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Doppler-free optogalvanic spectroscopy using an infrared color center laser
Volume 37, number 1
OPTICS COMMUNICATIONS
1 April 1981
DOPPLER-FREE OPTOGALVANIC SPECTROSCOPY USING AN INFRARED COLOR CENTER LASER ~ D.J. JACKSON l, H. GERHARDT 2 and T.W.H.~NSCH Department of Physics, Stanford University, Stanford, California 94305, USA Received 23 December 1980
We have used a cw color center laser near 2.6 tzm to study highly excited states in helium and neon atoms by Dopplerfree intermodulated optogalvanic spectroscopy in a hollow cathode discharge tube. For helium n = 4 to 6 transitions, the resolution was limited to about 320 MHz (FWHM) by Holtzmark broadening due to the presence of charged particles in the discharge. Lines as narrow as 60 MHz were observed for neon 3ss-SPlo.
A color laser center has been used for nonlinear high resolution spectroscopy of excited helium and neon atoms in a DC gas discharge. Color center lasers have extended the arsenal of broadly tunable and highly monochromatic lasers to include near infrared wavelengths out to 3 tam [ 1]. This has opened up a wealth of possible spectroscopic applications at wavelengths which are convenient to study absorption lines from highly excited atomic levels or molecular absorption bands [ 2 - 4 ] . In an earlier work [2] a number of infrared transitions between sparsely populated states in helium were observed Dopplerlimited for the first time by an optogalvanic detection scheme. Doppler-free measurements of transitions between states of high principal quantum number can provide information on fine structure intervals and the interaction of the atoms within the discharge environment. Therefore, we have explored the feasibility of Doppler-free experiments at infrared wavelengths in helium and neon and this paper describes the results and their implications for future experimental work. Four watts all lines from a Spectra Physics argon ion laser were used to pump a Spectra Physics 375 a Work supported by the National Science Foundation under Grant PHY80-10686, and U.S. Office of Naval Research under Contract N00014-78-C-0403. I Bell Laboratories Fellow. 2 Heisenberg Fellow.
dye laser which is operated with rhodamine 6G dye and no internal cavity etalons. The dye laser wavelength was tuned to optimize the coupling of the laser light into the KC1 : Li crystal of a Burleigh FCL20 color center laser. A dye laser power of 750 mW produced 10 mW of infrared light near the 2.65 tzm wavelength. A reliable continuous, tunable singlemode output from the color center laser was only achieved after the insertion of an additional etalon which was solid and 5 mm thick. At the output of the color center laser, 10% of the infrared light is picked off and sent through a 150 MHz interferometer for frequency calibration. The finesse of the interferometer is 20; after careful alignment of the laser light through the interferometer, we have established that the laser linewidth is 5 MI-lz or better. A 2.5 GHz single mode scan was obtained by applying - 5 0 0 V to 500 V from a high voltage power supply to the piezo driven folding mirror in the color center laser cavity. Another 10% of the beam is used to generate an error signal in a feedback loop which adjusts the etalon envelope function as the laser is tuned. This is achieved by modulating the etalon free spectral range at a frequency of 4.4 kHz. The tuning arm of the laser was flushed with dry nitrogen to reduce the effects of mode hopping in the laser due to water vapor absorption. 23
Volume 37, number 1
OPTICS COMMUNICATIONS
The Doppler.free measurements used the intermodulated optogalvanic technique [5,6] depicted in fig. 1. A beamsplitter is used to divide the remainder of the laser light into two parts. Forty percent of this light is chopped at 840 Hz and enters the discharge cell from one direction while the other beam is chopped at 600 Hz and sent through the cell in the opposite direction; the beam diameter inside the cell is 1.5 mm, and the overlap of the two beams is maximized. Optogalvanic detection of the Doppler-free component of the absorption is accomplished by monitoring a modulation of the discharge current at the 1440 Hz sum frequency. A halfwave plate is used to rotate the polarization 90 ° in one arm of the experiment thus eliminating feedback disturbances from the return of the counterpropagating beams to the laser. This means that one can adjust the opposite beams exactly collinear without serious feedback problems. As an added measure, one of the alignment mirrors in the experiment was mounted on a small loud speaker and dithered to randomize the phase of the light returning to the laser. Occasionally when operating in the wings of a water vapor absorption line, a tent was built around the experimental setup and flushed with dry nitrogen. A number of Doppler-free measurements have
1 April 1981
been carried out on helium transitions in the n = 4 to n = 6 complex near 2.62/am which were reported earlier [2]. Initial measurements, performed in a positive column discharge, suffered from very large Stark shifts and considerable line broadening because of the presence of large radial electric fields in the small diameter discharge tube [7,8]. A 1.6 cm diameter aluminum hollow cathode design was therefore adopted for the Doppler-free experiments to reduce the strength of the radial fields [7]. Infrared silica windows were attached to the pyrex body of the discharge tube via a glass to quartz transition and the whole tube was discharged cleaned [8]. The length of the negative glow region was 10 cm. The linewidth measured in this cell at 0.5 torr pressure was still much larger than the natural linewidth of a few MHz. By observing a single line, the singlet 4p 1po_6d 1D transition, we directly measured a width (FWHM) of 320 MHz. We have determined experimentally that Stark shifts by radial or axial fields in the discharge tube make no substantial contributions to the linewidth. In addition the power densities used in these experiments, between 150 mW/ cm 2 and 300 mW/cm 2, were small compared to the saturation parameter of the observed transition, therefore power broadening could not produce this line-
. . FEEDBACK •
] E'ECTRO 'CS I
f
SO" ROTATOR ' I I
\
/
f,~ ~ /~IX
, = I SPECTRUM
%,..,, ql
DISCHARGE ~ ~ ~ * ~ " " ~
v+ V
].
~
I~,. S~.
| REFERENCE I 'r~
r 150MHz FSR INTERFEROMETER
ILOCK-INI I I
Fig. 1. Schematic diagram of the experimental set up.
24
/
/ /
Volume 37, number 1
OPTICS COMMUNICATIONS
width. The only remaining broadening mechanism is collisional bkoadening caused either by helium atoms or by charged particles. The former would include excitation transfer collisions [9] and the latter the local effects of perturbations induced by ionic and electronic fields in the plasma which is termed Holtzmark broadening [10]. It is evident from the work of Collins et al. [11] on helium, that at 0.5 torr, there is almost no contribution to the measured linewidth from the helium atoms. Therefore the 320 MHz linewidth is mainly due to Holtzmark broadening, a phenomenon which is important in many other areas of physics; specifically explaining phenomena in gaseous plasmas in both interstellar space and stellar atmospheres and is also used as a benchmark for determining plasma electron and ion densities in plasmas [9]. The experimentally determined values for the halfwidth, Av, and the saturation intensity, Isat can be used to determine the longitudinal relaxation time, r, via the formula [12] 2rr hc Av /sat - r Au._,~ X3 ,
(1)
where A u ~ ~ is the Einstein coefficient. Determination of the parameter, r, gives information on the sum of all excitation processes including transfer collisions. To establish whether the large Holtzmark broadening is peculiar to helium transitions, a few Dopplerfree scans of neon were made. The 1.6 cm hollow cathode was filled with 0.5 torr of a 7 : 1 heliumneon gas mixture. Using the same experimental setup described earlier, the Doppler-free scan of the neon 5s(1½)-6p(½) J 2 - 1 transition, Paschen notation 3Ss-5pl 0, shown in fig. 2 was obtained. The intensity ratio of the lines from the two different isotopes is proportional to the abundance ratio of the natural isotope mixture. The 20Ne signal strength corresponds to a 10 -6 change in the discharge current. In the upper trace of the figure, frequency markes with a 150 MHz spacing are shown. The nonlinear response of the piezo driven mirror is evidenced by the nonuniform spacing of these intervals; therefore the 485 MHz isotope shift between 20Ne and 22Ne is determined with a +5 MHz error. From this measurement one can extrapolate a specific mass shift for a transition between higher transition quantum numbers. At a laser power density of 15 mW/cm 2, a 60 MHz linewidth is measured. When the power density
1 April 1981
150 MHz
NEON 5s 5 - 5Plo (2.55/~m)
•- Z ~ y ~ I 0 0 MHz
22Ne
2ONe
Fig. 2. Doppler-free scan of the neon 3ss-5Plo (Paschen notation) transition. The relative strengths of the lines from the two different isotopes are proportional to the natural abundances. The upper trace displays frequency markers with a 150 MHz spacing. is increased to 60 mW/cm 2, the line is power broadened to the 100 MHz width depicted in fig. 2. The 60 MHz linewidth is comparable to those which have been reported by other researchers [13]. A major difference between the neon and helium states is the proximity of energy levels available for the mixing of states. In helium where the states of the n = 4 and 6 levels are closely spaced, perturbations by charged particles will cause significant line broadening. Even though the transition in neon is between highly excited states, the perturbations by charged particles are not as pronounced because states with the same principal quantum number are more widely separated. Clearly, in helium the perturbing influence of these fields and other phenomena indigenous to the discharge makes optogalvanic detection unsuitable for resolving high resolution structural details in these highly excited states. As a tool for studying the complexities of discharge phenomena, however, the technique is potentially very powerful. In summary, we have demonstrated that: (1) Using the method of intermodulated opto25
Volume 37, number 1
OPTICS COMMUNICATIONS
galvanic spectroscopy with a color center laser, it is possible to make nonlinear Doppler-free spectroscopic measurements of transitions between highly excited atomic levels in the near infrared. The feedback disturbances of the high gain laser are minimized by using a polarization rotator in conjunction with dithering one o f the mirrors in the experimental setup. (2) The resolution o f these measurements for helium atoms in a hollow cathode discharge tube has been improved such that the linewidths are only limited by Holtzmark broadening in the plasma. This broadening prevents accurate measurements of the fine structure intervals between excited helium levels. The measurements on neon give a clear indication that this broadening is not a serious problem in all atoms but is related to the spacing of nearby levels of opposite parity; a Doppler-free optogalvanic spectroscopy can therefore be a powerful tool for the study o f excited atomic states. (3) On the other hand, studies involving helium or hydrogen conveniently provide in vitro probes of plasma properties. Holtzmark broadening and the DC Stark effect of infrared transitions at intermediate strength static fields can be studied below the Dopplerwidth. This is important because in a plasma, one is often left to guess the temperature when deconvoluting the plasma broadened linewidth from the Doppler width. Direct application of this technique to the study of both plasmas and flames should be helpful in obtaining information on the dynamics of these systems. One can also see, from eq. (1), that the measured saturation parameter can be used to determine the total nonradiative deexcitation rates.
26
1 April 1981
References
[1] L.F. Mollenauer, in: CRC Handbook Series on Laser Science and Technology, Vol. I, ed. M. Weber (CRC Press, 1981). [2] D.J. Jackson, E. Arimondo, J.E. Lawler and T.W. Hansch, Optics Comm. 33 (1980) 51. [3] G. Lift'm, C.R. Pollack, R.E. Curl and F.K. Tittel, J. Chem. Phys. 72 (1980) 6602. [4] T.E. Cough, D. Gravel, R.E. Miller and G. Scoles, in: Eleventh Intern. Quantum Electronics Conf.: Digest of Technical Papers, Boston, Massachusetts (1980), p. 665. [5] M.S. Sorem and A.L. Schawlow, Optics Comm. 5 (1972) 148.
[6] J.E. Lawler, A.I. Ferguson, J.E.M. Goldsmith, D.J. Jackson and A.L. Schawlow, Phys. Rev. Letters 42 (1979) 1046. [7] A. Von Engel, Ionized gases (Oxford, Clarendon Press, 1965). [8] J.D. Jackson, Thesis,G.L. Report No. 3207, Stanford University ( 1980). [9] C. Manus, in Proc. XII Intern. Conf. on Phenomena in ionized gases II, eds. J.A. Holscher and D.C. Schram (North-Holland Publishing Copany, 1976) pp. 165-181. [10] H. Griem, Spectral line broadening by plasmas (Academic Press, New York, 1964). [11] C.B. Collins, B.W. Johnson and M.J. Shaw, J. Chem. Phys. 57 (1972) 5310. [12] K. Shimoda, in: High resolution laser spectroscopy, ed. K. Shimoda (Springer-Verlag, New York, 1976) pp. 1148.
[13] F. Bixaben, G. Grynberg, E. Giac0bino and J. Bauche, Phys. Lett. 56A (1976) 441; F. Bixaben, E. Giacobino and G. Grynberg, Phys. Rev. A12 (1975) 2444.
OPTICS COMMUNICATIONS
1 April 1981
DOPPLER-FREE OPTOGALVANIC SPECTROSCOPY USING AN INFRARED COLOR CENTER LASER ~ D.J. JACKSON l, H. GERHARDT 2 and T.W.H.~NSCH Department of Physics, Stanford University, Stanford, California 94305, USA Received 23 December 1980
We have used a cw color center laser near 2.6 tzm to study highly excited states in helium and neon atoms by Dopplerfree intermodulated optogalvanic spectroscopy in a hollow cathode discharge tube. For helium n = 4 to 6 transitions, the resolution was limited to about 320 MHz (FWHM) by Holtzmark broadening due to the presence of charged particles in the discharge. Lines as narrow as 60 MHz were observed for neon 3ss-SPlo.
A color laser center has been used for nonlinear high resolution spectroscopy of excited helium and neon atoms in a DC gas discharge. Color center lasers have extended the arsenal of broadly tunable and highly monochromatic lasers to include near infrared wavelengths out to 3 tam [ 1]. This has opened up a wealth of possible spectroscopic applications at wavelengths which are convenient to study absorption lines from highly excited atomic levels or molecular absorption bands [ 2 - 4 ] . In an earlier work [2] a number of infrared transitions between sparsely populated states in helium were observed Dopplerlimited for the first time by an optogalvanic detection scheme. Doppler-free measurements of transitions between states of high principal quantum number can provide information on fine structure intervals and the interaction of the atoms within the discharge environment. Therefore, we have explored the feasibility of Doppler-free experiments at infrared wavelengths in helium and neon and this paper describes the results and their implications for future experimental work. Four watts all lines from a Spectra Physics argon ion laser were used to pump a Spectra Physics 375 a Work supported by the National Science Foundation under Grant PHY80-10686, and U.S. Office of Naval Research under Contract N00014-78-C-0403. I Bell Laboratories Fellow. 2 Heisenberg Fellow.
dye laser which is operated with rhodamine 6G dye and no internal cavity etalons. The dye laser wavelength was tuned to optimize the coupling of the laser light into the KC1 : Li crystal of a Burleigh FCL20 color center laser. A dye laser power of 750 mW produced 10 mW of infrared light near the 2.65 tzm wavelength. A reliable continuous, tunable singlemode output from the color center laser was only achieved after the insertion of an additional etalon which was solid and 5 mm thick. At the output of the color center laser, 10% of the infrared light is picked off and sent through a 150 MHz interferometer for frequency calibration. The finesse of the interferometer is 20; after careful alignment of the laser light through the interferometer, we have established that the laser linewidth is 5 MI-lz or better. A 2.5 GHz single mode scan was obtained by applying - 5 0 0 V to 500 V from a high voltage power supply to the piezo driven folding mirror in the color center laser cavity. Another 10% of the beam is used to generate an error signal in a feedback loop which adjusts the etalon envelope function as the laser is tuned. This is achieved by modulating the etalon free spectral range at a frequency of 4.4 kHz. The tuning arm of the laser was flushed with dry nitrogen to reduce the effects of mode hopping in the laser due to water vapor absorption. 23
Volume 37, number 1
OPTICS COMMUNICATIONS
The Doppler.free measurements used the intermodulated optogalvanic technique [5,6] depicted in fig. 1. A beamsplitter is used to divide the remainder of the laser light into two parts. Forty percent of this light is chopped at 840 Hz and enters the discharge cell from one direction while the other beam is chopped at 600 Hz and sent through the cell in the opposite direction; the beam diameter inside the cell is 1.5 mm, and the overlap of the two beams is maximized. Optogalvanic detection of the Doppler-free component of the absorption is accomplished by monitoring a modulation of the discharge current at the 1440 Hz sum frequency. A halfwave plate is used to rotate the polarization 90 ° in one arm of the experiment thus eliminating feedback disturbances from the return of the counterpropagating beams to the laser. This means that one can adjust the opposite beams exactly collinear without serious feedback problems. As an added measure, one of the alignment mirrors in the experiment was mounted on a small loud speaker and dithered to randomize the phase of the light returning to the laser. Occasionally when operating in the wings of a water vapor absorption line, a tent was built around the experimental setup and flushed with dry nitrogen. A number of Doppler-free measurements have
1 April 1981
been carried out on helium transitions in the n = 4 to n = 6 complex near 2.62/am which were reported earlier [2]. Initial measurements, performed in a positive column discharge, suffered from very large Stark shifts and considerable line broadening because of the presence of large radial electric fields in the small diameter discharge tube [7,8]. A 1.6 cm diameter aluminum hollow cathode design was therefore adopted for the Doppler-free experiments to reduce the strength of the radial fields [7]. Infrared silica windows were attached to the pyrex body of the discharge tube via a glass to quartz transition and the whole tube was discharged cleaned [8]. The length of the negative glow region was 10 cm. The linewidth measured in this cell at 0.5 torr pressure was still much larger than the natural linewidth of a few MHz. By observing a single line, the singlet 4p 1po_6d 1D transition, we directly measured a width (FWHM) of 320 MHz. We have determined experimentally that Stark shifts by radial or axial fields in the discharge tube make no substantial contributions to the linewidth. In addition the power densities used in these experiments, between 150 mW/ cm 2 and 300 mW/cm 2, were small compared to the saturation parameter of the observed transition, therefore power broadening could not produce this line-
. . FEEDBACK •
] E'ECTRO 'CS I
f
SO" ROTATOR ' I I
\
/
f,~ ~ /~IX
, = I SPECTRUM
%,..,, ql
DISCHARGE ~ ~ ~ * ~ " " ~
v+ V
].
~
I~,. S~.
| REFERENCE I 'r~
r 150MHz FSR INTERFEROMETER
ILOCK-INI I I
Fig. 1. Schematic diagram of the experimental set up.
24
/
/ /
Volume 37, number 1
OPTICS COMMUNICATIONS
width. The only remaining broadening mechanism is collisional bkoadening caused either by helium atoms or by charged particles. The former would include excitation transfer collisions [9] and the latter the local effects of perturbations induced by ionic and electronic fields in the plasma which is termed Holtzmark broadening [10]. It is evident from the work of Collins et al. [11] on helium, that at 0.5 torr, there is almost no contribution to the measured linewidth from the helium atoms. Therefore the 320 MHz linewidth is mainly due to Holtzmark broadening, a phenomenon which is important in many other areas of physics; specifically explaining phenomena in gaseous plasmas in both interstellar space and stellar atmospheres and is also used as a benchmark for determining plasma electron and ion densities in plasmas [9]. The experimentally determined values for the halfwidth, Av, and the saturation intensity, Isat can be used to determine the longitudinal relaxation time, r, via the formula [12] 2rr hc Av /sat - r Au._,~ X3 ,
(1)
where A u ~ ~ is the Einstein coefficient. Determination of the parameter, r, gives information on the sum of all excitation processes including transfer collisions. To establish whether the large Holtzmark broadening is peculiar to helium transitions, a few Dopplerfree scans of neon were made. The 1.6 cm hollow cathode was filled with 0.5 torr of a 7 : 1 heliumneon gas mixture. Using the same experimental setup described earlier, the Doppler-free scan of the neon 5s(1½)-6p(½) J 2 - 1 transition, Paschen notation 3Ss-5pl 0, shown in fig. 2 was obtained. The intensity ratio of the lines from the two different isotopes is proportional to the abundance ratio of the natural isotope mixture. The 20Ne signal strength corresponds to a 10 -6 change in the discharge current. In the upper trace of the figure, frequency markes with a 150 MHz spacing are shown. The nonlinear response of the piezo driven mirror is evidenced by the nonuniform spacing of these intervals; therefore the 485 MHz isotope shift between 20Ne and 22Ne is determined with a +5 MHz error. From this measurement one can extrapolate a specific mass shift for a transition between higher transition quantum numbers. At a laser power density of 15 mW/cm 2, a 60 MHz linewidth is measured. When the power density
1 April 1981
150 MHz
NEON 5s 5 - 5Plo (2.55/~m)
•- Z ~ y ~ I 0 0 MHz
22Ne
2ONe
Fig. 2. Doppler-free scan of the neon 3ss-5Plo (Paschen notation) transition. The relative strengths of the lines from the two different isotopes are proportional to the natural abundances. The upper trace displays frequency markers with a 150 MHz spacing. is increased to 60 mW/cm 2, the line is power broadened to the 100 MHz width depicted in fig. 2. The 60 MHz linewidth is comparable to those which have been reported by other researchers [13]. A major difference between the neon and helium states is the proximity of energy levels available for the mixing of states. In helium where the states of the n = 4 and 6 levels are closely spaced, perturbations by charged particles will cause significant line broadening. Even though the transition in neon is between highly excited states, the perturbations by charged particles are not as pronounced because states with the same principal quantum number are more widely separated. Clearly, in helium the perturbing influence of these fields and other phenomena indigenous to the discharge makes optogalvanic detection unsuitable for resolving high resolution structural details in these highly excited states. As a tool for studying the complexities of discharge phenomena, however, the technique is potentially very powerful. In summary, we have demonstrated that: (1) Using the method of intermodulated opto25
Volume 37, number 1
OPTICS COMMUNICATIONS
galvanic spectroscopy with a color center laser, it is possible to make nonlinear Doppler-free spectroscopic measurements of transitions between highly excited atomic levels in the near infrared. The feedback disturbances of the high gain laser are minimized by using a polarization rotator in conjunction with dithering one o f the mirrors in the experimental setup. (2) The resolution o f these measurements for helium atoms in a hollow cathode discharge tube has been improved such that the linewidths are only limited by Holtzmark broadening in the plasma. This broadening prevents accurate measurements of the fine structure intervals between excited helium levels. The measurements on neon give a clear indication that this broadening is not a serious problem in all atoms but is related to the spacing of nearby levels of opposite parity; a Doppler-free optogalvanic spectroscopy can therefore be a powerful tool for the study o f excited atomic states. (3) On the other hand, studies involving helium or hydrogen conveniently provide in vitro probes of plasma properties. Holtzmark broadening and the DC Stark effect of infrared transitions at intermediate strength static fields can be studied below the Dopplerwidth. This is important because in a plasma, one is often left to guess the temperature when deconvoluting the plasma broadened linewidth from the Doppler width. Direct application of this technique to the study of both plasmas and flames should be helpful in obtaining information on the dynamics of these systems. One can also see, from eq. (1), that the measured saturation parameter can be used to determine the total nonradiative deexcitation rates.
26
1 April 1981
References
[1] L.F. Mollenauer, in: CRC Handbook Series on Laser Science and Technology, Vol. I, ed. M. Weber (CRC Press, 1981). [2] D.J. Jackson, E. Arimondo, J.E. Lawler and T.W. Hansch, Optics Comm. 33 (1980) 51. [3] G. Lift'm, C.R. Pollack, R.E. Curl and F.K. Tittel, J. Chem. Phys. 72 (1980) 6602. [4] T.E. Cough, D. Gravel, R.E. Miller and G. Scoles, in: Eleventh Intern. Quantum Electronics Conf.: Digest of Technical Papers, Boston, Massachusetts (1980), p. 665. [5] M.S. Sorem and A.L. Schawlow, Optics Comm. 5 (1972) 148.
[6] J.E. Lawler, A.I. Ferguson, J.E.M. Goldsmith, D.J. Jackson and A.L. Schawlow, Phys. Rev. Letters 42 (1979) 1046. [7] A. Von Engel, Ionized gases (Oxford, Clarendon Press, 1965). [8] J.D. Jackson, Thesis,G.L. Report No. 3207, Stanford University ( 1980). [9] C. Manus, in Proc. XII Intern. Conf. on Phenomena in ionized gases II, eds. J.A. Holscher and D.C. Schram (North-Holland Publishing Copany, 1976) pp. 165-181. [10] H. Griem, Spectral line broadening by plasmas (Academic Press, New York, 1964). [11] C.B. Collins, B.W. Johnson and M.J. Shaw, J. Chem. Phys. 57 (1972) 5310. [12] K. Shimoda, in: High resolution laser spectroscopy, ed. K. Shimoda (Springer-Verlag, New York, 1976) pp. 1148.
[13] F. Bixaben, G. Grynberg, E. Giac0bino and J. Bauche, Phys. Lett. 56A (1976) 441; F. Bixaben, E. Giacobino and G. Grynberg, Phys. Rev. A12 (1975) 2444.