Second-harmonic Response Research Articles

Spectral dependence of time-resolved coherent and incoherent second-harmonic response of ferromagnetic Gd(0001)

Second-harmonic (SH) generation at the ferromagnetic Gd(0001) surface was investigated with 35-fs laser pulses and photon energies between 1.44 and 1.68 eV. In pump-probe mode the SH field was measured during the first 3 ps, and the time evolution, including contributions from coherent phonons and magnons, was analyzed. All data reveal strong changes with wavelength, which correlate with the SH magnetic contrast and can be well explained by the Gd(0001) exchange-split surface state. In SH interferometry an energy splitting of 13 meV was found that is attributed to a frequency modulation of the surface state by coherent phonons of approximately 3 THz.

Ab initiocalculation of optical second harmonic generation at the rutileTiO2(110)surface

Optical second harmonic (SH) response of rutile $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surface was studied by the self-consistent full potential linearized augmented plane-wave method within the local-density approximation. The calculated SH response of the relaxed surface agrees with the measured SH intensity as a function of the excitation photon energy and the sample rotation angle around its surface normal. In order to clarify the origin of the large surface SH response at the SH photon energy of about $2\ensuremath{\hbar}\ensuremath{\omega}=4\phantom{\rule{0.2em}{0ex}}\mathrm{eV}$, SH intensities from various Ti-O bonds on the surface were calculated. The result revealed that the zigzag chains consisting of bridging oxygen and neighboring sixfold coordinated titanium atoms on the top surface layer dominantly contribute to the SH response of the $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surface.

Optical second harmonic imaging of zinc oxide thin films grown by metal organic chemical vapour deposition (MOCVD)

We demonstrate optical second harmonic (SH) imaging using femtosecond laser pulses (80 fs, 1.59 eV, 80 MHz) as a suitable probe to spatially investigate the structural homogeneity of zinc oxide (ZnO) thin films grown by metal organic chemical vapour deposition (MOCVD). We find that, despite obvious differences in crystalline structure as well as lattice mismatch, ZnO films grown on the three different substrates Si(100), GaAs(100) and amorphous glass under equal growth conditions show a very similar crystalline structure, which appears to be of polycrystalline nature. Our study on films of different thickness reveals that initially micro-crystallites are deposited randomly on the substrate and possibly act as seeds for the further growth process. Furthermore, SH images of ZnO films grown on GaAs show sample regions (20–50 μm in size), where the SH response vanishes, indicating that the strong SH signal generated in the underlying substrate material is suppressed. We attribute these regions to presumably amorphous particles deposited during the growth process, which have light absorbing properties and suppress the SH generation process. Hence SH imaging is an applicable method to obtain detailed information about some aspects of the film deposition process. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical second harmonic imaging: a versatile tool to investigate semiconductor surfaces and interfaces

Optical second harmonic (SH) imaging using femtosecond laser pulses (782 nm, 80fs) is applied to investigate the structural quality of SiC and ZnO thin films grown by chemical vapour deposition (CVD) on Si(100) substrates. We find a spatially uniform SH response from the SiC sample as well as a regular 4-fold symmetry (3C-SiC-(001)) in the rotational SH anisotropy. The ZnO film is poly-crystalline as indicated by a strongly position dependent SH response and the missing regularity in the rotational SH anisotropy. The estimated grain size is ∼ 30-50 μm. Furthermore our time dependent SHG measurements on native Si/SiO 2 for laser peak intensities up to 100 GW/cm 2 indicate that hole injection and trapping into the ultrathin SiO 2 layer contributes to the SH signal above > 45 GW/cm 2 . The SHG images of defects irreversibly induced in Si/SiO 2 under laser irradiation are reproduced by scanning electron microscopy (SEM).

Charge-carrier dynamics and trap generation in nativeSi/SiO2interfaces probed by optical second-harmonic generation

Employing femtosecond laser pulses [1.59 eV, $(80\ifmmode\pm\else\textpm\fi{}5)\mathrm{fs},$ 80 MHz], we perform time-dependent second-harmonic (SH) measurements to probe the dynamics of charge-carrier transfer processes across native ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ interfaces in a new peak intensity regime up to $100{\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ The SH signal increases steadily during irradiation up to $45{\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ laser peak intensity, but changes its temporal evolution above $45{\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}:$ as a different phenomenon, the SH signal surpasses a maximum, decreases slowly, and appears to approach a steady state level. We assign this observed signal decrease to the light induced injection of holes into the ${\mathrm{SiO}}_{2}$ thin layers involving a four-photon process, which opposes the electron contributions (three-photon process). Furthermore, we investigate the SH signal development after irradiation interrupts and find that preirradiated samples show a drastically accelerated SH response. Both, the electron and the hole injection processes appear to be about two orders of magnitude faster in preirradiated than in ``virgin'' samples. We assign this acceleration to the laser induced generation of permanent defects, which affect electron as well as hole trapping in the ${\mathrm{SiO}}_{2}$ layers. In addition the SH signal amplitude is shown to be a function of the dark period duration, and the typical time constant for the depopulation of hole trap sites under dark conditions has been extracted and amounts to $(110\ifmmode\pm\else\textpm\fi{}15)\mathrm{s}.$ We present an empirical model involving four exponential functions, taking into account the electron and hole contributions to the time-dependent SH response. This model fully reproduces the recorded experimental data. It supports our interpretation of opposing three- and four-photon processes being involved in electron and hole injection, respectively. Finally, we employ SH imaging in comparison with scanning electron microscopy to visualize sample areas with laser defects.

Time evolution of the microwave second-harmonic response of MgB 2 superconductor

We report on transient effects in the microwave second-order response of two ceramic MgB 2 samples. The time evolution of the second-harmonic signal is investigated for about 500 s after the sample has been exposed to a variation of the dc magnetic field. We suggest that during the first seconds the response is determined by diffusive motion of fluxons, while in the time scale of minutes it is ruled by magnetic relaxation over the surface barrier. © 2001 Elsevier Science. All rights reserved.

Second-Harmonic Optical Response from First Principles

We present a full formalism for the calculation of the linear and second-order optical response for semiconductors and insulators. The expressions for the optical susceptibilities are derived within perturbation theory. As a starting point a brief background of the single and many particle Hamiltonians and operators is provided. As an example we report calculations of the linear and nonlinear optical properties of the mono-layer InP/GaP (110) superlattice. The features in the linear optical spectra are identified to be coming from various band combinations. The main features in the second-order optical spectra are analyzed in terms of resonances of peaks in linear optical spectra. With the help of the strain corrected effective-medium-model the interface selectivity of the second-order optical properties is highlighted.

Large optical nonlinearities in BiMnO3 thin films

Large third-order optical nonlinearities were observed in epitaxial thin films of BiMnO3 grown on SrTiO3 substrates. Using 140 fs laser pulses at 900 nm, a negative nonlinear refractive index (nI∼−0.53 cm2/GW) and a nonlinear absorption coefficient (αI∼−0.08 cm/kW) for BiMnO3 are measured at room temperature. Large electric-field-induced enhancement of three to four orders of magnitude in the second-harmonic response at 450 nm is observed with effective nonlinear coefficients of deff∼40(115) pm/V at 300 (473) K under applied fields of ∼707 (177) V/mm, respectively, from a 110 nm multivariant thin film.

Spin response in nonlocal nonlinear optics

A novel effect of the electron spin in nonlinear optics is predicted. Starting from the Pauli Hamiltonian, we present the influence of the electron spin on the nonlinear optical response of a homogeneous electron gas in the low temperature limit. It is shown that the longitudinal second-harmonic response which results from a transverse incoming field differs significantly from the usual paramagnetic response when spin effects are included. The calculation is done both with and without screening.

Second-order nonlinear optical properties of chiral materials

This paper reviews work on the second-order nonlinear optical properties of chiral thin films and surfaces. We give an overview of the theoretical formalism and experimental techniques to describe the second-harmonic generation (SHG) from chiral thin films and surfaces. We also discuss the second-harmonic response of thin films and surfaces composed of chiral synthetic materials and biological molecules. In addition, we point out possible applications and the future prospects in this field.

Investigation of Adsorbed Halide Layers on Single-Crystal Silver Electrodes by Second-Harmonic Generation

The second-harmonic response of single-crystal silver electrodes to an incident wavelength of 1064 nm was measured in the presence of adsorbed chloride and bromide ions. The adsorbate markedly affects the magnitude and phase of the isotropic part of the signal. In particular, the structural transitions in the adsorbed layer can be detected; transitions to dense layers are often indicated by a drop in the phase of the signal. For the case of Ag(111), we have obtained the Rudnick and Stern parameter a, which characterizes the electronic response perpendicular to the surface. These optical measurements were supplemented by impedance spectroscopy.

Second-harmonic study of polar symmetry and domain structure in SrTi18O3

The ferroelectric domain structure of SrTi18O3 is studied via the azimuthal dependence of the second-harmonic (SH) light intensity below Tc≈25 K. Tensor analysis of the SH response reveals locally variant mixtures of eight triclinic polar domains, which transform into an orthorhombic single domain under an electric field E applied perpendicularly to the tetragonal axis, with d22 being the dominant coupling constant.

Substrate effects in the magneto-optical second-harmonic generation from first principles: Fe/Cu(001)

We compute the nonlinear optical response of an Fe monolayer placed on top of 1 to 4 monolayers of Cu(001). Our calculation is based on ab initio eigenstates of the slab, which are obtained within the full-potential linearized augmented plane-wave method. The ground-state spin-polarized electronic structure is converged self-consistently to an accuracy better than 0.1 mRy. Subsequently, we take the spin-orbit interaction into account within a second variational treatment. The new set of eigenstates allows us to calculate the magneto-optical transition matrix elements. The second-harmonic response is determined in the reflection geometry with magnetization perpendicular to the surface (the so-called polar configuration) using the surface-sheet model. Adding layers of a noble metal (Cu) to the Fe monolayer gives a new degree of freedom for the inclusion of nonmagnetic Cu d bands to the nonlinear magneto-optical response of the slab, and the energy bands show that such an addition converges essentially to an addition of d states and a small broadening of the d band with growing number of Cu layers. The screened nonlinear optical susceptibility is calculated and converges quite well with a growing number of Cu layers. Our first-principles results confirm that the magnetic tensor elements of the nonlinear optical response tensor are roughly of the same order of magnitude as the nonmagnetic ones (in contrast to linear optics, where the magnetic response is only a minor correction).

Demagnetization following optical excitation in nickel and permalloy films

Time-resolved pump–probe second-harmonic (SH) response following optical excitation has been studied in nickel and permalloy films. Experimental data indicate that the odd SH field represents the transient magnetization, i.e. the behavior of the spin subsystem, while the electron subsystem generates the even SH field. Delays between odd and even SH fields of about 200 fs and 650 fs were detected for nickel and permalloy films, respectively. We found that the dynamics of the odd SH field is influenced by the polarization of the pump pulse. This dependence on excitation conditions suggests a difference in surface and bulk magnetization dynamics.

Mode matching for second-harmonic generation in photonic crystal waveguides

The influence of localized photonic eigenstates on the second-harmonic generation efficiency in planar photonic crystal waveguides is studied with a simple, yet realistic model. We show that $g~{10}^{6}$ enhancements in the external conversion efficiency in specular reflection can be achieved due to a combination of ``generalized phase-matching'' and strong local fields in these periodically textured, nonlinear dielectric structures. The second-harmonic response involving the resonant modes of these membranelike photonic crystals is essentially a resonant cavity effect, and hence is strongly influenced by the modal quality factor.

Absolute intensity measurements of the optical second-harmonic response of metals from 0.9 to 2.5 eV

The absolute intensity of the optical second-harmonic response and its spectral (ωfund≈0.9–2.5 eV) dependence has been measured for Ag(111), polycrystalline Ag, 4-Aminothiophenol/Ag (4-ATP/Ag) and decanethiol/Ag (DT/Ag) surfaces in contact with a liquid electrolyte. Preliminary spectra are also reported for polycrystalline Au and Cu(111) samples. For second-harmonic energies below the plasmon resonance, the magnitude of the nonlinear optical response of clean Ag samples increases as electrode potentials are made more positive. This trend reverses itself for energies above the plasmon resonance. The adsorbate-covered surfaces show a weak or nonexistent potential dependence. A unique feature is found in the 4-ATP/Ag spectra which could possibly be due to a surface charge-transfer state. The Ag results are discussed in the context of a free-electron response from which the spectral and potential dependence of the complex microscopic parameter, a(ω), are extracted. The features in the Au and Cu(111) spectra are not adequately described by this free-electron model and must be related to the effects of interband transitions on the nonlinear optical response.

Enhancement of Second-Harmonic Response and Photocurrent Generation from a Benzothiazolium Styryl Dye LB Film through an Interfacial Self-Assembly Reaction

An interface self-assembly reaction between HgCl2 and an amphiphilic heteroaromatic styryl dye, 2-(4-dihexadecylaminostyryl) benzothiazole methiodide (I), was studied. Its Langmuir−Blodgett films Ia, Ib, and Ic fabricated from different subphases (pure water, 5 × 10-8 M HgCl2, and 5 × 10-7 M HgCl2, respectively) were characterized by using UV−vis spectra and XPS measurements. Compared to those of film Ia, the seond-harmonic responses and photocurrent generation from film Ib and Ic were obviously enhanced with the addition of HgCl2 into the subphase. Under ambient conditions (0.5M KCl), the quantum yields of photoelectric conversion are 0.73%, 0.95%, and 2.45% for Ia, Ib, and Ic respectively. The effects of some factors such as bias voltage, methyl viologen (MV2+) and hydroquinone (H2Q) on the photocurrent generation have also been investigated.

Nonquadratic second-harmonic generation from semiconductor-oxide interfaces

Second harmonic generation on ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ interfaces shows unusual nonmonotonic, nonquadratic behavior. This behavior depends strongly on laser wavelength and surface termination. It is present for oxide-covered samples at second-harmonic wavelengths of 373 nm (3.32 eV), but not at 415 nm (2.99 eV). It is absent at both wavelengths for H-terminated samples. This behavior is consistent with photomodulation of an interfacial electric-field-induced contribution to the second-harmonic response of the system.

Giant second harmonic generation in microcavities based on porous silicon photonic crystals

Summary form only given. In this paper the electrochemical fabrication of coupled cavities microstructures based on porous silicon photonic crystals with the cavity modes located inside the photonic band gap is reported. The enhancement of the second-harmonic (SH) response of such structures in the vicinity of cavity modes is experimentally observed.

Static and Dynamic Solvation at the Air/Water Interface

The electronic excited-state solvation dynamics of coumarin 314 (C314) adsorbed at the air/water interface was investigated by femtosecond time-resolved second-harmonic generation (TRSHG). The second-harmonic spectrum of C314 at the air/water interface was determined using a tunable optical parametric generator, giving a spectral peak at 419 ± 3 nm. The solvatochromic shift is in good agreement with the recently proposed interfacial polarity scale, which predicts that the empirical polarity parameter of the interface is the arithmetic mean of the two bulk phases. The time-resolved second-harmonic response to the excited-state solvation was measured at two fundamental wavelengths (840 and 900 nm) and with two pump polarizations (S and P). The results indicate that the static spectral peaks of the S- and P-pumped distributions are offset ∼5 nm, while the dynamic interfacial solvation proceeds with two time constants, τ1 = ∼200 fs and τ2 = 1.2 ps, which are independent of C314 interfacial orientation. The sl...