Nonlinear Birefringence Research Articles

Stability and symmetry breaking of soliton bound states

With the variational method we analytically study rotating vector soliton bound states in Kerr media. A standard linear stability analysis reveals that, in the presence of nonlinear birefringence, these soliton bound states exhibit a symmetry-breaking instability. In the absence of nonlinear birefringence, that is, in the Manakov system, the bound states are shown to be neutrally stable and not robust against soliton collisions. We show that these stability properties are not influenced by saturation of the nonlinearity, a result that is relevant to recent experimental observations of soliton bound states in photorefractive media.

Spontaneous Faraday rotation due to strong laser radiation in a plasma

Theoretical studies on intense laser radiation interacting with plasma reveal an induced nonlinear birefringence which turns out to be spontaneous Faraday rotation. Here, this rotation is called nonlinear Faraday rotation (NFR). The expressions for the nonlinear refractive indices of the laser fields are calculated both in relativistic and non-relativistic limits for a two-component plasma. The NFR angles due to nonlinear induced birefringence are derived in the absence of externally imposed magnetic fields. It is shown that, in the relativistic limit, the electron motion plays an important role in producing NFR and so in the generation of induced magnetic fields.

Enhanced optical nonlinearity by photoinduced molecular orientation in absorbing liquids

We present a theoretical and experimental study of the molecular orientation induced in a light-absorbing liquid of rodlike molecules by a laser pulse that explains the large enhancement of anisotropic optical Kerr nonlinearity observed in liquids in which a small amount of dye has been dissolved. Our model is based on the assumption that a molecule changes significantly its rotational kinetic and equilibrium behavior when it is electronically excited by the absorption of a photon. The anisotropy of the absorption probability then results in a corresponding orientational anisotropy of the molecules involved in electronic transitions, which in turn orient the whole liquid via intermolecular interactions. To test our model, we performed nanosecond pump-and-probe measurements of nonlinear birefringence at varying pump pulse energy and temperature. In our experiments the host was a liquid-crystal mixture in its isotropic phase and the dye was an anthraquinone derivative. Both the decrease in the nonlinearity enhancement observed for increasing pulse energies and the pretransitional behavior occurring for temperatures approaching the isotropic-nematic transition point are in good agreement with the predictions of our model. The contribution of secondary effects related to light absorption and light-induced heating has been also taken into account in order to properly compare the nonlinearity of an absorbing liquid with that of a transparent one.

Two-dimensional noise-sustained structures in optical parametric oscillators

The problem of two-dimensional (2D), transverse, noise-sustained pattern formation is theoretically and numerically studied, in the case of an optical parametric oscillator, for negative signal detuning. This gives a complete analysis of a 2D, convective, pattern forming system which is also relevant to more general 2D physical systems. For the optical parametric oscillator, the transversal walk-off due to the nonlinear crystal birefringence, exploited to phase match the frequency down-conversion process, turns the instability to convective up to a certain threshold. In this regime, noise-sustained patterns can be observed. These structures are a macroscopic manifestation of amplified microscopic noise which, in the context of optics, can be of quantum nature. Directly observable properties of the near and far field as well as statistical properties of the spectral intensity help to distinguish noise- from dynamics-sustained structures. Moreover, the analysis indicates that the walk-off term breaks the rotational symmetry of the 2D model. This causes a preferential selection of the stripe orientation, which would be otherwise random, the modulus of the wave vector being the only restricted value. At the convective threshold an entire set of spatial modes becomes unstable, whereas the threshold of absolute instability depends on the relative orientation of the mode. Beyond the threshold for absolute instability, this causes the coexistence, in the linear regime of evolution, of modes that are absolutely unstable, and others that are only convectively unstable. The numerical solutions of the dynamical equations of the system under study confirm the analytical predictions for the value of the instability thresholds and the kind of pattern selected. Moreover, they allow us to investigate the nonlinear regime showing qualitatively the coexistence of modes with different types of instability and giving a quantitative characterization of the transition from noise-sustained to dynamics-sustained structures.

Domain walls of linear polarization in isotropic Kerr media

We present a new type of domain-wall vector solitary waves in isotropic self-defocusing Kerr media. These domain walls consist of localized structures separating uniform field domains of orthogonal linear polarizations. They result from the interplay between diffraction, self-phase modulation and cross-phase modulation in cases where the nonlinear birefringence coefficient B = χxyyx(3)χxxxx(3) is negative. Numerical simulations show that these new vector solitary waves are stable.

Pump–probe reflective polarization-sensitive nonlinear optics

We report on the phenomenological theory of frequency degenerate pump–probe polarization-sensitive wave interactions at the boundary with a nonlinear medium. We consider media with local and nonlocal optical responses. The general analytical solutions for polarization-plane azimuth rotation and ellipticity of the transmitted and reflected probe waves are obtained. It is found that the pump-intensity-dependent change of the polarization state of transmitted and reflected probe waves has coherent and incoherent contributions. Nonlinear polarization phenomena of the specular inverse Faraday effect, the specular optical Kerr effect, the specular nonlinear anisotropic effect, specular nonlinear optical activity, and specular nonlinear nonlocal birefringence are discussed.

Theory of a fibre ring laser mode-locked by cross-phase modulation

We have developed a computer model of a novel fibre ring laser, optically mode-locked by cross phase modulation. We have analysed the modulation process in detail, showing that, in the context of the earlier experiment, the nonlinear birefringence induced by cross-phase modulation gives rise to a loss modulation component. Numerical results suggest that this feature is essential if efficient mode-locking is to occur. The importance of attenuation and group delay dispersion in the modulator fibre has been demonstrated.

Nonlinear birefringence of optical fibers

The nonlinear birefringence of low birefringent optical fibers has been studied experimentally. It is found that the experimental results are consistent with theoretical predictions. Earlier discrepancies between experimental results and theory could be due to the failure to compensate for the linear birefringence adequately.

Third order optical non–linearities of nematic liquid crystals

Abstract Liquid crystals can exhibit large third order optical non-linearities. Using the Z-scan technique, we have measured the intensity dependence of the non-linear absorption and non-linear refractive indices on time-scales ranging from milliseconds to picoseconds for nematic liquid crystals. This method allows the determination of the non-linear absorption coefficients as well as the signs and magnitudes of the non-linear refractive indices for different polarizations. A two pulse technique further enables estimation of the response time of the dominant mechanism. Using CW argon and Q-switched and mode-locked pumped dye lasers, we have carried out Z-scan measurements on aligned liquid crystal samples as a function of temperature, as well as wavelength. In the geometries studied, director reorientation is not expected to take place. On the nanosecond time-scale, all materials studied were self-focusing for polarization perpendicular to the director, and self-defocusing for parallel polarization. On the picosecond timescale, the samples were self-focusing for all polarizations, but strong non-linear birefringence was typically observed. An attempt is made to relate the bulk response to the molecular structures.

Photovisco-elastoplastic behavior of polycarbonate material under creep and tension tests

Polymers are widely used as photomechanical models of a prototype material (often a metal). Photoplasticity is one of the methods used in order to show the behavior of plastic materials stressed beyond the linear elastic limit. To illustrate this process we have analyzed the photovisco-elastoplastic behavior of polycarbonate as a photoplastic material. In this paper a technique for local and simultaneous measurement of birefringence and principal strains is presented. The mechanical and optical properties, at room temperature, have been evaluated by means of uniaxial tension tests. A series of creep tests has been carried out in order to study the photovisco-elastoplastic behavior of polycarbonate. In two different experiments we analyzed nonlinear birefringence and the amplitude of the corresponding strains. We could thus evaluate the distribution of strains and the distribution of uniaxial stress for each birefringence state and vice versa.

Self-sustained mode locking using induced nonlinear birefringence in optical fibre

A numerical study of self-sustained mode locking due to optically induced nonlinear birefringence is presented. The model incorporates a polarizer, a gain medium, a half-wave plate and a single-mode passive fibre in a ring cavity configuration. The mechanism exploits polarization instability in optical fibre with a weak, linear, intrinsic birefringence.

Propagation of Short Light Pulses in Nonlinear Birefringent Fibre

Abstract An analytical approach is developed to obtain the solution of coupled Schrödinger equations for polarization components of a solitary wave. Variational analysis allows us to describe optical pulse propagation in fibres with nonlinear birefringence. Conditions for polarization channel switching and locking are formulated.

Optical multistability in a nonlinear cavityless system

We describe an all-optical cavityless multistable system that does not necessitate coherent beams. The experimental demonstration showing bistability was done by using a strontium barium niobate crystal that has a nonlinear birefringence of thermal origin.

Self-starting passively mode-locked fibre ring laser exploiting nonlinear polarisation switching

Nonlinear birefringence effects in an all-fibre ring laser cavity have been exploited to produce self-starting passive mode-locking with pulse durations lying in the nanosecond region.

Shaping of clean, femtosecond pulses at 1053 μm for chirped-pulse amplification

We report the generation of 200-fs optical pulses at 1.053 μm using two stages of fiber-grating compressors. The usual intensity background associated with this compression technique has been reduced to less than 10−7 of the peak intensity by using nonlinear birefringence in the second fiber. These pulses are used at the front end of our chirped-pulse amplification terawatt Nd:glass laser system.

Selfstarting passively mode-locked fibre ring soliton laser exploiting nonlinear polarisation rotation

Nonlinear birefringence effects in a fibre ring laser cavity have been exploited to produce selfstarting, passive mode-locking to give 1.5 ps soliton pulses.

Polarization-dependent optical nonlinearities in fractional-layer superlattices

We measure the room-temperature polarization-dependent nonlinear absorption and refractive-index spectra of a (Al0.5Ga0.5As)1/2(GaAs)1/2 fractional-layer superlattice (FLS) structure grown by metalorganic chemical vapor deposition. The anisotropic nonlinear effects between the directions parallel and perpendicular to the superlattice give rise to a nonlinear optical birefringence in the plane of the growth surface. From our measurements using a femtosecond optical pulse, we derive the magnitude and spectral shape of the nonlinear optical birefringence. We describe the basis of an all-optical polarization rotation switch using the FLS structure.

Quantum Dynamics of a Nonlinear Birefringent Model with Anticorrelated Field States

Abstract In this paper, anticorrelated field states with a binomial distribution over orthogonal polarizations, interacting with a nonlinear birefringent medium, are discussed. The field states are the SU(2) coherent states, where the angular momentum operators are realized as bilinear products of the boson operators associated with the two polarizations. The system discussed here provides an optical realization of a nonlinear rotator. We study the quantum dynamics of the statistical properties of the field and the evolution of the degree of polarization.

On steady state solution of nonlinear Kerr electric birefringence and polarization for AC and NC superimposed fields

The steady state response of the nonlinear Kerr effect to a small alternating field superimposed on a unidirectional field is analyzed using a second order perturbation theory. We draw inferences from the appearance of simple and double harmonics as the fundamental frequencies of the nonlinear Kerr effect. The study is based on the Smoluchowski equation of rotational Brownian motion.

Nonlinear Birefringence in Electrically Biased Hybrid Aligned Nematics

Abstract The laser induced nonlinear. birefringence in electrically biased hybrid aligned nematics is investigated. It is either enhanced or suppressed by the quasi-static electric field depending on the sign of the dielectric anisotropy of the nematics. An analytical solution is obtained for small distortion. The experimental results is in good agreement with theoretical prediction.