Patrick Sebbah Nicolas Bachelard, Sylvain Gigan Institut Langevin, ESPCI ParisTech CNRS UMR 7587, Paris A. Christian Vanneste, Xavier Noblin LPMC – Université de Nice– CNRS UMR 6622, Nice, France Jonathan Andreasen University of Arizona, Optical Sciences, Tucson (AZ) Kiran Bhaktha Indian Institute of Technology Kharagpur, India Supported by the Agence Nationale de la Recherche (ANR GLAD)

In a conventional laser light scattering introduces additional loss, thus increases lasing threshold Gain Medium : Light amplification Optical Cavity : Feedback Pour la Science n°396, Oct 2010

Multiple scattering :  dwell time increases  enhanced light amplification Lethokov, Sov. Phys. JETP 26, 835 (1968). Review: Wiersma, Nature Physics, 4, 359(2008) Wiersma, Nature, 406, 132(2000) Mirrorless laser : ASE or lasing with resonant feedback ?

H. Cao et al., Appl. Phys. Lett. 76, 2997 (2000)

Spectrum Emission

H. Cao et al., Appl. Phys. Lett. 76, 2997 (2000) Spectrum Emission

H. Cao et al., Appl. Phys. Lett. 76, 2997 (2000) Spectrum Emission

Feedback for lasing is phase sensitive (coherent) and therefore frequency dependent (resonant). (not ASE)  How lasing can occur in a fully open structure ?  How is coherent feedback possible in a random structure where phases are randomized ?

J. Andreasen et al., “Modes of Random Lasers”, Advances in Optics and Photonics, Vol. 3 Issue 1, pp (2011).

Reduced scattering (smaller n S ) Anderson Localization 2D random collection of scatterers with refractive index n S in [1.05,2] in a matrix with n 0 =1

FDTD Method to simulate Maxwell equations coupled to the population equations of of a four-level atomic structure Laser Field Amplitude Min Max Time evolution Time Intensity Emission spectrum Frequency Intensity Vanneste et al. PRL87 (2001), Sebbah et al. PRB66 (2002) n S = 2

Time evolution Time Intensity Emission spectrum Frequency Intensity Vanneste et al. PRL98 (2007) Laser Field Amplitude Min Max Vanneste et al., PRL98, (2007) n S = 1.25

 Random lasing occurs even in the diffusive regime (extended modes – no confinement). Threshold depends on mode confinement  Lasing modes are built on the resonances/quasinormal modes of the passive cavity These resonances are selected by the gain True in the singlemode regime Vanneste et al. PRL87 (2001), Sebbah et al. PRB66 (2002), Vanneste et al. PRL98 (2007)

K. Bhaktha et al., "An optofluidic random laser", APL 101, (2012)

IN OUT 3 mm Rhodamine 6G Δn = 0.06 Weak scattering Modes are extended PDMS K. Bhaktha et al., "An optofluidic random laser", APL 101, (2012)

IN OUT 3 mm K. Bhaktha et al., "An optofluidic random laser", APL 101, (2012)

All characteristics of classical lasers (threshold, narrow emission lines, Poissonian photon statistics) +  Random emission spectrum  Non-directive laser emission  Complex structure of lasing modes  Strong dependence on pumping area

If design is greatly simplified, control over directionality and frequency emission is lost  Can control over random lasing emission be regained ?  Idea : spatial shaping of the optical pump  Inspired from spatial shaping methods recently employed for coherent light control  Iterative method without prior knowlegde of the lasing modes.

N. Bachelard et al., "Taming random lasers", PRL 109, (2012)

N. Bachelard et al., "Active control of random laser emission", in preparation

 Numerical model valid only below threshold  Does not include  Spectrum to spectrum fluctuations  Gain saturation  Mode competition  Laser instabilities

Starting from uniform pumping IN OUT 3 mm

IN OUT 3 mm

IN OUT 3 mm

IN OUT 3 mm

 Singlemode operation at any desired mode  Optimal redistribution of the gain  Reduced threshold

 Optimization of random laser directivity  Optimization of pulse duration  Extension to control of other type of lasers  Organic 2D lasers  Broad area lasers  …

 For fundamental interest :  Nature of the lasing modes J. Andreasen et al., AOP 3 (2011)  Revisiting laser equation in absence of a cavity H. Tureci et al., Science 320 (2008)  Multimode regime & Nonlinear phenomena J. Andreasen et al., JOSAB28 (2011), PRA84 (2011) ……  For possible applications :  where mirrors are not available H. Cao, Optics & Photonics News (2005)  in bio & chemical sensing K. Bhaktha et al., ", APL 101 (2012)  as intense, spatially incoherent light sources B. Redding et al., Optics Lett. 36 (2011) ……

R. Kaiser, Cold atoms J. Fallert et al. Nature Photonics, 279 (2009) C. López, Photonic Glass RL Garcia et al., PRB 82 (2010) Sapienza et al., Science 327 (2010) Wiersma, PRL 93, (2004)