Abstract

Spontaneous emission is not an intrinsic property of an atom but rather a consequence of the coupling between this atom and the optical field. Thus, by tailoring the electromagnetic modes in a cavity whose size is comparable to the wavelength of the light, one can modify the spontaneous emission rate, spatial distribution and transition energy of an emitter placed in the cavity[1]. Moreover, low-dimensional semiconductor nanostructures such as quantum wires (QWRs) exhibit unique features due to the carrier quantum confinement in two dimensions (sharper density of states, enhanced excitonic effects and optical non linearities). To investigate the combined effect of carrier and photon confinements, we have incorporated strained InGaAs/GaAs QWRs in a one wavelength long (λ) planar microcavity. The QWRs are obtained by in situ seeded self-organized organometallic chemical vapor deposition growth on a substrate corrugated with a V-groove array, which yields wires with high interface quality[2]. The cavity mirrors consist of high reflectivity AlAs/AlGaAs planar Distributed Bragg Reflectors (DBRs). The full structure is grown in 2 steps : first, the growth is interrupted after deposition of the bottom DBR mirror in order to pattern the surface with a 0.25µm pitch V-groove array; second, the QWRs and top DBR mirror are regrown on the non planar surface. A transmission electron microscopy image of a typical structure is shown in fig. 1; the small sized (10nm) crescent-shaped QWRs are connected by side and top quantum wells (QWs) and located at the center of the λ GaAs spacer, which is planarized before the growth of the top DBR mirror. The dashed curves on figure 2 show typical low temperature photoluminescence (PL) spectra of a reference QWR structure without cavity. The narrow (8meV) and intense peak at low energy originates from the QWRs, and the high energy peak from the QWs. The inset of figure 2 shows the room temperature reflectivity of the full microcavity structure. The narrow (1.4meV) cavity mode observed exhibits an optical field enhancement factor ((Q-factor) of about 950. At low temperature, the QWR emission is resonant with the cavity mode as evidenced by PL spectra (solid lines on figure 2; note that the cavity and the reference structure have different thicknesses and emission wavelengths). For a detection normal to the cavity plane, only the QWR emission is visible. Its linewidth is reduced to 2meV, a value comparable to the cavity mode linewidth, and its intensity is about 25 times higher than the corresponding one for the reference QWR without cavity, indicating a significant enhancement of the spontaneous emission normal to the cavity. For a detection angle of 30° from the normal, the emission from the QWR in the cavity exhibits an expected blue shift and its intensity is decreased by a factor of about 80 compared to the normal emission, and becomes weaker than the reference QWR emission detected at 30°. This shows a clear redistribution of the spontaneous emission in favor of the highly directional mode of the microcavity. At resonance, the observed strong in plane polarization anisotropy of the QWR emission is a signature of the one dimensional nature of the emitter in the cavity[2]; this effect is not present for QWs in planar microcavities.

© 1998 IEEE