Effective Second Harmonic Generation Coefficient Research Articles

LiAlS2: A promising infrared frequency-conversion material with ultrawide band gap and high laser-induced damage threshold

The development of infrared (IR) nonlinear optical (NLO) crystals could foster their wide applications in civil and military fields. Thus, IR NLO materials with a wide band gap, high laser-induced damage threshold, and lack of two-photon absorption are urgently needed. This work reports the comprehensive optical properties of LiAlS2 through systematic experimental characterizations and the first principle study. The results confirmed the congruently melting feature of LiAlS2, type-I phase matching (PM) performance, and its second-harmonic generation (SHG) response (0.2 × AgGaS2 at 2.09 µm). Notably, LiAlS2 shows the widest band gap of 5.13 eV among the selected IR NLO materials and an ultrahigh laser-induced damage threshold about 28 times that of AgGaS2. The structure-property relationship analysis indicates that the [AlS4] tetrahedra determine the optical band gap and primarily contribute to the SHG response. Moreover, the calculated effective SHG coefficient deff, the refractive index dispersion relation, and the shortest PM wavelength are reported. The results of this study indicate that LiAlS2 can be a promising IR NLO material in high-energy laser systems.

Study on a multifunctional crystal LiMnBO3

Single crystal of high temperature phase of LiMnBO3 in the hexagonal (h) lattice has been grown by slow-cooling method with LiBO2 as flux. The transmission spectrum reveals that h-LiMnBO3 is highly transparent from 680 to 3000nm and its absorption bands in the visible and ultraviolet regions are assigned to d–d transitions of Mn2+ ions. Second-harmonic generation (SHG) was measured with a fundamental light at 2090nm using the Kurtz and Perry technique, which showed that its effective SHG coefficient was about half that of LiB3O5(LBO). AC impedance spectroscopy shows that along the crystallographic b axis the conductivity presumably due to Li ion migration is 1.1×10−7Scm−1 at 394°C and the activation energy Ea is 0.599eV. Together with its chiral magnetic ground state, present study shows that h-LiMnBO3 is an interesting multifunctional material to possess also nonlinear optic and ionic conducting properties.

Structure and optical properties of a noncentrosymmetric borate RbSr4(BO3)3

A new noncentrosymmetric borate, RbSr4(BO3)3 (abbreviated as RSBO), has been grown from Rb2OB2O3RbF flux and its crystal structure was determined by single crystal x-ray diffraction. It crystallizes in space group Ama2 with cell parameters of a=11.128(10)Å, b=12.155(15)Å, c=6.952(7)Å, Z=4. The basic structural units are isolated planar BO3 groups. Second harmonic generation (SHG) test of the title compound by the Kurtz–Perry method shows that RSBO can be phase matchable with an effective SHG coefficient about two-thirds as large as that of KH2PO4 (KDP). Finally, based on the anionic group approximation, the optical properties of the title compound are compared with those of the structure-related apatite-like compounds with the formula “A5(TOn)3X”.

Origin of Second-Order Nonlinear Optical Effects of Nonpoled Donor–Acceptor Chromophores on Surface

Thin films of alkyl conjugated donor–acceptor (D–A) molecules, prepared by a high-yielding addition reaction between electron-rich alkynes and tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ), were investigated by ultraviolet–visible–near infrared (UV–vis–NIR) spectroscopy, in-plane and out-of-plane X-ray diffraction (XRD), and second-harmonic generation (SHG) measurements. The variable self-assembling ability of D–A chromophores produced different molecular orientations in the thin films. The TCNQ-adducted D–A molecule formed a centrosymmetric bilayer structure based on the strong interactions between the chromophores, while the film of the TCNE-adducted counter molecule was composed of interdigitated alkyl layers and randomly oriented chromophores due to their weak interactions. The interfacial interactions between a substrate and a TCNE-adducted molecular layer were expected to induce a noncentrosymmetric chromophore orientation in the thin film, which produced an effective SHG coefficient (deff) of ca. 21 pm V-1 without electric poling.

Second harmonic generation in composites of ellipsoidal particles with core–shell structure

We study the enhancement of the second-harmonic generation (SHG) coefficient in a random composite consisting of ellipsoidal particles with a core–shell structure in a linear dielectric host. The material making up the ellipsoidal core is assumed to be dielectric, but with a nonlinear susceptibility for SHG. The coating material is assumed to be metallic with a linear susceptibility. The effective SHG coefficient is derived and its expression is related to various local field factors. The numerical calculations of the effective SHG response per unit volume of nonlinear material can be greatly enhanced at certain frequencies. For coated ellipsoidal particles, the core–shell structure and the particle shape allow for tuning of the resonance through the choice of material parameters and/or the ratio of the core to shell volume fraction and the depolarization factor of the particles.

Second harmonic generation in random composites of particles with core-shell structure

We study the effective second harmonic generation (SHG) coefficient in a random composite consisting of particles with a core-shell structure embedded in a linear dielectric host. The material making up the core of the particles is assumed to be nonconducting, but with a nonlinear susceptibility for SHG. The coating material is assumed to be linear and metallic. An expression for the effective SHG coefficient is obtained, in terms of various local field factors. The effective SHG response per unit volume of nonlinear material is found to be greatly enhanced at certain frequencies. For coated particles, the core-shell structure allows for tuning of the resonance through the choice of material parameters and∕or the ratio of the core to shell volume fraction.

Second-harmonic generation for a dilute suspension of coated particles

We derive an expression for the effective second-harmonic coefficient of a dilute suspension of coated spherical particles. It is assumed that the coating material, but not the core or the host, has a nonlinear susceptibility for second-harmonic generation (SHG). The resulting compact expression shows the various factors affecting the effective SHG coefficient. The effective SHG per unit volume of nonlinear coating material is found to be greatly enhanced at certain frequencies, corresponding to the surface plasmon resonance of the coated particles. Similar expression is also derived for a dilute suspension of coated discs. For coating materials with third-harmonic (THG) coefficient, results for the effective THG coefficients are given for the cases of coated particles and coated discs.

Determination of the nonlinear optical coefficients of YCa_4O(BO_3)_3 crystal

The nonlinear optical coefficients of YCa4O(BO3)3 (YCOB) have been calculated for the first time to the authors’ knowledge by use of the revised Gaussian 92 program on the basis of the anionic group theory. The d11,d12,d32, and d33 coefficients of the crystal were determined by the Maker fringes method at λ=1064 nm, and d13, and d31 were also evaluated by measurement of the Maker fringes. The coefficients determined by the Maker fringes are in agreement with the theoretical values. On the basis of the dij coefficients and the Sellmeier equations, the effective second-harmonic generation (SHG) coefficient deff of YCOB as a function of phase-matching angles θ and Φ was determined. It is interesting that the maximum effective SHG coefficient deff of YCOB is not in the principal plane but is located out of the X–Z plane in the positions θ=65.9°,Φ=36.5° and θ=66.3°,Φ=143.5°. It is also worth noting that the largest deff of YCOB in the region of 0°<θ<90°,90°<Φ<180° is larger than that in 0°<θ<90°,0°<Φ<90°. These calculated results have been proved by the effective SHG coefficient measurements of YCOB and conversion-efficiency measurements of frequency-doubling experiments on Nd:YCOB.

Second-harmonic generation in Ce:BaTiO_3 nanocrystallites grown by pulsed laser deposition

Ce:BaTiO3 nanocrystallites were prepared on fused quartz substrates by pulsed laser deposition. The structural properties of the nanocrystallites were studied by scanning electron microscopy and x-ray diffraction. The refractive indices and absorption edges were obtained from the optical transmission spectra with an approximate formula. The incidence and polarization angle–dependent second-harmonic generation (SHG) were measured. The effective SHG coefficient was reduced by decreasing the deposition temperature. Some possible mechanisms of the correlation between the SHG coefficient and the deposition temperature are discussed.

Local normal-mode analysis of second harmonic generation in a periodic waveguide

A new theoretical analysis method based on the local normal-mode expansion is proposed for analysis of optical second harmonic generation (SHG) under quasi-phase-matching in a waveguide with a periodic structure. Nonlinear coupled mode equations, SHG efficiency, and effective SHG coefficient are derived. In addition, SHG devices quasi-phase-matched with corrugation and with periodic domain inversion are studied analytically and numerically. The results are compared to those obtained by the conventional ideal normal-mode expansion. It is shown that in a corrugated waveguide the local normal-mode expansion method presents a different result from the ideal normal-mode expansion since the periodic variation of the modal field profile is dominant in the quasi-phase-matching process. On the other hand, in the periodically domain-inverted ferroelectric waveguide, it is shown that the variation of the modal field profile is sufficiently small and both expansion methods coincide with each other.

Stable and Large Second Harmonic Generation in Sol-Gel-Processed Poled Silica Waveguides Doped with Organic Azo Dye

Sol-gel-processed silica films doped with organic azo dye, Disperse Red 1 (DR1) in a higher concentration of 21.7 wt%, were prepared at a higher baking temperature with corona poling of higher voltage than in other reports to obtain stable and large second harmonic generation (SHG) and film strength. A large effective SHG coefficient of 75 pm/V was measured for the film of 0.2-µm thickness at a fundamental wavelength of 1064 nm, and the value was unvaried for 700 h with no relaxation. Moreover, a waveguide SHG device of four-layer structure with DR1-doped sol-gel film as a top layer was fabricated for the first time, and Cherenkov-type phase-matched SHG was observed successfully.