Ross Leyman

Continuous wave terahertz radiation from an InAs/GaAs quantum-dot photomixer device

Generation of continuous wave radiation at terahertz (THz) frequencies from a heterodyne source based on quantum-dot (QD) semiconductor materials is reported. The source comprises an active region characterised by multiple alternating photoconductive and QD carrier trapping layers and is pumped by two infrared optical signals with slightly offset wavelengths, allowing photoconductive device switching at the signals? difference frequency ~1 THz.(C) 2012 American Institute of Physics.

Multimodal spectral control of a quantum-dot diode laser for THz difference frequency generation

Generation of stable dual and/or multiple longitudinal modes emitted from a single quantum dot (QD) laser diode (LD) over a broad wavelength range by using volume Bragg gratings (VBGs) in an external cavity setup is reported. The LD operates in both the ground and excited states and the gratings give a dual-mode separation around each emission peak of 5 nm, which is suitable as a continuous wave (CW) optical pump signal for a terahertz (THz) photomixer device. The setup also generates dual modes around both 1180m and 1260 nm simultaneously, giving four simultaneous narrow linewidth modes comprising two simultaneous difference frequency pump signals

Towards an efficient spin-photon interface with NV centres in diamond (Conference Presentation)

The negatively charged nitrogen vacancy centre in diamond is known for its coherent spin properties and optical interface, and thus is regarded a promising candidate for quantum information applications [1]. Realisation of an efficient spin-photon interface with the NV centre is made challenging however by the fact that, in bulk diamond, only 3-4% of spontaneously emitted photons occur in the zero phonon line (ZPL). Placing NV centre in an optical cavity is being explored by several groups [2][3][4] as an effective way to selectively enhance the coherent emission of NVs and thereby increase the efficiency of the coherent spin-photon coupling. Previous work reported successful coupling of the NV in nano-diamond to an open access micro-cavity and observed enhanced ZPL emission [5]. However the NV centres in nano-diamond suffer from broadened zero phonon transition and poor spin coherence. By fabricating NV centres in a ~micrometre thick membrane of high purity single crystal material we can take advantage of the tunability of open access cavities, and at the same time, provide close-to-bulk crystal environment to maintain the coherent spin properties of the NV centres. Here we report our work on the tunable cavity coupling of the ZPL of a NV centre in a 1.2micrometre-thick diamond membrane at 4K. The diamond membrane is fabricated from a 0.5mm-thick E6 CVD diamond plate where ion implantation is carried out on both surfaces to create NV centres at the depth of around 70nm. The plate is then machined into 30micrometre-thick slices, and thinned by ICP-RIE with a combination of Ar/Cl[6] and pure oxygen plasma etching recipes. The open cavity consists of a concave mirror (99.99% reflectivity) deposited on a template fabricated using Focused Ion Beam (FIB) milling[7] and a planar mirror (99.8% reflectivity) which supports the membrane. For bare cavities with mirror radii of curvature (RoC) of 12micrometre, we measured a finesse of F~2000 and mode volume as small as 0.75micrometre^3. In-situ tuning of the cavity resonance is achieved with piezoelectric actuators. When mounted in our bath cryostat the cavity modes have dominant Lorentzian line profiles which indicate a passive stability of the cavity length of better than 0.15nm. No active locking is currently deployed. With the presence of a diamond membrane inside the cavities, the measured finesse and mode volume of a cavity with 12micrometre RoC are found to be around 300 and 3 micrometre^3, respectively. We attribute the reduction in finesse to scattering at the membrane-air and membrane-mirror interfaces. On coupling to the ZPL of a target NV centre, we record a factor of 4 increase in the saturated intensity of ZPL fluorescence compared to that measured from the same NV centre in absence of the concave mirror. This result is consistent with the calculated Purcell factor of 16 combined with a relatively low efficiency of light extraction (estimated to be around 19%) from the cavity due to the scattering losses.

Tunable open-cavity coupling to the zero phonon line of a nitrogen-vacancy defect in diamond (Conference Presentation)

Recent demonstrations of entanglement between two remote Nitrogen-Vacancy centers, have opened the way for their use in distributed quantum networks. An efficient spin-photon interface will now be required to help realize this system as a technology. Here we demonstrate the tunable enhancement of the zero phonon line of a single nitrogen-vacancy colour centre in nanodiamond at cryogenic temperatures. A plano-hemispherical open cavity, fabricated using focused ion beam milling provides mode volumes as small as 1.25 cubic microns and quality factor Q ~ 3000. It will be shown how the open geometry and independently adjustable mirrors allows for precise placement of the emitter in the centre of the cavity mode, and crucially enables in-situ tuning of the cavity resonances. At optimal coupling, the signal from individual zero phonon line transitions is enhanced by a factor of 6.25 through the Purcell effect and the overall emission rate of the NV- centre is increased by 40% compared with that measured from the same centre in the absence of cavity field confinement. This Purcell enhancement is mapped out as a function of cavity mode volume. These results represent a proof of principle for a tunable cryogenic spin-photon interface. However by far the best NV optical and spin coherences are to be found in bulk material and efforts towards the production of diamond membranes are currently being made, with dimensions suitable for open-cavity coupling. Efforts towards this and preliminary results will also be discussed.

Quantum Dot-Based Terahertz photoconductive antennas

We present novel Terahertz (THz) emitting optically pumped Quantum Dot (QD) photoconductive (PC) materials and antenna structures on their basis both for pulsed and CW pumping regimes.