The effect of the aspect ratio of rectangular holes on the transmissivity of periodic arrays of subwavelength holes in optically thick metal films is investigated. The transmissivity is found to be highly dependent on the aspect ratio of the holes. Moreover, the wavelengths of maximum transmissivity show a monotonous shift as a function of the aspect ratio of the holes. We attribute the enhanced transmission of the periodic arrays to an interplay of surface plasmons at the surface of the metal and shape resonances ͑also known as localized modes͒ inside the holes. The importance of the shape resonances was confirmed by a comparison of transmission through periodic hole arrays and through randomly distributed holes. Dispersion curves of periodic and random hole arrays confirmed the existence of shape resonance as well. We suggest that the localized modes effectively act as waveguides and increase the coupling efficiency of surface plasmons between both sides of the film, which results in a higher transmissivity. The shift of the maxima of the transmissivity may in part be explained by the spectral position of the localized modes in the individual holes. Finally measurements on similar patterns in Ni and Ag revealed that the occurrence of shape resonances is independent of the material of the film.

We have experimentally studied the distribution of the spatial extent of modes and the crossover from essentially single-mode to distinctly multimode behavior inside a porous gallium phosphide random laser. This system serves as a paragon for random lasers due to its exemplary high index contrast. In the multimode regime, we observed mode competition. We have measured the distribution of spectral mode spacings in our emission spectra and found level repulsion that is well described by the Gaussian orthogonal ensemble of random-matrix theory.

We show that the extraordinary transmission of light through an array of square subwavelength holes is strongly influenced by the size of the holes. For small, square holes (air fraction below 20%), the dependence of the normalized transmission (transmissivity) on hole width greatly exceeds the expectations on the basis of conventional aperture theory. For larger holes, the transmissivity saturates. Moreover, the positions of the transmission maxima shift when the size is varied.

We present a quantitative experimental and theoretical study of intensity fluctuations in the emitted light of a random laser that has different realizations of disorder for every pump pulse. A model that clarifies these intrinsic fluctuations is developed. We describe the output versus input power graphs of the random laser with an effective spontaneous emission factor ͑␤ factor͒.

We propose the use of an optically trapped, dye doped polystyrene microsphere for spatial probing of the refractive index at any position in a fluid. We demonstrate the use of the dye embedded in the microsphere as an internal broadband excitation source, thus eliminating the need for a tunable excitation source. We measured the full width at half maximum of the TE and TM resonances, and their frequency spacing as a function of the refractive index of the immersion fluid. From these relations we obtained an absolute sensitivity of 5 × 10 −4 in local refractive index, even when the exact size of the microsphere was not a priori known.Microspheres can act as high Q resonators in the optical regime 1, 2 and can store electromagnetic waves by repeated total internal reflection. Constructive interference results in an enhanced internal field which is called a whispering gallery mode (WGM). Because of the evanescent interaction between the WGM and the surrounding medium the WGM's resonance frequency is sensitive to changes in the refractive index of the sphere's surroundings. This sensitivity makes microspheres an useful tool in sensory applications. Recently Hanumegowda et al. demonstrated the use of microspheres in refractometric sensing. 3 Moreover Vollmer et al. 4 and White et al. 5 demonstrated protein detection using microspheres. In all these studies 3-5 narrowband light from a tunable diode laser was evanescently coupled to the microsphere via an optical fiber. Spatial mapping of the refractive index was not possible because the microsphere could not be freely moved through the sample.In this letter we demonstrate the use of a dye-doped polystyrene microsphere as a refractometric sensor. The polystyrene microsphere was trapped with optical tweezers, by which it could be positioned anywhere within the fluid. It can therefore be used to sense the refractive index in spatially inhomogeneous media, for example near interfaces or in mixing flows. In this system we did not need a tunable laser to excite WGMs, instead we used the dye embedded in the microsphere itself as a broadband excitation source. The absolute frequency of the WGM, which was used to probe the refractive index in previous studies, 3-5 is very sensitive to the size of the sphere. Microspheres synthesized in bulk always have a considerable size polydispersity, and therefore a wide distribution of absolute resonance frequencies. Measurements of the absolute resonance frequency of a particle that has not been individually calibrated does not provide information about the refractive information of the medium. In contrast, we show that both the full width at half maximum (FWHM) of the emission peaks and the frequency spacing between adjacent transverse electric (TE) and transverse magnetic (TM) modes are sensitive and robust probes of the refractive index of the sphere's surroundings. We show that a calibration of the individual microspheres is not needed in this case.The FWHM of a WGM is determined by the loss upon total internal reflection at the curv...

We prescribe the minimal set of experimental data and parameters that should be reported for random-laser experiments and models. This prescript allows for a quantitative comparison between different experiments, and for a criterion whether a model predicts the outcome of an experiment correctly. In none of more than 150 papers on random lasers that we found these requirements were fulfilled. We have nevertheless been able to analyze a number of published experimental results and recent experiments of our own. Using our method we determined that the most intriguing property of the random laser (spikes) is in fact remarkably similar for different random lasers.

We show that time-independent scattering coefficients calculated from the standard extrapolation of Mie theory to the gain regime have physical meaning up to the laser threshold. The theoretical width of a resonance decreases linearly with increasing gain and becomes zero at the laser threshold. We performed experiments on dielectric microspheres with gain, trapped with optical tweezers. The width of the mode was measured to narrow as a function of the gain up to the lasing threshold, confirming both the validity of the extrapolation of Mie theory to the gain regime below threshold and our interpretation of its point of divergence as the laser threshold.

We have measured the evolution of the light intensity of a random laser during a nanosecond pump pulse. Relaxation oscillations in a titania random laser were observed in the time trace of the total emitted intensity. We compare our experimental results with a simple model, based on the four-level rate equations for a single mode laser.