Doppler-free radiofrequency optogalvanic spectroscopy

Volume 38, number 1

OPTICS COMMUNICATIONS

1 July 1981

DOPPLER-FREE RADIOFREQUENCY OPTOGALVANIC SPECTROSCOPY "~ D.R. LYONS, A.L. SCHAWLOW and G-Y. YAN * Department of Physics, Stanford University, Stanford, CA 94305, USA Received 16 March 1981

Doppler-free optogalvanic spectra of two neon lines have been observed, with a radiofrequency oscillator used to both excite the discharge and detect the laser-induced change in ion density. Both intermodulated and polarization-intermodulated (POL1NEX) methods have been used. The sensitivity is similar to that obtained with fluorescence detection.

We have succeeded in observing Doppler-free optogalvanic spectra [ 1 - 3 ] by a remarkably simple radiofrequency technique. Excited atoms are provided by a discharge driven by a vacuum tube oscillator. The change o f ion density, and hence of discharge impedance, produced by laser irradiation is detected by the reaction on the oscillator. As with other methods of detection, Doppler-free signals are obtained from those atoms which absorb simultaneously from two oppositely-directed laser beams. We have employed both the intermodulation [3,6] and polarization intermodu-. lated (POLINEX) [7] methods, and have observed Doppler-free spectra in neon. The sensitivity is comparable to that obtained with fluorescence spectroscopy. Regenerative detectors have long been employed to detect small impedance changes, particularly for measuring phenomena such as nuclear magnetic and nuclear quadrupole resonances [5]. Any oscillator automatically adjusts its level of oscillation so that losses are just balanced by the regenerative amplification. If the losses are increased slightly, the oscillator amplitude is correspondingly decreased, and this produces an observable change in the oscillator's anode current. The use of a regenerative detector for Doppler-free optogalvanic spectroscopy, in which the * Supported by the National Science Foundation under Grant PHY80-10689. * On leave from East China Normal University, Shanghai, The People's Republic of China.

conductivity of a gas discharge is altered by laser excitation, was proposed previously [3]. Regenerative detection has been reported for Doppler-broadened lines by Stanciulescu et al. [4]. Radio-frequency oscillators have the advantage that they can maintain discharges at low gas pressures, minimizing pressure broadening. Electrodeless radiofrequency discharges are particularly suitable for spectroscopy of scarce or corrosive gases. In our experiments, we have used a Colpitts regenerative oscillator with a triode 6A4F (fig. 1), to excite a discharge in neon. Typically, the plate voltage and current o f the vacuum tube were about 70 volt and 10 mA respectively. Although the input power was

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Volume 38, number 1

OPTICS COMMUNICATIONS

small, it easily excited a discharge in a cell containing neon at a pressure of about 200 mTorr. The oscillatory circuit consisted o f an 9.3 pF capacitor and a coil with an inductance of 2.4 pH. The oscillator frequency was about 32 MHz. The diameter of the 15turn coil was 30 mm. A cell (120 mm long and 14 mm in diameter) containing naturally abundant neon was placed inside the coil. The plate voltage decreased 10 volts when the discharge began. When the sample was irradiated by a beam from a C.W. dye laser whose frequency is tuned to near resonance with some neon transition, the signal could be indicated by the voltage change across the 7.8 k~2 load resistor of the oscillator. The diameter of the beam irradiating the sample was 5 mm to provide a reaction region of suitable size. For ordinary Doppler-broadened absorption measurements, changes on the order of 10 mV have been observed on several transitions using tens of milliwats of laser power. The sensitivity for the Ne 2 P 4 - 1 s 5 transition at 594.5 nm, as shown by the oscilloscope trace in fig. 2, is similar to that reported for D.C. optogalvanic spectroscopy [8] . To get a Doppler-free radiofrequency optogalvanic spectrum in neon, we first used intermodulation of two oppositely-directed beams from the same dye laser. Before doing so, we tested the linearity of the detector to ensure that intermodulation would come only from the interaction of the beams with the saturable or pumpable atoms. This was done by measuring si-

1 July 1981

multaneously the signal from the radiofrequency detector and the fluorescence change produced by the same chopped laser beam. The 5852 A line of neon was used whose lower level ls 2 is not metastable. Fig. 3 shows that the radiofrequency and optical methods of detection gave the same dependence on laser intensity. Thus we could expect that the intermodulated radiofrequency optogalvanic spectrum would be Doppler-free and without troublesome background. When the two oppositely-directed laser beams were used, the intermodulated detector signal shown in fig. 4a was obtained. For comparison the same spectrum, observed by intermodulated fluorescence, is shown in fig. 4b. It is apparent that the two different methods give very similar results. This shows that this R.F. technique can be used as a linear detection method over a wide range of irradiation intensity. For some other transitions, particularly those from metastable levels like the 5882 A transition of neon, the intermodulation method cannot completely eliminate the intense Doppler-broadened background, which comes from velocity-changing collisions. Recently, however, a new Doppler-free spectroscopic technique termed polarization intermodulated excitation spectroscopy (POLINEX) has been demonstrated by Hansch et al. [7]. POLINEX can be used to completely eliminate the background in such transitions. In the radiofrequency POLINEX method (fig. 5), two counter-

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Fig. 2. R.F. optogalvanic signal of the Ne 2p4-1s5 transition at 594.5 nm with one chopped beam. The time scale (horizontal axis) is 0.2 ms/div, while the voltage scale (vertical axis) is 10 mV/div. 36

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Volume 38, number 1

OPTICS COMMUNICATIONS

2ONe

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propagating beams are linearly polarized but their directions of polarization are rotated, with spinning frequencies f l and/2 respectively. To achieve this, the laser beams are first circularly polarized and then passed through plane polarizers which rotate at these frequencies. The intensities of the beams are thus unmodulated. When the laser frequency is away from the center of the Doppler profile, the two beams interact with different atoms. The number of ions created by the irradiating beams is not modulated with time, even though these ions originate from ionization of differently oriented atoms depending on the instantaneous polarization of the beams. However, when the dye laser is tuned to the center of the Doppler profile, both beams can interact with the same atoms. That is, the beams interact with those atoms which have nearly zero velocity components along the axial direction. These atoms alternately see the field of the two beams with either parallel polarization or orthogonal polarizations. In the former case, both beams interact preferentially

1 July 1981

with atoms of the same orientation, whereas in the latter, the two beams interact with atoms whose orientations are orthogonal to each other. Therefore, the transsitions are saturated more deeply in the former than in the latter case. Even though the intensities of the beams are not modulated with time, the number of ions created is modulated with a frequency of either 2(fl + f 2 ) or 2 ( f 1 - / 2 ) depending on the relative rotation direction of the linear polarizers. Thus, we can use the lock-in amplifier to pick up signals at the intermodulation frequency. In our experiment, the light from a linear CW dye laser (Coherent Radiation Model CR 599-21) was circularly polarized. It was then divided into two beams with nearly equal intensity by a beam splitter. As shown in fig. 4, they passed in opposite directions through two spinning linear polarizers, whose rotational frequencies were 25 Hz and 112 Hz respectively. The reference signal at a frequency of 275 Hz was then picked from either of the two spit beams which, having passed through both polarizers and consequently through the cell, were reflected off either mirrors M 1 or M2, of the beam splitter, and finally onto the photodiode detector. Imperfections of the cell windows, polarizers and the presence of the mirrors (M 1 and M2) could cause modulations in the intensities of the two beams as the polarizers rotate. The compensators serve to reduce such modulation. In the radiofrequency method, we do not need to be concerned with the angular distribution of the fluorescent light from atoms excited with polarized light. Fig. 5 shows the results obtained from

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Volume 38, number 1

OPTICS COMMUNICATIONS

1 July 1981

The results reported here have demonstrated that a combination of the radiofrequency detection technique with intermodulated or polarization intermodulated excitation spectroscopy provides a new useful and convenient tool for high resolution laser spectroscopy. It is clear that this technique may be used for detecting two-photon transitions for investigating higher levels of atoms or molecules. For studies requiring the low pressures of electrodeless discharges, the advantages of this method are especially apparent.

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We are grateful to T.W. Hansch for several helpful discussions.

References

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Fig. 6. Doppler-free spectrum of the Ne 2p2-1Ss transition at 588.2 rim. recorded by (a) POLINEX R.F. optogalvanic spectroscopy (b) intermodulated R.F. optogalvanic detection.

intermodulated and POLINEX methods of radiofrequency optogalvanic spectroscopy. The Doppler background originating from velocity changing collisions in the 5882 A transition is completely eliminated in the latter method.

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[1] R.B. Green, R.A. Keller, G.G. Luther, P.K. Schenk and J.C. Travis, Appl. Phys. Lett. 29 (1976) 727. [2] J.E. Lawler, A.I. Ferguson, J.E.M. Goldsmith, D.J. Jackson and A.L. Schawlow, Phys. Rev. Lett 42 (1979) 1046. [3] J.E.M. Goldsmith, A.I. Ferguson, J.E. Lawler and A.L. Schawlow, Optics Lett. 4 (1979) 230. [4] C. Stanciulescu, R.C. Bobulescu, A. Surmeian, D. Popescu, Iovitzu Popescu, and C.B. Collins, Appl. Phys. Lett. 37 (1980) 888. [5] A.L. Schawlow, J. Chem. Phys. 22 (1954) 1211. [6] M.S. Sorem and A.L. Schawlow, Optics Comm. 5 (1972) 148. [7] T.W. Hansch, D.R. Lyons, A.L. Schawlow, A. Siegel, Z-Y Wang and G-Y Yan, Optics Comm. 38 (1981 ) 47. [8] K.O. Smyth and P.K. Schenk, Chem. Phys. Lett. 55 (1978) 466.