Reflective Second Harmonic Generation Research Articles

The SHG-response of different phases in proton exchanged lithium niobate waveguides

Reflection second-harmonic generation from the polished waveguide end face is used to investigate the second-order nonlinear optical properties of as-exchanged and annealed proton-exchanged (PE) waveguides in different H/sub x/Li/sub 1-x/NbO/sub 3/ phases. A detailed correlation is done between the nonlinear properties, the processing conditions, the refractive index changes, and the optical losses of the waveguides. It is found that for the direct PE samples, where the /spl beta//sub 4/, /spl beta//sub 3/, and /spl beta//sub 1/ phases are generated at the surface, the nonlinearity in the guide is strongly reduced by more than 85% of its bulk value, while for waveguides prepared in the /spl beta//sub 2/ phase, the nonlinear coefficient is about 55% of the bulk one. A consequence is that the step-like /spl beta//sub i/-phase PE LiNbO/sub 3/ waveguides with large refractive index increase are advantageous for efficient SHG in Cherenkov configuration. The nonlinearity, strongly reduced after the initial proton exchange, is found to be restored and even increased after annealing. However, this apparent increase of the nonlinearity is accompanied by a strong degradation of the quality of the second-harmonic generation reflected beam in the region of initial waveguides due to beam scattering. The graded proton exchange technique and dilute melt proton exchange have been shown to produce high-quality waveguides with essentially undergraded nonlinear optical properties. It has been also shown that the nonlinear properties of annealed proton exchanged LiNbO/sub 3/ waveguides can be effectively recovered by the reverse proton exchange technique. The results obtained are important for the design, fabrication, and optimizing of guided-wave nonlinear optical devices in LiNbO/sub 3/.

Magnetization dynamics of Ni and Co films on Cu(001) and of bulk nickel surfaces

The ultrafast magnetization dynamics of thin Ni and Co films on Cu(001) and of polycrystalline Ni surfaces was studied by pump-probe reflection second-harmonic generation, utilizing 150 fs/800 nm laser pulses. In all cases no delay between electron excitation and magnetization breakdown was observed within the experimental time resolution. An upper limit of such delay is 50 fs in case of bulk Ni surfaces. The recovery of magnetization follows the electron temperature relaxation during the first few picoseconds and thereafter cooling by regular thermal diffusion.

Extracting the Free Energy of Adsorption and the Base-Ionization Constant of Triethylamine at the Silica/CH3CN Interface Using Nonlinear Optical Molecular Probes

By using reflection second-harmonic generation (SHG), the adsorption of triethylamine (TEA) at the silica/CH3CN interface has been studied with the help of two molecular probes, trans-4-[4-(dibutylamino)styryl]-1-methylpyridinium iodide (DMP+I-) and trans-4-[4-(dibutylamino)styryl]-1-(3-sulfopropyl)pyridinium (DP). At the silica/CH3CN interface, DMP+ cations are adsorbed on SiO- sites by charge−charge interaction, while DP molecules are adsorbed on SiOH sites by forming hydrogen bonds. The saturation surface density is 3.10 × 1013 cm-2 for DMP+ cations and is 1.85 × 1014 cm-2 for DP molecules. The free energy of adsorption ΔGads is −25.5 kJ mol-1 for DMP+ cations and is −22.0 kJ mol-1 for DP molecules. The addition of TEA into the solution causes dramatically different changes in the interfacial densities of the two probes: The surface density of DP molecules is decreased by a factor of 5 when the TEA concentration reaches 2 mM, while that of DMP+ cations is increased by a factor of 5 under the same condition. From the interfacial behavior of the two molecular probes that are coadsorbed with TEA and TEAH+ at the silica/CH3CN interface, the free energy of adsorption and the base-ionization constant of TEA at the silica/CH3CN interface have been extracted, ΔGTEA = −20.8 kJ mol-1 and Kb = 0.0173.

Anisotropic second-harmonic generation from chromophoric self-assembled monolayers upon oxide overlayers of Si(111) surfaces

The nonlinear optical response of stilbazolium-based organic monolayers self-assembled upon silicon(111) substrates, which have oxide layers 4–860 nm thick, is studied by reflection second-harmonic generation at an input wavelength of 1.053 µm. All samples, irrespective of oxide thickness, display clear threefold symmetry about the azimuth, which is shown to arise from interference between an isotropic component generated by the chromophoric monolayer, and an anisotropic component of sixfold symmetry, which originates from the Si(111) substrate. Maximum second-harmonic intensity and orientation relative to the silicon lattice vary with different oxide thicknesses as a consequence of multiple internal reflections within the oxide layer, which can be expressed as thickness-dependent local field corrections. Evaluation of the principal tensor components permits estimation of the molecular tilt angle from the surface normal as ∼25° for monolayers self-assembled upon thick oxide overlayers (>30 nm).

Femtosecond time-resolved reflection second-harmonic generation on polycrystalline copper

Optical Second-Harmonic Generation (SHG) in reflection from a polycrystalline copper surface in air was studied using femtosecond time-resolved pump and probe measurements at λ=625 nm. The observed time dependence of second-harmonic yield from the probe beam demonstrates, that SHG is a very sensitive technique for measuring transient electron temperatures of metals even when these are covered by an oxide layer. For polycrystalline copper, an electron-phonon energy transfer time of 2 ps was observed, corresponding to a coupling constant of 3.75×1017 W/m3 K at average lattice temperatures of about 500 K. The analysis of experimental data indicates that the time dependence of SHG is governed by the linear dielectric function e which, in turn, is affected by the electron temperature. There is no evidence for a temperature dependence of the nonlinear susceptibility χ(2).

Femtosecond time-resolved reflection second-harmonic generation on polycrystalline copper

Femtosecond time-resolved reflection second-harmonic generation on polycrystalline copper

Total internal reflection second-harmonic generation: Probing the alkane water interface

Total internal reflection Second-Harmonic Generation (SHG) has been used to study a series of neat n-alkane/water interfaces. Polarization and incident angular-dependent measurements of the SH response show good agreement with theoretical predictions. Analysis of the incident and polarization angular-dependent SH response allows for determination of the nonlinear optical properties of molecules comprising the interfacial region. Based on Kleinman symmetry, the measured surface nonlinear susceptibilities suggest a high degree of interfacial order for octane and decane with less order indicated by the odd carbon n-alkanes examined, heptane and nonane. The SH response in reflection and transmission has been measured under a Total Internal Reflection (TIR) of the fundamental. The measured nonlinear susceptibilities in each case are found to be identical.

Determination of molecular orientation in molecular monolayers by second-harmonic generation

A detailed comparison is given of several theoretical expressions commonly used in the determination of the molecular orientation in monolayers by reflection second-harmonic-generation measurements. The experimental data obtained from the Langmuir films of a hemicyanine dye are used to demonstrate that the deduced molecular-orientation angle depends sensitively on the linear dielectric constants of the monolayer. It is also pointed out that an unambiguous determination of the nonlinear susceptibilities of a monolayer by reflection second-harmonic generation requires measurements that use at least three different polarization combinations.