Cw Dye Laser Beam Research Articles

Nonlinear ring structures in forward scattering of resonant monochromatic laser radiation.

We present experimental and theoretical investigations of a Gaussian laser beam interacting with Zeeman-tuned J=1\ensuremath{\rightarrow}J=0 atomic systems. In our experiment a single-mode cw dye-laser beam traverses a neon discharge in a longitudinal magnetic field. The excited neon transitions used for these studies are 1${\mathit{s}}_{4}$(J=1,g=1.464)\ensuremath{\rightarrow}2${\mathit{p}}_{3}$(J=0) and 1${\mathit{s}}_{2}$(J=1,g=1.034)\ensuremath{\rightarrow}2${\mathit{p}}_{1}$(J=0). In the laser-induced forward-scattered light, dark and bright ring-shaped structures are observed. The ring structures are investigated as a function of the magnetic detuning of the m=\ifmmode\pm\else\textpm\fi{}1 sublevels and as a function of the laser-beam power. We find that the line shapes of the spectra, i.e., the forward-scattered light intensities as a function of the magnetic detuning, are sensitive to the transverse intensity distribution of the laser beam. Analytical expressions based on a semiclassical theory are derived and the calculated results on the ring structures and the spectra are compared with the experimental results. Good agreement between the calculated and the experimental results are obtained only when the transverse effects are taken into account.

Electron capture by doubly charged ions from laser-excited alkali atoms: II. He2+-Li*(2p) collisions

For pt. I, see ibid., vol. 26, p. 2137-51 (1993b). Single electron capture in collisions of slow (keV amu-1) He2+ ions with aligned Li*(2p) atoms has been investigated by means of translational energy spectroscopy. A single mode cw dye laser beam has been modified by using an electro-optical modulator in order to pump both hyperfine ground states of Li simultaneously, thus maximizing the fraction of excited target atoms. Cross sections for electron capture from Li(2s), Li*(2p Sigma ) and Li*(2p Pi ) initial states into He+(n = 3 4, 5) final states have been determined for different impact energies and compared with 68-state atomic orbital (AO) close coupling calculations.

Squeezing experiments in sodium vapor

We describe our efforts to observe squeezing, and related nonclassical effects, in three forward four-wave mixing experiments using sodium vapor as a nonlinear medium. In a single-beam experiment with homodyne detection, we found optically phase-sensitive noise, which was radio-frequency phase insensitive, in a cw dye-laser beam that had propagated through sodium vapor. The minima of the phase-sensitive noise were always at or above the coherent-state value. In our forward four-wave mixing experiment with probe–conjugate-beam direct detection and correlation, we found that the sodium-vapor interaction produced positive noise correlation between the probe and conjugate beams. The correlation, however, did not exceed the excess noises on these beams, so that a nonclassical behavior was not demonstrated. Finally, our forward four-wave mixing experiment with probe–conjugate-beam combination and homodyne detection did demonstrate the generation of squeezed-state light in our setup. Optically phase-sensitive noise with a minimum falling 4% below the coherent-state level was observed.

A redetermination of the lifetime of the 4s 5s ( 3S 1) level in Ca by high-rate electron—photon stepwise pulse excitation

The radiative lifetime of the 4 s 5 s ( 3 S 1) level in Ca was measured using two-step excitation of atoms in a beam which traverses a cold-cathode Penning discharge, where, by electron impact the atoms are excited to the metastable 4 s 5 p ( 3 P 2, 1, 0) states. Immediately thereafter, the beam is crossed with a pulse-modulated cw dye laser beam to populate the final state selectively. Using a pulse-modulation frequency of 0.5 MHz, a preliminary value of 12.4 ± 0.5 ns was obtained with a statistical error of 0.4%. The wide limits of systematic error reflect an assumed uncertainty in the excitation function and possible distortion in the fluorescence decay requiring further study.

Accurate (0.25 mrad) perpendicular alignment of a continuous-wave single-mode dye laser beam and an atomic beam

Two simple methods are described for an accurate perpendicular alignment of a laser beam and an atomic beam, which is required for a velocity independent, i.e., Doppler-free interaction, of the laser beam with the atomic beam. With the first method a free running single-mode cw dye laser beam and an atomic beam are aligned perpendicular with an accuracy of 0.25 mrad. This alignment takes typically 2 h. The second method needs a laser beam that is absolutely stabilized to the investigated atomic transition. With this method a 0.2-mrad accurate perpendicular alignment is obtained within only 5 min.

A hollow cathode arc in neon: Simultaneous laser probing and molecular-beam sampling of metastable atoms as a plasma diagnostic

The mean free path of metastable rare-gas atoms in a plasma can be determined by simultaneous sampling of a beam of metastable atoms through an orifice in the wall and optically pumping of the metastable atoms in the plasma with a cw dye laser beam perpendicular to the atomic beam axis. This method gives very accurate results for the absolute value and the velocity dependence (using a time-of-flight detection technique) of this mean free path. With a simple model the contributions of collisions with electrons and with heavy particles to the experimental mean free path can be determined from the velocity dependence. We have applied this method to the external plasma column of a hollow cathode arc in neon. The experimentally determined mean free path of 2–10 mm points unambiguously to a surprisingly low degree of ionization of 2% or less. At these conditions the plasma column is fully transparent for the background atoms in the vacuum chamber. This all means that the character of the plasma of a hollow cathode arc in neon is totally different from argon and helium, which is probably caused by the isotope composition of neon.

Copper Vapor Laser Pumped Dye Amplifier of a CW Dye Laser

The characteristics of a pulsed dye amplifier of a cw dye laser beam pumped by a copper vapor laser were investigated. The results show that the system is an efficient tunable source for laser spectroscopy with high average power, good spectral purity and excellent frequency tunability. Using Rh6G, RhB and DCM, the two-stage amplifier covered the spectral range of 575–670 nm with a second-stage conversion efficiency greater than 7%.

Natural radiative lifetimes in the interacting5sndD21,3sequences in Sr

Natural radiative lifetimes have been determined in the $5snd^{1,3}D_{2}$ sequences of strontium in the perturbed region around $n=15$. Stepwise laser excitations and pulse modulation of a cw dye-laser beam were employed. A plot of the measured lifetime values versus the effective quantum number reveals complex configuration interaction.

Hyperfine structure of the [formula omitted] state of 27Al

The hyperfine structure of the 4s 2S 1 2 state of 27Al was measured using high-resolution laser spectroscopy on a collimated atomic beam. The investigated state was populated using a UV CW dye-laser beam. For the magnetic-dipole interaction constant the result a(4 s 2 S 1 2 ) = 421(15) MHz was obtained.

Natural radiative lifetimes in the perturbed 6snd 1 D 2 sequence of barium

Natural radiative lifetimes in the perturbed 6s n s1D2 (n=12−30) sequence of barium have been measured using the PUMOLS (PUlse-MOdulated Laser Spectroscopy) method, which incorporates pulse-modulation of a CW dye-laser beam and delayed coincidence techniques. Principal perturbers are the 5d 7d3F2 and1D2 states. Lifetimes were also measured for these shortlived valence states, which through configuration mixing cause drastic decreases in lifetime values in the region of perturbation of the 6s n d1D2 sequence. Two-step laser excitation from the ground 6s21S0 state via the 6s 6p1P1 state was employed. Special care was taken to determine the influence of black-body radiation transitions on the measured lifetimes.

Focusing and defocusing of neutral atomic beams using resonance-radiation pressure

We demonstrate strong focusing and defocusing of a sodium atomic beam using the transverse resonance-radiation pressure of a superimposed cw dye laser beam tuned near the atomic resonance. Focal spot diameters of about 60 μm and a 30-fold increase of the on-axis atomic beam intensity have been obtained. For defocusing, the on-axis intensity can be reduced to less than 10−2 of its original value.

Dynamic response of a cw laser-produced Cs plasma to laser modulations

We present a first experimental investigation of the response of a continuous laser-produced Cs plasma to weak modulation of the laser beam. The cw dye laser beam at 6010.49 Å, of ∼0.1 W power, excites the Cs (6P1/2→8D3/2) atomic transition in the present study. At low modulation frequency, the fractional modulation of the transmitted laser beam is greatly reduced from that of the incident beam. At high modulation frequency, the two fractional modulations are nearly equal. We also demonstrate that the normal amplitude fluctuation of the laser beam is greatly reduced after transmission through the plasma. These features of the dynamic response are explained in a simple rate equation analysis.

Lifetime measurements for excited states of rare-earth atoms using pulse modulation of a cw dye-laser beam

By pulse-modulating a cw dye-laser beam and employing the delayed-coincidence technique, we have determined the natural radiative lifetimes of a number of atomic states in rare-earth atoms Eu, Dy, and Yb. In the measurements, which were performed on an atomic beam, exponential decay curves with good statistics were obtained using short measuring times. The general features of the technique are discussed. When possible, oscillator strengths are deduced and compared with literature values.

Strong amplification of sidebands in self-focused laser beams in an atomic vapor

A cw dye-laser beam tuned to the high-frequency wing of the Na ${D}_{2}$ line (at 2-6 GHz from the center of the ${D}_{2}$ line) is focused into a Na vapor, forming a self-focused laser filament. The incident laser beam, which is mainly single frequency, contains very weak sidebands at about \ifmmode\pm\else\textpm\fi{} 1 GHz from the main frequency and of intensity \ensuremath{\sim} ${10}^{\ensuremath{-}3}$ of the main frequency. Such sidebands are found to be strongly amplified by the vapor. Amplification factors in excess of 100 have been observed at suitable Na density, laser power, and detuning. Such strong sideband amplification, previously unknown, is explained as an off-resonant stimulated Raman scattering process.

Preferential excitation transfer in Cs ∗(6D [formula omitted])- Cs(6S [formula omitted]) collisions

A cw dye laser beam, tuned near the Cs 8761 Å resonance (Cs ∗(6P 1 2 ) → Cs ∗(6D 3 2 ) transition) is focused into a Cs vapor. At a Cs density higher than 5 × 10 15 cm -3, we observe a greater 6P 1 2 population when the laser is on resonance than when it is off resonance. However, at a lower Cs density, the reverse is observed. This phenomenon is explained as due to the preferential excitation transfer process: Cs ∗(6D 3 2 ) + Cs(6S 1 2 ) → Cs ∗(6P) + Cs ∗(6P), and the corresponding cross secti on is estimated to be (1.5 +1.5 -0.7) × 10 -14 cm 2 by fitting the experimental results to an approximate rate-equation analysis.

An efficient method for measuring atomic and molecular lifetimes using a modulated or deflected CW dye laser beam

By pulse modulating or deflecting a CW dye laser beam, cascade-free lifetime determinations at high spectral resolution can be performed with the decayed-coincidence technique using measuring times of about 1 minute. The method is illustrated by lifetime measurements for the 7 2P 3 2 1 2 levels of cesium.

Transient behavior of optically excited vapor atoms near a solid interface as observed in evanescent wave emission

Using a cw dye laser beam normally incident on a glass-Na-vapor interface, fluorescence excitation spectra of the ${D}_{1}$ line are measured. By observing the fluorescence transmitted into the glass above the angle of total reflection, only evanescent light emitted within about a wavelength distance from the interface is observed. The measured spectra show a cutoff on the high-frequency side which is direct evidence for the transient behavior of the induced polarization of resonantly excited vapor atoms which have collided at the solid interface.

CW self-focusing and self-trapping of light in sodium vapor

We report steady-state self-focusing, self-trapping, and self-defocusing of a cw dye laser beam in sodium vapor for frequencies within several Doppler widths of the $D$-line resonance transitions. We measured the variation of the beam profile as the light propagates through the vapor. We observed a 20-mW beam self-trapped in a 12-cm-long filament having a half-power diameter of 70 \ensuremath{\mu}m.

A crossed beam experiment for the inelastic scattering of slow electrons by excited sodium atoms

Superelastic and inelastic cross section measurements of electrons scattered by excited sodium atoms in the 32P3/2 state are reported. A crossed beam apparatus is used with a tunable CW dye laser beam intersecting the atom and electron beam in the scattering region. Details of the experimental method are described, allowing the extension to heavy particle excited atom scattering. The dependence of the superelastic cross section on the polarization of the incident light is investigated experimentally.

Cw Self-Focusing and Self-Trapping of Light in Sodium Vapor

We report steady-state self-focusing, self-trapping, and self-defocusing of a cw dye laser beam in sodium vapor for frequencies within several Doppler widths of the $D$-line resonance transitions. We measured the variation of the beam profile as the light propagates through the vapor. We observed a 20-mW beam self-trapped in a 12-cm-long filament having a half-power diameter of 70 \ensuremath{\mu}m.