Coherent spectroscopy in Potassium vapor with amplitude modulated light

Silvia Gozzini*a, Stefka St. Cartalevab, Dimitar G. Slavovb, Luca Marmugia , Alessandro Lucchesinia

aIPCF-CNR Area della Ricerca, via Moruzzi 1, 56124 Pisa, Italy; bInstitute of Electronics, BAS, boul. Tzarigradsko shosse 72, 1784 Sofia, Bulgaria

ABSTRACT

We present the first experimental observation of Coherent Population Trapping (CPT) in Potassium, obtained with kHz- frequency modulation of the laser light amplitude. It is performed by acousto-optical amplitude modulation of the radiation from an external cavity diode laser, matching the D1 line of K. The CPT resonances are detected both through K absorption and fluorescence. The resonances are studied in three kinds of K cells: i) pure-evacuated, ii) polydimethylsiloxane (PDMS)-coated-evacuated and iii) Ne-gas buffered. In all cases CPT-resonance narrowing with cell temperature is observed. . In the pure-evacuated cell we registered the lowest contrast and the highest width of the resonance, while in buffered/coated cells a strong enhancement of the CPT resonance contrast up to 15% is observed. This behavior is the opposite to the one exhibited by Cs and Rb. The observed contrast enhancement in K is accompanied by more than two orders of magnitude reduction of the resonance width. The results here presented prove the advantage of using Potassium in CPT-based applications.

Keywords: coherent population trapping, electromagnetically induced transparency, magnetometry, spectroscopy

1. INTRODUCTION CPT phenomenon has been studied mainly in Rb and Cs vapors, due to the availability of conventional diode lasers matching their resonance lines. Different approaches have been utilized for CPT-resonance observation: most frequently two ground-state hyperfine-structure (hfs) levels of alkali atoms are coupled to a common excited hfs level by means of two coherent light fields, provided by laser frequency modulation in the GHz range1. For some applications it is advantageous to reduce the modulation frequency down to the kHz region, coherently coupling Zeeman sub-levels within single ground-state hfs level (for simplicity further noted as “kHz-CPT”) 2, 3. In the kHz-CPT approach, however, the optical pumping to the ground-state hfs level non-interacting with the laser field causes strong losses in the formation of the CPT resonance. The hfs optical pumping (OP) is particularly efficient when using noble-gas-buffered or anti-relaxation-coated cells filled with alkali atoms. However, buffered and coated cells introduce real advantage and are used very often because they provide long-living ground-state coherence.

In order to overcome the problem with the hfs-OP we propose to perform experiment on K atoms, whose ground-level frequency difference (461.7 MHz) is much less than the Doppler width of optical transitions (~765 MHz at room temperature). In this case, the overlapping of the Doppler profiles of the transitions4, starting from both ground-state hfs-levels can provide the re-population of the resonantly excited by the light ground hfs-level, hence can enhance the efficiency of the CPT resonances.

In this communication, we present the first experimental observation of the kHz-CPT resonance in K. The light irradiating K atoms is amplitude modulated by Acousto-Optical Modulator (AOM). A similar amplitude modulation of the light has been used for CPT-study in Rb, registered as non-linear magneto-optical rotation of the light polarization5. In our experiment, the kHz-CPT resonances are detected in the K fluorescence and transmission, by scanning the laser modulation frequency around the Larmor frequency. The resonances are studied in three different pyrex-glass cells filled with K: i) pure-evacuated, ii) with PDMS coating of the walls and iii) buffered by Ne.

2. EXPERIMENTAL SET-UP AND POTASSIUM LEVEL CONFIGURATION Fig. 1a shows a diagram of the used experimental set-up. The ECDL (Toptica DL100) radiation at 770.1 nm is amplitude modulated by an AOM at frequency fMod. The modulation input of the AOM is controlled by a radio frequency Function

*silvia@ipcf.cnr.it, phone 050/315.2541; fax 050/315.2230; www.ipcf.cnr.it

15th International School on Quantum Electronics: Laser Physics and Applications, edited by Tanja DreischuhElena Taskova, Ekaterina Borisova, Alexander Serafetinides, Proceedings of SPIE Vol. 7027, 70270K

© 2008 SPIE · CCC code: 0277-786X/08/$18 · doi: 10.1117/12.822458

Proc. of SPIE Vol. 7027 70270K-1

Generator (FG). During the main part of the experiment the frequency of the generator is kept constant. The first-order beam of the AOM is circularly polarized by λ/4 plate and directed to the K-cell.

The circularity of the light polarization is precisely adjusted using rotating polarizer and photo-detector. Every K cell is placed in the same housing containing a pair of Helmholtz coils positioned around the K cells in a way to produce a magnetic field B orthogonal to the laser beam. The magnetic field is continuously varied around B = 0. The input of an optical fiber (used for K fluorescence collection) is precisely centered along the Helmholtz-coils axis and touches the cell wall. All these elements are placed inside a single-layer μ-metal cylinder to reduce the influence of stray magnetic fields. The magnetic field shielding is situated in a temperature controlled oven able to heat the K-cell from room temperature up to about 1000C. The optical fiber output is connected to a photo-detector (PhD1). A second photo-detector is used (PhD2) to detect the signal transmitted thought the K cell. In the present experiment, the kHz-CPT resonances are mainly registered in the transmitted light. The laser power used in the experiment is 175μW and the beam cross-section 0.02cm2, corresponding to 8.75 mW/cm2 light intensity.

Fig. 1. a) Experimental set-up diagram. ECDL-External Cavity Diode Laser, FG-Function Generator, AOM-Acousto-

Optical Modulator, λ/4-quarter-wave plate, PhD- Photo-Detector; Iin, Itr – incident and transmitted through the cell light intensities; b) Left: Energy level diagram of the 39K D1 line with the excitation scheme for circularly polarized laser light (σ+). The arrows representing the σ+ transitions point in-between hfs levels to illustrate the overlapping of the optical transitions; Right: Simplified energy-level diagram resulting by the overlapping of the transition profiles.

In Fig. 1b (left), the energy level diagram of K is presented with the excitation scheme for circularly polarized laser light. In order to illustrate the atom-light interaction that results in the kHz-CPT feature formation, we can transform the real atomic system to one that is formed only by Fg = 2 and Fe = 2 (Fig. 1b-right). The large Doppler broadening makes possible simultaneous interaction of the light with atoms of different velocity classes from both ground hfs levels. For the excited state, the overlapping is considerably stronger, thus allowing us to consider the hfs as a single excited level.

The CW σ+ polarized light pumps the atomic population into the Zeeman sublevel mF with the maximal projection of angular momentum: mF=2 (commonly denoted as Zeeman OP). Atoms which are accumulated on this sublevel do not interact with the laser light. In the case of degenerated ground-state (when no magnetic field is applied), the efficiency of this process is high and therefore the absorption/fluorescence of the atoms has a minimum (dark state). With a magnetic

57.7 MHz

461.7 MHz

Fg=2

Fg=1

mF = -2 -1 0 1 2

39K D1 line

770.1 nm σ+

b)

σ+

-2 -1 0 1 2

Fe=2

Fe=1

ΩL

4 2P1/2

4 2S1/2

AOM

0-order

λ/4

FG

Oven K-cell

μ-metal shielding

Helmholtz coils

Optical fiber

PhD1

ECDL

a)

PhD2

Iin Itr

Proc. of SPIE Vol. 7027 70270K-2

field applied orthogonally to the atomic orientation the Zeeman OP decreases6, due to the atomic precession around the magnetic field. However, if the laser light is amplitude (or frequency) modulated at the Larmor frequency ΩL, a high-efficiency synchronous optical pumping of the atoms to Fg = 2, mF = +2 will take place again. The created atomic spin orientation puts the atomic ensemble in dark state and the sample increases its transparency. This transparency resonance can be observed in two ways: (i) by scanning the frequency of Zeeman-sublevels splitting by varying the strength of the applied magnetic field while keeping the light modulation frequency constant or (ii) by sweeping the modulation frequency around the value corresponding to ΩL.

3. KHZ-CPT RESONANCES IN POTASSIUM In this section, first we present in detail the experimental results obtained by scanning the magnetic field applied to the K atoms. At the end of the section, the second approach of the kHz-CPT observation will be briefly illustrated.

Concerning the first approach, we keep the modulation frequency constant and sweep the magnetic field value around B = 0. In this case, a CPT resonance is observed centered at zero magnetic field, and this resonance is accompanied by two kHz-CPT resonances on both sides of the magnetic field scan. The side resonances are centered at the magnetic field values creating Larmor precession of the magnetic moments with frequency ΩL = fMod (see Fig. 2). The resonance, produced at zero magnetic field is related to the well-known Hanle resonance, which is obtained without amplitude (frequency) modulation of the light.

To realize the second approach, we keep the magnetic field constant and sweep the modulation frequency in a region around the frequency of Larmor precession ΩL. In this case only the kHz-CPT resonance is observed. Here, only the resonance observed in the PDMS-coated cell will be illustrated and compared with that obtained by the first approach. A detailed study and parameter dependences of the kHz-CPT resonances obtained by scanning the modulation frequency will be presented in a future paper.

3.1 kHz-CPT resonance observation at constant modulation frequency

Fig.2 shows the CPT resonances in absorption obtained for different values of the amplitude modulation of the light. The laser frequency is fixed at the maximum of the fluorescence profile of the K D1 line. The magnetic field B is varied around B = 0 in an interval of 473 mG.

95 kHz

200

150

40

0

Bort=+/- 236 mG

Abs

orpt

ion

Time

Amod, mV

Fig. 2. CPT resonances in K observed without (Amod = 0 upper trace) and with amplitude modulation of the laser beam. The

four cases correspond to different modulation depths from 0 to 100% at Amod = 200 mV. The kHz-CPT resonance appears at magnetic field B = 136 mG corresponding to Larmor precession frequency equal to the modulation one (95 kHz).

Initially, the frequency generator FG applies no voltage to the AOM modulation input (Amod = 0, see Fig.2 upper trace). At B = 0, the Zeeman sub-levels of the ground-state are degenerate and the circularly polarized light pumps the atoms to the non-interacting with the light Zeeman sub-level which has maximal projection of the angular momentum (Fg=2, mF=2). Thus, at B = 0 the atomic ensemble shows reduced absorption and therefore reduced fluorescence. With B ≠ 0, the atomic magnetic momenta start precession around the magnetic field vector and the efficiency of the Zeeman OP decreases6. Hence the atomic population evolves to the other Zeeman sub-levels of Fg=2 resulting in an increased

Proc. of SPIE Vol. 7027 70270K-3

absorption. In this case (Amod = 0), the conventional Hanle-resonance is observed, showing transmission maximum centered at B = 0.

As a further step, a 95 kHz sinusoidal voltage of amplitude up to Amod=200 mV is applied to the AOM modulation input corresponding to 100% of amplitude modulation of the light field. The plots of K absorption signals are presented for three values of the sinusoidal input to the AOM (Fig.2). It can be seen that in case of the modulated optical pumping, along with the Hanle resonance described above a second resonance appears centered at about |B| = 136 mG (95 kHz Larmor frequency). This is due to the fact that increasing the magnetic field, the Larmor frequency increases to the frequency of the light amplitude modulation. Thus, the efficiency of the Zeeman OP increases again creating a second transparency peak.

We have performed a measurement of both types of resonances, for a series of temperatures in the range between 30 0C and 90 0C. This measurement was repeated for each of the three types of K-cells: a) pure evacuated cell, b) PDMS-coated and evacuated cell, and c) buffered by 30 Torr of Ne cell. Figure 3 presents the experimental traces obtained in the three cells, for three different level of cell heating. For any cell temperature, the resonances obtained from the pure-evacuated cell show significantly larger resonance widths (of the order of one Gauss) in comparison with the PDMS-coated and the Ne-buffered cells (showing milli-Gauss order of the resonances widths). The absorption signal A is

calculated as: in

trin

IIIA −

= , where Iin and Itr are the incident to- and the transmitted through- the K cell laser intensities

(see Fig. 1a). From the absorption signal at the resonance center (Ares for the kHz-CPT resonance and A’res for the Hanle-resonance) and that out of the resonance (A0), we calculate the resonance amplitude (A0 -Ares) and the resonance contrast (A0 -Ares)/A0 for the kHz-CPT resonance, and ((A0 –A’res) and (A0 –A’res)/A0 for the Hanle-resonance).

0.320.330.340.350.360.370.38

-10 -8 -6 -4 -2 0 2 4 6 8 10

0.026

0.028

0.030

0.032

0.034

0.070

0.075

0.080

0.085

0.090

0.095

A, abs.u.

fmod

= 4 MHz

71 0C

Magnetic field, G

47 0C

A)

Pure evacuated cell

56 0C

0.80

0.82

0.84

0.86

-0.2 -0.1 0.0 0.1 0.2

0.1200.1320.1440.1560.1680.1800.192

0.36

0.40

0.44

0.48

0.52

0.56

B)

A, abs.u.

fmod

=110 KHz

70 0C

Magnetic field, G

43 0C

PDMS-coated evacuated cell

58 0C

0.2

0.3

0.4

0.5

0.6

-0.2 -0.1 0.0 0.1 0.2

0.030.040.050.060.070.080.090.100.04

0.08

0.12

0.16

0.20

0.24

A, abs.u.

Ares

A'res

fmod=110 KHz

71 0C

Magnetic field, G

A0

42 0C

C)

30 Torr Ne cell

55 0C

Fig. 3. Coherent resonances observed at constant frequency of the light amplitude modulation, for three different cells filled

with K. A) pure-evacuated cell: fmod = 4.0 MHz and magnetic field scan of 25 G; B) PDMS-coated and evacuated cell: fmod = 110 kHz and magnetic field scan of 429 mG; C) buffered by 30 Torr of Ne cell: fmod = 110 kHz and magnetic field scan of 429 mG.

Proc. of SPIE Vol. 7027 70270K-4

In Fig. 4 we present a comparison of the kHz-CPT resonance parameters – contrast (A0 -Ares)/A0, amplitude (A0 -Ares) and width (full width at half maximum - FWHM) obtained in the three types of cells as function of the temperature. The pure-evacuated cell shows the lowest kHz-CPT resonance contrast (Fig. 4A) and amplitude (Fig. 4B). Raising the cell temperature we observe an increase of the resonance amplitude (Fig. 4B), due to the increased K-vapor density in the cell. For the coated and the buffered cells, we observe a different behavior. The resonance amplitude (Fig. 4B) shows a maximum with the cell temperature: this behavior can be related to the increased level of the total absorption for high temperatures (88% for the coated- and ~ 60% for the buffer-gas cell at 70oC), as shown in Fig. 3B, C. Therefore at such temperatures the atomic vapor is not anymore optically thin and the increased absorption of the light intensity along the cell reduces the resonance amplitude. The maximum contrast and amplitude of the resonance in the coated- and the buffered-cell are higher than those observed in the evacuated cell. In particular, the resonance amplitude (Fig. 4B) is higher by a factor of 4 for the coated cell and 5 for the buffer-gas cell in comparison with the pure evacuated cell.

Fig. 4C shows the widths of the resonance profiles (FWHM) observed in the coated- and buffered cells. For the evacuated cell, the width of the resonance profile is presented on a separate plot (Fig. 6), due to its higher value than that of the two previous cells. For the PDMS-coated- and the 30 Torr Ne buffered cells we obtain resonances more than two orders of magnitude narrower than that in the pure evacuated cell (Fig. 4C and Fig. 6).

This huge increase in the contrast and amplitude of the kHz-CPT resonance accompanied with its strong narrowing (as a result of the protection against spin relaxation collision with the cell-walls) represents a particular advantage of the Potassium application for the CPT resonance preparation. Under similar conditions Rb and Cs atoms show lower contrast and amplitude7-11 as a result of the very efficient optical pumping process. Therefore, in case of Rb and Cs we can benefit of the reduced resonance width only at the price of significantly compromised signal-to-noise ratio. As we discussed in the Introduction Sect., due to the overlapped spectral profiles of the transitions starting from both hfs ground levels of K, the mono-mode laser light can perform a partial “re-pumping” of atoms to the ground level resonant with the light, thus greatly compensating the losses due to the hfs optical pumping.

40 50 60 70 80 900

3

6

9

12

15

A)

kHz-CPT resonance 30 Torr Ne PDMS coated pure-evacuated

Contrast, %

T, 0C40 50 60 70 80 90

0.00

0.01

0.02

0.03

0.04

0.05

0.06

B)

kHz-CPT resonance 30 Torr Ne PDMS pure-evacuated

A0-Ares, abs.u.

T, 0C40 50 60 70 80 90

0

5

10

15

20

25

30

C)

kHz-CPTresonance 30 Torr Ne PDMS

Width, mG

T, 0C Fig. 4. kHz-CPT resonance parameters: A) resonance contrast (A0 -Ares)/A0, B) resonance amplitude (A0 -Ares) and C)

resonance width (FWHM) presented for the three types of cells (squares - pure evacuated cell; triangles - PDMS-coated and evacuated cell; circles - buffered by 30 Torr of Ne cell).

Fig. 5 shows the temperature dependence of the parameters of Hanle-resonance (centered at B = 0) observed in the three types of cells. Here we present the results related to the Hanle resonance obtained by amplitude modulated light field. The general conclusions derived when comparing the kHz-CPT resonances in the same cells are also valid here. The quantitative comparison between the Hanle-type and the kHz-CPT resonances shows that the amplitude and contrast are higher by a factor of ~10 in the first case than those for the second one (see Fig. 5A, B and Fig. 4A, B). In both cases a resonance narrowing with the cell temperature is observed (see also Fig. 6 for the pure evacuated cell). The width of the Hanle-resonance (centered at B = 0) is lower by a factor two than that of the kHz-CPT resonance (centered at the Larmor frequency). We explain this different behavior by the differences in the atom-light interaction in the two cases. At B = 0, the magnetic moments of the atoms are not precessing and the atomic ensemble interacts with the amplitude-modulated light (fMod = 110 kHz in our case) with relatively high efficiency determined by the laser frequency detuning from the optical transition. The atoms are optically pumped to the magnetic sub-level with higher magnetic moment (mF=2, see

Proc. of SPIE Vol. 7027 70270K-5

Fig. 1b). In the interval of magnetic fields between the two resonances, the atomic magnetic moments precess with Larmor frequency ΩL ≠ fMod. This leads to a reduced Zeeman OP efficiency and redistribution of the atoms among Zeeman sub-levels. However, when ΩL approaches fMod the Zeeman OP efficiency increases again. Thus, the kHz-CPT resonance appears.

It is worth to note that in the last case, the Zeeman OP efficiency depends not only on the laser frequency detuning but also on the phase mismatch of both frequencies (ΩL and fModul) as well as on the pulse duration of the amplitude-modulated intensity. As a result, some reduction of the OP-efficiency takes place6 leading to the resonance contrast reduction and to the broadening of the resonance profile.

30 40 50 60 70 80 900

102030405060708090

100

A)

Hanle resonance with modulation

30 Torr Ne PDMS coated pure-evacuated

Contrast, %

T, 0C30 40 50 60 70 80 90

0.0

0.1

0.2

0.3

0.4

0.5

0.6

B)

Hanle resonance with modulation

30 Torr Ne PDMS pure-evacuated

A0-A'res, abs.u.

T, 0C30 40 50 60 70 80 90

0

2

4

6

8

10

12

14

16

C)

Hanle resonance with modulation

30 Torr Ne PDMS

Width, mG

T, 0C Fig. 5. Hanle-resonance parameters: A) resonance contrast (A0 –A’res)/A0, B) resonance amplitude (A0 –A’res) and C)

resonance width (FWHM). Resonance parameters are presented for the three types of cells: squares - pure evacuated cell; triangles - PDMS-coated and evacuated cell; circles – buffered by 30 Torr of Ne cell.

50 60 70 80 900.0

0.5

1.0

1.5

2.0

2.5Width, G Pure-evacuated cell

kHz-CPT resonance Hanle resonance

T, 0C Fig. 6. Temperature dependence of the widths of Hanle and kHz-CPT resonances, for the case of pure-evacuated cell.

3.2 kHz-CPT resonance observation by sweeping the modulation frequency with applied constant magnetic field

The kHz-CPT resonance obtained by scanning the modulation frequency is more interesting for practical applications. In particular, using this approach a precise measurement can be performed of the magnetic field applied to the K atoms measuring the central frequency of the kHz-CPT resonance. Note that in this case, the kHz-CPT resonance is centered at the frequency fMod = ΩL. Thus, the modulation frequency of the light field amplitude is a measure of the magnetic field.

Proc. of SPIE Vol. 7027 70270K-6

In our experimental configuration, the light amplitude modulation frequency is related to the magnetic field as follow: fmod = γB, where γ=0.7 kHz/mG for K atoms and magnetic field B applied orthogonal to the laser beam.

Here we illustrate the kHz-CPT resonances obtained by scanning the modulation frequency at constant magnetic field of 280 mG. Fig. 7 shows resonances detected in the fluorescence collected from the PDMS-coated vacuum K-cell, at three different temperatures.

The resonance width corresponds to that obtained by scanning the magnetic field (Fig. 4). Narrowing of the resonance width with the cell temperature is also observed here. A detailed analysis of the kHz-CPT resonance (observed by the modulation frequency scanning) parameters in different types of K-cells is under preparation.

140 160 180 200 220 240

0,0

0,2

0,4

0,6

0,8

1,0 FWHM 13,9 kHz

A)

Fluorescence, a.u. 44oC

Modulation frequency, kHz 140 160 180 200 220 240

0,0

0,2

0,4

0,6

0,8

1,0 FWHM 13,7 kHz

B)

Fluorescence, a.u. 55oC

Modulation frequency, kHz 140 160 180 200 220 240

0,0

0,2

0,4

0,6

0,8

1,0 FWHM 13,2 kHz

C)

Fluorescence, a.u. 71oC

Modulation frequency, kHz Fig. 7. Comparison between the kHz-CPT resonances obtained in fluorescence from the PDMS-coated evacuated cell at:

A) 44oC, B) 55oC and C) at 71oC. The orthogonal magnetic field applied by the Helmholtz coils was Bort~278 mG. The modulation frequency was scanned in a region of 100 kHz. The laser frequency was centered at the fluorescence maximum.

4. CONCLUSIONS To our best knowledge, the Coherent Population Trapping resonances in K vapor, based on modulation of the light amplitude in the kHz range were obtained for the first time. The resonance parameters confirmed our initial expectation that, in the case of K atoms, the resonance contrast and amplitude will be significantly higher than those in Rb and Cs. When the reduction of spin-relaxing collisions with the cell walls was performed, either by adding a buffer gas or by coating the cell walls, the resonance width is reduced by more than two orders of magnitude accompanied with resonance contrast as high as 15%.

The kHz-CPT resonances were obtained using two approaches: (i) modulating the laser light amplitude with constant frequency and scanning the applied magnetic field and (ii) keeping the magnetic field constant while scanning the light modulation frequency around the Larmor frequency. CPT-resonances observed in Hanle-configuration are also analyzed. The observed influence of the light field amplitude modulation to the optical pumping efficiency and resonance width are discussed.

The presented experimental results show important advantages of using K-atoms (against Rb or Cs), for CPT-based methodologies and devices applications.

ACKNOWLEDGEMENTS

The authors wish to thank Mr. Badalassi and Mr.Tagliaferri for the technical assistance as well as Dr. V. Biancalana and Dr. Y. Dancheva for providing the acousto-optical modulator. Special thanks go to Prof. G. Alzetta for his precious and continuous suggestions. The work was done under collaboration program between the Italian National Research Council

Proc. of SPIE Vol. 7027 70270K-7

and the Bulgarian Academy of Sciences. We also acknowledge the partial support under the INTAS project (grant: 06-1000017-9001) and Indian-Bulgarian bilateral contract № БИн-2/07.

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