Atomic Sodium Vapor Research Articles

Observation of an ultraviolet diffuse band in the Na 2 molecule

We report the observation of an intense spontaneous emission of a Na 2 diffuse band in the 370–382 nm region. This emission band was observed in the forward direction along the propagating laser beam when the laser wavelength was tuned near the 3s-4d two-photon resonance transition of the sodium atom. Further, as the sodium atomic vapor density becomes higher than ∼10 15 cm -3, the UV diffuse band can be directly produced through multiphoton excitation of Na 2. The emission can be assigned to 3 1σ + u - X 1σ + g transition and is in good agreement with a recent absorption work of Schlejen et al. [J. Phys. B 20 (1987) 2691)]. The excitation functions, the dependences on the sodium vapor and laser power densities and the possible mechanisms for producing the diffuse band are discussed.

Stimulated emission from a quasienergy multiplet structure of sodium levels in a resonant optical field

Lasing was observed in a sodium atomic vapor near the absorption lines of the yellow doublet. Stimulated emission was achieved from quasilevels formed in a three-level system under the action of a resonant optical field. Calculations were made of the positions of the quasilevels formed under the action of the resonant field.

Behavior of sodium atomic vapor in optical cavities below the threshold for bistability.

A study is presented of the transmission characteristics of low-finesse ring and Fabry-Perot optical cavities containing sodium vapor and operated below the threshold for optical bistability. Novel features are observed in the transmission signals obtained as the laser light frequency is swept through resonance with a sodium D transition. The ring cavity transmission peaks display a broad absorption profile except for the peak which is tuned to the center of the profile which is almost completely transmitted. A similarly tuned cavity transmission peak in the Fabry-Perot \'etalon is split into three, with the central peak being very narrow. A theoretical description for each cavity is developed in terms of a \ensuremath{\Lambda}-type, three-state model for the atomic energy levels associated with the transition. Using parameter values based on the experimental conditions, theoretical transmission signals for the linear cavity are obtained which are in good detailed agreement with the experimental results. For the ring cavity, agreement is also good except for the degree of absorption of the central peak.

Studies of picosecond collisional dephasing in atomic sodium vapor using broad-bandwidth transient four-wave mixing

We have employed dye-laser pulses of 10-nsec duration and 1.5–10-nm bandwidth in a transient four-wave-mixing experiment to study high-perturber-pressure collisional dephasing in dilute atomic sodium vapor. Working at argon perturber pressures of several atmospheres, we are able clearly to resolve dephasing times over a thousand times shorter than our excitation pulse duration. Measured dephasing rates are in excellent agreement with those measured by more traditional methods. When both of the sodium D lines are excited, our signal displays quantum-beat-like oscillations at the 0.52-THz frequency difference between these two states. We provide a simple picture explaining the origin of these beats.

Two-photon conical emission

A two-photon resonantly enhanced four-wave mixing (FWM) process leading to the conical emission of two new frequency components has been observed in atomic sodium vapor. A dye laser tuned close to the 3s → 3d two-photon allowed transition produces broad-band emission near the frequencies of the 3d → 3p and 3p → 3s transitions. This radiation is emitted in the forward direction in the form of cones surrounding the transmitted laser beam. The dependence of the cone angle on the emission wavelength and atomic number density is in excellent agreement with the predictions of a model that ascribes the origin of the conical emission to a phase-matched four-wave mixing process.

Intracavity resonant degenerate 4-wave mixing in atomic sodium vapor: Bistability in phase conjugation

We report on intracavity phase conjugation via resonant degenerate 4-wave mixing in atomic sodium vapor with the use of transverse optical Zeeman pumping as a versatile nonlinear mechanism. Under appropriate conditions the phase conjugate wave is found to display bistability, polarization switching and magnetically-induced selfoscillations. Our measurements are shown to be in fair agreement with theoretical predictions based on a simple analytic treatment for a homogeneously broadened coupled 4-level system.

Angled-beam photon echoes

We have observed the effect of angled excitation beams on the photon echoes from the 22S1/2–22P1/2 transition of atomic lithium. The degradation of the echo from noncollinear excitations is not so great as had been feared. A calculation with the billiard-ball echo model is in good agreement with our experimental results. In a similar experiment in atomic-sodium vapor we have recorded a decay of echo intensity with increasing pulse separation over nearly 12 orders of magnitude, with the weakest signals coming from atoms that had been more than 23 lifetimes in their excited state.

Pulse shortening in degenerate four wave mixing in atomic sodium vapor

Phase conjugation is observed in sodium vapor near the resonance lines using a monomode pulsed dye laser. The divergence of the beam strongly varies during the pulse. By combining the curvature of the pumping beams and the diameter of a pinhole, it is possible to generate pulses which are shorter than the incident pulse by a factor as large as 20.

Measurement of optical dephasing by double phase switching

We show that the optical dephasing time ${T}_{2}$ can be obtained from a measurement of the time delay ${T}_{0}$ between two shifts of $\ifmmode\pm\else\textpm\fi{}\frac{\ensuremath{\pi}}{2}$ in the phase of the exciting laser for which a null occurs in the jump in the nonlinear transmission through a sample. The phase shift is imposed upon a single-mode cw laser by applying a rectangular voltage pulse to an electro-optic phase modulator placed in the laser beam. The time delay at which the null occurs is proportional to ${T}_{2}$, which is given by the expression ${T}_{2}=(\frac{2}{\mathrm{ln}2}){T}_{0}$. The method is demonstrated in atomic sodium and iodine vapor and the separation of third-order optical transients from linear transients induced by the phase shift is discussed. An analytic expression for the jump in the nonlinear transmission, induced by the second phase shift, as well as for the transient in transmission following a single phase shift of a laser which weakly saturates an optically thin sample is derived. It is also shown that optical dephasing rates may be inferred from the variation of the integral of the transmitted light in a double-phase-switching experiment.

Infrared laser emission in atomic sodium vapor pumped by collision assisted laser excitation

A collision-assisted two-step two-photon pump mechanism for generating a population inversion among the 5 2S and 4 2P levels in atomic sodium is reported. By tuning a pulsed dye laser to the 3 2P to 5 2S resonances IR laser emission on the 3.415 μm (5 2S 1/2-4 2P 3/2) and on the 3.408 μm (5 2S 1/2-4 2P 1/2) lines is observed. The conversion efficiency is e stimated to be 5% and up to 5W IR peak power is observed in the present experimental setup.

Novel chemiluminescence detector for determination of volatile polyhalogenated hydrocarbons by gas chromatography

Based on the chemiluminescence reaction with atomic sodium vapour under reduced pressure, a novel gas-chromatographic chemiluminescence detector (CLD) for the determination of volatile polyhalogenated hydrocarbons (VPHHCs) has been developed; the characteristics of the CLD are compared with those of the electron-capture detector (ECD). The CLD constructed is simple and unique in that it exhibits high sensitivity and good selectivity towards VPHHCs; CCl 4 ClCH 2CH 2Cl, ClCH 2CHCl 2, ClCH = CCl 2, and Cl 2C = CCl 2 have low detection limits of the order of 10 -13 g/sec (in the picogram range) and their molar response ratios to compounds other than VPHHCs exceed 10 4. The CLD can compete in terms of sensitivity with the ECD and also surpasses the ECD with regard to linear dynamic range and selectivity. In addition, it is free from the drawbacks associated with the ECD. These detector characteristics allow us to expect that the CLD will be used in place of the ECD, depending on the kinds of VPHHCs which are to be determined.

Narrow-band optical filter through phase conjugation by nondegenerate four-wave mixing in sodium vapor

An ultrahigh-Q, tunable optical filter with a FWHM bandwidth of 41 MHz is demonstrated. The filtering is produced by nondegenerate phase conjugation through four-wave mixing in atomic-sodium vapor. The filter is observed to have a maximum quantum efficiency of 4 x 10(-3). However, degenerate phase-conjugation experiments in sodium suggest that a quantum efficiency of greater than unity can be attained on a cw basis.

Effect of saturation on the spectrum of degenerate four-wave mixing in atomic sodium vapor

The spectrum of degenerate four-wave mixing in Na vapor is shown to be purely dispersive for Rabi frequencies between the natural linewidth and the Doppler width. This result is qualitatively explained by the fact that absorption saturates much faster than dispersion.

A new echo phenomenon in laser induced population grating in gases — LIPG echo

We report the observation of a new type of echo produced by applying two intersecting light pulses with different angles (θ and θ′) at t = 0 and τ. The laser induced population grating (LIPG) created by the first excitation and degraded due to the thermal motion of atoms is reformed by the second excitation at t e= τ+mτ (αn−M) , where m and n are integers and α sin ( θ 2 )= sin ( θ′ 2 ) . The experiment was performed in the ground state 3 2 S 1 2 of atomic sodium vapor. This echo is not related to the atomic coherence but to the population distribution, and has novel feature for studying velocity-changing collisions.

Interaction of high-power laser pulses with atomic media. II. Optical second-harmonic generation

It is theoretically and experimentally demonstrated that optical second-harmonic generation (SHG) is possible to be observed in atomic media irradiated with an intense laser pulse. The SHG is induced by a static electric field spontaneously generated in the interaction, and any special condition is not required for the SHG. The experiment to observe the SHG has been made using atomic sodium vapor and a mode-locked Nd-doped yttrium aluminum garnet laser system (pulse duration, 28 psec; output energy \ensuremath{\lesssim}50 mJ). The second harmonic (SH) polarization, spatial beam pattern, and dependences of SH signal on the sodium density and on the fundamental intensity have been examined. The experimental results agree well with the theoretical predictions and numerical results.

Interaction of high-power laser pulses with atomic media. I. Spontaneous fields in multiphoton ionization

A theoretical study of static electromagnetic field generation due to laser radiation force in multiphoton ionization of atomic media is presented. The static fields are spontaneously induced in atomic media and have electric field components in the radial and axial directions of the incident laser beam and an azimuthal magnetic field component. Multiphoton ionization process, which is inevitably apt to occur in the interaction, significantly enhances the spontaneous fields. Numerical calculations of the fields generated in atomic sodium vapor and helium gas have been made, assuming an incident picosecond Nd-doped yttrium aluminum garnet laser pulse. The results show the characteristic natures and magnitude of the spontaneous fields.

Laser Induced Population Grating in Sodium Vapor

By using two intersecting resonant pulses from a laser, we produced Laser Induced Population Gratings in atomic sodium vapor, and with an aid of a delayed probe pulse, observed “Zeeman beats,” anomalous effects of Na–Ar collision and a new type of echo. These new phenomena can be interpreted in terms of the dynamics of LIPG's. The description of the Zeeman beat experiment is given in some details.

Angular dependence of line shape and strength of degenerate four-wave mixing in a Doppler-broadened system with optical pumping

Results of experiments investigating the effects of optical pumping and atomic motion on cw degenerate four-wave mixing in atomic sodium vapor are presented. The signal strength is found to decrease as (sin θ)−2, where θ is the angle between the object and pump waves; however, the linewidth for generation is Doppler free and is independent of θ. Agreement is found with a simple model of the mixing process. Distortions of the atomic velocity distribution (produced in our experiments by optical pumping) are found to be essential for efficient cw generation. A new modulation scheme for the discrimination against background is used.

Spontaneous-Field-Induced Optical Second-Harmonic Generation in Atomic Vapors

It is shown that optical second-harmonic generation is possible in dispersive atomic vapors since a static electric field is spontaneously created by the fundamental laser beam. When multiphoton ionization of atoms occurs simultaneously in the interaction, the spontaneous field and second-harmonic generation are significantly enhanced. Our experimental results for atomic sodium vapor irradiated by picosecond Nd-doped yttrium-aluminum-garnet laser pulses agree well with the theoretical predictions.

Intensity saturation and polarization rotation of magnetically induced second-harmonic radiation

We have observed saturation of the intensity of magnetically induced second-harmonic radiation in atomic sodium vapor and rotation of the polarization angle of the second-harmonic radiation from that of the fundamental field at the magnetic field of about 1 kG. These phenomena have been fully analyzed, and explained by the Hanle effect of the second-harmonic quadrupole moments observed in the forward direction. A possibility of measuring the transverse relaxation times of the E2 allowed transition is suggested.