Fluid dynamical enstrophy and the number of optical vortices in a paraxial beam

Vortex-bearing optical beams have a tendency to maintain the maximum number of vortices during propagation. This tendency is reminiscent of the concept of enstrophy, which is a conserved quantity in two-dimensional fluid dynamics, and which is given in terms of the vorticity in the fluid. We derive the optical equivalent for the fluid vorticity and show that it represents the optical topological charge density in paraxial beams. It then follows that the optical equivalent of the enstrophy represents the total number of optical vortices on a cross-section of the beam. We then argue that this concept forms an important part of the tendency of paraxial beams to maintain their maximum number of vortices. As part of the derivation we provide a summary of some of the pertinent topological properties of phase functions.     

Cascade arrangement of irregular optical phased arrays

We present the principle of cascade arrangement of irregular optical phased arrays. The optical phased-array beam deflector comprises arrayed optical waveguides that are spaced irregularly and arranged in a two-stage cascade. Relations between optical path differences and corresponding center-to-center spacings among elements in each stage are found, and phase matches between the two stages are achieved. Simulation shows a wide scanning angle with dramatically suppressed sidelobes.     

Spatial coherence of an optical turbulent beam in a biased photorefractive crystal due to the spatiotemporal modulation instability

We report the experimental observation of the dynamic pattern formation of a broad coherent light beam in a biased photorefractive crystal due to the spatiotemporal modulation instability. When the nonlinearity exceeds a specific threshold, the coherent light beam not only breaks up into light spots due to the modulation instability but also fast fluctuates both spatially and temporally, forming an optical turbulent beam, which behaves as a quasi-homogeneous speckled beam or a partially incoherent beam. We investigate the spatial coherence property of an optical turbulent beam from the visibility of the averaged double-slit interference fringe. We also numerically demonstrate the visibility variation of the instantaneous interference fringe of an optical turbulent beam.     

Optical properties of lattice/magnetic small polarons from DMFT

We calculate the optical conductivity of small polarons in the Holstein and Holstein-t-J models, by applying the dynamical mean field theory. We show that the antiferromagnetic correlations tend to increase the region of the parameters where polaronic signatures are found in the optical spectra, and shift the polaronic absorption band to higher frequencies compared to the case of purely lattice polarons. On the other hand, the electron-lattice interaction is essential in order to have polaronic features in the optical absorption.     

Nonlinear optical studies of the transient coherence in the Quantum Hall system

We review recent investigations of the femtosecond nonlinear optical response of the two-dimensional electron gas (2DEG) in a strong magnetic field. We probe the Quantum Hall (QH) regime for filling factors ν∼1. Our focus is on the transient coherence induced via optical excitation and on its time evolution during early femtosecond timescales. We simultaneously study the interband and intraband coherence in this system by using a nonlinear spectroscopic technique, transient three-pulse four wave mixing optical spectroscopy, and a many-body theory. We observe striking differences in the temporal and spectral profile of the nonlinear optical signal between a modulation doped quantum well system (with the 2DEG) and a similar undoped quantum well (without a 2DEG). We attribute these qualitative differences to Coulomb correlations between the photoexcited electron-hole pairs and the 2DEG. We show, in particular, that intraband many-particle coherences assisted by the inter-Landau-level magnetoplasmon excitations of the 2DEG dominate the femtosecond nonlinear optical response. The most striking effect of these exciton-magnetoplasmon coherences is a large off-resonant four-wave-mixing signal in the case of very low photoexcited carrier densities, not observed in the undoped system, with strong temporal oscillations and unusually symmetric temporal profile.     

The cold atom Hubbard toolbox

We review recent theoretical advances in cold atom physics concentrating on strongly correlated cold atoms in optical lattices. We discuss recently developed quantum optical tools for manipulating atoms and show how they can be used to realize a wide range of many body Hamiltonians. Then, we describe connections and differences to condensed matter physics and present applications in the fields of quantum computing and quantum simulations. Finally, we explain how defects and atomic quantum dots can be introduced in a controlled way in optical lattice systems.     

Dynamic control of light propagation and optical switching through an RF-driven cascade-type atomic medium

We study nonlinear optical behaviors in pulse propagation through a medium consisting of four-level cascade-type cold atoms, where a radio-frequency (RF) field couples upper two-folded levels and double-dark resonances (DDRs) can arise. By numerically solving the coupled Bloch-Maxwell equations for atom and field simultaneously in space and time, we demonstrate dynamic control of light propagation and optical switching in such a four-level atomic medium. The proposed scheme may have potential applications in the design of optical switching and optical storage devices.     

Generation of Cluster-Type Entangled Coherent States via Cross-Kerr Nonlinearity

We propose an optical scheme for the generation of the cluster-type entangled coherent states in free travelling optical fields via cross-Kerr nonlinearity. The required resources for the generation are coherent state source, beam splitters, photodetectors, and Kerr media. We also discuss the implementation of the Hadamard gate operation for coherent states and the homodyne detection.     

Ultraslow optical solitons in a four-level tripod atomic system

We investigate possible formation and propagation of localized, shape-preserving nonlinear optical pulse in a resonant, lifetime-broadened four-level tripod atomic system via electromagnetically induced transparency (EIT). We prove both analytically and numerically that although in anomalous dispersion regimes near resonance a superluminal optical soliton may appear, such soliton suffers serious absorption. However, by choosing appropriate parameters to make the system work in normal dispersion regimes and within an EIT transparency window, ultraslow optical solitons with very low light intensity can form and propagate stably in the system.     

Soliton control in optical lattices with periodic modulation of nonlinearity coefficient

We address the dynamics of solitons in the optical lattices with periodic modulation of the nonlinearity coefficient. Based on the quasi-particle approach, the properties of fundamental soliton localized in optical lattices are theoretically analyzed and shown its potential application for controllable soliton switching. Moreover, the phenomena of multi-soliton splitting and the single-soliton constituent trapping in the optical lattices are illustrated and discussed.     

Asymmetrical shapes of optical line profiles in individual quantum dots

The experimental data on individual quantum dots show that the optical line can have the form of a very narrow spike accompanied by a shoulder. So far the shoulder has been found at the lower energy side of the narrow peak. In the present work, we study theoretically the origin of such a lineshape. We shall use a simple model of quantum dot and a simple approximation to the electronic excitation. The electronic system will be assumed to be coupled to the longitudinal optical phonons. We will show that the electronic multiple scattering on the optical phonons can then give us an explanation of the observed optical lineshape.     

Photorefractive spatial solitons as waveguiding elements for optical telecommunication

Spatial solitons permit optical waveguiding. This holds true for the soliton write beam (i.e. the driving laser beam), as well as for additional probe beams, which may carry optically encoded information. This feature of spatial solitons is of significant interest for applications in optical telecommunication. We present systematic experimental investigations on single and multiple spatial solitons in the infrared spectral regime (i.e. around optical telecommunication wavelengths), applied as controllable all-optical devices. In particular, we present the implementations of a Y-coupler as an optical signal divider, a switchable Y-coupler as an optical add multiplexer, and a novel design for a 1 × 3 optical beam switch, i.e. applied as a router for infrared signal beams. We report large waveguiding efficiencies up to 40% and transmission rates of 90 Tbit/s in our setups. The presented experimental data are confirmed by numerical simulations.     

Quantum theory of super-resolution for optical systems with circular apertures

We present the two-dimensional quantum theory of super-resolution, applicable for a large variety of optical systems with circular pupils. Our theory is formulated in terms of circular prolate spheroidal functions which form the eigen basis of two-dimensional imaging system with circular pupils. We provide, in particular, analytical and numerical results for the point-spread function characterizing reconstruction of optical objects with super-resolution from diffraction-limited images. We evaluate the super-resolution factor as a function of the signal-to-noise ratio in the input object for coherent light and multimode squeezed light.     

Configurable three-dimensional optical cage generated from cylindrical vector beams

We propose a new approach to generate a controllable three-dimensional (3D) optical cage by double-mode cylindrical vector beam in the vicinity of focus, by controlling the polarization state of beam. The simulation results show that the 3D optical cage of spheroid surface shape with a sharp dark region surrounded by a uniform optical barrier with a maximum deviation of 2% is achieved. The finite-difference time-domain calculation of optical force validates that such a kind of 3D optical cage has the trapping capability of the low-refractive-index particles with the size being much smaller than the light wavelength.     

Coherent ultrafast dynamics in the quantum Hall effect regime

We review recent investigations of the femtosecond non-linear optical response of the two-dimensional electron gas (2DEG) in the quantum Hall effect regime. We find that the time and frequency profile of the four-wave-mixing non-linear optical spectrum is strongly influenced by Coulomb correlations between the photoexcited electron-hole pairs and the 2DEG collective excitations. We discuss experimental and theoretical results showing non-Markovian memory effects in the polarization dephasing, and an optically induced time-dependent coupling between the two lowest Landau level magnetoexcitons.     

Low-energy signatures of charge and spin fluctuations in Raman and optical spectra of the cuprates

We calculate the optical and Raman response within a phenomenological model of fermion quasiparticles coupled to nearly critical collective modes. We find that, whereas critical scaling properties might be masked in optical spectra due to charge conservation, distinct critical signatures of charge and spin fluctuations can be detected in Raman spectra exploiting specific symmetry properties. We compare our results with recent experiments on the cuprates.     

Femtosecond hybrid mode-locked semiconductor laser and amplifier dynamics

We describe the generation of femtosecond high power optical pulses using hybrid passive-active mode-locking techniques. Angle stripe geometry GaAs/AlGaAs semiconductor laser amplifiers are employed in an external cavity including prisms and a stagger-tuned quantum-well saturable absorber. An identical amplifier also serves as an optical power amplifier in a stretched pulse amplification and recompression sequence. After amplification and pulse compression this laser system produces 200 fs, 160 W peak power pulses. We discuss and extend our theory, and supporting phenomenological models, of picosecond and subpicosecond optical pulse amplification in semiconductor laser amplifiers which has been successful in calculating measured spectra and time-resolved dynamics in our amplifiers. We have refined the theory to include a phenomenological model of spectral hole-burning for finite intraband thermalization time. Our calculations are consistent with an intra-band time of approximately 60 fs. This theory of large signal subpicosecond pulse amplification will be an essential tool for understanding the mode-locking dynamics of semiconductor lasers and for analysis of high speed multiple wave-length optical signal processing and transmission devices and systems based on semiconductor laser amplifiers.     

Optical trap with spatially varying polarization: application in controlled orientation of birefringent microscopic particle(s)

We report the use of the interference of two orthogonally polarized beams for generation of an optical trap with spatially varying polarization. The spatial variation of polarization in the optical trap has been used for demonstration of simultaneous rotation or orientation of multiple microscopic birefringent particles. Other potential applications of an optical trap with spatially varying polarization are also discussed.This revised version was published online in August 2005 with a corrected cover date.     

All-Optical RZ to NRZ Format Conversion Using Single SOA Assisted by Optical Band-Pass Filter

We propose a novel all-optical format conversion from the return-to-zero （RZ） to the non-return-to-zero （NRZ） based on single semiconductor optical amplifier （SOA） and optical band-pass filter （OBE）. We demonstrate the proof of the principle experiment at 10 Gbps by using the test SOA and OBF converter. The format conversion can be achieved with output extinction ratio of 11.51 dB. The BER is 5.5×10^-9 when the power of NRZ is - 10 dBm. The proposed scheme is robust and potential for applications in optical networks.     

Optical modes in a nonlinear double-channel waveguide

We analytically address different types of optical modes in a coupler composed by two nonlinear optical waveguides. It is shown that the coupler not only supports symmetry-preserving modes but also symmetry-breaking modes. In addition, the properties on the existence and bifurcation of those modes are analyzed in detail.