Faculté des sciences de base SB, Programme doctoral Photonique, Institut de photonique et d'électronique quantiques IPEQ (Laboratoire de physique des nanostructures LPN)

Light control and microcavity lasers based on quantum wires integrated in photonic-crystal cavities

Atlasov, Kirill ; Kapon, Elyahou (Dir.)

Thèse Ecole polytechnique fédérale de Lausanne EPFL : 2009 ; no 4359.

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    Novel light-emitting devices and micro-optical-circuit elements will rely upon understanding and control of light-matter interaction at the nanoscale. Recent advances in nanofabrication and micro-processing make it possible to develop integrable solid-state structures where the optical- and quantum-confinement effects determine the density and distribution of the energy states, allowing for mastering the output characteristics. In semiconductor nanostructures, such as quantum wires or quantum dots (sometimes referred even to as "artificial atoms") produced by epitaxy, with characteristic dimensions of 10÷20 nm, the quantization determines light absorbtion and emission spectra. Unlike the bulk matter, these important properties depend on the size and shape of the object, which is designed by nanotechnology. In photonic crystals and photonic-crystal micro-resonators, on the other hand, due to pronounced bandgap effects acting on light, unprecedented control over reflectivity, transmission and such a fundamental quantum-mechanical property as the density of electromagnetic vacuum-field fluctuations is achieved, the latter defining the rates of spontaneous emission of an embedded source. Based on these ideas, a number of passive and active optical and optoelectronic devices is anticipated practically, in particular, semiconductor microlasers with extremely low threshold pump-powers and ultimate conversion efficiency. Within the framework of this thesis we successfully integrated site-controlled quantum wires (QWRs) in 2D photonic-crystal (PhC) microcavities, examined the basic spectral and dynamics properties of the system, implemented the QWRs as a testing light source and probed interesting cavity configurations, and finally achieved stimulated emission and lasing. Starting from the previous studies of the QWR nanostructures, we, first, designed the geometry patterns adapted for implementation in the PhC-cavity system. Crystal growth (by metal-organic vapor-phase epitaxy) of InGaAs/GaAs QWRs on such patterns was verified; single and triple vertically stacked identical wires were obtained integrated within a 260-nm thin GaAs membrane. The basic properties of such QWRs were checked by photoluminescence spectroscopy. Spectra, power-dependent blueshift and temperature dependence consistent with previous studies were evidenced. Relatively long radiative lifetimes were found (at low – 20K – temperature) in transient spectroscopy, suggesting exciton localization effects and the effective dimensionality in between 0D and 1D. Identified as the most practically feasible way of exploiting the PhC bandgap effects for achieving high-Q truly single-mode resonators, the membrane approach in 2D photonic crystals was then implemented. In our nanofabrication effort we succeeded in incorporating the QWRs into such PhC cavities with very good site-control. The site control is apparently crucial, as light-matter coupling in an optical cavity and the spontaneous-emission properties are determined by the spatial and spectral matching. Cavity Q-factors of ∼ 5000÷6000 were reached. Our technology can be readily extended to schemes involving multiple site-controlled nanostructures in single or multiple (e.g. coupled) cavities that are currently of interest for various experiments in quantum physics. We then examined several interesting cavity configurations including coupled and 1D-like PhC cavities, exploiting QWRs as an embedded local light source. Such cavity geometries are relevant to on-chip photon-transfer lines, single-photon sources, coupled-cavity lasers and quantum-optics experiments. While with 1D-like cavities we were able to track the 0D-1D transition of the photonic states and observe important implications due to distributed disorder, we also found out experimentally and analyzed numerically that in the formation of the coupled states an important feature of loss splitting appears having implications on the energy transfer. On a more fundamental level, we examined PhC-cavity and bandgap effects on the QWR spontaneous emission. It was found experimentally that, at low temperature, the QWR spontaneous emission resonantly coupled to the cavity mode can be enhanced by factors of ∼ 2 ÷ 2.5. In addition, the off-resonance part is inhibited by a factor of ∼ 3. Such measured factors suggest that the output stems from an ensemble of emitters, which is consistent with a regular QWR inhomogeneous broadening and exciton localization picture. Nevertheless, the enhancement of the spontaneous emission into the cavity mode with respect to any other available modes is then a factor of ∼ 6, which is important for microcavity laser concept based on the spontaneous-emission control. Finally, multi- and single-mode lasing is experimentally demonstrated (for the first time). In order to verify the observation of the stimulated emission and lasing, complex analysis of spectral and photon-dynamics characteristics was undertaken and compared to a rate-equation model. Significantly low threshold values of ≲ 1 μW (incident power) were achieved, with relatively high spontaneous-emission coupling factors of ∼ 0.3.