Laser Cooling

of Thulium Atoms

P.N. Lebedev Physical Institute

Moscow Institute of Physics and Technology

Russian Quantum Center

N. Kolachevsky

D. Sukachev

E. Kalganova

G.Vishnyakova

A. Sokolov

A. Akimov

V. Sorokin

optical atomic clocks – a new era

of clocks

Laboratory frequency measurements

10-9

10- 01

10- 11

10- 21

10- 31

10- 41

10-15

10-16

1950 1960 year

rela

tive u

ncert

ain

ty

1970

Cs fountain

Cs beam

Re-definition of the second

1980 1990 2000 2010

Cs Essen

iodine stabilized He-Nelaser

H

Ca

H

HYb

+Hg

+

Sr+

SrHg

+

Hg+ vs Al

+

Al+ vs Al

+

Sr

10-17

Laser cooling of Lanthanides

Electronic structure of lanthanides (Yb, Dy, Er, Tm) is similar to

alkali-earth elements (Ca, Sr, Mg) due to a closed outer electronic

s-shell.

Laser cooling of lanthanides is more challenging because of the

absence of closed strong cooling transitions. All of strong

transitions possess decay channels.

• strong rate > 107 s-1

• cycled

• accessible for laser sources with a power of > 1 mW

Requirements for an efficient laser cooling transition:

Редкоземельные элементы(группа лнтаноидов)

Very recently a significant progress in laser cooling and

trapping of some lanthanides, including hollow-shell

ones, is achieved. They are intensively studied and

successfully implemented in

• precision spectroscopy and optical frequency

metrology

• study of interactions in quantum regime, study of

quantum gases

Applications

Ytterbium optical lattice clock are

unprecedentedly stable

N. Hinkley at al., arXiv:1305.5869v1

Comparison of two identical

Yb clocks is performed at

NIST, 2013 in the group of

Andrew Ludlow

Magnetic gases

Cr:

dipole-dipole

interactions

T. Lahaye,et al., Phys Rev Lett.

080401 (2008)

Optical

lattice+microscope for

degenerate gases?

W. S. Bakr, J. I. Gillen, A. Peng, S.

Foelling, M. Greiner

Nature 462, 74-77 (2009)

Rb

Hollow-shell Lanthanides

Vacancies in the hollow 4f-shell (e.g. Er, Dy, Tm) provide

big magnetic moment in the ground state.

Magnetic moment of Dy equals 10 mB, of Er - 6mB. Strong

dipole-dipole interactions between ground state atoms. Dipole-

interacting condensates and quantum simulators.

M. Lu, N.Q. Burdick, S.H. Youn, and B.L. Lev Phys. Rev. Lett. 107 190401 (2011)

K. Aikawa et al. Phys. Rev. Lett. 108 210401 (2012)

Feshbach resonance in Er

134 f 26sTm:

shell s p d f

L 0 1 2 3

- one vacancy in the 4 f shell

- relatively simple level structure

- fine splitting of the ground state

3

1 2

L

S

Large J causes large magnetic moments of the ground state

Thulium electronic structure

4ground Bm m

7 / 2J

5/ 2J

Similar polarizabilities of the ground-state fine structure components

To what extent the transition frequency remains

unperturbed? Calculations needed!

(V.D. Ovsyannikov, 2010 )

Optical

lattice

L S

L S

M1 transition = 1.14 mm, ~ 1 Hz

2f Ry

Shielding of the 4f shell levels

26s4 nf

collisions

Because of the closed 6s2 shell, the

inner shells are shielded to the external

perturbations.

The shileding was first demonstrated

experimentally by E.B. Alexandrov in

1983

J. Doyle at al. measured for He-Tm

collisions (Nature, 2004)

For Tm-Tm collisions in specific

magnetic state the shielding disappears

(PRA, 2010)

E.B.Aleksandrov et al., Opt. Spektrosk., 54, 3, (1983)

C.I. Hancox et al. Nature 431, 281 (2004)

C.B.Connolly et al., Phys. Rev. A 81, 010702 (2010)

5410in el -

He

Tm

The M1 transition in Tm atom

Spectroscopy on the ground state sublevels in lanthanides

is not yet performed

Thulium: = 1.14 mm, ~ 1 Hz

- suppression of the external electric fields perturbations

- small black-body shift

- loading in the optical lattice with small perturbation of

the clock transition

- strong -dependency 2f Ry

Laser cooling of Thulium

13 24 6f s 7/ 2J

13 24 6f s 5/ 2J

M1

transition

5d , =9/26s J

5d , =7/26s J

5d , =11/26s J

5d , =9/26s J

5d , =7/25/26s2

J

6s26p , =11/21/2 J

2 9413 cm-1

2 8733 cm-1

22560 cm-1

23335 cm-1

22468 cm-1

22420 cm-1

{

decay9/ 2J

410 nm17ns

Cooling

transitions

13 2

3/24 5 6f d s

Cooling transitions in Tm

531 nm440 ns1.14 mm

12 2

5/ 24 5 6f d s

Oven

(1100 K)

Zeeman slower

MOT

chamber

temperature Co

vap

or

pre

ssure

, m

bar

melt

boil

Zeeman slowing

slower

2D molasses

oven

410 nm

initial beam ~ 10 at/s9

slowing 1% atoms

beam size 1 cm2

flux (typ) 10 at/s cm7 2

~v4

0 200 400 600 800

0

2

4

6

co

unts

[10

]6 s

-1

velocity [m/s]

4 - 5

3 4-

beam 45

Dopplerprofilewithoutslowing

slowedatoms25 m/s

Slower operation

Oven design

Magneto-optical trap (2010)

106 atoms Tm-169

t, seconds

310 cm

3(2) 10s

-

The life time of Tm atoms

in the MOT

Binary collisions in the MOT

“Dark” MOT implemented

Temperature of atoms

0 1 2 3 4 5

0

1

2

3

4

0.5 red detuning

0.40.0

I0 [mW/cm ]2

dec

ay r

ate

[s

]t

-1-1

28 s-1

24 s-1

G 18 1 s-1

G0 [ s-1

]

0.5

1.0

1.5

G1= 22(6) s-1

5 times more atoms

Temperature measurements

time, ms0 5

Ballistic expansion of the atomic cloud

to measure temperature

Temperature in Tm MOT

IT

F

IT

F

min

240

25DT K

T K

m

m

Detuning

Tem

per

atu

re m

K

Saturation parameterT

emp

erat

ure

m

K

Tmin = 25 mK

TD = 240 мкК

Magnetic field

Due to specific level structure of Tm atom

(degeneracy of the Landé g-factors) sub-

Doppler mechanism IS EFFICIENT even

in the presence of magnetic field

Doppler:

sub-Doppler

BD e

B

kgv

m

BS g

B

kgv

m

Role of Landé g-factors

Rb: gg = 1/2, ge = 2/3

M. Walhout, J. Dalibard, S.L.Rolston, and W.D. Phillips, J. Opt.Soc. Am. 9, 1997 (1992)

Tm: gg = 1.14, ge = 1.12

(2% difference) (30% difference)

Magnetic trap for Tm

MOT

B = 0

MT

B ≠ 0

20 G/cm

t, ms0 5 10

t, ms10 30 500100 30020

40 000 atoms,

life time 0.5-1s,

T~ 40 mK

Magnetic trap(in presence of gravitation)

IF = 4

B

U

z

effectivepotential

m F = 4

m F = 4

I

E Bm -

z

Magnetic trap(in presence of gravitation)

IF = 4

B

U

z

effectivepotential

m F = 2

m F = 2

I

E Bm -

z

Magnetic trap profile

density [arb. un.]

0 50 100 150 200 2500

1

2

3

4

5

Ver

tica

l co

ord

inate

, m

m

g

Only atoms in mF=+2,3,4 states are trapped

time, ms

0 200 400 600 800 1000

tem

per

atu

re,

Km

0

20

10

30

40

50

60

Temperature

First stage cooling at 410 nm

TD=240 mK

Second stage cooling at 530.7 nm

TD=9 mK

Frequency-doubled laser diode

radiation is used

Second stage cooling

Second stage cooling

0 1 2 3 4 5 6 7 ms

• Efficient cooling recapture directly from

Zeeman slower

• Number of atoms similar to blue MOT due

to Zeeman slower design

• Recapture efficiency from blue MOT

100%

•To reach lower temperatures we need to

narrow the diode laser line width (lower

than 100 kHz)

0 2 4 6 8 10 1230

40

50

60

70

red detuning,

0

tem

pe

ratu

re,

Km

Optical trapping

MOT

mirror

trapping

laser

w0=30 mm

Red detuning Blue detuning

Spectroscopy of Tm clock transition in

the optical lattice

Dipole optical trap with a

standing wave

Excitation of the clock

transition in trapped atoms

One trapping beam

Optical lattice

530.660 nm

532.0 nm

13 24 6f s 7/ 2J

530.7 nm

12 2

5/ 24 5 6f d s

trapping

• Loading from MOT at 100 mK

• Laser Verdi G-12 blue detuned!

• Optical trap depth 1 mK

• Strong blue transitions mainly

contribute to the polarizability

• About 1% of atoms is

recaptured

13 24 6f s 7/ 2J

530.7 nm

12 2

5/ 24 5 6f d s

trapping

• Loading from MOT at 100 mK

• Laser Verdi V-8 red detuned!

• Optical trap depth 1 mK

• Optical trapping is more

efficient for red detuning

• Trapping depends on

polarization => lattice effect!

Intermediate conclusions

- Tm atoms are trapped in an optical lattice and prepared for

spectroscopy of clock transition at 1.14 mm

- Temperature of atoms is still too high for efficient recapture into

the shallow lattice => further cooling is necessary

- Narrow line lasers for second stage cooling (530.7 nm) and

studying of metrological transition (1.14 mm)

- Increasing of the number of atoms

Stabilized laser systems at Lebedev Institute

1 10

10-14

100 1000averaging time [s]

Alla

n d

evia

tion

10-13

Vertical cavity F=60000

Comparison of two systems

designed for 698 nm

Laser systems for optical clocks at Lebedev

InstituteTransportable setup

Vertical cavity

Compensation of temperature fluctuations

9 210 ( )cl l T T - -

ULE thermal expansion coefficient

8 10 12 14 16 18 20 22 24

50

55

60

65

70

75

80

Температура [ C]o

Tc

Частота

сигнала биений

МГц

[]

Изм

енение

длины

резонатора

нм

[]

0

5

10

15

45

Temperature [oC]

Be

atn

ote

fre

qu

en

cy M

Hz]

Ca

vity le

ng

th v

aria

tio

n [n

m]

Spectral line width

Fourier frequency [Hz]

Sp

ectr

al p

ow

er

density [arb

. un.]

0.5 Hz

Beatnote between two independent cavities(972 nm, for hydrogen spectroscopy)

J. Alnis, A. Matveev, N. Kolachevsky, Th. Udem, and T. W. Hänsch, Phys. Rev. A 77, 053809 (2008)

GaN diode lasers @ 410.6 nm

Diode PHR-803T

(HD-DVD, Blue-Ray)

+70 oC

Injection locking gives up to 150 mW @ 410.6 nm

Shuji Nakamura

Fraction of power in a single

frequency >95%

Saturation absorption signal in Tm

time [s]

Sig

nal

Tm cloud trapped by

diode laser radiation

slower

2D molasses

oven

410 nm

2D molasses for Tm beam collimation

• Slave diode SF-BW512P

• Max power 500 mW

• Central wavelength:

405 nm @ 25°C

410 nm @ 70°C

• Seed power < 1 mW

• Output power 120 mW

Power in 2D molasses, mW

nu

mb

er

of

ato

ms,

arb

.un

.

up to threefoldincrease in thenumber of atomsin the MOT

Thank you for attention!

Cooling transitions in Tm