Two-photon resonant parametric generation in alkali metal vapours

Two-photon resonant four-wave mixing has been studied both theoretically and experimentally. Tunable infrared radiation between 2 and 20 μm has been generated in potassium and caesium vapours. The mixing efficiency is found to be severely limited by competition with Raman scattering.     

Observation of two-photon absorption resonance in spacially separated fields

The paper presents the results obtained in investigating the lineshape of two-photon absorption in the field of two spacially separated standing waves at the 3S–4D transition of Na. Measurements were carried out by using a CW dye laser. This paper was presented, in part, by one of the authors (V.P.Ch.) at the V Vavilov Conference on Nonlinear Optics, Novosibirsk, USSR (June 1977).     

Light-pressure cooling of trapped ions in three dimensions

We formulate a Fokker-Planck theory for the three-dimensional cooling of a trapped ion by means of a single laser beam. Both the steady state and the dynamics of the cooling are discussed, and the correlations between the vibrational modes due to the light pressure are elucidated. A few comments on the actual experiments are presented.     

Two-photon resonant four-wave mixing processes in atomic barium

By means of a N₂-laser pumped dye-laser (P _L ≈20kW, Δv _L < 10 GHZ) different two-photon resonant four-wave coupling processes in Ba vapour (n _Ba≈10¹⁶ cm⁻³) using the Ba states 6s8s ¹ S _o and 7s5d ¹ D ₂ were investigated. Coherent line radiation with conversion rates up to 10⁻³ was generated within the range of λ=190–200 nm by sumfrequency mixingv _UV=2v ₁+v ₂ of 3 laser photons and within the range of λ=250–380 nm by couplingv _UV=2v ₁±v _IR of 2 laser photons with one photonv _IR stimulated emitted in laser-induced Ba transitions. For the second coupling type the various nonlinear processes contributing to the formation of the coupling components are discussed. The power of the UV-component as function of inensity and resonance detuning of the laser as well as on the phase-mismatch was calculated on the basis of the small signal theory and compared to the experimental data.     

Generation of coherent radiation at 89.6 nm through two-photon resonant phase-matched tripling of fourth-harmonic Nd: Glass laser radiation in Hg vapors

Coherent radiation at 89.6 nm has been obtained through third-harmonic conversion of the 268.8 nm pulses in mercury vapors. Tunability of the fourth-harmonic frequency of the Nd: glass laser radiation to the 6s ¹ S ₀–8s ¹ S ₀ double quantum resonance of Hg atoms has been used for the resonant enhancement of the twelfth-harmonic output. Possibility to phase match the generated radiation and driving polarisation in pure vapors without employing of bufer gas has been demonstrated.     

J-dependence of intensity-dependent polarization change

J-dependence of intensity-dependent polarization change of the elliptically polarized light in atomic resonance lines is studied both theoretically and experimentally.     

Interference effects between raman and parametric stimulated emission

The interaction between stimulated electronic Raman scattering and four wave parametric emission in potassium has been studied on the basis of common real states and emission channels. The modified intensity Stokes dependence and higher saturation pump intensity have been accounted for by the model developed. Two tunable pump waves are used to initiate the two steps, involving a dipole forbidden 4S – 3D transition.     

Excitation of multilevel molecular systems by laser ir field

Analytical results have been obtained for some aspects of polyatomic molecular interaction with resonant ir laser radiation. The following effects are investigated by use of Schr?dinger's equation solution for natural multilevel models: the role of dynamic Stark effect in the transient region from molecular low levels to the quasi-continuum of highly excited vibrational levels; broadening of multiphoton transitions because of relatively fast decay into the quasi-continuum; threshold excitation property of multilevel systems with random detunings. Possible applications to constructing models for numerical calculations dealing with the molecule excitation problem are discussed.     

Laser cooling of trapped particles III: The Lamb-Dicke limit

In this paper we consider the laser cooling of a trapped particle when the discrete nature of the quantum levels must be taken into account. We particularize the equations to the Lamb-Dicke limit when the oscillational, amplitude is much less than the optical wavelength. Then the quantum levels are coupled only to the neighbouring ones, and we can also find a small parameter to use for adiabatic elimination of rapidly varying terms. There emerges one rate equation for the probability distribution over the quantum states. The ultimate distribution is found to be of the Planckian type independently of the initial one. We solve the equation, generally and obtain the time evolution for an arbitrary initial distribution. In particular we look at an initially thermal, Poisson, or pure state distribution. For the thermal one we find that its shape is preserved during the cooling. The results are discussed.     

Broad band resonant light pressure

This paper treats the mechanical pressure of resonant light acting on a two-level system, where the degenerate magnetic sublevels are taken into account. The theory is developed with arbitrary relations between the quantization axis and the propagation and polarization of the light. Rate terms are obtained both for spontaneous and induced transitions; the requirements of incoherence put restrictions on the possible geometries of the experiment. The rate equations are restricted to motion along a light beam only; this one-dimensional case is simpler to handle. For small recoil velocities a Fokker-Planck equation is derived, and an adiabatic elimination procedure enables us to derive an equation for the velocity distribution of the total population. The assumptions and approximations are formulated and discussed.     

Laser cooling of trapped particles I: The heavy particle limit

We discuss laser cooling of a trapped ion. The problem is formulated for a harmonic trapping well and one-dimensional cooling. We carry out an adiabatic elimination and a Fokker-Planck expansion in the limit of a slowly oscillating particle. The ensuing Fokker-Planck equation is discussed and solved in certain limits. Especially the final cooling stages and ultimate width are of interest. The approach is related to earlier work and its validity is discussed.     

The identification of nonlinear molecular systems by spectroscopic methods

The Wiener functional expansion method for the analysis of nonlinear systems is applied to identify and analyze both nonlinear and linear molecular systems by spectroscopic methods. As the sampling filter (monochromator) of any spectroscopic apparatus may be defined by a Weber-Hermite polynomial, an analysis of the refracted or scattered light by orthogonal polynomials is easily achieved. Time averaging obtains the Weber-Hermite coefficients which permit the characterization of the molecular system with respect to the polarization of the incident and scattered light. In the case of two series of measurements made with incident and emerging light polarized in different directions: the identification of the JonesM matrices for the molecular system irradiated is possible. In the case of three series of measurements made, for example, with incident and emerging light (a) circularly polarized corotating, (b) circularly polarized contrarotating, and (c) plane polarized perpendicular: the identification of the molecular system's McClain invariants related to the vibrational symmetry group for Raman inelastic light scattering is possible. The analysis presents a unified picture of elastic and inelastic light scattering and one-photon and two-photon processes. The apparatus described would detect those instances in molecular systems for which Beer's law does not apply.     

Laser irradiation of a three-level gas system: Continued-fraction theory and applications

The general steady-state solution of the density matrix equations for a Doppler broadened three-level system irradiated by two resonant standing-wave laser beams of arbritrary intensity is analyzed. The solution is expressed in a matrix continued fraction form, that involves 4×4 matrices in important configurations and is convenient for numerical computations. Some representative cases including the absorption spectra for a probe laser of arbitrary intensity, the Doppler-free multiphoton resonances and the optically pumped lasers are analyzed numerically in connection with previous experimental investigations.     

Experimental investigation of the line center of Hg intercombination spectral line 253.7 nm perturbed by Kr

The collision broadening and shift of the Hg intercombination spectral line 253.7 nm (6¹S₀–6³P₁) perturbed by Kr has been investigated using a high-resolution scanning Fabry–Perot interferometer. The values of the pressure broadening and shift coefficients β and δ, respectively, for the studied line have been obtained. The obtained coefficients β and δ are compared with their corresponding published experimental values and also those calculated using Lindholm–Foley impact theory.     

Shot-by-shot frequency calibration of CARS spectra: Application to the measurement of the collisional line shift in oxygen

A technique using a Fabry-Perot interferometer has been developed to calibrate high-resolution spectra obtained by Coherent Anti-Stokes Raman Spectroscopy (CARS). This technique was used to measure simultaneously the Raman frequency and the Raman signal at each laser shot. We demonstrate the accuracy of the method by measuring theQ(15) line shifts of molecular oxygen due to collisions with oxygen and water vapour.     

Frequency chain towards the Ca intercombination line based on laser diodes: First step

Stable, narrow linewidth operation of red and 1.3 µm free-running laser diodes with external gratings in non-Littrow geometry is demonstrated. The resonance of the saturated fluorescence of an atomic beam with a contrast of 25% and a linewidth of 400 ± 50 kHz of the Ca intercombination line 4¹ S ₀–4³ P ₁ ( = 657 nm) is shown. A high-power (110 mW) single-mode external cavity laser diode at 1.3 µm is used for second-harmonic generation in a KTP crystal. The beat signal (signal to noise ratio about 25 dB) of 10 nW second-harmonic radiation at 1.3 µm and the radiation of a laser diode in the visible spectrum, as a step to realize a frequency chain, is observed.     

Spatial weak infrared-light bright and dark solitons in semiconductor quantum-dot nanostructures

The formation of spatial weak infrared-light solitons through interband and intersubband transitions in semiconductor quantum dots (QDs) is theoretically investigated with feasible parameters. Our analysis is shown using GaAs-Al-GaAs QD structures that allow operation at communication wavelength. Such a three-level QD solid-state system for the infrared-light-soliton operation is much more practical than that in gaseous media as a result of its flexible design and the wide adjustable parameters and thus facilitate more practical applications in optical signal processing and telecommunication.     

Ratio of intercombination to resonance line intensities for He-like oxygen ions in TFR tokamak plasmas

The ratio of intercombination to resonance line intensities has been measured in TFR tokamak plasmas, using space-and time-resolved soft X-ray spectroscopy. The experimental ratio values are compared to those obtained with available theoretical atomic models.     

Quantum non-demolition measurement of photon number using weak nonlinearities

We propose an alternative method for the quantum non-demolition measurement of photon numbers wherein weak cross-Kerr nonlinearities are to be used. The usual approach to quantum non-demolition measurements of quantum number involves encoding the photon number, through a cross-Kerr interaction, into a phase shift of a probe coherent state which is then detected through balanced homodyning. Weak nonlinearities produce small phase shifts which are difficult to detect and distinguish. In the method we propose, unbalanced homodyning acts as a displacement operator on the probe beam coherent state such that the cross-Kerr interaction encodes the photon number into the amplitude of a new coherent state. The value of the photon number can be determined by inefficient photon counting on the new coherent state. Our proposed method requires fewer resources than does the usual approach.