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Collective Nonlinear Optical Effects in an Ultracold Thermal Vapor Joel A. Greenberg, Daniel J. Gauthier Duke University, Physics Department and the Fitzpatrick Institute for Photonics · Durham, NC 2 Effects Collective Introduction Nonlinear Optics (NLO) with Cold Atoms • Few-photon NLO elements are critical for quantum information applications, but large atom-photon interaction strengths are needed • We obtain large nonlinear couplings in cold atoms by controlling the atoms’ internal and external states • Collective nonlinear effects allow for a drastic enhancement of the atom-photon coupling strength over single-atom effects, and may lower NLO thresholds to the single-photon limit We observe SF light generated along the trap’s long axis in both the forward and backward directions3 Collective optical effects occur when the radiative properties of an atom are effected by the presence of additional atoms Amplified Spontaneous Emission (ASE) Spontaneous Emission (SE) Experimental Setup Superfluorescence (SF) Pump (B) SF Thresh ~ 10 SF light The influence of the radiators on one another can take on a continuum of values (described by a collectivity parameter). On one end, atoms radiate independently (SE) – on the other, all atoms release their energy at the same time (SF) SF light Pump (F) Cold atoms Forward Our magneto-optical trap (MOT) uses lasers and magnetic fields to trap and cool atoms tSFtSE/N Power tSE tD Cell Magnets Example: Absorption atom Trapping laser beam configuration atom p Photo of MOT setup before 1) J.A. Greenberg, M. Oria, A.M.C. Dawes, D.J. Gauthier, Opt. Express 15, 17699 (2007) 2) M. Malcuit, Univ. of Rochester, PhD Dissertation (1987) 3) J.A. Greenberg and D.J. Gauthier, OSA Opt. Photonics Cong. Tech. Digest, ISBN 978-1-55752-873-5 (2009) Funding NSF AMO Grant # PHY-0855399; DARPA Slow Light Contract PO #412785-G-2 The degree of atomic organization affects the radiation field, thus producing a nonlocal atom-atom coupling. The net result is a runaway process that gives rise to the collective emission of light Scattering enhances grating Grating enhances scattering Ppeak (mW) SF light Atomic density grating Citations PF/B (mW) SF Power vs N CCD image of trapped atoms P PF/B (mW) 3 cm • Length: 3 cm, Radius: 150 mm • Optical Depth ~55 (Iout/Iin = e-OD) • Density 7x1010 atoms/cm3 • Temperature ~30 μK • 87Rb trapped on F=2F’=3 PF / B 1/ 2 F/B after The forces exerted on atoms by multiple light beams give rise to a global spatial organization of the atoms MOT Characteristics: cold atoms Laser timing scheme We find good agreement with the predictions of superfluorescent collective atomic recoil lasing (CARL) theory An atom recoils when it absorbs or a emits a photon Cooling MOT beams SF Light Trends Self-organization Mirror Probe F/B Pump beams Ppeak (mW) z • SF light is nearly degenerate with pump frequency • Light persists until atomic density falls below threshold • F/B SF temporal correlations • ~1 photon emitted/atom 3 cm MOT x on time off Vacuum y • Cooperative emission produces a short, intense pulse of light • Ppeak N2 (N times larger than SE!) • Delay time (tD) before pulse occurs • Threshold density/ pump power t (ms) MOT Setup: Trapping Backward tD (ms) Anisotropic SF Characteristics Power (mW) Ppeak Detector (F) SF Light Observed on Detectors Collective Emission Characteristics 1 MOT • Counter-propagating pump beams • Detect emitted light in forward (F) and backward (B) directions Detector (B) Collectivity 1 Goal: Single-photon NLO Superfluorescence Exp(N ) ( N Nt ) 2 We may be seeing a nonlinear (N2) scaling of the peak SF power with atom number OD N Applications • New insight into free electron laser dynamics • Possible source of correlated photon pairs • Optical/Quantum memory