The recently developed fluorescence confocal polarizing microscopy (FCPM) imaging technique allows 3D images of
the director structure in a liquid crystal cell to be resolved with sub-micron resolution. Results are presented on imaging
the response of 5-micron pitch cholesteric liquid crystals to an in-plane electric field applied between two silver
electrodes. The results show, in exquisite detail, how the application of an in-plane field causes the cholesteric helix to
tilt through 90° either within, or immediately adjacent to, the electrode gap depending on the sign of the dielectric
anisotropy of the liquid crystal. Furthermore, imaging the cholesteric material above the silver electrodes reveals a
previously unreported optical intensity enhancement. This phenomenon is discussed along with its possible benefits to
the existing imaging technique. The effects of the point spread function of the system are discussed and a ray optics
model is used to produce model data highlighting the influence of this phenomenon on the recorded results.
The field of plasmonics has historically been a playground exclusively for the optics community. Primarily this is because
the response of metals becomes dominated by their large conductivities at much lower frequencies, making it difficult
to exploit the unique properties of surface plasmon (SP) modes. Indeed SPs on flat, perfectly conducting substrates
are better described as simple surface currents or grazing photons. However the realization that one can form metal-dielectric
composites to support surface waves with plasmon-like properties has opened the field of plasmonics to the
terahertz and microwave domains. Pendry et al. [Science, 305, 847 (2004)] were among the first to speculate about an
extension of plasmonics into long wavelength regimes. They demonstrated that the perforated surface of a perfect conductor
can support a SP-like mode whose behavior is determined purely by the geometry of the substrate. Beginning
with our initial experimental verification of these SP-like modes excited via grating-coupling, we present an overview of
some of our recent microwave studies. We progress to study the classical method of prism coupling and also consider
the enhanced transmission phenomenon (mediated by plasmon-like surface modes) through hole arrays. Finally the first
experimental evidence of coupled SP-like modes between two such perforated metal substrates placed in close proximity
will be presented.
Fabry-Perot cavities are perhaps the best known of the optical transmission resonators, with cavity field enhancement
accomplished by two parallel and partially reflecting planes. Recently it has been shown that arrays of narrow slits cut into a
metal substrate are similarly able to exhibit resonant transmission modes. Here, the transmission of normally incident plane
wave microwaves through a single stepped sub-wavelength slit in a thick metal plate is explored. The presence of the step
substantially increases the radiation wavelength, which may be resonantly transmitted to well beyond twice the plate
thickness. Insight into the resonant behaviour of the stepped slit is provided through the analysis of the field solutions
produced by a finite element model. This model also predicts resonant transmission which is in excellent agreement with the
When perforated metal films are sufficiently thin, in addition to exciting surface plasmon-polariton
(SPP) modes by conventional two dimensional grating scattering, there is also the possibility of
coupling to the localised modes associated with the holes. Here, experimental transmission spectra are
obtained from focused ion beam fabricated hole arrays exhibiting localized modes in the visible
frequency region. We employ both analytical and numerical (finite element) modeling to understand
the fundamental properties of the localized mode. Finally, the sensitivity of the optical response to
changes in refractive index is explored, and its potential for sensing applications is discussed.
Using liquid crystals to control the propagation of microwaves is a potentially interesting technology. By incorporating small amounts of liquid crystal in thin slat metal structures through which the microwaves may resonantly pass a whole new range of voltage tuned microwave devices are becoming available. Metallic sub-wavelength slit structures at microwave frequencies have been constructed which show Fabry-Perot type resonances in very thin slits. If the dielectric in such thin slits is an aligned liquid crystal it is found possible to voltage-control the resonant frequencies. Novel selective filters and structures for microwave beam steering have been fabricated leading to a new generation of liquid crystal controlled devices.
It is well established that much more radiation may be transmitted through a set of apertures in a metallic screen than a simple calculation from the transmission through the aperture area alone would predict. There has been substantial debate regarding the exact cause of this enhanced transmission, and confusion over the difference between the behaviors of subwavelength apertures as opposed to subwavelength slits. In this study we have analyzed the transmission response of individual slits, using microwave radiation to ensure that transmission is in no part due to direct passage through the metal screen itself. A set of resonant transmission peaks is caused by the excitation of standing-wave-coupled surface plamsons in the finite length slit. It is also found that the high but finite value of the metals’ conductivity influences the transmission response of such slit channels when they are less than 100 microns in width. Indeed there is a strong decrease in transmitted resonant frequency, remarkably tending to zero as the slit width decreases. In addition we have explored the effect of misalignment of the two metal plates that comprise the slit. This modifies resonant frequencies and transmitted intensities through the changing boundary conditions at the slit ends.
It is shown that microwave radiation can be transmitted through a wall of aluminum-alloy bricks even though the width of the gaps between the metallic elements is less than 5% of the radiation wavelength. Up to 90% of the radiation made incident upon the wall is transmitted, with both linear polarizations being passed. Experimental results are compared to theoretical predictions. Proving that the transmission mechanism relies upon self-coupled surface plasmon resonances in what are effectively Fabry-Perot cavities.
Surveys of the natural world reveal an extensive array of optical effects in a broad range of animal and insect species. While the aesthete may simply take delight in such phenomena, students of photonics have increasingly been prepared to look more closely; deriving understanding and inspiration from nature's optical ingenuity. By describing specific structural color examples in detail, within a general context of Lepidopteran microstructure classification, this review paper seeks to present an introduction to current work on butterfly photonics. Additionally, new but preliminary results and analysis are presented, that describe the structural color of a butterfly that exhibits a distinct 3D photonic crystal structure.
In this work sinusoidal diffraction gratings with a range of pitches and amplitudes are used to algin nematic liquid crystal layers in a twisted homogeneous configuration. The grating profiles are accurately characterized using optical surface plasmon polariton spectroscopy which then allows a calculation of the anchoring energy as predicted by the simple Berreman expression. The experimental Rapini-Papoular anchoring energy is also obtained by a measurement of the director twist away from the alignment direction at room temperature. A linear relationship is found between the two anchoring energies except when it falls below 4 X 10<SUP>-7</SUP>Jm<SUP>-2</SUP>. Noticeably, the correlation between the two theories is hot unity, if room temperature elastic constants are used in the calculation. This apparent inconsistency is explained if the effect of surface memory on the system is considered. Indeed if elastic constant, corresponding to a higher temperature, at which surface memory effects are absent, are used in the Berreman expression, good agreement between the predicted and experimentally measured energies is found.
In this work, the electro-optic response of 6CB liquid crystal is studied using a sensitive differential technique. The liquid crystal layer is held at a temperature just above the nematic to isotropic (N-I) phase transition. Transverse magnetic (p) polarized light incident on the cell is coupled to guided modes in the liquid crystal layer using either grating or prism coupling. In both cases the modes manifest themselves as sharp resonances in the reflectivity as the angle of incidence is scanned. When a low frequency sinusoidal voltage is applied to the cell the resonance shapes and positions are altered at a frequency which is twice that of the applied field, resulting in a modulation of the reflectivity for a given angle of incidence. By carefully observing the modulated signal, and comparing the data with modelling generated from multilayer optics theory, two effects are quantified. The first of these is an induced birefringence, (delta) n, varying quadratically with applied voltage, which is well understood and can be expressed in terms of Landau De Gennes theory. The second is a field induced perturbation in the imaginary part of the optical permittivity, (delta) (epsilon) <SUB>i</SUB>, which implies a modification of the light scattering properties of the liquid crystal. The measurement of the latter effect is, as far as we know, a novel one, being only made possible by the remarkable sensitivity of the synchronous differential technique.
Electromagnetic modes may be excited by photons using grating coupling to provide the necessary extra momentum. Rotation of the grating so that the grooves are no longer perpendicular to the plane of incidence (conical diffraction) breaks the symmetry of the system and introduces polarization conversion. This conversion is strongly enhanced when a mode is excited. Reflectivity data are presented showing polarization enhancement by the excitation of surface plasmon polaritons (SPPs), long range surface plasmon polaritons, and resonant guided modes on surface modulated gratings. The dependence of this conversion on angle of incidence, azimuthal angle, and groove depth has been studied in detail. When the grooves are at 45 degree(s) to the plane of incidence, maximum conversion is recorded for the SPP. However, for guided modes the maximum conversion occurs at angles other than 45 degree(s). The conversion has been seen to rise monotonically with groove depth and efficiencies of over 50% are easily obtained via SPPs and guided modes. Theoretical comparisons with data are made using a rigorous differential method applicable for conical diffraction. This formulation uses a coordinate transformation between the grating surface and a planar system. The comparison between data and theory is very good and further theoretical modeling predicts that conversion efficiencies can approach 100% for appropriate circumstances.