Nonlinear Optical Absorption In InGaAs/InP And InGaAs/InA1As Multiple Quantum Well Structures

Abstract

Nonlinear optical absorption in InGaAs/InP and InGaAs/InAlAs multiple quantum well structuresD.A.H. Mace, M.A. Fisher, M.G. Burt, E.G. Scott, J.N. Tothill, K.J. Monserrat and MJ. AdamsBritish Telecom Research Labs, Martlesham Heath, Ipswich, UK, IPS 7REABSTRACTWe have measured the intensity dependence of the room temperature optical transmission of an multiple quantum well (MQW1) and an In 53Ga ^Asfln 52A1 4gAs multiple quantum well (MQW2) from 1.5)zm to 1.7pm. The absorption is calculated from the transmission taking into account the wavelength dependence of the reflection coefficients. At the edge of the 1H-1C transition absorption band the absorption is found to fall by 1/2 if the intensity is 16kWcm2 and ISkWcm2 gl the surface of MQW1 and MQW2 respectively and is completely saturated at intensities exceeding 10' Wcmf2 , showing no nonsaturable absorption. A theoretical model is described, which fits the intensity dependence of the absorption right up to saturation at two wavelengths with an assumed carrier lifetime of 0.66ns for MQW1 and 0.75ns for MQW2.1. INTRODUCTIONSemiconductor multiple quantum wells show promise for nonlinear optical devices because their optical properties may be tailored. Some assessment of optical nonlinearities may be made by using intensity dependent optical absorption (IDA) spectroscopy. In this technique, the absorption is calculated from the measured optical transmission. The multilayer structures that have been studied using this technique are AlGaAs/GaAs1'2^, InGaAs/InP4'5, and InGaAs/InAlAs . We report further IDA spectra on InGaAs/InP and InGaAs/InAlAs multiple quantum well structures at different wavelengths and over intensities sufficient to saturate the band edge absorption. In addition we present a simple theory, which has been fitted to the IDA spectra with the recombination time of the carriers as the only adjustable parameter.2. THE MQW STRUCTURESMQW1 consists of 100 periods of undoped S.lnm In53Ga4yAs wells and lO.Onm InP barriers and MQW2 is 66 periods of 10.9nm In^Ga^yAs wells and 2.7nm ^57^48^ oarriers followed by a similar multilayer structure where the width of the wells andparriers are 5.5nm and 2.7nni respectively. This latter multilayer structure whose band edge was at 1.46pm, was not studied in the following measurements. Both MQW1 and MQW2 were grown lattice matched onto InP substrates. MOCVD and MBE were used for the MQW1 and MQW2 growth respectively. The total period thickness was measured using X-ray diffraction. The InP was carefully polished to minimise laser scatter and both epilayer and substrate surfaces were anti- reflection coated with 220nm of silicon nitride to minimize Fabry-Perot effects between 1.5-1.7pm.3. MEASUREMENT TECHNIQUEThe transmission spectrometer, shown schematically in figure 1, was used to obtain the IDA spectra. Continuously tunable pulsed infra-red radiation from 1.48-2.0pm is generated by mixing 1.064pm radiation from a Q-switched Nd-Yag laser with radiation from a dye laser (using DCM dye) in lithium niobate. The pulses, generated at lOHz are focussed onto the sample and the transmitted light is collimated and focussed onto a Ge detector (D^). The intensity at the sample surface is altered by placing calibrated neutral density filters either side of the sample. Fluctuations of ± 20% in the intensity occur because of multimode instabilities in the lasers. Consequently a portion of the incident