Transient enhancement of the nonlinear atom

Report
FIP
Superfluorescence in an
Ultracold Thermal Vapor
Joel A. Greenberg and Daniel. J. Gauthier
Duke University
7/15/2009
Superfluorescence (SF)
Pump
W
N
L
W2/Ll~1
‘endfire’ modes
Dicke, Phys. Rev. 93, 99 (1954); Bonifacio & Lugiato, Phys.
Rev. A 11, 1507 (1975), Polder et al., Phys. Rev. A 19, 1192
(1979), Rehler & Eberly, Phys. Rev A 3, 1735 (1971)
SF Threshold
Amplified Spontaneous
Emission (ASE)
Spontaneous
Emission
Superfluorescence (SF)
Cooperativity
1
SF Thresh
Ppeak
• Cooperative emission produces short,
intense pulse of light
tSFtsp/N
Power
• PpeakN2
• Delay time (tD) before pulse occurs
tsp
tD
• Threshold density/ pump power
time
Malcuit, M., PhD Dissertation (1987); Svelto, Principles of Lasers, Plenum (1982)
New Regime: Thermal Free-space SF
Detector (B)
* Counterpropagating,
collinear pump beams1
* Large gain path length2
Pump (B)
 ~ 10
Cold atoms
Pump (F)
- PF/B~4 mW
- DF2F’3=-5G
NO CAVITY!
≠ Slama et al.
Detector (F)
- T=20 mK
- N~109 Rb atoms
- L=3 cm, R=150 mm F=R2/lL~1
NOT BEC!
≠ Inouye et al.
1) Wang et al. PRA 72, 043804; 2) Yoshikawa PRL 94, 083602
Inouye et al. Science 285, 571 (1999); Slama et al. PRL 98, 053603 (2007)
Results - SF
Power (mW)
Forward
Backward
• SF light nearly degenerate
with pump frequency
• Light persists until N falls
below threshold
• F/B temporal correlations
• ~1 photon/atom  large
fraction of atoms participate
t (ms)
on
F/B Pumps
off
MOT
beams
Results - SF
•Density/Pump power thresholds
Ppeak
Power
•PpeakPF/B
• tD (PF/B)-1/2
time
 PF / B
tD (ms)
Ppeak (mW)
tD
PF/B (mW)
Consistent with CARL
superradiance*
PF/1B/ 2
PF/B (mW)
*Piovella et al. Opt. Comm. 187, 165 (2001)
Probe Spectroscopy
What is the mechanism
responsible for SF?
Probe Spectroscopy
What is the mechanism
responsible for SF?
Probe
Pump (B)
Detector (B)
(wp =w+d)
 ~ 10
Cold atoms
Pump (F)
- PF/B~4 mW
- DF2F’3=5G
- T=20 mK
- L=3 cm, R=150 mm
- N~109 Rb atoms
Detector (F)
Recoil-Induced Resonance
• Atom-photon interaction modifies the energy and momentum of an
atom
• Energy + momentum conservation result in resonance
Absorption:
p
atom
Emission:
p 2 / 2m
E
atom
p
atom
atom
2p
p
2p
Probe Spectroscopy
RIR
Backward Detector (FWM)
RIR
PCR
Pout/Pin
Forward Detector
Raman
Raman
dSF
d (kHz)
dSF
d (kHz)
Probe Gain
PRIR/Pprobe
Typical SF gain threshold are Pout/Pin~exp(10)=104
SF Threshold
F/B Pump Power (mW)
Self-Organization
RIR leads to spatial organization or atoms
Backaction between atoms and photons leads to
runaway process  Lower SF threshold
Scattering enhances grating
Grating enhances scattering
Conclusions
• Observe free-space superfluorescence in a cold, thermal
gas
• Temporal correlation between forward/backward
radiation
• Spectroscopy and beatnote imply RIR scattering as
source of SF
Applications
• New insight into free electron laser dynamics
• Possible source of correlated photon pairs
• Optical/Quantum memory
Resonant Processes
Recoil-Induced
Resonance (RIR)
Vibrational Raman
E
E
p
z
Initial state
Final state
atom
atom
p
Probe Spectroscopy
Rayleigh pump
beam alignment
Raman pump
beam alignment
Rayleigh
Raman
250
SF signal
0
dSF
250
Probe Power
Backward Detector (FWM)
SF Power
Probe Power
Forward Detector
0
250
0
d (kHz)
250
100
time (ms)
200
Beatnote
Power (F)
Look at beatnote between probe beam and SF light as
probe frequency is scanned
700
500
d (kHz)
300
Beatnote
Look at beatnote between probe beam and SF light as
probe frequency is scanned
Df~450kHz fSF~-50kHz
1/Df
170 172 174 176
time (ms)
700
500
d (kHz)
300
Weak probe
Backward
Pumps (w)
Probe (wp=w+d)
Forward
Forward
400 200 0
d (kHz)
200 400
Backward
400 200 0
d (kHz)
200 400
Coherence Time
Power
1
PR
on
time
toff
F/B Pumps
PR
off
1.0
0.8
0.6
0.4
0.2
0.0
0 1 2 3 4 5 6
toff
Lin || Lin
Backward
Pumps (w)
Power
Forward
100 200 300
time (ms)
Results - SF
Power
Ppeak
Ppeak (mW)
tD
0.20
0.15
0.10
0.05
0.00
0
time
 Exp(N )
 ( N  Nt ) 2
5 10 15 20 25
OD  N
*Piovella et al. Opt. Comm. 187, 165 (2001)
CARL Regimes
Quantum:
wr>G
MIT
(1999)
Quantum
CARL
Tub (2006)
Semiclassical:
wr<G
Bad Cavity: k>wr
Tub (2006)
MIT
(2003)
Tub (2003)
In resonator
Free space
Slama Dissertation (2007)
Thermal Ultracold Atoms/BEC
Good Cavity: k<wr

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