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 upto 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 what is 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.Comment: LaTex 19 pages, 10 ps figures and 8 eps figures. psfig.sty included. (submitted to Optics Commumications

Anomalous transport regime in non-Hermitian, Anderson-localizing hybrid systems

In a disordered environment, the probability of transmission of a wave reduces with increasing disorder, the ultimate limit of which is the near-zero transmission due to Anderson localization. Under localizing conditions, transport is arrested because the wave is trapped in the bulk of the sample with decaying-exponential coupling to the boundaries. Any further increase in disorder does not modify the overall transport properties. Here, we report the experimental demonstration of a hitherto-unrealized anomalous transport of hybrid particles under localizing disorder in a non-Hermitian setting. We create hybrid polariton-photon states in a one-dimensional copper sample with a comb-shaped periodic microstructure designed for microwave frequencies. Metallic dissipation realizes the necessary non-Hermiticity. Disorder is introduced by deliberate alterations of the periodic microstructure. Direct measurement of wave-functions and phases was achieved by a near-field probe. At a particular disorder, We observe the onset of Anderson localization of the hybrid states endorsed by exponential tails of the wavefunction. However, at stronger disorder and under conditions that support localization, an unexpected enhancement in the transmission was facilitated by an emergent mini-band. The transmission was traced to the hopping of the hybrid particle over multiple co-existing localized resonances that exchange energy due to the non-orthogonality. These emergent states are manifested in all configurations under strong disorder, suggesting the formation of a novel transport regime. This is verified by measuring the averaged conductance which endorses an anomalous transport regime in the hybrid, non-Hermitian environment under strong disorder. These experimental observations open up new unexplored avenues in the ambit of disorder under non-Hermitian conditions.Comment: 7 pages, 5 figure

Supplementary document for Neural networks for calculation of complex fields from far-field intensities in mesoscopic photonic systems - 6337548.pdf

Elaboration on transfer matrix methods, FDTD simulations, layer information for 1D and 2D networks, and error information for 2D networks

Evanescent-wave-excited quasi-two-dimensional random lasing

We report on random lasing from a dye-scatterer system directly excited by evanescent modes formed over a dielectric-dielectric interface. The system is quasi-two-dimensional because of the subwavelength exponential decay of the excitation energy. Spectral narrowing by a factor of $\sim 13$ was obtained under diffusive scattering conditions. The peak intensity exhibited a unique dual-saturation behavior due to the localized nature of the excitation. We observed a reduction of threshold for samples with a higher dye concentration because of a larger excitation-state occupation number within the same interaction volume

Evanescent-wave-excited quasi-two-dimensional random lasing

We report on random lasing from a dye-scatterer system directly excited by evanescent modes formed over a dielectric-dielectric interface. The system is quasi-two-dimensional because of the subwavelength exponential decay of the excitation energy. Spectral narrowing by a factor of ~13 was obtained under diffusive scattering conditions. The peak intensity exhibited a unique dual-saturation behavior due to the localized nature of the excitation. We observed a reduction of threshold for samples with a higher dye concentration because of a larger excitation-state occupation number within the same interaction volume