Elimination of variables in simple laser equations

The usual way of obtaining rate equations (RE) and a single equation for the field amplitude (EFA) from the semiclassical laser equations (Lorenz-Haken model) is reexamined by undertaking a systematic elimination procedure developed in synergetics. The RE and EFA are justified in the case 1 (, ) and case 2 (, ), respectively. We show that, because the eliminated variable happens to contain a considerable contribution from an unstable mode, the usual elimination technique in the case 3 (, ) leads to an inconsistency. As important by-products we obtain the RE and EFA for arbitrary cavity relaxation constant (). Some remarks are given on the direct elimination technique in the non-diagonal representation in the study of instabilities.     

Excited-state absorption and third-order optical nonlinearities in symmetric π-electron organic molecules

Excited-State Absorption (ESA), Two-Photon Absorption (TPA) and the third-order polarizability (;,, – ) have been investigated for a model dichloride derivative of a symmetrically substituted benzylidene analine (SBAC), using a multielectron configuration-interaction procedure. The calculations indicate that SBAC exhibits ESA across the visible region of the spectrum, but that it is not as extensive as for molecules such as the phthalocyanines. The magnitude of the third-order polarizability is dominated by resonance enhancement from a very strongA _g B _u one-photon absorption. The calculated off-resonance value for (;,, – ) suggests that SBAC is a potential candidate for ultrafast switching applications.     

Quantum correlated twin beams

We discuss recent progress in the study of the non-classical properties of light beams generated by non-degenerate parametric splitting in ⁽²⁾ nonlinear birefringent crystals, with special emphasis on their quantum correlation (twin beams). We describe experimental results using successively pure parametric fluorescence, parametric amplification of a weak signal beam pumped by a pulsed laser, and parametric oscillation in a cavity pumped by a cw laser. In this review, we compare the respective advantages and drawbacks of the different approaches.     

Fatigue of Sb-Cs photocathodes

This experimental work deals with the influence of fatigue on the properties of the antimony cesium (Cs₃Sb) photocathode sensitized with oxygen. Photocells of special construction were prepared for the measurement and they were fatigued by illuminating them with white light from a tungsten lamp. The effect of fatigue on the spectral response curve, on the optical properties, values of activating energies, type of conductivity, current-voltage curves and integral sensitivity were investigated. In connection with the fatigue a shift of the long wavelength threshold towards shorter wavelengths, a decrease of response in the whole spectral range, changes in absorption and a drop in the value of the refractive index were observed. The maximum values of the activation energies, determined from measurements of the temperature dependence of the dark conductivity did not change. The conductivity of the photocathode was found to be of thep-type. The fatigue had certain effects on the current-voltage curves, the starting point of the anode current and the saturation point showing a certain shift towards lower positive anode voltages with fatigue. The relation between the fatigue and temperature rise of the photocathode due to the absorption of radiation was investigated. Finally some problems related to the mechanism of photoemission from the photocathodes are discussed.

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Glueball spectroscopy in strongly coupled lattice gauge theories

We study the mass spectrum up to –7 (1–) log of pure three-dimensional lattice gauge theories with action (g _P) for real irreducible and small . Besides the lowest excitationm ₀–4log, we find two nearly degenerate excited statesm ₁,m ₂ withm _i–6log (i=1, 2) and (m ₁–m ₂) at leastO().Work partially supported by CNPq (Brasil)     

Branched polymers in a wedge geometry in three dimensions

We investigate the statistical and dimensional properties of uniform star polymers attached by the branching vertex of degreef in a wedge geometry in three dimensions and described by the wedge angles and. We show that the growth constant is equal to ^f , where is the self-avoiding walk limit. Thef and (, ) dependences of the corresponding critical exponent _f (, ) are studied using Monte Carlo techniques. In the casef=1, our results are compared with existing predictions obtained from series expansion and renormalization group methods. We have also estimated the amplitudes for the mean square radius of gyration and the mean square end-to-end branch length. Our results for the ratio of the mean square radius of gyration of anf-star to that of a linear polymer of the same degree of polymerization attached in a similar wedge, and the analogous ratio for the mean square end-to-end branch length, are consistent with these ratios being lattice-independent quantities.     

Tunable UV radiation at short wavelengths (188–240 nm) generated by sum-frequency mixing in lithium borate

Sum-frequency mixing (₃=₁+₂) of UV laser radiation (₁=266 nm and 213 nm) and tunable coherent infrared light (₂=1.2–2.6 m) in lithium borate (LBO) generates radiation at short wavelengths (₃=188–242 nm). The UV radiation at ₁ is produced by the fourth and fifth harmonic of a pulsed Nd-YAG laser. The infrared light is generated with an optical parametric oscillator of beta barium borate. The phase-matching angle is measured as function of ₃ and compared with calculated values. For UV laser radiation at shorter wavelengths (173 nm₁213 nm) the calculations predict an extension of the tuning range of the sum-frequency generated at ₃ to wavelengths as short as the LBO transmission cutoff at 160 nm.     

Second-harmonic generated by self-squeezed light in isotropic medium. The role of light intensity-dependent effects and molecular-statistical structure

An isotropic medium with electrically removed centre of symmetry in which a fundamental wave at frequency generates the second harmonic at 2 is considered. The two waves give rise to self-acting effects modifying the refractive indices at and/or 2 (selfinduced ellipse rotation, optical Kerr effect). For this physical situation, an effective interaction Hamiltonian is introduced involving nonlinear coupling parameters, discussed versus the dc electric field and temperature as well as the density, concentration and molecular structure of the medium. The solutions of the respective quantum equations for the field operators at and 2 permit, in particular, the calculation of the variances for a novel process of second-harmonic generation by light, self-squeezed in an isotropic medium. It is shown that squeezing in the out-of-phase component of the second harmonic beam follows, with some delay, after self-squeezing in the in-phase component of the fundamental beam.Sponsored by the Polish Academy of Sciences, Project CPBP 01.12     

Basic Properties of Quantum Automata

This paper develops a theory of quantum automata and their slightly more general versions, q-automata. Quantum languages and -quantum languages, 0<1, are studied. Functions that can be realized as probability maps for q-automata are characterized. Quantum grammars are discussed and it is shown that quantum languages are precisely those languages that are induced by a quantum grammar. A quantum pumping lemma is employed to show that there are regular languages that are not -quantum, 0<1.     

χ(3) scattering noises in nonlinear optical phase conjugation

Phase conjugation by Four-Wave Mixing (FWM) is treated quantum mechanically as a multiphoton scattering process. The spontaneous scattering of two counter-propagating photons gives rise to a background noise, whereas the stimulated scattering in the presence of the probe beam generates the phase conjugate signal. In addition, the stimulated scattering also contributes to a background noise which has never been discussed. Compared with Rayleigh scattering noise, the ⁽³⁾ scattering noises are relatively small for intensity up to 10⁸ W/cm². For higher intensities, the ⁽³⁾ scattering noises, in conjunction with the Rayleigh scattering noise, will affect the spatial fidelity of the optical phase conjugation.     

Optical bistability in the semilinear phase-conjugate mirror

It is shown theoretically that the semilinear phase conjugate mirror exhibits bistability when it is seeded by a weak external pump beam. Full hysteresis in the output intensity is obtained when the seed intensity is swept through the two-stable-states region and the coupling strength is below the self-oscillation threshold.     

Manifestation of circular photogalvanic current by dynamic holography in BaTiO3

We report the first measurement of the photo-galvanic circular current antisymmetric tensor component in BaTiO₃: Co. The measurement gives a value of this coefficient, for extraordinary beam amplification, of 4×10^–9 A/W using a nonstationary measurement technique at a wavelength of 0.632 m.Professor N. Kukhtarev is on leave from the Institute of Physics in the Ukraine and is supported by the NSFDr. G. Dovgalenko is on leave from the IRPA Rotor-Control research laboratory of the Ukraine and is supported by the University of Arkansas     

Quantum non-demolition measurements in optics: a review and some recent experimental results

We review the schemes which have been implemented, in order to achieve quantum non-demolition (QND) measurements in the optical domain. The simplest schemes can be obtained using the optical Kerr effect, which yields a crossed-phase modulation coupling between two light beams. Other schemes use either independently generated squeezed light, or coupled-mode parametric amplifiers. These various schemes can be characterized using three criteria, which describe, respectively, the quality of the quantum measurement, the non-destruction of the signal, and the conditional variance of the output signal beam, given the output meter beam (quantum-state preparation criterion). We show that quantitative limits can be defined with respect to these criteria, delimiting classical and quantum domains of operation. Then we present in more detail a new experimental implementation of QND measurements, using three-level atoms inside a doubly-resonant optical cavity.     

Lattice theory,quadratic spaces,and quantum proposition systems

A quadratic space is a generalization of a Hilbert space. The geometry of certain kinds of subspaces (closed, splitting, etc.) is approached from the purely lattice theoretic point of view. In particular, theorems of Mackey and Kaplansky are given purely lattice theoretic proofs. Under certain conditions, the lattice of closed elements is a quantum proposition system (i.e., a complete orthomodular atomistic lattice with the covering property).     

Coherent raman spectroscopy of nitrogen molecules and clusters in supersonic jets

Coherent Stokes and anti-Stokes Raman scattering CSRS and CARS have been employed to study the spectroscopy of nitrogen molecules and clusters in the expansion of a supersonic jet. In the vibrational spectrum, at strong stagnation conditions, an intense redshifted peak is observed which can be assigned to the intramolecular vibrations in large N₂ clusters having adopted the -phase structure. Another weak feature is assigned to nitrogen clusters in the -phase. In the rotational region of the spectrum only monomer features have been observed. The failing to observe librational motions is consistent with the finding that the nitrogen clusters are predominantly in the orientationally unordered -phase. The low rotational temperature suggests supercooling of the -phase.     

Probing interface properties of nanocomposites by third-order nonlinear optics

We describe the exploitation of third-order nonlinear optical response — particularly nonlinear absorption and the nonlinear index of refraction — to probe interface dynamics, modifications and relaxation processes in granular materials consisting of metal quantum dots embedded in such dielectrics as fused silica and sapphire. Many features of these materials can be interpreted in terms of the quantum-mechanical model of the particle-in-a-box. Electronic and thermal relaxation processes in these novel nanocomposites are dominated by interactions of conduction-band electrons at the boundary between the quantum dot and its surrounding host material. Experimental examples presented include measurements of thermal and electronic relaxation rates, dephasing due to electron collisions at the nanocluster surface, effects of local structural order, changes in the saturation parameter due to chemical modification of the substrate, and one-and two-dimensional heat-transfer effects.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994.     

Pb1−x Fe x films prepared by vapor condensation

ZF, LF and TF SR experiments with antiferromagnetic (AF) ceramical samples La_2–x Sr _x CuO_4–y have been performed in the temperature range 10–300 K. Zero field muon spin polarization functions obtained below the Neel temperature clearly show a nonzero initial precession phase-–0.35 rad. We propose an explanation based on existence of the dynamical magnetic fields on the muon.We thank Drs. A.G. Chistov and A.M. Brjazkalo from RSC Kurchatov Institute for the preparation the #2 La₂CuO_4–y sample.     

Influence of Solvent Polarity and Viscosity on Nonradiative Processes in Oligothiophenes with Intramolecular Charge Transfer

By the methods of luminescence, picosecond spectroscopy, and quantumchemical calculations the mechanisms of electron excitation energy deactivation in some oligothiophenes with intramolecular charge transfer depending on the solvent polarity and viscosity have been investigated. While for 2Npiperidino5(2,2dicyanovinyl)thiophene (PDCVT) the main channel of nonradiative deactivation is the transition to a lower intermediate state with a twisted double bond controlled by the medium viscosity, in the case of (E){2[25piperidino2thienyl]6(trifluoridemethyl)4H4pyranylidene}propanedinitryl (PTFDN) fluorescence quenching is initiated by the solvent polarity. For two other oligothiophenes, 2Npiperidino5cyanothiophene (PCT) and 2Npiperidino5cyanoterthiophene (PCTT), differing in the length of the thiophene chain, we have revealed, along with the effective quenching of fluorescence in shortchain PCT (independent of the solvent polarity and viscosity), an increase in the radiation capacity in PCTT with increasing polarity of the solvent. The possible mechanisms of nonradiative deactivation in the investigated oligothiophenes are discussed.     

The influence of oscillations on ionization in a discharge path

The paper solves the problem of gas ionization in a discharge path in a very dilute gas, where the free path of the electrons is much larger than the dimensions of the path and the transit time of the electrons between the electrodes is of the order of the period of the applied h-f voltage. It was found that for a certain ratio of the transit time of the electrons between the electrodes in the discharge path to the period of the h-f oscillation, resonance occurs when the wattless current component is zero. The electron density rises in the path and thus also the gas ionization.

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In conclusion, the author would like to thank F. Benda for preparing the equipment, M. Kivánek for preparing the equipment and some of the measurements, and A. Hrdá for the measurements and for working out the case with equally large a-c and d-c voltages within the framework of her thesis.     

Optical second-harmonic generation of metal-island films

In metal-island films consisting of nanometer particles on a transparent substrate irradiated light fields can be locally enhanced by electron-plasma resonances. Therefore, nonlinear optical processes should be enhanced dramatically. However, second-order nonlinear processes as second-harmonic generation occuring in the surface are strongly reduced by the centrosymmetric shape of the metal particles. It is found that this surface-specific contribution to second-harmonic generation is less enhanced, as is expected from the field enhancement. The bulk contribution, at smooth metal surfaces known to be much weaker than the contribution from the real surface, is strongly enhanced by the plasma resonances without symmetry restrictions and becomes comparably important.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994.