Ian R. Hooper

Directionally Controlled Fluorescence Emission in Butterflies

Recently developed, high-efficiency, light-emitting diodes use two-dimensional photonic crystals to enhance the extraction of otherwise internally trapped light and multilayer reflectors to control the direction of light emission. This work describes the characterization of a naturally evolved light-extraction system on the wing scales of a small group of Papilio butterflies. The efficient extraction of fluorescence from these scales is facilitated by a two-dimensional photonic crystal slab that uses a multilayer to help control emission direction. Its light-extraction function is analogous to that of the light-emitting diode.

Experimental method for reliably establishing the refractive index of buprestid beetle exocuticle

In this study we apply an existing optical characterisation technique to establish reliably the complex refractive indices of layers comprising a natural multilayer reflector in the beetle Chrysochroa raja. Its reflector characteristics, ultrastructure and layer thicknesses were established using electron and optical microscopy. We recorded a significant number of wavelength dependent optical data sets from the same regions of sample using both linear polarisations and from a variety of different angles. These optical data sets were modelled simultaneously in order to significantly reduce the degeneracy of the fitting process. For the C. raja sample in question, the fitted complex refractive indices of both layer types were determined to be n=1.68 k=0.03 and n=1.55 k=0.14.

Transmission of light through thin silver films via surface plasmon-polaritons

We report results of measurements and calculations that help to clarify the role of surface plasmon-polariton modes in the transmission of light through thin continuous films of silver. Our experimental data show that there is an optimum silver film thickness for which transmission is maximal. We offer an explanation of this phenomenon in terms of competition between increasing absorption in the metal and increasing optical field-enhancement due to surface plasmon-polariton excitation as the metal film thickness is increased. We find no need to invoke the regeneration of evanescent waves as has recently been suggested.

Sensing using differential surface plasmon ellipsometry

In this work a differential ellipsometric method utilizing surface plasmons (SPs) for monitoring refractive index changes, which could be used in chemical and biological sensors, is presented. The method is based upon determining the azimuth of elliptically polarized light reflected from a Kretschmann SP system, resulting from linearly polarized light containing both p and s components incident upon it. The sensitivity of this azimuth to the refractive index of a dielectric on the nonprism side of the metal film is demonstrated both experimentally and theoretically. The smallest refractive index change which is resolvable is of the order of 10−7 refractive index units, although it is believed that this could be improved upon were it not for experimental constraints due to atmospheric changes and vibrations. The method requires the Kretschmann configuration to be oriented at a fixed angle, and the SP to be excited at a fixed wavelength. With no moving parts this method would be particularly robust from an ...

Differential ellipsometric surface plasmon resonance sensors with liquid crystal polarization modulators

Differential ellipsometric interrogation of surface plasmon (SP) resonances is a technique that gives ultrahigh sensitivity to refractive index changes, and it may provide the basis for chemical and biological sensors. In this study, a liquid crystal polarization modulator has been developed to provide such a differential technique. A refractive index sensitivity of 2×10−7 refractive index units is demonstrated, which is at least as sensitive as more established SP sensing techniques. The use of a liquid crystal modulator allows for low-voltage signal modulation and also feedback locking to zero. Possibly more important, it leads to pixelization for array sensing and for potential imaging.

Broadband polarization-converting mirror for the visible region of the spectrum

A mirror structure that enables the polarization of linearly polarized light to be rotated by 90 degrees over the entire visible region of the spectrum is presented. Theoretical modeling is used to show that this phenomenon occurs for light that is normally incident upon a metal grating consisting of a series of high and narrow ridges that are oriented at 45 degrees to the polarization angle. This broad polarization-conversion band is shown to arise from mode mixing of surface plasmon polariton-mediated polarization-conversion bands and interference-mediated polarization-conversion bands.

Magneto-optic behaviour in the presence of surface plasmons

Insight into the mechanism responsible for the enhanced level of magneto-optical (MO) activity observed from magnetic surfaces when supporting propagating surface plasmons (SPs) is provided by studies of ferromagnetic Ni and Co gratings where SPs are generated and interact magneto-optically at the air/ferromagnetic interface. These ferromagnetic gratings were subsequently coated with semi-transparent Ag layers. In these systems only some 30% of the light incident on the air/Ag interface is reflected; most is converted to surface plasmons whilst the remainder propagates to reflect off the ferromagnetic surface where it undergoes a MO interaction before returning through the Ag. The precise nature of the beam returned from the ferromagnetic and the MO signature imprinted on it depends on the relative magnitude of two driving sources: the E-field of the light wave and the E-field associated with SPs penetrating through the Ag to the ferromagnetic. Evidence is obtained indicating that the degree of MO enhancement correlates strongly with the magnitude of the intense electric field associated with the presence of SPs at the ferromagnetic surface, confirming previous experimental studies postulating that enhanced MO effects are not solely a consequence of the reduction in reflection that is experienced at surface plasmon resonance.

Surface plasmon polaritons on narrow-ridged short-pitch metal gratings in the conical mount

Recent investigations into high-aspect-ratio short-pitch metal grating structures have shown that it is possible to excite surface plasmon polaritons (SPPs) even in the zero-order region of the spectrum. The predominant reason this is possible is that extremely large bandgaps occur in the SPP dispersion curves, which are caused by the large depths, and heights, of the structures. The form of the resultant dispersion curves has also been found to be highly dependent on the shape of the grating profile. We present an extension to a previously published paper that described the nature of the SPPs excited on narrow-ridged short-pitch metal gratings in the classical mount by considering the case in which the radiation is incident at nonzero azimuthal angles (the conical mount). In particular, we consider the case of 90° and 45° azimuthal angles and discuss the coupling to the SPP modes and the way in which polarization conversion is evident on such structures.

Some considerations on the transmissivity of thin metal films

As interest in plasmonics grows the optical properties of thin metal films becomes increasingly significant. Here we explore the transmissivity of thin metal films at normal incidence, from the ultraviolet to microwaves, and show how, contrary to simplistic treatments, the microwave transmissivity may be much less than the optical transmissivity for films which are well below the skin depth in thickness. This arises because the film is acting as a zero order Fabry-Perot with very high reflectivity at each interface. The skin depth then becomes irrelevant for thin metal films at microwave frequencies. We also note in passing that the expected exponential dependence on thickness at higher thicknesses has an asymptotic limit at zero thickness which may be as high as four times the input intensity.

Massively Sub-wavelength Guiding of Electromagnetic Waves

Recently a new form of ultra-thin flexible waveguide consisting of a conducting comb-like structure with a thickness of the order of 1/600th of the operating wavelength was presented. However, whilst the thickness of the guide was massively sub-wavelength, the remaining dimensions (the height and period of the comb) were much longer. In this paper we propose, and experimentally verify, that a modified guiding geometry consisting of a chain of ultra-thin conducting spirals allows guiding of electromagnetic waves with wavelengths that are many times (40+) longer than any characteristic dimension of the guide, enabling super-sub-wavelength guiding and localisation of electromagnetic energy.