Dense Sodium Vapor Research Articles

Light-induced atomic desorption for loading a sodium magneto-optical trap

We report studies of photon-stimulated desorption, also known as light-induced atomic desorption, of sodium atoms from a vacuum-cell glass surface used for loading a magneto-optical trap (MOT). Fluorescence detection was used to record the trapped atom number and the desorption rate. We observed a steep wavelength dependence of the desorption process above $2.6 \mathrm{eV}$ photon energy, a result significant for estimations of sodium vapor density in the lunar atmosphere. Our data fit well to a simple model for the loading of the MOT dependent only on the sodium desorption rate and residual gas density. Up to $3.7\ifmmode\times\else\texttimes\fi{}{10}^{7}$ Na atoms were confined under ultrahigh-vacuum conditions, creating promising loading conditions for a vapor-cell-based atomic Bose-Einstein condensate of sodium.

The near infrared (0.8−2.6µm) absorption spectrum of a dense sodium vapor and possible mechanisms of the spectrum formation

The absorption coefficient of a dense sodium vapor (N 0 ∼ 1017–1018 cm− 3) in the near infrared spectral range (0.8–2.6 µm) was measured in a homogeneously heated isolated cell. In the range of parameters studied, the sample exhibits significant absorption. Neither the observed spectral features nor the measured absorption coefficients can be explained using the existing notions of the possible absorption mechanisms (absorption due to a forbidden intercombination transition, collision-induced processes, the trimer vapor component, and many-particle effects) and the available data.

Infrared absorption in dense sodium vapor

The absorption spectra of a dense resonance medium were experimentally studied for the example of thermally heated dense sodium vapor. Several mechanisms that might cause substantial absorption and enhanced intensity of emission in the IR spectral region, λ τ; 0.9 μm, were considered. For the first time, a detailed study of the structure of the absorption spectra of sodium vapor in the specified wavelength range was performed to determine the influence of the kind and pressure of the buffer gas. It was found that buffer gas characteristics had a substantial effect on the absorption coefficient of vapor. The presence of the molecular component (dimers and trimers) in sodium vapor could not explain the experimental dependences of absorption in the infrared region. Possible influence of microparticles formed in condensation of convective sodium vapor flows in heated cells on the optical properties of vapor was considered. Microparticles could contribute to the observed absorption, but were incapable of explaining the substantial intensity of vapor radiation reported earlier. Possible many-particle effects on the absorption in the far spectral line wing were discussed. For the first time, the method of molecular dynamics was used to show for the example of the distribution function of ionic microfields in a dense plasma that such effects were in principle capable of substantially raising the profile of the line and increasing absorption in the region of large detunings from the resonance compared with the simple quasi-static model in the nearest-neighbor approximation.

Coherent population trapping studied through energy transfer and energy-pooling collisions

The coherent population trapping phenomenon in sodium is probed through energy transfer and energy pooling collisions. Quenching of the excited state population (dark resonance) is measured through the detection of the collisional induced fluorescence up to sodium densities as large as 10 15 cm −3. Two dark resonances have been studied and a different dependence on the sodium vapor density has been observed. For the first time an enhancement of the upper level population and, hence, of the collisional energy transfer processes has been measured correlated to the double coherent excitation.

Excitation of sodium atoms by 330-nm laser pulses

) atoms in a dense sodium vapour irradiated by nanosecond laser pulses tuned near the 3S→4P transition was investigated. It was observed that the population of Na(4P) atoms remained high only within the laser pulse, in spite of the relatively long lifetime of the 4P level (110.ns)The 3P1/2 level, which is populated as a result of cascade transitions from the higher levels, reached the highest population several nanoseconds after the laser shot. The fast population changes are explained by cascade-stimulated transitions between the excited atomic levels.

Spectral and spatial evolution of a conical emission in Na vapor

A detailed study on yellow conical emission observed in dense sodium vapor following near-resonant one-photon excitation of the Na 42P levels was performed. The origins of conical emission are suggested to be due to Raman resonant four-wave parametric mixing processes. In order to describe spectral and spatial features of off-axis emission, we introduce self-phase modulation and induced-phase modulation as important additional nonlinear processes, explaining the nonmonotonic detuning of the wavelength of the conical radiation with the exiting laser wavelength in the vicinity of resonance transitions. The spatial evolution of emission generated via four-wave mixing is modeled by solving the two-dimensional nonlinear Schrodinger equation describing the spatial evolution of light beams in a nonlinear media. Experimental and theoretical results are presented and discussed.

Distortion-free gain and noise correlation in sodium vapor with four-wave mixing and coherent population trapping

We observed optical gain as great as 30 with nearly distortion-free beam propagation in optically dense sodium vapor, using four-wave mixing. Moreover, 15-dB classical noise correlations were seen in the amplified probe and conjugate beams. To achieve this performance in such a strongly absorbing medium, one must suppress unwanted absorption and self-focusing effects. This is accomplished with coherent population trapping.

Nonlinear emission in sodium vapour upon pulsed-laser excitation of the 42 P levels

Parametric four-wave mixing, stimulated electronic Raman scattering and amplified spontaneous emission cascades have been studied experimentally by single-photon resonant and near-resonant excitation of the 42P levels of dense sodium vapour. The dependence of emission intensities on laser detuning and exciting laser power is discussed.

Role of radiation trapping in degenerate four-wave-mixing experiments.

The process of degenerate four-wave mixing is studied experimentally in dense sodium vapor in a rare-gas atmosphere. The influence of trapped fluorescence light on the ground-state orientation is shown to be responsible for the observed strong reduction of saturation phenomena with increased sodium density. The interpretation is based on a simple model and is supported by results obtained by suppressing the fluorescence.

Diffusion of resonance radiation in optically saturated sodium vapour

The Holstein-Biberman equation describing the evolution of the excited atom population in a dense sodium vapour optically saturated by a short resonant light pulse was solved numerically. Results were compared with the solutions for a weakly excited vapour. It was found that in a strongly excited medium the decay of the Na(3P) atoms is faster than in the case of the weak excitation. A subnatural decay of the resonance fluorescence signal for some experimental geometries was predicted.

Resonantly enhanced two-photon excitation of sodium atoms in a heat pipe

We report the results of a study of two-color laser excitation, ω1 + ω2, in dense sodium vapor where ω1 was tuned in the vicinity of the 3P fine structure doublet and ω2 was tuned to highernl states. Resonant excitation by ω2 of higherns andnd states from one member of the 3P doublet when ω1 is tuned near the other is attributed to collisional excitation transfer. Anomalous dipole forbiddenp-p transitions are observed when ω2 is tuned to highnP states (≥20) and are believed to be DC electric field induced. Sample spectra for the processes considered are calculated and compared with experiment.

Sodium vapor picosecond optical parametric oscillator

A four-photon parametric light oscillation in sodium vapor within optical cavity enhanced by the resonant third order nonlinearity has been investigated. For synchronous bichromatic pumping of the 3D sodium level near-bandwidth-limited picosecond light pulses were used. Dependences of the main parameters of the optical parametric oscillator (OPO) on the cavity length and sodium vapor density were determined.

Na2 Near Infrared and Violet Excimer Emissions by Electron Beam Excitation.

A near infrared broadband excimer emission band and an amplified sponaneous emission (ASE) of the Na2 violet excimer band have been observed by electron beam excitation of a dense sodium vapor. The continuum from 820 to 900 nm is attributed to the Na2 first bound-free transition band c3Σ+g→ a3Σ+u and the violet excimer band centered at 436 nm is attributed to the Na2 23IIg→ a3Σ+u bound-free emission which was already observed in the optical pumping. The mechanisms for the c3Σ+g state formation are proposed to be the three-body collision recombination and the atom-molecule collisional energy transfer. The formation of the 23IIg state is proposed by a dissociative recombination of Na3+ which is efficiently yielded by electron beam. In addition, a discrete structure band observed on the long wavelength side of the violet bands is believed from the Na2 21Σu+→ X1Σg+ bound-bound transitions.

Ionization of sodium vapour by nanosecond resonant laser pulses tuned to 3S → 4P transition

We present a model of dense sodium vapour ionization induced by nanosecond resonant laser pulses exciting the 3S → 4P transition. The results of calculations are compared with those obtained by means of a similar model for the case of 3S → 3P excitation. The possibility of population inversion formation between excited sodium levels is also discussed.

Two-photon excitation of dense sodium vapor near the n d 2D5/2, 3/2 (n=3, 4, 5) levels: Na2 1 3Σ+g→1 3Σ+u excimer emission

Laser excitation and ionization processes in dense (1–10 Torr) sodium vapor have been studied for laser wavelengths near the two-photon allowed nd 2D(n=3, 4, 5) and ns 2S(n=4, 5, 6) states. In particular, the 1 3Σ+g→1 3Σ+u excimer emission in Na2, predicted in 1980 by Konowalow and Julienne and observed recently by Dinev et al, was studied here in greater detail. Strong excimer emission (∼830 nm) was observed for two-photon pumping to both sides of the unresolved 4D states, and weak excimer emission was seen when pumping near the 5D levels. The excimer emission exhibits a complicated pump laser profile with a pronounced ‘‘dip’’ at the 4d 2D two-photon resonance. Similarly, [2+1] photon ionization via the 3d 2D and 4d 2D states shows a dramatic decrease as the sodium density increases. These results can be attributed either to depleted 3d 2D or 4d 2D population due to stimulated electronic Raman scattering (SERS) or to the interference effects recently reported by Malcuit et al. and Krasnikov et al. and treated theoretically by Manykin and Afanas’ev and by Agarwal. It is argued that both mechanisms are operative. Strong ionization and SERS signals were observed at the hybrid resonances corresponding to 3p 2P3/2, 1/2 →4d 2D transitions; however, no excimer lasing at 830 nm was detected. No excimer emission was detected upon two-photon pumping near or at the 3d 2D or ns 2S(n=4, 5, 6) states. Based on these and other observations, the 1 3Σ+g→1 3Σ+u excimer emission is attributed to a molecular Raman process involving stimulated emission or six-wave mixing via a pathway of the type 1 3Σ+u→2hν 3Δu→ j 3Πg→(k 3Πu, l 3Σ +u)→1 3Σ+g→1 3Σ +u .

Spectral and energy characteristics of four-photon parametric scattering in sodium vapor

An analysis was made of four-photon interaction processes under conditions of two-photon resonance excitation of the 3D sodium level by two-frequency radiation from a single-pulse picosecond YAG:Nd laser with frequency doubling and a KDP optical parametric oscillator. The spatial and frequency spectra of the four-photon parametric scattering (FPPS) were recorded and studied experimentally at various sodium vapor densities (1015–1017 cm −3) and also using collinear and noncollinear excitation. It was found that the observed conical structure of the FPPS radiation may be interpreted by an analysis of whether the frequency and spatial phase-matching conditions were satisfied. Dependences of the intensity of the FPPS radiation on the intensity of the exciting radiation, the sodium vapor density, and the detuning of the exciting radiation frequency from the two-photon resonance were obtained.

Anomalous blue shifted emission near the D1transition from laser-excited sodium vapour

The observations of anomalous emission near the D1 transition of a dense (>or=1014 cm-3) sodium vapour illuminated by a laser detuned to the blue with respect to the D2 transition is studied and reported. A discussion of existing models is presented. The authors propose a type of parametric down-conversion process to explain this emission. This process involves a laser photon splitting into two photons by the three-level system of sodium, assisted by a magnetic dipole transition.

The absorption and emission observations of the sodium near-infrared spectrum

The absorption and emission spectra of dense sodium vapour have been measured between 790 nm and 1000 nm. In the emission spectrum from the high-pressure sodium lamp we observed the characteristic continuum structure at 882 nm which we interpreted as a satellite band originating from the 1 3Σ + g−1 3Σ + u transition. The complementary absorption measurements in very dense sodium vapour revealed only certain structures near the head of heads of the A–X molecular band, but not a continuous band at 882 nm.

Production of dense, cool plasmas by resonance pumping of sodium vapor.

High-electron-density, low-temperature plasmas have been produced with a minimum of 1 electron per Debye sphere by high-power, short-pulse resonant laser pumping of dense (${10}^{16}$--4\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) sodium vapor. The recombining plasmas were diagnosed by time-resolved emission spectroscopy. Electron densities between 0.5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ and 2.2\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ were inferred from widths and shifts of allowed lines and intensities of forbidden components. Electron temperatures between 0.2 and 0.3 eV were calculated from relative line intensities, since all states above 3d were shown to be in partial local thermodynamic equilibrium. Such low temperatures coupled with other experimental evidence suggested that laser-induced associative ionization involving atoms in the 3p state was the dominant ionization mechanism.

Effect of Na–Na spin-exchange collisions on spin relaxation of optically pumped Na in Xe gas

We report the first detection of the effect of Na–Na spin exchange collisions on spin relaxation of optically pumped Na in Xe gas by studying the relaxation in the "dark" of the variable ⟨Sz⟩ in a mixture of dense sodium vapor (number density [Na] ~ 1013 cm−3), and buffer gas helium, xenon and nitrogen as a function of [Na] and xenon pressure.