Local second-harmonic generation enhancement on gold nanostructures probed by two-photon microscopy

The localization of surface second-harmonic generation (S-SHG) enhancements from granular gold structures that exhibit local plasmon resonance was investigated. A two-photon microscopy technique was used to perform high spatial resolution S-SHG imaging. The magnitude and the spatial density of S-SHG enhancement confined in submicroscopic regions are strongly dependent on the morphology of the gold's surface. Polarization measurements of local S-SHG responses reveal the local field anisotropy in enhancement regions and furthermore prove the incoherent and strongly depolarized nature of the emission, which is attributed to ultrafast fluctuations of the enhancement location in the focal volume.     

Melting-induced enhancement of the second-harmonic generation from metal nanoparticles

We report on the generation of second-harmonic signals by irradiating monolayers of high purity Ga nanoparticles, embedded in a SiOx matrix, with femtosecond laser pulses at 800 nm. A remarkable melting-induced enhancement of the second-harmonic generation is observed in correspondence of the phase transition. In addition, the hysteresis cycle of nonlinear transmittance is shown to be amplified a factor of 80-100 with respect to the linear response and interpreted in the framework of a nonlinear effective-medium model.     

Giant enhancement of second-harmonic generation from a nanocavity metasurface

Nonlinear plasmonic metasurfaces are compatible with complementary metal oxide semiconductor technology and highly promising for on-chip optical switching and modulations and nanoscale frequency conversions. However, the low nonlinearoptical response of metasurface devices limits their practical applications. To circumvent this constraint, we propose the design of a nanocavity plasmonic metasurface, in which the strong light localization in the nanocavity can be used to boost the efficiency of second-harmonic generation. Compared with the single-layer counterpart, experimental results show that the intensity of the second-harmonic waves in the nanocavity metasurface is enhanced by ~790 times. The proposed nanocavity plasmonic metasurfaces in this work may open new routes for developing highly efficient nonlinear metacrystals for on-chip nonlinear sources,nonlinear image encryption, information processing, and so on.     

Near-field second-harmonic microscopy of thin ferroelectric films

We present a near-field optical technique for second-harmonic imaging by use of tapered optical fiber tips externally illuminated with femtosecond laser pulses. Enhancement of the electric field at the tip of the fiber results in enhanced second-harmonic (SH) generation from the sample region near the tip. This SH emission is collected by the same tapered fiber. The spatial distribution and polarization properties of SH generation from thin ferroelectric films and a poled single crystal of BaTiO(3) have been studied. A spatial resolution of the order of 80 nm was achieved. Symmetry properties of the near-field SH signal allow us to recover the local poling direction of individual ferroelectric domains in the film. Thus the technique provides a novel tool for nanometer-scale crystal analysis of polycrystalline samples.     

Disorder-correlated enhancement of second-harmonic generation in strongly photonic porous gallium phosphide

We report an order of magnitude enhancement of second-harmonic generation (SHG) from porous gallium phosphide relative to SHG in crystalline gallium phosphide. Optical heterodyning measurements of photon free-path length reveal a correlation between SHG enhancement and disorder of the porous material.     

Polarization-vortex-driven second-harmonic generation

Polarization vortices exist in the focus of a class of vector beams, the lowest order of which possess full vector symmetry about the axis of propagation of the beam. At high numerical apertures these beams are known to exhibit large, local, longitudinal fields in the focal region. At an interface these fields can be many times stronger than the largest available transverse component and are therefore candidates for a variety of different experiments in surface physics. The observation of vortex-driven surface second-harmonic generation at smooth metal and semiconductor surfaces and thin films is reported. By comparing the response to that of a purely transverse field, we show that the smooth surface responds primarily to the longitudinal field component.     

Near-field second-harmonic imaging of ferromagnetic and ferroelectric materials

A novel scanning probe technique that is able to image surface magnetic and electric properties has been developed. It is based on the near-field microscopy of surface second-harmonic generation. We have demonstrated the capability of the technique by imaging the domain structure of a Ni single crystal and a piezoelectric ceramic.     

Non-phase-matched enhancement of second-harmonic generation in multilayer nonlinear structures with internal reflections

Traditional notions of second-harmonic generation rely on phase matching or quasi phase matching to achieve good conversion efficiencies. We present an entirely new concept for efficient second-harmonic generation that is based on the interference of counterpropagating waves in multilayer structures. Conversion efficiencies are an order of magnitude larger than with phase-matched second-harmonic generation in similar multilayer structures.     

Role of optical interference effects in the enhancement of magnetization-induced second-harmonic generation

Received: 14 October 1998     

Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field

We present a method of producing single attosecond pulses by high-harmonic generation with multicycle driver laser pulses. This can be achieved by tailoring the driving pulse so that attosecond pulses are produced only every full cycle of the oscillating laser field rather than every half-cycle. It is shown by classical and quantum-mechanical model calculations that even a minor addition (1%) of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high-harmonic generation leads to a major (15%) difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.     

Probing interfacial properties by optical second-harmonic generation

Optical second-harmonic generation from surfaces has recently proven to be a very effective and versatile probe for surface studies. It allows, among other things, direct probing of molecular adsorption on a solid substrate from a liquid. In this paper we review the most important features of this new technique. We discuss the possibility of applying it to the study of adsorption of ionic surfactants at solid–liquid interfaces, commonly used in a great number of scientific and technological fields including industrial applications. Moreover, we describe the preliminary results of an experiment currently in progress.     

Monitoring the thermally induced structural transitions of collagen by use of second-harmonic generation microscopy

The thermal disruption of collagen I in rat tail tendon is investigated with second-harmonic generation (SHG) microscopy. We investigate its effects on SHG images and intensity in the temperature range 25 degrees-60 degrees C. We find that the SHG signal decreases rapidly starting at 45 degrees C. However, SHG imaging reveals that breakage of collagen fibers is not evident until 57 degrees C and worsens with increasing temperature. At 57 degrees C, structures of both molten and fibrous collagen exist, and the disruption of collagen appears to be complete at 60 degrees C. Our results suggest that, in addition to intensity measurement, SHG imaging is necessary for monitoring details of thermally induced changes in collagen structures in biomedical applications.     

Quantum theory for plasmon-assisted local field enhancement

We applied quantum theory for nonlocal response and plasmon-assisted field enhancement near a small metallic nanoscale antenna in the limit of weak incoming fields. A simple asymmetric bio-inspired design of the nanoantenna for polarization-resolved measurement is proposed. The spatial field intensity distribution was calculated for different field frequencies and polarizations. We have shown that the proposed design the antenna allows us to resolve the polarization of incoming photons.     

Colored conical emission by means of second-harmonic generation

We predict that the combination of space and time modulational instabilities that occur by means of parametric wave mixing in quadratic media leads to colored conical emission. This phenomenon should be observed under conditions usually employed in second-harmonic experiments.     

Studies of buried interfaces by optical second-harmonic generation

Optical Second-Harmonic Generation (SHG) has been used to study the technologically very important buried semiconductor-metal and magnetic multilayer interfaces. For the case of GaAs-Au, the SHG intensity is shown to depend on the applied bias and Schottky barrier height, and is strongly affected by the sweepout of the carriers generated by the fs excitation pulses. For the M/Co/M multilayers, with M=Cu or Au, the SHG signals appear to depend strongly on the magnetization and can be shown to be interface specific.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Squeezing by second-harmonic generation in a monolithic resonator

We have measured the intensity fluctuations of the second-harmonic mode generated in a MgO:LiNbO₃ external monolithic cavity pumped by a Nd:YAG laser. The cavity has mirror coatings for both the fundamental and the second-harmonic mode. We scan the cavity using the electro-optic effect of the crystal and observe that the second-harmonic beam of 2 mW exhibits a quantum noise reduction of 40(±5)%. In addition, we report on the active frequency stabilization of the monolithic device used in our squeezing experiments. Several fast tuning parameters such as the electro-optic effect, the photo-elastic effect, and the laser frequency have been investigated. With these tuning parameters the monolithic resonator can be locked on double-resonance at the phase-matching temperature, which is a prerequisit for observing squeezing in a cw-regime.     

Three-dimensional electric field visualization utilizing electric-field-induced second-harmonic generation in nematic liquid crystals

An electric-field-induced second-harmonic-generation signal in a nematic liquid crystal is used to map the electric field in an integrated-circuit-like sample. Since the electric-field-induced second-harmonic-generation signal intensity exhibits a strong dependence on the polarization of the incident laser beam, both the amplitude and the orientation of the electric field vectors can be measured. Combined with scanning second-harmonic-generation microscopy, three-dimensional electric field distribution can be easily visualized with high spatial resolution of the order of 1 microm.