Nigel P. Johnson

Ultra high quality factor one dimensional photonic crystal/photonic wire micro-cavities in silicon-on-insulator (SOI)

We present experimental results on photonic crystal/photonic wire micro-cavity structures that demonstrate further enhancement of the quality-factor (Q-factor)--up to approximately 149,000--in the fibre telecommunications wavelength range. The Q-values and the useful transmission levels achieved are due, in particular, to the combination of both tapering within and outside the micro-cavity, with carefully designed hole diameters and non-periodic hole placement within the tapered section. Our 2D Finite Difference Time Domain (FDTD) simulation approach shows good agreement with the experimental results.

Asymmetric split ring resonators for optical sensing of organic materials

Asymmetric Split Ring Resonators are known to exhibit resonant modes where the optical electric field is strongest near the ends of the arms, thereby increasing the sensitivity of spectral techniques such as surface enhanced Raman scattering (SERS). By producing asymmetry in the structures, the two arms of the ring produce distinct plasmonic resonances related to their lengths - but are also affected by the presence of the other arm. This combination leads to a steepening of the slope of the reflection spectrum between the resonances that increases the sensitivity of the resonant behavior to the addition of different molecular species. We describe experimental results, supported by simulation, on the resonances of a series of circular split ring resonators with different gap and section lengths--at wavelengths in the mid-infra red regions of the spectrum--and their utilization for highly sensitive detection of organic compounds. We have used thin films of PMMA with different thicknesses, resulting in characteristic shifts from the original resonance. We also demonstrate matching of asymmetric split ring resonators to a molecular resonance of PMMA.

Observation of Bragg reflection in photonic crystals synthesized from air spheres in a titania matrix

Three-dimensional photonic crystals made of close-packed air spheres in an interconnected titania matrix have been fabricated using a self-organized template of polystyrene microspheres of 400 nm diameter. The matrix was obtained by vacuum-assisted infiltration of a precursor, with subsequent removal of the microspheres by calcination. Electron microscopy has confirmed the ordering of the structure and the presence of features likely to enlarge the photonic band gap. The reflection spectra of the crystal measured at different angles of incidence are consistent with Bragg’s law and with theoretical calculations, confirming the photonic nature of the material.

Thin film photonic crystals: synthesis and characterisation

The results of an investigation of the major factors that influence colloidal self-assembly of thin film photonic crystals are reported. The effect of temperature, relative humidity, sphere diameter, colloidal concentration and substrate angle were investigated: the results establish clearly that temperature is the most critical factor. Quantitative analysis of the results using Design of Experiments methodology has identified the optimum conditions for the growth of large area, low defect density thin film photonic crystals.

Synthesis and optical properties of opal and inverse opal photonic crystals

We describe the synthesis of two complementary photonic crystals: opal and an inverted opal structure consisting of air spheres in a titania matrix. From optical measurements and comparison with the literature, the mechanical strength and effective refractive index are increased by sintering for opal and because of compression for the inverse opal. However, this is at the expense of reducing the PBG in the [1 1 1] direction.

Enhancement of the photonic gap of opal-based three-dimensional gratings

The “semimetallic” photonic band gap formed in a synthetic opal has been increased by depositing a layer (InP or TiO2) with high refractive index on the inner surface of opal voids. Reflectance spectra of the composites (nanolayers assembled within grating voids) are correlated with both the photonic structure of the opal and electronic structure of the semiconductor.

Magnetic response of split ring resonators (SRRs) at visible frequencies

In this paper, we report on a substantial shift in the response of arrays of similarly sized Split Ring Resonators (SRRs), having a rectangular U-shaped form--and made respectively of aluminium and of gold. We also demonstrate that it is possible to obtain the polarization dependent LC peak in the visible spectrum--by using SRRs based on aluminium, rather than gold. The response of metallic SRRs scales linearly with size. At optical frequencies, metals stop behaving like nearly perfect conductors and begin displaying characteristically different behaviour, in accord with the Drude model. The response at higher frequencies, such as those in the visible and near infra-red, depends both on their size and on the individual properties of the metals used. A higher frequency limit has been observed in the polarization dependent response (in particular the LC resonance peak) of gold based SRRs in the near infrared region. By using aluminium based SRRs instead of gold, the higher frequency limit of the LC resonance can be further shifted into the visible spectrum.

Impact of titanium adhesion layers on the response of arrays of metallic split-ring resonators (SRRs)

At higher frequencies (visible and infrared) both the dimensions and the individual metal properties play an important role in determining the resonant response of arrays of SRRs. As a result, a substantial difference between the responses of gold- and Al-based SRR arrays has been observed. Additionally, deposition of gold SRRs onto a substrate typically involves the use of an additional adhesion layer. Titanium (Ti) is the most common adhesive thin-film material used to attach gold onto dielectric/semiconductor substrates. In this paper we investigate the impact of the Ti adhesion layer on the overall response of Au-based nano-scale SRRs. The results quantify the extent to which the overall difference in the resonance frequencies between Au- and Al-based SRRs is due to the presence of the Ti. We show that even a 2-nm-thick Ti layer can red-shift the position of SRR resonance by 20 nm. Finally, we demonstrate that by intentional addition of titanium in the Au-based SRRs, their overall resonant response can be tuned widely in frequency, but at the expense of resonance magnitude.

Dry etching damage in III–V semiconductors

Dry etching using ions can cause damage to the underlying semiconductor. This paper discusses damage in III–V semiconductors and presents examples of etching conditions under which it can be effectively eliminated. A distinction between surface and sidewall damage is made and methods of measuring both parameters are reviewed. It is noted that the noble gases cause relatively deep damage, while under the correct circumstances, etchants that have a marked chemical effect can cause much less damage. The present state of understanding of the mechanisms for the damage is discussed.

Synthesis and characterisation of photoniccrystals

The formation of two types of photonic crystal is described. Normal opal is formed by close packing of silica spheres. The synthesis of the silica spheres is described and some of the factors that influence the size of the spheres in the product are discussed. Three different methods for the preparation of close packed arrangements of the spheres are described and the photonic properties of the crystals produced are demonstrated. Inverse opal is based on a close packed array of air spheres in a dielectric matrix. Different methods for the formation of this ordered microporous solid based on titanium dioxide are described and the photonic properties of this material are discussed.