Patrick Sebbah

Modes of random lasers

In conventional lasers, the optical cavity that confines the photons also determines essential characteristics of the lasing modes such as wavelength, emission pattern, directivity, and polarization. In random lasers, which do not have mirrors or a well-defined cavity, light is confined within the gain medium by means of multiple scattering. The sharp peaks in the emission spectra of semiconductor powders, first observed in 1999, has therefore lead to an intense debate about the nature of the lasing modes in these so-called lasers with resonant feedback. We review numerical and theoretical studies aimed at clarifying the nature of the lasing modes in disordered scattering systems with gain. The past decade has witnessed the emergence of the idea that even the low-Q resonances of such open systems could play a role similar to the cavity modes of a conventional laser and produce sharp lasing peaks. We focus here on the near-threshold single-mode lasing regime where nonlinear effects associated with gain saturation and mode competition can be neglected. We discuss in particular the link between random laser modes near threshold and the resonances or quasi-bound (QB) states of the passive system without gain. For random lasers in the localized (strong scattering) regime, QB states and threshold lasing modes were found to be nearly identical within the scattering medium. These studies were later extended to the case of more lossy systems such as random systems in the diffusive regime, where it was observed that increasing the openness of such systems eventually resulted in measurable and increasing differences between quasi-bound states and lasing modes. Very recently, a theory able to treat lasers with arbitrarily complex and open cavities such as random lasers established that the threshold lasing modes are in fact distinct from QB states of the passive system and are better described in terms of a new class of states, the so-called constant-flux states. The correspondence between QB states and lasing modes is found to improve in the strong scattering limit, confirming the validity of initial work in the strong scattering limit.

Waves and Imaging through Complex Media

Preface. Part I: Introduction. 1. Wave Chaos and Multiple Scattering: a Story of Coherence O. Legrand. 2. Towards Optical Biopsy: a Brief Introduction A.C. Boccara, P.M.W. French. Part II: Multiple Wave Scattering. 1. Coherent Multiple Scattering in Disordered Media E. Akkermans, G. Montambaux. 2. Statistical Approach to Photon Localization A.Z. Genack, A.A. Chabanov. Part III: Wave Chaos and Multiple Scattering. 1. The Semiclassical Approach for Chaotic Systems D. Delande. 2. Random Matrix Theory of Scattering in Chaotic and Disordered Media J.-L. Pichard. 3. Wave Chaos in Elastodynamics R.L. Weaver. 4. Time-reversed Acoustics and Chaotic Scattering M. Fink, J. de Rosny. Part IV: Imaging in Heterogeneous Media: Ballistic Photons. 1. Imaging Biological Tissue using Photorefractive Holography and Fluorescence Lifetime N.P. Barry, et al. 2. Simultaneous Optical Coherence and Two-photon Fluorescence Microscopy E. Beaurepaire, et al. 3. Low Coherence Interferometric Technique G. Brun, et al. 4. Laser Optical Feedback Tomography E. Lacot, et al. 5. Imagery of Diffusing Media by Heterodyne Holography M. Gross, et al. 6. Imaging through Diffusing Media by Image Parametric Amplification E. Lantz, et al. Part V: Imaging in Heterogeneous Media: Diffuse Light. 1. Detection of Multiply Scattered Light in Optical Coherence Microscopy K.K. Bizheva, et al. 2. Scattering by a Thin Slab: Comparison between Radiative Transfer and Electromagnetic Simulation J.-J. Greffet, et al. 3. Methods for the Inverse Problem in Optical Tomography S.R. Arridge. 4. Inverse Problem for Stratified Scattering Media J.-M. Tualle, et al. 5. Scattering on Multi-scale Rough Surfaces C.A. Guerin, et al. Part VI: Dynamic Multiple Light Scattering. 1. Imaging of Dynamic Heterogeneities in Multiple Light Scattering G. Maret, M. Heckmeier. 2. Diffuse Laser Doppler Velocimetry from Multiple Scattering Media and Flowing Suspensions P. Snabre, et al. 3. High Resolution Acousto-optic Imaging S. Leveque-Fort. Part VII: Speckle Correlations in Random Media. 1. Correlation of Speckle in Random Media R. Pnini. 2. Dynamic Speckle Correlations F. Scheffold, G. Maret. 3. Spatio-temporal Speckle Correlations for Imaging in Turbid Media S.E. Skipetrov. 4. Speckle Correlations and Coherent Backscattering in Nonlinear Random Media R. Bressoux, R. Maynard. Index. Author Index.

Taming Random Lasers through Active Spatial Control of the Pump

Active control of the spatial pump profile is proposed to exercise control over random laser emission. We demonstrate numerically the selection of any desired lasing mode from the emission spectrum. An iterative optimization method is employed, first in the regime of strong scattering where modes are spatially localized and can be easily selected using local pumping. Remarkably, this method works efficiently even in the weakly scattering regime, where strong spatial overlap of the modes precludes spatial selectivity. A complex optimized pump profile is found, which selects the desired lasing mode at the expense of others, thus demonstrating the potential of pump shaping for robust and controllable single mode operation of a random laser.

Adaptive pumping for spectral control of random lasers

Random lasers generate the optical feedback required for stimulated emission by scattering light from disordered particles. Their inherent randomness, however, makes controlling the emission wavelength difficult. It is now shown that this problem can be remedied by carefully matching the pump laser to the specific random medium. The concept is applied to a one-dimensional optofluidic device, but could also be applicable to other random lasers.

Optofluidic random laser

Random lasing is reported in a dye-circulated structured polymeric microfluidic channel. The role of disorder, which results from limited accuracy of photolithographic process, is demonstrated by the variation of the emission spectrum with local-pump position and by the extreme sensitivity to a local perturbation of the structure. Thresholds comparable to those of conventional microfluidic lasers are achieved, without the hurdle of state-of-the-art cavity fabrication. Potential applications of optofluidic random lasers for on-chip sensors are discussed. Introduction of random lasers in the field of optofluidics is a promising alternative to on-chip laser integration with light and fluidic functionalities.

Spatial-field correlation: The building block of mesoscopic fluctuations

The absence of self-averaging in mesoscopic systems is a consequence of long-range intensity correlations. Microwave measurements suggest, and diagrammatic calculations confirm, that the correlation function of the normalized intensity with displacement of the source and detector, Delta R and Delta r, respectively, can be expressed as the sum of three terms, with distinctive spatial dependences. Each term involves only the sum or the product of the square of the field correlation function, F identical with F(2)(E). The leading-order term is the product, F(Delta R)F(Delta r); the next term is proportional to the sum, F(Delta R)+F(Delta r); the third term is proportional to F(Delta R)F(Delta r)+[F(Delta R)+F(Delta r)]+1.

Flat lens for pulse focusing of elastic waves in thin plates

Flat lens concept based on negative refraction proposed by Veselago in 1968 has been mostly investigated in monochromatic regime. It was recently recognized that time development of the super-lensing effect discovered in 2000 by Pendry is yet to be assessed and may spring surprises: Time-dependent illumination could improve the spatial resolution of the focusing. We investigate dynamics of flexural wave focusing by a 45\degre-tilted square lattice of circular holes drilled in a Duraluminium plate. Time-resolved experiments reveal that the focused image shrinks with time below diffraction limit, with a lateral resolution increasing from 0.8

Localized Modes in Open One-Dimensional Dissipative Random Systems

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Complexity of two-dimensional quasimodes at the transition from weak scattering to Anderson localization

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Field and intensity correlation in random media

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