Fast Recovery Dynamics of GaSbBi-Based SESAMs for High-Fluence Operation

Semiconductor saturable absorber mirrors (SESAMs) are essential for passive modelocking in compact ultrafast lasers. Modelocked laser sources operating in the short wave-infrared (SWIR) are peaking significant interest for various spectroscopic applications [1]. In this spectral region, the GaSb-based material system provides a rich playground for band-gap engineering. While InGaSb/GaSb 2-μm SESAMs have been used to sucessfully modelock various laser systems [2], they exhibit peculiar properties which make it difficult to increase the low rollover (F2) parameter without also significantly increasing the absorption recovery time [3]. To this end, this study introduces GaSbBi quantum wells (QWs) heterostructures as a novel absorber material for SESAMs operating above <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2 \mu \mathrm{m}$</tex>. Two GaSbBi-based SESAM structures (#1 and #2) were grown by molecular beam epitaxy (MBE). The target operation wavelength was at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.06\mu \mathrm{m}$</tex>. The design of SESAM #1 is shown in fig. 1a) where the absorber consists of two 10-nm thick GaSb<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.92</inf>Bi<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.08</inf> QWs embedded in GaSb and followed by a 21.5-pair lattice matched GaSb/AlAs<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.08</inf>Sb<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.92</inf> distributed Bragg reflector (DBR). A λ/4 Si<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>N<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> AR coating was PECVD deposited on top. For SESAM #2 the structure was grown identical to SESAM #1, however the absorber barrier material was changed from GaSb to AlAs<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.08</inf>Sb<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.92</inf> to increase the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{F}_{2}$</tex> parameter. Figure 1b) shows the measured reflectance and room temperature photoluminescence (PL) spectrum of SESAM #1. The reflectance is near 100 %, illustrating the high reflectivity of the DBR together with low absorbance of the GaSbBi QWs. The PL spectrum is very broad, essentially covering the full stop band of the DBR.