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Chinese Physics Letters

Ultrafast Modulation Spectroscopy in Four-Level System

Zuhe Yu, Xin Mi, Qian Jiang, Xiaofeng Li, Panming Fu

Published Jun 1, 1998• 2 citations
Publisher

Abstract

Based on the interference between two two-photon processes, we reported a four-level ultrafast modulation spectroscopy in Na vapor, where 3S, 3P, 5S, and 4D formed a four-level system. We studied the temporal behavior of the four-wave mixing signal intensity and obtained signal modulation with period 25 fs, corresponding to the splitting between 5S and 4D. This technique can achieve Doppler-free precision in the measurement of the energy level splitting between two excited states which are dipolar forbidden from the ground state.

DOI
10.1088/0256-307x/15/6/012

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Based on the interference between two two-photon processes, we reported a four-level ultrafast modulation spectroscopy in Na vapor, where 3S, 3P, 5S, and 4D formed a four-level system. We studied the temporal behavior of the four-wave mixing signal intensity and obtained signal modulation with period 25 fs, corresponding to the splitting between 5S and 4D. This technique can achieve Doppler-free precision in the measurement of the energy level splitting between two excited states which are dipolar forbidden from the ground state.
Based on the polarization interference between one-photon and two-photon processes, we demonstrated an ultrafast modulation spectroscopy in a cascade three-level system (UMSCTS) in Na vapor, which showed a modulation of the four-wave mixing signal intensity with a 55-fs period, corresponding to the beating between the resonant frequencies of the transitions from 3S1/2{\mathit{S}}_{1/2}S1/2​ to 3P3/2{\mathit{P}}_{3/2}P3/2​ and from 3P3/2{\mathit{P}}_{3/2}P3/2​ to 4D3/2,5/2{\mathit{D}}_{3/2,5/2}D3/2,5/2​. We have also discussed the possibility of using UMSCTS to measure the energy levels of the excited states with Doppler-free precision.
We have studied theoretically a phase-conjugate ultrafast modulation spectroscopy (PCUMS) in a Doppler-broadened system. The pump beams consist of two frequency components which drive two different transitions. It is found that the four-wave-mixing (FWM) signal exhibits damping oscillation as the relative time delay between two pump beams is varied, from which the energy-level splitting can be deduced. We have considered the cases that pump beams have either narrow band or broadband linewidth and found that for both cases a Doppler-free precision in the measurement of the energy-level splitting can be achieved. The advantage of PCUMS is that the energy-level splitting between states can be widely separated. Finally, the relationship between PCUMS and other Doppler-free techniques in frequency and time domains has also been discussed.
We study theoretically ultrafast modulation spectroscopy in Doppler-broadened cascading three-level and four-level systems. The pump beams consist of two frequency components that drive two-pathway excitation. The four-wave mixing (FWM) signal exhibits damping oscillation as the relative time delay τ between the two pump beams is varied, from which the resonant frequencies between energy levels can be deduced. Furthermore, Doppler-free precision in the measurement of the resonant frequencies can be achieved when the pump beams have either narrow-band or broadband linewidths. We study two-photon resonant FWM induced by time-delayed correlated fluctuating fields in a Doppler-broadened cascading three-level system. It is related intrinsically to the sum-frequency trilevel echo when the pump beams have broadband linewidths. However, unlike the corresponding incoherent-light stimulated photon echo, the maximum of the FWM signal is shifted from τ≃0. If we consider the fluctuations of the pump beams, the FWM signal exhibits damping oscillation when the laser frequency is off resonant from the two-photon transition.
We have observed a four-wave mixing process in sodium vapor in which two of the four waves are resonantly enhanced by electric quadrupole transitions. The atom is pumped via a quadrupole transition from the 3s to the 4d state with a single photon at 2894 Å. Intense coherent emission at 3427 Å corresponding to the 3d-3s transition can be observed exiting the Na cell only in the direction parallel to the input beam. Also observed coming from the cell is intense coherent light at 2.34 µm corresponding to the 4d-4s transition in Na.
Photon-echo modulation spectroscopy is extended into a new regime to study coherences between states that are widely separated in frequency compared with the bandwidth of the exciting laser pulses. Time-delayed four-wave mixing (TDFWM) is observed on the Na DDD doublet by use of 7-nsec pulses of light spectrally peaked at each of the DDD-line transitions. As the two double-frequency, 7-nsec light pulses are delayed with respect to each other a 1.9-psec beating of the integrated TDFWM signal is observed, even for delays greater than the excitation pulse width.