Optical pumping of helium-3 with a frequency electromodulated laser

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Optical pumping of helium-3 with a frequency

electromodulated laser

M. Elbel, C. Larat, P.J. Nacher, M. Leduc

To cite this version:

Optical

pumping

of helium-3 with

a

frequency

electromodulated

laser

M. Elbel

_(1),

C. Larat

_(2),

P.J. Nacher

₍₂₎

and M. Leduc

₍₂₎

(1)Fachbereich

Physik, Philipps

Universität

_Marburg,

_{Renthof 5,}

D355

_Marburg

_1,

F.R.G.

(2)Laboratoire

de

_{Spectroscopie}

Hertzienne de

l’École

Normale

_Supérieure,

24 rue

Lhomond,

75231

Paris Cedex

_05,

France

(Reçu

le 23

_juin

_1989,

_accepté

le 28

_septembre

₁₉₈₉₎

Résumé. 2014 Nous _avons

ajouté

des bandes latérales à la structure de modes du laser utilisé _pour le _pompage

_optique

d’un _{gaz d’hélium-3} au _moyen d’un modulateur

_{électro-optique}

externe. Nous

avons _{ainsi montré que l’efficacité du processus} de _pompage

_optique

_peut être améliorée _par une

meilleure

_répartition

de la

_puissance

du laser sur le

_{profil d’absorption Doppler}

des atomes d’hélium. Cette amélioration _peut être

_expliquée

_par l’effet des collisions

_purement

_élastiques

dans le _gaz. Cette

interprétation

est _{confirmée par} le résultat d’une

_expérience

de _pompage

_optique

sélectif en vitesse. Abstract. 2014 We have

operated

an

_{electrooptical}

modulator to _generate sidebands to the laser used for

_{optical pumping}

of a helium-3 _gas.

_Spreading

the laser _power over the

_{Doppler absorption profile}

of the metastable helium atoms can enhance the

_efficiency

of the

_{optical pumping}

_process. This

im-provement is discussed in terms of

_{velocity changing}

collisions and

_compared

to the result of a

_velocity

selective

_{optical pumping experiment.}

Classification

Physics

Abstracts

34.40 - 4260

1. Introduction.

Optical

pumping

is a

_powerful

_technique

used for

_creating

electronic or nuclear

_{spin polarisation}

in

a

_gas

or a beam of atoms

_[1].

It results from the interactions between the atoms and a resonant

_light

source,

usually a

tunable laser. When one deals with a

_gas,

the atoms have a

_{Maxwellianvelocity}

distribution

_{corresponding}

to an

_absorption

_{frequency profile}

which can be much

broader

than the

linewidth of the laser. As a

_result,

_only

a small fraction of the atoms are

_directely

excited

_by

the laser.

_Actually,

at

_{high enough}

_pressure,

_velocity

_changing

collisions are

_likely

to

_bring

different

atoms into resonance with the laser

_frequency,

thus

_increasing

the number of atoms

_effectively

excited

_by

the

_pumping

_light.

Such collisional

_processes

have been studied in

_détail,

for instance

in the case of

_krypton

metastable atoms

_[2].

At usual

_pressures,

_they

are not efficient

_enough

to

allow

_pumping

of the whole

_velocity

distribution

_{of a gas.}

This feature is considered in the

_present

_article,

in the context of

_optical

_pumping

of helium-3. A

motivation for this work is the

_growing

number of domains in which

_highly

_polarised

helium-3 is

40

required,

such as the field of

_quantum

fluids

_[3],

_polarised

_targets

in nuclear

_physics

_[4],

and helium

spin

filters for neutron beams

_[5].

Optical

polarisation

is obtained

_through

a

_two-step

_process.

It relies on

_{optical pumping}

of the

23S,

metastable state of

helium;

_{metastability exchange}

collisions ensure the transfer of nuclear

polarisation

to the

_ground

state atoms

_[6].

In order to excite a number of metastable atoms as

large

as

_possible

with a

_laser,

one can first

_rely

on the above mentionned collisional

_processes.

However,

optimal

conditions for

_creating

metastable atoms limit the

_operating

_pressure

to a few

torr. Another

_possibility

is to increase the number of

_longitudinal

laser modes within the

_Doppler

absorption

profile

of the _gas, so as to excite more atoms in different

_velocity

classes.

_Preliminary

experiments

have shown that a multimode laser

_polarises

helium more

_efficiently

than a

single-mode one

_[7],

_especially

at

_high

laser

_power.

This result is

_fairly

well

_interpreted

_by

the model

derived in

_[7]

for the kinetics of

_{optical pumping.}

In the

_present

_article,

we

_report

the results of an

_experiment

in which the number of metastable

velocity

classes excited

_by

the laser was increased

_using

an

_{electrooptical}

modulator outside the laser

_cavity

to

_generate

sidebands to each

_longitudinal

mode. This

_set-up

_produced

an enhanced

efficiency

of the helium-3

_{optical pumping, depending}

on the

_spacing

of the sidebands. This result is

_interpreted

in terms of collisional redistribution of atomic velocities. It is consistent with other results obtained in a

velocity

selective

_{optical pumping experiment,}

which allowed

_measuring

cor-relations between

_velocity

and

_polarisation

created

_by

a

_singlemode

laser. The latter

_experiment

is also

_briefly

described in this article.

2.

_Modulating

tlie

_frequency

of a LNA laser.

Optical pumping

of helium-3 uses laser

_light

at _{1.083 Mm,}

_{corresponding}

to the

23S

--->

23P

transi-tion. A tunable laser at this

_Bvavelength

was first obtained

_using

NaF

_crystals

coloured with

_(F+)

centers

_[8];

a

_{singlemode ring}

version of this laser was built

_[9]

and was used in the

_experiment

described in

_paragraph

5. More

_recentely,

a

_neodymium

_doped

solid-state

_material,

called LNA

(La1-xNdxMg Al11O19) ,

proved

to be tunable around the

_{required wavelength,}

and to

_polarise

helium-3 nuclei

_up

to 60% in a two-mode linear version

_[10].

The latter laser was used for the sideband

_generation.

It was

_{pumped by}

an ion _argon

laser,

and included selective elements to

ensure both

_tuning

and

_operation

on two consecutive

_longitudinal

modes,

250 MHz

_apart.

The

output

beam was first

_{passed through}

a

_telescope,

which increased its diameter

_by

a factor of

_3,

but decreased its

_divergence.

It was then focused

_through

the

_{electrooptical}

modulator

_by

a lens

of 30 cm

_{focal length.}

This

_{electrooptical}

modulator

_(EOM)

consisted of a lithium tantalate

_crystal

of dimensions 0.65 x 0.50 x 25.0

mm3.

The laser beamwas

_{passed along}

the

_long

axis which was identical with

_{the y}

axis

in Yariv’s nomenclature

_[11].

The z axis was directed

_along

the

_{height (0.65 mm), perpendicular}

to the

_polarisation

of the laser. The

_crystal

base was

_glued

to a

_copper

bed with a 1 mm indium

inlay.

The

_glue

was

_{electrically conducting.}

On

_top,

a fine

_gold

wire was attached

_by

the same

glue,

which was

_spread

to cover the whole surface. This EOM

_crystal

had been

_previously

used at

the

_wavelength

À=596 nm for which its faces were antireflection coated. At the

_wavelength

À=

1083

_nm,this

non

_{adequate coating}

caused a 30% loss on the transmitted

_power.

At 596 nm, 10 W of

_{radiofrequency}

_power

_produced

a

_large

modulation of the laser

mode,

with

complete disappearance

of the carrier

_[12].

In this case, the EOM was

_just

used as a

_piece

of a

50 Q cable. This

_simple

_set-up

was not

_appropriate

_here,

as a similar modulation at a =1083 nm

requires

more

_{radiofrequency}

_power.

Instead the EOM was used as a

_part

of a

_resonating

_circuit,

as shown in

_figure

1. For

_frequencies

below 175

MHz,

tank circuits were suitable:

_figure

la shows the

_{corresponding}

_set-up.

For

_{higher frequencies,}

the EOM

_crystal

was used as a

_part

of a

_a/4

Figure

2

_displays

a

_sequence

of

_spectra

observed with a confocal

_Fabry

Perot

_analyser.

The

_upper

trace shows the double mode structure of the LNA laser in the absence of modulation. The lower

traces

_correspond

to

_increasing

values of the

_{radiofrequency}

_power

_applied

to the EOM. 1Wo sidebands on both sides of each carrier mode could thus be

_easily

_produced.

Fig. 1.

Fig.

2.

Fig.

1.- Two resonator circuits used to create sidebands to the laser modes with the EOM

_{crystal. a)}

’Iank circuit used for

_frequencies

_up to 175 MHz.

_b)

_À/4

resonator used for

_{higher frequencies,}

the EOM

_being

part of a 50 Q wave

_guide.

The dimensions of the resonator are: 75 mm in

_length,

25 mm and 10 mm for the external and internal diameters

_{respectively.}

The

_{eigenfrequency}

is about 200

_MHz,

or 400 MHz when the end of the EOM is short circuited to make a _A/2 resonator.

Fig.

2.- Mode structure of the modulated laser

_light.

The _sprectra were obtained

_using

a

_{piezo-scanned}

Fabry-Perot

_analyser.

The _upper trace was obtained without modulation. The lower traces were obtained with

increasing

values of the modulation _power. The modulation

_frequency

was 370 MHz. For each of the laser

modes,

as the first and second sidebands

_(F

and

_S)

_grow with the

_{radiofrequency}

_power, the

_amplitude

of the carrier

_(C)

decreases.

3.

_Polarising

helium-3 with the modulated LNA laser.

The laser source described above was used in an

_{optical pumping experiment.}

The helium-3 cell

was a

_{cylinder (5}

cm in

diameter,

5 cm in

_length)

filled at a

_pressure

of 0.3 torr and held in a

homogeneous magnetic

field of 6 Gauss. The laser beam was

_expanded

to cover a

_large

_part

of the cell area,

_{circularly polarised}

with a

_quarter-wave

_{plate, passed through}

the cell

_along

the field axis and reflected back into the cell

_by

a metallic mirror. The laser

_frequency

was tuned on the

Cs

component

of the

_absorption

line

_(23S1,

F =

1/2

--;

23Po transition),

which is best suited for

42

the circular

_polarisation

of the line at À=668 nm emitted

_by

the

_{discharge, according}

to the method described in

_[13].

Figure

3 shows the time

_build-up

of the nuclear

_polarisation

when the laser is

_applied

to the cell. At first there was no modulation of the laser

_frequency.

The

_polarisation

amounted to

_50%,

as can

be

_expected

for a two-mode laser of 250

_{mW power}

_[10].

The

_modulating

_{radiofrequency}

was then switched on, at a

_frequency

of 175

_MHz,

and with an

_{amplitude corresponding}

to the

_generation

of sidebands on each side of each

carrier,

as shown

_by

the lowest trace in

_figure

2. The nuclear

polarisation consequentely

increased,

reaching

a new

_equilibrium

value after a time

_comparable

to that of the initial

_build-up.

The

_{radiofrequency}

was then switched off and the

_polarisation

de-creased back to its

_original

value. A relative enhancement of about 8% of the nuclear

_polarisation

resulted from the

_spreading

of the laser

_power

over a

_larger

number of modes.

The same

_experiment

was

_repeated

for various sideband

_spacings,

_generated

_using

the devices

shown in

_figure

1. The

_amplitude

of the modulation was

_adjusted

to

_keep

the ratio of carrier to

sideband

_amplitudes

constant. The relative increase in nuclear

_{polarisation resulting}

from the modulation is shown in

_figure

4 as a function of the

_frequency

of modulation. Above 50

MHz,

the observed effect does not show _any

_{significant dependance}

on the

_frequency

of the

_modulating

voltage.

This could be

_expected

since the

_{frequency spreading}

of the modulated beam remained

within the

_{absorption profile}

in all cases.

_Conversely,

for the same estimated sideband

_amplitude,

the enhancement was smaller when the

_frequency

of the modulation was below 20 MHz.

However,

due to the limited resolution of our

_spectrum

_analyser,

it was not

_possible

to measure the relative

amplitudes

of the carrier and

sidebands,

and we did not

_get

_quantitative

data to

_plot

in

_figure

4.

Fig.

3.

_{Fig. 4.}

Fig.

3.- Time evolution

of 3He

nuclear

_polarisation

)B1. It first _grows from zero to its

_asymptotic

value when the unmodulated

_{pumping light}

is

_applied.

When the

_{radiofrequency}

is switched on, the

polarisation

increases. After modulation has been turned

_off,

Af comes back to its initial value. The

_experiment

was

_repeated

twice.

4. Discussion.

The

_{velocity dependance}

of the laser excitation tends to create correlations between atomic veloc-ities and internal atomic

variables,

whereas collisions

_constantly

tend to

_destroy

these correlations.

The orientation of each

_velocity

class of metastable

23S

atoms thus results from several

compet-ing

processes,

and so does the nuclear

_polarisation

transferred to the

_ground

state. The model

developped

in

_[7]

_simply

assumes that a fraction

_nm/nm

of the atoms in the metastable state is

effectively interacting

with the

_{pumping light. Figure}

5 shows the

_computed

_equilibrium

polarisa-tions as a function of the laser

_power

for different values of that

fraction,

as well as the results of the

_experiment

described above.

Fig.

5.-

_{Computed equilibrium polarisation}

M as a function of the laser _power.

_a)

Ideal broadband

_pumping,

matching

the whole

_{Doppler absorption profile}

_{(n* m /nm}

=

1). b)

_Singlemode

laser

_{pumping, taking}

into

account

_{velocity changing}

collisions

_(nm/nm

=

1/120)

_{c) Singlemode}

laser

_{pumping, assuming}

no

_velocity

changing

collisions occur

_{(n* m /n.}

=

1/600).

The data

_{corresponding}

to the _present

_experiment

are also shown

_{(A :}

without

_{modulation; 0}

:with

_modulation).

Curve

_(a)

is for

_{nro/nm -}

_1,

_{corresponding}

to an ideal situation where a broadband

_pumping

light

would

_just

be

_matching

the

_{Doppler absorption profile.}

Curve

_(c)

would

_correspond

to a

monochromatic

_{pumping light}

in the absence of

_{velocity changing}

_collisions;

the value

_{of n* m /n,}

is

set

_equal

to the ratio between the linewidth associated to the excited

23P

state lifetime

_(3.3MHz),

and the

_Doppler

linewidth

_{(2 Ghz).}

Curve

_(b)

is for

_{n§£ /n m}

set to five times the

_previous

value,

and

was found in

_[7]

to fit the

_experimental

results for monochromatic

_(single

mode

_{laser) pumping}

in

presence

of

velocity changing

collisions for a

_pressure

of 0.3 Torr. As

_expected,

the results of the

present

experiment

lie between curves

_(a)

and

_(b),

since the laser had more than one

_longitudinal

mode. The

_polarisation

obtained when

_modulating

lies above that obtained without modulation:

the

_larger

the number of laser modes within the

_{Doppler profile,}

the

_higher

the value of the fraction

n*/n.,

of

_{effectively pumped}

atoms.

The difference between the calculated curves can be

_{qualitatively}

understood. At low

_pumping

transi-44

tion occurs: the

_{effectively pumped}

fraction

_{n*m /nm}

is irrelevant to the

result,

and all curves have the same

_slope

at the

_origin.

On the other

_hand,

at

_high

laser _power, it is easier to saturate the transition and be limited to low nuclear

_polarisation

if that fraction is

small,

due to monochro-matic

_pumping

_{and/or poor}

collisional

_{velocity changes.}

As a

_consequence,

sidebands

_generation

increases the

_efficiency

of the

_{optical pumping}

_process

as soon as

_they

increase the

_effectively

pumped

fraction

_nm/nm,

that is as soon as

_they

differ in

_frequency

from the carrier

_by

more than the width associated with collisional redistribution. For a _pressure of 0.3

_Thrr,

that width can be

directely

measured

_{(see below),}

or estimated from the value of

_{n*m /nm}

used for curve

_(b)

which best fits

_experimental

results for

_single

mode

_pumping.

Both methods lead to a value of the order of 20 MHz for the collisional

_width,

which is consistent with the

_{frequency dependance}

of the

po-larisation enhancements obtained in the

_previous

section. That width can be attributed to

_velocity

changing

collisions,

as it is much

_larger

than the natural linewidth

_(the

saturation

_broadening

is

negligible

when

_using

an

_{expanded beam).}

5.

_Velocity

sélective

_{optical pumping}

in helium-3.

Velocity

selective

_{optical pumping}

is a method to measure correlations between

_velocity

and po-larisation of the metastable atoms. It was first demonstrated with metastable neon atoms

_[14].

Our

_experimental

_set-up

is shown in

_figure

6. The

_light

source was the

_{singlemode ring}

colour

Fig.

6.-

_Set-up

of the

_velocity

selective

_{optical pumping experiment.}

The _pump beam ,

parallel

to the

_magnetic

field

B,is

circularly polarised by

the

_quarter-wave

_{plate Ll.}

It

_{polarises 3He}

nuclei in a cell where a

_discharge

populates

the

_pumped

metastable state. The circular

_polarisation

of the

_probe

beam is

_periodically

_reversed,

due to the rotation

_(at

a

_{frequency Q )}

of _quarter-wave

_{plate L2.}

The difference between

signals

recorded in

Ci

and

C2

is

_proportional

to the _average

_absorption

of the

_cell,

and modulated at

_frequency

Q. The

_resulting

AC

_signal

is

_{phase detected,}

and

_proportional

to the

_polarisation

of the

_probed

metastable atoms.

_{Sl , S2}

and

centre laser described in

_[9].

The

_pump

beam was weak

_enough

so that the nuclear

_polarisation

was a linear function of the laser

_power.

A small amount of

_power,

deflected from the

_pump

_beam,

became a

_{counterpropagating probe}

beam. The

_pump

beam was

_circularly

o,+

_polarised

while the

circular

_polarisation

of the

_probe

beam was

_periodically

reversed. We measured the

_absorption

of the

_probe

_beam,

whose modulation was

_proportional

to the

_polarisation

of the metastable atoms.

Figure

7 shows the recorded

_signal

obtained

_by

_sweeping

the laser

_frequency

across the

_Doppler

profile

of the metastable _atoms,

_{slowly enough}

to

_keep

the nuclear

_polarisation

at its

_equilibrium

value.

Fig.

7.- Polarisation

of 3He

metastable atoms as a function of laser

_frequency,

recorded in the VSOP exper-iment shown in

_figure

6.

_a)

All

_velocity

classes of the atoms are

_{explored b)}

Detail of the central

_peak.

Its width is determined

_by

_{velocity changing}

collisions

_{(see text).}

The

_pump

and

_probe

beams

_{being counterpropagating, they}

interact with atoms which have

oppo-site velocities

_along

the laser direction: these atoms are

_{différent, except}

at the center of the

_profile

where this

_velocity

is zero. Outside the narrow central

_peak,

one thus observes the

_polarisation

of metastable atoms not

_interacting

with the

_pump

_beam;

their

_polarisation

is in

_equilibrium

with that of the

_ground

state atoms

_through

the

_{metastability exchange}

collisions. The central narrow

_peak

shows that atoms

_{effectively interacting}

with the laser are

_{overpolarised.}

The width of this central

peak

is

_roughly

20 MHz. It

_gives

a value for the

_broadening

of the

_velocity

class

_effectively

inter-acting

with the laser due to

_{velocity changing}

collisions. This value is consistent with the results obtained in section

_3,

as discussed in section 4.

6. Conclusion.

In

_practice,

the use of the

_{electrooptical}

modulator can increase the nuclear

_polarisation

obtained

by

laser

_{optical pumping}

of helium 3. The

_frequency

of the

_generated

sidebands is not critical and

can _range between 50 and 400 MHz. An increase of

_polarisation

from 50% to 55% was observed at

46

Optimisation

of the LNA laser and of the EOM

_crystal

_(for

instance

_by

_appropriate

anti-reftection

coating)

should lead to over 60% of nuclear

_{polarisation.}

Our

_present

results can be

_compared

to

the

_polarisation

rates obtained in other

_{optical pumping experiments performed}

with

_frequency

modulated multimode

_dye

lasers on

_argon

metastable beams

_[15].

The main difference is that a

gas

exibits a much broader

absorption

profile

than a beam. Since each laser mode interacts

_only

with a narrow fraction of this

profile, generating

sidebands to the laser

_appears

_particularly

well suited in the case of

_{a gas}

such as helium 3.

Acknowledgements.

One of the authors

_(M.E.)

received

_support

from the Deutsche

_{Forschungsgemeinschaft (DFG).}

References

[1]

See for instance:

KASTLER

_{A., J.}

_Phys.

Rad. 11

₍₁₉₅₀₎

255. HAPPER

W.,

Rev. Mod.

_Phys.

14

₍₁₉₇₂₎

169.

[2]

BRECHIGNAC

_C.,

VETTER R. and BERMAN

P.R.,

Phys.

Rev. A 17

₍₁₉₇₈₎

1609.

[3]

LHUILLIER C. and LALOE

F., J.

Phys.

France 40

₍₁₉₇₉₎

239.

[4]

Workshop

on "Polarized helium-3 beams and

_targets",

AIP

_proceedings

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