Investigation into the coupling of quantum dots to photonic crystal nanocavities at telecommunication wavelengths

Recently, the emission of single photons with emission wavelength in the 1.3 µm telecommunication window was demonstrated for InAs quantum dots. This makes them strong candidates for applications such as quantum cryptography, and in a longer term, quantum computing. However, efficient extraction of the spontaneous emission from semiconductors still represents a major challenge due to total internal reflection at the semiconductor/air interface. In particular, single photon sources based on quantum dots are plagued by low extraction efficiency and poor coupling to single-mode fibers, typically on the order of 10-3 ~ 10-4, which prevents their application to quantum communication. To seek a solution to this problem, this thesis work explores the integration of quantum dots, with emission at 1.3 µm, in photonic crystal microcavities. Photons emitted in a mode of the cavity are funneled out of the semiconductor, and thus bypass the total internal reflection. In addition, the modified density of electromagnetic states in the cavity affects the emission lifetime of a weakly coupled emitter: in resonance, we assist to an increase of the emission rate, known as the Purcell effect, that would allow faster data transmission. Photonic crystal microcavities conveniently address this objective as they provide modes with the required small volumes and high quality factors. They also allow the engineering of the farfield pattern of the cavity modes, and thus of the collection efficiency. In the following pages, after brie y reviewing single photon emitters, the Purcell effect, and photonic crystal cavities, we present our results on the coupling of quantum dots to photonic crystal cavities. We report on the different strategies we used to control the tuning between the cavity mode and the quantum dot emission frequency. We also show our efforts in improving the collection of coupled photons by engineering the shape of the microcavity. Finally, we present our time-resolved measurements demonstrating the Purcell effect under optical and electrical operation

Enhanced spontaneous emission from quantum dots in short photonic crystal waveguides

We report a study of the quantum dot emission in short photonic crystal waveguides. We observe that the quantum dot photoluminescence intensity and decay rate are strongly enhanced when the emission energy is in resonance with Fabry-Perot cavity modes in the slow-light regime of the dispersion curve. The experimental results are in agreement with previous theoretical predictions and further supported by three-dimensional finite element simulation. Our results show that the combination of slow group velocity and Fabry-Perot cavity resonance provides an avenue to efficiently channel photons from quantum dots into waveguides for integrated quantum photonic applications.Comment: 12 pages, 4 figure

Exciton recombination dynamics in a-plane (Al,Ga)N/GaN quantum wells probed by picosecond photo and cathodoluminescence.

International audienceWe present a combined low-temperature time-resolved cathodoluminescence and photoluminescence study of exciton recombination mechanisms in a 3.8 nm thick a-plane (Al,Ga)N/GaN quantum well (QW). We observe the luminescence from QW excitons and from excitons localized on basal stacking faults (BSFs) crossing the QW plane, forming quantum wires (QWRs) at the intersection. We show that the dynamics of QW excitons is dominated by their capture on QWRs, with characteristic decay times ranging from 50 to 350 ps, depending on whether the local density of BSFs is large or small. We therefore relate the multiexponential behavior generally observed by time-resolved photoluminescence in non-polar (Al,Ga)/GaN QW to the spatial dependence of QW exciton dynamics on the local BSF density. QWR exciton decay time is independent of the local density in BSFs and its temperature evolution exhibits a zero-dimensional behavior below 60 K. We propose that QWR exciton localization along the wire axis is induced by well-width fluctuation, reproducing in a one-dimensional system the localization processes usually observed in QWs

Tuning of photonic crystal cavities by controlled removal of locally infiltrated water

We present a spectral tuning mechanism of photonic crystal microcavities based on microfluidics. The microinfiltration with water of one or few cavity holes and its subsequent controlled evaporation allow us to tune the cavity resonances in a spectral range larger than 20 nm, with subnanometer accuracy, and we also observe that the addition of water in the microcavity region improves its quality factor Q. (C) 2009 American Institute of Physics. [doi:10.1063/1.3247894

Nonlinear optical tuning of photonic crystal microcavities by near-field probe

We report on a nonlinear way to control and tune the dielectric environment of photonic crystal microcavities exploiting the local heating induced by near-field laser excitation at different excitation powers. The temperature gradient due to the optical absorption results in an index of refraction gradient which modifies the dielectric surroundings of the cavity and shifts the optical modes. Reversible tuning can be obtained either by changing the excitation power density or by exciting in different points of the photonic crystal microcavity. (C) 2008 American Institute of Physics

Spectral tuning and near-field imaging of photonic crystal microcavities

We experimentally observe a sizable and reversible spectral tuning of the resonances of a two-dimensional photonic crystal microcavity induced by the introduction of a subwavelength size glass tip. The comparison between experimental near-field data, collected with lambda/6 spatial resolution, and results of numerical calculations shows that the spectral shift induced by the tip is proportional to the local electric field intensity of the cavity mode. This observation proves that the electromagnetic local density of states in a microcavity can be directly measured by mapping the tip-induced spectral shift with a scanning near-field optical microscope