Optical second-harmonic generation by supported metal clusters: Size and shape effects

Optical Second Harmonic Generation (SHG) by metal clusters has been investigated. For this purpose clusters were generated by the deposition and nucleation of metal atoms on a LiF(100) single crystal surface under ultrahigh vacuum conditions. The size and shape of the metal particles was characterized by optical transmission spectroscopy. The SHG intensity was detected in situ as a function of cluster size during the nucleation. Fundamental wavelengths of =1064 and 532 nm were used and the SHG signal was measured for different polarization combinations of the incident and registered light. SH radiation is detectable for particles as small as approximately 1 nm. The signal grows monotonically as a function of particle size, passes a maximum and finally drops off. This behavior is discussed in terms of resonant enhancement of the signal by surface plasmon excitation and changes of ⁽²⁾ as a function of particle size and shape. In further experiments the chemisorption of oxygen on the surface of the metal particles was studied. The SH signal decreases as a function of oxygen coverage and amounts to only about 15% of the initial value upon chemisorption of one monolayer. This indicates that the SH signal originates almost exclusively from the surface of the clean clusters and that higher order bulk contributions are negligible.     

Coherent phonon and electron spectroscopy on surfaces using time-resolved second-harmonic generation

pump (ω)E_probe ^*(ω)|E_probe(ω)|². This is much easier to detect than transient grating, photon echo, or four-wave mixing schemes that use higher-order nonlinearities. We have applied this technique to measure the energy gap and dephasing time of the dangling bond interband transition on the GaAs(110)-relaxed (1×1) surface. Surface-carrier/surface-phonon interaction plays an important and perhaps dominant role in surface carrier dephasing consistent with the larger electron-phonon coupling on the surface compared to the bulk. Received: 13 October 1998     

Enhanced second-harmonic generation for multiple wavelengths by defect modes in one-dimensional photonic crystals

We present an effective design of aperiodically stacked layers of nonlinear material and air, sandwiched by two truncated photonic crystals, in terms of the simulation annealing method. The constructed structure can achieve multiple-wavelength second-harmonic generation (SHG) at the preassigned wavelengths. We derive a general solution of SHG in 1D inhomogeneous systems and apply it to evaluate the SHG conversion efficiency. Numerical simulations show that the conversion efficiency of SHG can be significantly enhanced when the fundamental wave frequencies are assigned to the designed defect states.     

Enhanced second-harmonic generation by means of high-power confinement in a photovoltaic soliton-induced waveguide

We present the first experimental demonstration of enhanced second-harmonic generation (SHG) by means of power confinement with a femtosecond laser in a photovoltaic soliton-induced waveguide. A dark spatial soliton created with a weak cw laser beam in a photovoltaic lithium niobate crystal induces an efficient waveguide for SHG, leading to a 60% enhancement of the conversion efficiency.     

Quasi-particle spectra of perovskites: Enhanced Coulomb correlations at surfaces

Photoemission spectra of the perovskites Ca_xSr_1-xVO₃, Ca_xLa_1-xVO₃, and SrRuO₃ indicate that Coulomb correlations are more pronounced at the surface than in the bulk. To investigate this effect we use the dynamical mean field theory combined with the Quantum Monte Carlo technique and evaluate the multi-orbital self-energy. These systems exhibit different degrees of band filling and range from metallic to insulating. The key input in the calculations is the layer dependent local density of states which we obtain from a tight-binding approach for semi-infinite cubic systems. As a result of the planar character of the perovskite t_2g bands near the Fermi level, the reduced coordination number of surface atoms gives rise to a significant narrowing of the surface density of those subbands which hybridize preferentially in planes normal to the surface. Although the total band width coincides with the one in the bulk, the effective band narrowing at the surface leads to stronger correlation features in the quasi-particle spectra. In particular, the weight of the quasi-particle peak near E_F is reduced and the amplitude of the lower and upper Hubbard bands is enhanced, in agreement with experiments.     

Second-harmonic generation from metal surfaces: Beyond jellium

The generation of Second-Harmonic (SH) radiation from simple metals has been the subject of numerous investigations for almost three decades. The use of clean metal surfaces in the experiments and increasingly sophisticated jellium-based theoretical models has led to a convergence on the magnitude and some of the dispersion characteristics of SH generation in free-electron metals like aluminum. However, the sensitivity of the SH-generation process to crystal symmetry, surface morphology, and surface electronic states indicates that nonlinear optical properties are influenced by processes which cannot be explained using a model based on an isotropic electron gas. Here, we review some recent experimental work indicating where deviation from jellium-based models occurs and attempts to understand these.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Enhanced second-harmonic generation in AlGaAs/AlxOy tightly confining waveguides and resonant cavities

We demonstrate second-harmonic generation (SHG) from sub-micrometer-sized AlGaAs/AlxOy artificially birefringent waveguides. The normalized conversion efficiency is the highest ever reported. We further enhanced the SHG using a waveguide-embedded cavity formed by dichroic mirrors. Resonant enhancements as high as approximately 10x were observed. Such devices could be potentially used as highly efficient, ultracompact frequency converters in integrated photonic circuits.     

Thin noble metal films on Si (111) investigated by optical second-harmonic generation and photoemission

Thin noble metal films (Ag, Au and Cu) on Si (111) have been investigated by optical second-harmonic generation (SHG) in combination with synchrotron radiation photoemission spectroscopy. The valence band spectra of Ag films show a quantization of the sp-band in the 4-eV energy range from the Fermi level down to the onset of the d-bands. For Cu and Au the corresponding energy range is much narrower and quantization effects are less visible. Quantization effects in SHG are observed as oscillations in the signal as a function of film thickness. The oscillations are strongest for Ag and less pronounced for Cu, in agreement with valence band photoemission spectra. In the case of Au, a reacted layer floating on top of the Au film masks the observation of quantum well levels by photoemission. However, SHG shows a well-developed quantization of levels in the Au film below the reacted layer. For Ag films, the relation between film thickness and photon energy of the SHG resonances indicates different types of resonances, some of which involve both quantum well and substrate states. Received: 16 October 2001 / Revised version: 14 March 2002 / Published online: 29 May 2002     

Multicrystal, continuous-wave, single-pass second-harmonic generation with 56% efficiency

We report a simple and compact implementation for single-pass second-harmonic-generation (SP-SHG) of cw laser radiation, based on a cascaded multicrystal (MC) scheme, that can provide the highest conversion efficiency at any given fundamental power. By deploying a suitable number of identical 30-mm-long MgO:sPPLT crystals in a cascade and a 30W cw Yb-fiber laser at 1064nm as the fundamental source, we demonstrate SP-SHG into the green with a conversion efficiency as high as 56% in the low-power as well as the high-power regime, providing 5.6W of green output for 10W and 13W of green output for 25.1W of input pump power. The MC scheme permits substantial increase in cw SP-SHG efficiency compared to the conventional single-crystal scheme without compromising performance with regard to power stability and beam quality.     

Characterization of silicon carbide surfaces of 6H-, 15R- and 3C-polytypes by optical second-harmonic generation in comparison with X-ray diffraction techniques

Received: 16 December 1996/Accepted: 24 March 1997     

Simultaneous four-photon luminescence, third-harmonic generation, and second-harmonic generation microscopy of GaN

We demonstrate what is to our knowledge the first example of four-photon luminescence microscopy in GaN and apply it to quality mapping of bulk GaN. The simultaneously acquired second- and third-harmonic generation can be used to map the distribution of the piezoelectric field and the band-tail state density, respectively. Through spectrum- and power-dependent studies, the fourth power dependence of the band edge luminescence is confirmed. The superb spatial resolution of the four-photon luminescence modality is also demonstrated. This technique provides a high-resolution, noninvasive monitoring and tool for examining the physical properties of semiconductors.     

Membrane imaging by simultaneous second-harmonic generation and two-photon microscopy

We demonstrate that simultaneous second-harmonic generation (SHG) and two-photon-excited fluorescence (TPEF) can be used to rapidly image biological membranes labeled with a styryl dye. The SHG power is made compatible with the TPEF power by use of near-resonance excitation, in accord with a model based on the theory of phased-array antennas, which shows that the SHG radiation is highly structured. Because of its sensitivity to local asymmetry, SHG microscopy promises to be a powerful tool for the study of membrane dynamics.     

Enhanced second-harmonic generation by use of the photonic effect in a ferroelectric smectic C* liquid crystal

Special phase matching of second-harmonic generation (SHG) by use of the photonic effect was experimentally confirmed for an obliquely propagating fundamental beam in the helical smectic C>(*) liquid-crystal phase, as suggested by Belyakov [JETP Lett. 70, 811 (1999)]. The enhancement of SHG occurs when the second-harmonic wave is close to a photonic bandgap (half-pitch band) and consequently the fundamental wave is close to another photonic bandgap (full-pitch band). The enhanced signal was observed from both the transmitted and the reflected directions and was compared with our recently developed simulation.