Intensity-dependent Refractive Index Change Research Articles

Optically tunable scattering using epsilon-near-zero-enabled nonlinear nanoantennas

Optical nanoantennas provide extraordinary capabilities in controlling light at subwavelength scale. It attracts increasing interest that how to achieve tunable nanoantennas for flexible and multifunctional miniaturized optical applications. Here, a sandwich-type nonlinear nanoantenna with a thin layer of epsilon-near-zero (ENZ) material is proposed to dynamically tune the far-field scattering via an all-optical approach. By synthetically combining the varying dependence of four fundamental Mie modes on refractive index and giant intensity-dependent refractive index change caused by nonlinear optical effect in ENZ material, the far-field scattering of the proposed nonlinear nanoantenna can be dynamically tuned from bidirectional to transverse or unidirectional scattering by readily controlling the intensity of incident light. We believe that the proposed all-optical nonlinear optical nanoantennas will motivate more promising applications of all-optical light modulation for photonic devices.

All-optical polarization tuning based on an intensity-dependent nonlinear metasurface

Active control of light polarization at the nanoscale is essential for integrated photonic devices. Here, an all-optical approach is proposed to tune the polarization state of near-infrared light using a nonlinear metasurface. Based on the large intensity-dependent refractive index change of epsilon-near-zero (ENZ) materials, a phase difference between two orthogonal electric fields at ENZ wavelength can be continuously tuned in a range larger than 0.61π by varying the incident light power. The polarization state of the reflected light can thus be actively tuned from linear to circular state via an all-optical approach. Notably, abundant polarization states can be obtained by altering the polarization angle of incident light. The proposed all-optical approach is promising for tunable photonic functionalities of practical applications.

Structure-dependent optical nonlinearity of indium tin oxide

We use post-deposition vacuum annealing of epsilon-near-zero (ENZ) indium tin oxide (ITO) nanolayers in order to modify their structural properties and enhance the third-order optical nonlinear response around the ENZ wavelength. We find that room temperature magnetron sputtering deposition results in polycrystalline thin films with an intrinsic tensile strain and a ⟨110⟩ fiber axis preferentially oriented normal to the substrate. Moreover, we demonstrate that post-deposition vacuum annealing treatments produce a secondary anisotropic phase characterized by compressive strain that increases with the annealing temperature. Finally, we use the Z-scan optical technique to accurately measure the complex nonlinear susceptibility χ(3) and the intensity-dependent refractive index change Δn for samples with different structural properties despite featuring similar ENZ wavelengths. Our intensity-dependent analysis demonstrates that an enhancement of the optical nonlinearity can be achieved by tuning the structure of ENZ nanolayers with values as large as χR(3)=(5.2 ± 0.3)×10−17m2/V2. This study unveils the importance of structural control and secondary phase formation in ITO nanolayers with ENZ optical dispersion properties for the engineering of integrated highly nonlinear devices and metamaterials that are compatible with the scalable silicon photonics platform.

Observation of stimulated Mie-Bragg scattering from large-size-gold-nanorod suspension in water

Highly directional backward stimulated scattering has been observed from large-size-gold nanorods suspended in water, pumped with $\ensuremath{\sim}$816 nm and $\ensuremath{\sim}$10 ns laser pulses. In comparison with other known stimulated scattering effects, the newly observed effect exhibits the following features. (i) The scattering centers are impurity particles with a size comparable in order of magnitude to the incident wavelength. (ii) There is no frequency shift between the pump wavelength and the stimulated scattering wavelength. (iii) The pump threshold can be significantly lower than that of stimulated Brillouin scattering in pure water. The nonfrequency shift can be explained by the formation of a standing-wave Bragg grating induced by the interference between the forward pump beam and the backward Mie-scattering beam. The low pump threshold results from stronger initial Mie-scattering (seed) signals and the intensity-dependent refractive-index change of the scattering medium enhanced by metallic nanoparticles.

Spontaneous Pattern Formation Due to Modulation Instability of Incoherent White Light in a Photopolymerizable Medium

Spontaneous pattern formation due to modulation instability was observed in a broad uniform beam of incoherent white light propagating in an optically isotropic, photopolymerizable organosiloxane. Pattern formation originates from intensity-dependent refractive index changes due to polymerization, which cause competition between the natural diffraction (broadening) and self-induced refraction of the beam. Under these nonlinear conditions, weak intensity modulations in the beam, noise, that would be negligible under linear conditions are amplified. The amplified patterns become unstable over time and spontaneously divide into individual self-trapped filaments of white light of essentially identical diameter (76 +/- 3 microm), which propagate through the medium without diffracting. In the case of noise with a weak 1-D periodic modulation, for example, the uniform beam transformed into a 1-D periodic array of self-trapped lamellae, which in turn formed a 2-D array of self-trapped cylindrical filaments. Although the rate of pattern formation varied inversely with optical power (measured from 8.4 to 59.8 mW), the uniform beam always split into discrete filaments, demonstrating that they are the most stable form of light propagation under the nonlinear conditions created by polymerization. Each filament of light retained the spectral composition and incoherence of white light, which showed that the entire polychromatic, incoherent and unpolarized wavepacket collectively participated in pattern formation. These findings are consistent with recent theoretical models of nonlinear white light propagation and with experimental observations of pattern formation in coherent and partially coherent light. Because refractive index changes due to polymerization are permanent, pattern formation imparts microstructure to the organosiloxane. Optical micrographs revealed that, after pattern formation, the initially homogeneous medium consisted entirely of a closely packed array of narrow channel waveguides induced by self-trapped filaments.

Measurement of Pump-Induced Absorption and Refractive Index Changes in GaInAsP/InP Waveguides Using an Optical Loop Mirror Interferometer

We investigated the direct measurement of intensity-dependent refractive index changes in GaInAsP/InP ridge waveguides with ps-order pump pulses below the band gap using an optical loop mirror interferometer. The temporal response of the probe in GaInAsP (λg=1410 nm) waveguides was shown to be instantaneous with respect to 4 ps pump pulses. The intensity-dependent refractive index coefficient was measured to be -2.0±0.6×10-12 cm2/W. In addition, pump-induced absorption changes were studied.

Visible laser self-focusing in hybrid glass planar waveguides

We report that self-focusing occurs with simultaneous self-inscription of a cylindrical waveguide when 514.5-nm light from a cw argon-ion laser propagates in a solgel-derived silica methacrylate hybrid glass planar waveguide. Spatially localized free-radical polymerization of methacrylate substituents is initiated in the path of the guided wave. This causes intensity-dependent refractive-index changes that lead to self-lensing and focusing. A channel waveguide evolves in the matrix, which supports fundamental and higher-order optical modes and suppresses diffraction of the beam.

The intensity dependent refractive index change of bacteriorhodopsin measured by the Z-scan and phase-modulated beams techniques

Abstract A comparative analysis of intensity dependent refractive index change of a thin polymer bacteriorhodopsin (bR) film by the Z -scan and phase-modulated beams techniques is given. The analysis is conducted at two excitation wavelengths, λ =543 and 633 nm, corresponding to the opposite slopes of the bR ground-state absorption band resulting in opposite signs of the refractive index change. A three-level model of the bR photo-cycle is proposed to describe the experimental data.

Liquid crystal photorefractivity – towards supra-optical nonlinearity

A review of the basic mechanisms of optically induced refractive index modification in liquid crystalline materials is presented. We describe how the use of methyl-red dye or an azobenzene liquid crystal will dramatically enhance the nonlinear optical responses of typical nematic liquid crystals (NLC). The intensity dependent refractive index changes in these materials are characterized by nonlinear coefficients of over 1 cm 2 / W . The principal contributing mechanisms are orientational photorefractivity associated with photo-induced space charge fields, dopant-nematic molecular torque, and order parameter modification caused by trans – cis isomerization of the azo-dopants.

Application of the polyconjugated main chain polymer DPOP-PPV for ultrafast all-optical switching in a nonlinear directional coupler

The observation of ultrafast all-optical switching in a polyconjugated main chain polymer (DPOP-PPV), nonlinear directional coupler at a wavelength with an energy below two-photon resonances is reported. The effect responsible for the switching is clearly the nonresonant instantaneous intensity dependent refractive index change. To our knowledge these results represent the first demonstration of an ultrafast device using a polyconjugated polymer, where the switching is not drastically limited by absorption processes.

The intensity dependent refractive index change of photochromic proteins

We report the nonlinear refractive index behaviour of bacteriorhodopsin and photoactive yellow protein films. A Michelson interferometer with a probe and excitation beams was used in the measurements. By using this technique instead of the Z-scan we can easily separate the index change caused by photochromism and the thermal expansion. Sigmoidal index changes in the logarithmic intensity scale were observed.

Optical Nonlinearities of Bis(4-dimethylaminodithiobenzil)-nickel Solution in the Nanosecond Regime

Nonlinear absorption and nonlinear refraction of the saturable absorber BDN (bis(4-dimethylaminodithiobenzil)-nickel) in a nitrobenzene solution are studied using a nonlinear transmission method and z-scan methods, respectively, under irradiation with 23 ns laser pulses at 1.047 µm. The z-scan experiment shows that, in contrast to the previous work by Zhu and Garmire [IEEE J. Quantum Electron. 19 (1983) 1495], the main origin of the intensity-dependent refractive index change in the dye solution is the thermal effect even in the nanosecond regime. The buildup of the thermal lens is attributed to the photoacoustic effect and its rise time is proportional to the beam spot size. For a tightly focused beam, the rise time can be as fast as about ten nanoseconds. The fall time of the thermal lens is determined by thermal diffusion and is generally on the order of milliseconds. Moreover, nanosecond optical limiting and pulse narrowing have been demonstrated using strong self-defocusing induced by the photoacoustic effect.

Optical solitons in a dispersion-flattened fiber with linear and quadratic intensity-dependent refractive-index changes

Optical-soliton propagation in a dispersion-flattened fiber is investigated, of which third-order dispersion is nil and fourth-order dispersion exists with linear and quadratic intensity-dependent refractive-index changes. For four possible sign combinations of the second-order dispersion and the Kerr-effect terms, we found that there are two types of bright-soliton solutions and two types of dark-soliton solutions. The magnitude of the fourth-order dispersion parameter is related to the quadratic intensity-dependent nonlinearity coefficient, and their signs are opposite. The peak power and the period of the soliton are determined by the magnitude of the fourth-order dispersion parameter. We numerically show that the bright-soliton solution in anomalous second-order dispersion and the positive Kerr coefficient regime is stable and becomes quasi stable when the Raman effect is considered.

Self-focusing in Terbium Gallium Garnet using Z-scan

When illuminated near its resonance with an Ar ion laser beam ( λ=488 nm), laser induced thermal self-focusing is observed in Terbium Gallium Garnet. The crystal exhibits a strong intensity dependent refractive index change Δ n. The Z-scan technique is used to study the beam waist change due to Δ n. The refractive index is found to be well described by a quadratic spatial distribution model. Both the sign and the distribution coefficient of Δ n are determined.

Secondary bistable solitons in non-linear gradient index media

Secondary bistable solitons have been calculated for a non-linear optical inhomogeneous medium with parabolic gradient index and quadratic intensity-dependent refractive index change. The remarkable effect of inhomogeneities on the existence and stability of bistable solitons has been studied. The results obtained are compared with those from homogeneous solitons.

Bistable bright solitons in a non-linear monomode step-index optical fibre

For a non-linear optical fibre with quadratic intensity-dependent refractive index change, bistable solitons have been calculated. Bistability has been considered as the existence of two different amplitude shapes for the same soliton temporal width. It is demonstrated that boundary conditions (finite radius of the fibre) have a remarkable effect on the existence of bistable solitons. Results are compared with those obtained for the homogeneous case, where the effect of boundaries was not considered.

Non-linear optics for transducers: Principles and materials

This paper concentrates on intensity-dependent refractive-index changes due to the third-order optical non-linearity. Materials exhibiting such effects are good candidates for applications in all-optical devices. The discussion will be on these materials, and characterization techniques and an overview will be given of proposed and realized all-optical devices. Of the latter, the non-linear Y-junction and Mach-Zehnder interferometer will be treated in considerable detail.

Measurement of ultrafast optical nonlinearities using a modified Sagnac interferometer

A method for the measurement of fast, intensity-dependent refractive-index changes with the use of a modified Sagnac ring interferometer is presented. The measurement is not degraded by slowly responding background index changes. Nonlinear refractive-index changes in an undoped silicon wafer, and in poly-bis toluene sulfonate polydiacetylene and dye-doped polymethyl methacrylate waveguides, were measured with the use of a cw mode-locked Nd:YAG laser.

Dynamics of switching total internal reflection to transmission in a dielectric-cladded nonlinear film

The time dependence of switching from total internal reflection to the transmission mode of a cw step turned-on laser is analyzed. The nonlinear thin film is bounded by linear dielectric media and possesses an intensity-dependent refractive index. Our theory accounts for the intense reflection and feedback interferences from the two interfaces at the total-internal-reflection state and for the nonlinear medium’s finite response time. We have explicitly evaluated the dependence of turn-on time on various optical and geometrical factors. The theory can be generalized to describe pulsed lasers and various forms of intensity-dependent refractive-index changes. Our theoretical results are important for constructing practical total-internal-reflection switching devices.

Thermal index changes by optical absorption in group III-V semiconductor waveguides

Large fast-relaxing intensity-dependent refractive index changes would make certain all-optical signal processing devices in waveguides feasible. In III-V compound materials, light-induced index changes can arise from any of several physical mechanisms which have different strengths and relaxation times. Index changes from 1.06-μm pulsed and CW Nd: YAG laser excitation were measured in single-mode low-loss InP and GaAs waveguide Fabry-Perot structures. The dominant effect was found to be due to heating of the waveguide volume which accompanies an extremely small amount of free-carrier absorption.