Microcavity Lasers as Efficient Light Emitters
 

Since the advent of the laser, the field of quantum optics and optoelectronics has made great progress and vastly transformed the communication and music recording industries. The realization of fiber optical communication, the compact disk (CD) and the Digital Versatile Disk (DVD) recording with the use of the semiconductor laser are examples of the profound influence. In order to process a vast amount of information, further innovation in the size and power consumption of laser devices has been aggressively pursued by contemporary scientists and researchers. More recently, novel concepts such as quantum optical control of the spontaneous emission by an optical microcavity have been proposed, resulting in the emergence of microcavity lasers.

Figure 1: The reflectance spectrum of a planar cavity, photoluminescence (PL) 
spectra of quantum dots (QDs) in free space and in cavity.

A microcavity laser has a structure and lasing characteristics fundamentally different from those of the conventional edge emitting laser with horizontal stripe cavity. A conventional edge-emitting laser with a cavity length of hundreds of microns usually lases with several longitudinal modes simultaneously, and thus only a small portion of the spontaneous emission couples into a single state of the electromagnetic field sustained by the laser cavity. In a microcavity laser, the photons emitted by the active media resonate in a much smaller cavity ranging from several microns to submicrons (a "quantum dot" for photons), which allows only a single cavity mode to operate. Thus, a microcavity laser can be very compact in size and very efficient in emission. The microcavity laser will have a key role in photonics, for example, as an ultra-low power-consumption and highly efficient light emitter in an optical interconnect. Meanwhile, the concept of spontaneous emission control in microcavities presents an ultimate and novel performance of microcavity lasers.

Collaborating with Dr XH Zhang and Dr JR Dong from the Institute of Materials Research & Engineering (IMRE), self-assembled quantum dots were employed as light-emitting materials and the enhancement and/or retardation of the spontaneous emission of quantum dots ultilizing microcavities are being investigated. The objective is to study the physics and new phenomena in optical microcavities, such as to enhance or inhibit spontaneous emission and its relation to the laser oscillation, and to fabricate very compact and efficient microcavity lasers, through the unification of electronic confinement by quantum dots and photonic confinement by microcavities. Figure 1 gives some experimental results obtained thus far, which show that the emission of the quantum dots has been successfully tuned to the cavity mode and that, in so doing, the emission linewidth is reduced by an order of magnitude.