Laser technology, optical atomic clocks and new cooling and trapping schemes

Flavio C. CruzGleb Wataghin Physics Institute – UNICAMP

flavio@ifi.unicamp.brhttp://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