Nonlinear optical characteristics of nanoparticles in suspensions and solid matrices

The study of the nonlinear optical parameters of suspensions and glass matrices doped with various nanoparticles (Ag, Cu, Co, Au, Pt, GaAs, CdS, As₂S₃) are presented. Various techniques for the preparation of nanoparticles in suspensions and solid matrices, in particular laser ablation, chemical methods, and ion implantation are discussed. We present our measurements of the nonlinear refractive indices, nonlinear absorption coefficients, saturation intensities, and nonlinear susceptibilities of nanoparticles-containing media using laser pulses generating in various spectral and temporal ranges. PACS 42.65.An; 42.65.Hw; 42.65.Jx; 61.46.Df; 78.67.Bf     

Nonlinear refractive properties of Yb3+-doped KY(WO4)2 and YVO4 laser crystals

The nonlinear refractive indices of Yb³⁺: KY(WO₄)₂ and Yb³⁺:YVO₄ laser crystals are characterized using a Z-scan technique at a wavelength of 1.08 μm for different polarizations. The results reveal that the nonlinear refractive index of Yb³⁺:YVO₄ is superior to that of Yb³⁺:KY(WO₄)₂ and is found to be 1.9×10^-15 cm²/W and 1.5×10^-15 cm²/W for E⊥c and E∥c polarizations, respectively. PACS 42.65.Hw; 42.55.Rz; 42.65.Re; 42.70.Hj     

Characterization of the nonlinear refractive index of the laser crystal Yb:KGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We present results of the characterization of the nonlinear refractive index of the laser crystal Yb:KGd(WO₄)₂ using a z-scan technique over the 800–1600 nm wavelength range. Based on our experimental and theoretical results we conclude that Yb:KGW crystal is a good candidate for efficient Kerr-lens mode locking. PACS 42.65.An; 42.65.Hw; 42.55.Rz; 42.65.Re     

Temperature and wavelength dependent refractive index equations for MgO-doped congruent and stoichiometric LiNbO<Subscript>3</Subscript>

We present wavelength- and temperature-dependent refractive index equations for 5% MgO-doped congruent PPLN and for 1% MgO-doped stoichiometric PPLN crystals valid for a wide spectral and temperature range. The dispersion equations were derived from quasi-phase-matched nonlinear interactions with these two crystal compositions in the near and mid-infrared. The results show a good agreement with previously published frequency conversion experiments. PACS 42.65.Ky; 42.65.Yj; 42.70.Mp     

Linear and nonlinear optical properties of yttrium formate dihydrate,Y(HCOO)<Subscript>3</Subscript>·2H<Subscript>2</Subscript>O

Yttrium formate dihydrate, Y(HCOO)₃·2H₂O (point group 222), a promising new material for Raman laser frequency converters, has been reinvestigated with respect to the linear and nonlinear optical (second-harmonic generation, SHG) properties. High-precision data of refractive indices and their dispersion in the wavelength range 365–1083 nm are given, together with the three independent components of the tensor of nonlinear optical susceptibility (SHG) [d_ijk ^SHG]. A calculation of the macroscopic nonlinear optical susceptibility from the hyperpolarizabilities of the formate groups results in good agreement with the experimental values. This signals that the nonlinear optical interaction in Y(HCOO)₃·2H₂O can mainly be attributed to the formate groups. PACS 42.65.Ky; 42.70.Mp; 78.20.Ci     

Discrete and dipole-mode gap solitons in higher-order nonlinear photonic lattices

We investigate the formation of fundamental discrete solitons and dipole-mode gap solitons in triangular photonic lattices imprinted in photorefractive nonlinear media. These lattices are strongly affected by the photorefractive anisotropy, resulting in orientation-dependent refractive index structures with reduced symmetry. It is demonstrated that two different orientations of the lattice wave enable the formation of fundamental discrete solitons in the total internal reflection gap. Furthermore, it is shown that one lattice orientation additionally supports dipole-mode solitons in the Bragg reflection gap. The experimental results are corroborated by numerical simulations using the full anisotropic model. PACS 42.65.Tg; 42.65.Wi; 42.70.Qs     

Nonlinear optical characteristics of self-assembled films of ZnO

In the present work, we have investigated the nonlinear optical properties of self-assembled films formed from ZnO colloidal spheres by z-scan technique. The sign of the nonlinear component of refractive index of the material remains the same; however, a switching from reverse saturable absorption to saturable absorption has been observed as the material changes from colloid to self-assembled film. These different nonlinear characteristics can be mainly attributed to ZnO defect states and electronic effects when the colloidal solution is transformed into self-assembled monolayers. We investigated the intensity, wavelength and size dependence of saturable and reverse saturable absorption of ZnO self-assembled films and colloids. Values of the imaginary part of third-order susceptibility are calculated for particles of size in the range 20–300 nm at different intensity levels ranging from 40 to 325 MW/cm² within the wavelength range of 450–650 nm. PACS 81.16.Dn; 42.65.-k; 42.65.An; 42.70.-a; 42.70.Nq; 78.20.Ci     

Streak formation as side effect of optical breakdown during processing the bulk of transparent Kerr media with ultra-short laser pulses

Femtosecond lasers have been successfully used to perform refractive surgery, by cutting within the bulk of the corneal tissue. As a side effect to the laser cutting there, a streak-like discoloration is observed in histological sections above and below the cutting plane, incident with the direction of laser propagation. These streak-shaped alterations of tissue are believed to originate from low free-electron densities not sufficient to cause optical breakdown. To understand the generation of the streaks, the nonlinear interaction of ultra-short laser pulses with water, as an approximation to corneal tissue, is simulated numerically using a model that simultaneously describes both the nonlinear pulse propagation and the generation of free electrons. The model is used to calculate spatial free electron density distributions generated by ultra-short pulses. Areas of high free-electron density correspond to optical breakdown, whereas areas of low density can be related to streaks. The numerical code can be adapted to practically any transparent dielectric Kerr medium, whose nonlinear optical parameters are known. PACS 72.20.Jv; 42.65.Jx; 42.65.Sf; 42.62.Be     

Oscillatory behavior of spatial soliton in a gradient refractive index waveguide with nonlocal nonlinearity

Oscillatory behavior of spatial solitons in a transverse parabolic gradient refractive index distribution (GRIN) waveguide with both local and nonlocal nonlinearity is investigated. Dynamics of such solitons are analyzed by the effective-particle approach method. For weak nonlocal nonlinearity, solitons oscillate in transverse direction periodically during propagation. The normalized width and maximum of refractive index variation of the waveguide play a key role in determining the oscillating period while the position of soliton oscillatory center is slightly influenced by the nonlocal nonlinearity. Stronger nonlocal nonlinearity leads to instability of the oscillatory solitons. Furthermore, the dynamics of the solitons are simulated numerically and good agreements are obtained between the analysis and numerical results. This behavior may be used in all-optical routers, switches etc. PACS 42.65.Tg; 42.65.Jx; 42.65.Wi     

Measurement of nonlinear optical coefficients by the z-scan technique: Correctness of the technique and investigation of a new compound-lutetium diphthalocyanine complex

Correctness of z-scan measurements of nonlinear refractive indices for optical media is considered. It is shown that accuracy of these measurements is greatly affected by the thickness of the nonlinear medium layer (as compared with the diffraction length of the laser beam). Nonlinear refractive indices of nitrobenzene are measured at different laser pulse durations (7 ns and 350 ps); the nonlinearity mechanism is discussed. It is shown that at the pulse duration 350 ps the orientation mechanism of nonlinearity is less effective than at the pulse duration 7 ns due to its time lag. Nonlinear refractive indices are measured for the solution of a new synthesized compound-hexadecabutyl-substituted lutetium diphthalocyanine.     

Origins of waveguiding in femtosecond laser-structured LiNbO<Subscript>3</Subscript>

Femtosecond laser-induced structural changes in LiNbO₃ are studied. Depending on the laser processing parameters two different types of modification are identified and their origin is discussed. Both types of modification can be described within the framework of induced lattice defects. For strong material damage a refractive index increase can be obtained due to the induced stress field. By appropriate tailoring of this stress field thermally stable and highly symmetric waveguides can be obtained well suited for nonlinear integrated-optical applications. PACS 61.80.Ba; 77.84.Dy; 42.65.Re; 42.82.Et     

Femtosecond laser writing of optical waveguides with controllable core size in high refractive index glass

Optical waveguides have been produced by femtosecond-laser writing in a high linear and nonlinear refractive index glass (SF57). Light guiding occurs nearby a central damaged zone due to the collateral densification caused by shockwaves generated in the focal volume. High pulse energies are required to induce a modified region capable of supporting a guided mode. An alternative processing method, based on using multiple structures, has been successfully used for the production of waveguides with controllable core size at low energies. PACS 42.82.Et; 42.65.Re; 42.70.Ce; 81.20.–n     

A spectrophotometric method for determining linear and predicting nonlinear optical properties of glass substrates

In this work, an expression for the interface reflectance in terms of both the total transmittance and the total reflectance of the glass substrate is derived. Using this expression a nondestructive procedure for deducing the complex refractive index of glass bulk material is suggested. Complex refractive indices of four glassy substrates with different thicknesses and compositions are experimentally measured across a spectral range 200–2000 nm. The errors of measurements for the suggested procedure depend on the reflectance and transmittance errors. They are calculated to be of about ±2.751 × 10^?8 for imaginary and ±0.008 for real refractive indices. Dispersive parameters of the real refractive indices and optical energy gaps for direct and indirect allowed optical transitions are deduced. Predicted nonlinear refractive indices and susceptibilities are evaluated.     

Temperature-dependent dispersion relations for RbTiOPO<Subscript>4</Subscript> and RbTiOAsO<Subscript>4</Subscript>

We have measured the first and second derivatives of the refractive index with respect to temperature along the z- and y-axes for two nonlinear optical materials, RbTiOPO₄ and RbTiOAsO₄. The measurements were done using an interferometric setup in the wavelength range of 532–1580 nm and in the temperature range of 25–200 °C. In addition, the thermal expansion coefficients were measured in the x direction. Experimental results of nonlinear frequency conversion in RbTiOAsO₄ were used to explore the temperature dependence of the refractive index in the z-axis near 3.5 microns. We have derived thermal-dependent dispersion equations for the two materials. Calculations based on these dispersion equations are in good agreement with previously reported measurements. The thermal dependence of the refractive index of RbTiOAsO₄ is greater than that of RbTiOPO₄ by approximately 15% and 70% in the z- and y-axes, respectively. Hence RbTiOPO₄ is more appropriate for applications that require immunity to thermal lensing, whereas RbTiOAsO₄ is suitable for realizing temperature-tuned nonlinear devices. PACS 42.65.Ki, 78.20.Cy, 78.20.Nv     

Chaos in Fiber-Optic Stimulated Brillouin Scattering without External Feedback in Large Nonlinear Refractive Index Regime

Stokes dynamics in stimulated Brillouin scattering generated in optical fibers is analyzed over a wide range of nonlinear refractive indices under a no feedback condition. The Stokes fluctuation becomes complicated as the nonlinear refractive index increases. Chaotic behavior appears without external feedback at a large nonlinear refractive index of 1×10^-19 m²/V² with pump power larger than 0.3 W.     

Ultra-fast reconfigurable spatial switching between a quadratic solitary wave and a weak signal

Experimental and numerical investigations of an ultra-fast reconfigurable spatial switch based on the nonlinear interaction between a weak wave (the signal) and a solitary wave (the control) at 1548 nm are reported. The non-collinear interaction in a quadratic nonlinear film waveguide gives birth to a third switched optical beam (the idler). This beam could be steered according to the transverse spatial position of the control beam. PACS 42.65.Ky; 42.65.Re; 42.65.Wi     

Nonlinear optical and optical limiting studies of alkoxy phthalocyanines in solutions studied at 532 nm with nanosecond pulse excitation

We report our results on the nonlinear optical and optical limiting properties of two alkoxy phthalocyanines namely 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine and 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine zinc(II) studied at a wavelength of 532 nm using 6 ns pulses. Using the standard Z-scan technique we observed that both the phthalocyanines exhibited negative nonlinearity as revealed by the signature of closed aperture data. The magnitude of the nonlinear refractive index n₂ evaluated from the closed aperture data was ∼ 1.61×10^-11 cm²/W for the free-base phthalocyanine and ∼ 1.56×10^-11 cm²/W for the metallic phthalocyanine. Open aperture Z-scan data indicates strong nonlinear absorption in both the phthalocyanines with measured nonlinear coefficients of ∼ 1650 cm/GW and ∼ 1850 cm/GW respectively. We also report optical limiting properties of these phthalocyanines with limiting thresholds (I_1/2) of ∼ 0.5 J/cm². Our studies suggest that these phthalocyanines are one of the best molecules for nonlinear optical applications studied recently. PACS 42.65.-k; 42.70.Jk, 42.65.Jx     

Self-recovery of wavefront in a nonlinear light scattering medium

Both theoretical and experimental studies of wavefront self-recovery of intense light beam propagating through heterogeneous nonlinear scattering medium are described. The self-recovery of the light beam wavefront is shown to be observed in the definite range of the light intensities when a mismatch of the linear refractive indices of the medium hetero components is compensated by nonlinear light-induced changes of refractive indices.     

掺半导体玻璃(SDG)波导非线性饱和的导波测量方法

掺半导体玻璃波导的折射率分布是通过多模波导有效折射率测量得到的.当耦合进入波导的激光功率密度变化时,可在不同模深度处,测量到波导的非线性折射率系数,并获得波导非线性折射率系数与功率密度的关系.从而得到波导的非线性饱和值为1.54×10-4.     

Precise measurement of group refractive indices and temperature dependence of refractive index for Nd-doped yttrium orthovanadate by intracavity spontaneous mode locking

We report on a novel method based on intracavity spontaneous mode locking to precisely measure the group refractive indices and temperature dependence of refractive index of Nd:YVO(4) crystal at the wavelength of 1064 nm. All the experimental results are found to agree very well with the most recent measured values. We also confirm that the developed method is applicable to measuring the group refractive indices and the temperature dependence of the refractive indices of other vanadate crystals, as well as nonlinear crystals.