Y.-L. D. Ho

Strongly enhanced photon collection from diamond defect centers under microfabricated integrated solid immersion lenses

The efficiency of photon collection from optically active defect centers in bulk diamond is greatly reduced by refraction at the diamond-air interface. We report on the fabrication and measurement of a geometrical solution to the problem; integrated solid immersion lenses (SILs) etched directly into the surface of diamond. An increase of a factor of 10 was observed in the saturated count-rate from a single negatively charged nitrogen-vacancy (NV−) within a 5 μm diameter SIL compared with NV−’s under a planar surface in the same crystal. Such a system is potentially scalable and easily adaptable to other defect centers in bulk diamond.

Nanofabricated solid immersion lenses registered to single emitters in diamond

We describe a technique for fabricating micro- and nanostructures incorporating fluorescent defects in diamond with a positional accuracy better than hundreds of nanometers. Using confocal fluorescence microscopy and focused ion beam etching, we initially locate a suitable defect with respect to registration marks on the diamond surface then etch a structure using these coordinates. We demonstrate the technique by etching an 8 μm diameter hemisphere positioned with single negatively charged nitrogen-vacancy defect lies at its origin. Direct comparison of the fluorescence photon count rate before and after fabrication shows an eightfold increase due to the presence of the hemisphere.

Orbital angular momentum vertical-cavity surface-emitting lasers

Harnessing the orbital angular momentum (OAM) of light is an appealing approach to developing photonic technologies for future applications in optical communications and high-dimensional quantum key distribution (QKD) systems. An outstanding challenge to the widespread uptake of the OAM resource is its efficient generation. In this work we design a new device that can directly emit an OAM-carrying light beam from a low-cost semiconductor laser. By fabricating micro-scale spiral phase plates within the aperture of a vertical-cavity surface-emitting laser (VCSEL), the linearly polarized Gaussian beam emitted by the VCSEL is converted into a beam carrying specific OAM modes and their superposition states, with high efficiency and high beam quality. This new approach to OAM generation may be particularly useful in the field of OAM-based optical and quantum communications, especially for short-reach data interconnects and QKD.

Diamond-based structures to collect and guide light

We examine some promising photonic structures for collecting and guiding light in bulk diamond. The aim of this work is to optimize single photon sources and single spin read-out from diamond color centers, specifically NV centers. We review the modeling and fabrication (by focused ion beam and reactive ion etching) of solid immersion lenses, waveguides and photonic crystal cavities in monolithic diamond.

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.

Frequency-Domain Model of Longitudinal Mode Interaction in Semiconductor Ring Lasers

A general and comprehensive frequency-domain model of longitudinal mode interactions in semiconductor ring lasers (SRLs) is presented, including nonlinear terms related to third order nonlinear susceptibilities χ3 and also linear terms due to back scattering between counter-propagating modes. The model can handle a large number of modes and complex third order nonlinear processes such as self-suppression, cross-suppression and four wave mixing occurring due to both interband and intraband effects. Every aspect of the lasing characteristics of SRLs, including lasing spectra, light-current curves and lasing direction hysteresis, can be reproduced by the model. To assess the performance and validity of the model, several miniaturized SRLs are designed, fabricated and tested. Stable unidirectional lasing in SRLs is also demonstrated by introducing asymmetric feedback from external facets. Good agreement between theoretical and experimental results is demonstrated.

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.

Three-Dimensional FDTD Simulation of Micro-Pillar Microcavity Geometries Suitable for Efficient Single-Photon Sources

We present the results of calculations of the microcavity mode structure of distributed-Bragg-reflector (DBR) micro-pillar microcavities of group III-V semiconductor materials. These structures are suitable for making single photon sources when a single quantum dot is located at the center of a wavelength scale cavity. The 3-D finite difference time domain (FDTD) method is our primary simulation tool and results are validated against semi-analytic models. We show that high light extraction efficiencies can be achieved (>90%) limited by sidewall scattering and leakage. Using radial trench DBR microcavities or 2-D photonic crystal structures, we can further suppress sidewall emission, however, light is then redirected into other leaky modes

Efficient out-coupling and beaming of Tamm optical states via surface plasmon polariton excitation

We present evidence of optical Tamm states to surface plasmon polariton (SPP) coupling. We experimentally demonstrate that for a Bragg stack with a thin metal layer on the surface, hybrid Tamm-SPP modes may be excited when a grating on the air-metal interface is introduced. Out-coupling via the grating to free space propagation is shown to enhance the transmission as well as the directionality and polarization selection for the transmitted beam. We suggest that this system will be useful on those devices, where a metallic electrical contact as well as beaming and polarization control is needed.

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.