.

Members of jury :

• Maria AMANTI, Rapporteur & Examinatrice, Maîtresse de conference (HDR), MPQ Paris

• Sukhdeep DHILLON, Rapporteur & Examinateur, Directeur de recherche (HDR), LPENS Paris

• Aloyse DEGIRON, Examinateur, Directeur de recherche (HDR), MPQ Paris

• Virginie TRINITÉ, Examinatrice, Docteure Thales Palaiseau

Fig. The intersubband transition of a III-V semiconductor quantum well is embedded in an array of metal-metal microcavity resonators. The optical response of the sample is measured under low intensity - showing the appearance of intersubband polaritons in strong coupling - and high intensity - showing the absorption saturation of the intersubband transition. Pumping at the upper polariton frequency, the reflectivity of the device is measured varying the intensity of a quantum cascade laser. The result is the optical response of a semiconductor saturable absorber mirror (SESAM) tunable in the whole mid-infrared.