Michael D W Grogan

Tapered fibers embedded in silica aerogel

We embedded thin (down to 2 µm diameter) tapered fibres in silica aerogel with low loss. The aerogel is rigid but behaves refractively like air, protecting the tapered fibre without disturbing light propagation along it.

Emission and nonradiative decay of nanodiamond NV centers in a low refractive index environment

The nitrogen vacancy (NV) center is the most widely studied single optical defect in diamond with great potential for applications in quantum technologies. Development of practical single-photon devices requires an understanding of the emission under a range of conditions and environments. In this work, we study the properties of a single NV center in nanodiamonds embedded in an air-like silica aerogel environment which provides a new domain for probing the emission behavior of NV centers in nanoscale environments. In this arrangement, the emission rate is governed primarily by the diamond crystal lattice with negligible contribution from the surrounding environment. This is in contrast to the conventional approach of studying nanodiamonds on a glass coverslip. We observe an increase in the mean lifetime due to the absence of a dielectric interface near the emitting dipoles and a distribution arising from the irregularities in the nanodiamond geometry. Our approach results in the estimation of the mean quantum efficiency (~0.7) of the nanodiamond NV emitters.

Stable low-loss optical nanofibres embedded in hydrophobic aerogel

Nanofibres, optical fibres narrower than the wavelength of light, degrade in hours on exposure to air. We show that encapsulation in hydrophobic silica aerogel (refractive index 1.05) provides protection and stability (over 2 months) without sacrificing low attenuation, strong confinement and accessible evanescent field. The measured attenuation was <0.03 dB/mm, over 10 × lower than reported with other encapsulants. This enables many nanofibre applications based on their extreme small size and strong external evanescent field, such as optical sensors, nonlinear optics, nanofibre circuits and high-Q resonators. The aerogel is more than a waterproof box, it is a completely-compatible gas-permeable material in intimate contact with the nanofibre and hydrophobic on both the macroscopic and molecular scales. Its benefits are illustrated by experiments on gas sensing (exploiting the aerogels porosity) and supercontinuum generation (exploiting its ultra-low index).

Acetylene frequency references in gas-filled hollow optical fiber and photonic microcells

Gas-filled hollow optical fiber references based on the P(13) transition of the ν1+ν3 band of 12C2H2 promise portability with moderate accuracy and stability. Previous realizations are corrected (<1σ) by using proper modeling of a shift due to line-shape. To improve portability, a sealed photonic microcell is characterized on the 12C2H2 ν1+ν3 P(23) transition with somewhat reduced accuracy and stability. Effects of the photonic crystal fiber, including surface modes, are explored. Both polarization-maintaining (PM) and non-PM 7-cell photonic bandgap fiber are shown to be unsuitable for kilohertz-level frequency references.

Radially polarized Bessel-Gauss beams: decentered Gaussian beam analysis and experimental verification

We derive solutions for radially polarized Bessel-Gauss beams in free-space by superimposing decentered Gaussian beams with differing polarization states. We numerically show that the analytical result is applicable even for large semi-aperture angles, and we experimentally confirm the analytical expression by employing a fiber-based mode-converter.

Porous silicon nanocrystals in a silica aerogel matrix.

Silicon nanoparticles of three types (oxide-terminated silicon nanospheres, micron-sized hydrogen-terminated porous silicon grains and micron-size oxide-terminated porous silicon grains) were incorporated into silica aerogels at the gel preparation stage. Samples with a wide range of concentrations were prepared, resulting in aerogels that were translucent (but weakly coloured) through to completely opaque for visible light over sample thicknesses of several millimetres. The photoluminescence of these composite materials and of silica aerogel without silicon inclusions was studied in vacuum and in the presence of molecular oxygen in order to determine whether there is any evidence for non-radiative energy transfer from the silicon triplet exciton state to molecular oxygen adsorbed at the silicon surface. No sensitivity to oxygen was observed from the nanoparticles which had partially H-terminated surfaces before incorporation, and so we conclude that the silicon surface has become substantially oxidised. Finally, the FTIR and Raman scattering spectra of the composites were studied in order to establish the presence of crystalline silicon; by taking the ratio of intensities of the silicon and aerogel Raman bands, we were able to obtain a quantitative measure of the silicon nanoparticle concentration independent of the degree of optical attenuation.

Large-core acetylene-filled photonic microcells made by tapering a hollow-core photonic crystal fiber

We report on kagomé-lattice photonic microcells with low losses, large outer diameters, and large cores. The large (40-70microm) cores are accommodated by tapering the fibers and splicing the reduced ends to a single-mode fiber. We demonstrate the repeatability of this process and obtain splice losses of 0.6dB by optimizing the taper transition length. Narrow electromagnetically induced transparencies and saturable absorption are demonstrated in an acetylene-filled photonic microcell.

Effect of vector asymmetry of radially polarized beams in solid immersion microscopy

We theoretically and experimentally investigate the effect of imperfect vector symmetry on radially polarized beams focused by an aplanatic solid immersion lens at a numerical aperture of 3.3. We experimentally achieve circularly symmetric focused spot with a full-width-half-maximum of ~λ0/5.7 at λ0 = 1,310 nm, free-space wavelength. The tight spatial confinement and overall circular symmetry of the focused radially polarized beam are found to be sensitive to perturbations of its cylindrical polarization symmetry. The addition of a liquid crystal based variable retarder to the optical path can effectively ensure the vector symmetry and achieve circularly symmetric focused spots at such high numerical aperture conditions.

Sensing with optical vortices in photonic-crystal fibers

We demonstrate optical polarization vortex generation in a photonic-crystal fiber (PCF) by means of a CO(2) laser-induced long period grating. Vortices are a special subclass of fiber modes that result in polarization-insensitive resonances even when grating perturbations are asymmetric, as is the case with structural perturbations in single-material PCFs. The physics of vortex generation, combined with the use of structural perturbations alone, in single-material fibers, opens up a new schematic for realizing harsh-environment sensors. We show that the temperature and polarization stability of our vortex devices is maintained for prolonged periods of time (tested up to 34 h) at temperatures exceeding 1000 °C. We envisage that this demonstration opens up a new way of realizing high-temperature sensors in a cost-effective manner.

Structure of plasmonic aerogel and the breakdown of the effective medium approximation.

A method for making aerogel doped with gold nanoparticles (GNPs) produces a composite material with a well-defined localized surface plasmon resonance peak at 520 nm. The width of the extinction feature indicates the GNPs are well dispersed in the aerogel, making it suited to optical study. A simple effective medium approximation cannot explain the peak extinction wavelengths. The plasmonic field extends on a scale where aerogel cannot be considered isotropic, so a new model is required: a 5 nm glass coating on the GNPs models the extinction spectrum of the composite material, with air (aerogel), methanol (alcogel), or toluene filling the pores.