Saturation characteristics of a 1.55 /spl mu/m GaSb/AISb MQW Fabry-Perot mirror

Using 1.55 μm, 150 fs pulses we have measured the excitonic saturation intensity and recovery time of a GaSb/AlSb MQW Fabry Perot mirror to be 25 MW/cm^2 and 600 fs, illustrating possible applications for mode locking lasers. Saturation Characteristics of a 1.55 μm GaSb/AISb MQW Fabry Perot Mirror K.C. Hall, Yu. Yashkir and H.M. van Driel Department of Physics, University of Toronto and Ontario Laser and Lightwave Research Centre, Toronto, Canada, M5S I A7 and A. Kost Hughes Research Laboratories 3011 Malibu Canyon Rd. Malibu CA 90265 The need for convenient passive mode locking elements in solid state and fiber lasers has led to the development of saturable absorbers based on quantum well structures incorporated into Fabry Perot mirrors [1]. Considerable success has been reported for GaAs/AlGaAs systems for use near 800 nm. Here we report measurements of the saturation and recovery characteristics of a GaSb/AlSb system which has favorable characteristics for mode locking lasers operating near 1.55 μm. Our sample consisted of 5 layers of an A1(0.37)Ga(0.63)Sb/AlSb as a quarter wave stack grown on a GaSb substrate; this structure provides a maximum reflectivity of 80%, although this can easily be increased by adding additional layers. Six GaSb QWs of thickness 8 nm with 8 nm AlSb barrier layers were grown on the surface of the Bragg mirror, followed by a cap layer of AlGaSb to protect the surface from oxidation; the MQWs gives rise to a heavyhole exciton absorption peak at 1.55 μm. Bulk GaSb is a weakly direct gap semiconductor with a 70 meV separation between and L conduction band edges. Confinement effects are expected to make the two edges nearly degenerate for our well widths. Our experiments employed a standard pump probe reflection geometry. The optical source is a Coherent Laser Systems model 9800 optical parametric generator, providing 150 fs pulses at 250 KHz, with 60 nJ/pulse. Timeresolved reflectivity measurements for pump / p robe wavelengths between 1.4 and 1.65 μm were carried out at room temperature. Fig. 1 shows that the transient reflectivity changes at 1.55 μm for a 90 MW/cm^2 peak pump intensity. The partial saturation of the exciton resonance is followed by a two component recovery with time constants of 600 fs and ~ 12 ps. The former is attributed to exciton ionization as induced by zone center LO phonons or perhaps even zone- boundary phonons taking electrons into Lvalley states. The longer time constant may reflect carrier recombination or evolution of exciton screening characteristics as free carriers cool. No significant pump induced change in reflectivity is observed for pump photon energies below the exciton energy as expected. Direct activation of continuum states by 1.45 μm photons yields a trace which is similar to that shown at 1.55 μm, although the peak of the signal is nearly 10 x smaller and temporally shifted. Similarity of the temporal characteristics of the two traces indicates that free carrier kinetics probably dominate exciton saturation recovery. Fig. 2 shows that the exciton resonance is saturated at a peak intensity of 25 Mw/cm^2; this intensity is close to what might be expected based on the need to provide one photon per exciton during its 600 fs lifetime and using an exciton radius of 11 nm for the 2D structures [2]. The fast recovery and favorable saturation characteristics point to application of these nonlinear mirrors as modelocking element in high repetition rate, short pulse solid state lasers operating at 1.55 μm. Unlike other IIIV based materials, to reduce recovery time the GaSb/AlSb system does not require extra ion implantation or low temperature growth procedures which can degrade sample quality. The ease of making the AlSb/GaSb mirrors on GaSb also offers advantages over other materials such as InAsP for which considerable difficulty exists in making good, lattice matched layer pairs with large refractive index differences on InP substrates. References [1] S. Tsuda, W.H. Knox, E.A. de Souza, W.Y. Jan and J. E. Cunningham, Optics Lett. 20,1406 (1995); L.R. Bovelli, U.Keller and T.H. Chiu, JOSA B 12, 311 (1995). [2] E. O. Gobel and K. Ploog, Prog. Quant. Electr. 14, 289 (1990)