Laser technology, optical atomic clocks and new cooling and trapping schemes
Flavio C. CruzGleb Wataghin Physics Institute – UNICAMP
[email protected]://www.ifi.unicamp.br/lasers
Escola Jorge André Swieca de Óptica Quântica e Não Linear – 11.02.2008
“Lasers and Applications” Grouphttp://www.ifi.unicamp.br/lasers
Antonio A. Soares (D, 2005-)Bruno Roque (IC, 2007-)
Giovana T. Nogueira (D, 2003-2007, Collab., 2007-)Joseph D. Topomondzo (PD, 2007-)
Luciano S. Cruz (PD, 2005-07)Mayerlin N. Portela (M, 2007-)
Milena Sereno (IC, 2007-)Silvania A. Carvalho (D, 2005-)
Prof. Luis E. de AraujoProf. D. Pereira
Outline
1. Laser cooling and trapping of alkaline-Earth atoms.
2. Optical atomic clocks: ultra-narrow continuos lasers and ultra-broadband short-pulse lasers.
3. Two-photon cooling with three-level transitions.
4. Deep optical traps.
1. Laser cooling and trapping of alkaline-Earth atoms.
1D2
3P2
2180
40
2100
3P0
- 2 electron atoms: singlet and triplet states;2 electron atoms: singlet and triplet states;2 electron atoms: singlet and triplet states;2 electron atoms: singlet and triplet states;
- Spin forbidden Spin forbidden Spin forbidden Spin forbidden 1111SSSS0 0 0 0 ---- 3333PPPP1 1 1 1 ”clockclockclockclock”””” transition:transition:transition:transition:2100 photons/s2100 photons/s2100 photons/s2100 photons/s ô ∆ν∆ν∆ν∆ν = 370 Hz; Q = 10= 370 Hz; Q = 10= 370 Hz; Q = 10= 370 Hz; Q = 1012121212; ; ; ; 100 % detection efficiency100 % detection efficiency100 % detection efficiency100 % detection efficiency by electron by electron by electron by electron shelving;shelving;shelving;shelving;
-3333PPPP2222: naturally populated and magnetically : naturally populated and magnetically : naturally populated and magnetically : naturally populated and magnetically trapped in a MOT;trapped in a MOT;trapped in a MOT;trapped in a MOT;
• No hyperfine structure for No hyperfine structure for No hyperfine structure for No hyperfine structure for bosonicbosonicbosonicbosonic isotopes: isotopes: isotopes: isotopes: study of cold collisions; PAS; tests of Doppler study of cold collisions; PAS; tests of Doppler study of cold collisions; PAS; tests of Doppler study of cold collisions; PAS; tests of Doppler study of cold collisions; PAS; tests of Doppler study of cold collisions; PAS; tests of Doppler study of cold collisions; PAS; tests of Doppler study of cold collisions; PAS; tests of Doppler cooling theory; EIT; new cooling schemescooling theory; EIT; new cooling schemescooling theory; EIT; new cooling schemescooling theory; EIT; new cooling schemescooling theory; EIT; new cooling schemescooling theory; EIT; new cooling schemescooling theory; EIT; new cooling schemescooling theory; EIT; new cooling schemes
• All optical BEC: All optical BEC: All optical BEC: All optical BEC: achieved only for achieved only for achieved only for achieved only for YbYbYbYb; ; ; ; nonmagnetic ground state; nonmagnetic ground state; nonmagnetic ground state; nonmagnetic ground state; metastablemetastablemetastablemetastable 3333PPPP2222statestatestatestate
Cooling and trapping of alkaline-Earth atoms
657 nm
“clock”
1S0
1P1
1S0
3P1
423 nm
“cooling”
8
2.1
x1
0
300
96
1034 nm
3.0
x10
7
Level diagram forLevel diagram for 4040CaCa
Trapped and Cold Calcium• Calcium Fluorescence at
423 nm• Magneto-Optical Trap
§ 107 trapped atoms;
§ T ≈ 1 milliKelvin.JOSA B 20, 5 (2003); Appl. Phys. B., 78, 1, (2004); Braz. J. Phys. (2004)
Our new compact system: MOT loaded from decelerated beam
1 meter
60 cm
Old system (2003): glass tube and
MOT chamber: no Eddy currents
but vacuum limitations
New system (2006): all-steel chamber,
high vacuum (<10-10 mbar background).
Technical challenge: need powerful
blue or UV lasers• 285 nm for Magnesium, 423 nm for Calcium (at least 50
mW), 400 nm for Ytterbium, 461 nm for Strontium, deep
UV for Zn, Cd, Hg.
• Our approach: near-infrared laser at 846 nm, frequency
doubled in external resonant optical cavities (using
KNbO3 cristals).
Previous systems:
Diode lasers orTi:sapphire
Opt. Communications, 201 (2002) ; Opt. Engineering, 43, 6 (2004) Opt. Engineering, 41, 5 (2002)
Diode laser
Doubling cavity
Single-frequency blue laser for calcium cooling
0 1 2 3 4 5 6 7 8
0
100
200
300
400
500
600
700
b
c
a
Pow
er
at
420
nm
(m
W)
Power at 532 nm (W) L. S. Cruz and F. C. Cruz , Opt. Express, 15, 11913 (2007).
• SHG with BIBO crystal: critical phase matching
at room temperature (angle tuning);• 10 cm long, AR-coated crystal• Nonlinear coefficient 9 times bigger than LBO• Photorefractive effect observed• BBO and LBO will be tested
• Other option: PPLI
Intracavity
frequency doubling
of Ti:sapphire laser
a: BRF
b: BRF+OD
c: BRF+OD+etalon
2. Optical atomic clocks: ultra-narrow continuous lasers and ultra-broadband
short-pulse lasers.
Optical atomic clocks
• Optical clocks will be the
next generation of atomic clocks;
• A very stable laser is locked to a narrow optical atomic
transition;
• The frequency of such laser
(hundreds of THz, 1014 -1015
Hz) is measured;
• Important for navigation,
telecommunications, high precision measurements
(relativity, change of fundamental constants).
Oscillator: Laser
Feedback
Optical Frequency
counter
Atom
Frequency Stabilized Laser
• Diode laser in Littman cavity
• Stabilized to high finesse cavity (F >
100000)
• High finesse cavity (Q > 3x1010): ULE
cilinder with optically contacted mirrors;
• Resonance linewidth < 15 kHz
Isolating the Optical Cavity from Seismic and Thermal Noise
• ULE optical cavities supported by passive platform, based on negative stiffness
(resonance frequency ≈ ½ Hz);• High vacuum for thermal isolation;
How to measure optical frequencies
(e.g. frequencies >1014 Hz)?
Optical Frequency Combs(1999)
(Nobel Prize in 2005)
Time and Frequency domain pictures of fsec lasers
Our Optical Frequency Combs
Homemade fsec laser, 750 MHz, chirped mirrors, ring cavity
IC
OC
Ti:S
Supercontinuum generation in microstructuredphotonic fibers
-400 -300 -200 -100 0 100 200 300 400
0.0
0.2
0.4
0.6
0.8
1.0
-400 -300 -200 -100 0 100 200 300 400
-0.4
-0.2
0.0
0.2
0.4
0.6
phase (
rad)
Inte
nsity
time (fs)
150 fs
FROG
Broadband femtosecond laser• Spectrum from 600-1200 nm
• No need for photonic fiber:
stable long-term operation;
• Repetition rate: 1 GHz,
extended to 2.12 GHz
• fceo stabilization by 3f-2f
scheme
600 800 1000 1200 14001E-6
1E-4
0,01
1
100
-80
-70
-60
-50
-40
-30
po
wer (dB
m )
po
wer ( µµ µµW
/mo
de
)
λλλλ(nm)
565nm
(a)
(b)
(c)
1130nm
532 nm pump
0 1 2 3 4 5 6 7 8
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6 CW
ML 1GHz
ML 1GHz
ML 2GHz
Ou
tpu
t P
ow
er
(W)
Pump Power (W)
G. T. Nogueira, F. C. Cruz, Opt. Lett. 31 (2006) 2069-2071; Opt. Express, submit., 2008
Achieving transform-limited pulses: 6 femtoseconds
-Our broadband laser had its spectral phase measured and corrected using MIIPS
(multiphoton intrapulse interference pulse shaping; in collaboration with Prof. Marcos
Dantus at Michigan State University)
- We realized the fastest laser with the shortest pulses so far: 5.9 fsec, 2 GHz !!
650 700 750 800 850 900 950 1000 1050
0.0
0.5
1.0
-40
-20
0
20
40
60
80
100
120
140
Ph
ase
/ra
d.
Inte
nsity /
arb
. u
nit
Wavelength /nm
Spectrum and phase(before correction)
650 700 750 800 850 900 950 1000 1050
0.0
0.5
1.0
-3.14
-1.57
0.00
1.57
3.14
Ph
ase
/ra
d.
Inte
nsity /
arb
. u
nit
Wavelength /nm
Spectrum and phase(after correction)
Pulse width:5.9 fsec
-30 -20 -10 0 10 20 30
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsit
y (
arb
. u
nit
)
Time (fs)
3. Two-photon cooling with three-level transitions.
Temperature limitations in laser cooling of alkaline-Earth atoms
-Alkaline-Earth atoms are relatively
hot : few milliKelvin;
- Absence of hyperfine structure
does not allow sub-Doppler schemes;
- Alternative : 2nd stage cooling for
pre-cooled atoms using a narrow
transition.
- works well for 88Sr and 174Yb;- Difficult for 40Ca and 24Mg;
- Are there other techniques ??
34 MHz
370 Hz
Two photon transitions can provide better cooling?
• W. C. Magno, R. L. Cavasso- Filho, and F. C. Cruz, Phys. Rev. A 67, 043407 (2003)
• Josh W. Dunn, Jan W. Thomsen, Chris H. Greene, and Flavio C. Cruz,
Phys. Rev. A 76, 011401 (Rapid Commun.), (2007).
• Giovana Morigi and Ennio Arimondo, Phys Rev. A 75, 051404 (Rapid Commun.),
(2007).
Theory
Cooling with 3-level transitions
δ1(MHz)
δ2= −γ1
-100 -50 0 50 100
δ2= −γ1Autler-Townes
splitting
δ1
δ2
Semiclassical treatment
● Density matrix approach (coherences are included):
d
dt
iH
ρ ρ ρ= − +h
, Γ Solve for ρijForce operator
= ρ,^^
FTrF
Find damping and diffusion coefficients: α = -dF/dv and D kT = D/α
Problems: 1) treatment fails near recoil limit, 2) diffusion coefficient is difficult to calculate, 3) temperatures can be even negative!
● Rate equations
Calculate single and two-photon transition ratesCalculate momentum transfer
(force)
Find damping and diffusion coefficients: F= -αv ; d(p2)/dt = 2D kT = D/α
Exploring parameter space
Full quantum treatment:- momentum states are
used, - sparce-matrix techniques
S1 << 1, S2 = 1
S1
S2
Saturation
intensities
First Experimental Results
• Magnesium – Copenhagen
N. Malossi, S. Damkjær, P. L. Hansen, L. B. Jacobsen, L. Kindt, S. Sauge,
F. C. Cruz, M. Allegrini, E. Arimondo, J. W. Thomsen.
Phys. Rev. A 72, 051403 R (2005).
• Magnesium - Hannover
T.E. Mehlstäubler et al, Proceedings of the Les Houches Workshop, Feb. 14-17
(2005); III workshop on ultracold group II atoms, Smithsonian Observatory,
Harvard, Sep., 2006.
Theory
• W. C. Magno, R. L. Cavasso- Filho, and F. C. Cruz, Phys. Rev. A 67, 043407 (2003)
• Josh W. Dunn, Jan W. Thomsen, Chris H. Greene, and Flavio C. Cruz,
Phys. Rev. A 76, 011401 (Rapid Commun.), 2007.
• Giovana Morigi and Ennio Arimondo, Phys Rev. A 75, 051404 (Rapid Commun.), 2007.
4. Deep optical traps
1D Deep Optical Dipole Trap
-0.00005
-0.000025
0
0.000025
0.00005
radial direction HmL
-0.005
-0.0025
0
0.0025
0.005
Axial direction
-0.1
-0.05
0
Potential Depth HKelvin L
-0.00005
-0.000025
0
0.000025radial direction HmL
Motivations:
▪ trap “hot” atoms at
few milliKelvin, such as Mg
and Ca;
▪ Powerful visible
lasers are commercially
available and are red
detuned for alkaline-Earth
atoms (ex.: 532 nm, 515 nm,
488 nm, single-frequency );
▪ Possibility of
evaporative cooling in the
optical trap or sideband
cooling in trap or lattice:
easier to reach resolved
sideband limit;
▪ Larger waist sizes
can be used for dipole trap
→ larger trap volumes and
better loading.
Potential depths (mK), oscillation frequencies and residual scattering rates
1.9348Hz / 41.1MHz
129 s-1
1.2270 Hz /
31.8MHz
77.2 s-1
0.4156Hz / 18.4MHz
25.7 s-1
0.2110Hz /
13.0MHz
12.9 s-1
200 µµµµm
7.7985Hz /82.3MHz
515 s-1
4.5 763 Hz /
63.7MHz
308.6 s-1
1.5440Hz / 36.8MHz
103 s-1
0.8312Hz /
26.0MHz
51.4 s-1
100 µµµµm
312.8kHz / 165MHz
2058 s-1
192.2kHz /127MHz
1235 s-1
6.2 1.2kHz /
73.6MHz
412 s-1
3.1 881Hz /
52.1MHz
206 s-1
50 µµµµm
5000 W3000 W1000 W500 WPower
Waist
size
Optical Cavity for a Deep 1D Lattice
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
100
200
300
400
500
600
-30 -20 -10 0 10 20 30
0
2
4
6
8
Erro
r Sig
nal (V
)
Cavi
ty tra
smis
sion (arb
.unit)
Cavity detuning (MHz)
-2
-1
0
1
2
Intr
aca
vity P
ow
er
(W)
Incident Power (W)
• Fabry-Perot cavity locked to 532 nm
laser by Pound-Drever-Hall technique;
• 100 microns waist size: higher trap volume
yields better atom transfer efficiency from MOT
• Intracavity power of 550 Watts
has been obtained for 3.2 Watt of
input power;
• Corresponds to 0.8 milliKelvin depth.
Luciano S.Cruz, Milena Sereno, Flavio C. Cruz, Opt. Express, accepted, 2008
AC-Stark shifts and “magical” wavelengths
• AC-Stark shifts (in MHz) produced by a
1000 W, 532 nm laser with a waist size of
100 microns.
• AC-Stark shift for a second laser in
the presence of a 532-nm laser
producing a 50 microKelvin depth;
• Shift cancellation of calcium clock
transition occurs for a second laser
at 613 nm.
600 605 610 615 620 625 630
-4
-2
0
2
∆ν (
MH
z)
λ (nm)
4s2 1S
0 (m= 0, π)
4s4p 3P1 (m= 0, π)
4s4p 3P1 (m= 0, σ+
)
4s4p 3P1 (m= 1, σ+
or m=-1, σ-)
4s4p 3P1 (m= 1, σ-
or m=-1, σ+)
Luciano S.Cruz, Milena sereno, Flavio C. Cruz, Opt. Express, accepted, 2008
Conclusion & Prospects
• New MOT for calcium should be compatible with BEC
experiments;
• Need for special lasers led to development of laser technology:Ti:sapphire lasers, frequency doubling, solid state lasers (green,
red);
• Optical frequency combs have been developed, in particular
using ultra-broadband fsec lasers and without using photonic fibers;
• Pulses have been compressed, and we obtained the fastest and
shortest laser ever produced: 6 fs at 2 GHz. Demonstrated use for
secure communications. Other applications include 2-photon microscopy and CARS;
Conclusion & Prospects
• New cooling scheme based on 2-photon excitation of 3-level
transitions should allow enough further cooling for direct optical trapping;
• Optical lattice using power enhancement cavity allows for a few
kWatts of intracavity power, leading to trap depths of several milliKelvin, and larger trapping volumes.
Prospects: cooling with femtosecond lasers?
• Suitable for 2-photon transitions.
• Possibility of laser cooling with UV transitions;
• Use calcium 1S0-1P1-
1S0 system, driven at 423 nm+1034 nm from optical frequency comb ?
• Decay rate of intermediate 1P1 state: 5 ns; 2 GHz fsec laser gives 500 ps between pulses: allows coherent accumulation.
• Spectral shaping can be done with MIIPS.
Prospects: ultra stable solid state laser at 657 nm
• Diode pumped Nd:YLF laser at 1.314 nm
• Intracavity frequency doubled to 657 nm
• Higher power than diode lasers
• Small intrinsic noise: less servo bandwidth for frequency stabilization
• Fundamental light at 1.314 nm can be transmitted through fiber networks (remote clock distribution and synchronization)
Support
• FAPESP, CEPOF
• CNPq, CAPES
• FAEPEX (UNICAMP)
• US – Air Force Office Scientific Research
• Oportunidades para Mestrado, Doutorado, e Pós-Doutorado
(bolsas FAPESP).• http://www.ifi.unicamp.br/lasers
• Projetos em andamento no grupo:
– Temático – FAPESP
– CEPOF – FAPESP
– Milênio - CNPq
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