Optical Second-Harmonic Generation as a Semiconductor Surface and Interface Probe

Abstract

Optical second‐harmonic generation, although a sophisticated technique, is making a significant contribution to the characterization of surfaces and interfaces. Advantages include: all pressure ranges being accessible; insulators being studied without the problem of charging effects; solid–solid and solid–liquid interfaces being characterized by utilizing the large penetration depth of optical radiation. Single wavelength studies yield information on the crystallographic and magnetic structure of surfaces and interfaces, and are particularly sensitive to symmetry changes. Spectroscopic and time‐resolved studies, which have recently become simpler due to the availability of high pulse power, tunable, broad band laser sources, additionally provide electronic structure and dynamics information, although interpretation may not be simple. Recent examples from well‐characterized semiconductor interfaces are discussed, and it is shown that the technique is now well understood at the phenomenological level, although theoretical calculations are proving to be difficult.

New Tools for Surface Second-Harmonic Generation

“…144,149,150 Simultaneous acquisition of both the second-harm onic spectrum and phase has recently been demonstrated by Wilson et al by frequency-domain interferometry with ultra-short (15 fs) broad-band pulses. 151 Experimental studies have utilized SHG spectroscopy and phase m easurem ents w ith g re at success to probe the energetics of semiconductor surfaces [151][152][153][154][155][156][157][158] and to characterize the spectral response of thin fullerene surface lms. 159 -163 In contrast, comparatively little has been done with the use of SHG to characterize electronic spectroscopy in organic thin lms.…”

Time-, Energy-, and Phase-Resolved Second-Harmonic Generation at Semiconductor Interfaces

“…Within the electric dipole approximation, the generation of SH is forbidden in materials with inversion symmetry. 1,2 At a surface or interface, inversion symmetry is broken and this gives rise to an SH polarization originating from the first few layers of the interface. 3 Application of an electric field also breaks inversion symmetry through a process called electric field-induced second-harmonic generation (EFISH), which makes SH generation sensitive to interfacial electric fields.…”

“…17 The polymer was deposited on a glass slide with a thickness of ∼780 nm through spin-coating and poled at the glass transition temperature of 120 °C under an electric field of 100 V/μm. The poled films showed a Pockels coefficient of 35 pm/V at a wavelength of 830 nm, which corresponds to a large χ (2) of 146 pm/V. After poling, the top gold electrode was etched off by the gold etchant, and the obtained films were used for SH measurements.…”

“…Second-harmonic (SH) generation is a second-order nonlinear process which provides a unique spectroscopic tool for investigating surfaces and interfaces. Within the electric dipole approximation, the generation of SH is forbidden in materials with inversion symmetry. , At a surface or interface, inversion symmetry is broken and this gives rise to an SH polarization originating from the first few layers of the interface . Application of an electric field also breaks inversion symmetry through a process called electric field-induced second-harmonic generation (EFISH), which makes SH generation sensitive to interfacial electric fields. − These fields can be established on ultrafast time scales from interfacial charge transfer; thus, time-resolved pump–probe EFISH can be used as a particularly sensitive probe of the charge-transfer process. , However, pump-induced changes in the SH field can arise from multiple contributions, including excitons, free carriers, and space charge fields, − which, in general, can change the amplitude and phase of the emitted SH field.…”

Ab initiodescription of second-harmonic generation from crystal surfaces

“…Adapted from Ref. [74]. (b) A sheet of nonlinear polarization is considered at z = 0 − , surrounded by an isotropic and homogeneous linear medium, characterised by a macroscopic dielectric function (ω).…”