Frequency Of Laser Beam Research Articles

Stark-effect study of excited states in sodium using two-photon spectroscopy

We have used two-photon absorption from counterpropagating laser beams of different frequencies to measure Stark shifts and splittings of the $6^{2}S$, $7^{2}S$, $8^{2}S$, $5^{2}D$, and $6^{2}D$ states in sodium. Beams from a fixed-frequency, single-mode ${\mathrm{Ar}}^{+}$ laser and a tunable, single-frequency cw dye laser were focussed into a coarse sodium atomic beam and the fluorescence was monitored. The collimation of the atomic beam reduced the residual Doppler width inherent in using two different frequencies. Using the two lasers allowed two-photon transitions to be observed at frequencies unattainable with the dye laser alone, and enhanced the transition rates due to the near resonance of the dye laser with the strong Na $3^{2}S\ensuremath{\rightarrow}3^{2}P$ transition. Our experimental results for the scalar and tensor polarizabilities, ${\ensuremath{\alpha}}_{0}$ and ${\ensuremath{\alpha}}_{2}$, agree with calculations in the Bates and Damgaard Coulomb approximation within 2% for ${\ensuremath{\alpha}}_{0}$ and 9% for ${\ensuremath{\alpha}}_{2}$. As a by-product the hyperfine-interaction constants $a$ for the $S$ states and the fine-structure intervals $\ensuremath{\delta}\ensuremath{\nu}=\frac{({E}_{\frac{3}{2}}\ensuremath{-}{E}_{\frac{5}{2}})}{h}$ for the $D$ states were obtained.

An analytical investigation of the optical mixing of two laser beams of identical frequencies in a spatially homogeneous collisionless plasma

Two laser beams of the same frequency and intensity propagating in opposite directions are mixed in a uniform collisionless plasma.A standing longitudinal wave of wave number k0 = 2kL (kL being the wave number of one of the lasers) is excited and traps charged particles. Including trapped particle dynamics and making use of the properties of the Jacobi elliptic functions, an expression for the power P(t) and the energy. ΔU0 absorbed by the plasma electrons is derived. It is found that the power varies sinusoidally with time. Then the energy density ΔU0 is plotted as a function of the variable E10/kL, E10 being the electric field intensity of the lasers, and the validity of the results is discussed.

Direct Observation of Atomic Energy Level Shifts in Two-Photon Absorption.

We report direct observation of optically induced shifts of atomic energy levels in sodium vapor that occur in two-photon absorption. These shifts must be considered in high-resolution two-photon spectroscopy. Two cw dye laser beams of different frequencies and propagating in opposite directions are utilized to obtain high-resolution spectra. The intensity and frequency dependence of the level shifts are examined.

Direct Observation of Atomic Energy Level Shifts in Two-Photon Absorption

We report direct observation of optically induced shifts of atomic energy levels in sodium vapor that occur in two-photon absorption. These shifts must be considered in high-resolution two-photon spectroscopy. Two cw dye laser beams of different frequencies and propagating in opposite directions are utilized to obtain high-resolution spectra. The intensity and frequency dependence of the level shifts are examined.

Multiple absorption of laser photons by atoms

Applying a space translation operation, the Schrodinger equation for an atom in an electromagnetic field is solved with sufficient accuracy to obtain probabilities for multiple absorption of photons from a monochromatic laser beam of arbitrary intensity or frequency. It is shown that the derived expression for the N-photon T-matrix contains the usual single photon matrix elements given by the perturbation theory and that the perturbative result is obtained in the limit of low intensity. Other explicit examples are considered. The conditions of applicability of the method are specified.

Multiphoton ionization in hydrogen-like atoms by intense linearly polarized light

A non-perturbative theoretical study shows that optimal frequency and intensity of laser beam inducing multi-photon ionization of hydrogen-like atoms, a result not to be expected from usual perturbation calculations.

Optical mixing in InSb

Abstract In this paper, the authors have derived the expression for the combination frequency components ω3 = 2ω1-ω2 in the current density by solving the appropriate Boltzmann transfer equation for electrons, when two laser beams of frequencies of ω1 and ω2 are incident on a degenerate nonparabolic semiconductor viz. InSb; the nonlinearity due to collision mechanism as well as the nonparabolicity of conduction band has been taken into account. The ionized impurity scattering has been considered to be the sole mechanism of electron scattering. The expression for the current density is further substituted in the wave equation to obtain the expression for the amplitude of combination frequency wave in the transmitted component. The calculated value of output power of frequency ω2 is found to be 1/10th of the experimental value (Patel, Slusher and Fleury, 1966), (7) and thus is in better agreement as compared to earlier investigators (Wolff and Pearson, 1966; Kaw, 1968). (5, 10)

Second-Order Optical Processes and Harmonic Fields in Solids

A consistent method has been developed to calculate induced electromagnetic fields and optical transitions of electrons in a solid, in response to an incident laser beam of (circular) frequency $\ensuremath{\omega}$. The analysis is based upon the independent-particle Schr\"odinger equation for electrons and Maxwell's equations for the electromagnetic fields. General expressions for linear and bilinear currents as well as second-order optical transition probabilities have been derived. It is shown that the second-order transition probability, which is proportional to the fourth power in the incident field, contains two different types of terms, describing double-photon transitions of the incident frequency $\ensuremath{\omega}$ and single-photon transitions of the harmonic frequency $2\ensuremath{\omega}$. An estimate has been made to show that in the case of centrosymmetric solids like metals, the relative contribution due to the single second-harmonic photon transition is of the order ${(\frac{{e}^{2}}{\ensuremath{\hbar}c})}^{2}\ensuremath{\ll}1$ in the optical region, compared with the double-fundamental-photon transition. However, in the case of solids lacking inversion symmetry, the contributions due to these two processes are estimated to be of the same order in magnitude.

Time-resolved spectroscopy of laser-generated microplasmas

Time-resolved spectrograms have been obtained of the radiation emitted by various target materials when placed at the focus of a 1 MW Q-switched ruby laser beam. A small, highly ionized plasma is formed near the target surface. During each pulse of laser light strong continuous emission occurs, extending well into the ultra-violet. Ge IV and Sn IV lines have been recorded: these appear only briefly. Less highly ionized atoms are more persistent: lines from neutral atoms are emitted for several microseconds after the laser pulse ends. It is shown that absorption of the laser beam by free-free transitions is an effective plasma heating process above a critical `take-off' temperature which depends on the laser beam power density and frequency and on the ionization potential of the target material.

The interaction of transverse electromagnetic plane waves and a moving ionizing shock wave in the presence of a magnetic field

Propagation of a transverse electromagnetic wave along a d.c. magnetic field and interaction with a moving shock wave is investigated. The direction of propagation is normal to the shock front. Solutions to electromagnetic fields, gas velocities and Doppler shifts in frequency are found in both the ionized and un-ionized gases. A frequency is obtained at which electromagnetic reflexion from the shock front is minimized. In the ionized gas behind the shock front a fast and a slow electromagnetic wave result. By adjusting the shock velocity the frequency of the slow wave can be either raised or lowered. This frequency change, however, is not the Doppler shift and, consequently, can be made much larger than the Doppler shifts encountered at non-relativistic velocities. The slow wave attenuates much less than the ordinary fast wave, and applications to diagnostics and communications through plasmas and laser-beam frequency multiplication may be possible.