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NTT Develops CoherentLight Source Based on Semiconductor Microcavities Nippon Telegraph and Telephone Corporation (NTT) has announced that a research group at its Basic Research Laboratories, headed by Yoshihisa Yamamoto, (presently a visiting professor at Stanford University) is close to realizing a novel semiconductor light source, which it calls the "Controlled SpontaneousEmission Diode" (CSD). This new coherent-light source is based on the control of spontaneously emitted light using three-dimensional semiconductor microcavities. The CSD is different from conventional semiconductor laser diodes (LDs) and light-emitting diodes (LEDs) due to its extremely high conversion efficiency, but has the high directionality of LDs and the low-current drive capability of LEDs. NTT says that CSDs should be useful for a wide range of applications. Yamamoto's group has demonstrated that spontaneous emission in a semiconductor structure, which is usually in random directions, can be made highly directional by controlling the vacuum field fluctuations surrounding the atoms, thus making "spontaneous emission" become, in a sense, "stimulated emission." This is done by coupling the semiconductor gain medium to an optical microcavity, rather than to the vacuum field fluctuations, the basic principle of the CSD. Semiconductor microcavities have recently been fabricated on the order of the optical wavelength, comprising a thin active layer sandwiched between multilayer reflectors. An important advance is a microlens structure, embedded in one of the multilayer reflectors, that acts to confine the optical field to the lateral direction. Experiments have confirmed that the microlens structure acts to control spontaneous emission; for example, the coupling efficiency of the total spontaneous emission to the fundamental modes of the cavity (i.e., the two orthogonal polarized modes) was measured at 10%, an increase of four orders of magnitude over that of conventional laser diodes. A further enhancement of the coupling efficiency is theoretically expected through improved cavity design. According to Shigenobu Yamakoshi of Fujitsu Laboratories Ltd., the main feature of this device is the curved dielectric Bragg mirror introduced to a previous design by Yamamoto. He explains that since this CSD still has no cavity structure in the horizontal plane, it is not a "real" threedimensional microcavity, the most important point in controlling spontaneous emission. Thus, since external spontane-

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ous light can easily penetrate into the cavity from along the horizontal plane, spontaneous emission cannot be completely controlled. On the positive side, Yamakoshi says that controlling spontaneous emission is an interesting and important topic, and this new CSD should be a good, basic advancement in this field for scientists to consider, while pursuing a "real" three-dimensional microcavity. Still, the CSD, in contrast to conventional laser diodes, does produce spatiallycoherent light, even when operating below the laser oscillation threshold. This o