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AIP conference proceedings

Intensity dependent phase shifts in frequency modulation spectroscopy

H.-R. Xia, J. I. Cirac, Stephen D. Swartz, Bern Kohler, Daniel S. Elliott, J. L. Hall, P. Zoller

Published Jan 1, 1993• 1 citation
Publisher

Abstract

We discuss experimental observations of an induced phase shift of the rf signal produced in FM saturated absorption measurements under the conditions of large probe beam intensities. This phase shift can be understood in terms of the phase difference between the phase modulation of the optical driving field and the steady state macroscopic polarization of the medium. This phase difference is relatively insensitive to detuning of the laser from the atomic resonance, such that essentially the same spectrum can be observed by changing the rf detection phase. We also present theoretically calculated spectra which are based on the optical Bloch equations, and which are in excellent agreement with the experimental results.

DOI
10.1063/1.45062

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We discuss experimental observations of an induced phase shift of the rf signal produced in FM saturated absorption measurements under the conditions of large probe beam intensities. This phase shift can be understood in terms of the phase difference between the phase modulation of the optical driving field and the steady state macroscopic polarization of the medium. This phase difference is relatively insensitive to detuning of the laser from the atomic resonance, such that essentially the same spectrum can be observed by changing the rf detection phase. We also present theoretically calculated spectra which are based on the optical Bloch equations, and which are in excellent agreement with the experimental results.
We discuss experimental observations of an induced phase shift of the rf signal produced in frequency-modulated saturated-absorption measurements under the conditions of large probe-beam intensities. This phase shift can be understood in terms of the phase difference between the phase modulation of the optical driving field and the steady-state macroscopic polarization of the medium. We also present theoretically calculated spectra that are based on the optical Bloch equations and that are in excellent agreement with the experimental results.
We Study a modification of classical FM spectroscopy in the cases where several electromagnetic fields are FM modulated, each in a different manner. This complex spectrum scans a multi-photon resonant atomic medium with the output detected by a phase-sensitive scheme. The demodulated output signal reveals the spectroscopic features of the probed medium. The case in which two different carriers are FM modulated at the same frequency and index but with an opposite phase with respect to each other is analyzed theoretically. This configuration is essential for probing Coherent Population Trapping (CPT) resonances induced by a directly modulated diode laser. Employing a macroscopic model to describe the physical properties of CPT leads to a superb fit between predicted and measured CPT characteristics.
We report the first observation of the geometric phase of a quantum system driven by laser fields. The observable geometric phase effects in optical FM spectroscopy, here distinguished from various known lineshape changes due to spectroscopic or technical causes, are characterized by the following aspects: (1) Dramatic lineshape variation of the optical heterodyne signal due to a monotonic signal phase change with increasing probe power; (2) The lineshape changes are associated with a nearly pure phase-change effect; and (3) The phase-change rate with probe power depends on rf sideband frequency, and less strongly on the rf modulation index.
A spatial dependence of the demodulated signal over the cross-section of a laser beam used for saturation spectroscopy has been observed. If the detector is not exposed to the full beam this spatial dependence causes a distortion of the line-shape. In extreme cases this distortion can lead to frequency shifts as large as 15% of the full width of the resonance. A model is presented in which the self-focusing effect acts to separate spatially the different frequency components of the modulated laser beam and thus allows for a spatial inhomogeneity of the demodulated signal. Numerical evaluation of the model agrees well with the experimental data provided a strong enhancement of the beam convergence in the absorption medium is applied.
The interaction of a strong bichromatic field with a two-level atom has been the subject of numerous experimental and theoretical investigations. We performed an experimental study of the Interaction of an intense 100% amplitude-modulated (AM) field with an ensemble of homogeneously broadened two-level atoms. An optically pumped sodium atomic beam is used as the atomic medium. The field is created by interfering two intense frequency-stabilized laser fields. The frequency of one field is held fixed while another field of equal amplitude is scanned in frequency. The beat frequency produced by interfering the two laser fields is equal to twice the modulation frequency of a 100% AM field. We monitor the time-averaged fluorescent intensity of the atoms as the frequency separation between the two fields is varied. A similar experiment was done in the past in the rf regime.1 For the first time, we report the observation of multiple, well-resolved resonances in the absorption spectrum of the field in the optical regime. We present data and theory for several values of detuning between the fixed field and atomic resonance. The experimental results are in good agreement with theory.