176 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 2, FEBRUARY 1997 Waveguide Microcavity Based on Photonic Microstructures Thomas F. Krauss, Brigitte V¨ ogele, Colin R. Stanley, and Richard M. De La Rue Abstract—A waveguide based microcavity exhibiting a quality factor 2500 has been realized by incorporating a phase shift into a 1-D photonic microstructure. The microstructure has an overall length of 3 m, consists of a deeply etched grating with very narrow (75 nm) air-gaps and exhibits a third-order stop band in the 800–900 nm wavelength regime. A comparison between measurement and simulation suggests that there is a thin (approximately 18 nm) skin of oxidized material at the etched semiconductor–air interfaces. Index Terms— gratings, integrated optics, microcavity, pho- tonic microstructures, semiconductor waveguides. I. INTRODUCTION T HE QUEST for miniaturization in optoelectronics is approaching its fundamental limit, the wavelength of light. Following the size reduction achieved by vertical cavity surface emitting lasers (VCSEL’s) [1], microdisks [2], and microrings [3], micrometer-size waveguide devices based on photonic microstructures have recently appeared. Examples include air bridge microcavities or photonic wires [4], [5] and very compact DBR mirrors [6], [7]. One of the major goals of these devices is to control much of the internally generated spontaneous emission and channel it into the cavity emission mode, thereby obtaining a large -factor [8], [9]. Other applications for photonic microstructures are nonlinear devices [10] and very low loss waveguides [11], [12]. The microcavity described here consists of a defect in a high contrast grating or 1-D photonic bandgap (PBG) structure. The waveguide core and cladding consist of semiconductor material since we are aiming at devices with current injection capability. Following the proof-of-principle of this approach [7], our objective was to determine to what extent high - factor microcavities could be realized with deeply etched Bragg mirrors in a waveguide geometry. Our approach differs from that of others since we fabricate very narrow slots to form the grating in order to increase the confinement of the optical mode and to maximize the fraction of material with potential for light emission [7], [13]. II. DESIGN AND FABRICATION The structure was designed to exhibit a third-order stopband in the 800–900 nm wavelength range. Since the stopband was Manuscript received August 30, 1996; revised October 31, 1996. The work of T. F. Krauss was supported by a Royal Society Research Fellowship. The authors are with the Optoelectronics Research Group, Department of Electronics and Electrical Engineering, Glasgow University, Glasgow G12 8LT, Scotland, U.K. Publisher Item Identifier S 1041-1135(97)01203-2. Fig. 1. Micrograph of the photonic lattice based microcavity. The grating region is 3 m 4 m wide and the waveguide is etched 1.5 m deep; the etch depth for the narrow slots is approximately half as much, i.e., 0.7 m. Fig. 2. Simulated transmission of the 440 nm grating of Fig. 1 between 700–1100 nm for TE polarization. expected to be wider than the tuning range of 820–900 nm readily available from our Ti Sapphire laser, we varied the periodicity in the range 400, 420, , 480 nm. All structures consist of 7 “fins” of semiconductor separated by 75-nm slots, the center fin being 60 nm wider in order to form a ( ) defect state in the center of the stop band (Fig. 1). The defect is barely discernible in the micrograph because it is only 15% wider than its neighbors. Fig. 2 shows the expected transmission characteristic of the 440-nm structure which was simulated using a 1-D transfer matrix routine (multilayer stack theory) [14]. The features on either side of the stopband (peaks at wavelengths of 770 and 970 nm) can be considered as bound band-edge states and only appear so clearly because the lattice consists of very few elements. The defect state can be seen as a sharp spike in the center of the stopband at 845 nm. The MBE-grown wafer consists of a 0.4 m Al Ga As waveguide core ( ) on top of an Al Ga As cladding ( ) [7]. Since the waveguide core layer is at the surface of the epitaxial material, the top cladding is air. The pattern 1041–1135/97$10.00  1997 IEEE