Sushil Mujumdar

Plasmonically enhanced diffusive and subdiffusive metal nanoparticle-dye random laser

We report on surface plasmon (SP)-enhanced random laser emission from a suspension of silver nanoparticles in a laser dye operating at diffusive and subdiffusive scattering strengths. SP resonance enhances the scattering cross section, while the geometrical cross section remains small, thus providing a large gain volume. The localized electromagnetic field near the particle surface leads to enhanced absorption of excitation light and larger amplification of fluorescence. The metal-nanoparticle-based random laser yields larger linewidth narrowing at lower pump fluence threshold than a dielectric-scatterer-based random laser under equivalent conditions. These findings open the door to studies of applications related to light amplification assisted by SP in metallic nanoparticles.

Optics of nanostructured dielectrics

We discuss the optical transport properties of complex photonic structures ranging from ordered photonic crystals to disordered strongly-scattering materials, with particular focus on the intermediate regime between complete order and disorder. We start by giving an overview of the field and explain the important analogies between the transport of optical waves in complex photonic materials and the transport of electrons in solids. We then discuss amplifying disordered materials that exhibit random laser action and show how liquid crystal infiltration can be used to control the scattering strength of random structures. Also we discuss the occurrence of narrow emission modes in random lasers. Liquid crystals are discussed as an example of a partially ordered system and particular attention is dedicated to quasi-crystalline materials. One-dimensional quasi-crystals can be realized by controlled etching of multi-layer structures in silicon. Transmission spectra of Fibonacci type quasi-crystals are reported and the (self-similar) light distributions of the transmission modes at the Fibonacci band edge are calculated and discussed.

Near-field imaging and frequency tuning of a high-Q photonic crystal membrane microcavity.

We discuss experimental studies of the interaction between a nanoscopic object and a photonic crystal membrane resonator of quality factor Q=55000. By controlled actuation of a glass fiber tip in the near field of the photonic crystal, we constructed a complete spatio-spectral map of the resonator mode and its coupling with the fiber tip. On the one hand, our findings demonstrate that scanning probes can profoundly influence the optical characteristics and the near-field images of photonic devices. On the other hand, we show that the introduction of a nanoscopic object provides a low loss method for on-command tuning of a photonic crystal resonator frequency. Our results are in a very good agreement with the predictions of a combined numerical/analytical theory.

Temperature-tunable random lasing: numerical calculations and experiments

We report numerical studies on temperature-tunable, multiple-scattering media with gain. We describe Monte Carlo simulations that model the behavior of such a system through a three-dimensional random walk of light in a temperature-dependent disordered medium with amplification. We compare the results of our model with previous experimental results on a disordered dielectric for which the scattering strength could be tuned by changing the external temperature. The agreement between the numerical and experimental results enables us to predict the spectral features of the emission from the tunable random laser under various conditions. Results obtained from new experimental data are consistent with the predictions of the simulations.

Identification of statistical regimes and crossovers in coherent random laser emission

We measure intensity statistics and identify statistical regimes and crossovers in random lasers based on nonresonant feedback. A single parameter extracted from an α-stable Levy fit is used to characterize the intensity distributions in all regimes. Measurements made over a range of scattering strengths, excitation energies, and sample sizes enable us to demarcate three regimes of intensity statistics and the corresponding crossovers. An initial subthreshold Gaussian regime abruptly transits into a Levy regime at the random lasing threshold, which is followed by a continuous gradual crossover toward a second Gaussian regime. We find that the prominence of the Levy regime depends upon the sample size.

Use of a graded gain random amplifier as an optical diode.

The spectral characteristics of liquid amplifying media have been used to design and experimentally realize an optical device that prevents the propagation of a band of wavelengths in one direction and permits it in the opposite direction, thus acting as an optical diode. The addition of random scattering centers is shown to narrow the width of the forbidden band. A model is proposed to explain the observations and is verified by Monte Carlo simulations.

Experimental and numerical investigation of terahertz transmission through strongly scattering sub-wavelength size spheres

We report on experimental and numerical studies of terahertz propagation in strongly scattering random media. The experimental variations of the terahertz pulse group delay and scattering-induced effects such as temporal pulse distortion, spectral decay, and power attenuation as a function of sample thickness are in excellent agreement with those predicted from a Monte Carlo photon migration model. The transmitted pulses are analyzed with a classical effective medium approximation. Due to the subwavelength size of the random scatterers, it is found that the effective medium approximation underestimates the accumulated pulse phase acquired by the high frequencies during pulse propagation.

Statistical fluctuations of coherent and incoherent intensity in random lasers with nonresonant feedback

We report on intensity fluctuations of a coherent random laser based on incoherent feedback via nonresonant multiple scattering. We quantify the spectral line shape fluctuations in terms of correlations of an individual spectrum with the ensemble-averaged spectrum, which infers the signature of the gain profile of the medium. These correlations are studied in relation to the intensity of the highest coherent modes. We evaluate the distribution of the ratio of the coherent and incoherent fractions in the emission, after independently assessing their statistics. Finally, these intensity fluctuations are graphically represented in a single scatter plot, the centroid of which can be used as a characterization parameter for the laser.

Nano-Optomechanical Characterization and Manipulation of Photonic Crystals

We describe the application of scanning near-field optical microscopy (SNOM) for the high-resolution visualization of light propagation in photonic crystal structures. We also demonstrate that nanoscopic elements such as sharp tips could be used for the mechanical manipulation of the optical properties of photonic crystals. In particular, our theoretical and experimental results show that narrow resonances of a photonic crystal cavity can be tuned without a substantial influence on its quality factor. Furthermore, we discuss the modification of the fluorescence of a nanoscopic emitter as a function of its location close to a photonic crystal

Imaging in turbid media using quasi-ballistic photons

We study by means of experiments and Monte Carlo simulations, the scattering of light in random media, to determine the distance up to which photons travel along almost undeviated paths within a scattering medium, and are therefore capable of casting a shadow of an opaque inclusion embedded within the medium. Such photons are isolated by polarisation discrimination wherein the plane of linear polarisation of the input light is continuously rotated and the polarisation preserving component of the emerging light is extracted by means of a Fourier transform. This technique is a software implementation of lock-in detection. We find that images may be recovered to a depth far in excess of that predicted by the diffusion theory of photon propagation. To understand our experimental results, we perform Monte Carlo simulations to model the random walk behaviour of the multiply scattered photons. We present a new definition of a diffusing photon in terms of the memory of its initial direction of propagation, which we then quantify in terms of an angular correlation function. This redefinition yields the penetration depth of the polarisation preserving photons. Based on these results, we have formulated a model to understand shadow formation in a turbid medium, the predictions of which are in good agreement with our experimental results.