Second Harmonic Generation Experiments Research Articles

Second harmonic generation study of myoglobin and hemoglobin and their protoporphyrin IX chromophore at the water/1,2-dichloroethane interface

Second harmonic generation (SHG) from the proteins myoglobin (Mb) and hemoglobin (Hb) adsorbed at the interface between 1,2-dichloroethane (DCE) and a pH 7 aqueous buffer solution is reported and compared to the response from the Mb and Hb chromophore Fe(III) protoporphyrin IX (PpIX) freely adsorbed at the interface. Resonant SHG experiments were performed probing the PpIX π–π* transition and large SHG signals were obtained for bulk aqueous concentrations of proteins as low as 2 μM for Mb and 350 nM for Hb. Both protein adsorption isotherms demonstrated interactions between the proteins at the interface. A Gibbs free energy of adsorption of −82 ± 8 kJ mol−1 was determined for Mb and of −121 ± 12 kJ mol−1 for Hb at high surface coverage. From a light polarization analysis of the SHG signal, the angle of orientation of the free PpIX chromophore at the water/DCE interface was found to be 43 ± 2° with respect to the interface normal direction, assuming a Dirac delta function for the angle distribution, and 55 ± 2° for Mb, independently of the protein surface coverage. The bulk aqueous buffer solution pH was varied and a clear modification in the Hb monolayer was observed at a pH of 4, possibly the protonation of the PpIX carboxylic groups.

Colored conical emission by means of second-harmonic generation

We predict that the combination of space and time modulational instabilities that occur by means of parametric wave mixing in quadratic media leads to colored conical emission. This phenomenon should be observed under conditions usually employed in second-harmonic experiments.

Numerical analysis of conversion efficiency in second-harmonic generation in new crystal CsLiB6O10

In this paper, a detailed theoretical analysis of the second-harmonic generation (SHG) efficiency as a function of polarization ratio and crystal length of CsLiB6O10 (CLBO) in the phase matching using a computer simulation is presented. The SHG efficiency was as high as 80% when the pumping radiation was a plane wave. The efficiency is 65% and 80% for the pumping waves of type I and type II Gaussian beams, respectively. An important result is that the phase matching of type II is perfectly adopted in the SHG experiment with CLBO.

About the depth sensitivity of second-harmonic radiation in ultra-thin metal films

Second-harmonic generation (SHG) results for Ni and Co films on Cu (001) have been reinvestigated regarding the depth sensitivity of this technique for thin films. We find that tangential components of the nonlinear susceptibility are much more sensitive to real film properties than normal components, which are confined to surfaces and interfaces. In consequence, for SHG experiments on ultra-thin metal films, polarization combinations should be favored that possess only tangential susceptibility components, even though the yield is weaker. Additional phase measurements are necessary to obtain full information about the film.

SHG anisotropy in Au/Co/Au/Cu/vicinal Si(1 1 1)

The second harmonic generation (SHG) reflectivity on magnetic multilayers is a very sensitive technique to reveal the crystallography of buried interfaces. We have used the azimuthal anisotropy of SHG to demonstrate that the vicinal character of Si(1 1 1) substrate is duplicated in the metallic multilayer Au/Co/Au/Cu. The magnetic properties of these multilayers as anisotropy and magneto-optic polar Kerr rotation were studied by linear magneto-optic effects in correlation with SHG experiments, by varying the Co and Au buffer thicknesses as well as the Cu buffer deposition condition.

Optical second-harmonic spectra of Si(001) with H and Ge adatoms: First-principles theory and experiment

We present calculated second-harmonic-generation (SHG) spectra of the Si(001) surface based on a first-principles description of eigenvalues and eigenvectors using ab initio pseudopotentials. We also present SHG spectra for Ge-covered Si(001). The theoretical results explain all essential features of recent experimental SHG spectra of the $\mathrm{Si}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surface with low coverages of hydrogen and/or germanium, which alter the ${E}_{1}$ resonance in contrasting ways. The strong adatom specificity of the spectra results from redistribution of the adatom-related electronic states on the surface.

Study on the Second-Harmonic Generation of YCa4O(BO3)3 Crystals along Various Phase-Matching Directions

Large size and high-quality YCa 4 O(BO 3 ) 3 (YCOB) crystal can be grown using the conventional Czochralski pulling method. The theoretical calculations of the effective coefficient (d eff ) and the walk-off angles at 530 nm of YCOB along various phase-matching (PM) directions are reported. The PM direction along (0 = 64.5°, φ = 144.5°) for YCOB crystal has the largest d eff- Second-harmonic generation (SHG) experiments were performed on YCOB crystal along various PM directions. The results show that in 6 mm long YCOB crystal, the SHG conversion efficiency is 39% along the PM direction (θ = 64.5°, φ = 144.5°) at 80 MW/cm 2 input power density, which is larger than that along (0 = 64.5°, φ = 35.5°) and the PM direction in the XZ main plane. The experimental result is in good agreement with the theoretical calculations.

Polarons, oxygen vacancies, and hydrogen in BaxSr1–xTiO3

Abstract The role of hydrogen in strontium titanate and barium strontium titanate is analysed, for both ceramics and single crystals. New infrared and second-harmonic generation (SHG) experiments show the relationship between hydrogen and oxygen vacancies. The infrared data exhibit strong absorption in the 3000–6000 cm −1 region due to polarons. The SHG data exhibit rather strong signals at room temperature (nominally forbidden). These data are correlated with the temperature dependence of activation enthalpies from Steinsvik (Oslo), and it is found that the latter are characterised by a universal exponent of 7 4 for the dependence upon vacancy concentration, in agreement with the predictions for a cubic lattice in percolation theory.

Effect of Strontium Ion on the Growth and Second-HarmonicGeneration Properties of GdCa4O(BO3)3 Crystal

Large sized and optical quality GdCa4O(BO3)3 (GdCOB)crystal with calcium partiallysubstituted by strontium (Sr) ions was grown for the first time.The x-ray powderdiffraction shows that the Sr ion has an effect on the cell parametersof the GdCOBcrystal. Second-harmonic generation (SHG) experiments were performedon theSr:GdCOB and GdCOB crystals along the spatial phase-matching direction. Theresults show that the cw green laser output power is 1.04 Wwhen the laser diodepump power is 10 W in the Nd:YVO4/Sr:GdCOB intracavity laser. Considering theslope efficiency of Nd:YVO4 and the fundamental laser in cavity,the intracavity SHGconversion efficiency of the GdCOB crystal is 21% while it is 24% forthe Sr:GdCOBcrystal, the SHG conversion efficiency is improved by 14%. The effectof the Sr ionson the nonlinearity of the GdCOB crystal is also discussed by using theanion group theory. PACS: 42. 65. Ky, 78. 20. -e, 78. 55. Hx

Second-order susceptibility tensor of a monolayer at the liquid–air interface: SHG spectroscopy by compression

The complete expression for the macroscopic second-order susceptibility (SOS) tensor χ (2) for a monolayer with C ∞ symmetry at the air–liquid interface is derived as functions of the molecular SOS tensor α (2) and the orientational order parameters. The chiral (non-chiral) terms of χ (2) are distinguished by their association with even (odd) order parameters, by which a second-harmonic generation (SHG) circular-dichroism effect can be better understood. An SHG experiment of a 4-cyano-4′-5-alkyl-biphenyl monolayer on an air–water interface by monolayer compression is also discussed.

Optical Second Harmonic Generation Study of Malachite Green Orientation and Order at the Fused-Silica/Air Interface

Optical second harmonic generation (SHG) studies of malachite green (MG) adsorbed at the fused-silica/air interface have been conducted. The concentration dependence of the second harmonic intensity indicates the formation of ordered monolayers normal to the interface. Polarization SHG experiments have been used to extract the averaged molecular orientation in adjacent molecular layers and to characterize the breadth of the orientation distribution. MG adsorbs in an upright orientation with its molecular plane near normal to the surface. This orientation is consistent with the interaction of the polar surface and the polar dimethylamino substituents of the adsorbate. Molecular orientation in the second monolayer resembles that in the first, but also reflects some additional broadening of the orientation distribution function. The onset of parallel, sandwich-type aggregates has been observed within the first monolayer. The formation of parallel, sandwich-type, intralayer aggregates is distinct from the int...

Optical second harmonic spectra of silicon-adatom surfaces: theory and experiment

A recently-developed ab initio theory of second harmonic generation (SHG) spectroscopy of semiconductor surfaces is used to develop a new ‘modified’ semi-empirical tight binding model (SETBM) of SHG spectroscopy. Some key quantitative features of experimental SHG spectra for H- and Ge-covered Si(001) are not well explained by a simple two-parameter SETBM, despite its success in explaining linear semiconductor surface spectra. The ab initio theory identifies the source of this short-coming as the stronger contribution of optical transitions involving surface atom/adatom states to the SHG response, and is then used to quantify additional SETBM parameters describing these transitions, resulting in a complete, accurate, and convenient description of surface orbital hybridization and experimental results.

Zwitterionic polymers for nonlinear optics

Good optical quality thin films could be obtained with zwitterionic polymaleimides grafted by pyridinium–phenoxide zwitterions and also with polymethylmethacrylate (PMMA) doped by the same chromophores. Zwitterionic grafted polymaleimides presented low nonlinear efficiencies. Conversely, orientation of the PMMA doped films by the corona method could easily be performed and led to efficient systems for second-harmonic generation. Orientation within the doped films and susceptibility coefficients were very stable with time. Since doped polymers readily loose their orientation generally, the stability of these zwitterionic systems must be highlighted. The efficiency of zwitterionic doped PMMA films was of the same order as that of classical guest–host systems of PMMA such as DR 1, DO 3, DCV and TCV. Illumination of zwitterionic grafted or doped polymers in the chromophore absorption band and during the corona poling led to a reversible depoling of thin films. The poling was restored when the illumination was switched off. A cycling of poling–depoling, monitored by the external light illumination, led to an enhanced polar orientation as well as to an enhanced temporal stability of the induced order, as checked by the in situ second-harmonic generation experiments. This perfectly reversible effect, accompanied by the light-induced refractive index variation of thin film, should find numerous practical applications.

Linear and nonlinear optical properties of Langmuir–Blodgett multilayers from chromophore-containing maleic acid anhydride polymers

Organized polymer films prepared from amphiphilic side-chain polymers were investigated with respect to their application in passive or active photonic devices. The polymers were synthesized by means of polymer analogous reaction of azo chromophores with a maleic acid anhydride polymer which bears saturated hydrophobic side chains. The resulting polymer contains, besides hydrophilic groups and hydrophobic side chains, azo chromophores with high second-order hyperpolarizability which are attached to the main chain via a spacer. In order to increase the hydrophobic character of the chromophore, a fluorinated moiety was chosen as acceptor of the chromophore. Langmuir–Blodgett (LB) technique was applied for preparation of multilayer structures with distinct supramolecular architecture from these polymers. They are characterized by high anisotropy and a special combination of linear and nonlinear optical (NLO) properties. The polymers were deposited both in Y-type and Z-type LB layers. The Y-type layers of the azopolymer show high birefringence and dichroism but small nonlinear response which originates from the first deposited layer only. Multilayers of the azopolymer deposited in Y-type configuration are characterized by high internal order which can be concluded from a pronounced Bragg-peak and Kiessig-fringe patterns as obtained from gracing incidence X-ray scattering. Experiments to deposit the active polymer directly in Z-type configuration by passing a blank water compartment before layer deposition in a double compartment LB-trough led to the result that well-ordered, anisotropic films are obtained only if the number of deposited monolayers is limited to a value of about 30. The layers deposited in Z-type manner show low order. This can be concluded from the fact that only a weak Bragg-peak is observed in the X-ray scattering curves, and Kiessig-fringes are very weak or missing. The same conclusion can be drawn from ellipsometric and second-harmonic generation (SHG) experiments. A second kind of Z-type multilayers was prepared by alternate deposition of the active azopolymer and of another maleic acid anhydride polymer without chromophores. They show similar linear optical properties like Y-type layers but due to the noncentrosymmetric internal structure, a NLO second-order response grows proportional to the square of the sample thickness. Again, thick layers of the azopolymer prepared by alternate deposition LB-technique were prepared, which can be used as active waveguide for second-order NLO application.

Phase-matching temperature shifts in blue generation by frequency doubling of femtosecond pulses in KNbO_3

We have demonstrated temperature rises that result from nonlinear absorption in single-pass frequency-doubling experiments using femtosecond pulses and a 3-mm-thick KNbO3 crystal. These temperature changes shift the phase-matching curve and must be accounted for to optimize the conversion efficiency. We obtained a maximum second-harmonic generation (SHG) efficiency of 66% at an input power of 107 mW and a slope efficiency of ∼1.5%/mW at low input powers. We have investigated, for the first time to our knowledge, the focusing dependence of the phase-matching temperature. We have also found that the temperature-dependent SHG efficiency in femtosecond SHG experiments is significantly different from that obtained for frequency doubling of continuous-wave light.

Determination of Nonlinear Hyperpolarizability of Bent-Shaped Molecules using Hyper-Rayleigh Scattering

The first-order hyperpolarizability β was determined for bent-shaped molecules P-8-PIMB and P-8-O-PIMB using the Hyper Rayleigh scattering method. βs of these materials were found to be relatively large, about 30×10−30 esu. These results are consistent with the second-harmonic generation experiments performed in their liquid crystalline phases.

Second-harmonic generation in single-crystal thin films of 4-aminobenzophenone

Single-crystal thin films of 4-aminobenzophenone have been prepared by a modification of the shear method, and the significant elements of the second-order optical polarizability tensor have been measured by the transverse second-harmonic generation (SHG) technique. Films with thickness less than the coherence length allowed measurements without significant effects due to phase-mismatch. Detailed SHG experiments with different polarization conditions were performed to determine the d coefficients. The magnitudes of the d coefficients at the fundamental wavelength of 1064 nm were found to be d23=7.3±0.4 pm/V and d22=0.73±0.04 pm/V. Directions for both type-I and type-II phase-matched propagation parallel to the film were identified. The results showed that guided-wave phase-matched SHG (type I and type II) should be possible using the films.

Second-harmonic generation properties of 2-amino-5-nitropyridinium-dihydrogenarsenate and -dihydrogenphosphate organic-inorganic crystals

We present a comparative study of the linear and nonlinear optical properties of two isomorphous organic-inorganic crystals: the 2-amino 5-nitropyridinium-dihydrogenphosphate (2A5NPDP) and -dihydrogenarsenate (2A5NPDAs). We determine accurate equations for the dispersion in wavelength of their principal refractive indices from direct prism measurements and from angular noncritically phase-matched second-harmonic generation (SHG) experiments. We deduce the quadratic nonlinear coefficients d/sub 15/ and d/sub 24/ of each compound from the phase-matched conversion efficiency of SHG experiments and find that they are close to those of KTiOPO/sub 4/.

Calculation of macroscopic first- and third-order optical susceptibilities for the benzene crystal

Starting from a set of high-level ab initio frequency-dependent molecular first- and third-order polarizabilities, the macroscopic first-order (linear) and third-order (cubic) susceptibilities of the benzene crystal are calculated. Environmental effects are taken into account using a rigorous local-field theory and are compared with the anisotropic Lorentz field factor approach. The experimentally determined first-order susceptibility of crystalline benzene is accurately reproduced. Dispersion curves for the first-order susceptibility and results for electric-field-induced second-harmonic generation and third-harmonic generation experiments are predicted. Comparison with similar calculations conducted in the course of molecular simulations of liquid benzene shows that the theoretical results for the two phases are of comparable accuracy. Overall, the results show that for the fairly compact nonpolar benzene molecules, environmental effects on the effective molecular response are small.

Molecular orientation determined by second-harmonic generation: Self-assembled monolayers

Monolayers of 12-(4-nitroanilino)-dodecane thiol $[{\mathrm{O}}_{2}{\mathrm{N}--\mathrm{C}}_{6}{\mathrm{H}}_{4}{\mathrm{\ensuremath{-}}\mathrm{N}\mathrm{H}(\mathrm{C}\mathrm{H}}_{2}{)}_{12}\mathrm{SH}](\mathrm{NAT})$ adsorbed on polycrystalline gold substrates were investigated in a gaseous environment and in ethanol. Contributions of comparable magnitude from the metal substrate, the adsorbate-substrate interaction, and the p-nitro aniline moiety $(p\mathrm{NA})$ require phase-sensitive and polarization-dependent second-harmonic-generation (SHG) measurements. The susceptibility of the $p\mathrm{NA}$ end group is separated by comparative experiments with n-alkane thiols. Evaluation of the orientation of the $p\mathrm{NA}$ moiety requires phase information about the SHG signal. Intensity data alone produce ambiguous results. The tilt angle determined by SHG is shown to be strongly dependent on the model applied. For the NAT film in an inert gas atmosphere, the tilt angle \ensuremath{\vartheta} of the $p\mathrm{NA}$ group calculated from the SHG experiments is compared with the value of 53\ifmmode^\circ\else\textdegree\fi{} obtained by linear techniques. Agreement is only achieved if local-field factors are neglected and if a two-layer model is assumed to describe the linear optical properties of the system. Immersion of the NAT film in ethanol causes a change of the tilt angle of the $p\mathrm{NA}$ end group to a more upright position of about \ensuremath{\vartheta}=36\ifmmode^\circ\else\textdegree\fi{}. A detailed discussion reveals that the accuracy and reliability of SHG data to determine the orientation of molecules depend critically on the precision by which molecular properties are known and on the model describing the linear optical properties.