S. Brand

Tamm plasmon polaritons: Slow and spatially compact light

We report on the first experimental observation of Tamm plasmon polaritons (TPPs) formed at the interface between a metal and a dielectric Bragg reflector (DBR). In contrast to conventional surface plasmons, TPPs have an in-plane wavevector less than the wavevector of light in vacuum, which allows for their direct optical excitation. The angular resolved reflectivity and transmission spectra of a GaAs∕AlAs DBR covered by Au films of various thicknesses show the resonances associated with the TPP at low temperatures and room temperature. The in-plane dispersion of TTPs is parabolic with an effective mass of 4×10−5 of the free electron mass.

Electronic structure calculations on nitride semiconductors

A series of calculations have been performed on group-III nitrides (GaN, AlN and InN) in both zinc-blende and wurtzite structures. Three different levels of computation have been performed in an integrated programme of study: first-principles total energy calculations, semi-empirical pseudopotential calculations and calculations. Bandstructures are obtained from each method in a consistent manner, and used to provide effective masses and parameters for planned work on the electronic structure of alloys and quantum well heterostructures.

The metrics of surface adsorbed small molecules on the Young's fringe dual-slab waveguide interferometer

A method for analysing thin films using a dual-waveguide interferometric technique is described. Alternate dual polarization addressing of the interferometer sensor using a ferroelectric liquid crystal polarization switch allowed the opto-geometrical properties (density and thickness) of adsorbed layers at a solid?liquid interface to be determined. Differences in the waveguide mode dispersion between the transverse electric and transverse magnetic modes allowed unique combinations of layer thickness and refractive index to be determined at all stages of the layer formation process. The technique has been verified by comparing the analysis of the surface adsorption of surfactants with data obtained using neutron scattering techniques, observing their behaviour on trimethylsilane coated silicon oxynitride surfaces. The data obtained were found to be in excellent agreement with analogous neutron scattering experiments and the precision of the measurements taken to be of the order of 40?pm with respect to adsorbed layer thicknesses. The study was extended to a series of surfactants whose layer morphology could be correlated with their hydrophilicity/lipophilicity balance. Those in the series with longer alkyl chains were observed to form thinner, denser layers at the hydrophobic solid/aqueous liquid interface and the degree of order attained at sub-critical micelle concentrations to be correlated with molecular fluidity.The technique is expected to find utility with those interested in thin film analysis. An important and growing area of application is within the life sciences, especially in the field of protein structure and function.

Diffraction and transmission of light in low-refractive index Penrose-tiled photonic quasicrystals

We report the measurements of the diffraction pattern of a two-dimensional Penrose-tiled photonic quasicrystal, obtained by etching air cylinders in a silica substrate, and the modelling of the light propagation and dispersion relations of photons inside such a structure. The calculated transmission spectra exhibit dips whose positions are insensitive to the direction of propagation and whose depth increases with increasing structure length. An approach is developed for the calculation of the dispersion relations which is based on a set of reciprocal vectors defined by the diffraction pattern. The dispersion curves exhibit gap-like features at frequencies corresponding to the dips in the transmission spectra.

Localized defects in III-V semiconductors

A Greens-function approach, closely related to that of Koster and Slater, is developed, within the one-electron pseudopotential formalism. This is applied to single vacancies, the divacancy and vacancy-oxygen pairs in GaAs, and single vacancies in GaP and InSb. A number of localized states are found. It is shown that the properties of the host-crystal band structure, the electron-electron interaction, and lattice relaxation play an important part in the formation of these states. In contrast with the Koster-Slater calculations, our results are not very sensitive to the strength of the vacancy potential. Our calculations indicate that As and Ga vacancies in GaAs should behave as single donors and acceptors, respectively. The

Hybrid states of Tamm plasmons and exciton polaritons

{V}_{\mathrm{Ga}}\ensuremath{-}\mathrm{O}

Two-dimensional Penrose-tiled photonic quasicrystals: from diffraction pattern to band structure

complex behaves as a deep neutral center. The wave functions associated with the levels introduced into the band gap are highly localized. Since they contain admixtures of both valence- and conduction-band functions, and exhibit large lattice distortion, they are likely to take part in nonradiative recombination processes. Finally, we note that nonradiative recombination via some deep neutral centers is likely to be very efficient since our calculation also indicates a possibility of localized excited states introduced by these centers.

Real time, high resolution studies of protein adsorption and structure at the solid?liquid interface using dual polarization interferometry

Channeling of exciton polaritons in the plane of semiconductor microcavities can be achieved by the deposition of metallic mesas on the top of the semiconductor structure. We show theoretically that the regime of strong coupling between cavity polaritons and Tamm surface plasmons is possible in such structures. The effect is favorable for the spatial confinement of polaritons and the formation of hybrid one-dimensional plasmon-polariton modes.

Two-dimensional Penrose-tiled photonic quasicrystals : diffraction of light and fractal density of modes.

We report measurements of the diffraction pattern of a two-dimensional photonic quasicrystal structure and use the set of plane waves defined by the diffraction pattern as the basis of a theoretical approach to calculate the photonic band structure of the system. An important feature of the model is that it retains the essence of the rotational and inflational properties of the quasicrystal at all levels of approximation: properties lost in approximate models which artificially introduce elements of periodicity. The calculated density of modes of the quasicrystals is found to display a weakly depleted region analogous to the bandgap that occurs in a periodic system. The calculated transmission spectra for different polarizations and directions of propagation show features that correlate with the behaviour of the density of modes.

Terahertz frequency bandpass filters

A novel method for the analysis of thin biological films, called dual polarization interferometry?(DPI), is described. This high resolution (<1??), laboratory-based technique allows the thickness and refractive index (density) of biological molecules adsorbing or reacting at the solid?liquid interface to be measured in real time (up to 10 measurements per second). Results from the adsorption of bovine serum albumin (BSA) on to a silicon oxynitride chip surface are presented to demonstrate how time dependent molecular behaviour can be examined using DPI. Mechanistic and structural information relating to the adsorption process is obtained as a function of the solution pH.