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Physical Review A

Optical mixed-state transient effects by Stark switching

J. A. Kash, Sun Tao-Heng, Erwin Louis Hahn

Published Nov 1, 1982• 0 citations
Publisher

Abstract

The superposition of vibrational states produced in gaseous $^{13}\mathrm{C}$${\mathrm{H}}_{3}$F by 9.6-\ensuremath{\mu}m infrared laser radiation results in anomalous echo-envelope and free precession-beat effects. The effects arise when pulsed Stark electric fields alter the state mixing among nearly degenerate spin-rotation states. The initial eigenstates are governed by the internal spin-rotation interaction and by applied dc bias Stark electric and Zeeman magnetic fields. Unresolved modulation of the photon echo envelope caused by sudden changes of state mixtures is confirmed by analytic theory. Separate measurement of ground- and excited-state spinrotation interactions is possible for future investigations with increase in apparatus resolution. A new anomalous coherent Raman beat also caused by altering the state mixtures is investigated. The reduction of state mixing by local collisional electric fields which increase with gas pressure is qualitatively confirmed.

DOI
10.1103/physreva.26.2682

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 30, 2002•4 citations
The superposition of vibrational states produced in gaseous ^{13}\mathrm{C}$${\mathrm{H}}_{3}F by 9.6-\ensuremath{\mu}m infrared laser radiation results in anomalous echo-envelope and free precession-beat effects. The effects arise when pulsed Stark electric fields alter the state mixing among nearly degenerate spin-rotation states. The initial eigenstates are governed by the internal spin-rotation interaction and by applied dc bias Stark electric and Zeeman magnetic fields. Unresolved modulation of the photon echo envelope caused by sudden changes of state mixtures is confirmed by analytic theory. Separate measurement of ground- and excited-state spinrotation interactions is possible for future investigations with increase in apparatus resolution. A new anomalous coherent Raman beat also caused by altering the state mixtures is investigated. The reduction of state mixing by local collisional electric fields which increase with gas pressure is qualitatively confirmed.
Photon-echo interference damping and anomalous coherent Raman beats are observed due to the spin-rotation interaction from laser-excited vibration-rotation transitions in 13C^{13}\mathrm{C}13CH3{\mathrm{H}}_{3}H3​F at 9.6\ensuremath{\mu}m. Pulsed Stark fields superimposed upon a constant magnetic field control the intrinsic mixture of spin-rotation states. The spin-rotation interaction for spin I=12I=\frac{1}{2}I=21​ is confirmed from photon-echo measurement and theory.
The authors study the phenomenon of coherent Raman beats in 15N^{15}\mathrm{N}15NH3{\mathrm{H}}_{3}H3​ using a CO2{\mathrm{O}}_{2}O2​ laser and Stark switching. In particular, a three-level system in which the two upper levels always remain split by a few MHz is prepared by switching the transitions into resonance with a short Stark pulse whose bandwidth is large enough to coherently excite both transitions. When one of the coherently excited levels remains in resonance with the laser after the Stark pulse, this system exhibits a qualitatively new effect which has not been seen heretofore. For Stark shifts on the order of the upper-state splitting or smaller, the Raman beat is amplitude modulated at a frequency related to the optical nutation frequency. This is interpreted as being due to an interaction between the two-photon coherent Raman beat process and a single-photon optical nutation process which occurs simultaneously. Numerical calculations as well as a simple analytic model are presented to support this interpretation. By reducing the laser power, one can make the modulation of the Raman beat disappear. From the Raman signal in this regime the permanent electric dipole moment of 15N^{15}\mathrm{N}15NH3{\mathrm{H}}_{3}H3​ in an excited vibrational state is determined and the Raman beat decay rate measured. By comparing the latter result with a delayed optical nutation measurement we show that phase-changing collisions are negligible for the transition studied.
The phenomenon of coherent Raman beats observed recently by Shoemaker and Brewer is analyzed through solutions of the coupled Maxwell-Schr\"odinger equations. The effect arises in coherently prepared molecular samples where the level degeneracy is suddenly removed by Stark-pulse switching. The problem divides into two parts: (a) a steady-state preparative regime whereby the initially degenerate quantum levels are placed in superposition by a resonant laser field, and (b) a transient regime in which coherent forward Raman scattering decays in the presence of the same laser field during a nonresonant condition. The laser and Raman light thus propagate together and produce at a detector a coherent beat with a frequency that corresponds to the level splitting between initial and final states. The beat signal possesses the remarkable property of being entirely independent of longitudinal molecular velocity in the case of a plane-wave laser beam and of being almost so for milliradian divergence. It follows that relaxation involving velocity-changing collisions and that Doppler dephasing effects are absent in coherent Raman beats, in agreement with experimental measurements of the decay rate. The effect of off-resonance excitation on the dephasing time and on the period of the beat signal is also discussed, and the possibility of a small rotation of the plane of polarization of the scattered light is included. The subject presents a new aspect of Raman scattering which can now yield precise radio-frequency splittings directly and offers a selective way of examining relaxation phenomena.
Photon echo and optical nutation have been easily observed in C13{\mathrm{C}}^{13}C13H3{\mathrm{H}}_{3}H3​F and NH2{\mathrm{H}}_{2}H2​D by applying Stark pulses which shift the molecular levels into resonance with cw laser radiation. Numerical computations of the nutation effect agree with observation, and echo characteristics closely follow predictions of existing theories. The $T_{2}^{}{}_{}{}^{\ensuremath{'}}$ pressure dependence, from infrared-echo measurements, indicates that C13{\mathrm{C}}^{13}C13H3{\mathrm{H}}_{3}H3​F relaxes primarily by rotational energy transfer.
Coherent transient effects are the optical analogs of the many transient phenomena seen in pulsed nuclear magnetic resonance experiments on spin systems. For example, photon-echo and optical nutation are the respective optical equivalents of spin echo and transient nutation of nuclear magnetic resonance. In Stark-switching and frequency-switching techniques the laser field and the molecules are brought into resonance in a sequence of pulses, the rest of the time they remain well off-resonance. So far it has been assumed that the off-resonance field does not have any measureable influence on the experimental results and is utilized to implement a very efficient detection scheme. This work discusses how the off-resonance field affects the coherent transient effects. It is shown here how this field, by inducing changes in the index of refraction as small as 10⁻⁶ produces easily observable effects in photon echo and delayed optical nutation.
Coherent transient phenomena can be generated by the abrupt changes of parameters either of the electromagnetic field or of the matter. Comparative analysis of these two approaches, made on the base of experimental research in molecular gases SF6, 15NH3, HFCO and 13CH3F, excited by the radiation of CW CO2 and 13CO2 lasers, is presented. The first approach of photon echo (and its modifications) generation, based on electro-optic pulses formation from the resonant CW exciting radiation, has proven to be successful for intense infrared molecular transitions. The coherent responses are sufficiently powerful to be applied in high resolution spectroscopy, the spectral markers creation ,and molecular collisions investigations. However, this technique is not very effective for the free polarization decay investigations. Another approach based on the Stark switching molecular levels of polar gas, irradiated by CW laser, was demonstrated for free polarization decay and photon echo generation in 13CH3F. Free polarization decay has shown the pronounced frequency shift because of dynamic Stark effect in the CW radiation field. This specific feature of coherent transients, created by Stark switching, limits the high resolution spectroscopy by the low intensity range of exciting radiation. From other viewpoint, it gives unique possibility to investigate coherent transients generated at the dressed molecular levels.