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Light-induced drift of an optically thick cloud of sodium vapor
26 September 1994 PHYSICS LETTERS A
Physics Letters A 193 (1994) 179-182
ELSEVIER
Light-induced drift of an optically thick cloud of sodium vapor S.N. Atutov 1, S.V. Plotnikov, S.P. Pod'yachev, A.M. Shalagin Institute of Automation and Electrometry, Universitetskii Prospekt 1, Novosibirsk 630090, Russian Federation
Received 6 June 1994; acceptedfor publication 26 July 1994 Communicatedby B. Fricke
Abstract
The experimental observation of light-induced drift in a buffer gas of an optically thick sodium vapor cloud is presented. Compression of the optically thick cloud under light-induced drift is demonstrated.
1. Introduction
The essence of light-induced drift (LID) [ 1 ] consists of the arising of a drift of absorbing particles in a buffer gas in the field of a travelling wave with a frequency smoothly detuned from the absorption line centre. After prediction [ 1 ] and first observation [ 2 ] much experimental and theoretical work connected with LID was done. In most LID experiments the mixture of absorbing and buffer gases was placed in a capillary with an optical window and the distribution of the absorbing particles along the capillary was measured. Usually the concentration of absorbing particles was chosen so that the length of absorption was greater than the capillary length (optically thin medium) [2,3 ]. When the length of absorption was essentially less than the capillary length (optically thick medium) the behaviour of the absorbing particles distribution under LID changes qualitatively [ 4 ]. An outstanding example of this behaviour is the "optical piston" [4,5 ]. The experimental set-up described in Ref. [ 6 ] permits making a direct observation of the movement of a Na-vapor cloud by the LID effect. In Ref. [ 6 ] for i E-mail:Atutov@iaie-lid. nsk.su. ElsevierScienceB.V. SSDI 0375-9601 (94) 00628-8
drift velocity measurements the vapor cloud was optically thin. In this case the vapor cloud as a whole moves with the drift velocity and simultaneously spreads because of diffusion. As it has been calculated in Ref. [ 7 ] a nonspreading steady form is possible for the optically thick vapor cloud during lightinduced drift. The process of the establishment of this steady state of the absorbing particles distribution in the cloud was considered in Ref. [ 8 ]. Let the vapor cloud move in the direction of the light propagation. If the saturation of light absorption by the particles is not big, then the drift velocity is proportional to the intensity of the light. Due to light absorption its intensity on the front edge of cloud is less than at the rear. Therefore the drift velocity at the rear edge will be higher than at the front. This means that light-induced drift compresses the distribution of absorbing particles in the cloud. An alternative process is the diffusive spreading of the distribution. In principle they can compensate each other, as a result at a given width of the vapor cloud a steady distribution can exist. The aim of this work was the experimental observation of the compression of the vapor cloud as it moves under the LID effect.
180
S.N. Atutov et al. /Physics Letters A 193 (1994) 179-182
2. Experimental set-up and method of measurement
Basically our set-up was like the one described in Ref. [6]. Some modification was made, as result of which a high density of Na-vapor in a cloud at room temperature was achieved. The main elements of the set-up are presented in Fig. 1. The experimental cell, containing the buffer gas, consists of two communicating capillaries: the main one (1) and the auxiliary (2), 60 cm and 8 cm in length. The inner capillary diameter is 1 mm. At a distance of 1.5 cm from the main capillary, a heated reservoir (3) with metallic Na is connected to the auxiliary capillary (2). The main capillary was kept at room temperature. In order to reduce the physical adsorption the inner surfaces of the capillaries have been covered with a thin paraffin layer [ 9 ]. It was found that this layer also allowed a suitable decay time for oversaturated Na-vapor at room temperature
remainder part of the light was used for shutting the sodium vapor in the auxiliary capillary. The density of the sodium vapor was controlled by the temperature of the Na-reservoir. The decay time of the sodium vapor in the main capillary was 0.3-0.5 s. We registered the fluorescence distribution of the Na-vapor along the main capillary. The photodetector (6) with the microscope is movable along the capillary (1) (X-axis). The spatial resolution of the system is accurate to less than 1 mm. The signal from the photodetector was observed on the oscilloscope (7) and given to the box-car averager (8). The gate time was 0.1 ms, the delay time counter triggered by the pulse from the rotated mirror (5). The box-car signal was given to the Y-input of the X- Y recorder. The X-input received a signal proportional to the xcoordinate of the photodetector. Absorption of the laser beam by the sodium cloud was measured by photodetector (10).
[10]. The source of light was cw dye-laser (4) with power I00 mW and spectrum width ~ 1 GHz. The frequency of the laser light was detuned to the longer wavelength region of the absorption centre of the D 2line. In this way, the LID effect leads to a movement of the sodium atom in the direction of the light propagation. We formed the cloud by means of the LID effect. A rotated mirror (5) periodically directed the laser beam into the auxiliary capillary, which pushed the sodium vapor into the main capillary during 200 ms. After part of the laser beam directed into main capillary had drawn the cloud along that capillary, the 5
4
[
<
2
10
I L L
/
<
I
×
>
8
Fig. 1. E x p e r i m e n t a l set-up.
3. Experimental result and discussion
The evolution of the Na-vapor cloud is determined by two factors: diffusive spreading with characteristic time rl = 1 2 / ~ and light-induced compression with characteristic time T2= l/Ju. Here l is the character cloud width, ~ is the diffusion coefficient in the buffer gas and ~u is the velocity difference because of the absorption of the laser beam by the vapor cloud. If "C1 > r 2 ( r I < z 2 ) , then the original distribution particles into the cloud is getting narrower (wider). In a steady case rl ~ r2 and the characteristic distribution width will be ~ / ~ u . We tried to minimalize this size in our experiments. Taking into account the results of Ref. [ 6 ], Xe at 8 Torr pressure was taken as the buffer gas. The detuning of the laser frequency from the absorption line centre was chosen for maximum drift velocity. Measurement of the dependence of the drift velocity on the light intensity showed that the optimal laser beam power in the main capillary is cloud ( ~ 130 cm/s), if the light intensity increased the drift velocity continues to increase, but because of saturation ~u begins to decrease. The theoretical treatment of the achieving of a steady form of vapor cloud in Ref. [ 8 ] was given for the case that the decay time was essentially larger than the other characteristic times. Under our experimen-
S.N. Atutov et al. / Physics Letters A 193 (1994) 179-182
tal conditions the decay time was of the same order or less than the time needed for the achievement of a steady form of cloud. For this reason we did not try to make a detailed comparison of the theory and experiment and would like only to demonstrate the qualitative changing of the evolution of the vapor cloud form during the transition from an optically thin to an optically thick vapor cloud. The evolution of the cloud form at different optical thicknesses observed in our experiments confirmed our qualitative speculations. When the light absorption by the vapor cloud was less than 5% the evolution of the cloud form was diffusive. At 15%-20% light absorption, the cloud spreading was essentially slower than diffusive. When the vapor cloud absorbed 50% of the light we observed the compression of the cloud instead of the spreading. An example of this evolution is given in Fig. 2. As can be seen, during the drift the width of the vapor cloud decreased from 6 cm to 0.4 cm. It should be noted that because of such a strong compression of the sodium vapor the concentration in the distribution maximum has increased, although the total amount of vapor has decreased. The dependence of the squared cloud width on drift time is given in Fig. 3. The first curve shows the ordinary diffusive spreading of the optically thin Navapor cloud with diffusion coefficient 9 = 11 + 1.2 cm2/s. This value agrees with data measured earlier [ 11 ]. The second curve in Fig. 3 presents the evolution of the optically thick vapor cloud. Note that at the beginning the cloud was broadening. This was due to the asymmetry of the cell. The input window is sealed to the capillary of a glass tube with 6 m m diameter and there is a relatively large volume at a dis20
Fluorescence,
rel.un.
16
10
i 10
20
i
/ 3o
i
i
X, cm
Fig. 2. Drift of optically thick Na-vapor cloud. Absorption of light is 50%.
24
181
- a 2, c m 2 /
/
1,~ // 16 /e
~" i /
~j/
\
/#
x\
N"
\\ \ \ k \ x 100
200
SO0
,
mc
Fig. 3. Deperidences of the squared cloud width on the drift time. Explanation in the text. 20 - Fluorescence
rel.un.
15
10
\ 10
20
'
3o
X,
crn
Fig. 4. Drift of two vapor clouds at the same time.
tance of 2 cm from the auxiliary capillary. During of the cloud formation a large amount of Na-vapor at a relatively low density accumulated in the near window volume. During the ~ 140 ms of cloud drift, the vapor from this volume was being pumped into the cloud because the cloud velocity is less than the drift velocity on its "tail". This time the cloud was broadening, and only after the sum distribution began to compress. Something like that can be seen in Fig. 4 when two clouds are drifting simultaneously. In Ref. [ 8 ] this situation was called inelastic cloud collisions and the analogy between the behaviour of shock-waves and nonlinear light-induced drift of optically thick clouds was demonstrated. The cloud which was created first interacts with less powerful radiation due to the light absorption by the second cloud. So it moves more slowly and spreads. Above 250 ms the clouds will have joined and drift as one cloud. The results of our experiments demonstrate that LID gives us the opportunity to form a nonspreading
182
S.N. Atutov et al. / Physics Letters A 193 (1994) 179-182
compact vapor cloud propagating in gas. The processes of formation, drift and "collisions" of this cloud agree with theoretical notions. We hope that this new method of transporting a compact cloud of one gas c o m p o n e n t through another will find applications in different branches of science a n d in practice.
Acknowledgement This work was supported by the Netherlands Organization for Scientific Research ( N W O ) a n d the Russian F o u n d a t i o n for F u n d a m e n t a l Research (RFFR).
References [ 1] F.Kh. Gcl'mukhanov and A.M. Shalagin, Pis'ma Zh. Eksp. Teor. Fiz. 29 ( 1979) 773; JETP Lett. 29 (1979) 71 I.
[2] V.D. Antsygin, S.N. Atutov, F.Kh. Germukhanov, G.G. Telegin and A.M. Shalagin, Pis'ma Zh. Eksp. Teor. Fiz. 30 (1979) 243. [3] P.L. Chapovsky, A.M. Shalagin, V.N. Panfilov and V.P. Strunin, Opt. Commun. 40 ( 1981 ) 129. [4] F.Kh. Germukhanov and A.M. Shalagin, Zh. Eksp. Teor. Fiz. 78 (1980) 1672;Sov. Phys. JETP 29 (1979) 711. [5] H.G.S. Werij, J.P. Woerdman, J.J.M. Beenakker and I. Kucser, Phys. Rev. Lett. 52 (1984) 2237. [6] S.N. Atutov, St. Lesjak, S.P. Pod'jachev and A. M. Shalagin, Opt. Commun. 60 (1986) 41. [7] G. Nienhuis, Phys. Rev. A 31 (1985) 1636. [8] F.Kh. Germukhanov and A.I. Parkhomenko, Zh. Eksp. Teor. Fiz. 92 (1987) 813. [ 9 ] S.N. Atutov, Phys. Lett. A 119 (1986) 121. [ 10] S.N. Atutov and S.P. Pod'yachev,Opt. Spectrosc. 62 (1987) 979. [ 11 ] S.N. Atutov, I.M. Ermolaev and A.M. Shalagin, Zh. Eksp. Teor. Fiz. 92 (1987) 1215;Soy. Phys. JETP 65 (1987) 679.
Physics Letters A 193 (1994) 179-182
ELSEVIER
Light-induced drift of an optically thick cloud of sodium vapor S.N. Atutov 1, S.V. Plotnikov, S.P. Pod'yachev, A.M. Shalagin Institute of Automation and Electrometry, Universitetskii Prospekt 1, Novosibirsk 630090, Russian Federation
Received 6 June 1994; acceptedfor publication 26 July 1994 Communicatedby B. Fricke
Abstract
The experimental observation of light-induced drift in a buffer gas of an optically thick sodium vapor cloud is presented. Compression of the optically thick cloud under light-induced drift is demonstrated.
1. Introduction
The essence of light-induced drift (LID) [ 1 ] consists of the arising of a drift of absorbing particles in a buffer gas in the field of a travelling wave with a frequency smoothly detuned from the absorption line centre. After prediction [ 1 ] and first observation [ 2 ] much experimental and theoretical work connected with LID was done. In most LID experiments the mixture of absorbing and buffer gases was placed in a capillary with an optical window and the distribution of the absorbing particles along the capillary was measured. Usually the concentration of absorbing particles was chosen so that the length of absorption was greater than the capillary length (optically thin medium) [2,3 ]. When the length of absorption was essentially less than the capillary length (optically thick medium) the behaviour of the absorbing particles distribution under LID changes qualitatively [ 4 ]. An outstanding example of this behaviour is the "optical piston" [4,5 ]. The experimental set-up described in Ref. [ 6 ] permits making a direct observation of the movement of a Na-vapor cloud by the LID effect. In Ref. [ 6 ] for i E-mail:Atutov@iaie-lid. nsk.su. ElsevierScienceB.V. SSDI 0375-9601 (94) 00628-8
drift velocity measurements the vapor cloud was optically thin. In this case the vapor cloud as a whole moves with the drift velocity and simultaneously spreads because of diffusion. As it has been calculated in Ref. [ 7 ] a nonspreading steady form is possible for the optically thick vapor cloud during lightinduced drift. The process of the establishment of this steady state of the absorbing particles distribution in the cloud was considered in Ref. [ 8 ]. Let the vapor cloud move in the direction of the light propagation. If the saturation of light absorption by the particles is not big, then the drift velocity is proportional to the intensity of the light. Due to light absorption its intensity on the front edge of cloud is less than at the rear. Therefore the drift velocity at the rear edge will be higher than at the front. This means that light-induced drift compresses the distribution of absorbing particles in the cloud. An alternative process is the diffusive spreading of the distribution. In principle they can compensate each other, as a result at a given width of the vapor cloud a steady distribution can exist. The aim of this work was the experimental observation of the compression of the vapor cloud as it moves under the LID effect.
180
S.N. Atutov et al. /Physics Letters A 193 (1994) 179-182
2. Experimental set-up and method of measurement
Basically our set-up was like the one described in Ref. [6]. Some modification was made, as result of which a high density of Na-vapor in a cloud at room temperature was achieved. The main elements of the set-up are presented in Fig. 1. The experimental cell, containing the buffer gas, consists of two communicating capillaries: the main one (1) and the auxiliary (2), 60 cm and 8 cm in length. The inner capillary diameter is 1 mm. At a distance of 1.5 cm from the main capillary, a heated reservoir (3) with metallic Na is connected to the auxiliary capillary (2). The main capillary was kept at room temperature. In order to reduce the physical adsorption the inner surfaces of the capillaries have been covered with a thin paraffin layer [ 9 ]. It was found that this layer also allowed a suitable decay time for oversaturated Na-vapor at room temperature
remainder part of the light was used for shutting the sodium vapor in the auxiliary capillary. The density of the sodium vapor was controlled by the temperature of the Na-reservoir. The decay time of the sodium vapor in the main capillary was 0.3-0.5 s. We registered the fluorescence distribution of the Na-vapor along the main capillary. The photodetector (6) with the microscope is movable along the capillary (1) (X-axis). The spatial resolution of the system is accurate to less than 1 mm. The signal from the photodetector was observed on the oscilloscope (7) and given to the box-car averager (8). The gate time was 0.1 ms, the delay time counter triggered by the pulse from the rotated mirror (5). The box-car signal was given to the Y-input of the X- Y recorder. The X-input received a signal proportional to the xcoordinate of the photodetector. Absorption of the laser beam by the sodium cloud was measured by photodetector (10).
[10]. The source of light was cw dye-laser (4) with power I00 mW and spectrum width ~ 1 GHz. The frequency of the laser light was detuned to the longer wavelength region of the absorption centre of the D 2line. In this way, the LID effect leads to a movement of the sodium atom in the direction of the light propagation. We formed the cloud by means of the LID effect. A rotated mirror (5) periodically directed the laser beam into the auxiliary capillary, which pushed the sodium vapor into the main capillary during 200 ms. After part of the laser beam directed into main capillary had drawn the cloud along that capillary, the 5
4
[
<
2
10
I L L
/
<
I
×
>
8
Fig. 1. E x p e r i m e n t a l set-up.
3. Experimental result and discussion
The evolution of the Na-vapor cloud is determined by two factors: diffusive spreading with characteristic time rl = 1 2 / ~ and light-induced compression with characteristic time T2= l/Ju. Here l is the character cloud width, ~ is the diffusion coefficient in the buffer gas and ~u is the velocity difference because of the absorption of the laser beam by the vapor cloud. If "C1 > r 2 ( r I < z 2 ) , then the original distribution particles into the cloud is getting narrower (wider). In a steady case rl ~ r2 and the characteristic distribution width will be ~ / ~ u . We tried to minimalize this size in our experiments. Taking into account the results of Ref. [ 6 ], Xe at 8 Torr pressure was taken as the buffer gas. The detuning of the laser frequency from the absorption line centre was chosen for maximum drift velocity. Measurement of the dependence of the drift velocity on the light intensity showed that the optimal laser beam power in the main capillary is cloud ( ~ 130 cm/s), if the light intensity increased the drift velocity continues to increase, but because of saturation ~u begins to decrease. The theoretical treatment of the achieving of a steady form of vapor cloud in Ref. [ 8 ] was given for the case that the decay time was essentially larger than the other characteristic times. Under our experimen-
S.N. Atutov et al. / Physics Letters A 193 (1994) 179-182
tal conditions the decay time was of the same order or less than the time needed for the achievement of a steady form of cloud. For this reason we did not try to make a detailed comparison of the theory and experiment and would like only to demonstrate the qualitative changing of the evolution of the vapor cloud form during the transition from an optically thin to an optically thick vapor cloud. The evolution of the cloud form at different optical thicknesses observed in our experiments confirmed our qualitative speculations. When the light absorption by the vapor cloud was less than 5% the evolution of the cloud form was diffusive. At 15%-20% light absorption, the cloud spreading was essentially slower than diffusive. When the vapor cloud absorbed 50% of the light we observed the compression of the cloud instead of the spreading. An example of this evolution is given in Fig. 2. As can be seen, during the drift the width of the vapor cloud decreased from 6 cm to 0.4 cm. It should be noted that because of such a strong compression of the sodium vapor the concentration in the distribution maximum has increased, although the total amount of vapor has decreased. The dependence of the squared cloud width on drift time is given in Fig. 3. The first curve shows the ordinary diffusive spreading of the optically thin Navapor cloud with diffusion coefficient 9 = 11 + 1.2 cm2/s. This value agrees with data measured earlier [ 11 ]. The second curve in Fig. 3 presents the evolution of the optically thick vapor cloud. Note that at the beginning the cloud was broadening. This was due to the asymmetry of the cell. The input window is sealed to the capillary of a glass tube with 6 m m diameter and there is a relatively large volume at a dis20
Fluorescence,
rel.un.
16
10
i 10
20
i
/ 3o
i
i
X, cm
Fig. 2. Drift of optically thick Na-vapor cloud. Absorption of light is 50%.
24
181
- a 2, c m 2 /
/
1,~ // 16 /e
~" i /
~j/
\
/#
x\
N"
\\ \ \ k \ x 100
200
SO0
,
mc
Fig. 3. Deperidences of the squared cloud width on the drift time. Explanation in the text. 20 - Fluorescence
rel.un.
15
10
\ 10
20
'
3o
X,
crn
Fig. 4. Drift of two vapor clouds at the same time.
tance of 2 cm from the auxiliary capillary. During of the cloud formation a large amount of Na-vapor at a relatively low density accumulated in the near window volume. During the ~ 140 ms of cloud drift, the vapor from this volume was being pumped into the cloud because the cloud velocity is less than the drift velocity on its "tail". This time the cloud was broadening, and only after the sum distribution began to compress. Something like that can be seen in Fig. 4 when two clouds are drifting simultaneously. In Ref. [ 8 ] this situation was called inelastic cloud collisions and the analogy between the behaviour of shock-waves and nonlinear light-induced drift of optically thick clouds was demonstrated. The cloud which was created first interacts with less powerful radiation due to the light absorption by the second cloud. So it moves more slowly and spreads. Above 250 ms the clouds will have joined and drift as one cloud. The results of our experiments demonstrate that LID gives us the opportunity to form a nonspreading
182
S.N. Atutov et al. / Physics Letters A 193 (1994) 179-182
compact vapor cloud propagating in gas. The processes of formation, drift and "collisions" of this cloud agree with theoretical notions. We hope that this new method of transporting a compact cloud of one gas c o m p o n e n t through another will find applications in different branches of science a n d in practice.
Acknowledgement This work was supported by the Netherlands Organization for Scientific Research ( N W O ) a n d the Russian F o u n d a t i o n for F u n d a m e n t a l Research (RFFR).
References [ 1] F.Kh. Gcl'mukhanov and A.M. Shalagin, Pis'ma Zh. Eksp. Teor. Fiz. 29 ( 1979) 773; JETP Lett. 29 (1979) 71 I.
[2] V.D. Antsygin, S.N. Atutov, F.Kh. Germukhanov, G.G. Telegin and A.M. Shalagin, Pis'ma Zh. Eksp. Teor. Fiz. 30 (1979) 243. [3] P.L. Chapovsky, A.M. Shalagin, V.N. Panfilov and V.P. Strunin, Opt. Commun. 40 ( 1981 ) 129. [4] F.Kh. Germukhanov and A.M. Shalagin, Zh. Eksp. Teor. Fiz. 78 (1980) 1672;Sov. Phys. JETP 29 (1979) 711. [5] H.G.S. Werij, J.P. Woerdman, J.J.M. Beenakker and I. Kucser, Phys. Rev. Lett. 52 (1984) 2237. [6] S.N. Atutov, St. Lesjak, S.P. Pod'jachev and A. M. Shalagin, Opt. Commun. 60 (1986) 41. [7] G. Nienhuis, Phys. Rev. A 31 (1985) 1636. [8] F.Kh. Germukhanov and A.I. Parkhomenko, Zh. Eksp. Teor. Fiz. 92 (1987) 813. [ 9 ] S.N. Atutov, Phys. Lett. A 119 (1986) 121. [ 10] S.N. Atutov and S.P. Pod'yachev,Opt. Spectrosc. 62 (1987) 979. [ 11 ] S.N. Atutov, I.M. Ermolaev and A.M. Shalagin, Zh. Eksp. Teor. Fiz. 92 (1987) 1215;Soy. Phys. JETP 65 (1987) 679.