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Infrared laser emission in atomic sodium vapor pumped by collision assisted laser excitation
Volume 45, number 6
INFRARED
OPTICS COMMUNICATIONS
15 May 1983
LASER EMISSION IN ATOMIC SODIUM VAPOR PUMPED
BY COLLISION ASSISTED LASER EXCITATION
W. MOLLER and I.V. HERTEL Institut fuer Molekuelphysik,
Freie Universitaet Berlin,
Dl 000 Berlin 33, Fed. Rep. Germany
Received 12 January 1983
A collision-assisted two-step twophoton pump mechanism for generating a population inversion among the 5 ‘S and 4 ‘P levels in atomic sodium is reported. By tuning r) pulsed dye laser to the 3 ‘P to 5 ‘S resonances IR laser emission on the 3.415 pm (5 2S1,24 2PJ,2) and on the 3.408Mm (5 2S rj2-4 2P,,2) lines is observed. The conversion efficiency is estimated to be 5% and up to SW IR peak power is observed in the present experimental setup.
In their original proposal for infrared (IR) and visible lasers Schawlow and Townes first suggested resonant optical pumping of atomic alkali vapors as a mechanism to generate a population inversion [ 11. In the mean time, alkali dimers have proved to be efficient laser media in the visible and near infrared [2] and by using alkali atoms stimulated Raman scattering and various optical parametric mixing processes [3] have become powerful tools for down conversion of visible light into the IR region. But although alkali atoms are probably the most intensively studied objects in atomic physics [4], for sodium as an example only superradiance [5] and optically stimulated emission [6] after resonant excitation has been exploited up until now. Recently, we have demonstrated two possibilities of creating a population inversion among higher excited atomic levels in a dense sodium vapor. Resonant excitation of the sodium 3 2P levels and subsequent energy pooling is one possible mechanism [8], direct two photon excitation 3 2S-5 2S another one [9]. In both cases, the 5 2S112-4 2P3/2 1/2 transitions are observed to lase when studied in A IR laser resonator, however, with low efficiency only. In the first case competing ionization processes arise from the high density of excited 3 2P atoms and destroy the population inversion, while the latter mechanism is based on an only moderate excitation cross section, the intermediate 3 2P level being 364 cm-l off resonance. 400
In the present paper we report a new and significantly more efficient two photon excitation mechanism to create a population inversion between the 5 2S and 4 2P levels by tuning the pump laser into resonance with the 3 2P-.5 2S transitions. This process is similar to the light-induced collision used in a binary CaSr vapour reported by Falcone et al. [7]. The experimental setup is similar to that described in refs. [8,9]. Briefly, an undercritically operated heat pipe oven with a vapor zone of effectively 4 cm length sealed by CaF2 Brewster windows is mounted inside an IR laser resonator. The latter is formed by a gold mirror (3 m radius) and a dielectrically coated CdTe mirror (10 m radius, 30% reflectivity at 3.4 pm) and has a length of 1.35 m. A flashlamp pumped rhodamine 6G dye laser with a pulse duration of 1 ys, a linewidth of 0.6 cm-l, a pulse energy up to 5 mJ and a beam waist of 0.3 cm is coupled into and out of the IR resonator by two Ge-plates set at Brewster angle for the IR radiation. A fast photodiode monitors the pump-laser while the IR laser output is detected by a cooled InSb detector with a risetime of 50 ns. The IR wavelengths are measured by a 0.5m grating monochromator. Intensity measurements are performed by a boxcar integrator and the time dependences of the output signals are recorded by an oscilloscope or alternatively by a transient digitizer averager combination. IR laser emission is observed on the 3.415 pm 0 030-4018/83/0000-OOOO/$
03.00 0 1983 North-Holland
Volume 45, number 6
15 May 1983
OPTICS COMMUNICATIONS
.
Fig. 1. Schematic illustration of the collision assisted two step laser excitation of the 5 2 S level and the observed IR laser transitions (left side). IR output power of the 3.415 urn line as a function of pump laser wavelengths (right side).
/
t 1 0011 0 001
001 PUMPPULSE
and on the 3.408 pm (5 2S1,2 c52s,,,4 2p3,2) 4 2P,,2) lines but also, on a much weaker level, the cascading 2.209 nm (4 2P,,Z-4 2S1,2) transition. These laser oscillations are observed when the pump laser is tuned on resonance with either of the two red 3 2p1/2 3/2- 52S1~2transitionsat615.42and616.07nm, respectively. Fig. 1 shows a schematic of this excitation mechanism together with an experimental recording of the 3.415 pm IR output as a function of the pump laser wavelength *. Laser action is observed for sodium pressures in between a fraction of a mbar and some tens of mbars. The dependence of the time integrated IR output power on the pump pulse energy follows for several orders of magnitude an essentially quadratic power law as shown in fig. 2 for a Na vapor pressure of 10 mbar. We thus conclude that the mechanism for the 5 2S,,2 excitation is really a two step process as proposed, the first step being the 3 2P excitation in the far red wing of the self broadened Na D-lines. (Similar IR output powers are obtained for 1 mbar of Na in the presence of 1000 mbar of Ar.) The 3 LP Na atoms excited in this collision assisted absorption process are
* Because of the mode structure and limited tunability of the pumplaser the intensity ratio of the two lines seen in fig. 1 is of no physical significance. It depends on how well the frequency of one of the laser modes coincides with the collision broadened linewidth of the red 3 2P-5 2S lines.
01 ENERGY
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1
Fig. 2. Time integrated 3.415 pm (5 2S1n-4 2Ps,z) laser output power as a function of pumppulse energy above threshold (0.01 mJ) showing a quadratic power law as indicated by the straight line.
then resonantly pumped in a second step to the 5 2S level. It should be noted that the detuning off exact resonance for the first step is approximately 727 cm-l while at a pressure of 10 mbar the line width of the self broadened Na D-lines is 1 cm-l [lo]. Assuming a lorentzian line profile one estimates that the 3 2S to 3 2P absorption coefficient at a wavelength of 6 16 nm is 4 X 10e3 cm-l. Thus at our experimental conditions roughly 2% of the pump laser light is absorbed in the first step. Experimentally, at this wavelength absorption due to nearby molecular lines prevents an exact determination of the total power fed into the laser medium. We thus assume the overall absorption in the proposed two step pump process to be a few percent corresponding to around 100 W for a typical dye laser output peak power of 4 kW. The corresponding experimentally observed IR laser output has a peak power of around 5W. Thus we conclude the IR laser to have an conversion efficiency of about 5% on the 3.415 ,um line which has to be compared with an maximum possible efficiency of 18% for converting two visible photons of 616 nm into one infrared photon of 3.415 pm. In addition we observe the 3.408 401
OPTICS COMMUNICATIONS
Volume 45, number 6
-1
I-
100ns
TIME
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Fig. 3. Time dependence of the observed IR laser output pulses: 3.415 pm 5 2 S ij2-4 2Ps,, (full line), 3.408 pm 52s r/a-4 2Pt,2 (dashedlin)e and 2.209 pm 4 2Paja-42Si/a (dashed dotted line) in comparison with the dye laser pump pulse (dotted line). The pump wave length is 616.07 nm, the pump pulse energy 1 mJ. Top part of the figure: 10 mbar Na and 1 mbar Ar, lower part: 1 mbar Na and 1 mbar AI. Note the different relative intensities as indicated at the pulse traces refering to the 3.4 15 urn line in the top part.
2PI,2 line with roughly half that power. An even more detailed insight into the lasing process we obtain by a time dependent study. Fig. 3 shows pulse traces for the three IR laser transitions observed at two different Navapor pressures. At 1 mbar (lower part of fig. 3) all three IR pulses follow essentially the pump laser pulse once threshold has been reached. Since the cross section for stimulated emission for the 5 2S1/2-4 2P3/2 transition is twice that for the 5 2SI,2-42P1/2 transition one expects the former process to reach threshold first, as observed. Before steady state can be reached this laser process populates the 4 2P3/ 2 level so that an inversion between 4 2P3,2 and 4 2S112 builds up immediately
15 May 1983
which is sufficient to bring this transition above laser threshold as well. While at 1 mbar Na vapor pressure this cascading process is terminated only when the pump pulse ends, at 10 mbar it has a duration of less than 100 ns. We explain this behaviour by competing collision process which redistribute the population among the 4 2P and 3 2D levels *. These collisional losses of population in the 4 2P levels also explain the significantly lower IR output in the 2.209 pm cascade transition as compared to initial 5 2SI,2-4 2P3/2 line. In contrast, the 5 2S1,2-4 2P1,2 line has approximately half the power of the other doublet component for both Na pressures. In conclusion we have demonstrated an efficient way to generate IR laser emission between higher levels in atomic sodium by a collision assisted two step laser excitation. It circumvents the difficulties arising from low excitation probabilities in direct two photon excitation processes as well as the problems originating from strong ionisation [ 121 due to excess presence of sodium 3 2P atoms in the energy pooling laser [8]. We have not attempted in the present work to optimise the ratio of IR laser output power to total pump pulse power. By increasing the length of the Na vapor zone and possibly by choosing suitable buffer gases one can hope to absorb a much larger fraction of the pump laser input without increasing the collisional destruction of the population inversion. 200 W IR output power appears feasible without great difficulties. Partial support of this work by the Deutsche Forschungsgemeinschaft through Sfb 16 1 is greatfully acknowledged. Stimulating discussions with G. Jamieson have been very helpful.
pm 5 2SI,2-4
402
’ The energy gap between the 4 2P and the 3 2D levels is 1097 cm-l corresponding to roughly 2kT. It is interesting to note that also for the 5 ‘S and 4 2 D lines with an even larger mismatch of the energies (1348 cm-‘) strong collisional mixing is observed in our experiment by visual observation of the 4 2D-3 ‘P fluorescence perpendicular to the IR resonator. Similar observations in flames [ 1 l] support our findings.
References [l] A.L. Schawlow and C.H. Townes, Phys. Rev. 112 (1958) 1940.
Volume 45, number 6
OPTICS COMMUNICATIONS
[2] H. Welling and B. Wellegehausen, in: Laserspectroscopy III, eds. J.L. Hall and J.L. Carlson, Springer Series in Optical Sciences, Vol. 7 (Springer, Berlin, 1977) p. 365. [3] See e.g. J.J. Wynne, P.P. Sorokin, in: Topics in applied physics, Vol. 61 (Springer, Berlin, 1977) p. 159; C. Hanna, M.A. Yaratich and D. Cotter, Nonlinear optics of free atoms and molecules, Springer Series in Optical Sciences, Vol. 17 (Springer, Berlin, 1979). [4] W.C. Stwalley and M.E. Koch, Opt. Eng. 19 (1980) 71; A. Kopystynska and L. Moi, Phys. Rep. 92 (1982) 135. [5 ] M. Gross, C. Fabre, P. Pellet and S. Haroche, Phys. Rev. Lett. 36 (1976) 1035.
15 May 1983
[6] W. Hart&, Appl. Phys. 15 (1978) 472. [7] R.W. Falcone, W.R. Green, J.C. White, J.F. Young and S.E. Harris, Phys. Rev. Al5 (1977) 1333. [S] W. Mueller, J.J. McClelland and I.V. Hertel, Appl. Phys. B, to be published. [9] W. Mueller and I.V. Hertel, Appl. Phys. 24 (1981) 33. [lo] J. Huennekens and A. Gallagher, to be published. [ 111 C.A. van Dijk, Ph.D. thesis, Rijksuniversiteit Utrecht (1978). [ 121 T.B. Lucatorto and T.J. McIlrath, Appl. Optics 19 (1980) 3948.
403
INFRARED
OPTICS COMMUNICATIONS
15 May 1983
LASER EMISSION IN ATOMIC SODIUM VAPOR PUMPED
BY COLLISION ASSISTED LASER EXCITATION
W. MOLLER and I.V. HERTEL Institut fuer Molekuelphysik,
Freie Universitaet Berlin,
Dl 000 Berlin 33, Fed. Rep. Germany
Received 12 January 1983
A collision-assisted two-step twophoton pump mechanism for generating a population inversion among the 5 ‘S and 4 ‘P levels in atomic sodium is reported. By tuning r) pulsed dye laser to the 3 ‘P to 5 ‘S resonances IR laser emission on the 3.415 pm (5 2S1,24 2PJ,2) and on the 3.408Mm (5 2S rj2-4 2P,,2) lines is observed. The conversion efficiency is estimated to be 5% and up to SW IR peak power is observed in the present experimental setup.
In their original proposal for infrared (IR) and visible lasers Schawlow and Townes first suggested resonant optical pumping of atomic alkali vapors as a mechanism to generate a population inversion [ 11. In the mean time, alkali dimers have proved to be efficient laser media in the visible and near infrared [2] and by using alkali atoms stimulated Raman scattering and various optical parametric mixing processes [3] have become powerful tools for down conversion of visible light into the IR region. But although alkali atoms are probably the most intensively studied objects in atomic physics [4], for sodium as an example only superradiance [5] and optically stimulated emission [6] after resonant excitation has been exploited up until now. Recently, we have demonstrated two possibilities of creating a population inversion among higher excited atomic levels in a dense sodium vapor. Resonant excitation of the sodium 3 2P levels and subsequent energy pooling is one possible mechanism [8], direct two photon excitation 3 2S-5 2S another one [9]. In both cases, the 5 2S112-4 2P3/2 1/2 transitions are observed to lase when studied in A IR laser resonator, however, with low efficiency only. In the first case competing ionization processes arise from the high density of excited 3 2P atoms and destroy the population inversion, while the latter mechanism is based on an only moderate excitation cross section, the intermediate 3 2P level being 364 cm-l off resonance. 400
In the present paper we report a new and significantly more efficient two photon excitation mechanism to create a population inversion between the 5 2S and 4 2P levels by tuning the pump laser into resonance with the 3 2P-.5 2S transitions. This process is similar to the light-induced collision used in a binary CaSr vapour reported by Falcone et al. [7]. The experimental setup is similar to that described in refs. [8,9]. Briefly, an undercritically operated heat pipe oven with a vapor zone of effectively 4 cm length sealed by CaF2 Brewster windows is mounted inside an IR laser resonator. The latter is formed by a gold mirror (3 m radius) and a dielectrically coated CdTe mirror (10 m radius, 30% reflectivity at 3.4 pm) and has a length of 1.35 m. A flashlamp pumped rhodamine 6G dye laser with a pulse duration of 1 ys, a linewidth of 0.6 cm-l, a pulse energy up to 5 mJ and a beam waist of 0.3 cm is coupled into and out of the IR resonator by two Ge-plates set at Brewster angle for the IR radiation. A fast photodiode monitors the pump-laser while the IR laser output is detected by a cooled InSb detector with a risetime of 50 ns. The IR wavelengths are measured by a 0.5m grating monochromator. Intensity measurements are performed by a boxcar integrator and the time dependences of the output signals are recorded by an oscilloscope or alternatively by a transient digitizer averager combination. IR laser emission is observed on the 3.415 pm 0 030-4018/83/0000-OOOO/$
03.00 0 1983 North-Holland
Volume 45, number 6
15 May 1983
OPTICS COMMUNICATIONS
.
Fig. 1. Schematic illustration of the collision assisted two step laser excitation of the 5 2 S level and the observed IR laser transitions (left side). IR output power of the 3.415 urn line as a function of pump laser wavelengths (right side).
/
t 1 0011 0 001
001 PUMPPULSE
and on the 3.408 pm (5 2S1,2 c52s,,,4 2p3,2) 4 2P,,2) lines but also, on a much weaker level, the cascading 2.209 nm (4 2P,,Z-4 2S1,2) transition. These laser oscillations are observed when the pump laser is tuned on resonance with either of the two red 3 2p1/2 3/2- 52S1~2transitionsat615.42and616.07nm, respectively. Fig. 1 shows a schematic of this excitation mechanism together with an experimental recording of the 3.415 pm IR output as a function of the pump laser wavelength *. Laser action is observed for sodium pressures in between a fraction of a mbar and some tens of mbars. The dependence of the time integrated IR output power on the pump pulse energy follows for several orders of magnitude an essentially quadratic power law as shown in fig. 2 for a Na vapor pressure of 10 mbar. We thus conclude that the mechanism for the 5 2S,,2 excitation is really a two step process as proposed, the first step being the 3 2P excitation in the far red wing of the self broadened Na D-lines. (Similar IR output powers are obtained for 1 mbar of Na in the presence of 1000 mbar of Ar.) The 3 LP Na atoms excited in this collision assisted absorption process are
* Because of the mode structure and limited tunability of the pumplaser the intensity ratio of the two lines seen in fig. 1 is of no physical significance. It depends on how well the frequency of one of the laser modes coincides with the collision broadened linewidth of the red 3 2P-5 2S lines.
01 ENERGY
IO I E _ ETH I
IO0 I ml
1
Fig. 2. Time integrated 3.415 pm (5 2S1n-4 2Ps,z) laser output power as a function of pumppulse energy above threshold (0.01 mJ) showing a quadratic power law as indicated by the straight line.
then resonantly pumped in a second step to the 5 2S level. It should be noted that the detuning off exact resonance for the first step is approximately 727 cm-l while at a pressure of 10 mbar the line width of the self broadened Na D-lines is 1 cm-l [lo]. Assuming a lorentzian line profile one estimates that the 3 2S to 3 2P absorption coefficient at a wavelength of 6 16 nm is 4 X 10e3 cm-l. Thus at our experimental conditions roughly 2% of the pump laser light is absorbed in the first step. Experimentally, at this wavelength absorption due to nearby molecular lines prevents an exact determination of the total power fed into the laser medium. We thus assume the overall absorption in the proposed two step pump process to be a few percent corresponding to around 100 W for a typical dye laser output peak power of 4 kW. The corresponding experimentally observed IR laser output has a peak power of around 5W. Thus we conclude the IR laser to have an conversion efficiency of about 5% on the 3.415 ,um line which has to be compared with an maximum possible efficiency of 18% for converting two visible photons of 616 nm into one infrared photon of 3.415 pm. In addition we observe the 3.408 401
OPTICS COMMUNICATIONS
Volume 45, number 6
-1
I-
100ns
TIME
-
Fig. 3. Time dependence of the observed IR laser output pulses: 3.415 pm 5 2 S ij2-4 2Ps,, (full line), 3.408 pm 52s r/a-4 2Pt,2 (dashedlin)e and 2.209 pm 4 2Paja-42Si/a (dashed dotted line) in comparison with the dye laser pump pulse (dotted line). The pump wave length is 616.07 nm, the pump pulse energy 1 mJ. Top part of the figure: 10 mbar Na and 1 mbar Ar, lower part: 1 mbar Na and 1 mbar AI. Note the different relative intensities as indicated at the pulse traces refering to the 3.4 15 urn line in the top part.
2PI,2 line with roughly half that power. An even more detailed insight into the lasing process we obtain by a time dependent study. Fig. 3 shows pulse traces for the three IR laser transitions observed at two different Navapor pressures. At 1 mbar (lower part of fig. 3) all three IR pulses follow essentially the pump laser pulse once threshold has been reached. Since the cross section for stimulated emission for the 5 2S1/2-4 2P3/2 transition is twice that for the 5 2SI,2-42P1/2 transition one expects the former process to reach threshold first, as observed. Before steady state can be reached this laser process populates the 4 2P3/ 2 level so that an inversion between 4 2P3,2 and 4 2S112 builds up immediately
15 May 1983
which is sufficient to bring this transition above laser threshold as well. While at 1 mbar Na vapor pressure this cascading process is terminated only when the pump pulse ends, at 10 mbar it has a duration of less than 100 ns. We explain this behaviour by competing collision process which redistribute the population among the 4 2P and 3 2D levels *. These collisional losses of population in the 4 2P levels also explain the significantly lower IR output in the 2.209 pm cascade transition as compared to initial 5 2SI,2-4 2P3/2 line. In contrast, the 5 2S1,2-4 2P1,2 line has approximately half the power of the other doublet component for both Na pressures. In conclusion we have demonstrated an efficient way to generate IR laser emission between higher levels in atomic sodium by a collision assisted two step laser excitation. It circumvents the difficulties arising from low excitation probabilities in direct two photon excitation processes as well as the problems originating from strong ionisation [ 121 due to excess presence of sodium 3 2P atoms in the energy pooling laser [8]. We have not attempted in the present work to optimise the ratio of IR laser output power to total pump pulse power. By increasing the length of the Na vapor zone and possibly by choosing suitable buffer gases one can hope to absorb a much larger fraction of the pump laser input without increasing the collisional destruction of the population inversion. 200 W IR output power appears feasible without great difficulties. Partial support of this work by the Deutsche Forschungsgemeinschaft through Sfb 16 1 is greatfully acknowledged. Stimulating discussions with G. Jamieson have been very helpful.
pm 5 2SI,2-4
402
’ The energy gap between the 4 2P and the 3 2D levels is 1097 cm-l corresponding to roughly 2kT. It is interesting to note that also for the 5 ‘S and 4 2 D lines with an even larger mismatch of the energies (1348 cm-‘) strong collisional mixing is observed in our experiment by visual observation of the 4 2D-3 ‘P fluorescence perpendicular to the IR resonator. Similar observations in flames [ 1 l] support our findings.
References [l] A.L. Schawlow and C.H. Townes, Phys. Rev. 112 (1958) 1940.
Volume 45, number 6
OPTICS COMMUNICATIONS
[2] H. Welling and B. Wellegehausen, in: Laserspectroscopy III, eds. J.L. Hall and J.L. Carlson, Springer Series in Optical Sciences, Vol. 7 (Springer, Berlin, 1977) p. 365. [3] See e.g. J.J. Wynne, P.P. Sorokin, in: Topics in applied physics, Vol. 61 (Springer, Berlin, 1977) p. 159; C. Hanna, M.A. Yaratich and D. Cotter, Nonlinear optics of free atoms and molecules, Springer Series in Optical Sciences, Vol. 17 (Springer, Berlin, 1979). [4] W.C. Stwalley and M.E. Koch, Opt. Eng. 19 (1980) 71; A. Kopystynska and L. Moi, Phys. Rep. 92 (1982) 135. [5 ] M. Gross, C. Fabre, P. Pellet and S. Haroche, Phys. Rev. Lett. 36 (1976) 1035.
15 May 1983
[6] W. Hart&, Appl. Phys. 15 (1978) 472. [7] R.W. Falcone, W.R. Green, J.C. White, J.F. Young and S.E. Harris, Phys. Rev. Al5 (1977) 1333. [S] W. Mueller, J.J. McClelland and I.V. Hertel, Appl. Phys. B, to be published. [9] W. Mueller and I.V. Hertel, Appl. Phys. 24 (1981) 33. [lo] J. Huennekens and A. Gallagher, to be published. [ 111 C.A. van Dijk, Ph.D. thesis, Rijksuniversiteit Utrecht (1978). [ 121 T.B. Lucatorto and T.J. McIlrath, Appl. Optics 19 (1980) 3948.
403