Post on 18-Dec-2015
William Guerin
A random laser with cold atoms
Institut Non Linéaire de Nice (INLN)CNRS and Université Nice Sophia-Antipolis
William Guerin 2OCA, Nice, May 2014
Two ingredients for a standard laser :
1) An amplifying material
2) An optical cavity
Role of the optical cavity:
- To provide feedback
Chain reaction: intensity grows until gain saturation
- Fabry-Perot interferometer
Mode selection: spatial and temporal coherence properties
random
Multiple scattering
Multiple scattering
What is a laser ?
??
William Guerin 3OCA, Nice, May 2014
Two ingredients for a random laser :
1) An amplifying material
2) Multiple scattering
Role of the multiple scattering:
- To provide feedback
Chain reaction: intensity grows until gain saturation
What is a random laser ?
William Guerin 4OCA, Nice, May 2014
Photons make a random walk between scatterers Diffusion process
ℓt = transport length = mean-free-path for isotropic scattering ℓsc
Interference effects are ignored !!!
Model justified for L >> ℓsc
With gain ?
Diffusion model with gain
V. S. Letokhov, Sov. Phys. JETP 26, 835 (1968).
“Photonic bomb”
Threshold on the system size:
William Guerin 5OCA, Nice, May 2014
Review: J. Andreasen et al., Adv. Opt. Photon. 3, 88 (2011).
Mode and coherence properties
Random lasers are complex systems: open, highly multimode and nonlinear
What are the mode and coherence properties of random lasers ?
New theoretical approaches have been developed
The nature of the ‘modes’ has been a long debate in the last years…
Türeci, Ge, Rotter & Stone, Science 2008
William Guerin 6OCA, Nice, May 2014
Experiments on the coherence properties of random lasers
Link to this workshop (I)
Cao et al., PRL 2001
Poissonian photon statistics and G(2)(0) = 1 above threshold
temporal coherence
But without spatial coherence:
William Guerin 7OCA, Nice, May 2014
Link to this workshop (II)
Amplification of radiation by stimulated emission (“laser” for astrophysicists) is known in space.
- “Space masers” are common
- Far IR amplification in MWC349A (H)
- Amplification at 10 µm in the atmospheres of Mars and Venus (C02)
- Amplification in the near IR in Carinae (FeII and OI)
Multiple scattering (radiation trapping) is also common (e.g. in stars).
A random laser could happen naturally in space
William Guerin 8OCA, Nice, May 2014
• Cold atoms are clean and well-controlled systems:- Simple system (“easy” to model)- All the same (monodisperse sample)- Almost no Doppler effect- No absorption (but still inelastic scattering )- Well isolated from environment (quantum effects ?)
• Cold atoms are different: strong resonance / very dispersive
• Disorder-configuration averaging is easy (even unavoidable )
• Cold atom are versatile: -The scattering cross-section is tunable- Several gain mechanisms are possible
• Cold atoms are gas (≠ cond. mat.) closer to astrophysical systems
A random laser with cold atoms ?
Possibility of ab initio models
William Guerin 9OCA, Nice, May 2014
Introduction
The two necessary ingredients
Both together ? The quest for the best gain mechanism
Experimental signature of random lasing
Multiple scattering in cold atoms
Gain and lasing with cold atoms
Outline
William Guerin 10OCA, Nice, May 2014
Typically, on resonance, b0 = 10 – 100
With some efforts: up to b0 ~ 200
Rubidium 85
= 780 nm
/2 = 6 MHz
MOT parameters:
N ~ 108-1010 atom
T ~ 50-100 µK
L ~ 1-5 mm
n ~ 1011 at/cm3
Experimental setup
William Guerin 11OCA, Nice, May 2014
Phys. Rev. Lett. 91, 223904 (2003).
Radiation trapping in cold atoms
William Guerin 12OCA, Nice, May 2014
Gain with cold atoms
Several mechanisms are possible
Mollow gain:
- Two level atoms + one pump
- 3 photon transition (population inversion in the dressed-state basis)
pump pump
Raman gain:
- Three-level atoms + one pump
- 2 photon transition (population inversion between the two ground states)
- Hyperfine levels or Zeeman levels
R
pump
Parametric gain:
- Two-level atoms + two pumps
Degenerate four-wave mixing (DFWM)
William Guerin 13OCA, Nice, May 2014
Laser radiation 300 µW
Cold atoms inside !
Phys. Rev. Lett. 101, 093002 (2008).
- Mollow laser for small pump detuning.
- (Zeeman) Raman laser for larger pump detuning, single pump.
- DFWM laser for larger pump detuning and two pumps.
A laser with cold atoms (& cavity)
William Guerin 14OCA, Nice, May 2014
Introduction
The two necessary ingredients
Both together ? The quest for the best gain mechanism
Experimental signature of random lasing
Criterion: random laser threshold
Comparison between different gain mechanisms
Outline
William Guerin 15OCA, Nice, May 2014
The scatterers and the amplifiers are the same atoms !
Is it possible to get enough scattering and gain simultaneously ?
Gain
Saturation elastic scattering
inelastic scattering
Pumping
Combining gain and scattering ?
Gain and scattering do not occur at the same frequency !!!
William Guerin 16OCA, Nice, May 2014
What is measured in transmission experiments:
with the extinction length
Letokhov’s diffusive model (interference effects are ignored)
= mean free path(sphere geometry)
= linear gain length
Letokhov’s threshold
Both lengths are related to the same atomic density n. We can use cross-sections :
William Guerin 17OCA, Nice, May 2014
b0 is an intrinsic parameter of the sample and is easily measured.
depends on the pumping parameters and of the frequency.
Criterion to compare the different gain mechanisms
Phys. Rev. Lett. 102, 173903 (2009).
Letokhov’s threshold with atoms
On-resonance optical depth :
= on-resonance atomic cross-section
= polarizability (~ : dimensionless)
William Guerin 18OCA, Nice, May 2014
Let’s compare
[1] Phys. Rev. Lett. 102, 173903 (2009).[2] Opt. Express 17, 11236 (2009).
Gain mechanism Evaluation method
b0cr Validity of the diffusion approx.
Other problem Ref.
Mollow gainAnalytical ~ 300 Pump
penetration [1]
NDFWMExp. & Num. ∞ Inelastic
scattering
Raman gain (Zeeman) Exp. ~ 200 Detection [2]
Raman gain (Hyperfine) Num. ~ 90
William Guerin 19OCA, Nice, May 2014
Let’s compare
[1] Phys. Rev. Lett. 102, 173903 (2009).[2] Opt. Express 17, 11236 (2009).
Gain mechanism Evaluation method
b0cr Validity of the diffusion approx.
Other problem Ref.
Mollow gainAnalytical ~ 300 Pump
penetration [1]
NDFWMExp. & Num. ∞ Inelastic
scattering
Raman gain (Zeeman) Exp. ~ 200 Detection [2]
Raman gain (Hyperfine) Num. ~ 90 Raman gain (Hyperfine) + additional scattering
Num. ~ 30
William Guerin 20OCA, Nice, May 2014
Introduction
The two necessary ingredients
Both together ? The quest for the best gain mechanism
Experimental signature of random lasing
Outline
Raman gain between hyperfine levels with additional scattering
Experimental observations
William Guerin 21OCA, Nice, May 2014
Raman gain between hyperfine levels
with additional scattering
William Guerin 22OCA, Nice, May 2014
Experiment
• We sweep slowly (steady-state) the Raman laser (no probe) around the frequency where Raman gain is on resonance with the |2> |1’> transition.
• The random laser emission:
- is not spatially separated from elastic scattering from the external lasers
- is very hard to spectrally separate
We look at the total fluorescence (= pump depletion)
• We change b0 with a constant atom number.
changes are only due to collective effects
William Guerin 23OCA, Nice, May 2014
Observations
1- Overall increase of fluorescence Amplified spontaneous emission
William Guerin 24OCA, Nice, May 2014
Observations
2- Increase of fluorescence around = 0
1- Overall increase of fluorescence Amplified spontaneous emission
combined effect of gain and multiple scattering
William Guerin 25OCA, Nice, May 2014
Signature of random lasing
Fit of the wings we can subtract the “ASE” background
More visible bump (Gaussian shape)
The amplitude has a threshold with b0
Nature Phys. 9, 357 (2013).
William Guerin 26OCA, Nice, May 2014
Qualitative ab initio modeling
For ASE, OBE + ballistic amplification (scattering neglected, saturation effects included):
For the RL-bump, OBE + Letokhov’s threshold (ASE neglected, saturation effects included)
Nature Phys. 9, 357 (2013).
William Guerin 27OCA, Nice, May 2014
Conclusion and outlook
First evidence of random lasing in atomic vapors
The observations agree qualitatively with ab initio modeling based on Letokhov’s threshold.
- Acquire more data (larger b0, different pump parameters)
- Study the dynamics
- Other signature of the transition (e.g. excess noise at threshold) ?
Short term projects (work in progress):
William Guerin 28OCA, Nice, May 2014
Outlook (longer term)
Quantitative agreement with more evolved models (ASE + RL) ?
Coherence / spectrum of the random laser ?
Random laser in hot vapors ? Closer to astrophysical systems…
b0=7b0=14
- Use a Fabry-Perot to filter the random laser light and look at the photocount statistics or the correlation function.
- Make a beat note with the Raman laser to access the spectrum.
- Comparison with theory ?
William Guerin 29OCA, Nice, May 2014
From cold atoms to astrophysics
Cold and hot atomic vapors: a testbed for astrophysics?Q. Baudouin, W. Guerin and R. Kaiser, in Annual Review of Cold Atoms and Molecules, vol. 2, edited by K. Madison, Y. Wang, A. M. Rey, and K. BongsWorld Scientific, Singapour, 2014 (in press, preprint hal-00968233)
• Light diffusion / radiation trapping / radiative transfer
• Polarization of the scattered light: work in progress with M. Faurobert
• Frequency redistribution due to the Doppler effect in hot vapors Superdiffusion (Lévy flights)
• Light-induced long range forces plasma physics, gravity
• Gain and lasing in atomic vapors, random lasers (?)
William Guerin 30OCA, Nice, May 2014 € : ANR, DGA, PACA, CG06, INTERCAN
People currently involved in this project at INLN:
• Robin Kaiser
• William Guerin
• Samir Vartabi Kashani (PhD)
• Alexander Gardner (joint PhD)
Past contributions:
• Quentin Baudouin (PhD, 2013)
• Djeylan Aktas (Master, 2013)
• Nicolas Mercadier (PhD, 2011)
• Verra Guarrera (Post-doc, 2011)
• Davide Brivio (Master, 2008)
• Frank Michaud (PhD, 2008)
Past collaborators:
R. Carminati (Paris)
L. Froufe-Pérez (Madrid)
S. Skipetrov et al. (Grenoble)
Collaborators:
Dmitriy Kupriyanov et al. (St-Petersburg)
Stefan Rotter (Vienna)
Chong Yidong (Singapour)
William Guerin 31OCA, Nice, May 2014
Publications related to this projectMechanisms for Lasing with Cold Atoms as the Gain MediumW. Guerin, F. Michaud, R. Kaiser,Phys. Rev. Lett. 101, 093002 (2008).
Threshold of a Random Laser with Cold AtomsL. Froufe-Pérez, W. Guerin, R. Carminati, R. Kaiser,Phys. Rev. Lett. 102, 173903 (2009).
Threshold of a random laser based on Raman gain in cold atomsW. Guerin, N. Mercadier, D. Brivio, R. Kaiser,Opt. Express 17, 11236 (2009).
Towards a random laser with cold atomsW. Guerin et al.,J. Opt. 12, 024002 (2010).
Steady-state signatures of radiation trapping by cold multilevel atomsQ. Baudouin, N. Mercadier, R. Kaiser,Phys. Rev. A 87, 013412 (2013).
A cold-atom random laserQ. Baudouin, N. Mercadier, V. Guarrera, W. Guerin, R. Kaiser,Nature Physics 9, 357 (2013).
http://www.inln.cnrs.fr/content/atomes_froids/publications
William Guerin 32OCA, Nice, May 2014
Optical pumping due to radiation trapping
Multiple scattering radiation trapping
The intensity changes inside the sample.
Could it change the equilibrium population such that it increases the fluorescence ?
YES, this is the dominant effect very close to the |3> |2> transition.
But it is negligible around = 0 (-5 from the |3> |2> transition).
Phys. Rev. A 87, 013412 (2013).