GaAs microdisk lasers with holes pierced through the disk surface are investigated for their threshold characteristics. Disks are fabricated with either a single hole or two diametrically opposite holes at various distances from the disk outer edge. Even though the disk area is reduced by only 1%, we find that the lasing threshold for a disk with one hole is reduced by up to 50% compared to a disk with no hole. We attribute this reduction to the perturbation of nonlasing modes, which decreases the amplification of spontaneous emission in these modes and makes more carriers available to contribute to lasing. © 1999 American Institute of Physics. ͓S0003-6951͑99͒00301-0͔Semiconductor microdisk lasers are of interest for micron-scale low-threshold laser devices. Their laser modes approximate whispering-gallery ͑WG͒ modes, for which the high reflectivity at the curved disk boundary gives lowthreshold operation. The laser threshold is a function of the optical confinement, gain/absorption balance, scattering processes, and device dimensions. The optical confinement in these structures is high because of the large dielectric discontinuity between the disk and the surrounding air, while carrier absorption and scattering processes are difficult to modify. Threshold characteristics can be calculated, 1-3 and various research groups have sought to achieve lowthreshold operation by shrinking the device dimensions to reduce spontaneous emission into nonlasing modes; 4-6 the smallest microdisk laser reported having 1.6 m diameter. 5 Other groups have concentrated on optimizing the edge quality of their devices to improve the optical confinement, resulting in further reductions of the laser threshold. 7 In this letter we present an approach for decreasing the threshold in a large semiconductor disk (Rӷ ) based on reducing the amplified spontaneous emission ͑ASE͒ into competing nonlasing modes. We achieve this reduction by removing material from the interior of the disk, which reduces the photon lifetimes for modes with field in this region. Since the ASE depends on the photon lifetimes of the corresponding modes, fewer carriers emit into these modes, making more carriers available to contribute to lasing. The introduction of one small hole, comprising approximately 1% of the disk area, can reduce the laser threshold by a factor of 2. Figure 1 shows an electron micrograph of a typical microdisk structure with small holes etched through the top surface ͑Fig. 1͒. The disks contain four 10 nm GaAs quantum wells ͑QWs͒ separated by Al 0.28 Ga 0.72 As barriers, and are defined by electron-beam lithography and reactive ion etching. The pedestals ͑500 nm Al 0.65 Ga 0.35 As) are created by undercutting with a selective wet etch. Finally, the structures are passivated in ammonium sulphide and stabilized with a 30-nm-thick silicon nitride encapsulation. 8 The disks have a diameter of 12.6 m with 0.5 m square holes. Disks have been fabricated with one hole, or two holes placed opposite each other, separated from the disk outer edge by 0.5,...
GaAs microdisk laser structures with modified resonator geometry have been investigated. Device fabrication employs electron-beam lithography and etch procedures for generating disks with notches, projections, and holes, and a novel ring-laser structure. Directional emission from the notched microdisk lasers has been investigated using external scattering structures, which may also be used as couplers for the laser emission
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