The unstable regime, experimentally observed with different reflectivity coefficients in a ring Er${}^{3+}$doped fiber laser, drives the field to generation of periodic pulses, with a period equal to the round-trip time of the cavity, via a self-mode locking mechanism. We investigate this instability using the standard Maxwell-Bloch equations for a resonant two-level laser with a ring cavity under conditions of homogeneous broadening. The results of the linear stability analysis and the numerical simulations are strongly suggestive that the observed pulsations correspond to those of the classic scenario in the multimode laser instability predicted by Risken and Nummedal, Graham and Haken.

The polarization properties of the light emitted by vertical-cavity surface-emitting semiconductor lasers are studied by means of a four-level scheme which includes misaligned linear phase and amplitude anisotropies. First we investigate the system stability characteristics, demonstrating that upon increasing the pumping level a polarization switching may take place both from the lower- to the higher- and from the higher- to the lower-frequency states. Then we study the laser stationary states identifying the role played by nonlinear effects and demonstrating that noticeable deviations from linear behavior are induced by the simultaneous presence of misaligned phase and amplitude anisotropies with comparable magnitude.

We performed single-photon interference experiments with neutral density filters and slow beam choppers to confirm the difference between stochastic and deterministic absorption and obtained results that agreed with the case of neutron-interference experiments and quantum theory.

An approach for calculating fourth-order coherence functions between fields produced in the parametric down-conversion process is proposed. It takes into account the transverse correlations between twin photons and is used to calculate the visibility of the interference patterns obtained in double-slit experiments with coincidence detection. The finite dimension of the light source and detectors are shown to be responsible for a kind of partial coherence which affects the visibility of the fringes. The results explain known features and predict other ones.

The multiconfiguration relativistic random-phase approximation theory (MCRRPA) is applied to the photoionization processes of Mg. Double-electron excitations as well as core-electron excitations are included. The core-electron excitations account for the core-valence correlations, specifically the core-polarization effects, and the double-electron excitations account for the intravalence correlations. We obtain a prediction of the ${3s}_{1/2}$ threshold photoionization cross section that is in excellent agreement with experiment. It shows that the core-polarization effects in addition to the double-electron excitations play an important role in the near- threshold photoionization processes of Mg and the MCRRPA provides an accurate account of these effects.

The nonlinear amplification of X-ray channeling radiation in crystal is discussed on the basis of the self-consistent set of the Maxwell and Dirac equations. Both planar and axial channeling for relativistic electron beams are considered. Two stationary regimes of nonlinear amplification are discussed.

We examine subnatural phase-dependent linewidths in the fluorescence spectrum of a three-level atom damped by a narrow-bandwidth squeezed vacuum in a cavity. Using the dressed-atom model approach of a strongly driven three-level cascade system, we derive the master equation of the system from which we obtain simple analytical expressions for the fluorescence spectrum. We show that the phase effects depend on the bandwidths of the squeezed vacuum and the cavity relative to the Rabi frequency of the driving fields. When the squeezing bandwidth is much larger than the Rabi frequency, the spectrum consists of five lines with only the central and outer sidebands dependent on the phase. For a squeezing bandwidth much smaller than the Rabi frequency the number of lines in the spectrum and their phase properties depend on the frequency at which the squeezing and cavity modes are centered. When the squeezing and cavity modes are centered on the inner Rabi sidebands, the spectrum exhibits five lines that are completely independent of the squeezing phase with only the inner Rabi sidebands dependent on the squeezing correlations. Matching the squeezing and cavity modes to the outer Rabi sidebands leads to the disappearance of the inner Rabi sidebands and a strong phase dependence of the central line and the outer Rabi sidebands. We find that in this case the system behaves as an individual two-level system that reveals exactly the noise distribution in the input squeezed vacuum.

For a thin layer of thickness $d<\mathrm{}{l}_{\mathrm{coherence}},$ we have observed a nonlinear Brewster angle in transmission by time-resolved second-harmonic (SH) generation. The associated nonlinear polarization is different from zero and oscillates in the plane of incidence. We interpret the vanishing SH signal as the result of a field-induced nonlinear polarization whose axis of oscillation is directed towards the observer. A nematic liquid-crystal mixture, which was contained in a cell whose electrode polarity could be switched, served as the sample.

The photon emission probability in a micromaser changes drastically when the kinetic energy of the pumping atoms is comparable to the atom-field interaction energy. In this situation, the atomic center-of-mass motion has to be treated quantum mechanically and the de Broglie wavelength of the atom inside the cavity is an important physical parameter. The interplay between reflection and transmission of the atoms leads to a new mechanism for induced emission. A photon is emitted by an excited atom when the de Broglie wavelength fits resonantly into the cavity. These resonances lead to the process of microwave amplification via $z$-motion-induced emission of radiation (mazer). We derive and illustrate a general expression for the emission probability and a master equation for the mazer. We note that the probability for emission by an excited thermal atom (stimulated maser emission) is very different from the emission probability as given by the de Broglie resonances (induced mazer emission).

The quantum theory of the mazer is extended by giving exact analytical solutions for a smooth potential and comparing them with the mesa potential considered earlier. Furthermore, WKB solutions for sinusoidal mode functions are found, leading to interesting features such as a state-changing or state-preserving mirror for atoms. Experimental parameters are taken into account to show that it is in principle possible to realize the mazer experimentally.

We calculate the spectrum of the micromaser with quantized motion of the pump atoms [M. O. Scully, G. M. Meyer, and H. Walther, Phys. Rev. Lett. 76, 4144 (1996)] in different velocity regimes. We show that when the kinetic energy of the pump atoms is comparable to the atom-field interaction energy the spectrum of the cavity field is no longer centered at the cavity frequency. In the case of very slow atoms huge broadenings and shifts occur in the vicinity of the mazer resonances. We present approximate analytical results for the spectrum that allow a comparison with the exact numerical calculations.

We describe how a quantum system composed of a cavity field interacting with a movable mirror can be utilized to generate a large variety of nonclassical states of both the cavity field and the mirror. First we consider state preparation of the cavity field. The system dynamics will prepare a single mode of the cavity field in a multicomponent Schrödinger-cat state, in a similar manner to that in a Kerr medium. In addition, when two or more cavity modes interact with the mirror, they can be prepared in an entangled state, which may be regarded as a multimode generalization of the even and odd coherent states. We show also that near-number states of a single mode may be prepared by performing a measurement of the position of the mirror. Second we consider state preparation of the mirror and show that this macroscopic object may be placed in a Schrödinger-cat-like state by a quadrature measurement of the light field. In addition, we examine the effect of the damping of the motion of the mirror on the field states inside the cavity and compare this with the effect of cavity field damping.

We propose a scheme by which signals carried by a coherent optical source close to the quantum noise limit can be amplified, making them more robust against losses, and then reconstructed at a remote site. The amplitude signal-to-noise ratio, the absolute amplitude noise level, and optical coherence are preserved. The scheme utilizes electro-optic feedforward and laser injection locking. We investigate the performance of the scheme as a quantum nondemolition device.

Laguerre-Gaussian modes of various order are frequency doubled. The azimuthal phase structure of the second-harmonic light is measured directly by interfering the beam with its mirror image. We show that the orbital angular momentum per photon is doubled, so conserving the orbital angular momentum in the light beam. The frequency-doubled output beam is shown to have a Gegenbauer-Gaussian amplitude distribution at the beam waist. The beam can be described as a summation of Laguerre-Gaussian modes that interfere so that it changes form with propagation, but the distribution at the beam waist is reproduced in the far field.

The decay rate of an excited molecule (atom) embedded in a dispersive and absorbing planar cavity is derived by using a recently obtained compact form of the Green’s function for a multilayer. As a by-product, a hint is provided for a straightforward extension of the results obtained for lossless cavities of other shapes to the corresponding absorbing cavities. The decay rate in an absorbing cavity consists of the spontaneous emission rate and of the nonradiative rates caused by the near-field interaction of the molecule with the cavity medium and, for nearby molecules, with the cavity mirrors. Only the spontaneous emission rate is satisfactorily described in the macroscopic approach adopted. The theory is applied to an analysis of the effects of the weak cavity absorption on the decay rate in a dielectric microcavity formed by two metallic mirrors. As expected, dissipation in the cavity medium spoils the conditions for controlled spontaneuos emission and strongly suppresses the intensity of spontaneous emission. However, its effect on the spontaneous emission rate is much less pronounced.

We study the properties of wave-packet states with vortex structure. In particular we present the characteristics of a two-mode radiation field characterized by a configuration-space wave function ${\psi }_v=(x-{iy)}^m{e}^{-{(x}^2{+y}^2)/2\mathrm{}{\sigma }^2}.$ We show the generation of such states by the interaction of Λ systems with squeezed radiation in a two-mode cavity. The two modes are correlated due to entanglement, which also produces the mixed state of each mode even though the state of the two-mode field is a pure state.

This is a theoretical study of the interplay of optical phase conjugation and multiple scattering. We calculate the intensity of light reflected by a phase-conjugating mirror when it is placed behind a disordered medium. We compare the results of a fully phase-coherent theory with those from the theory of radiative transfer. Both methods are equivalent if the dwell time ${\tau }_{\mathrm{dwell}}$ of a photon in the disordered medium is much larger than the inverse of the frequency shift $2\Delta \omega$ acquired at the phase-conjugating mirror. When ${\tau }_{\mathrm{dwell}}\Delta \omega \lesssim 1$, in contrast, phase coherence drastically affects the reflected intensity. In particular, a minimum in the dependence of the reflectance on the disorder strength disappears when $\Delta \omega$ is reduced below $1/{\tau }_{\mathrm{dwell}}.$ The analogies and differences with Andreev reflection of electrons at the interface between a normal metal and a superconductor are discussed.

This paper presents the results of a comparative experimental study of low-temperature Zn and Cu line plasmas created on slab targets by 400-ps laser pulse producing irradiance from $4\times {10}^9$ to ${10}^{11}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}.$ The aim was to examine the nanosecond-scale postpulse evolution of plasmas created in conditions equivalent to those produced by prepulses in collisional x-ray lasers, of elements that have neighboring atomic numbers but very different material properties. The plasmas were interferometrically probed at 4 and 10 ns next to the driving pulse, using geometry that made it possible to obtain an authentic two-dimensional (2D) electron density pattern in the plane perpendicular to the plasma axis. VIS-IR spectroscopy and imaging were used to provide an indication of the electron temperature and volume of the plasma layer near the target. We observe that over the whole range of the applied irradiances the characteristics and/or the expansion history of the Zn and Cu plasmas are very different. For irradiance exceeding a threshold specific to each element the density patterns exhibit an unexpected structure characterized by symmetrical flanks strongly localized in space, suggesting plasma is generated in addition to that produced within the laser pulse duration. The results imply that during the postpulse time the energy coupling between the plasma and the target is substantial for the plasma flow that exhibits a complex 2D character. A comparison of the data and results of a 1.5D hydrodynamic simulation for ${10}^{11}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}$ is made, indicating reasons for problems of such models in the treatment of the plasmas in question, and thus in the treatment of small-prepulse action in some x-ray laser systems.

We develop a method to precisely propagate short optical pulses through dispersive media with a cubic self-focusing nonlinear polarization. We show that above the critical cw self-focusing power, onset of pulse splitting into pulselets separated in time occurs, and for a certain regime of parameters a cyclic series of pulse splitting (into pulselets separated in time) and pulse recombination occurs for diffraction length smaller than dispersion length. At higher power, another threshold for noncyclic temporal and spatial pulse splitting is manifest. The physics of these phenomena are described and delineated.

Refined measurements and analysis of time-delayed second-harmonic generation on the ${6D}_{3/2}{-6S}_{1/2}$ transition in Cs vapor are presented that resolve the disparity between the measured and calculated time delays reported in its initial observation. We extend our work to the ${6D}_{5/2}{-6S}_{1/2}$ transition which, in agreement with our analysis, is characterized by a slightly smaller delay. For both transitions we also measure the amplitude of the second-harmonic intensity and find it close to what we estimate it should be.