Second Harmonic Generation Performance Research Articles

Polar structures of bent-core liquid crystals with tetrathiafulvalene units

We report on a series of three bent-core liquid crystals containing tetrathiafulvalene units. Their characterization consisted of electro-optical observations, second-harmonic generation (SHG) analysis, and x-ray diffraction measurements. Compound I exhibits a lamellar optically isotropic phase on cooling from the isotropic liquid and undergoes a nonreversible field-induced transition to a birefringent state. The two others present two-dimensional structural periodicities, being only compound II switchable. Additionally, the electron density maps of compounds II and III have been obtained based on the analysis of the x-ray diffraction intensities. According to SHG measurements the ground state is antiferroelectric in the three compounds. A quite good SHG performance is found in compounds I and II.

Chemical growth of ZnO nanorod arrays on textured nanoparticle nanoribbons and its second-harmonic generation performance

On the basis of the highly oriented ZnO nanoparticle nanoribbons as the growth seed layer (GSL) and solution growth technique, we have synthesized vertical ZnO nanorod arrays with high density over a large area and multi-teeth brush nanostructure, respectively, according to the density degree of the arrangement of nanoparticle nanoribbons GSL on the glass substrate. This controllable and convenient technique opens the possibility of creating nanostructured film for industrial fabrication and may represent a facile way to get similar structures of other compounds by using highly oriented GSL to promote the vertical arrays growth. The growth mechanism of the formation of the ordered nanorod arrays is also discussed. The second-order nonlinear optical coefficient d 31 of the vertical ZnO nanorod arrays measured by the Maker fringes technique is 11.3 times as large as that of d 36 KH 2PO 4 (KDP).

Fabrication of polyurea thin films for optical second-harmonic generation by vapor deposition polymerization. Effects of poling field and monomer structure on reactivity and SHG performance

Aromatic PU (polyurea) thin films were fabricated for second-order nonlinear optical application by vapor deposition polymerization using MDI (4,4-diphenylmethane diisocyanate) and DDM (4,4-diaminodiphenylmethane) or DNB (p-diamino nitro benzene). The former was abbreviated to PU (DDM) and the latter to PU (DNB). In PU (DDM) fabricated by VDP (vapor deposition polymerization), the SH (second-harmonic) intensity showed orientation stability up to 200/spl deg/C due to polymerization and simultaneous orientation under the dc field (the poling process). In PU (DNB) fabricated by VDP, although the enhanced optical nonlinearity due to the introduction of the NO/sub 2/ group was achieved, a drastic decay of SH intensity was observed at 200/spl deg/C. The decay was attributed to hindrance of the polymerization by the dc field, suggesting that the strong poling field reduces reactivity of some monomers. In PU (DNB) fabricated by EVDP (VDP under electric field), SHG (second-harmonic generation) was observed. Applying a higher electric field was effective in improving the optical nonlinearity. Film fabrication at higher temperature was also effective in improving the orientation stability and the optical nonlinearity, due to further polymerization during fabrication. We attributed this to the increase in the number of urea groups. The use of appropriate monomers that polymerize under high electric fields would result in improvement of SHG characteristics.

Optical second harmonic generation: A probe of atomic structure and bonding at Si–SiO2 interfaces, and other chemically modified Si surfaces

Optical second harmonic generation (SHG) is a highly surface-sensitive probe for studying crystalline Si surfaces because the inversion symmetry is broken and electric dipole optical SHG processes forbidden in the bulk are allowed. The polarized optical SHG from a perfectly oriented Si surface is inherently anisotropic, varying periodically as the in-surface projection of the polarization vector of the incident laser is rotated about a normal to the surface. The harmonic contributions to the angular anisotropy from the surface are characteristic of the surface bonding, and are modified by misorientation, chemical termination, as well as thermal treatments. This paper reviews the results of our previously reported optical SHG studies on Si(111) wafers with misorientations of 0°–5°±0.5° in the [112̄] direction for Si–H or Si–O terminated surfaces. Azimuthal anisotropy data are compared with an empirical model for the SHG intensity that is based on (i) the nonlinear response of anharmonic oscillators, and (ii) a phenomenological theory of azimuthal anisotropies expected for different surface orientations. This model is used as a framework for estimating ’’effective‘‘ resonance energies from single wavelength experiments, and in particular, for providing insights into the microscopic mechanisms that can contribute to the changes in these resonance energies with respect to different processing conditions. For example, important differences between thermally grown and plasma-oxidized interfaces are identified, and correlations between SHG and electrical performance of the Si–SiO2 interfaces are discussed.

Simultaneous blue and green second harmonic generation in quasiphase matched LiNbO3 waveguide

We report the first simultaneous quasiphase matched second harmonic generation (SHG) of blue (433 nm) and green (532 nm) light in LiNbO3 channel waveguides. Up to 0.45 mW (0.25 mW) of blue (green) light was obtained stimultaneously with an efficiency of 22%/W (28%/W). This result is made possible by using a 6.8 μm period domain inversion grating which serves as a first-order (second-order) grating for green (blue) generation simultaneously. We also show that further enhancement in the SHG performance can be obtained by choosing the appropriate grating depth.

Room-temperature 106–053-μm second-harmonic generation with MgO:LiNbO_3

Room-temperature 1.06-0.53-microm second-harmonic generation (SHG) achieved with LiNbO(3) doped with 7 mol. % MgO has been studied. Phase matching was achieved with angle tuning. SHG conversion efficiency of 45% was obtained with a 12-mm-long crystal and a fundamental peak-power density of 140 MW/cm (2). SHG performance of MgO:LiNbO(3) is compared with that of KTP and LBO crystals. Various phase-matching parameters of MgO:LiNbO(3) were calculated as functions of the fundamental wavelength, using the experimentally determined Sellmeier equations. It was found that room-temperature, noncritically-phase-matched Type I SHG can be achieved in this crystal at 1.053 microm, where Nd:YLF lasers operate.