Optical Kerr Nonlinearity Research Articles

Dynamics of the guest-host orientational interaction in dye-doped liquid-crystalline materials

We present a comprehensive study on the dynamics of laser-induced molecular reorientation in a dye-doped liquid crystalline (LC) medium that exhibits significant enhancement of the optical Kerr nonlinearity due to guest-host interaction. Using various techniques, we separately characterized the dynamical responses of the relevant molecular species present in the medium following photoexcitation and, thus, were able to follow the transient process in which photoexcitation of the dye molecules exert through guest-host interaction a net torque on the host LC material, leading to the observed enhanced optical Kerr nonlinearity. Experimental results agree quantitatively with the time-dependent theory based on a mean-field model of the guest-host interaction.

Multistability and hysteresis phenomena in passively mode-locked fiber lasers

A passively mode-locked fiber laser is theoretically investigated. The mode locking is achieved using the nonlinear polarization technique. We consider the practical case of the ytterbium-doped fiber laser operating in the normal dispersion regime. The effect of the phase plates is explicitly taken into account. The resulting model reduces to one iterative equation for the optical Kerr nonlinearity, the phase plates and the polarizer, and one partial differential equation for the gain and the dispersion. Numerical simulations allow us to describe several features observed in passively mode-locked fiber lasers such as bistability between the mode lock and the continuous regime, multiple pulse behavior, hysteresis phenomena. The dynamics of the number of pulses as a function of the pumping power is also reported. Pump power hysteresis is demonstrated.

Determination of optical Kerr nonlinearity of a photonic bandgap structure by Z-scan measurement

Through analysis of the dispersion relation in a photonic bandgap structure, the effective optical Kerr nonlinearity that determines a Z-scan profile particularly near the stop-band edges, is derived. Near and inside the stop band, the nonlinear optical phase change that originates from an off-resonant response is converted into a change in nonlinear optical intensity through Bragg reflection. The Z-scan measurement of a cholesteric liquid-crystal photonic bandgap structure confirmed that off-resonant Kerr nonlinearity is responsible for the characteristic open-aperture Z-scan profiles near the stop-band edges.

Subwavelength Nonlinear Plasmonic Nanowire

We investigate numerically, by means of the finite-difference time-domain method, the propagation characteristics of surface plasmon-polariton (SPP) modes excited in an optical nanowire consisting of a chain of Ag spheres embedded in dielectric shells made of materials with optical Kerr nonlinearity. It is demonstrated that, in the linear limit, the nanowire supports two SPP modes, a transverse and a longitudinal one, separated by Δλ = 20 nm. Furthermore, the dependence of the transmission characteristics of these SPP modes, on both the pulse peak power and Kerr coefficient of the dielectric shell, is investigated. Nonlinear optical phenomena, such as power dependence of mode frequency, switching, or optical limiting, are observed and discussed.

Manifestation of optical quadratic nonlinearity in gas mixtures

In optical media, particularly in gases, there existseveral types of quadratic optical nonlinearity. Thisphenomenon is manifested in a nonlinear (quadratic)dependence of the refractive index n of the opticalmedium on the light wave amplitude. The best knownis the Kerr optical nonlinearity, for which the increasein the refractive index is associated with orienting mol-ecules in the light wave field. The other example is thephenomenon of electrostriction, in which the light wavecompresses the medium and elevates its density,thereby increasing n [1]. It turns out that, in gas mix-tures, there exists one more mechanism of manifestingoptical nonlinearity. We are implying the fact that themolecules of a gas with the highest n are attracted to theregion of the most intense light. As a result, the refrac-tive index of the mixture is higher compared to theequilibrium mixture in the absence of optical radiation.Thus, quantity n inside the radiation field is dependenton the radiation intensity. In the case of increasing n ,the work spent to separate molecules in a gas mixturecan be smaller than that spent for the same increase in n by means of compression of the same gas mixture bythe electrostriction mechanism. Therefore, the effectobserved can be even stronger than in the case of elec-trostriction.As is well known, the separation of a mixture isassociated with an energy consumption determined bythe expression W = T ∆ S , where T is the temperature ofthe gas mixture and ∆ S is the rise of the entropy whenthe gases are mixed [2]. If this process is adiabatic, theincrement of entropy is zero. Therefore, when the heatinflow to the system under consideration is negligible,the work needed for separation of the gas mixture isclose to zero, and the nonlinearity coefficient of the gasmixture can be rather high. However, the time of occur-ring transition processes accompanied by the suction ofmolecules of a gas with the highest n is also relativelylong, because it is associated with transport phenom-ena. Therefore, in the case of instantaneous appearanceof radiation, e.g., in a gas discharge, there is no domainfrom which molecules can be sucked, since the radia-tion is present in the entire volume of the mixture.However, this does not imply that the effect of the indi-cated mechanism is not manifested.In the same manner as the condensation of saturatedvapor can be accompanied by the appearance of a fogconsisting of small droplets of liquid, the molecules ofa gas with the highest n are concentrated by intenseradiation in a local domain. In contrast to droplets ofliquid—where molecules occupy the entire droplet vol-ume—in a gas mixture, molecules are concentrated in athin spherical layer (TSL) having an increased refrac-tive index. The TSL plays the role of a bent planar lightguide directing the light that circulates in it over all pos-sible paths. Intense light, in turn, provides the concen-tration in the TSL of molecules with the highest n .Thus, in a gas mixture, intense light condenses asthough into light fog consisting of a set of different-sizeTSLs.Figure 1 illustrates the stages of the appearance ofthese formations. We assume that, in a certain domain,a fluctuation of the density and/or of the concentrationhas occurred. In this case, the boundary of the domainwith an elevated refractive index is convex. Then, inaccordance with the eikonal equation, a beam propagat-ing along the tangent to the boundary is bent toward themaximum refractive index. The radius of curvature isdetermined by the expression R

Optical bistability in subwavelength slit apertures containing nonlinear media

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.

Linear and Nonlinear Transmission of Surface Plasmon Polaritons in an Optical Nanowire

ABSTRACTPolymer-metal composites offer the possibility of strongly enhanced nonlinear optical properties, which can be used for ultrasmall photonic devices. In this paper, we investigate numerically, by means of the finite-difference time-domain (FDTD) method, the propagation characteristics of surface plasmon polariton (SPP) modes excited in an optical nanowire consisting of a chain of either metallic cylinders or metallic spheres embedded in dielectric shells made of polymers (or other material) with optical Kerr nonlinearity. Our FDTD calculations incorporate both the nonlinear optical response of the dielectrics as well as the frequency dispersion of the metals, which is considered to obey a Drude-like model. It is demonstrated that, in the linear limit, the nanowire supports two SPP modes, a transverse and a longitudinal one, separated by Δλ = 20 nm. Furthermore, the dependence of the transmission of these SPP modes, on both the pulse peak power and Kerr coefficient of the dielectric shell, is investigated. Nonlinear optical phenomena, such as power-dependent mode frequency, switching, or optical limiting, are observed.

Effect of chromatic dispersion on nonlinear phase noise.

We consider the combined effects of amplified spontaneous emission noise, optical Kerr nonlinearity, and chromatic dispersion on phase noise in an optical communication system. The effect of amplified spontaneous emission noise and Kerr nonlinearity were considered previously by Gordon and Mollenauer [Opt. Lett. 15, 1351 (1990)], and the effect of nonlinearity was found to be severe. We investigate the effect of chromatic dispersion on phase noise and show that it can either enhance or suppress the nonlinear noise amplification. For large absolute values of dispersion the nonlinear effect is suppressed, and the phase noise is reduced to its linear value. For a range of negative values of dispersion, however, nonlinear phase noise is enhanced and exhibits a maximum related to the modulation instability found in amplitude fluctuations. Nonlinear phase noise is quenched by these effects even in dispersion-compensated systems; the degree of suppression is sensitively dependent on the dispersion map. We demonstrate these results analytically with a simple linearized model.

Stimulated Raman scattering of intense laser pulses in air.

Stimulated rotational Raman scattering (SRRS) is known to be one of the processes limiting the propagation of high-power laser beams in the atmosphere. In this paper, SRRS, Kerr nonlinearity effects, and group velocity dispersion of short laser pulses and pulse trains are analyzed and simulated. Fully time-dependent, three-dimensional, nonlinear propagation equations describing the Raman interaction, optical Kerr nonlinearity due to bound electrons, and group velocity dispersion are presented and discussed. The effective time-dependent nonlinear refractive index containing both Kerr and Raman processes is derived. Linear stability analysis is used to obtain growth rates and phase matching conditions for the SRRS, modulational, and filamentation instabilities. Numerical solutions of the propagation equations in three dimensions show the detailed evolution of the Raman scattering instability for various pulse formats. The dependence of the growth rate of SRRS on pulse duration is examined and under certain conditions it is shown that short (approximately psec) laser pulses are stable to the SRRS instability. The interaction of pulses in a train through the Raman polarization field is also illustrated.

Early dynamics of guest-host interaction in dye-doped liquid crystalline materials.

We have studied in detail the early dynamics of laser-induced molecular reorientation in a dye-doped liquid crystalline (LC) medium that exhibits a significant enhancement of the optical Kerr nonlinearity due to guest-host interaction. Experimental results agree quantitatively with theory based on a model in which the anisotropic dye excitation helps reorient the LC molecules through a mean-field intermolecular interaction.

Enhancement of Kerr nonlinearity by multiphoton coherence.

We propose a new method of resonant enhancement of optical Kerr nonlinearity that uses multilevel atomic coherence. The enhancement is accompanied by suppression of the other linear and nonlinear susceptibility terms of the medium. We show that the effect results in a modification of the nonlinear Faraday rotation of light propagating in an 87Rb vapor cell by changing the ellipticity of the light.

Kerr-stabilized super-resonant modes in coupled-resonator optical waveguides

We investigate the effects of the optical Kerr nonlinearity in a coupled-resonator optical waveguide (CROW). Under certain conditions, there exists a stationary spatial distribution of the field whose envelope does not change with time-a super-resonant mode. The analysis does not indicate the existence of traveling hyperbolic-secant solitons of the Schrödinger type.

Highly nonlinear Ge-As-Se and Ge-As-S-Se glasses for all-optical switching

We have synthesized Ge-As-Se and Ge-As-S-Se chalcogenide glasses designed to have large optical nonlinearities. Measurements reveal that these glasses offer optical Kerr nonlinearities greater than 500 times that of fused silica and figures of merit for all-optical switching >5 at 1.25 and 1.55 /spl mu/m.

Highly nonlinear As–S–Se glasses for all-optical switching

We have synthesized a series of chalcogenide glasses from the As-S-Se system that is designed to have strong nonlinearities. Measurements reveal that many of these glasses offer optical Kerr nonlinearities greater than 400 times that of fused silica at 1.25 and 1.55mum and figures of merit for all-optical switching greater than 5 at 1.55mum .

Large deuterium isotope effect in the optical nonlinearity of dye-doped liquid crystals.

Deuterium-hydrogen isotopic substitution was found to approximately double the magnitude of the giant optical nonlinearity of dye-doped nematic liquid crystals arising from photoinduced molecular reorientation. In accordance with the predictions of our model, this doubling is well correlated with the increase in a characteristic decay time of the electronically excited dye anisotropy, as measured with time-resolved fluorescence. A similar isotopic effect is predicted for the optical Kerr nonlinearity of dyed isotropic liquids.

Approach for the study of soliton in optical fiber

In the anomalous group velocity dispersion regime, under the influence of optical Kerr nonlinearity, the monomode optical fiber supports stable propagation of an initial soliton pulse. In this paper, we present a singular perturbation approach for the study of various perturbations acting on this initial soliton pulse, In contrast with the previous perturbation theory for this subject, this approach gets rid of dependence on any tool of the sophisticated inverse scattering transform.

Optical Stark effect and Kerr nonlinearities of atomic media in nonlinear optical processes

It is shown that the resonant enhancements of the optical Kerr effect and the optical Stark effect of the same order are caused by the same change in the state of an atomic system. The analysis of the known experimental data demonstrates the identity of their influence on nonlinear optical processes. It is shown that a separate analysis of the influence of Stark shift and resonantly enhanced Kerr nonlinearities leads to incorrect results.

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.

Nonlinear optics of coupled semiconductor microcavities

In a coupled microcavity configuration with Kerr optical nonlinearities in the external as well as in the central distributed Bragg reflectors, the energies of the closely spaced doublet of delocalized photon eigenmodes change with increasing light intensity. One beam optical bistability as well as all optical switching in a pump and probe configuration can be realized. The nonlinearity can induce a novel kind of Rabi anticrossing between a photonic mode and a nearly resonating quantum well exciton. Realistic numerical simulations of such effects in AlGaAs based microstructures are presented.

Interesting effect of optical Kerr nonlinearity in expanding single-mode regime of optical fibers using dispersion-flattened profiles

An interesting effect of optical Kerr nonlinearity is reported from theoretical investigation on the fundamental and first higher mode cutoff frequencies in single-mode fibers doped with highly nonlinear materials like CdS x Se 1− x with W-type dispersion-flattened profiles. It is observed that whereas the single-mode regime decreases absolutely for dispersion-shifted profiles due to reduction of the first higher order mode cutoff frequency as is evident from earlier studies, it increases for W-type profiles for such nonlinearity taking its effect on the nonzero fundamental mode cutoff frequency into consideration. Thus, one can go further down below the fundamental mode cutoff frequency while using W-fibers in single mode nonlinear fiber devices involving evanescent field coupling in the low V region.