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Journal de Physique II

From linear amplification to triggered superradiance : illustrative examples of stimulated emission and polarization spectroscopy for sensitive detection of a pulsed excited forbidden transition

Marie-Anne Bouchiat, Jocelyne Guéna, Ph. Jacquier, Michel Lintz, L. Pottier

Published Apr 1, 1992• 8 citations
Publisher

Abstract

This paper presents our experimental strategy for detecting an electric-field controlled pulsed-excited highly forbidden transition such as the 6S-7S transition of the Cs atom. The amplification of a pulsed probe beam close to resonance for one hfs component of the 7S – 6P3/2 transition is monitored. Furthermore the polarization modification experienced by the probe beam as a result of its interaction with the vapor is meticulously analysed. In convincing examples we point out the high sensitivity and complete selectivity of the method for the angular anisotropy of the 7S state, such as orientation or alignment created in the 6S-7S excitation process. We illustrate the flexibility of the method by varying the pump-probe delay, or by changing the relative positions of the beams or the magnitude of the E-field that directly controls the optical gain. A smooth transition from linear amplification to triggered superradiance is observed which necessitates readjustment of the probe intensity for optimal detection. We demonstrate the vapor-induced enhancement of the amplification anisotropy, expected at high optical density.

DOI
10.1051/jp2:1992162

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Proposal for high-precision atomic-parity-violation measurements by amplification of the asymmetry by stimulated emission in a transverse electric and magnetic field pump-probe experiment

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Amplification by stimulated emission of radiation provides an interesting means for increasing the sensitivity of atomic-parity-violation (APV) measurements in a pump-probe configuration well adapted to the 6S-7S Cs transition. It takes advantage of the large number of atoms excited along the path of the pump beam. In the longitudinal electric field configuration currently exploited in our ongoing APV measurement, this number is limited only by the total voltage sustainable by the Cs vapor. To overcome this limit we consider, both theoretically and experimentally, the possibility of performing the measurements in a transverse electric field configuration requiring a much lower voltage. We discuss the necessarily different nature of the observable and the magnetoelectric optical effects that come into play. These condition modifications of the experimental configuration with, in particular, the application of a transverse magnetic field. We suggest the possibility of rotating the transverse direction of the fields so as to suppress systematic effects. With a long interaction length a precision reaching 0.1% in a quantum-noise-limited measurement can be expected, limited only by the necessity of operating below the threshold of spontaneous superradiant emission of the excited medium. Were we to approach this limit, however, we could greatly amplify the asymmetry using triggered superradiance.

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symmetry discrimination and measurements of parity violating and magnetoelectric dichroisms in a Cs vapor

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We detect atomic parity violation (APV) by using stimulated emission, in the form of a chiral optical gain of a cesium vapor on the 7S-6P3/2 probe transition, consecutive to linearly polarized 6S-7S excitation. The method relies on a pump-probe scheme in which the excited 7S state is populated, in zero magnetic field, by an intense, pulsed excitation beam, with a linear polarization [epsilon]ex and a wave vector [k] parallel to the applied electric field [E]. The atoms are then probed by a second, collinear pulsed laser beam, tuned to resonance with the 7S - 6P3/2 transition. The probe beam is amplified by stimulated emission and its linear polarization [epsilon]pr undergoes a small change of direction, due to the presence of the pseudoscaler PV term in the gain. This dichroism, with axes at +/-45° from the ([epsilon]ex,[E]) plane of symmetry of the experiment, is smaller than the parity-conserving, Stark linear dichroism by the ratio of the Stark to the PV 6S-7S transition amplitudes [beta]E/ E1pv, approximately 10^6. As a result the gain axes are tilted by an angle [theta]pv = E1pv/[beta]E, approximately 1µradian.This is the relevant APV quantity, measured by balanced-mode, two channel polarimetry, with dedicated real-time calibration. This detection method has been validated by a measurement of expected sign and magnitude with a 9% accuracy [ref. 1], now improved by more than a factor of two, thanks essentially to a dichroic polarization-tilt magnifier [ref. 2]. Rotational symmetry around the common pump-probe beam axis is used to reduce systematic effects [ref. 3], among which the parity-conserving Jones dichroism due to transverse E and B fields. This latter kind of magneto-optical effects has already been observed in especially chosen liquids thanks to the application of intense (>10T) magnetic fields. Here, symmetry discrimination, applied to the strongly forbidden 6S-7S transition, allows its detection with applied magnetic fields 10^5 to 10^7 times weaker, in a dilute vapor, where the mechanisms involved are clearly understood [ref. 3]. Besides, we discuss a novel kind of magneto-optical effects manifesting parity violation, as possible observables for APV measurements to the 0.1% precision level on the 6S-7S Cs transition. They consist in observing a parity violating circular dichroism in the presence of transverse electric and magnetic fields [ref. 4]. The advantage, as compared to the longitudinal E field configuration, is that the interaction length can be made much longer without increasing the applied potentials. Thanks to the much larger optical density of the excited vapor, stimulated emission gives rise to considerable amplification of the polarimetric signal during the propagation of the probe beam. Even moderate transverse electric fields would allow to reach the triggered superradiance regime, for further enhancing the PV asymmetry. Finally we suggest a means of rotating the transverse direction of the fields so as to restore the cylindrical symmetry and to suppress systematic effects, as in the longitudinal E field configuration. [1] J. Guéna et al.: Phys. Rev. Lett. 90, 143001, (2003) [2] D. Chauvat et al.: Opt. Commun. 138, 249 (1997) [3] M.-A. Bouchiat, J. Guéna, M. Lintz: Eur Phys J D28, 331 (2004) [4] J. Guéna, M. Lintz & M.-A. Bouchiat: arXiv:physics/0403141 & JOSA B, to be published

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Observation of the 6S1/2-7S1/2 single-photon transition of cesium induced by an external d. c. electric field

Marie-Anne Bouchiat, L. Pottier

We have observed the direct single-photon electric dipole excitation of the 7 2S1/2 state of cesium, by exciting the 6 2S1/2 ground state in an external d. c. electric field of 10 to 1 000 V/cm with a single-mode, c. w., tunable dye laser. The resonance was detected by monitoring the decay fluorescence from the excited state. We verified the selection rules obeyed by the hyperfine transitions which depend on the relative orientation of the linear polarization of the beam with respect to the field. When they are orthogonal, the line pattern reflects the existence of a spin-dependent part of the polarizability induced by spin-orbit effects, the size of which was measured with respect to that of the scalar part. Results in very weak fields show that the oscillator strength of the 6S-7S transition is smaller than 2 x 10-13.

Journal de Physique Lettres•Jan 1, 1975•23 citations
This paper presents our experimental strategy for detecting an electric-field controlled pulsed-excited highly forbidden transition such as the 6S-7S transition of the Cs atom. The amplification of a pulsed probe beam close to resonance for one hfs component of the 7S – 6P3/2 transition is monitored. Furthermore the polarization modification experienced by the probe beam as a result of its interaction with the vapor is meticulously analysed. In convincing examples we point out the high sensitivity and complete selectivity of the method for the angular anisotropy of the 7S state, such as orientation or alignment created in the 6S-7S excitation process. We illustrate the flexibility of the method by varying the pump-probe delay, or by changing the relative positions of the beams or the magnitude of the E-field that directly controls the optical gain. A smooth transition from linear amplification to triggered superradiance is observed which necessitates readjustment of the probe intensity for optimal detection. We demonstrate the vapor-induced enhancement of the amplification anisotropy, expected at high optical density.
Amplification by stimulated emission of radiation provides an interesting means for increasing the sensitivity of atomic-parity-violation (APV) measurements in a pump-probe configuration well adapted to the 6S-7S Cs transition. It takes advantage of the large number of atoms excited along the path of the pump beam. In the longitudinal electric field configuration currently exploited in our ongoing APV measurement, this number is limited only by the total voltage sustainable by the Cs vapor. To overcome this limit we consider, both theoretically and experimentally, the possibility of performing the measurements in a transverse electric field configuration requiring a much lower voltage. We discuss the necessarily different nature of the observable and the magnetoelectric optical effects that come into play. These condition modifications of the experimental configuration with, in particular, the application of a transverse magnetic field. We suggest the possibility of rotating the transverse direction of the fields so as to suppress systematic effects. With a long interaction length a precision reaching 0.1% in a quantum-noise-limited measurement can be expected, limited only by the necessity of operating below the threshold of spontaneous superradiant emission of the excited medium. Were we to approach this limit, however, we could greatly amplify the asymmetry using triggered superradiance.
We detect atomic parity violation (APV) by using stimulated emission, in the form of a chiral optical gain of a cesium vapor on the 7S-6P3/2 probe transition, consecutive to linearly polarized 6S-7S excitation. The method relies on a pump-probe scheme in which the excited 7S state is populated, in zero magnetic field, by an intense, pulsed excitation beam, with a linear polarization [epsilon]ex and a wave vector [k] parallel to the applied electric field [E]. The atoms are then probed by a second, collinear pulsed laser beam, tuned to resonance with the 7S - 6P3/2 transition. The probe beam is amplified by stimulated emission and its linear polarization [epsilon]pr undergoes a small change of direction, due to the presence of the pseudoscaler PV term in the gain. This dichroism, with axes at +/-45° from the ([epsilon]ex,[E]) plane of symmetry of the experiment, is smaller than the parity-conserving, Stark linear dichroism by the ratio of the Stark to the PV 6S-7S transition amplitudes [beta]E/ E1pv, approximately 10^6. As a result the gain axes are tilted by an angle [theta]pv = E1pv/[beta]E, approximately 1µradian.This is the relevant APV quantity, measured by balanced-mode, two channel polarimetry, with dedicated real-time calibration. This detection method has been validated by a measurement of expected sign and magnitude with a 9% accuracy [ref. 1], now improved by more than a factor of two, thanks essentially to a dichroic polarization-tilt magnifier [ref. 2]. Rotational symmetry around the common pump-probe beam axis is used to reduce systematic effects [ref. 3], among which the parity-conserving Jones dichroism due to transverse E and B fields. This latter kind of magneto-optical effects has already been observed in especially chosen liquids thanks to the application of intense (>10T) magnetic fields. Here, symmetry discrimination, applied to the strongly forbidden 6S-7S transition, allows its detection with applied magnetic fields 10^5 to 10^7 times weaker, in a dilute vapor, where the mechanisms involved are clearly understood [ref. 3]. Besides, we discuss a novel kind of magneto-optical effects manifesting parity violation, as possible observables for APV measurements to the 0.1% precision level on the 6S-7S Cs transition. They consist in observing a parity violating circular dichroism in the presence of transverse electric and magnetic fields [ref. 4]. The advantage, as compared to the longitudinal E field configuration, is that the interaction length can be made much longer without increasing the applied potentials. Thanks to the much larger optical density of the excited vapor, stimulated emission gives rise to considerable amplification of the polarimetric signal during the propagation of the probe beam. Even moderate transverse electric fields would allow to reach the triggered superradiance regime, for further enhancing the PV asymmetry. Finally we suggest a means of rotating the transverse direction of the fields so as to restore the cylindrical symmetry and to suppress systematic effects, as in the longitudinal E field configuration. [1] J. Guéna et al.: Phys. Rev. Lett. 90, 143001, (2003) [2] D. Chauvat et al.: Opt. Commun. 138, 249 (1997) [3] M.-A. Bouchiat, J. Guéna, M. Lintz: Eur Phys J D28, 331 (2004) [4] J. Guéna, M. Lintz & M.-A. Bouchiat: arXiv:physics/0403141 & JOSA B, to be published
We have detected, by using stimulated emission, an atomic parity violation (APV) in the form of a chiral optical gain of a cesium vapor on the 7S-6P(3/2) transition, consecutive to linearly polarized 6S-7S excitation. We demonstrate the validity of this detection method of APV, by presenting a 9% accurate measurement of expected sign and magnitude. We stress several advantages of this new approach which fully exploits the cylindrical symmetry of the setup. Future measurements at the percent level will provide an important cross-check of an existing more precise result obtained by a different method.
With a view to improved parity violation measurements in Cs, we have considered the 6S-7S-6P3/2 « forbidden three-level system », in which one laser excites the forbidden 6S-7S transition while a second, colinear laser probes the excited 7S atoms. This paper presents an analytical calculation of the 7S → 6P ½ fluorescence intensity and of the probe amplification as functions of the laser frequencies. The collisional processes in the 6P 3/2 resonance level, as well as the level multiplicity and the laser polarizations are taken into account. The quantitative agreement with experiment is good ; in particular, the observed sub-Doppler structures are correctly described. The damping rates of the 7S-6P3/2 cohérence and of the trapped 6P3/2 atoms are extracted. New détection schemes for detecting a 7S orientation, with direct possible extension to an alignment, are demonstrated.
We demonstrate polarization spectroscopy of an excited state transition in room-temperature cesium vapor. An anisotropy induced by a circularly polarized pump beam on the D2 transition is observed using a weak probe on the 6P(3/2)→7S(1/2) transition. At high pump power, a subfeature due to Autler-Townes splitting is observed that theoretical modeling shows is enhanced by Doppler averaging. Polarization spectroscopy provides a simple modulation-free signal suitable for laser frequency stabilization to excited state transitions.
We have observed the direct single-photon electric dipole excitation of the 7 2S1/2 state of cesium, by exciting the 6 2S1/2 ground state in an external d. c. electric field of 10 to 1 000 V/cm with a single-mode, c. w., tunable dye laser. The resonance was detected by monitoring the decay fluorescence from the excited state. We verified the selection rules obeyed by the hyperfine transitions which depend on the relative orientation of the linear polarization of the beam with respect to the field. When they are orthogonal, the line pattern reflects the existence of a spin-dependent part of the polarizability induced by spin-orbit effects, the size of which was measured with respect to that of the scalar part. Results in very weak fields show that the oscillator strength of the 6S-7S transition is smaller than 2 x 10-13.