Harmonic Generation In Reflection Research Articles

Nonlinear propagation effects in high harmonic generation in reflection and transmission from gallium arsenide.

High harmonic spectroscopy in solids is emerging as a new tool to investigate ultrafast electron dynamics in the presence of strong optical fields. However, the observed high harmonic spectra do not usually reflect the microscopic origin of high harmonic generation (HHG) because of nonlinear and/or linear propagation effects. Here, we systematically investigate the HHG in reflection and transmission from gallium arsenide exposed to intense mid-infrared optical pulses. In transmission geometry, we find that the properties of high harmonics are drastically changed by nonlinear effects during the propagation of even tens of micrometers. Especially, the nonlinear absorption and/or nonlinearly induced ellipticity of the drive pulses as well as a cascade nonlinear mixing significantly alter the high harmonic signals in the case of the transmission geometry, making an extraction of the microscopic electron dynamics of gallium arsenide difficult. On the contrary, in reflection geometry, we obtain HHG spectra that are free from propagation effects, opening a general approach for high harmonic spectroscopy.

Layer- and frequency-dependent second harmonic generation in reflection from GaSe atomic crystals

We report optical second-harmonic generation (SHG) in reflection from GaSe crystals of 1 to more than 100 layers using a fundamental picosecond pulsed pump at 1.58 eV and a supercontinuum white light pulsed laser with energies ranging from 0.85 to 1.4 eV. The measured reflected SHG signal is maximal in samples of $\sim$20 layers, decreasing in thicker samples as a result of interference. The thickness- and frequency-dependence of the SHG response of samples thicker than $\sim$7 layers can be reproduced by a second-order optical susceptibility that is the same as in bulk samples. For samples $\lesssim$7 layers, the second-order optical susceptibility is reduced compared to that in thicker samples, which is attributed to the expected bandgap increase in mono- and few-layer GaSe.

Light scattering from randomly rough surfaces.

A survey is given of a variety of effects that can arise in the scattering of electromagnetic waves from one- and two-dimensional randomly rough surfaces. The focus is primarily on multiple-scattering effects such as enhanced backscattering, enhanced transmission, satellite peaks, new features in speckle correlations and in second harmonic generation in reflection. Theoretical treatments of these phenomena are outlined, and experimental results illustrating them are presented.

Theory of multiple harmonic generation in reflection from a metal surface

Theory of multiple harmonic generation in reflection from a metal surface

Harmonic generation in thin films and multilayers

A general method for computing harmonic generation in reflection and transmission from planar nonmagnetic multilayer structures is described. The method assumes plane waves and treats harmonic generation in the parametric approximation. The method is applied in studying the second- and third-harmonic generation properties of thin crystal silicon layers surrounded by thermal oxide. Most independent components of the nonlinear susceptibility tensor have unique signatures with silicon layer thickness $d$, allowing their strength to be determined in principle by measuring harmonic generation as a function of $d$. Surface and bulk contributions to third-harmonic generation are cleanly distinguished, with the bulk signal dominating. Four of six nonvanishing components of ${\ensuremath{\chi}}^{(2)}$ are independent. An approximate value for the bulk susceptibility component ${\ensuremath{\delta}}^{\ensuremath{'}}$, which is accessible only in multibeam experiments and has not previously been measured, is obtained.

Theory of multiple harmonic generation in reflection from a metal surface

A microscopic theory is presented for multiple harmonic generation (MHG) from a metal surface in the jellium approximation and for laser intensities below the critical value for plasma formation. Calculations are presented for the second and up to the sixth harmonic, in the case of a gold surface and for laser wavelengths 800 and 1064 nm, with femtosecond and picosecond laser pulses. The peaks of the harmonic pulses are delayed relative to the peak of the incident laser pulse by about 20 to 30 fs due to the finite electron-relaxation times. The calculated relative peak intensities of the various harmonics are most sensitive to the values of the relaxation times, and MHG provides a way of determining these times.

New approach to the theory of second harmonic generation in reflection and transmission of light

A new approach to the calculation of the nonlinear optical response of a crystalline slab is presented. Analytical expressions for the reflected and transmitted second harmonic wave are derived, in a form allowed by symmetry of a medium including time-reversal symmetry. The main advantage of the approach is that it takes into account both the non-local behavior of the optical susceptibilities and inhomogeneity of a medium. The theory is especially suited to the analysis of the experiments in which the symmetry of the medium should be taken into account exactly, e.g., in the case of magnetic crystals. It is shown that a consistent derivation of the optical response of the surface with a regular structure requires of using the space group of the crystal but not the point group.

Temporal and spatial investigation of 6 H-SiC by picosecond pulse excitation

Sensitive linear and nonlinear optical methods of picosecond transient reflectivity, fluorescence, and second harmonic generation in reflection are applied for spatial and temporal characterization of 6 H-SiC. Two different substrate crystals grown by the Lely method and a 5 μm thick layer grown by liquid phase epitaxy are tested. High spatial resolution time-resolved measurements reveal a significant variation of the effective carrier lifetime on a nanosecond time scale, related to the random distribution of crystal imperfections. Second harmonic generation (SHG) in reflection measurements reveals pronounced structural and crystalline quality differences of the tested samples. The roles of non-equilibrium carrier radiative, nonradiative, and enhanced surface recombination processes are discussed on the basis of the experimental data.

Wavelength dependent study of laser induced metallic properties of Si(111) by surface second harmonic generation

A wavelength dependent study of optical second harmonic generation in reflection from Si(111) is presented. Broadly tunable high power laser pulses with photon energies exceeding the indirect band gap of Si were used to probe the dispersion characteristics of the nonlinear response in the vicinity of bulk absorption. Changes in the second harmonic anisotropy have been attributed to the generation of an isotropic hot carrier plasma in the silicon surface. A phenomenological explanation of the observed changes is given based on a jellium model that is used to describe second harmonic generation from a free electron metal surface.

Femtosecond dynamics of laser-induced phase transition of the GaAs surface layer to a centrosymmetric phase

Recently we observed ultrafast laser-induced phase transition (on a time scale less than 100 fs) to a centrosymmetric semiconductor-like phase at the GaAs surface [1,2] by using time-resolved second harmonic generation in reflection. A phenomenological model describing this phase transition is developed. The new phase exists during the first 1 ps after laser excitation due to high plasma carrier density and lattice stress, the lattice temperature remaining well below the melting threshold.

Optical second harmonic generation in reflection from silicon carbide films

AbstractSecond harmonic generation in reflection has been used to investigate the crystal properties of silicon carbide surfaces. By using a passively mode‐locked pulsed Nd3+: YAG laser the variation of the crystalline, polycrystalline and amorphous phase of silicon carbide at the surface has been investigated. The method is shown to be a sensitive and practical method to investigate the departure from perfect crystalline order at the surface.

Effects of surface magnetism on optical second harmonic generation.

We report on the first experiments showing the influence of surface magnetization on optical second harmonic generation in reflection at a Fe(110) surface. The magneto-optical Kerr effect modifies the hyperpolarizability of the surface in the optical field, leading to a dependence of the second harmonic yield on the direction of magnetization relative to the light fields. For the clean surface an effect of 25% was determined, which decays exponentially with surface contamination by the residual gas, thus demonstrating the high surface sensitivity of this technique.

Laser nonlinear-optical probing of silicon/SiO2 interfaces: Surface stress formation and relaxation

A nonlinear optical technique based on optical second harmonic generation in reflection is shown to provide information on the surface layer structure of semiconductor crystals, thin films and layered systems. The second harmonic intensity is sensitive to inhomogeneous stress in centrosymmetric materials via spatial selection rules and the appearance of an electric dipole contribution to the second order nonlinear optical susceptibility. The technique is used to monitor mechanical stress relaxation in the SiO2/Si interface during several annealing procedures.

Polarized adsorption of H2O on NaCl(100) in air, observed by second harmonic generation

We have observed a pronounced m-symmetry in the azimuthal dependence of optical second harmonic generation in reflection from NaCl(100) in air at room temperature, in spite of the fact that the C4v symmetry of this surface should not exhibit any anisotropy. The observed anisotropy builds up and then disappears again with time of exposure to ambient air. The high solubility of NaCl in water, together with the high polarity of the water molecule, suggests that the dipolar surface hyperpolarization, responsible for second harmonic generation, is caused by adsorbed H2O at a certain range of coverage. The m-symmetry then reflects a macroscopic polarization due to a partially parallel orientation of the adsorbed water molecules. In accord with tentative results of molecular dynamics calculations, this indicates polymerization of water on NaCl(100) at room temperature.

Second harmonic generation in reflection from crystalline GaAs under intense picosecond laser irradiation

The emission of second harmonic radiation in reflection from crystalline GaAs irradiated with 20-ps, 530-nm laser pulses has been measured for incident laser fluences far exceeding the threshold fluence Fth for permanent reflectivity changes. The results are consistent with the occurrence of surface melting during the laser pulse. Detailed analysis of the second harmonic signals reveals an upper limit of 2 ps for the structural transition associated with the melting of the surface.

Study of Optical Third Harmonic Generation in Reflection,

This paper reports studies of optical third-harmonic generation reflected from the boundary of solids including metals, semiconductors, alkali halides, and glass. The harmonic wave generated in reflection was found to interfere with those generated in air and in various optical components along the path of the incident laser beam. By studying the interference pattern, reliable relative values for the third-harmonic coefficients were deduced for alkali halides and compared with previous measurements of the coefficients for nonresonant three-wave mixing. It is found that the optical anisotropy is small for these cubic crystals. Preliminary results for metals and semiconductors indicate that the coefficients for these materials are of the correct order of magnitude expected of the nonlinearities associated with the conduction and bound electrons.