J. A. Timpson

Control of polarized single quantum dot emission in high-quality-factor microcavity pillars

Small size microcavity pillars with elliptical cross section and high quality factors Q are reported and are shown to provide nearly 100% linearly polarized single photon sources. It is shown that the polarization of the emission of quantum dots embedded within the pillars can be controlled by using the coupling of the dot emission with the photonic modes. A notable dependence of the Q value is found on the polarization of the mode even though calculations of the mode profiles show that the electric field distribution is very similar.

High Q modes in elliptical microcavity pillars

The degenerate fundamental mode of a microcavity pillar structure with circular cross section splits into two linearly polarized modes when the shape is changed to elliptical. The quality factor Q of these modes is very different. This letter demonstrates that the high Q mode provides better values of the figure of merit for strong coupling applications, Q∕V), where V is the modal volume, compared to values obtainable in circular structures. The difference in Q is shown to be a consequence of the polarization dependence of the losses through the microcavity mirrors.

SINGLE PHOTON SOURCES BASED UPON SINGLE QUANTUM DOTS IN SEMICONDUCTOR MICROCAVITY PILLARS

Semiconductor microcavity pillars with both circular and elliptical cross-section containing semiconductor quantum dots are shown to be good candidates for efficient single photon sources. Pillars with small diameters are shown to have exceptionally high quality factors and the reduction in the measured quality factor as the pillar diameter is reduced is shown to agree well with finite difference time domain simulation. These pillars exhibit a Purcell enhancement of the quantum dot emission when the dots are on-resonance with the cavity mode and strong photon antibunching. The use of the polarized modes of an elliptical micropillar allows the polarization of the emitted single photons to be selected.

Quantum key distribution system in standard telecommunications fiber using a short wavelength single photon source

A demonstration of the principles of quantum key distribution (QKD) is performed using a single-photon source in a proof of concept test-bed over a distance of 2 km in standard telecommunications optical fiber. The single-photon source was an optically-pumped quantum dot in a microcavity emitting at a wavelength of 895 nm. Characterization of the QKD parameters was performed at a range of different optical excitation powers. An investigation of the effect of varying the optical excitation power of the quantum dot microcavity on the quantum bit error rate and cryptographic key exchange rate of the system are presented.

Control of polarization and mode mapping of small volume high Q micropillars

We show that the polarization of the emission of a single quantum dot embedded within a microcavity pillar of elliptical cross section can be completely controlled and even switched between two orthogonal linear polarizations by changing the coupling of the dot emission with the polarized photonic modes. We also measure the spatial profile of the emission of a series of pillars with different ellipticities and show that the results can be well described by simple theoretical modeling of the modes of an infinite length elliptical cylinder.

Focused ion beam etching for the fabrication of micropillar microcavities made of III-V semiconductor materials

The authors demonstrate a simple approach for the construction of single photon sources utilizing focused ion beam (FIB) etching, a maskless fabrication technique. Here they use FIB with gas-assisted etching to fabricate micropillar microcavities from a GaAs∕AlGaAs distributed Bragg reflector planar cavity containing self-assembled InAs quantum dots. Using a 1.5μm square pillar, they demonstrate a single photon source where the two photon emission is suppressed by a factor of 3.8. They believe this to be the first example of a FIB fabricated pillar single photon source.The authors demonstrate a simple approach for the construction of single photon sources utilizing focused ion beam (FIB) etching, a maskless fabrication technique. Here they use FIB with gas-assisted etching to fabricate micropillar microcavities from a GaAs∕AlGaAs distributed Bragg reflector planar cavity containing self-assembled InAs quantum dots. Using a 1.5μm square pillar, they demonstrate a single photon source where the two photon emission is suppressed by a factor of 3.8. They believe this to be the first example of a FIB fabricated pillar single photon source.

Experiments Versus Modelling in Quantum Dot Pillar Microcavities

Recently, single photon sources have been realised by coupling InAs quantum-dots into circular micro-pillar microcavities based on distributed Bragg reflectors (DBRs). These sources can be highly efficient because the high semiconductor refractive index collects a large fraction of the spontaneous emission into the waveguide mode. We have modelled emission from circular, square, elliptical and rectangular pillars using the finite difference time domain (FDTD) method and see enhanced emission into the cavity mode and improved efficiency for coupling light out of the microcavity. The cavity Q-factors can be very high even when the pillar diameter (dimension) is comparable to the emission wavelength. In the elliptical and rectangular cavities the modes separate (in frequency) into a high-Q resonance with polarisation parallel to the long axis and a lower Q-factor resonance with polarisation orthogonal to the long axis. We compare our modelling with preliminary measurements made on micro-pillar microcavity samples containing a layer of low density InAs dots at the cavity centre

CQED-Enhanced Single Photon Sources From InGaAs Quantum Dots

Historically non-classical sources of single photons have been used to test quantum mechanics and to develop the field of quantum optics. Nowadays the single photon source has become a critical device for quantum communications and optical quantum information processing [Knill, et al., 2001]. A single photon source emits one photon at a time, and its quality can be evaluated by three parameters: quantum efficiency, multi-photon probability, and photon distinguishability. Among the different types single photon sources, InGaAs/GaAs quantum dots (QDs) are a promising solid state candidate showing high quantum efficiency, no bleaching effect, long-term stability, capable of high repetition rate, and compatible with standard semiconductor processing techniques. Furthermore, QDs can be embedded in micro-cavities or photonic crystal nano-cavities by in situ growth, and thus cavity quantum electrodynamics (CQED) can be exploited to improve the performance of QD-based single photon sources in all aspects mentioned above [Santori, et al., 2002].

High Q values and polarised emission from microcavity pillars with elliptical cross-section

Benefits of elliptical micropillars are reported including polarisation control of dot emission; and increased quality factors as one of the loss mechanisms found in circular micropillars is removed.