Light-induced Refractive Index Changes Research Articles

Spectrokinetic characterization of photoactive yellow protein films for integrated optical applications

In this paper, the photocycle of the dried photoactive yellow protein film has been investigated in different humidity environments, in order to characterize its nonlinear optical properties for possible integrated optical applications. The light-induced spectral changes of the protein films were monitored by an optical multichannel analyser set-up, while the accompanying refractive index changes were measured with the optical waveguide lightmode spectroscopy method. To determine the number and kinetics of spectral intermediates in the photocycle, the absorption kinetic data were analysed by singular value decomposition and multiexponential fitting methods, whose results were used in a subsequent step of fitting a photocycle model to the data. The absorption signals of the films were found to be in strong correlation with the measured light-induced refractive index changes, whose size and kinetics imply that photoactive yellow protein may be a good alternative for utilization as an active nonlinear optical material in future integrated optical applications.

Nonlinear propagation and quasi self-confinement of light in plasmonic resonant media

We study nonlinear propagation of light in colloidal suspension of metallic nanoparticles, in the regime of particles surface plasmon resonance. We show that the propagation exhibits features typical for purely defocusing media and the observed spatial confinement is not a real self-trapping, as for solitons, but rather than is caused by the phase modulation of the beam via nonlocal defocusing nonlinearity. We also show that the light-induced refractive index change in the suspension leads to stabilization of structured light beams. In particular, we demonstrate a stable nonlinear propagation of bright ring beams with complex states of polarization, including practically important radial and azimuthal states.

Soft-proton exchange on magnesium-oxide-doped substrates: A route toward efficient and power-resistant nonlinear converters

Despite their attractive features, integrated optical devices based on Congruent-melted Lithium Niobate (CLN) suffer from Photo-Refractive Damage (PRD). This light-induced refractive-index change hampers the use of CLN when high-power densities are in play, a typical regime in integrated optics. In bulk devices, the resistance to PRD can be largely improved by doping the lithium-niobate substrates with magnesium oxide. However, the fabrication of waveguides on MgO-doped substrates is not as straightforward as on CLN and either the resistance to PRD is strongly reduced by the waveguide fabrication process (as it happens in Ti-indiffused waveguides) or the nonlinear conversion efficiency is lowered (as it occurs in annealed-proton exchange). Here, we fabricate waveguides starting from MgO-doped substrates using the Soft-Proton Exchange (SPE) technique and we show that this combination represents a promising alternative. We demonstrate that, with a small adaptation of the exchange parameters, SPE allows producing MgO-doped LN refractive-index profiles almost identical to those produced in CLN without reducing the nonlinearity in the substrate. We also prove that the SPE does not affect substantially the resistance to PRD characteristics of MgO-doped substrates. Therefore, we think that SPE is the right recipe to outperform standard techniques and to fabricate robust and efficient waveguides for high-intensity-beam confinement.

Enhanced Nonlinear Refractive Index in ε-Near-Zero Materials.

New propagation regimes for light arise from the ability to tune the dielectric permittivity to extremely low values. Here, we demonstrate a universal approach based on the low linear permittivity values attained in the ε-near-zero (ENZ) regime for enhancing the nonlinear refractive index, which enables remarkable light-induced changes of the material properties. Experiments performed on Al-doped ZnO (AZO) thin films show a sixfold increase of the Kerr nonlinear refractive index (n_{2}) at the ENZ wavelength, located in the 1300nm region. This in turn leads to ultrafast light-induced refractive index changes of the order of unity, thus representing a new paradigm for nonlinear optics.

Dynamically Tunable by Kerr Effect Multichannel Filter Based on Plasmon Induced Transparencies at Optical Communication Range

Dynamically tunable multichannel filter based on plasmon-induced transparencies (PITs) is proposed in a plasmonic waveguide side-coupled to slot and rectangle resonators system at optical communication range. The slot and rectangle resonators in this system can be regarded as radiative or dark resonators as same as the radiative or dark elements in the metamaterial structure with the help of the evanescent coupling. The multiple PIT responses which can enable the realization of nanoscale filter with four channels are originated from the direct near-field coupling and indirect phase couple through a plasmonic waveguide simultaneously. Moreover, the magnitudes and bandwidths of the filter can be efficiently tuned by controlling of the geometric parameters such as the coupling distances and the pump light-induced refractive index change of the Kerr material which is embedded into the metal-dielectric-metal waveguide between the radiative resonators.

Types of filamentation in tapered diode amplifiers: their causes and features

Software for calculation of gain–current relation, gain saturation, light-induced refractive index change and light propagation in diode amplifiers and lasers is developed. Three orders of filamentation are found in tapered (flared) amplifiers within the typical ranges of the parameters. While the 1st order is caused by small external perturbations, the 2nd and 3rd orders are generated by the optical beam itself. The causes of the different types of filamentation are elucidated and their dependence on the amplifier geometry is investigated. Practical guidelines for filamentation suppression are suggested.

In situ visualizing the evolution of the light-induced refractive index change of Mn:KLTN crystal with digital holographic interferometry

The light-induced refractive index change in Mn:KLTN crystal, illuminated by focused light sheet, is visualized in situ and quantified by digital holographic interferometry. By numerically retrieving a series of sequential phase maps from recording digital holograms, the spatial distribution of the induced refractive index change can be visualized and estimated readily. This technique enables the observation of the temporal evolution of the refractive index change under different recording situations such as writing laser power, applied voltage, and temperature, and the photoconductivity of Mn:KLTN crystal can be calculated as well, the experimental results are in good agreement with the theory. The research results suggest that the presented method is successful and feasible.

On-chip plasmon-induced transparency based on plasmonic coupled nanocavities

On-chip plasmon-induced transparency offers the possibility of realization of ultrahigh-speed information processing chips. Unfortunately, little experimental progress has been made to date because it is difficult to obtain on-chip plasmon-induced transparency using only a single meta-molecule in plasmonic circuits. Here, we report a simple and efficient strategy to realize on-chip plasmon-induced transparency in a nanoscale U-shaped plasmonic waveguide side-coupled nanocavity pair. High tunability in the transparency window is achieved by covering the pair with different organic polymer layers. It is possible to realize ultrafast all-optical tunability based on pump light-induced refractive index change of a graphene cover layer. Compared with previous reports, the overall feature size of the plasmonic nanostructure is reduced by more than three orders of magnitude, while ultrahigh tunability of the transparency window is maintained. This work also provides a superior platform for the study of the various physical effects and phenomena of nonlinear optics and quantum optics.

Measurement of refractive index change induced by dark reaction of photopolymer with digital holographic quantitative phase microscopy

Light-induced refractive index change in photopolymer is quantified by a digital holographic microscope. The refractive index change is induced as the dark reaction which proceeds inside the photopolymer after a writing beam is stopped. Time-lapse phase distribution across the photopolymer is measured by the off-axis digital holographic microscope which enables us to retrieve the 2-D phase map from a single hologram. It is found that the initial phase profile does not coincide with the illumination intensity distribution. This observation suggests that the rate of the refractive index change in dark reaction is not proportional to the illumination intensity in case the exposure energy becomes high.

低功率近红外光的偏振方向对光致折射率变化影响的实验研究

The influence of the polarization direction of light-induced refractive index change(Δn) induced by light irradiation in In:Fe:LiNbO3 crystals were investigated by using ordinary and extraordinary recording polarization(opol/epol) laser beams of low-power near infrared light.The experimental results show that the curves are similar of the refractive index distribution,but they have opposite signs,and a further difference is that the saturation refractive index change is clearly larger for ordinary recording polarization,changing to extraordinary recording polarization result only about one-third.The refractive index change in photorefractive crystals induced by two polarization directions of near infrared light is differ from visible light.The crystal illuminated with extraordinary polarized light in the middle of the irradiation region,one can see a strong increased of the refractive index.Moreover,the results can be very useful for fabrication of tailored nonlinear optical device with different refractive index distributions by changing different the polarization directions of near infrared beam.

Suppression of photorefractive damage with aid of steady-state temperature gradient in nominally pure LiNbO3 crystals

Photorefractive damage (PRD) in as-grown, oxidized, and slightly reduced nominally pure LiNbO3, and iron-doped crystals with different compositions has been studied with the closed-aperture Z-scan and pseudo-Z-scan techniques at uniform temperature distribution, as well as at a steady-state temperature gradient ∇T, created by two external thermoelectric elements in the transverse direction to the light beam trajectory. The most important experimental finding consists of the demonstration of the possibility of a full PRD suppression in nominally pure crystals with the aid of a relatively small temperature gradient (12.5–84 K/cm), if ∇T is above a threshold value specific for each crystal studied. The threshold ∇Tth decreases significantly with the increase in the so-called bipolaron absorption band (center at 470–500 nm) in the optical spectra, and it is not correlated with the composition of the crystals within the studied range of compositions ([Li]/[Nb]=0.946–0.983). No any partial suppression of PRD was observed in iron-doped ([Fe]≥0.01 wt %) lithium niobate crystals, even at the largest temperature gradient (85 K/cm) used in our study. To explain these experimental results, we use theoretical model taking into account local changes of spontaneous polarization (i.e., polarization charge field) at photoionization of intrinsic defects (polarons, bipolarons, and hole polarons). It has been shown that a steady-state temperature gradient may induce a local thermoelectric current of light-induced charge carriers and their instant recombination, resulting in a decrease in the polarization space-charge field and hence, a light-induced refractive index change. The latter mechanism may induce the full PRD suppression, if macroscopic charge separation (space-charge field effect) gives insignificant contribution to the total light-induced electric field, as in as-grown and reduced nominally pure LiNbO3 crystals.

Second harmonic generation of 2.6W green light with thermoelectrically oxidized undoped congruent lithium niobate crystals below 100°C

We demonstrate that thermoelectric oxidization suppresses unwanted light-induced refractive index changes in undoped congruent periodically poled lithium niobate crystals used for single-pass continuous-wave frequency doubling of 1030nm light. The maximum second harmonic output power shows a strong dependence on the phase matching temperature. For 90°C, a total of 2.6W green light is generated with an efficiency of 22% corresponding to a normalized conversion efficiency of 1.8%∕W.

Comparative Study of Composition Dependences of Photorefractive and Related Effects in LiNbO3 and LiTaO3 Crystals

The results of a comparative study of composition dependences of the light-induced refractive index change, photorefractive sensitivity, polaron luminescence, and intrinsic optical absorption in nominally pure LiNbO 3 and LiTaO 3 crystals are reported. Our investigation demonstrates that the particular light-induced defects traced by the photoluminescence are secondary photorefractive centers itself. The experimental results obtained are well described by light-induced dissociation of bipolarons into small polarons that are unstable at room temperature.

Charge transport in highly iron-doped oxidized lithium niobate single crystals

The photorefractive properties of highly iron-doped lithium niobate crystals in as-grown and completely oxidized states are investigated. Iron concentrations range from 0.5 to 3 wt. %. Measurements of light-induced refractive-index changes, two-beam coupling gain, bulk-photovoltaic current densities, and conductivities are performed with visible light. Absorption spectra and the bulk photovoltaic currents are used to determine the Fe2+ concentration. The dark conductivity confirms the tunneling model for charge-carrier migration. Beam-coupling experiments as well as photocurrent measurements with highly oxidized samples indicate that the one-center model is valid in such highly doped crystals, but that hole conductivity dominates the charge transport.

Transient grating measurements of refractive-index changes in intensively pumped Yb-doped laser crystals

Mechanisms of transient light-induced refractive-index changes in Yb-doped crystals Yb:KYW, Yb:KGW and Yb:YVO4 were studied. Relative amplitudes and lifetimes of the electronic component of phase gratings caused by the polarizability difference of the excited and unexcited Yb3+ ions and the thermal component arising from thermalization of the pump energy were measured. Polarizability difference values of the ground 2F7/2 and the excited metastable 2F5/2 levels at the testing wavelength of 633 nm were determined for all crystals. The thermal diffusivity of the samples was estimated for selected directions of the thermal gradient. An athermal direction, along which the thermal component of refraction disappears, was found in Yb:KYW crystal under specific experimental conditions.

Photorefractive properties of undoped lithium tantalate crystals for various composition

Lithium tantalate crystals of compositions ranging from 48.3mol%to50.0mol% lithium oxide are fabricated by vapor transport equilibration. Light-induced refractive index changes of the crystals are investigated with holographic methods at usual cw-laser intensities (≈105W∕m2) and with a single focused laser beam at high light intensities up to 2×107W∕m2. In stoichiometric crystals the index changes are reduced by more than two orders of magnitude when compared with congruently melting ones. Simultaneously, the normalized photoconductivity σph∕I, where I is the light intensity, increases by nearly two orders of magnitude. Therefore, stoichiometric lithium tantalate is an attractive material for applications such as frequency conversion via quasi-phase matching.

Photon sensitive high index metal oxide films

In this work, we review the electrical and optical properties ofInOx and ZnO films, and their use as dynamic optical materials. The transport properties ofthese films are analysed and correlated with the growth parameters. We havestudied the permanent light-induced refractive index changes in our dc-sputteredInOx thin films using pulsed ultraviolet laser radiation at 193 nm.Non-permanent holography recording of information has been achieved inInOx films upon illumination with UV radiation (325 nm). The recording, which appears to beindependent of the electrical state of the material, disappears in the absence of UV lightwith decay times that depend on the material’s specific properties, as these are influencedby the film fabrication conditions. We also review our work on waveguide reflectionsubmicron relief gratings on multilayer waveguides operating near 1550 nm, fabricated byexcimer laser ablation at 248 nm.

Fabrication and characterization of UV-written channel waveguides in Bi2O3-based glass

Abstract Light-induced refractive index change was investigated in an erbium-doped Bi2O3-based bulk glass using direct UV laser writing at 244 nm. Single mode channel waveguides were obtained with a positive refractive index change of 4 × 10−4 at 633 nm. An upper limit for propagation losses at 980 nm was estimated at 4 dB/cm. A morphological study of the exposed regions revealed that strong interactions occurred between the UV beam and the glass sample, causing physical damage and stresses in the material. To our knowledge, this is the first paper reporting the use of 244 nm laser beam to produce waveguides in a Bi2O3-based glass.

Experimental study of a faster responsive effect of light induced refractive index change

We have observed an effect of light-induced refractive index change in LiNbO3∶Fe crystal sheet,which is different from the photorefractive effect. It has the characters of faster response (<0.1s),isotropy and function of information storage. We have also discussed the physical mechanism of the effect.

Visualizations of Light-induced Refractive Index Changes inPhotorefractive Crystals Employing Digital Holography

We propose a novel approach to visualize the light-induced refractiveindex changes in photorefractive crystals employing digital holography.The holograms formed in a Mach-Zehnder interferometer are recorded by atwo-dimensional CCD camera. From these holograms, the phasedifferences, which contain the information of the index changes inphotorefractive crystals, are determined by utilizing digitalholographic interferometry. Then the two-dimensional visualizations ofindex changes in the crystals can be obtained. This method issuccessfully demonstrated in LiNbO3:Fe, KNSBN:Ce and SBN:Crcrystals.