Ravitej Uppu

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.

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.

Programmable two-photon quantum interference in 10^3 channels in opaque scattering media

We investigate two-photon quantum interference in an opaque scattering medium that intrinsically supports a large number of transmission channels. By adaptive spatial phase modulation of the incident wave fronts, the photons are directed at targeted speckle spots or output channels. From 10 3 experimentally available coupled channels, we select two channels and enhance their transmission to realize the equivalent of a fully programmable 2×2 beam splitter. By sending pairs of single photons from a parametric down-conversion source through the opaque scattering medium, we observe two-photon quantum interference. The programed beam splitter need not fulfill energy conservation over the two selected output channels and hence could be nonunitary. Consequently, we have the freedom to tune the quantum interference from bunching (Hong-Ou-Mandel-like) to antibunching. Our results establish opaque scattering media as a platform for high-dimensional quantum interference that is notably relevant for boson sampling and physical-key-based authentication.

Aerosol-based coherent random laser

We demonstrate coherent random lasing from an aerosol of dye-doped microdroplets in air. The aerosol is in the form of a linear array of polydisperse, arbitrarily shaped, and randomly spaced microdroplets with average dimensions of about 30 μm. Upon optical excitation, ultranarrow lasing modes were observed in the emission along the axis of the linear array, while the transverse emission exhibited intrascatterer resonance peaks. Direct spatiospectral imaging and lasing threshold studies confirmed the origin of the lasing peaks to be from spatial modes that extended over the array of the polydisperse microdroplets.

Persistent coherent random lasing using resonant scatterers.

We study the frequency behavior of coherent random lasers consisting monodisperse scatterers with single-particle resonances. A three-dimensional photon propagation model is employed to compute the wavelength-sensitive path length distribution of fluorescence photons in this system. We observe that a persistence interval of wavelengths exists for the coherent random lasing modes, corresponding to the Mie resonances of the individual resonant scatterer. Within the interval, characteristic pulse to pulse fluctuations continue to be observed from the system. The gain competition in the random laser suppresses likely coherent modes in other regions of the emission band, thereby reducing the wavelength fluctuations in the random laser. We further illustrate the tunability of this persistence interval by varying the size parameter of the resonant scatterers.

Transmitting more than 10 bit with a single photon

Encoding information in the position of single photons has no known limits, given infinite resources. Using a heralded single-photon source and a spatial light modulator (SLM), we steer single photons to specific positions in a virtual grid on a large-area spatially resolving photon-counting detector (ICCD). We experimentally demonstrate selective addressing any location (symbol) in a 9072 size grid (alphabet) to achieve 10.5 bit of mutual information per detected photon between the sender and receiver. Our results can be useful for very-high-dimensional quantum information processing.

Single-mode, quasi-stable coherent random lasing in an amplifying periodic-on-average random system

We experimentally demonstrate single-mode coherent random lasing in a linear array of monodisperse amplifying microresonators, which behaves as an amplifying periodic-on-average random system. We theoretically analyse the frequency distribution of lasing modes under weak and strong configurational disorder. We show that the tuning of the microresonator diameter can match the frequency interval of the lasing modes with the gain maximum, thus achieving spectral mode-matching. We implement this experimentally and demonstrate that the spectral mode-matched system yields single-mode coherent random lasing with 76% probability of the modes restricted to an interval of width ∼1.2 nm, thus offering quasi-stability in the emission.

Collective lasing from a linear array of dielectric microspheres with gain

We experimentally study the optical emission behavior of a linear array of dielectric microspheres with gain. The microspheres are randomly arranged and well-separated, and can only couple via radiative modes. We observe resolution-limited, ultra-narrowband modes in the longitudinal emission, which constitutes collective lasing from the entire array, inferred from the observation of a lasing threshold. The lasing modes show wavelength selectivity, wherein the lasing probability is large only in specific frequency bands while being inhibited at other wavelengths, a behavior which is independent of the degree of configurational randomness. Analysis of the frequency bands indicates the participation of Fabry-Perot resonances of the individual microspheres in the collective emission.

Quantum optics of lossy asymmetric beam splitters

We theoretically investigate quantum interference of two single photons at a lossy asymmetric beam splitter, the most general passive 2×2 optical circuit. The losses in the circuit result in a non-unitary scattering matrix with a non-trivial set of constraints on the elements of the scattering matrix. Our analysis using the noise operator formalism shows that the loss allows tunability of quantum interference to an extent not possible with a lossless beam splitter. Our theoretical studies support the experimental demonstrations of programmable quantum interference in highly multimodal systems such as opaque scattering media and multimode fibers.

Physical manifestation of extreme events in random lasers.

We report our studies on exponentially-tempered Lévy sums that explain coherent random lasers based on nonresonant feedback. We investigate the hierarchy in the sums and identify the contribution of the extremes over a wide range of excitation energies and disorder strengths. Subsequently, we carry out experiments in which the physical manifestation of these extremes is revealed. At the appropriate gain and disorder, the extremes manifest as the sharp ultranarrow modes in the spectrum. At stronger excitation and disorder, the peaks disappear due to the reduced rarity of the extremes, compounded by the decreased magnitude effected by the tempering.