Anjani Kumar Tiwari

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.

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.

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.

Coherent random lasing in diffusive resonant media

We investigate diffusive propagation of light and consequent random lasing in a medium comprising resonant spherical scatterers. A Monte‐Carlo calculation based on photon propagation via three‐dimensional random walks is employed to obtain the dwell‐times of light in the system. We compare the inter‐scatterer and intra‐scatterer dwell‐times for representative resonant and non‐resonant wavelengths. Our results show that more efficient random lasing, with intense coherent modes, is obtained when the gain is present inside the scatterers. Further, a larger reduction in frequency fluctuations is achieved by the system with intra‐scatterer gain.

Experimental demonstration of small-angle bending in an active direct-coupled chain of spherical microcavities

We experimentally demonstrate collective amplified modes along bent chains of directly coupled, amplifying spherical microdroplet resonators. The chains, comprising ∼40 non-contacting resonators, were bent through angles up to ∼25°. The modal probability of the system shows a sharp drop upon bending through small angles (∼10°), and thereafter changes minimally under further bending. The frequency response is significantly maintained under bending. We numerically study the transmittance of a chain of non-contacting amplifying resonators using finite-difference-time-domain calculations, and observe that nanojet filamentation influences coupling at the bend. A self-correcting mechanism of propagation is observed, originating from the lensing effect of the spherical resonator.

Amplifying periodic-on-average random systems: Route to Anderson-localization random lasers

We report a periodic-on-average random system with gain, that realizes frequency-controlled random lasing. We show how the disorder favors Anderson localization, and also reduces lasing threshold compared to a periodic system.

Photon-localization induced random lasing from an amplifying periodic-on-average random system

Summary form only given. Lasing from random media continues to generate significant interest[1]. A particular random system of interest is the periodic-on-average random system (PARS, in short), which comprises a random structure that has an underlying periodicity[2]. In this case, the passband and stopband characteristics of the underlying structure influence the modes of the random system. Despite the interesting transport properties of such systems[3,4], a PARS-based amplifying system has so far not been studied. In this paper, we present a practical implementation of an amplifying PARS system (aPARS), and analyze the random lasing properties thereof. Our experimental observations are in excellent agreement with transfer matrix calculations.We created an aPARS system by generating an array of monodisperse microresonators by using a vibrating orifice aerosol generator. The microresonators were in the form of liquid droplets of Rhodamine 6G in methanol. The aerosol generator could create arrays with varying degrees of monodispersity, as shown in Fig1[A]. The longitudinally emitted radiation upon optical excitation exhibited frequency-sensitive properties. As shown in Fig1[B], under polydisperse conditions (blue lines), lasing modes at arbitrary wavelengths were generated. Consequently, the histogram shown in bottom panel, taken over 200 spectra, shows a continuous distribution of the lasing wavelengths. Under monodisperse conditions (red lines), the lasing modes occur at restricted wavelength intervals, as seen from the bunches in the histograms. We analyzed the spectra features by using transfer matrix calculations on a one-dimensional system comprising a multilayer with refractive index profiles similar to those seen in Fig1[A]. The results are shown in the Fig1[C]. Under monodisperse conditions (red lines), sharp lasing peaks occurred at specific wavelengths, as against the spectra from polydisperse system shown in blue. The corresponding histograms are shown underneath, which exactly reproduced the experimental observations. Further transfer matrix analysis shows that the frequency sensitivity arises from the bandstructure effect of the underlying periodic lattice. The lasing modes are essentially the perturbed band-edge modes that migrate to the stopband, and are restricted to a small frequency neighbourhood near the edge. The modes here have a high quality factor, and also a small localization length, which leads to strong random lasing. The strongly polydisperse system washes out the photonic band structure, and hence does not qualify as an aPARS system.

Signature of band-edge-induced lasing observed in self-assembled photonic crystals

We report experimental demonstration of band-edge-induced lasing from Rhodamine B-dyed self-assembled all-solid photonic crystals. We discuss the origin of lasing as due to the enhancement of density of states and field distribution at band edge frequencies.