Nonlinear optics for surface phase transitions

A general approach of second-harmonic generation (SHG) studies for surface phase transitions (PTs) is presented, with a thermodynamic classification of surface PTs and their relation to SHG parameters. The symmetry aspects of SHG near a surface PT are discussed, including issues connected with separation of surface and bulk contributions and the role of atomic and mesoscopic inhomogeneities. This approach is illustrated by applying it to two systems revealing a (near-) surface PT: single-crystalline SrTiO₃ near a bulk structural PT and single-crystalline Au in an electrochemical cell revealing an order–disorder and a reconstructive PT. Received: 15 January 2002 / Published online: 11 June 2002     

Decay and dephasing of image-potential states due to surface defects and disorder

Received: 19 October 1998     

Photoemission study of clean and c(4×2)-2CO-covered Pt (111) using high-harmonic radiation

An extreme ultraviolet (EUV) laser light source based on high-harmonic generation is presented. Coherent radiation in the photon energy range hν=20–120 eV is produced in the conversion media argon, neon and helium. High-harmonic radiation in the energy range 20–50 eV is applied to investigate photoemission spectra of Pt (111) and CO/Pt (111). In the photoemission spectra of the clean surface, new secondary electron emission structures are found which influence the cross section analysis of the CO states. When taking these Pt resonances into consideration, the 4σ and 5σ CO shape resonances are found at photon energies of 37 eV and 28 eV, respectively. Additionally, a resonance at hν=31 eV is also observed for the CO 1π state, in contrast to formerly published experimental data. Experimental and theoretical data suggest that this resonance is not connected to the well-known shape resonances in the σ-channel. Based on theoretical approaches, it is identified as an autoionization resonance. Received: 8 April 2002 / Accepted: 22 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-251/833-3604, E-mail: kutzner@uni-muenster.de     

Studies of third-order nonlinear optical properties of C60 by the optical Kerr effect

60 at 1.064 μm is determined by the optical Kerr effect in toluene solution. Values of -6.5×10^-33 esu and 2.0×10^-32 esu for the real and imaginary parts of γ, respectively, are obtained. Based on the experimental results and symmetry constraint, it is concluded that a three-level model is not adequate to describe the third-order nonlinearity of C₆₀. It is also inferred that the non-resonant γ of C₆₀ should be positive in sign, which is in agreement with reported off-resonant measurements of γ of C₆₀ by third-harmonic generation. Received: 3 September 1997/Revised version: 1 December 1997     

First Z-scan n2 measurements on crystal hosts for ultraviolet laser systems

We report the results of the first measurements of the non-linear Kerr refractive index, n₂, for LiBaF₃ and LiLuF₄ crystal hosts, known as prospective UV-emitting tunable laser media when doped with Ce³⁺ ions. These n₂ values (2.7×10^-16 and 1.5×10^-16 cm²/W at 532 nm, respectively), obtained using the well-established Z-scan technique, are important for the characterization of new optical materials particularly in relation to their potential ultrafast applications. Received: 6 June 2000 / Published online: 5 October 2000     

DC-electric-field-modified second-harmonic generation at the Si(100) surface

1 resonances for clean and H covered surfaces shift as a function of the dc field in agreement with experiment. This suggests the presence of built-in electric fields whose strength depends on the H coverage, and which are strongly localized in the subsurface region. Received: 20 September 1998     

Surface nonlinear optics: a historical overview

The availability of widely tunable dye lasers and optical parametric oscillators permit the detection of surface specific electronic and vibrational states. Polarization dependent studies yield information about the orientation of molecular monolayers and surface specific bonds. The use of picosecond and femtosecond pulse–probe techniques permits time-resolved studies of surface phase transformations, desorption, and melting. A few examples from the rapidly growing literature are selected to illustrate this historical evolution. Received: 20 September 1998     

Nonlinear surface photoeffect in multi-color laser field: effect of the finite width of the surface potential barrier

Received: 20 September 1998     

Spin splitting of image-potential states on fcc Fe/Cu(100)

Received: 19 October 1998     

Phase and population decay of image-potential states – the influence of defects

Quantum-phase and population decay of image-potential states have been investigated by two-photon photoemission with femtosecond time resolution. The influence of steps and defects on quasielastic and inelastic scattering processes is illustrated for a vicinal Cu(119) surface and diluted adsorbate layers of CO and Cu on Cu(001). Received: 19 April 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Simultaneous measurements of second-harmonic generation and two-photon photoelectric emission from Au

Received: 1 October 1998 / Accepted: 30 October 1998     

Nonlinear optical response of Ge nanocrystals in silica matrix with excitation of femtosecond pulses

We report an investigation of third-order optical nonlinearities in Ge nanocrystals (∼6 nm radius) embedded in silica matrix using the Z-scan and pump-probe techniques with femtosecond laser pulses at 780-nm wavelength. The nanocrystallite Ge samples were prepared using magnetron co-sputtering and post-thermal annealing at 800 °C. The nonlinear absorption coefficient and refractive index of the Ge nanocrystals were determined to be in the range from 1.8×10^-7 to 6.8×10^-7 cm/W and 1.5×10^-12 to 8.0×10^-12 cm²/W, respectively, which are proportional to the Ge atomic fraction in the matrix. Relaxation of the nonlinear response was found to have two characteristic time constants, 1.8 ps and 65 ps. The mechanisms responsible for the observed nonlinear response are discussed. Received: 21 August 2000 / Revised version: 17 January 2001 / Published online: 30 March 2001     

Laser microstructuring of steel surfaces for tribological applications

Laser technique is employed to improve substantially the tribological behavior of microstructured stainless steel surfaces. The laser-generated patterns consisting of precisely ablated microcraters were produced using a flash lamp pumped, Q-switched Nd:YAG laser. The rims of resolidified melt around the craters are typical features of laser-produced microstructures and the laser parameters were established where their extent was notably reduced. The remaining rims could easily be removed by gentle polishing. The laser-structured substrates were tribologically tested by a sliding tribometer under standard conditions. The test results show clearly that the microstructures improve the lifetime of the samples. Received: 29 December 1999 / Accepted: 7 January 2000 / Published online: 8 March 2000     

Simultaneous measurement of pure-optical and thermo-optical nonlinearities induced by high-repetition-rate, femtosecond laser pulses: application to CS2

A simple experimental technique is presented capable of separating the contribution of purely optical Kerr effects from that of thermo-optical effects in the nonlinear response of materials under high-repetition-rate laser irradiation. The technique has been realized by combining the single-beam Z-scan method with the single-beam thermal lens measurement method. We demonstrate this technique by analysing the nonlinear response at 770 nm of CS₂ which exhibits cumulative thermal effects when irradiated by very intense femtosecond laser pulses at a 76-MHz repetition rate. Received: 3 November 1998 / Revised version: 4 January 1999 / Published online: 2 June 1999     

Manipulation of soliton ensembles by spectral filters

It was shown recently that an ensemble of solitons can be created in a driven optical fiber resonator. The ensemble can exist in either an ordered or a disordered state; these have been referred to as soliton solid or gas (fluid), respectively. We demonstrate through numerical simulation that the transition from one state to the other or vice versa, and further manipulation of the ensemble, can be effected through the insertion of spectral filters. Received: 7 July 2000 / Revised version: 1 September 2000 / Published online: 30 November 2000     

Threshold reduction of stimulated Brillouin scattering (SBS) using fiber loop schemes

SBS mirrors as self-pumped and easy to handle non-linear optical devices are frequently used in high-power laser systems for improving the beam quality based on optical phase conjugation. Because of the non-linear behaviour, a certain pulse energy or power of incident light is needed to generate enough reflectivity for practical purposes. Therefore, reducing this “threshold” is still a main topic in the development of new schemes for optical phase conjugation. In addition to the taper concept reported earlier, this paper deals with loop schemes for reducing the power requirements. A reduction of the so-called “threshold” by a factor of between two and four was obtained with the schemes investigated using liquids and fibers. Received: 4 September 2001 / Revised version: 22 October 2001 / Published online: 23 November 2001     

Electronic band structures of low-dimensional adsorbate systems: Rare-gas adsorption on transition metals

Angle-resolved photoemission data are dis-cussed for five different Xe adlayers which exhibit electronic structures of different dimensionalities. Xe adsorption on Ni (110)-(1 × 2)-3Hand the (×) R30^° Xe layer on Ru (001) reveal two-dimensional (2D) Xe-derived band structures that are characteristic for hexagonal rare-gas layers. Different Xe 5p dispersion widths on Ni and on Ru are found due to the difference in the Xe-Xe nearest-neighbor distance. For three rare-gas systems (two different Xe coverages on hydrogen-modified Pt (110)-(1 × 2)-H and Kr step decoration on a Pt (997) surface) true one-dimensional (1D) band structures are found. For Xe step adsorption on Pt (997), electronic localized (0D) behavior is observed due to an enlarged Xe-Xe separation. The qualitative differences of the band structures in the case of 2D, 1D and 0D rare-gas systems are demonstrated and are explained by the different dimensionalities of the various structures. Received: 3 August 2000 / Accepted: 4 August 2000 / Published online: 7 March 2001     

Electron properties and surface effects of rare-earth systems

3 . Received: 21 March 1997/Accepted: 12 August 1997     

Second-harmonic spectroscopy of Ge/Si(001) and Si1-xGex(001)/Si(001)

0.9 Ge_0.1(001)/Si(001) films with SH photon energies 3.1<2hν<3.5 eV near the bulk E₁ critical point of Si(001) or Si_0.9Ge_0.1(001). Ge was deposited on Si(001) by using atomic layer epitaxy cycles with GeH₄ or Ge₂H₆ deposition at 410 K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 monolayers the SH signal increased uniformly by a factor of seven with no detectable shift in the silicon E₁ resonant peak position. SH signals from Si_0.9Ge_0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). Hydrogen termination of the Si_0.9Ge_0.1(001) and Ge/Si(001) surfaces strongly quenched the SH signals, which is similar to the reported trend on H/Si(001). We attribute the stronger signals from Ge-containingsurfaces to the stronger SH polarizability of asymmetric Ge-Si and Ge-Ge dimers compared to Si-Si dimers. Hydrogen termination symmetrizes all dimers, thus quenching the SH polarizability of all of the surfaces investigated. Received: 13 October 1998 / Revised version: 18 January 1999     

Nonlinear phase shift without cascaded second-order processes and third-order nonlinearity

A new mechanism for obtaining a nonlinear phase shift has been proposed and schemes are described for its implementation. As it is shown, the interference of two waves with intensity-dependent amplitude ratio coming from the second-harmonic generation should produce the nonlinear phase shift. The sign and amount of nonlinear distortion of a beam wavefront is dependent on the relative phase of the waves that is introduced by the phase element. The calculated value of n₂ ^eff exceeds that connected with cascaded quadratic nonlinearity, at the same conditions. Received: 1 November 1999 / Published online: 23 February 2000