Entanglement of the l- and the r-photons in an anisotropic crystal

Light incident onto an anisotropic crystal is divided into the ordinary and the extraordinary waves which vibrate in two perpendicular directions. Because of the tensor property of the dielectric constant, the direction of the electric displacement is not parallel to the vector of the incident electric field. An optical torque is induced by incidence of the linearly polarized light and propagating through the crystal. The optical torque tends to rotate the directions of eigenvibration which results in self-modulation of the ordinary and the extraordinary waves, and causes an energy splitting of the left (l)-, and the right (r)-handed circularly polarized waves in the crystal. The l- and the r-photons are correlated through the optical torque, which are found to be in an entanglement state.     

Structural geometry of grain boundaries in icosahedral quasicrystals

The mechanically induced gyrotropy is studied in crystal plates of hexagonal CdS near the frequency of isotropic point (in which the refractive indices of the ordinary and extraordinary waves are equal). The transmittance of light by torsion deformed crystal (when the torsional moment is orientated along the opticalc-axis) between two crossed polarizers is measured as well as the polarization parameters of elliptically polarized emergent light. In the discussion the hypothesis that the mechanical deformation induces gyrotropy is argued. The experimental results are interpretated in the frameworks of the theory of the elliptical birefringence.     

Laser-induced helical structure in the isotropic phase of nematic liquid crystal

We report on the first observation of laser-induced helical structure in the isotropic phase of a nematic liquid crystal. The helical structure was induced by two nearly counter-propagating waves circularly polarized in the same sense. Bragg reflection in the optical tunneling region was studied. We made an extensive investigation on the analogy between Bragg reflection from a cholesteric liquid crystal slab and four-wave mixing. It provides a deeper insight into the physics of the optical properties of cholesteric liquid crystal.     

Polarization-anisotropic scattering lines in LiNbO3

Lines of light-induced scattering with extraordinary polarization from two ordinary polarized pump waves in LiNbO₃:Fe crystals are observed. A nonlinear mixing of four copropagating waves in the crystal with photovoltaic charge transport is shown to be the reason for parametric amplification of the seed radiation scattered from optical imperfections of the sample.     

Giant Raman optical activity of the 128-cm-1 mode in α-quartz

The threshold intensity of stimulated Raman scattering along the c-axis in α-quartz was measured for the 128-cm^-1 optical lattice vibration as a function of pump laser polarization at T = 10 K. In a right-handed, optically active quartz crystal the Raman threshold intensity for left-handed, circularly polarized pump light was lower by a factor of about 1.5 than for right-handed, circularly polarized light. The difference in threshold intensity is discussed in terms of Raman optical activity.     

Magneto-optic effects in ferromagnetic films: Implications for spin devices

We present results on the magneto-optic properties of ferromagnetic films deposited on GaAs and SiO₂ substrates. Using left- and right-circularly polarized light, we have measured the polarization-dependent photoresponse and reflectivity of Co/GaAs, Fe/GaAs and NiFe/GaAs Schottky diodes and the polarization-dependent reflection and transmission of NiFe/SiO₂ and Co/SiO₂ structures as a function of ferromagnetic film thickness, reported here in the range of 7.5-15 nm. Films were prepared by sputtering and molecular-beam epitaxy. Measurements were made in the presence of magnetic fields ranging from −1.2 to +1.2 T both parallel and perpendicular to the sample surface. We find maximum polarization-dependent transmission and photoresponse effects (with respect to left- versus right-circularly polarized light) of 2-4% in magnitude. Taken together the work suggests that magneto-optic effects intrinsic to the films, rather than spin injection across the ferromagnetic/semiconductor interface, are responsible for the observed phenomenology. The work has direct implications for the interpretation of results in ferromagnetic/semiconductor spintronic systems.     

Optics of electrically controlled structurally chiral material with periodic transverse perturbation for polarization-universal bandgaps

An electro-optic structurally chiral medium - with periodic transverse perturbation such that its unit cell contains structurally left-handed and structurally right-handed halves that are otherwise identical - exhibits an electrically tunable, polarization-universal bandgap for normally incident plane waves.     

Self-modulation of infrared waves in rutile

Electromagnetic waves carry angular and linear momentums and exert torques on anisotropic dielectrics, arising from the fact of the tensor property of the dielectric constant, that is, the direction of electric displacement is not parallel to the electric field vector of the incident light. The torque per unit volume exerted on a wave plate is given by P×E$\mathit{P}\times \mathit{E}$, where P is the polarization and E is the electric field, which induces the rotations of eigenvibration direction in the crystals. The rotation angles increase with the intensity of the incident light and the dielectric constant of the crystals. Because of the large dielectric constants, self-modulation of the incident light in the infrared frequency region was clearly demonstrated in the infrared transmission spectra of ferroelectric and piezoelectric crystals. Rutile (TiO₂) is a non-ferroelectric and non-piezoelectric crystal, but it also has the large dielectric constants. Rotations of the vibration direction of the ordinary (o-ray) and the extraordinary (e-ray) waves were shown in the infrared transmission spectra recorded by incidence of the plane-polarized light and transmission through a rutile plate. Interference of the o-ray and the e-ray waves transmitted through the crystals confirms the rotations of eigenvibration direction, a self-modulation effect of light in the crystal of large dielectric constants and large birefringence in the infrared range.     

Soliton dynamics in chiral molecular chains with first- and third-neighbour interactions

We investigate the propagation and interaction of solitons associated with circularly polarized vibrations in gyrotropic media. The chirality of the structure yields different dispersion laws and hence different phase and group velocities for the left- and right-handed modes. The helical arrangement of the monomers is modelled through first- and third-neighbour interactions. The dynamics of the excitations is governed by a system of coupled discrete nonlinear Schr?dinger equations which is studied both analytically and numerically. Depending on the initial conditions and the interaction constants, different evolutionary patterns are obtained corresponding to unbound or bound one- and two-soliton solutions. The results can be applied to the process of energy transfer in helical polymers. Received 1st October 2001 / Received in final form 30 April 2002 Published online 2 October 2002 RID="a" ID="a"e-mail: krad@issp.bas.bg     

Electromagnetic unidirectionality and frozen modes in magnetic photonic crystals

Magnetic photonic crystals are periodic arrays of lossless materials, at least one of which being magnetically polarized. Magnetization, either spontaneous or induced, is associated with nonreciprocal effects, such as Faraday rotation. Magnetic photonic crystals of certain configuration can also display strong spectral asymmetry, implying that light propagates in one direction much faster or slower than in the opposite direction. This essentially nonreciprocal phenomenon can result in electromagnetic unidirectionality. A unidirectional medium, being perfectly transmissive for electromagnetic waves of certain frequency, freezes the radiation of the same frequency propagating in the opposite direction. The frozen mode has zero group velocity and drastically enhanced amplitude. The focus of our investigation is the frozen mode regime. Particular attention is given to the case of weak nonreciprocity, related to the infrared and optical frequencies. It appears that even if the nonreciprocal effects become vanishingly small, there is still a viable alternative to the frozen mode regime that can be very attractive for a variety of practical applications.     

Giant optical activity and circular dichroism in the terahertz region based on bi-layer Y-shaped chiral metamaterial

We present a bi-layer Y-shaped chiral metamaterial (CMM) that can realize a giant optical activity and circular dichroism (CD) effect to the incident linear polarization wave in the terahertz (THz) region. Numerical simulation results exhibit that the pronounced CD effect with a great difference between the transmission coefficients for the circularly polarized waves can be obtained at 5.06 THz, meanwhile the 90°-polarization rotation can be observed at 5.2 THz when a y-polarized wave is incident to this CMM propagating along the −z-axis. The mechanism of the optical activity and giant CD effect is illustrated by simulated surface current distributions. Further, the influences of the structural parameters of the proposed CMM to the optical activity and CD effect have been investigated numerically.     

Torque equations for spin and orbital angular momenta of radiation fields and Faraday effect

The spin angular momentum S of light has never been linked to the Faraday rotation of light traveling in an optically active medium possessing a rotational invariance of a crystal, because there was no helicity term associated with the phase shift in the previous torque equation for S. In order to relate the change in S with time to the Faraday rotation, therefore, we derived an exact torque equation for S. As a result, a magnetic helicity term appeared in a new torque equation for S, so that one-half of the phase shift derived from the helicity term was equivalent to the Faraday rotation angle. However, the orbital angular momentum L had no relation to the Faraday rotation. It was thus clarified that the change in S with time is related to the Faraday rotation angle of light traveling in an optically active medium, owing to the appearance of the helicity term without a rotational invariance around the optical axis. It was also demonstrated theoretically that the Faraday rotation is accompanied by a torque acting on the crystal so that the total angular momentum of light and matter is conserved.     

Mirrorless parametric oscillation in BaTiO3

We report on the first observation of mirrorless oscillation in BaTiO₃ pumped by two nearly counterpropagating mutually incoherent light waves, both containing an ordinary and an extraordinary component. The oscillation waves appear at a certain angle defined by the phase matching condition for four-wave mixing of orthogonally polarized waves.     

A phase sensitive optical rotation measurement in a scattered chiral medium using a Zeeman laser

A novel circular polarized optical heterodyne interferometer using a Zeeman laser to measure optical rotation both in nonscattered and scattered chiral medium is proposed. A pair of correlated orthogonal circular polarized light waves of different temporal frequency propagating in the chiral medium at different speed is studied. This results in phase retardation between circular polarized light waves of which the phase difference is proportional to the optical rotation angle of a linear polarized light in a chiral medium. In the mean time, two orthogonal circular polarized light waves can be treated as a circular polarized photon pair that is able to reduce the scattering effect in a scattered chiral medium. Then the optical rotation angle can be measured in the scattering medium. In addition, a common-path configuration with respect to circular polarized light waves immune the background noise. This further improves the sensitivity on optical rotation measurement based on phase difference detection.     

Negative refraction imaging of solid acoustic waves by two-dimensional three-component phononic crystal

By using of the multiple scattering methods, we study the negative refraction imaging effect of solid acoustic waves by two-dimensional three-component phononic crystals composed of coated solid inclusions placed in solid matrix. We show that localized resonance mechanism brings on a group of flat single-mode bands in low-frequency region, which provides two equivalent frequency surfaces (EFS) close to circular. The two constant frequency surfaces correspond to two Bloch modes, a right-handed and a left-handed, whose leading mode are respectively transverse (T) and longitudinal (L) modes. The negative refraction behaviors of the two kinds of modes have been demonstrated by simulation of a Gaussian beam through a finite system. High-quality far-field imaging by a planar lens for transverse or longitudinal waves has been realized separately. This three-component phononic crystal may thus serve as a mode selector in negative refraction imaging of solid acoustic waves.     

A Novel Method for Enhancing Goos-Hänchen Shift in Total Internal Reflection

Due to the tiny shift in order of optical wavelength for Goos-Hǎnchen （GH） shift, it is very difficult to directly measure and apply the GH shift. We develop a new method for enhancing GH shift of both TE and TM polarized waves. The method is based on a total reflection prism made of BK9 glass combined with a precise measurement of the resulting spatial displacement with a one-dimensional charge coupled device （CCD）. Measurements are performed to examine the validity of the method. Experimental and theoretical results indicate the feasibility of the method with an enhancement in optical wavelenghth shift at millimetre scale. The method is advantageous to application the GH shift in the optical domain, and is also meaningful for measuring even smaller changes in the refractive index of a liquid.     

Photorefraction and anisotropic light scattering in LiNbO3-Fe crystals

By holographic recording in LiNbO₃-Fe and LiTaO₃-Fe crystals a new light scattering effect has been observed with an optical indicatrix along the optical axis. The kinetics of the light scattering depends on the intensity, wavelength as well as the polarization of the incident light The holographic volume grating is created by the interference of an incident light and light scattered by crystal inhomogeneities.     

Photonic spin Hall effect and terahertz gas sensor via InSb-supported long-range surface plasmon resonance

We propose a simple one-dimensional grating coupling system that can excite multiple surface plasmon resonances for refractive index(RI)sensing with self-reference characteristics in the near-infrared band.Using theoretical analysis and the finite-difference time-domain method,the plasmonic mechanism of the structure is discussed in detail.The results show that the excited resonances are independent of each other and have different fields of action.The mode involving extensive interaction with the analyte environment achieves a high sensitivity of 1236 nm/RIU,and the figure of merit(FOM)can reach 145 RIU-1.Importantly,the mode that is insensitive to the analyte environment exhibits good self-reference characteristics.Moreover,we discuss the case of exchanging the substrate material with the analyte environment.Promising simulation results show that this RI sensor can be widely deployed in unstable and complicated environments.     

Photonic crystals with a chiral basis by holographic lithography

We demonstrate the use of holographic lithography to fabricate chiral photonic crystals. These structures are calculated to exhibit strong optical activity even though they are made from material that is not intrinsically optically active. By control of the polarizations of the interfering plane waves that are used to define the three-dimensional microstructure it is possible to create left- and right-handed and closely related non-chiral structures.     

Rotation of polarization and ellipticity of ultrasound in helicon-phonon resonance conditions in indium

Rotation of polarization as well as ellipticity of initially linearly polarized transverse ultrasonic mode have been observed and investigated in helicon-phonon resonance region in single crystal of indium. The peculiarities of the experimental curves of rotation angle and ellipticity have shown, that the considered phenomena are determined by the superposition of three circularly polarized waves: left-handed polarized phonon and two right-handed polarized coupled helicon-phonon modes.