Novel AlGaAs/CaF 2 SESAM Device for Ultrashort Pulse Generation
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Novel AlGaAs/CaF2 SESAM Device for Ultrashort Pulse Generation Silke Schön, Lukas Gallmann, Markus Haiml and Ursula Keller Swiss Federal Institute of Technology (ETH), Physics Department / Institute of Quantum Electronics ETH Zurich Hoenggerberg - HPT, CH-8093 Zurich, Switzerland ABSTRACT A novel ultrabroadband AlGaAs/CaF2 semiconductor saturable absorber mirror (SESAM) covering nearly the entire Ti:sapphire gain spectrum is demonstrated. This device supports sub10-fs pulse operation of a laser. In contrast to previous SESAMs of comparable bandwidth, our device can be monolithically grown by molecular beam epitaxy and requires no post-growth processing. GaAs is used as semiconductor saturable absorber material. The high defect concentration of the material is due to the lattice-mismatched growth on a fluoride surface with (111) orientation. With a time response of 1.2 ps for carrier trapping, a saturation fluence of 36 µJ/cm2 and a modulation depth of up to 2.2% , the GaAs saturable absorber is well-suited for alloptical switching in SESAM devices used for ultrashort pulse generation. INTRODUCTION Devices based on semiconductor materials have been found many applications in ultrafast all-optical switching, in femto- and picosecond laser pulse generation, and in optoelectronics [1]. For example, semiconductor saturable absorber layers were introduced in optical devices to explore their nonlinear optical property of an intensity depending absorption for all-optical switching [2]. A semiconductor saturable absorber mirror (SESAM) basically consists of a high reflection mirror with a reflectivity of at least 95%, and a saturable absorber material. SESAMs rely on the operation of the absorber layer as an all-optical switch that is based on changes of reflectivity due to absorption bleaching induced by a strong laser pulse allowing for the selfstarting of a laser. Conventional SESAMs have been successfully fabricated by molecular beam epitaxy (MBE) using AlGaAs/AlAs multilayer stacks and GaAs saturable absorber. Laser pulses as short as 34 fs were generated with Ti:sapphire lasers using an AlGaAs/AlAs Bragg mirror with an imbedded single GaAs quantum-well layer [3]. However, these conventional SESAMs suffer from a narrow bandwidth caused by a small difference in the refractive indices of AlAs and GaAs. Shorter pulses were obtained by replacing the AlGaAs/AlAs Bragg mirror by a silver mirror in order to increase the high reflection bandwidth. This kind of a low-finesse AFPSA device, where the top reflector is formed by the interface semiconductor/air, supported sub-6-fs pulses [4]. These pulses are some of the shortest ever generated. However, the semiconductor saturable absorber cannot be directly grown on top of a silver mirror by epitaxy. Post-growth processing has to be applied, which complicates the fabrication process and may cause additional nonsaturable losses. Moreover, the silver bottom mirror only provides a reflectivity of less than 97%. Therefore, methods are continuously in development to avoid post-growth pr
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