Fast recovery dynamics of GaSbBi-based SESAMs for high-fluence operation

Modelocked lasers operating in the 2–3 μm wavelength region are interesting for various spectroscopic applications. To this end, GaSb-based semiconductor saturable absorber mirrors (SESAMs) are developing fast as a practical technology for passive modelocking. Yet, such SESAMs suffer from either too high two-photon absorption or slow absorption recovery dynamics. This study introduces GaSbBi quantum wells (QWs) as a platform to ensure a larger material selection for engineering GaSb-based SESAMs with decreased two-photon absorption and ultrafast absorption recovery time. Three GaSbBi QW SESAM designs were fabricated to compare their performance against conventional GaInSb QW SESAMs. The first structure makes use of typical GaSb barriers and exhibits comparable characteristics to the conventional design, including a saturation fluence of 1.09 μJ cm−2, a modulation depth of 1.41%, and a fast interband recovery time of 6.03 ps. The second design incorporated AlAs0.08Sb0.92 barriers, achieving a reduced two-photon absorption, though at the cost of higher nonsaturable losses due to unintended Bi droplet formation during the growth of the AlAs0.08Sb0.92/GaSbBi QW heterostructure. Importantly, it maintained a fast interband recovery time (30 ps), overcoming the slow recovery dynamics exhibited by standard GaInSb QW SESAMs with AlAs0.08Sb0.92 barriers. The third design explored GaSbBi QWs with higher Bi content targeted for longer wavelength operation at 2.3 μm, which exhibited fast recovery times and good nonlinear reflectivity characteristics. However, the higher Bi content resulted in elevated nonsaturable losses. These results highlight the potential of GaSbBi QWs for short-wave infrared (SWIR) SESAMs, opening the path for further epitaxial optimization to enhance their performance.