Session 2: Silicon and Carbon Photonics (11:00 11:30, Huxley LT311)

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1 Session 2: Silicon and Carbon Photonics (11:00 11:30, Huxley LT311) (invited) Formation and control of silicon nanocrystals by ion-beams for photonic applications M Halsall The University of Manchester, UK Ion beams allow the formation of compounds under highly non-equilibrium conditions as part of a CMOS process flow. A simple example of this is the formation of silicon rich oxide on a silicon wafer. High temperature annealing of this materials causes the formation of nanoscale clusters of silicon embedded in a dielectric host. Such clusters can relax the conservation of momentum selection rules that prevent the emission of light from bulk silicon. Thus these materials have great potential for the rapidly developing field of silicon photonics which seeks to shrink optical telecommunication technologies onto a single chip fabricated by standard CMOS processes. In this presentation the author will discuss the formation, scale and structure of such nanoclusters. The control over the size of the clusters by rapid thermal processing is demonstrated and the use of dopants to affect the growth rate is also presented. The measured optical properties of nanoclusters is related to the structure and dielectric properties of the layers produced. Co-doping with rare-earth ions to produced on-chip light sources and amplifiers has also been achieved and the detailed structure of the resulting nanoclusters as studied by scanning transmission electron microscopy. Finally we will discuss potential future applications of such clusters in electronics and optoelectronics.

2 Session 2: Silicon and Carbon Photonics (11:30 11:45, Huxley LT311) Highly efficient directional coupling using a compact plasmonic-photonic coupler for integrated silicon photonic circuits T P H Sidiropoulos, M P Nielsen, T R Roschuk, S A Maier and R F Oulton Imperial College London, UK The rise of integrated silicon photonic circuits has created issues related to efficiently coupling external light sources in the telecommunication band into silicon photonic waveguides. The high efficiencies of grating couplers, with obtainable efficiencies in excess of 20%, come at the cost of complicated designs requiring extensive optimization schemes and bulky device sizes. In addition, grating couplers have high angular sensitivity, requiring complicated source to coupler arrangements, and suffer from problems of high dispersion and narrow bandwidths. For these reasons, the simplistic method of direct-end fire excitation has become the most common method of coupling despite efficiencies as low as 1%. We propose and demonstrate a compact plasmonic-photonic coupler that exhibits efficient broadband coupling between free-space radiation and a silicon photonic waveguide via localised surface plasmon (LSP) resonance. The coupler consists of gold nanoparticles positioned above a silicon-on-insulator (SOI) photonic waveguide, a schematic of which can be seen in Figure 1 below along with a depiction of the experiment. Numerical simulations were first used to design the coupler, and show, for 250nm wide nanoparticles near λ=1.5μm, coupling efficiencies of 13% for a single particle and 20% an identical particle pair into both directions. Directional coupling can be obtained by adjusting the relative phase between LSPs through varying the width of one particle. A maximum coupling efficiency of 27% into one direction is found for a relative phase of π/4 due to multiple scattering effects. This result is in general agreement with a coupled oscillator model. The numerical simulations also show a 500nm coupling bandwidth in the telecommunications band and low group delay dispersion, enabling the faithful coupling of pulses as short as 50fs. The couplers performance was verified experimentally through the addition of a single particle out-coupler to extract the light out of the waveguide back into free-space radiation. Directionality of the in-coupling was verified by varying the particle separation and relative phase difference. Measurement of the nonlinear effects of second harmonic generation (SHG) and third harmonic generation (THG) as seen at the in- and out-couplers was investigated to verify strong coupling of short femtosecond pulses. In-coupling efficiencies for the asymmetric particle pair as high as 21% were measured. The broadband spectral response of the plasmonic-photonic couplers was examined using a supercontinuum generated white light source and compared against the predicted spectrum. The effects of the buried oxide layer and cladding layer on coupling efficiency are explored. Figure 1. a) Schematic of the plasmonic-photonic coupler. b) Depiction of coupling when the input beam is off/on the in-coupler.

3 Session 2: Silicon and Carbon Photonics (11:45 12:00, Huxley LT311) Pair generation by spontaneous four wave mixing in a silicon waveguide G Sinclair University of Bristol, UK Having benefitted from an unparalleled level of research and investment from the electronics industry, silicon is in many respects the ideal platform for photonic applications. Nonetheless, challenges remain in transferring this wealth of development into the single-photon quantum information regime. One such challenge is presented by the generation of photon-pairs by spontaneous four mixing (SFWM). SFWM arises due to the high (real) effective nonlinearity in silicon. However, the imaginary part of the nonlinear can also give rise to unwanted processes, such as cross two-photon absorption between pump and signal (or idler) photons. This limits the efficiency with which photon pairs can be heralded at high pump intensities. Our work explores this process and suggests some potential solutions.

4 Session 2: Silicon and Carbon Photonics (12:00 12:15, Huxley LT311) Group IV photonics and low index waveguides integration F Y Gardes 1, D Bucio 1, C G Littlejohns 1, T K Debnath 2, and L O'Faolain 2 1 University of Southampton, UK, 2 University of St Andrews, UK Low fabrication error sensitivity, integration density, channel scalability, low switching energy and low insertion loss are the major prerequisites for future on-chip WDM systems and interfacing with optical fibres. A number of device geometries have already been demonstrated that fulfil these criteria, at least in part, but combining all of the requirements is still a difficult challenge. Here, we propose and demonstrate a novel architecture consisting of the interfacing of Photonic crystal modulators but also detectors connected by a silicon nitride bus waveguide. The architecture features very high scalability due to the small size of the devices ( ~ 100 micrometre square) and the modulators operate with an AC energy consumption of less than 1fJ/bit. Furthermore, we demonstrate a variety of silicon nitride low temperature deposition for waveguiding applications using PECVD and Hot Wire CVD. Fig 1: Photonic crystal modulator array

5 Session 2: Silicon and Carbon Photonics (12:15 12:30, Huxley LT311) Coupling nitrogen-vacancy centres in diamond to tunable, open-access optical microcavities S Johnson, P Dolan, A Trichet, D Coles, A Watt and J Smith University of Oxford, UK Nitrogen-vacancy (NV) colour centres in diamond display remarkable spin properties, even at room temperature, and thus show promise for future quantum technologies. Optical microcavities can provide a way to manipulate their spontaneous emission rate, and their spectral profile, in order to efficiently interface with photonic networks in the future. The open-access geometry allows great flexibility and tuning of the optical resonances. We report the coupling of NV centres in nanodiamond to tunable open-access optical microcavities. Formed from high reflectivity mirrors with a planar-concave geometry, these can be independently manipulated to allow for full tunability of the optical resonances. Arrays of concave features are constructed by Focused Ion Beam milling. The narrow ion beam diameter allows us to pattern features well below the wavelength scale. As a consequence we obtain smooth features with radii of curvature on the micron scale and thus ultra-small cavity mode volumes. This is essential for the enhancement of the light-matter interaction. When coupling to these small cavities, we observe profound changes in the observed emission spectrum of the NVs at room temperature. We demonstrate coupling to cavities with mode volumes down to 7 cubic wavelengths and Purcell enhancements of up to 5% are inferred at room temperature which is significant for an emitter that is well within the bad emitter regime of cavity QED. Continuing refinement of the fabrication process will allow access to even smaller mode volumes. However, much improved Purcell enhancements are expected when the zero-phonon line of the NV becomes narrower than the cavity linewidth, thus entering the good emitter regime of cavity QED. For this, low temperature cavity coupling experiments are required and efforts towards this will be discussed along with some preliminary results.