Resonant cavities based on whispering gallery modes (WGM) have high quality factor, Q. This property is highly
desirable for the design of a variety of devices. The quality factor is larger for larger systems, but, at the same time the
density of modes increases, and not as regularly spaced frequencies, because modes of different kind superpose. For a
dielectric cylinder, the modes are characterized by their azimuthal and radial behavior. The complex frequencies
corresponding to the modes have a strong dependence on the radial order. We use the finite difference time domain
method, to study the transmission of light by two prisms with a dielectric cylinder between them. The system acts as a
filter with sharp peaks of modes of high Q. The coupling to the modes can be controlled by changing the distance of
separation between the prisms. For large coupling, the full width at half maximum of the transmission peaks increases,
being more noticeable for the high Q modes.
Experimental results of reflection and light scattering measurements using the technique of attenuated total reflection (ATR) in the Kretschmann configuration of a system when it is excited an electromagnetic guided mode are reported. The system used is BK7 glass-prism/Dielectric1/Dielectric2/Dielectric1/air, where the refractive index of dielectric1 is less than the refractive index of the dielectric2. It was found a dip in the specular reflection as a function of the incident angle due to the excitation of a guided mode in the dielectric2 film. The guided mode was found for s polarization of the incident light. The angular dependence of the scattered light displays a peak caused by single-scattering and located approximately at the angles of excitation of the guided modes whose normalized wave numbers are less than the refractive index of the glass. Values of thickness, refractive index and absorption index obtained from Lorentz dispersion model of the film are reported.
We reported experimental results of the resonant scattering of light from a system prism-glass/Ag/MgF2/air in the ATR-Kretschmann configuration, for p-polarized light incident by the glass side. The thickness of the dielectric film is chosen in such a way that in the absence of roughness the system supports 3 transverse magnetic (TM) guided modes, at a wavelength λ = 632.8 nm of the incident light. The scattering is due to the natural roughness of each interface of the system, while the resonant character of the scattering is due to the excitation of the guided modes and their interaction with the interfaces roughness. The scattered light shows six peaks at angles given by θ1 = ± 61.65 0, θ2 = ± 53.69 0, and θ3 = ± 43.40 0, for any angle of incidence. These angles correspond to the excitation of the guided modes. The scattering response is enhanced when the angle of incidence is equal to one of the angles of excitation of the guided modes.
We obtain the reflection spectra of p-polarized light from metallic gratings fabricated by means of the interference technique. The gratings have constant period and heights are varied by changing the exposure time of the interference pattern on the sample. The experimental spectra show two minima due to the excitation of surface plasmons, and they are compared with the corresponding numerical results obtained with two models, the Rayleigh theory and the rigorous integral method. In order to demonstrate the validity of the Rayleigh hypothesis the amplitudes of the gratings are chosen close to the Rayleigh limit.
We study the resonant excitation of the electromagnetic modes in a planar waveguide of metallic walls - light incident on the guide from the air can transfer energy through the walls exciting normal modes of propagation. It is found theoretically that radiation propagates along the guide while the reflectivity presents a minimum. The energy of the incident radiation can be transferred to the guide almost completely when the thickness dm of the metallic wall is around two times the skin depth. Experimental evidence of the injection of light is presented for the system Ag/Al2O3/Ag that was grown by pulsed laser deposition.
The scattered light from a bi-layer system with a shallow random rough surface bounded by semi-infinite dissimilar optical media is calculated. Results are obtained by assuming a gaussian roughness spectrum for the random rough surface and the formalism is applied to simulate the scattering in the ATR-Kretschmann configuration, allowing the excitation of guided waves.
The reflected and transmitted fields scattered from a layered structure, build up of N thin films with shallow rough interfaces and bounded by two semi-infinite media are determined to analyze the effects of roughness in the optical response of a given multilayer system. Each layer is assumed to be homogeneous, isotropic, local, linear and characterized by a frequency dependent complex dielectric function. All rough interfaces are modeled by using a stationary random process, and the Rayleigh criteria is used in the mathematical treatment because of the shallowness of the defined roughness. Two independent integral equations relating the reflected and transmitted fields to the incident wave are found. These will allow a recursive calculation of the overall optical responses. The solution involves Fourier coefficients of functions dependent on the roughness profiles. This treatment is shown to be valid for both TM(p) or TE(s) states of polarization. Numerical results are obtained for the case of a two layer system, with two of the interfaces present Gaussian roughnesses and compared to some experimental data.
The sum of several periodic functions is used as an approach to simulate random interfaces in multilayers where a metal is coated with a dielectric thin-film or viceversa. Experimental deviations from reflectivity of such systems respect to those with smooth interfaces are well explained with the Rayleigh theory.