Second-harmonic Reflection Research Articles

Second-Harmonic Reflection Imaging of Normal and Accelerated Corneal Crosslinking Using Porcine Corneas and the Role of Intraocular Pressure

The aim of this study was to investigate morphological changes, using second-harmonic (SH) optical imaging, in the corneal stroma after normal- and high-intensity collagen crosslinking in postmortem enucleated porcine eyes at controlled intraocular pressures (IOPs). Reflection-mode SH optical imaging of the stroma was realized after standard collagen cross-linking (CXL) and accelerated crosslinking (AXL) of porcine corneas, and the results were compared with the results for untreated controls. Ultraviolet-X lamps (365 nm) were used during riboflavin crosslinking with intensities of 3, 10, 30, and 100 mW/cm2 for constant 5.4-J/cm2 exposure doses. The IOP was varied using saline solution injected through the optic nerve and monitored using a Schiotz tonometer. SH optical imaging was realized with an in-house build multiphoton microscope using an ultrafast dispersion-compensated Ti-sapphire laser. SH reflection images of CXL and AXL porcine corneas obtained at IOPs of 8, 11, 16, and 26 mm Hg showed a similar fibrillar structure of collagen lamellae. All crosslinked corneas showed an increased fibrillar contrast in comparison with untreated baseline images. At low IOPs, strong variations in the scattering were observed that reduced with an increase in the IOP, when fibrils tended to straighten out. At low and normal IOPs, no significant difference between CXL and AXL results could be observed. At very elevated IOPs, however, the impact of AXL was found to alter the fibrillar structure of the collagen becoming less apparent in SH images when compared with that of CXL. We found a strong influence of the IOP on SH reflection imaging of postmortem porcine corneal stroma. CXL and AXL led to similar SH images indicative of a similar tensile strength. Only at very elevated IOPs (26 mm Hg) did the results for AXL deviate from those of CXL, suggesting an IOP-related threshold for reliable applications of AXL.

Second-harmonic generation measured on a GaAs photonic crystal planar waveguide

Second-harmonic (SH) reflection and diffraction measurements are performed, at the wavelengths of a Ti:sapphire laser on a GaAs/AlGaAs photonic crystal waveguide patterned with a square lattice, the basis consisting of rings of air in the dielectric matrix. The measured angles of diffracted SH beams agree with those predicted from nonlinear diffraction conditions. Results for reflected and diffracted SH intensities as a function of incidence angle, polarization, and pump wavelength show that, due to the low air fraction of the photonic crystal, the reflected one is dominated by the crystalline symmetry of GaAs, while the diffracted one is related to the photonic crystal structure. The large diffraction-to-reflection ratio points to the importance of nonlinear diffraction in photonic crystals. Preliminary measurements in the 1500 nm range reveal explicit features related to photonic modes.

Non-linear optics and magneto-optics on nano-structured interfaces

The behaviour of the reflectivity and Kerr magneto-optic effects in the non-linear second-harmonic (SH) field diffracted from nano-scale structured ferromagnetic interfaces is reported. Measurements are made in both the linear and non-linear fields at two different fundamental wavelengths (1064 nm and 800 nm) and their associated harmonics. Resonant behaviour observed as a function of angle of incidence is identified with surface-plasmon production that is known to intensify the local field within the interface. Radiation incident at angles of incidence that optimise coupling to the electron plasma produces an increase in the SH field radiated in the vicinity of those angles. Similarly, at those angles of incidence where radiation at the SH wavelengths (532 nm and 400 nm) couples optimally to the electron plasma, troughs are seen in the angular spectrum of the generated SH radiation. Kerr magneto-optic measurements taken in both the linear field and the SH field both show very significant enhancement at angles meeting the plasma-resonance condition. The totality of experimental data presented allows the conclusion that intensification of the interface electric field due to plasmon creation enhances not only the SH reflection coefficient, as was already known, but also the magneto-optic reflection coefficients in both linear and SH fields.

Hyper-Rayleigh scattering in Gd-containing Langmuir–Blodgett superstructures

Incoherent optical second harmonic (SH) generation, or hyper-Rayleigh scattering (HRS), is used to study the structure and properties of Gd/C18H36O2 multilayer Langmuir–Blodgett (LB) films. SH reflection spectra (in combination with linear absorption spectra), HRS indicatrices, thickness dependence, and wave polarization diagrams are measured for s-polarized fundamental radiation. The SH radiation is entirely diffuse but predominantly s polarized, with the maximum of the HRS indicatrix shifted by 8° relative to the specular direction, which is interpreted as being due to the contribution of a fluctuating quadrupole to the nonlinear-optical response of the films. The estimated correlation length of spatial fluctuations of the nonlinear-optical polarization in the film is ∼400 nm. These properties are associated with an islandlike structure formed by Gd ions inside LB structural units.

Symmetry and phase determination of second-harmonic reflection from calcite surfaces

We perform second harmonic reflection (SHR) on the calcite surface perpendicular to the c axis. First, we record the signal as a function of the azimuthal angle of the sample, which clearly evidences the ${C}_{3v}$ symmetry of this surface, and we show that the nonvanishing background of the p-polarized SHR is an indication of a complex-valued nonlinear susceptibility. Then, we measure the relative phases of the susceptibility tensor components with a method that was used recently to study thin films. We vary the state of polarization of the fundamental beam with a rotating quarter waveplate which introduces some phase difference between the different polarization components. This method appears to be an easy and efficient way to measure the phase of SHR on a crystalline surface. Furthermore, comparison of both sets of experiments shows very good quantitative agreement, even though the origin of this phase difference is not perfectly clear.

Surface-enhanced second-harmonic diffraction: Experimental investigation of selective enhancement

Recent theoretical predictions demonstrating large enhancements of second-harmonic diffraction through the control of surface harmonic composition when the surface-plasmon polariton (SPP) is excited on a corrugated silver surface have been investigated experimentally for the specular order. Interpretation of the mechanism of enhancement in terms of a three-step selective scattering process involving only two spatial harmonics has been found to be justified. Fourier blaze holography, which permits the phase- and amplitude-controlled superposition of multiple harmonics in photoresist, has been used to fabricate the nanostructured, biperiodic gratings. Surfaces have been characterized by atomic force microscopy and Heitmann's method. As predicted, surfaces with optimized spatial frequency composition equal or exceed randomly rough surfaces in the enhancement of second-harmonic reflection relative to a flat silver surface. Enhancement produced by simultaneous excitation of counterpropagating SPP modes at the pump and second-harmonic frequencies has also been detected by varying grating wave vector to tune through a double resonance condition. Factors affecting the quantitative agreement between enhancement measurements and theoretical predictions are briefly discussed. \textcopyright{} 1996 The American Physical Society.

Probing Inhomogeneous Lattice Deformation at Interface of Si(111)/SiO2 by Optical Second-Harmonic Reflection and Raman Spectroscopy

Optical second-harmonic generation (SHG) and Raman spectroscopy have been applied to investigate surface strain/stress appearing at the interface between Si(111) and thermally grown SiO2 layers. From the frequency shift and spectral broadening of the optical phonon mode of Si(111) covered by a 608-Å-thick oxide layer, a tensile stress of 19 kbar was obtained. The azimuthal distribution of the reflected second-harmonic (SH) signal varies with the thickness of surface oxide. To deduce the strain in the lattice-deformed layer, a simple microscopic theory based upon the bond additivity model was proposed, and an agreement between the results of SHG and Raman spectroscopy was achieved. This study suggests that SHG is a sensitive technique for examining surface stress/strain between two lattice-mismatched layers. Therefore it can be useful for the study of the structure of Si1-x Gex and many other strained-layer systems.

Optical second-harmonic reflection from polycrystalline silicon and its relation to grain boundaries

Grain boundary contributions to optical second-harmonic generation give rise to an unusually high reflected second-harmonic intensity from polycrystalline silicon. At the grain boundaries, the dangling bonds affect the second order nonlinear susceptibility via the electric dipole contribution. The second-harmonic signal dependence on dangling bonds passivation by hydrogen is experimentally observed to support the grain boundary model.

Mechanisms determining third order intermodulation distortion in AlGaAs/GaAs heterojunction bipolar transistors

The third-order intermodulation distortion (IMD3) mechanisms of heterojunction bipolar transistors (HBTs) are analyzed using Volterra series theory. A T-equivalent circuit is used for the large-signal model of the HBT. The third-order nonlinear currents generated by the device nonlinearities are evaluated for this purpose and current cancellation is discussed. It is found that, even though the C/sub je/ and g/sub je/ related currents do not show pronounced cancellation, the total base-emitter current and the total base-collector current cancel partially. Second harmonic loading is addressed in view of IMD3 optimization while at the same time maintaining high gain through conjugate matching at the fundamental frequency. IMD3 is very sensitive to the nonlinear currents generated by g/sub je/ and alpha . Optimum IMD3 occurs at high second-harmonic reflection coefficients corresponding to open load conditions. Minimum and maximum IMD3 occurs for second-harmonic load reflection coefficient phases close to analogous extremes of the dominant nonlinear current of the device.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Second-Harmonic Reflection from Silicon Surfaces and Its Relation to Structural Symmetry

Second-harmonic reflection from Si(100) and Si(111) surfaces exhibits a strong dependence on the angle of rotation of the sample about its surface normal. This behavior can be related directly to the structural symmetry of the crystal and of the surface. Analysis of the results shows that the surface and bulk contributions to the observed second-harmonic signals from Si are generally of the same order of magnitude.

Experimental Verification of the Laws for the Reflected Intensity of Second-Harmonic Light

The intensity of the second-harmonic light generated in reflection from piezoelectric crystals of GaAs has been measured as a function of the angle of incidence of the primary ruby- or neodymium-glass-laser beam. The second-harmonic intensity has been measured for various crystallographic orientations of the GaAs mirror surface and for different directions of polarization. The results verify the theoretical predictions of Bloembergen and Pershan. In particular, the existence of a "Brewster angle" for second-harmonic reflection, with the harmonic electric field in the plane of reflection, has been observed.