Beginning from low level concepts the basic understanding for the optical excitation of surface plasmons is developed. Prism coupling using the attenuated total reflection technique is discussed as well as the less traditional grating coupling technique. A brief discussion of some recent developments using twisted gratings is also presented. Finally a short summary of the potential device applications is given.

Optical excitation of surface plasmons: An introduction

Observation of a spin symmetry breaking effect in plasmonic nanoscale structures due to spin-orbit interaction is presented. We demonstrate a nanoplasmonic structure which exhibits a crucial role of an angular momentum (AM) selection rule in a light-surface plasmon scattering process. In our experiment, the intrinsic AM (spin) of the incident radiation is coupled to the extrinsic momentum (orbital AM) of the surface plasmons via spin-orbit interaction. Due to this effect, we achieved a spin-controlled enhanced transmission through a coaxial nanoaperture.

Observation of Optical Spin Symmetry Breaking in Nanoapertures

A formalism is presented for calculating the ordinary and extraordinary refractive indices for a uniaxial film with optical properties equivalent to those of a conically mounted zeroth-order lamellar grating. In the quasi-static limit (grating-period-to-wavelength ratio → 0) this treatment is exact for both dielectric and metallic gratings. For period-to-wavelength ratios approaching unity, the effective anisotropic refractive indices are dependent on the polar and azimuthal incidence angles. The validity of this method as the period-to-wavelength ratio increases is studied by comparison of the method with the predictions of rigorous grating theories. Comparisons are then made with the predictions of other effective-medium theories. Examples of the use of this theory for conically mounted zeroth-order gratings are presented.

Effective-medium theory of zeroth-order lamellar gratings in conical mountings

An optical assaying method and system having a movable sensor is described. In one aspect, the present invention is a sensing system having a rotating sensor disk coated with indicator dyes sensitized to a variety of substances. In this configuration the sensing system further includes a detector for sensing spectral changes in light received from one or more of the indicator dyes. In another aspect, the present invention is a sensing system having a surface plasmon resonance sensor disk having grooves extending radially from a center of the disk. In yet another aspect, the present invention is a sensing system including a diffraction anomaly sensor disk having a dielectric layer that varies in thickness. The present invention allows for construction of an inexpensive sensing system that is capable of easily detecting a variety of substances either in a sample or a surrounding environment. Furthermore, the present invention provides a sensing system capable of sensing multiple substances without requiring multiple sensors.

Optical assaying method and system having movable sensor with multiple sensing regions

A good basis has been established for the development of a prototype gas sensor using the phenomenon of surface plasmon resonance. By exciting a surface plasmon on a metallic diffraction grating that is twisted azimuthally so its grooves are not perpendicular to the plane of incidence, and with suitable choice of input and output polarization, a resonance maximum is detected (as opposed to the usual resonance minimum). the operation of the sensor is based on the measurement of this resonance maximum on a background of weak signal and incorporates a sensing head made remote from both the source and detector by means of fibre optics. Its use is demonstrated by sensing remotely the condensation of ≈0.9 nm of isopropyl alcohol onto a silver-coated grating surface.

Development of a prototype gas sensor using surface plasmon resonance on gratings

Surface plasmon-polaritons may be excited by photons at a metal-air boundary if a grating is used to provide the necessary extra momentum. Rotation of the grating so that the grooves are no longer perpendicular to the plane of incidence reduces the coupling efficiency while at the same time producing some s-polarised radiation in the reflected beam. When the grooves are at 45° to the plane of incidence maximum p-s conversion is obtained. Conversion efficiencies of up to 66% have been recorded for silver-coated gratings of appropriate depth. By scanning the angle of incidence and using a crossed polariser on the detector a strong resonance peak is observed. The application of this phenomenon to optical sensing is considered.

G.P. Bryan-Brown

Papers

Polarisation Conversion through the Excitation of Surface Plasmons on a Metallic Grating