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17th Congress of the International Commission for Optics: Optics for Science and New Technology

Superslow optical relaxation induced by FM laser field in I2

H.-R. Xia

Published Sep 1, 1996• 0 citations
Publisher

Abstract

The approaches to realize superslow relaxation receive growing attention for its correspondence to sharp optical resonance in frequency domain and to long term of information storage in time domain. Dephasing time as long as 2.6 mS was observed in the condensed matter of crystal under the temperature of 1.4K [1]. In RF ion trap Yb ion has been kept in its metastable excited state for 8 days [2]. Population trapping is one of the interested approaches recently. While most papers discussed it for Λ- or V-type three-level systems, Agarwarl et al has theoretically predicted the possibility of population trapping been established by frequency modulated laser field in two-level system [3]. The authors has demonstrated population trapping in high-lying state doublet and in intermediate state doublet via quantum interferences between two sum-frequency two-photon excitations in Y- and in rhomb-type four-level systems respectively [4].

DOI
10.1117/12.2299048

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The approaches to realize superslow relaxation receive growing attention for its correspondence to sharp optical resonance in frequency domain and to long term of information storage in time domain. Dephasing time as long as 2.6 mS was observed in the condensed matter of crystal under the temperature of 1.4K [1]. In RF ion trap Yb ion has been kept in its metastable excited state for 8 days [2]. Population trapping is one of the interested approaches recently. While most papers discussed it for Λ- or V-type three-level systems, Agarwarl et al has theoretically predicted the possibility of population trapping been established by frequency modulated laser field in two-level system [3]. The authors has demonstrated population trapping in high-lying state doublet and in intermediate state doublet via quantum interferences between two sum-frequency two-photon excitations in Y- and in rhomb-type four-level systems respectively [4].
Resonant laser excitation of Yb ions confined in an rf ion trap is found to result in perfect population trapping in the highly metastable 2F7/2^{2}\mathrm{F}_{7/2}2F7/2​ state. The lifetime of this level is measured to be greater than 8 days. Population and depopulation of this level under narrow- and broad-band laser excitation conditions is investigated and its potentiality for ultimate-resolution laser spectroscopy indicated.
Ion traps are of current interest for high precision measurements of atomic splitting frequencies and they are widely discussed as new frequency standards [1, 2]. Recently they have also successfully been used to perform quantum jump experiments [3, 4]. The ion storage technique can successfully be applied to observe ions for long periods without perturbations and thus to eliminate transit time broadening and collisional effects. We were interested to make high resolution level splitting measurements with Yb ions which are proposed as a proper candidate in ion trap frequency standards [1]. When resonantly exciting the ions to the 6p2P1/2-state we measured a rapidly decreasing fluorescence signal. In order to explain this phenomenon we have investigated the excitation scheme of Yb+ in more detail (see Fig. 1).
In this paper we report the first observation of molecular population trapping in four level systems. Constructive and destructive quantum interferences between two sum-frequncy two-photon transitions in Y- and rhomb-type four-level systems, respectively, im sodium molecules have been experimentally achieved by using only one laser source. Their energy level schemes are featured by the extremely near-resonant enhancement of the equal-frequency two-photon transitions, sharing both the initial and the intermediate levels for the Y-type, and sharing both the initial and the final levels for the rhomb-type systems. Their novel spectral effects are to show seriously restrained Doppler-free UV peak at the nominal location of the induced two-photon transition with visible fluorescence in rhomb-type schems, and to show a strong extra UV peak but null visible fluorescence in the middle between the two dipole allowed two-photon transitions.
We report the first observation of molecular population trapping in a pair of coupled upper or coupled intermediate levels in Y-type or rhomb-type four-level systems, respectively. The demonstrations were done by means of equal-frequency two-photon excitations at about 587 and 640 run in sodium dimers. The mentioned coupled levels consist of a pair of wavefunction mixing levels due to spin-orbital perturbation with typical separation of a few GHz. The characteristic of the molecular trapping in a Y- type system was a strong extra UV excitation fluorescence peak against null visible emission in the middle between the two allowed two-photon absorption peaks. On the contrast, a pair of singlet-triplet mixing intermediate levels in a rhomb-type four-level system might provide significant destructive quantum interference for a two- photon transition, depending on their relative offsets. The characteristic of molecular population trapping in a rhomb-type four- level system was only two pieces of residual signals observable al the sides after completely eliminated of the center part of the two-photon lineshape.
We present a theoretical analysis of high-frequency optical heterodyne saturation spectroscopy in Doppler-broadened coupled three-level systems. The saturating beam, which is assumed to be amplitude modulated at frequency δ (double-sideband suppressed carrier amplitude modulation) is resonant for one of the transitions. Through non-degenerate four-wave mixing processes, this modulation is transferred to a probe beam, resonant for the coupled transition. Saturating and probe fields may be either co-propagating or counter-propagating. The lineshape of the probe modulation is analysed via a third-order perturbation expansion of the atomic density matrix with respect to the incident field amplitudes. The resulting signal is integrated over velocities, in the Doppler limit approximation. Both population saturation effects and coherent (Raman-type, or two-photon) processes contribute to the signal. The various contributions appear as Lorentzian-type resonance doublets. We show that, in the absence of relaxation processes (collisional dephasing, or radiative cascades), destructive interferences between population saturation and coherent two-photon processes are responsible for the disappearance of one resonance doublet. Phase-interrupting collisions are thus predicted to lead to the existence of a « pressure-induced extra-resonance » (PIER) doublet, which could yield information on collisional processes in the impact regime. The properties of the predicted PIER doublets are analysed in relation with other types of PIER signals studied in four-wave mixing, and in time-resolved saturation spectroscopy.