Directional light coupling from microdisk lasers

Recently we have investigated the possibility of fabricating optically active microstructures which may potentially be of use in future photonic or optoelectronic circuits. As a first step in this direction we demonstrated the concept of a whispering-gallery mode microdisk laser. ly2 The device makes use of optical modes at the edge of a thin semiconductor dielectric disk which is 2-10 pm in diameter and suspended in air or low refractive index material such as SiO,. Optical gain is provided by one or more optically or electrically pumped InGaAs quantum wells in the plane of the disk. The disk thicknesses are approximately ,%/4~2,~-0.15 ,um for a wavelength il = 1.5 pm. The effective’ disk index of refraction corresponding to this thickness is nCr=2.55. The high index contrast ratio between the disk and its surroundings is a key feature since it strongly confines active optical modes to the plane of the disk thereby ensuring large modal overlap with the quantum well gain region. The high index contrast ratios also enhance the Q value and mode selectivity of the microcavity. Our initial experiments’ and all of the results described here use optical pumping at liquid nitrogen temperatures to achieve lasing. However, we have also recently demonstrated lasing action in similar microdisk structures using electrical pumping at room temperature.l An ideal microdisk laser of the type described above emits radiation in a radially symmetric pattern which is confined to a small range of angles centered on the plane of the disk. However, to be of practical use, we believe that it is necessary to control both the direction and intensity of light output both in the disk’s plane and in the vertical direction perpendicular to the disk plane. The purpose of this letter is to report results of our initial experiments aimed at achieving this goal. For larger semiconductor whispering-gallery mode lasers with diameters in the 100 pm range, Y-couplers3*4 or cleaved faces’ can be used as output couplers. For the microdisk case these more traditional couplers cannot be fabricated at present. First consider the high quality microdisks that we have fabricated using photolithography and selective etching techniques. ‘Z These disks are very nearly circular but do exhibit some edge roughness in the form of variations in radius as large as 0.1 ,um with a scale length around the disk of -0.2 pm. A second major deviation from circular symmetry is the nearly square InP support post that can act as an effective grating scattering light out of whispering-gallery modes. This post can also introduce coupling between the whispering-gallery mode nearest the edge and lower order modes that penetrate further into the interior of the disk. It also damps the lower order modes that overlap the post region. It is convenient to parameterize the degree of damping in a given mode by its quality factor6 Q. The Q values for our microdisks with absorbing InGaAs quantum wells are typically between 100 and 750 as measured from the shape of the low power InGaAs photoluminescence spectrum near a microcavity resonance. As may be seen in Fig. 1, Q=il/A/2-1.5 ym/lO nrn2: 150, corresponding to a cavity finesse of F=FSR/ ALE 10 where the free spectral range is FSR100 nm for a 2.2 pm diam disk. These data are taken with optical excitation power as low as a factor of 25 below the laser threshold. In this regime we expect the Q value to be representative of that for the passive disk resonator, limited by both the quantum well absorption and losses from the optical cavity. In the absence of absorption and losses, ideal radiation from the whispering-gallery mode at the disk edge is estimated to limit the passive Q to approximately