Light-induced Refractive Index Research Articles

Field theory description of the non-perturbative optical nonlinearity of epsilon-near-zero media

In this paper, we introduce a fully non-perturbative approach for the description of the optical nonlinearity of epsilon-near-zero (ENZ) media. In particular, based on the rigorous Feynman path integral method, we develop a dressed Lagrangian field theory for light–matter interactions and discuss its application to dispersive Kerr-like media with order-of-unity light-induced refractive index variations. Specifically, considering the relevant case of Indium Tin Oxide (ITO) nonlinearities, we address the novel regime of non-perturbative refractive index variations in ENZ media and establish that it follows naturally from a scalar field theory with a Born–Infeld Lagrangian. Moreover, we developed a predictive model that includes the intrinsic saturation effects originating from the light-induced modification of the Drude terms in the linear dispersion of ITO materials. Our results extend the Huttner–Barnett–Bechler electrodynamics model to the case of non-perturbative optical Kerr-like media providing an intrinsically nonlinear, field-theoretic framework for understanding the exceptional nonlinearity of ITO materials beyond traditional perturbation theory.

Long-range interaction between solitons in strongly nonlocal nonlinear media with oscillatory response

The long-range interaction between two identical solitons in strongly nonlocal nonlinear media with the sine-oscillation response function is studied. It is found that the two solitons may attract each other, repel each other, or keep in parallel in the initial stage. The behavior of the soliton interactions depends periodically on the separation between solitons; it is essentially controlled by the superposition of the periodic light-induced nonlinear refractive index. The different versions of the soliton interactions can be interchanged by adjusting the separation between solitons.

Generation of regular optical patterns and photonic structures by a single Gaussian beam in a photorefractive LiNbO3:Fe crystal

We report 2D micrometric scale regular optical patterns formation induced by a laser beam propagating through a Fe doped lithium niobate (LN:Fe) crystal. A single mode CW laser radiation at 632.8 nm wavelength and beam powers from 0.5 to 10 mW and LN:Fe samples with 0.03 wt% Fe concentration and thicknesses of 4 mm and 10 mm were used in the experiments. The observed phenomenon of optical patterns formation in photorefractive crystal is explained with the light-induced complex refractive index variation with a central minimum (negative lens) and symmetric maxima (positive lenses) on each side due to photovoltaic effect in the crystal. Light-induced complex lenses produce beam defocusing from the beam center and focusing on the periphery. Mutual interference of the beams leads to a regular optical pattern formation. Regular optical pattern, in its turn, leads to the formation of quasi-crystalline photonic structure in LN:Fe crystal. Experimental results are supported by numerical simulations.

Dynamically tunable multifunctional QED platform

Solid-state quantum electrodynamics (QED) not only demonstrates basic principles of quantum physics, but also provides key support to future quantum technologies. However, the non-modifiability of the fabricated solid-state QED systems limits their flexibility and versatility in manipulating light-matter interaction, and severely hinders their practical applications. Here, we put forward an approach of multi-dimensionally manipulating light-matter interaction to realize a dynamically tunable multifunctional QED platform by combining the local light-induced refractive index modulation (LRIM) and strong dispersion characteristic of the photonic crystal (PC) waveguide. We demonstrate three significant functions of the platform as examples: switch control between weak and strong couplings on demand, distant quantum entanglement, and a directional single photon source with high brightness and efficiency. These functions are strongly robust against positioning error of the quantum emitter, and can be facilely realized only by local LRIM on one PC waveguide. Our work paves a new way for the realization of multifunctional quantum devices.

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.

Field-induced lifetime enhancement of photorefractive gratings in a Mn:Fe:KTN crystal.

We report the lifetime enhancement of light-induced refractive index grating by applying a bias field during writing. In comparison with the lifetime of about 10 hours of the photorefractive grating prepared without a bias field, the lifetime of the grating with a 4 kV/cm bias field can be prolonged to 7.5 years, which is obtained from the dynamic behavior of grating visualized and monitored with digital holographic microscopy. The higher the bias field is applied, the longer the dark decay time of grating can be achieved. The enhanced lifetime of phase grating is attributed to polar nanoregions oriented by external field. This effect is of great significance for electro-holographic device applications.

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.

Visualization of Spatial–Temporal Evolution of Light-Induced Refractive Index in Mn:Fe:KTN Co-Doped Crystal Based on Digital Holographic Interferometry

The dynamic refractive-index-change behavior of the light-induced process in an Mn:Fe:KTN crystal illuminated by a focused light sheet can be observed experimentally by digital holographic interferometry. By numerically retrieving a series of sequential phase maps from recording digital holograms, the spatial and temporal evolution of the light-induced refractive-index-change distribution inside the material is visualized in situ and monitored in a quantitative and in full field way. With this technique, the effect of recording parameters, such as writing laser power and polarization, bias voltage, temperature, and writing time, on the Mn:Fe:KTN crystal in the photorefractive effect can be explored. Therefore, optimized recording parameters will be achieved according to the dynamic behavior. The method provides an access to explore the evolution of the photorefractive (PR) effect of electrooptic crystal under various situations.

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.

Nonlinear change in refractive index and transmission coefficient of silicon at long-pulse, mJ-range, 1.54-μm excitation

An experimental study and theoretical modeling of the nonlinear changes in transmission coefficient and refractive index of mono-crystalline Silicon (Si) at long-pulse, mJ-range, single-beam Z-scan probing at 1.54 μm wavelength are reported. It is shown experimentally that at increasing pulse energy density the photo-induced darkening permanently increases in Si while its photo refraction properties demonstrate a more complicate character, being a product of various type nonlinearities. A theoretical analysis based on simple assumptions of a square-shape pulse in the time domain and Gaussian spatial distribution of the probe beam allows fitting of a whole of the experimentally measured open- and closed-aperture Z-scans through an account of the main contributions in the light-induced absorption and refractive index nonlinearities. These are revealed to originate from non-direct two-photon absorption and Kerr effect, induced absorption and dispersion of light-generated free carriers, and light-induced thermal lensing.

In situ Mapping of Light-Induced Refractive Index Gratings by Digital Holographic Microscopy

This work presents a technique for in situ measurement of light-induced refractive index gratings in epoxy resin using digital holographic microscopy (DHM). The reconstructed phase image derived from a digital hologram can exhibit the grating structure and refractive index profile of an epoxy resin hologram. Reconstruction properties of finite aperture effect in the DHM system are considered and analyzed theoretically. Due to the high spatial frequency components collected by the objective lens, the DHM system can measure fine grating structures and numerically determine the reconstructed images in increased detail. Grating formation and dynamic behavior during the light-induced holographic process can be demonstrated experimentally and characterized using the proposed scheme. Compared with an optical holographic readout, the proposed technique facilitates direct observation and substantial understanding of the holographic recording mechanism in a microscopic view.

Photopolymerizable nanocomposites for holographic recording and sensor application

Novel nanocomposites consisting of a water-soluble acrylamide-based photopolymer and colloidal zeolite nanoparticles of zeolite Beta and zeolite A were prepared. The interactions between the photopolymer components and zeolite nanoparticles in the photopolymerizable nanocomposites were characterized for the first time by (13)C nuclear magnetic resonance and visible spectroscopy. It was found that the zeolite Beta nanoparticles (up to 5 wt. %) behave as a noninert additive, resulting in an effective increase in layer thickness, which causes doubling of the diffraction efficiency of the nanocomposite in comparison to that of the undoped photopolymer. On the other hand, the nanocomposite containing zeolite A nanoparticles showed no evidence of interaction with the polymer matrix, had similar values of diffraction efficiency, and--up to a small addition of nanoparticles (up to 2.5 wt. %)--showed slightly higher light-induced refractive index modulation of the grating when compared to the undoped photopolymer. The good optical compatibility between the zeolite nanoparticles and the polymer allows a versatile design of photopolymerizable nanocomposites with different properties by selecting the adequate type of zeolite. The nanocomposite containing zeolite Beta nanoparticles demonstrates selective sensing behavior toward toluene and can be coated in either glass or plastic substrates and exposed directly to the environmental conditions.

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.