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AIP conference proceedings

Four-wave mixing spectroscopy of state selective collisions in gases and solids

Juan Francisco Lam

Published Jan 1, 1988• 0 citations
Publisher

Abstract

The quantum evolution from a closed to an open system, induced by collisional processes, is presented in the framework of nearly degenerate four‐wave mixing (NDFWM) spectroscopy. We show that an open system manifests itself by the appearance of a subnatural linewidth in the spectrum of the phase conjugate signal. Examples are described for sodium vapor in the presence of buffer gases and in Nd+3 doped β‘‐Na‐Alumina for high concentration of Nd+3 ions.

DOI
10.1063/1.37359

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Nearly degenerate four-wave mixing (NDFWM) within a closed degenerate two-level atomic transition is theoretically and experimentally examined. Using the model presented by Lezama et al. [Phys. Rev. A 61, 013801 (2000)] the NDFWM spectra corresponding to different pump and probe polarization cases are calculated and discussed. The calculated spectra are compared to the observation of NDFWM within the ${6S}_{1/2}(F=4)\ensuremath{\rightarrow}{6P}_{3/2}(F=5)$ transition of cesium in a phase conjugation experiment using magneto-optically cooled atoms.

arXiv:quant-ph/0005096•Physical Review A•Dec 8, 2000•36 citations
The quantum evolution from a closed to an open system, induced by collisional processes, is presented in the framework of nearly degenerate four‐wave mixing (NDFWM) spectroscopy. We show that an open system manifests itself by the appearance of a subnatural linewidth in the spectrum of the phase conjugate signal. Examples are described for sodium vapor in the presence of buffer gases and in Nd+3 doped β‘‐Na‐Alumina for high concentration of Nd+3 ions.
We report the first observation of the nearly degenerate four-wave--mixing spectrum of Nd3+{\mathrm{Nd}}^{3+}Nd3+-doped \ensuremath{\beta}''-Na-alumina. In a new application of continuous-wave high-resolution nearly degenerate four-wave mixing, a linewidth analysis of \ensuremath{\sim}1-kHz-wide optical resonances in heavily doped samples has been used to explain concentration-dependent, nonradiative decay of Nd ions which is obscured in conventional fluorescence-decay studies. Quantitative four-wave--mixing saturation studies also reveal a transition between a ``closed'' and an ``open'' quantum system due to pair interactions at a Nd density of \ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}1020{10}^{20}1020 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$.
Optical four-wave mixing resonances in sodium vapor induced by collisions with inert buffer gases (He, Ar, Xe, N2{\mathrm{N}}_{2}N2​) are characterized at high resolution ( 5 MHz) using lasers tuned near the DDD lines of Na. These resonances are interpreted in terms of coherent scattering from a population grating established by collisional absorption of the incident radiation. The roles of homogeneous and inhomogeneous contributions to the resonance line shapes are clarified. The line shapes exhibit structure sharper than the predicted linewidth, that characteristic of the 3P3P3P-to-3S3S3S transition rate. We show that this is caused by collision-induced optical pumping which introduces the ground-state lifetime into the grating relaxation physics. These long-lived gratings are exemplified dramatically by experiments in the presence of N2{\mathrm{N}}_{2}N2​ which collisionally quenches the Na 3P3P3P states. Similar results are obtained in Xe where it is argued that excimer formation with the 3P3P3P states plays a role in the population dynamics. Model calculations of the effects of sodium level degeneracy on the grating dynamics are presented to support these conclusions. Finally we present data illustrating interference between the collision-induced population grating resonances detailed here and collision-induced coherent Raman resonances between Zeeman sublevels of the ground state.
The purpose of this research is to study the dynamic behavior of a resonant system using four-wave-mixing spectroscopy (FWM). From this study, we have obtained rich information about the FWM spectral response of systems which have different state-specific reservoir couplings. Atomic sodium (Na) is used as our model system. It is shown that the FWM spectral response depends strongly on how each level couples to the vacuum radiation field (through spontaneous emission). This effect is illustrated in the line shape obtained from a nearly degenerate FWM (NDFWM) experiment for a simple two-level system in Na. A narrow spike on top of a wide resonance is observed at zero pump-probe detuning (δ\deltaδ = 0) when the upper-state decay rate $\gamma\sb2$ to the reservoir is larger than the lower-state decay rate $\gamma\sb1$. The width of the spike is 2$\gamma\sb1$ and the width of the wide resonance is determined by the total upper-level decay rate (2$\gamma\sbsp{2}{\rm t}$). From the NDFWM spectrum, all the decay rates characterizing a two-level system may be measured directly. If magnetic substate degeneracy is included, the NDFWM line shape has contributions arising from orientation and alignment gratings, as well as from population gratings. For a two-level system closed with respect to population, the orientation or alignment gratings may contribute a narrow resonance at δ\deltaδ = 0 in the NDFWM response. These features are demonstrated for the first time by using NDFWM. Reservoir coupling can also occur through collisions with externally added gas. Collisions can cause a closed two-level system to become open, resulting in NDFWM spectra that show a narrow resonance at δ\deltaδ = 0 with a width below the residual Doppler width (Dicke narrowing). This is also the first time that Dicke narrowing has been observed in population terms. Finally, we present a calculation of the NDFWM response for strong degenerate pump fields and a weak probe field. We compare the differences between NDFWM spectra of closed two- and three-level systems in the presence of strong pumps.
Recently, we have shown that the nearly degenerate four-wave mixing (NDFWM) process can provide a direct and simultaneous measurement of the longitudinal (1/T1) and transverse (1/T2) relaxation rates of a resonant two-level atom. Furthermore, we observed that in the presence of buffer gases, the linewidth of the resonance describing the longitudinal relaxation process experiences a significant narrowing, belowo that of the natural width. Using a theoretical model, the narrowing was accounted for as arising from collisional effects in the ground-state population. 1 These observations were made for the case where the radiation fields are co-polarized, and we concluded that NDFWM offers a unique technique for the measurement of the ground-state velocity-changing cross section.2
Nearly degenerate four-wave mixing (NDFWM) within a closed degenerate two-level atomic transition is theoretically and experimentally examined. Using the model presented by Lezama et al. [Phys. Rev. A 61, 013801 (2000)] the NDFWM spectra corresponding to different pump and probe polarization cases are calculated and discussed. The calculated spectra are compared to the observation of NDFWM within the ${6S}_{1/2}(F=4)\ensuremath{\rightarrow}{6P}_{3/2}(F=5)$ transition of cesium in a phase conjugation experiment using magneto-optically cooled atoms.