Second-harmonic generation at simple metal surfaces

Anisotropic second-harmonic generation from monocrystalline gold flakes

Noble metals with well-defined crystallographic orientation constitute an appealing class of materials for controlling light-matter interactions on the nanoscale. Nonlinear optical processes, being particularly sensitive to anisotropy, are a natural and versatile probe of crystallinity in nano-optical devices. Here we study the nonlinear optical response of monocrystalline gold flakes, revealing a polarization dependence in second-harmonic generation from the {111} surface that is markedly absent in polycrystalline films. Our findings confirm that second-harmonic microscopy is a robust and non-destructive method for probing the crystallographic orientation of gold, and can serve as a guideline for enhancing nonlinear response in plasmonic systems.

Giant magneto-refractive effect in mid-infrared second-harmonic generation from plasmonic antennas

Active modulation of nonlinear-optical response from metallic nanostructures can be realized with an external magnetic field. We report a resonant 20% magneto-refractive modulation in second-harmonic generation (SHG) from spintronic multilayer antennas in the mid-infrared. We discuss mechanisms of this modulation and show that it cannot be explained by an unequal enhancement of the electromagnetic field. Instead, we propose a novel, to the best of our knowledge, contribution to the nonlinear susceptibility, which relies on the spin-dependent electron mean free path. In contrast to magneto-optics in ferromagnets, our approach allows simultaneous observation of the enhanced SHG and its large modulation.

Mid-infrared second-order susceptibility of α-quartz and its application to visible-infrared surface sum-frequency spectroscopy

We provide the first account of the second-order susceptibility of quartz down to 10 mum (1000 cm(-1)) and show how this data may be used along with the sum-frequency response of an amorphous gold surface to elucidate the nonlinear susceptibility of any material in the mid-infrared region. Crystalline quartz is an established material for use in second-harmonic and sum-frequency generation studies of new systems, on account of its well-characterized linear and nonlinear optical properties. Previous knowledge of its nonlinear susceptibility has been limited to its transparent region, wavelengths shorter than about 3 mum. Longer wavelength chi((2)) values for quartz are particularly important for techniques such as vibrational sum-frequency spectroscopy which are expanding into the mid-IR with the increasing availability of widely tunable infrared laser sources.

Simulations of photon-assisted tunneling using the Fokker-Planck equation to model the scattering of electrons within the emitting metal tip

A method to simulate photon-assisted tunneling is developed, and applied to model laser-assisted field emission from metals. Our simulations show that most of the exchange of quanta between the electrons and the radiation occurs within the emitting metal tip. In typical experiments (lambda = 670 nm with tungsten metal) the depth of penetration for the radiation is four times the mean free path for electrons at the Fermi level, so it is necessary to allow for scattering. We use a Floquet expansion with the time-dependent Schrödinger equation to allow for the exchange of quanta between the electrons and the radiation field. Multiparticle effects are modeled with the density functional theory within the local density approximation for the Kohn-Sham exchange and correlation, and the Fokker-Planck formulation is used to determine the effects of scattering on the energy distribution of the electrons.

Optimization of second-harmonic generation microscopy

We describe the principles and characteristics of second-harmonic generation imaging (SHGI) and explore various methods for optimization of the technique. Second-harmonic imaging is optimized for ultrashort laser pulses, high numerical aperture microscope objectives, a highly sensitive non-descanned large area detector, pseudo-phase-matching, and specimens with large second-order non linearity or which exhibit surface plasmon enhanced phenomena. We also compare and contrast the techniques of SHGI and two-photon excited fluorescence imaging.

Surface properties in an external electric field

This paper reviews recent calculations on the effect of an external electric field on surface electronic properties, in particular using the embedding method for solving the Schrödinger equation at the surface. The shape of the screening charge and its field dependence are discussed, and the results are compared with experiments in which the image plane is determined. The force on the surface atoms in the field is given in terms of an effective charge, which also determines the work-function variation with surface displacements. This relationship can lead to a surface instability if the effective charge is big enough.