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Optical response of artificial composite nanoscale molecules comprising a closely spaced noble metal nanoparticle and a semiconductor quantum dot have been studied theoretically. We consider a system composed of an Au particle and CdSe or CdSe/ZnSe quantum dot and predict optical bistability and hysteresis in its response, which suggests various applications, in particular, all-optical processing and optical memory.

We show that electrons on liquid helium display intrinsic bistability of resonant inter-subband absorption. The bistability occurs for comparatively weak microwave power. The underlying giant nonlinearity of the many-electron response results from the interplay of the strong short-range electron correlations, the long relaxation time, and the multi-subband character of the electron energy spectrum.

We show that electrons on liquid helium display intrinsic bistability of resonant inter-subband absorption. The bistability occurs for comparatively weak microwave power. The underlying giant nonlinearity of the many-electron response results from the interplay of the strong short-range electron correlations, the long relaxation time, and the multi-subband character of the electron energy spectrum.

Some years ago, a number of people independently suggested the use of the saturable absorber to induce optical bistability (hereafter referred to as OB). The phenomenon was demonstrated and studied recently by Gibbs, McCall and Venkatesan who used a cw dye laser to excite atoms of sodium vapor in a cell between the plates of a Fabry Perot interferometer. Under certain experimental and material conditions, they observed a nonlinear dependency of the transmitted field as a function of the dye-laser input which exhibited hysteresis, differential gain, and bistable behavior. Interest in OB from the practical applications standpoint stems from obvious device applications as the optical analog of the transistor, optical clipper or limiter and digital optical memory element. The recent demonstration of the phenomenon has generated considerable interest from the fundamental point of view as a clear example of spontaneous ordering in an open, stationary system of matter interacting with light. This has led to a surge of recent theoretical activity. (Author)

Longitudinal spatial effects in the nonlinear medium are analyzed in a model for mirrorless optical bistability due to induced absorption that treats heat conduction, electromagnetic field propagation, and temperature-dependent absorption. We consider two specific cases: the temperature-induced shift of the absorption peak of bound I2 excitons in CdS single crystals and the temperature-induced shrinkage of the band gap in GaAs/GaA1As quantum-well material. For both cases it is shown that the inhomogeneous spatial distribution of the nonlinear absorption profile along the direction of propagation of the electric field undergoes a first-order-like phase transition commensurate with the transition of the output intensity from one value to another for fixed input intensity. Stability conditions are presented for the steady states of the output intensity as a function of the input intensity for the cases treated. There is a multiplicity of bistability curves, and the stability analysis produced the expected branches of stable solutions. Keywords: Single crystals; Quantum well material; Intrinsic optical bistability; Absorption profile; Gallium arsenides; Reprints.

Quantum coherence is one of the most intriguing applications of quantum mechanics, and has led to interesting phenomena and uncommon results. Here we show that in a stark contrast to the usual red-detuned condition to observe bistability in single-mode optomechanics, the optical intensities exhibit bistability for all values of cavity-laser detuning due to intermode coupling induced by the two-photon coherence. Interestingly, an unconventional bistability with "ribbon"-shaped hysteresis can be observed for blue-detuned laser frequencies. We also demonstrate that the two-photon coherence leads to a strong entanglement between the movable mirrors in the adiabatic regime. Surprisingly, the mirror-mirror entanglement is shown to persist for environment temperature of the phonon bath up to 12 K using experimental parameters.

Nonlinear optical responses of excitation-phonon-coupling systems near a reflective surface are investigated. For a polydiacetylene chain lying parallel to the surface, optical bistability induced by the surface is discovered. It is also found that the threshold and contrast of the bistability can be controlled by adjusting the distance of the system from the surface, and that vacuum fluctuations are reduced whenever the bistability occurs. The nature of the bistability and its origin are discussed. (Author) (TTL)

The affect of the coherent dipole-dipole interactions on the resonance fluorescence spectra of two, two-level atoms, driven by coherent external field, has been investigated. The explicit dependence of the coherent dipole-dipole interactions on the distance between the treatment of the spectra. For the case of two, two-level atoms with a fixed distance smaller than a wavelength an analytical result for the resonance fluorescence line shape has been obtained and the result has been compared with previous works. The Langevin Equations for Intrinsic Bistability, the effects of the Langevin noise terms on intrinsic bistability are analyzed for the case in which dephasing constant is large relative to that of the relaxation. Within this approximation one dimensional Ito differential equation for the inversion of population has been developed. The tunneling process and the passage times between the two stable states in the intrinsic bistability are studied.

We investigate the effects of atomic collisions as well as optomechanical mirror-field coupling on the optical bistability in a hybrid system consisting of a Bose-Einstein condensate inside a driven optical cavity with a moving end mirror. It is shown that the bistability of the system can be controlled by the s-wave scattering frequency which can provide the possibility of realizing a controllable optical switch. On the other hand, by studying the effect of the Bogoliubov mode, as a secondary mechanical mode relative to the mirror vibrations, on the cooling process as well as the bipartite mirror-field and atom-field entanglements we find an interpretation for the cooling of the Bogoliubov mode. The advantage of this hybrid system in comparison to the bare optomecanical cavity with a two-mode moving mirror is the controllability of the frequency of the secondary mode through the s-wave scattering interaction.

This report describes work performed on a program for fabrication and characterization of nonlinear optical properties of HgCdTe superlattices. We determined early on that performing nonlinear optical experiments (i.e., optical phase conjugation and optical bistability) on such superlattices requires the fabrication of thick or = 2 micron samples. The growth of thick superlattices with very thin individual layers, on the other hand, can best be accomplished by using an automated MBE (Molecular Beam Epitaxy) system. We launched a major effort to automate our existing MBE system which is due for completion by the end of July 1985. This report also describes the automation design, software, and the computer-controlled hardware.

We develop a self-consistent method to study the optical response of metallic gratings with nonlinear media embedded within their subwavelength slits. An optical Kerr nonlinearity is considered. Due to the large E-fields associated with the excitation of the transmission resonances appearing in this type of structures, moderate incoming fluxes result in drastic changes in the transmission spectra. Importantly, optical bistability is obtained for certain ranges of both flux and wavelength.

We study phase conjugation in a nonlinear composite slab when the counter propagating pump waves are completely absorbed by means of coherent perfect absorption. Under the undepleted pump approximation the coupling constant and the phase conjugated reflectivity are shown to undergo a substantial increase and multivalued response. The effect can be used for efficient switching of the phase conjugated reflectivity in photonic circuits.

We observe and characterize intrinsic optical bistability in a dilute Rydberg vapor. The bistability is characterized by sharp jumps between states of low and high Rydberg occupancy with jump up and down positions displaying hysteresis depending on the direction in which the control parameter is changed. We find that the shift in frequency of the jump point scales with the fourth power of the principal quantum number. Also, the width of the hysteresis window increases with increasing principal quantum number, before reaching a peak and then closing again. The experimental results are consistent with predictions from a simple theoretical model based on semiclassical Maxwell-Bloch equations including the effect of broadening and frequency shifts. These results provide insight to the dynamics of driven dissipative systems.

We analyze the resonant linear and nonlinear transmission through a photonic crystal waveguide side-coupled to a Kerr-nonlinear photonic crystal resonator. Firstly, we extend the standard coupled-mode theory analysis to photonic crystal structures and obtain explicit analytical expressions for the bistability thresholds and transmission coefficients which provide the basis for a detailed understanding of the possibilities associated with these structures. Next, we discuss limitations of standard coupled-mode theory and present an alternative analytical approach based on the effective discrete equations derived using a Green's function method. We find that the discrete nature of the photonic crystal waveguides allows a novel, geometry-driven enhancement of nonlinear effects by shifting the resonator location relative to the waveguide, thus providing an additional control of resonant waveguide transmission and Fano resonances. We further demonstrate that this enhancement may result in the lowering of the bistability threshold and switching power of nonlinear devices by several orders of magnitude. Finally, we show that employing such enhancements is of paramount importance for the design of all-optical devices based on slow-light photonic crystal waveguides.

We analyze the resonant linear and nonlinear transmission through a photonic crystal waveguide side-coupled to a Kerr-nonlinear photonic crystal resonator. Firstly, we extend the standard coupled-mode theory analysis to photonic crystal structures and obtain explicit analytical expressions for the bistability thresholds and transmission coefficients which provide the basis for a detailed understanding of the possibilities associated with these structures. Next, we discuss limitations of standard coupled-mode theory and present an alternative analytical approach based on the effective discrete equations derived using a Green's function method. We find that the discrete nature of the photonic crystal waveguides allows a novel, geometry-driven enhancement of nonlinear effects by shifting the resonator location relative to the waveguide, thus providing an additional control of resonant waveguide transmission and Fano resonances. We further demonstrate that this enhancement may result in the lowering of the bistability threshold and switching power of nonlinear devices by several orders of magnitude. Finally, we show that employing such enhancements is of paramount importance for the design of all-optical devices based on slow-light photonic crystal waveguides.

It is well-known that some nonlinear phenomena such as strong photon blockade are hard to be observed in optomechanical system with current experimental technology. Here, we present a coherent feedback control strategy in which a linear cavity is coherently controlled by an optomechanical controller in a feedback manner. The coherent feedback loop transfers and enhances quantum nonlinearity from the controller to the controlled cavity, which makes it possible to observe strong nonlinear effects in either linear cavity or optomechanical cavity. More interestingly, we find that the strong photon blockade under single-photon optomechanical weak coupling condition could be observed in the quantum regime. Additionally, the coherent feedback loop leads to two-photon and multiphoton tunnelings for the controlled linear cavity, which are also typical quantum nonlinear phenomenon. We hope that our work can give new perspectives in engineering nonlinear quantum phenomena.

We consider $N$ two-level atoms in a ring cavity interacting with a broadband squeezed vacuum centered at frequency $\omega_{s}$ and an input monochromatic driving field at frequency $\omega $. We show that, besides the central mode (at $\o$), many other {\em sideband modes} are produced at the output, with frequencies shifted from $\omega $ by multiples of $ 2(\omega -\omega_{s})$. Here we analyze the optical bistability of the two nearest sideband modes, one red-shifted and the other blue-shifted.

We perform a theoretical study of the nonlinear optical response of an ultrathin film consisting of oriented linear aggregates. A single aggregate is described by a Frenkel exciton Hamiltonian with uncorrelated on-site disorder. The exciton wave functions and energies are found exactly by numerically diagonalizing the Hamiltonian. The principal restriction we impose is that only the optical transitions between the ground state and optically dominant states of the one-exciton manifold are considered, whereas transitions to other states, including those of higher exciton manifolds, are neglected. The optical dynamics of the system is treated within the framework of truncated optical Maxwell-Bloch equations in which the electric polarization is calculated by using a joint distribution of the transition frequency and the transition dipole moment of the optically dominant states. This function contains all the statistical information about these two quantities that govern the optical response, and is obtained numerically by sampling many disorder realizations. We derive a steady-state equation that establishes a relationship between the output and input intensities of the electric field and show that within a certain range of the parameter space this equation exhibits a three-valued solution for the output field. A time-domain analysis is employed to investigate the stability of different branches of the three-valued solutions and to get insight into switching times. We discuss the possibility to experimentally verify the bistable behavior.

We theoretically study quadrature and polarization squeezing in dispersive optical bistability through a vectorial Kerr cavity model describing a nonlinear cavity filled with an isotropic chi(3) medium in which self-phase and cross-phase modulation, as well as four--wave mixing, occur. We derive expressions for the quantum fluctuations of the output field quadratures as a function of which we express the spectrum of fluctuations of the output field Stokes parameters. We pay particular attention to study how the bifurcations affecting the non-null linearly polarized output mode squeezes the orthogonally polarized vacuum mode, and show how this produces polarization squeezing.

A new analytical method is presented here, offering a physical view of driven cavities where the external field cannot be neglected. We introduce a new dimensionless complex parameter, intrinsically linked to the cooperativity parameter of optical bistability, and analogous to the scaled Rabbi frequency for driven systems where the field is classical. Classes of steady states are iteratively constructed and expressions for the diffusion and friction coefficients at lowest order also derived. They have in most cases the same mathematical form as their free-space analog. The method offers a semiclassical explanation for two recent experiments of one atom trapping in a high Q cavity where the excited state is significantly saturated. Our results refute both claims of atom trapping by a quantized cavity mode, single or not. Finally, it is argued that the parameter newly constructed, as well as the groundwork of this method, are at least companions of the cooperativity parameter and its mother theory. In particular, we lay the stress on the apparently more fundamental role of our structure parameter.

We report observations of novel dynamic behavior in resonantly-enhanced stimulated Raman scattering in Rb vapor. In particular, we demonstrate a dynamic hysteresis of the Raman scattered optical field in response to changes of the drive laser field intensity and/or frequency. This effect may be described as a dynamic form of optical bistability resulting from the formation and decay of atomic coherence. We have applied this phenomenon to the realization of an all-optical switch.

We explore the coherent control of nonlinear absorption of intense laser fields in four-level atomic systems. For instance, in a four-level ladder system, a coupling field creates electromagnetically induced transparency (EIT) with Aulter-Townes doublet for the probe field while the control field is absent. A large absorption peak appears at resonance as the control field is switched on. We show how such a large absorption leads to optical switching. Further, this large absorption gets diminished and a transparency window appears due to the saturation effects as the strength of the probe field is increased. We further demonstrate that the threshold of the optical bistability can be modified by suitable choices of the coupling and the control fields. In a four-level Y-type configuration, the effect of the control field on saturable absorption (SA) and reverse saturable absorption (RSA) is highlighted in the context of nonlinear absorption of the probe field. We achieve RSA and SA in a simple atomic system just by applying a control field.

We address a two level system in an environment interacting with the electromagnetic field in the dipole approximation. The resonant optical bistability induced by local-field effects is studied by considering the relationship between the population difference and the excitation field. The diversity of various systems is included by accounting for the system self-action via the surface part of the Green's dyadic in the general form. The bistability condition and the exact solution of the steady state optical Bloch equations at the absolute bistability threshold are derived analytically.

We report on a bistable light transmission through a planar metamaterial composed of a metal pattern of weakly asymmetric elements placed on a nonlinear substrate. Such structure bears the Fano-like sharp resonance response of a trapped-mode excitation. The feedback required for bistability is provided by the coupling between the strong antiphased trapped-mode-resonance currents excited on the metal elements and the intensity of inner field in the nonlinear substrate.

A quantum electrodynamical, many body treatment is used to show mirrorless optical bistability in terms of the spatial properties of coherent dipole dipole interactions among interacting two-level atoms. The general theory is applied to two special cases: (1) a thin sample of two-level atoms, with a width smaller than a resonance wavelength, and (2) a long sample of two atoms with dimensions very large relative to a resonance wavelength. While for the thin sample we are able to use a mean-field approximation with validity, for the long sample we are compelled to take into account retardation and propagation. In both cases bistability is found to be related to a renormalization of the frequency (or relaxation rate) that is inversion-dependent. For the long sample the frequency renormalization is significant for high atomic densities and for large oscillator strengths. Keywords: Reprints; Mirrorless optical bistability; Intrinsic optical bistability; Two-level atoms; Dipole-dipole interactions.

Transient behavior in optical bistability mediated by phonon mode and virtual exciton coupling is studied for an electron-phonon coupling system such as polydiacetylene-toluene-sulfonate. Numerical calculations are used to analyze the evolution between bistable states, and novel switching behavior and Rabi oscillations are found.

A proposal for an all-optical memory based on a bistability of single-mode states in a dual-mode diode laser with time-delayed optical feedback is presented. The system is modeled using a multimode extension of the Lang-Kobayashi equations with injected optical pulses. We uncover the bifurcation structure by deriving analytical expressions for the boundaries of the bistable region and demonstrate how the delay time in the external cavity determines an optimal pulse duration for efficient switching of the memory element. We also show the relevant role played by gain saturation and by the dual-mode solutions of the Lang-Kobayashi equations for the existence of the bistable regions. Our results demonstrate that feedback induced bistability can lead to significant performance improvements when compared to memory elements based on the injection locking bistability in dual-mode devices.

An optical resonator with intracavity Kerr nonlinearity can exhibit dispersive bistability suitable for all-optical switching. With nanophotonic elements it may be possible to achieve attojoule switching energies, which would be very attractive for ultra-low power operation but potentially problematic because of quantum fluctuation-induced spontaneous switching. In this manuscript I derive a quantum-optical model of two Kerr-nonlinear ring resonators connected in a coherent feedback loop, and show via numerical simulation that a properly designed `controller' cavity can significantly reduce the spontaneous switching rate of a bistable `plant' cavity in a completely embedded and autonomous manner.

Langevin equations for intrinsic optical bistability in a system of two-level atoms interacting by dipole-dipole interactions are treated, and the stochastic effects of quantum noise are analyzed. For cases in which the population relaxation is very slow (relative to the homogeneous dephasing rate of the complex dipole) we derive a one-dimensional Ito stochastic differential equation for the inversion of population. For such cases the probability distribution, diffusion process and the first passage times between the two metastable states are calculated. On the other hand, the quantum fluctuations in intrinsic bistability of a pure quantum mechanical system are calculated by using the Einstein relations. We find that a quantum noise which is above the standard quantum limit is inherent in the reaction field. We develop a theoretical analysis for the spectrum of the light scattered by two, two-level atoms driven near resonance by an external electromagnetic field.

Some fundamentally novel nonlinear optical devices were proposed and experimentally realized (in collaborative experiments with P.W. Smith, W.J. Thomlinson and J. Bjorkholm at Bell Laboratories). These results have been published in fourteen papers by the principal investigator (some of them in collaboration with researchers at Bell Laboratories, Holmdel, NJ, and Max-Planck-Institute fur Quantenoptick, Garching, Germany). The unified theory of plane-wave reflection and reflection at nonlinear interfaces was considered. Excitation of inhomogeneous traveling waves, which are peculiar features for negative nonlinearity, have been studied. It was shown analytically that in the case of limited (Gaussian) beam incidence, the nonlinear surface wave, observed in some recent computer simulations, is strictly forbidden, and an approximate theory of reflection in such a case has been developed. (Author)

Some fundamentally novel nonlinear optical devices were proposed and experimentally realized (in collaborative experiments with P.W. Smith, W.J. Thomlinson and J. Bjorkholm at Bell Laboratories). These results have been published in fourteen papers by the principal investigator (some of them in collaboration with researchers at Bell Laboratories, Holmdel, NJ, and Max-Planck-Institute fur Quantenoptick, Garching, Germany). The unified theory of plane-wave reflection and reflection at nonlinear interfaces was considered. Excitation of inhomogeneous traveling waves, which are peculiar features for negative nonlinearity, have been studied. It was shown analytically that in the case of limited (Gaussian) beam incidence, the nonlinear surface wave, observed in some recent computer simulations, is strictly forbidden, and an approximate theory of reflection in such a case has been developed. (Author)

We consider N driven two-level atoms interacting with a structured reservoir. By dressing the collective operators within a semiclassical approach, we derive a master equation and a mean-field single particle effective Hamiltonian. This Hamiltonian describes the optical bistability phenomenon occurring in the relation between an input electromagnetic field and the effective output generated by the N atoms. The dissipative part of the master equation and the effective single-particle Hamiltonian contain new terms due the reservoir structure of modes. In plotting the output field amplitude and phase, for a structured reservoir, as function of the input amplitude, one verifies the bistable behavior in both. We illustrate our results for two structured reservoirs: one having a Lorentzian shape for the distribution of modes, and the second is modeled as a photonic band-gap structure.

We consider N driven two-level atoms interacting with a structured reservoir. By dressing the collective operators within a semiclassical approach, we derive a master equation and a mean-field single particle effective Hamiltonian. This Hamiltonian describes the optical bistability phenomenon occurring in the relation between an input electromagnetic field and the effective output generated by the N atoms. The dissipative part of the master equation and the effective single-particle Hamiltonian contain new terms due the reservoir structure of modes. In plotting the output field amplitude and phase, for a structured reservoir, as function of the input amplitude, one verifies the bistable behavior in both. We illustrate our results for two structured reservoirs: one having a Lorentzian shape for the distribution of modes, and the second is modeled as a photonic band-gap structure.

The control of light transmission through a Fabry-Perot cavity containing atoms is theoretically investigated, when the cavity mode beam and an intersecting control beam are both close to specific atomic resonances. A four-level atomic system is considered and its interaction with the cavity mode is studied by solving for the time dependent cavity field and atomic state populations. The conditions for optical bistability of the atom-cavity system are obtained in steady state limit. For an ensemble of atoms in the cavity mode, the response of the intra-cavity light intensity to the intersecting resonant beam is understood for stationary atoms (closed system) and non-static atoms (open system). The open system is modelled by adjusting the atomic state populations to represent the exchange of atoms in the cavity mode, with the thermal environment. The solutions to the model are used to qualitatively explain the observed steady state and transient behaviour of the light in the cavity mode, in Sharma et. al. [1]. The control behaviour with three- and two-level atomic systems is also studied, and the rich physics arising out of these systems, for closed and open atomic systems is discussed.

It is well known that the squeezing spectrum of the field exiting a nonlinear cavity can be directly obtained from the fluctuation spectrum of normally ordered products of creation and annihilation operators of the cavity mode. In this article we show that the output field squeezing spectrum can be derived also by combining the fluctuation spectra of any pair of s-ordered products of creation and annihilation operators. The interesting result is that the spectrum obtained in this way from the linearized Langevin equations is exact, and this occurs in spite of the fact that no s-ordered quasiprobability distribution verifies a true Fokker-Planck equation, i.e., the Langevin equations used for deriving the squeezing spectrum are not exact. The (linearized) intracavity squeezing obtained from any s-ordered distribution is also exact. These results are exemplified in the problem of dispersive optical bistability.

We present a theoretical analysis of the nonlinear response of a metamaterial consisting of an array of split ring resonators coupled to a two level atomic system (2LS) without inversion. We calculated the total complex susceptibility and the linear, third and fifth-order nonlinear solution of the Maxwell Bloch equations that describe the coupled system. We analyzed the impact of the nonlinear effects on the dynamics of an active metamaterial/2LS compound in dependence of the magnitude of the external electric field when the 2LS is pumped by an external pump source and population inversion is achieved. We observed that nonlinear damping in the active metamaterial/2LS compound significantly reduces the effect of spiking. Finally, we investigated the conditions under which the system can exhibit the phenomenon of optical bistability. We observed that optical bistability can only be obtained when the atomic volume density of the 2LS attains unphysically high values and thus is not expected in the system under investigation.

Ion beam fabrication techniques have been used to form a wide range of elemental metal nanocrystals in high purity dielectrics. Implantation protocols and post processing treatments have been established to control the size and size distribution of the elemental nanocrystals. This control has allowed us to correlate the nanocrystal microstructure with the optical response and to show that the quantum size effects are present and can be observed in metal nanocrystal dielectric composites. Sequential implantation techniques have been developed to form intermetallic nanocrystals and metal core metal shell coated particles in dielectrics.

A new type of optical bistability mediated by phonon and virtual exciton coupling is suggested for polymers. Numerical results with parameters pertaining to polydiacetylene-toluene sulfonate are given as an illustration. The stability of the steady-state solution is examined, and the physical mechanism responsible for the bistability is discussed. Keywords: Light pulses, Pulsed lasers, Optical bistability.

This report summarizes our progress and results during this reporting period. Our experiments show that the growth of thick (approx 5 micrometers) superlattice samples of HgTe/CdTe requires the use of an MBE system with in situ diagnostic capability to assure stability of growth parameters for periods of several hours. Our efforts with our in house MBE system have not succeeded in growing thick superlattice samples needed for the optical experiments.

We present a mechanism able to show intrinsic bistable behaviour involving single Yb3+ ions embedded into bromide lattices, in which intrinsic optical bistability (IOB) has been observed. The mechanism is based on the experimentally found coupling between the Yb3+ ion and the totally symmetric local mode of vibration of the [YbBr6]3- coordination unit. The model reproduces the IOB observed in CsCdBr3:1% Yb3+ and allows to understand the experimentally found presence of the phenomenon in the other bromides, but its absence in Cs3Lu2Cl9:Yb3+.

Several novel implementations of optical bistability have been proposed and analyzed. Novel applications of optical nonlinearities in a ring have been proposed; particularly interesting is the use of nonlinear nonreciprocity of counterpropagating waves in a ring to enahnce the Sagnac Effect, and, thereby, increase the sensitivity of a passive ring optical gyro. The use of moderate energy electron beams traversing superlattices to generate coherent x-rays by the coherent addition of transition radiation has also been proposed and analyzed. This document is a collection of articles pertaining to such fields as: Solitons; Cyclotron resonance of free electrons; High energy electron beams. (Author)

Here we explore spin dependent quantum transport through a single quantum dot coupled to an optical microcavity. The spin current is generated by electron tunneling between a single doped reservoir and the dot combined with intradot spin flip transitions induced by a quantized cavity mode. In the limit of strong Coulomb blockade, this model is analogous to the Jaynes-Cummings model in quantum optics and generates a pure spin current in the absence of any charge current. Earlier research has shown that in the classical limit where a large number of such dots interact with the cavity field, the spin current exhibits bistability as a function of the laser amplitude that drives the cavity. We show that in the limit of a single quantum dot this bistability continues to be present in the intracavity photon statistics. Signatures of the bistable photon statistics manifest themselves in the frequency dependent shot noise of the spin current despite the fact that the quantum mechanical average spin current no longer exhibits bistability. Besides having significance for future quantum dot based optoelectronic devices, our results shed light on the relation between bistability, which is traditionally viewed as a classical effect, and quantum mechanics.

When laser beam are directed on an exciton-phonon coupling system inside an optical cavity, the cavity field intensity is investigated as a function of the driving field intensity for various distances between the sample and a metallic surface representing one of the mirrors of the cavity. Optical bistability is found to be induced by the surface when the cavity field is on resonance with the excitonic transition frequency. It is also found that at the threshold, the direction of switch of the optical bistability can be controlled by adjusting the distance from the surface. The investigation of optical properties of molecular systems near a solid surface has been of interest from both the fundamental and practical points of view. Exciton phonon coupling systems such as organic semiconductors have been considered very recently instead of single molecular systems located near ideal metal surfaces. Surface- induced optical bistability has been discovered and its nature and origin investigated.

We report on a new type of optical nonlinearity in a polariton p-i-n microcavity. Abrupt switching between the strong and weak coupling regime is induced by controlling the electric field within the cavity. As a consequence bistable cycles are observed for low optical powers (2-3 orders of magnitude less than for Kerr induced bistability). Signatures of switching fronts propagating through the whole 300 microns x 300 microns mesa surface are evidenced.

Scattering of resonant radiation in a dense two-level medium is studied theoretically with account for local field effects and renormalization of the resonance frequency. Intrinsic optical bistability is viewed as switching between different spectral patterns of fluorescent light controlled by the incident field strength. Response spectra are calculated analytically for the entire hysteresis loop of atomic excitation. The equations to describe the non-linear interaction of an atomic ensemble with light are derived from the Bogolubov-Born-Green-Kirkwood-Yvon hierarchy for reduced single particle density matrices of atoms and quantized field modes and their correlation operators. The spectral power of scattered light with separated coherent and incoherent constituents is obtained straightforwardly within the hierarchy. The formula obtained for emission spectra can be used to distinguish between possible mechanisms suggested to produce intrinsic bistability.

We study the injection locking bistability of a specially engineered two-color semiconductor Fabry-Perot laser. Oscillation in the uninjected primary mode leads to a bistability of single mode and two-color equilibria. With pulsed modulation of the injected power we demonstrate an all-optical memory element based on this bistability, where the uninjected primary mode is switched with 35 dB intensity contrast. Using experimental and theoretical analysis, we describe the associated bifurcation structure, which is not found in single mode systems with optical injection.

We study the injection locking bistability of a specially engineered two-color semiconductor Fabry-Perot laser. Oscillation in the uninjected primary mode leads to a bistability of single mode and two-color equilibria. With pulsed modulation of the injected power we demonstrate an all-optical memory element based on this bistability, where the uninjected primary mode is switched with 35 dB intensity contrast. Using experimental and theoretical analysis, we describe the associated bifurcation structure, which is not found in single mode systems with optical injection.

We report the observation of optical bistability in a microcavity pumped at the "magic angle". Experimental evidence is given in the form of a hysteresis cycle of the nonlinear emission as a function of the pump intensity or the position of the excitation spot. The results can be well understood with simple theoretical considerations that underline the fundamental analogy between our system and an optical parametric oscillator.

The automation of Molecular Beam Epitaxy system shutters was completed for the substrate and the Mercury, Cadmium Telluride and Telluride sources. As reported earlier, the automated shutter operations allow for much more precise thickness control of the superlattice layers during the deposition. Also, from signal-to-noise considerations, performing the nonlinear optical experiments requires the use of thick (2 micrometer) superlattice samples which involve several hundred layers of Mercury Telluride and Cadmium Telluride. This can best be accomplished by using an automated Molecular Beam Epitaxy system. The current superlattice design calls for alternate layers of HgTe and CdTe with thicknesses of 100-150 A. A p-type sample with this structure is expected to yield a third order susceptibility (x(3)) of 100 times larger than that of the bulk HgCdTe. A number of test runs were carried out with the automated MBE system. These runs resulted in five superlattice samples of various thicknesses. One sample is now being used in a preliminary experiment of optical phase conjugation to establish baseline data.

We demonstrate experimentally that a broad area laser-like optical oscillator (a nondegenerate photorefractive oscillator) with structured injected signal displays two-phase patterns. The technique (G. J. de Valc\'arcel and K. Staliunas, Phys. Rev. Lett. (105), 054101 (2010)) consists in spatially modulating the injection, so that its phase alternates periodically between two opposite values, i.e. differing by pi