Duke University, Physics Department and the Fitzpatrick Institute for Photonics · Durham, NC

Collective Nonlinear Optical Effects in an Ultracold Thermal Vapor

Anisotropic MOT1

Joel A. Greenberg, Daniel J. Gauthier

Introduction

Citations

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)

• Length: 3 cm, Radius: 150 m • Optical Depth ~55 (Iout/Iin = e-OD) • Density 7x1010 atoms/cm3

• Temperature ~30 μK • 87Rb trapped on F=2F’=3

Applications

Funding

NSF AMO Grant # PHY-0855399; DARPA Slow Light Contract PO #412785-G-2

Mirror

Cooling

Trapping

Probez

yx

MOT

Vacuum

CellMagnets

3 cm

Our magneto-optical trap (MOT) uses lasers and magnetic fields to trap and cool atoms

Collective Effects2

3 cm

cold atoms

MOT Characteristics:

MOT Setup:

Collective optical effects occur when the radiative properties of an atom are effected by the presence of additional atoms

Superfluorescence

• 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

Nonlinear Optics (NLO) with Cold Atoms

Goal: Single-photon NLO• 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

Trapping laser beam configuration Photo of MOT setup

CCD image of trapped atoms

1

Spontaneous Emission (SE)

Amplified Spontaneous Emission (ASE)

Superfluorescence (SF)

SF Thresh

Collectivity

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)

Pow

er

SFSE/N

SE

D

Ppeak • Cooperative emission produces a short, intense pulse of light• Ppeak N2 (N times larger than SE!)

• Delay time (D) before pulse occurs• Threshold density/ pump power

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

Ppe

ak (W

)

OD N

)(NExp2)( tNN

PF/B (mW)

D (s

)

2/1/

BFP

Ppe

ak (W

)

PF/B (mW)

BFP /

Self-organization

Collective Emission Characteristics

We observe SF light generated along the trap’s long axis in both the forward and backward directions3

t (s)

Pow

er (W

)

Forward

Backward

F/B Pump beams MOT beamson

off

• 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

SF Characteristics

Experimental Setup

10~

Pump (F)

Pump (B)

Cold atoms

Detector (B)

Detector (F)

SF light

SF light

• Counter-propagating pump beams• Detect emitted light in forward (F) and backward (B) directions

The forces exerted on atoms by multiple light beams give rise to a global spatial organization of the atoms

Atomic density grating

SF light

An atom recoils when it absorbs or a emits a photon

atom atomp

Example: Absorption

SF Light Observed on Detectors

SF Light Trends

before after

Laser timing scheme

We find good agreement with the predictions of superfluorescent collective atomic recoil lasing (CARL) theory

• New insight into free electron laser dynamics• Possible source of correlated photon pairs• Optical/Quantum memory

We may be seeing a nonlinear (N2) scaling of the peak SF power with atom number

SF Power vs N

time