Probing interfacial properties by optical second-harmonic generation

Optical second-harmonic generation from surfaces has recently proven to be a very effective and versatile probe for surface studies. It allows, among other things, direct probing of molecular adsorption on a solid substrate from a liquid. In this paper we review the most important features of this new technique. We discuss the possibility of applying it to the study of adsorption of ionic surfactants at solid–liquid interfaces, commonly used in a great number of scientific and technological fields including industrial applications. Moreover, we describe the preliminary results of an experiment currently in progress.     

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 =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 which, in turn, is affected by the electron temperature. There is no evidence for a temperature dependence of the nonlinear susceptibility ⁽²⁾.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Broadband second-harmonic generation in a double-tapered gallium arsenide slab using total internal reflection quasiphase matching

This article analytically describes broadband second-harmonic generation in a double-tapered gallium arsenide (GaAs) slab using total internal reflection quasi-phase matching technique. This double-tapered configuration ensures a combination of increasing and then decreasing bounce lengths which provides an extremely wide 3 dB bandwidth of 573.6 nm with a conversion efficiency of 1.929%, after considering reflection and absorption losses. Effect of varying the slab dimensions, viz., length and tapering angles, as well as the operating temperature on the performance parameters has also been incorporated in the analysis.     

Total internal reflection apodizers

A novel type of apodizer using total internal reflection is suggested for use in high-power laser systems.     

Transmission reflection anomaly in second-harmonic generation from a monolayer

We show experimentally that in second-harmonic generation from a monolayer the radiation propagating in transmission and that in reflection can have very different magnitudes. The origin of this difference lies in destructive and constructive interference of the components of the nonlinear polarization that drive the field at the second-harmonic frequency.     

The temperature dependence of surface second-harmonic generation from the air-water interface

An investigation of the polarization and temperature dependence of surface second-harmonic generation (SHG) at the air-water interface shows that the intensity of the SHG signal decreases over the range 20–70°C. Accounting for the changes in the SHG polarization behaviour with temperature to extract the relative temperature dependence of X_zzz shows that it is similar to that obtained from simulation studies. The role of the interfacial refractive index in the interpretation of the SHG signals is addressed.     

Probing nonequilibrium electron distributions in gold by use of second-harmonic generation

Second-harmonic radiation is generated at a gold surface by use of a laser pulse that is varied in duration from 14 to 29 fs and in intensity from 10(9) to 10(11)W/cm(2) . At laser intensities below 10(10)W/cm(2) , the second-harmonic signal has the expected quadratic dependence on pump-laser intensity; however, at higher intensities, the dependence is supraquadratic. This difference arises because the leading edge of the laser pulse interacts significantly with the gold electrons to create a nonequilibrium, photoexcited distribution. The second-harmonic generation process occurs before electron-electron or electron-phonon collisions can equilibrate the distribution and therefore serves as a probe of the nonequilibrium distribution.     

Pulsed-laser annealing of GaAs surface studied by time-resolved second-harmonic generation in reflection

Experimental results are presented on the time dependence of the second-harmonic generation (SHG) of a mode-locked Q-switched Nd: YAG laser in reflection from both amorphous and crystalline GaAs surfaces during the pulsed-laser annealing under ruby laser irradiation. A dramatic increase of SHG by more than two orders of magnitude due to recrystallization of the noncentrosymmetric GaAs lattice in the process of pulsed-laser annealing is observed and the duration of surface recrystallization is measured to be not greater than 30–40 ns. The results can be fully understood in terms of the melting model of laser annealing.     

Total internal reflection in gain medium slab

The problem of light propagation through a slab of gain medium is studied in terms of the transfer matrix pole distribution. Identifying the lasing by the existence of the poles in the upper half-space of the complex frequencies, we demonstrate that under the condition of total internal reflection lasing may be observed at a finite thickness of the slab; further increase in the thickness results in the quenching of lasing. However, in the latter case the amplification of reflected wave is possible.     

透射衍射光栅内全反射级次

当前高功率激光装置系统中, 国内使用透射式衍射光栅进行光束采样, 国外甚至使用光栅聚焦并同时结合谐波分离和采样功能. 高功率激光辐照下, 除了0级透射打靶光以及目标取样光外, 其他级次衍射光的影响也不可忽视, 可能造成诸如鬼像聚焦破坏、时间脉宽测量不稳定、近远场测量噪声等问题. 尤其是透射衍射光栅内部全反射级次会导致光栅出射一系列规律的衍射花样, 造成上述干扰和危害. 首先理论上针对透射式光栅(包括取样光栅和聚焦光栅)进行了相应的计算和分析, 然后从实验上观察并测量了取样光栅内部全反射级次的现象, 并且结合镀减反膜前后的衍射现象进行了讨论, 最后分析并给出了如何消除或规避全反射级次和其他冗余级次影响的方法.     

Total anti-specular reflection at an undulating dielectric interface

We have studied the diffraction of electromagnetic waves at a corrugated interface between two dielectrics by rigorously solving the integral equation for scattered waves. We show that total anti-specular reflection is possible when the radiation is incident from the denser dielectric, and we find the parameters for a corrugated surface needed to observe this phenomenon.Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 3, pp. 351–362, March, 1994.     

Broadening of the phase-matching bandwidth in quasi-phase-matched second-harmonic generation using GaN-based Bragg reflection waveguide

We present an analysis of a high index core symmetric Bragg reflection waveguide (BRW) design based on a GaN/AlxGa1-xN system for efficient quasi-phase-matched second-harmonic generation for broadband applications. By choosing the fundamental frequency to be a BRW mode and suitably tailoring the overall dispersion characteristics, the strong dispersion of the second-harmonic mode is partially canceled, leading to phase matching between the fundamental and second-harmonic over a broad range of wavelengths. The crucial interplay between the dispersive behavior of the fundamental and second-harmonic wave manifests as a broad acceptance bandwidth of approximately 33 nm accompanied with appreciable conversion efficiency (22.8%/W) for a 10 mm long waveguide. The impact of tailoring the dispersion characteristics on the conversion efficiency is also discussed.     

Total internal reflection of a Gaussian light beam

An approximate theoretical model for calculating the reflected and refracted fields of a Gaussian light beam at a plane interface between two isotropic media is formulated on the basis of a Fourier integral. In the vicinity of the critical angle of incidence (for total internal reflection) the model predicts the presence of two refracted beams, one displaced along the interface by an amount equal to the Goos-Hänchen shift; curvature of the phase fronts and nonalignment of the effective directions of energy and phase propagation occur for each beam, as in an anisotropic medium.     

Total internal reflection diffraction grating in conical mounting

The main conditions and parameters for obtaining surface relief total internal reflection diffraction gratings in conical mounting are presented. Calculated and experimental investigations reveal that there are ranges of grating periods, incidence angles, diffraction angles and gratings depths for which such gratings could be obtained, both for TE and TM polarizations. With optimized grating parameters the diffraction efficiency of the total internal reflection diffraction gratings can be greater than 90%.     

Non-phase-matched enhancement of second-harmonic generation in multilayer nonlinear structures with internal reflections

Traditional notions of second-harmonic generation rely on phase matching or quasi phase matching to achieve good conversion efficiencies. We present an entirely new concept for efficient second-harmonic generation that is based on the interference of counterpropagating waves in multilayer structures. Conversion efficiencies are an order of magnitude larger than with phase-matched second-harmonic generation in similar multilayer structures.     

Probing photo-induced alignment in poly(vinyl cinnamate) films by surface second-harmonic generation

Received: 20 September 1998 / Revised version: 18 December 1998     

Photonic-crystal waveguide for the second-harmonic generation

The possibility of efficient second-harmonic generation in the optical range in a planar dielectric waveguide with the active region in the form of a one-dimensional photonic crystal has been theoretically shown. The true phase matching can be achieved by controlling wave dispersion in the photonic crystal. The dispersion equations for the photonic crystal and three-layer waveguide have been self-consistently solved. It is shown that the coherence length may exceed 10 mm.