O.A. Aktsipetrov

Ultrafast Transport of Laser-Excited Spin-Polarized Carriers in Au/Fe/MgO(001)

Hot carrier-induced spin dynamics is analyzed in epitaxial Au/Fe/MgO(001) by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.

Plasmons reveal the direction of magnetization in nickel nanostructures.

We have applied the surface-sensitive nonlinear optical technique of magnetization-induced second harmonic generation (MSHG) to plasmonic, magnetic nanostructures made of Ni. We show that surface plasmon contributions to the MSHG signal can reveal the direction of the magnetization. Both the plasmonic and the magnetic nonlinear optical responses can be tuned; our results indicate novel ways to combine nanophotonics, nanoelectronics, and nanomagnetics and suggest the possibility for large magneto-chiral effects in metamaterials.

Optical second-harmonic generation induced by a dc electric field at the Si–SiO 2 interface

For what is to our knowledge the first time, electric-field-induced optical second-harmonic generation (SHG) is studied at the Si-SiO(2) interface by the use of a metal-oxide-semiconductor (MOS) structure. The crystallographic anisotropy of this phenomenon is studied for MOS structures. Experimental results indicate that the MOS technique of dc electric-field application to the Si-SiO(2) interface can be effectively used for studying electroinduced effects on SHG.

Giant microcavity enhancement of second-harmonic generation in all-silicon photonic crystals

Second-harmonic generation (SHG) spectra of single and coupled porous silicon-based photonic crystal microcavities are studied in both frequency and wave vector domains. For the fundamental field resonant to the microcavity mode the second-harmonic intensity is enhanced by 102 times in comparison with that outside the photonic band gap. SHG spectroscopy in identical microcavities coupled through the intermediate Bragg reflector reveals two SHG peaks if the fundamental field is in resonance with the splitted mode of coupled microcavities. The spatial confinement of the resonant fundamental radiation is directly probed at the microcavity cleavage by scanning near-field optical microscopy.

All optical poling and second harmonic generation in glasses: theory and experiment

Abstract Theoretical and experimental evidence of light driven electron–hole self-organization resulting in amplification of seed static electric field is presented. The amplification allowed us to perform all optical poling of conventional commercial “heavy flint” silica glass. The χ (2) -grating is optically recorded in the initially centrosymmetric glass and the second harmonic generation (SHG) is observed in poled noncentrosymmetric material. The fundamental and frequency doubled output of a YAG:Nd 3+ laser starts the poling process creating rectified DC-electric field. Then single wavelength radiation (either fundamental or frequency doubled output) proceeds with optical recording of nonlinear grating amplifying this initial DC-electric field.

dc-electric-field-induced and low-frequency electromodulation second-harmonic generation spectroscopy of Si(001)-SiO2 interfaces

The mechanism of dc-electric-field-induced second-harmonic ~EFISH! generation at weakly nonlinear buried Si(001)-SiO2 interfaces is studied experimentally in planar Si(001)-SiO 2-Cr MOS structures by optical second-harmonic generation spectroscopy with a tunable Ti:sapphire femtosecond laser. The spectral dependence of the EFISH contribution near the direct two-photon E1 transition of silicon is extracted. A systematic phenomenological model of the EFISH phenomenon, including a detailed description of the space-charge region ~SCR! at the semiconductor-dielectric interface in accumulation, depletion, and inversion regimes, has been developed. The influence of surface quantization effects, interface states, charge traps in the oxide layer, doping concentration, and oxide thickness on nonlocal screening of the dc-electric field and on breaking of inversion symmetry in the SCR is considered. The model describes EFISH generation in the SCR using a Green’s-function formalism which takes into account all retardation and absorption effects of the fundamental and second-harmonic ~SH! waves, and multiple reflection interference in the SiO 2 layer. The optical interference between field-dependent and -independent contributions to the SH field is considered as aninternal homodyne amplifier of the EFISH effects. Good agreement between the phenomenological model and our EFISH spectroscopic results is demonstrated. Finally, low-frequency electromodulated EFISH is demonstrated as a useful differential spectroscopic technique for studies of the Si-SiO 2 interface in silicon-based metaloxide-semiconductor structures. @S0163-1829~99!01836-6#

Surface-Enhanced Optical Third-Harmonic Generation in Ag Island Films

Surface-enhanced optical third-harmonic generation (THG) is observed in silver island films. The THG intensity from Ag nanoparticles is enhanced by more than 2 orders of magnitude with respect to the THG intensity from a smooth and homogeneous silver surface. This enhancement is attributed to a local plasmon excitation and resonance of the local field at the third-harmonic wavelength. The diffuse and depolarized component of the enhanced THG is associated with the third-order hyper-Rayleigh scattering in a two-dimensional random array of silver nanoparticles.

Second-harmonic generation in metal and semiconductor low-dimensional structures

Abstract In this article previous and recent results of our nonlinear optical studies of low-dimensional structures are surveyed. Size effect in the optical second-harmonic generation (SHG) from Ag nanocrystals in island films and CdSe quantum dots in glass matrices as well as thermo- and DC-electric-field-induced effects in SHG from SiSiO2 multiple quantum wells (MQWs) on Si substrate are studied experimentally and theoretically. Essential (by several orders of magnitude) enhancement of quadratic optical response upon decrease in the particle size is observed for both Ag nanocrystals and CdSe quantum dots. Regular oscillations in the second-harmonic intensity measured as functions of temperature and applied DC electric field are observed for MQWs. It is shown that the size effect for metal nanocrystals can result from the fluctuations of the particle size breaking the local inversion symmetry. Another mechanism studied (based on concepts of dynamic chaos) describes the observed size effects for both metal and semiconductor particles. The dependence of the second-harmonic intensity on the SiO2 layer thickness in MQWs is described with taking into account the retardation of the second-harmonic radiation in MQWs, whereas the thermoinduced effect is interpreted as resulting from the optical interference in the MQWs substrate having thickness dependent on temperature (due to the thermal expansion). A possible approach to explaining the observed electroinduced effect in MQWs is also discussed.

Nonlinear diffraction and second-harmonic generation enhancement in silicon-opal photonic crystals

Nonlinear diffraction in three-dimensional silicon-filled photonic crystals of opals is studied. Efficient backward second-harmonic generation (SHG) is observed in the specular direction upon the fundamental radiation reflection from the (111) face of the face-centered-cubic opal lattice. Tuning the fundamental wavelength across the photonic band gap (PBG) results in the 20-times increase of the second-harmonic intensity. The SHG peak has the width of approximately 20nm and is located at the long-wavelength edge of the PBG.

Giant optical second-harmonic generation in single and coupled microcavities formed from one-dimensional photonic crystals

The nonlinear optical properties of one-dimensional all-solid-state photonic-crystal microcavities (MCs) are experimentally studied by second-harmonic generation (SHG) spectroscopy in both the frequency and the wave-vector domains. The studied single and coupled MCs are formed by the alternating of mesoporous silicon layers of different porosities. When the fundamental radiation is in resonance with the MC mode the second-harmonic intensity is enhanced by a factor of approximately 102. The resonant SHG response is compared with the off-resonance response, as the fundamental wavelength is outside the photonic bandgap. The splitting of the modes of two identical coupled MCs is observed in the wave-vector domain spectrum of enhanced SHG. The SHG enhancement is attributed to the combined effects of the spatial localization of the fundamental field in the MC spacer and the fulfillment of the phase-matching conditions. The confinement of the resonant fundamental field is probed directly at the MC cleavage by a scanning near-field optical microscope. The role of the phase matching that is associated with the giant effective dispersion in the spectral vicinity of the MC mode is deduced from a comparison with the SHG peaks at both edges of the photonic bandgap.