Surface plasmon-assisted optical bistability in the quantum dot-metal nanoparticle hybrid system

We theoretically investigated optical bistability (OB) of a coupled excition–plasmon hybrid system in a unidirectional ring cavity. It is found that the threshold and the region of OB can be tuned by adjusting the center–center distance between the quantum dot and metal nanoparticle (MNP), the Rabi frequency of the control field and the radius of the MNP. Due to the significantly enhanced optical nonlinearity by the surface plasmon effect, the threshold of OB can be decreased greatly when the probe field is parallel to the major axis of the hybrid system. The enhanced OB may have promising applications in optical switching and optical storage.     

An effective-medium approach to the optical properties of heterogeneous materials with nonlinear properties

Abstract

We have derived an analytic approach, based on the hypothesis of the effective-medium theory, to evaluate the effective dielectric magnitudes, including the effective third order nonlinear optical susceptibility α(χ⁽³⁾), of a heterogeneous two-component medium in which one of the components (the host matrix or the embedded particle respectively) presents nonlinear behaviour. Our formulation allows us to solve the full nonlinear problem in the whole range of concentrations without treating the nonlinear effects as a small perturbation to the linear behaviour. Therefore, it can be considered as an upper limit of the commonly used low-field nonlinear approximations for heterogeneous materials. Under certain conditions, these composites can exhibit a bistable regime that the present theory properly describes in wide ranges of the concentration, shape, wavelength, dielectric contants and intrinsic nonlinear optical susceptibility of the nonlinear component as well as in terms of the intensity of the external electric field. The present approach has been used to calculate the nonlinear optical response of Cu—Al₂O₃ nanocomposites.     

Giant Electro‐Optical Effect through Electrostriction in a Nanomechanical Metamaterial

Electrostriction is a property of all naturally occurring dielectrics whereby they are mechanically deformed under the application of an electric field. It is demonstrated here that an artificial metamaterial nanostructure comprising arrays of dielectric nanowires, made of silicon and indium tin oxide, is reversibly structurally deformed under the application of an electric field, and that this reconfiguration is accompanied by substantial changes in optical transmission and reflection, thus providing a strong electro‐optic effect. Such metamaterials can be used as the functional elements of electro‐optic modulators in the visible to near‐infrared part of the spectrum. A modulator operating at 1550 nm with effective electrostriction and electro‐optic coefficients of order 10^?13 m² V^?2 and 10^?6 m V^?1, respectively, is demonstrated. Transmission changes of up to 3.5% are obtained with a 500 mV control signal at a modulation frequency of ≈6.5 MHz. With a resonant optical response that can be spectrally tuned by design, modulators based on the artificial electrostrictive effect may be used for laser Q‐switching and mode‐locking among other applications that require modulation at megahertz frequencies.     

Laser Excitation Effect on the Optical and Dielectric Properties of Ferroelectric KNbO3 Single Crystals

The effects of laser irradiation on the optical absorption (200- to 800-nm range) and dielectric properties (10²- to 10⁷-Hz range) of KNbO₃ single crystals in ferroelectric phase have been studied. All these properties are found to be reduced significantly at lower frequencies with an increase in laser irradiation time. However, the phase transition temperature remains unchanged. It is observed that the dielectric constant (K) and loss (tan ) values of KNbO₃ single crystals are decreased significantly at room temperature or at Curie temperature by laser excitation. Appreciable changes in the band gap (eV) and activation energy are also observed after laser excitation.     

Infrared Surface Plasmon-polaritons on Ni,Pd and Pt

Abstract

Surface plasmon-polaritons (SPP) have been excited on evaporated films of the transition metals nickel, palladium and platinum. Using the Otto geometry and a He/Ne laser operating at 3·391 μm we report the first characterisations of these metals using this technique. Adjusting the air-gap over the range 4 to 15 μm has allowed observation of > 95% coupling of p-polarised (TM) radiation to the SPP and precise quantification of the optical dielectric constants of the metal films. The relatively large imaginary components so obtained mean that these materials may not readily be used in the Kretschmann geometry at this wavelength. By contrast the SPP resonances observed using the Otto geometry are sharp and open up potential for catalytic studies with SPP excitation on these metals.     

Optical properties of binary composite materials with two nonlinear components

Abstract

A new formulation is presented for the calculation of effective dielectric magnitudes of two-component composites in which both components (the host matrix particles and the embedded particles) exhibit nonlinear behaviour of the Kerr type. It is predicted that, under certain conditions, two nonlinear component composites can exhibit optical bistable behaviour as a function of the shape and concentration of the embedded particles, the dielectric contants of the components, the intensity of the external electric field (power density) and the intrinsic third-order nonlinear optical susceptibilities χ⁽³⁾ _p and χ⁽³⁾ _m of the nonlinear components. It is also deduced that, as the power density increases, the effective third-order nonlinear optical susceptibility χ⁽³⁾ of the composite exhibits a clear transition from values close to χ⁽³⁾ _p (low power density) to χ⁽³⁾ _m (high power density). Therefore, it is shown that the optical response of binary composites dramatically changes at moderate and high power densities. A comparison is performed between the optical response of a real two nonlinear component composite and that of a composite with a single nonlinear component.     

Stabilization of an 852 nm extended cavity diode laser using the Zeeman effect

Abstract

We demonstrate the use of the Zeeman effect in Cs vapour to stabilize an ultra-compact extended cavity diode laser (ECDL) operating at 852 nm. We investigate the expected laser stabilization error signal for a range of magnetic fields and are able to tune the locked ECDL by variation of the magnetic field. We also study in detail the tuning of the laser frequency using optical methods. The ECDL has a linewidth of 520kHz and the drift, when locked, is of the order of 5 MHzh^?1.     

Electric Field Distribution in Multilayer Thin-film Polarizers

Abstract

The distribution of electric-field amplitude inside multilayer thin-film polarizers is evaluated for various polarizer designs. The polarizers, based on (H/2, L, H/2) as the basic stack, show higher field amplitude than those based on all quarter-wave layers. Here H and L represent quarter-wave layers of high- and low-index materials respectively. It has been pointed out that, to avoid damage to the polarizers in high-power laser systems, it is necessary to know the field distribution inside the layers both for radiation incident from the air-multilayer interface as well as the glass-multilayer interface. Some implications of the positioning of thin-film polarizers in Faraday isolators used in high-power laser chains are discussed in relation to the susceptibility of these polarizers to optical damage. The results indicate that the liquid-prism MacNeille polarizer shows a higher damage threshold, which can possibly be further improved by increasing the angle of incidence at the multilayer.     

Fast fabrication of silicon based microstructures using 355 nm UV laser

Abstract

In this study, a 355 nm UV Nd:YAG laser is used to process silicon wafers. In order to obtain microstructures with high aspect ratio, a dual prism optical system is set up to control the cutting linewidth of the UV laser beam. During the laser beam propagation through the prisms, the two prisms are rotated with the same angular velocity, which results in the focal spot of the laser beam moving in a circular path on the silicon substrates. When the laser beam moves relative to the holder (workstation), a laser cutting process can be carried out. With this laser system, the cutting linewidth is controllable ranging from 10 μm to 1 mm by adjusting the initial phase difference in the two prisms. The experimental results show that arbitrary shaped silicon based microstructures with high aspect ratio can be fabricated by this 355 nm UV laser system, and the aspect ratio over 10 can be obtained.     

Partially Coherent Imaging of a Straight-edged Object Associated with Periodic Structures of Acoustically Modified Spatial Coherence

Abstract

Spatial coherence of laser light can be modified by a progressive ultrasonic wave. The resultant optical mutual intensity has lateral and longitudinal periodicities in the near field behind the ultrasonic cell. Over the near field the longitudinal periodicity is lost in any plane parallel to the output plane of the cell but the lateral periodicity still remains. Such a plane serves as an object plane for partially coherent imaging. The imaging characteristics of an edge object, depending on various ultrasonic parameters, are described. Ringing suppression and contrast enhancement occur satisfactorily in the image if the ultrasonic parameters are properly controlled. Theoretical predictions are in good agreement with the experimental results.     

Getting effective permittivity and permeability equal to −1 in 1D dielectric photonic crystals

J.B. Pendry (Phys. Rev. Lett., 86 3966 (2000)) mentioned the possibility of making perfect lenses using a slab of left-handed material with relative permeability, permittivity and optical index equal to ?1. This kind of metamaterial has been made in the microwaves domain, using metal and dielectric materials. On the other hand, it has been shown that lenses made using 2D dielectric photonic crystals can generate similar imaging properties, but until now, the image contains only a small part of the incident light. The paper shows, using a very simple analytical model, that 1D dielectric photonic crystals can generate left-handed materials with relative permeability, permittivity and optical index rigorously equal to ?1. Of course, such photonic crystals cannot be used to make perfect lenses, but this conclusion leads to the conjecture that 2D or 3D dielectric photonic crystals could be used in the visible region to realize superlenses.     

Determination of the Director Alignment in a Ferroelectric Liquid Crystal Device by Observation of Optical Modes

Abstract

By observing the angular dependence of the reflectivity of a ferroelectric liquid crystal (FLC) cell fabricated from glass pyramids with 60° evaporated SiO aligning layers it has been possible to sensitively probe the configuration of the optical dielectric tensor in the FLC layer. It is found that the optical data can best be explained by a uniaxial slab twisted from the alignment axis; this is consistent with the recently proposed chevron structure.     

Influence of detuned injection locking on the relaxation oscillation frequency of a multimode semiconductor laser

Abstract

The injection locking characteristics of a multi-mode semiconductor laser are considered. A formalism is developed to investigate the stability properties of an arbitrary laser mode subject to optical injection. The formalism is used to show that the relaxation oscillation frequency (ROF) in a semiconductor laser subject to optical injection is increased relative to that of the free running laser diode. Methods of utilizing positive detuning to determine the best approach of increasing the ROF of a semiconductor laser via injection locking are considered.     

An All-optical Mechanism of Sub-poissonian Light Generation

Abstract

We investigate the interaction of two counterpropagating linearly polarized waves in a dielectric waveguide grating in the presence of an intensity-dependent contribution to the refractive index (optical Kerr effect). The resulting nonlinear distributed-feedback (DFB) mechanism responsible, from a classical point of view, for a marked limiting effect which stabilizes the intensity of one of the two waves, is shown to generate, once the problem is treated in a quantum-mechanical way, a field exhibiting sub-Poissonian photon statistics.     

Statistical distribution of the optical intensity obtained using a Gaussian Schell model for space-to-ground link laser communications

Abstract

Based on the characteristics of the laser device and the inevitable error of the processing technique, a laser beam emitted from a communication terminal can be represented by the Gaussian Schell model (GSM). In space-to-ground link laser communications, the optical intensity is affected by the source coherence parameter and the zenith angle. With full consideration of these two parameters, the statistical distribution model of the optical intensity with a GSM laser in both downlink and uplink is derived. The simulation results indicate that increasing the source coherence parameter has an effect on the statistical distribution of the optical intensity; this effect is highly similar to the effect of a larger zenith angle. The optical intensity invariably degrades with increasing source coherence parameter or zenith angle. The results of this work can promote the improvement of the redundancy design of a laser communication receiver system.     

Reflection and refraction of light at saturating active boundaries

Abstract

This study deals with the topic of reflection and refraction of light from the boundary of a high-gain saturating laser amplifier. Expressions for the electric field and intensity reflection and transmission coefficients are obtained for the case of a wave in a uniform dielectric that is incident normally on a nonlinear saturating active medium. Bistability effects are predicted even for the simplest case of zero frequency detuning from the line centre of the saturating transition.     

Measuring changing strain fields in composites with Distributed Fiber-Optic Sensing using the optical backscatter reflectometer

Background/purposeMeasurements of strains in critical components are often required in addition to finite element calculations when evaluating a structure.MethodsThis paper describes how standard optical fibers, bonded to the surface or embedded in a laminate, can measure strain fields along the entire length of the fiber, using the optical backscatter reflectometer.ResultsA strain field measurement can be much better compared to simulations than the more common single point measurements with strain gauges or Bragg Gratings. Changes of the strain field can be related to damage development and can be used for structural health monitoring. Practical aspects of using the fibers are also discussed.ConclusionDistributed Fiber-Optic Sensing was successfully embedded and bonded to a composite joint. Adhesive damage was identified and the strain field agreed well with FE-Analysis.     

Fringing fields in a liquid crystal spatial light modulator for beam steering

Abstract

Phase modulating spatial light modulators (SLMs) can be used to alter the shape of a laser wavefront to achieve a deflection or change in the shape of a laser beam. This paper reports the results of characterization, simulation and optimization of a one-dimensional liquid crystal (LC) SLM. The device has a large ratio between LC layer thickness and pixel pitch that results in a fringing field between pixels. In effect, the applied phase patterns will be lowpass filtered and the loss of high frequency components limits, for instance, the usable steering range. A method is presented where intensity measurements in the far field are used to determine how the phase modulation at the SLM is distorted. The inhomogeneous optical anisotropy of the device was determined by modelling the liquid crystal director distribution within the electrode-pixel structure. Finite-difference time-domain (FDTD) simulations were used to calculate the light propagation through the LC. The simulated phase distortion was compared with the experimental results. A voltage compensation scheme to improve the diffraction efficiency was developed utilizing the measured and simulated results. It is demonstrated that a modification of the voltage patterns can give a better realization of high frequency components in the phase distribution and an increase in maximum steering angle by a factor two.     

Fast photonic switches based on GaAs nanostructures

Fast photonic switches, in which light controls light, can be created based on high-voltage GaAs nanostructures with a thin (nanodimensional) surface dielectric layer. The switches offer a high operation speed that ensures optical data recording and transmission at a rate of 10⁴-10⁵ cps, possess a large modulation amplitude, and can operate at a relatively low optical control signal power (I<1 W/cm²). These devices can be used in systems of optical data processing, optical computation systems, all-optical communication lines with optical addressing of informative signals, etc.     

Interferometric Measurements of Phase Singularities in the Output of a Visible Laser

Abstract

We report the use of a simple interferometric technique which allows direct identification of phase singularities in laser fields. Phase singularities are observed in families of optical patterns formed via cooperative frequency mode locking in a continuous single longitudinal mode Na₂ ring laser. The interferometric technique complements a previously reported astigmatic imaging method, and is superior in that it can be used to elucidate the structure of the higher order stationary patterns.