Fabrication and performance of GaAs-MQW nonlinear directional coupler

Abstract

Modified fabrication technique has been developed to improve the performance of GaAs/A1GaAs MQW nonlinear directional couplers for all-optical picosecond and subpicosecond switching and modulation. A nonlinear directional coupler (NLDC) capable of switching data streams and demultiplexing signals could be an important component in switching networks. Reliable and reproducible performance depends on the proper technique used to fabricate these devices. Previously, we demonstrated all-optical switching in GaAs/AlGaAs multiple quantum well (MQW) nonlinear directional couplers using 10 ps pulses1 where the origin of the nonlinearities was due to photo-excited real carriers. The contrast of the PS switching was from 1 : 3 to 3 : 1 . We have also observed subpicosecond all-optical modulation in the same device using 150 fs pulses2 where the origin of such ultrafast modulation is attributed o optical Stark effect3. The contrast of the fs modulation was from 1 :2.3 to 1 : 1 .2. Recently, we improved the fabrication procedures. This results in improved performances in both the picosecond and subpicosecond regimes. The MQW waveguide structure was designed to sustain a single planar mode for wavelength close to the absorption edge using a four-layer waveguide model. The effective index method4 was then used to model the strip-loaded waveguide performance to ensure single mode operation. The sample is grown by molecular beam epitaxy (MBE) and has 1 .2 am thick guiding region which consists of 60 periods of alternating 100 GaAs wells/lOO Al023Ga72As barriers. The guiding in the direction perpendicular to the MQW is provided by the AlGaAs layers above and below the MQW region whereas the confinement in the horizontal direction is facilitated by the 2 ,am wide ridge etched into the top AlGaAs layer. The ridge structures are formed by first patterning the sample through contact-print photolithography using positive photoresist (KTI 820), and then reactive ion etching the top AlGaAs layer with photoresist mask. The etch process uses pure BC!3 as an etching gas flowing at 25 sccm with a pressure of 45 mTorr. The power density delivered is 0.43 W/cm2 and the self bias potential is 1 82 V. A Si wafer is laid on the bottom electrode on top of which the sample is placed. After etching, the couplers are cleaved on both ends to allow light to be end-fire coupled into the guides. We have since improved the fabrication procedures. First, previous studies show that photolithography is a problem when the guide separation is only 1 m whereas a guide separation of 4 m is too large for efficient coupling. A mask is thus made that consists only of guides separated by 2 or 3 'am. Second, more care is taken to prevent dust from falling on the sample during the photolithographic process in our non-clean lab environment. Third, a new cleaving device is assembled using a phonographic needle and an x-y-z translation stage. With the help of a stereo microscope, devices as short as 100 1am can be cleaved with high-quality end surfaces. See Fig. 1. These improvements not only enhance our yield so that nonlinear coupling behavior is observed in almost every pair of guides but also allows the light to be coupled into each guide with ease, greatly shortening the alignment time.