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Electrically controlled surface plasmon dispersion in hole arrays

A nanostructured optical system based on polaritonic crystals and a liquid crystal layer can modulate light transmission up to 100Hz.
5 May 2008, SPIE Newsroom. DOI: 10.1117/2.1200804.1101

The ability to confine, guide, and manipulate light in two dimensions on subwavelength scales is likely to herald a new era in photonics. The control of optical signals has been achieved by coupling them to coherent electronic excitations near a metal surface (surface plasmons). As a result, the field of nanoplasmonics is rapidly advancing, with the development of an impressive catalogue of subwavelength photonic components such as mirrors, lenses, and waveguides.

The enhanced optical properties of metal films periodically perforated with arrays of subwavelength-sized holes (plasmonic crystals) have recently been the object of intense research in surface plasmon optics. Plasmonic crystals1,2 are now considered basic materials for superlenses,3,4 metamaterials for negative refraction applications,5,6 and enhanced nonlinear metamaterials.7,8 These nanostructures are becoming important components of light-emitting devices, nanolasers, photodetectors, nanoscale light sources, and for optical data storage, imaging, and sensing.

The ability to engineer the optical transmission of plasmonic crystals by varying their geometry provides them with great flexibility. However, transforming these passive optical elements into actively controlled devices presents a challenge. Currently, manipulating the refractive index of the dielectric medium adjacent to the metal surface is the preferred method for tweaking surface plasmons and their associated optical properties. An electro-optically active dielectric medium is an obvious choice since electric signals can be guided within the same metallodielectric structure. The large broadband optical anisotropy of liquid crystal (LC) molecules makes them ideal candidates for electrically operated nanoplasmonic devices, as illustrated by several experiments performed on smooth films and nanoparticles.9–12

Figure 1. Optical transmission dispersion plots for the structures in liquid crystal without and with an applied electric field (12.55kV/cm) for the lines (a, b) and the holes (d, e). Change in frequency dispersion associated with the application of the electric field (c, f). The yellow arrow in (f) indicates where the dynamic response of the transmission was studied. Color bar intensities are normalized to 1. Au: Gold. Subs: Substrate. LC: Liquid crystal.

Figure 2. Electric field dependence of the optical transmission for the 2D array. (a) The change in transmission (color plot) for an increasing applied electric field. (b) Transmission spectra for several values of the applied electric field. The wavelength used to probe the switching dynamics is shown by a dashed orange line. (c, d) Light intensity transmitted through the 2D array at 670nm with the applied field modulated at 2.5 and 100Hz, respectively. E-field: Electric field.

We recently realized an electrically controlled nanostructured optical system based on surface plasmon polaritonic (SPP) crystals in contact with an LC layer. We studied the effect of modulating the LC layer on the surface plasmon dispersion and the related optical transmission.

To demonstrate the effect, we used structures consisting of both 1D and 2D arrays of subwavelength apertures having the same period (550nm). In the 1D case, the array lines had a width of ∼100nm. For the 2D structures, square arrays were created using a circular hole of diameter 180nm as the lattice basis. Figure 1 shows the dispersion plots hω=f(k) of the zero-order optical transmission of these nanostructures when the static electric field controlling the LC orientation in the cell has values of 0kV/cm (OFF) or 12.5kV/cm (ON). The polarization of the incident light is in the plane of incidence and perpendicular to the lines.

Voltage-induced changes in transmission T are highlighted in the differential dispersion plots shown in Figure 1(c, f), which represent the differential ratio (TONTOFF)/TOFF. When an electric field was applied to the structure, we observed a modification of the transmittance and spectral position of the resonances. As expected, transmission changes were most important close to the band edges of the SPP crystals. The different SPP bands are identified in Figure 1 by the values of (l, m) corresponding to the direction in reciprocal space responsible for their scattering and the supporting interface. When compared to the 1D case shown in Figure 1(a, b), the dispersion of the 2D crystals plotted in Figure 1(d, e) exhibits additional bands due to extra Bragg scattering channels along the (±1, ±1) directions that are not present in 1D crystals. This plot shows that the lower SPP band (on the gold/LC interface) is most sensitive to changes of the refractive index in the cell rather than to the close proximity of the interface. This suggests that the low-frequency bands, such as the (±1,0), penetrate further into the LC region than the higher bands, which are more confined to this interface.13

The dynamic response of the LC cell was investigated by monitoring the transmission of the 2D array (200nm-thick film) while the cell was subjected to a time-modulated electric potential. The energy and momentum of the incident light were selected to probe the mode at k∥=0 μm-1, hω=1.85eV, corresponding to a vacuum wavelength of 670nm. A large variation of transmitted light intensity was observed on application of the electric field, indicated by the yellow arrow in Figure 1(f). The change in optical transmission with increasing applied field is illustrated by the color plot shown in Figure 2(a): the conformational change of the LC layer occurs between 0.4 and 5kV/cm, resulting in unstable transmission. The spectral shift and change in amplitude of the transmission peaks observed with an applied field are also clearly visible in Figure 2(b).

The dynamic response of the transmission with applied field at frequencies of 2.5 and 100Hz shows the ability to switch and modulate light at a frequency corresponding to that of today's modern LC displays with a less than 10% reduction in amplitude. We anticipate that the application of LC molecules to plasmonic crystals may lead to the development of plasmonic devices with electronically controlled and enhanced transmission, reflection, and absorption properties for use as light modulators, switches, electronically tunable spectral filters, and diffraction gratings.

Future measurements in this area should concentrate on investigating the liquid crystal switching on optically thin gold films (<50nm). SPP coupling between the upper and lower interface is expected to be strong in this case, leading to a more pronounced change in the transmission and dispersion of the modes in the plasmonic crystals.

Wayne Dickson
Centre for Nanostructured Media
Queen's University Belfast
Belfast, UK

Wayne Dickson is a research fellow.