Second-harmonic Light Research Articles

Second-harmonic study of polar symmetry and domain structure in SrTi18O3

The ferroelectric domain structure of SrTi18O3 is studied via the azimuthal dependence of the second-harmonic (SH) light intensity below Tc≈25 K. Tensor analysis of the SH response reveals locally variant mixtures of eight triclinic polar domains, which transform into an orthorhombic single domain under an electric field E applied perpendicularly to the tetragonal axis, with d22 being the dominant coupling constant.

Optical Implications of Crystallite Symmetry and Structure in Potassium Niobate Tellurite Glass Ceramics

We present combined results of optical, scattering, and spectroscopic studies of (K2O)15(Nb2O5)15(TeO2)70 glass and glass ceramic and comment on possible mechanisms of the optical behavior in terms of existing theory. The glass ceramic is confirmed to generate second-harmonic light. Combined neutron and X-ray diffraction data have been indexed to an orthorhombic unit cell (a = 3.39 A, b = 4.78 A, c = 6.38 A). Additionally, we show that the previously proposed fluorite-based model is insufficient for the crystal phase using real-space analysis. The orthorhombic unit cell allows the possibility of a conventional explanation for the second-harmonic generation (SHG). Peak broadening and scattering length difference analyses are combined to formulate a model of anion disorder in the crystal phase. TEM and SAXS reveal domains of ≈15 nm in both the glass and glass ceramic. The role of liquid−liquid phase separation in the crystallization behavior of this material is discussed. Solid-state 93Nb MAS NMR and Raman ...

Simultaneous second-harmonic generation with two orthogonal polarization states in periodically poled KTP

Simultaneous generation of second-harmonic light with orthogonal polarizations in the blue spectral region is demonstrated by use of typeII and typeI quasi-phase matching for the nonlinear coefficients d(33) , d(24) , and d(32) in periodically poled KTP. The second-harmonic power ratio in both polarization states can be tuned by the fundamental polarization and (or) the periodically poled KTP temperature. Femtosecond pulse frequency-doubling efficiencies of as much as 39.5% W (-1) and 8.1% W (-1) are demonstrated in 7-mm periodically poled KTP for typeI and typeII processes, respectively. The efficiency limitations caused by various effects of the group-velocity mismatch are discussed.

The interference effects induced by two-color excitation in the photodissociation of IBr

We have investigated the interference effects in the two-color dissociation of IBr using fundamental (560 nm) and its second-harmonic (280 nm) light. We have performed one-dimensional photofragment translational spectroscopy in the presence of two-color excitation. In the strong-excitation regime for the fundamental light (∼ 1.3×10 10 W cm −2 ), the yield of the spin–orbit excited iodine atoms dissociated from IBr molecules shows an oscillating behavior that is dependent on the relative phase between the fundamental and the second-harmonic light. This phase-dependent behavior can be explained by the interference between a one-photon transition induced by the second-harmonic light and a dipole-forbidden two-photon transition induced by the strong fundamental light.

Second-Harmonic Generation of Blue Light in Epitaxial K3Li2-xNb5+xO15+2x Waveguides on K3Li2-x(Nb0.98Ta0.02)5+xO15+2x Substrates by Metalorganic Chemical Vapor Deposition

We report a new second-harmonic generation blue laser using a K3Li2-xNb5+xO15+2x single crystal film waveguide on a K3Li2-x(Nb0.98Ta0.02)5+xO15+2x crystal substrate by the metalorganic chemical vapor deposition method. Second-harmonic blue light with output power of 2 mW at 487 nm was successfully generated. The phase-matched wavelength bandwidth of the device was 1.3 nm, and the temperature coefficient of the phase-matched wavelength was 0.36 nm/°C. Crystallographic evaluation was also performed for the KLN thin film crystal. The (001) plane of KLN crystal film was well orientated to the (001) plane of the KLNT substrate, and the surface morphology of the film was very smooth. Moreover, it was revealed that an as-deposited KLN film crystal has a single domain structure.

Performance optimization of an external enhancement resonator for optical second-harmonic generation

We study the factors that ultimately limit the performance of an external enhancement resonator for optical second-harmonic generation (SHG). To describe the resonant SHG process we introduce a theoretical model that accounts for the intensity-dependent cavity loss that is due to harmonic generation and that also includes a realistic assumption about the shape and the frequency width of the laser mode. With the help of this model we optimized the performance of a doubling cavity based on a lithium triborate (LBO) crystal. This cavity was used for frequency doubling the output of a single-frequency titanium-doped sapphire laser at 850 nm. We were able to push the total second-harmonic conversion efficiency to 53% (a 1.54-W pump resulted in 820 mW of second-harmonic light), which to our knowledge is the best result ever reported for a LBO-based doubling cavity.

Optical second harmonic generation near a black hole horizon as possible source of experimental information on quantum gravitational effects

Optical second harmonic (SH) generation near a black hole horizon is suggested as a source of information on quantum gravitational effects. Localization phenomena in light scattering from random fluctuations of matter fields and space–time metric near the horizon produce a pronounced peak in the angular distribution of SH light in the direction normal to the horizon. This phenomenon is similar to the strong enhancement of diffuse SH emission from a randomly rough metal surface in the normal direction.

Magnetic-dipole nonlinearities in chiral materials

We propose a new measurement procedure to study the role of magnetic-dipole contributions to the second-order nonlinearity in chiral materials. The procedure is based on the measurement of the ellipticity of the second-harmonic light generated in the chiral material. A better insight in the magnetic-dipole nonlinearities of materials could eventually lead to the observation of new phenomena in the field of photochemistry and photophysics.

Second harmonic generation in Ga nanoparticle monolayers

Ga nanoparticle monolayers formed by evaporation-condensation in ultrahigh vacuum and embedded in a transparent SiOx matrix generate second harmonic (SH) signals in transmission and reflection when illuminated by a 150 fs, 800 nm laser pulses. The observed SH light exhibits a critical dependence on input and output polarizations, angle of incidence and azimuthal orientation of the samples. The results lead to a consistent picture of shape and orientation of the nanoparticles. Linear transmittance spectra in the visible range support these findings and the observed size dependence of the SH signal.

Angle-resolved second-harmonic light scattering from colloidal particles.

We report angle-resolved second-harmonic generation (SHG) measurements from suspensions of centrosymmetric micron-size polystyrene spheres with surface-adsorbed dye (malachite green). The second-harmonic scattering profiles differ qualitatively from linear light scattering profiles of the same particles. We investigated these radiation patterns using several polarization configurations and particle diameters. We introduce a simple Rayleigh-Gans-Debye model to account for the SHG scattering anisotropy. The model compares favorably with our experimental data. Our measurements suggest scattering anisotropy may be used to isolate particle nonlinear optics from other bulk nonlinear optical effects in suspension.

Enhanced Phase-Matched Second-Harmonic Generation in a Ferroelectric Liquid Crystal Waveguide

A binary mixture of crosslinkable ferroelectric liquid crystals (FLCs) was used for the design of a channel waveguide. The liquid crystal was aligned in layers parallel to the glass plates (homeotropic) in a sandwich geometry and then the arrangement was permanently fixed by photopolymerization which yielded a polar network possessing a high thermal and mechanical stability. The linear and nonlinear optical properties have been measured and all four independent components of the nonlinear susceptibility tensor d have been determined. The off-resonant d-coefficients are remarkably high and comparable to those of the best known inorganic materials. Phase-matching was achieved by taking advantage of the modal dispersion of the waveguide. A reversal of sign of χ(2) at the nodal plane of the electric field distribution of the first-order mode was needed to maximize the overlap integral between the fundamental and the second-harmonic (SH) light. In samples with χ(2) inversion the SH signal was 1000 larger that in samples without χ(2) inversion.

Phase-dispersion optical tomography

We report on phase-dispersion optical tomography, a new imaging technique based on phase measurements using low-coherence interferometry. The technique simultaneously probes the target with fundamental and second-harmonic light and interferometrically measures the relative phase shift of the backscattered light fields. This phase change can arise either from reflection at an interface within a sample or from bulk refraction. We show that this highly sensitive (~5 degrees ) phase technique can complement optical coherence tomography, which measures electric field amplitude, by revealing otherwise undetectable dispersive variations in the sample.

Apertureless near-field optical microscopy via local second-harmonic generation.

We describe an apertureless scanning near-field optical microscope (SNOM) based on the local second-harmonic generation enhancement resulting from an electromagnetic interaction between a probe tip and a surface. The imaging mechanisms of such apertureless second-harmonic SNOM are numerically studied. The technique allows one to achieve strongly confined sources of second-harmonic light at the probe tip apex and/or surface area under the tip. First experimental realization of this technique has been carried out using a silver-coated fibre tip as a probe. The experiments reveal a strong influence of the tip-surface interaction as well as polarization of the excitation light on images obtained with apertureless second-harmonic SNOM. The technique can be useful for studying the localized electromagnetic excitations on surfaces as well as for visualization of lateral variations of linear and nonlinear optical properties of surfaces.

Phase effects in magnetic second-harmonic generation on ultrathin Co and Ni films on Cu(001)

Phase relations in second-harmonic generation (SHG) have been studied systematically for Ni and Co films on Cu(001) as a function of thickness in the range 1--12 ML. Below two monolayers we observe, with s-polarized fundamental light and P-polarized second-harmonic light, phase shifts close to 180\ifmmode^\circ\else\textdegree\fi{} of both films against the Cu substrate. Such large phase shifts in this polarization combination appear to be characteristic of the transition from s- to d-band metals. In the ferromagnetic thickness range the relative phase between odd and even tensor elements becomes important. For Ni films it is independent of film thickness for both $\stackrel{\ensuremath{\rightarrow}}{p}P$ and $\stackrel{\ensuremath{\rightarrow}}{s}P$ polarizations, and leads to the ratio ${\ensuremath{\chi}}_{\mathrm{xyy}}^{\mathrm{odd}}/{\ensuremath{\chi}}_{\mathrm{zyy}}^{\mathrm{even}}=0.047\ifmmode\pm\else\textpm\fi{}0.013,$ in agreement with theoretical predictions. In the case of Co films the relative phase differs dramatically between these two polarization combinations. It is nearly independent of film thickness for $\stackrel{\ensuremath{\rightarrow}}{p}P$ polarized SHG. However, for $\stackrel{\ensuremath{\rightarrow}}{s}P$ polarization it varies from 135\ifmmode^\circ\else\textdegree\fi{} at 3 ML to 60\ifmmode^\circ\else\textdegree\fi{} at 10 ML, causing a sign change of the magnetic contrast at 90\ifmmode^\circ\else\textdegree\fi{} near 6 ML. The ratio of odd to even second-harmonic field amplitudes was found to vary from 22% at 3 ML to 12.5% above 6 ML. From these results we conclude that measurements of the relative phase are necessary for deriving reliable information from SHG about the magnetization state of a sample.

Photoinduced phase-matched second-harmonic generation in azodye-doped polymer films

Phase-matched second-harmonic generation (SHG) in azodye-doped polymethylmethacrylate films was demonstrated using the nonresonant optical poling technique. During the seeding process, samples were irradiated simultaneously by coherent superposition of the fundamental and the second-harmonic light of a femtosecond laser. Optical poling experiments were performed at three wavelengths of 1500, 1320, and 1300 nm, respectively. The quadratic dependence of SHG on the film thickness showed that a χ(2) grating that satisfied the phase-matching condition for SHG was optically induced in the polymer films. The SHG conversion efficiency of a 105 μm thick film was estimated to be about 2%. A relaxation retardation effect of the photoinduced χ(2) was also observed in the thick films.

Quasi-phase matched second-harmonic generation in an AlxGa1−xAs asymmetric quantum-well waveguide using ion-implantation-enhanced intermixing

Quasi-phase matched second-harmonic generation in the copropagating geometry is demonstrated in an asymmetric quantum-well waveguide. Modulation of the nonlinear susceptibility along the waveguide was achieved using a patterned quantum-well intermixing process. Photoluminescence measurements of the quantum-well bandedges indicate that a grating of alternating regions of intermixed and as-grown asymmetric quantum wells was produced for periods between 2 and 12 μm. The variation of the second-harmonic light intensity generated by guided incident light between λ=1480 and 1600 nm was measured. The resulting second-harmonic spectra show sharp quasi-phase matching resonances for grating periods near 3 μm, demonstrating that a periodic modulation of the quantum-well nonlinear susceptibility was produced.

Interferometric phase-dispersion microscopy

We describe a new scanning microscopy technique, phase-dispersion microscopy (PDM). The technique is based on measuring the phase difference between the fundamental and the second-harmonic light in a novel interferometer. PDM is highly sensitive to subtle refractive-index differences that are due to dispersion (differential optical path sensitivity, 5 nm). We apply PDM to measure minute amounts of DNA in solution and to study biological tissue sections. We demonstrate that PDM performs better than conventional phase-contrast microscopy in imaging dispersive and weakly scattering samples.

Second-order optical nonlinearity generated by doping the surface layer of silica with anions or cations

A method for generating second-harmonic light from silica glass by doping its surface layer with ions is presented. When anions CO32−, OH− or Cl− are introduced from the cathodic surface during poling, the second-harmonic signal generated near that surface is enhanced by two orders of magnitude with respect to the case of poling without doping. The penetration depths of CO32−, OH− and Cl− have been measured by the etching technique. In contrast, by doping the anodic surface with cations Li+, Na+ or K+, the second-harmonic signal drops to one tenth of that corresponding to the case of poling without doping. This indicates that in the anodic surface the main charges are negative, and the positive charges neutralize them to decrease the strength of the electric field that is at the origin of the second-order susceptibility. Secondary ion mass spectroscopy detection shows the concentration distribution of the injected lithium in the surface layer. The second-harmonic signal from the anodic surface decreases monotonically with increasing poling-doping time up to 300 min. Doping from interfaces far from the electrodes is inefficient for influencing the second-harmonic generation.

Properties of the peaks of second harmonic light through Fibonacci-class ferroelectric domains

After establishing the method of constructing a class of one-dimensional (1D) Fibonacci-class quasiperiodic (FC(n)) ferroelectric domains system, we have studied the properties of the electric field of the second harmonic generation (SHG) by means of the small-signal approximation in the case of the vertically transmission. It was found that only the second harmonic light (SHL) peaks which were indexed by two special integers q and p would be the brightest and the spectra whose positions were decided by successive FC(n) integers \(\) and \(\) were perfect self-similar without considering the dispersive effect of the refractive index on SHL. The effect of the vacancies for some special spectral lines was also studied generally. The analytic results were confirmed by the numerical simulations.

A novel method to characterize photorefractive damage in quasiphase-matched wavelength converters

A novel two-beam method is proposed and applied for the first time, to characterize photorefractive damage (PRD) in a LiNbO3 quasiphase-matched (QPM) wavelength converter. In the proposed method, irradiation light from a Ti sapphire laser and a broadband probe beam from an erbium-doped fiber amplifier are coupled into a LiNbO3 QPM waveguide. The PRD effect caused by the irradiation is studied by monitoring the generated second-harmonic light spectrum of the probe light. It is shown that PRD in the LiNbO3 QPM waveguide can be qualitatively characterized by the proposed method, and relevant information relating to the QPM wavelength conversion can be extracted directly.