Sum-frequency vibrational spectroscopy of a monolayer self-assembled on gold: interference between resonant and nonresonant contributions of nonlinear polarization

zzz ^(R) is dominant in the observed resonant signals, whereas all of the ijk components contribute to the nonresonant signal. The transition frequencies and the relative amplitude of resonant signals are also determined for the CH₃ vibrational modes of ODT on gold. Received: 21 August 1998 / Revised version: 16 September 1998 / Published online: 24 February 1999     

Polarisation control of light by light in a nonlinear polymer

Received: 11 December 1997/Revised version: 11 May 1998     

Two-wave-mixing dynamics and nonlinear hot-electron transport in transverse-geometry photorefractive quantum wells studied by moving gratings

The photorefractive response to an applied electric field is measured in a photorefractive quantum well, providing evidence in favor of the nonlinear transport in the device due to the hot electrons. The reduced mobility of the hot electrons limits the drift length, and thereby limits fringe overshoot. Thus the nonlinear transport prevents the slowing down of the grating writing rate for increasing fields which is common in bulk photorefractives. The photorefractive phase shift in transverse-field photorefractive quantum wells is measured as a function of the frequency offset between two laser writing beams that generate moving gratings. The two-wave mixing passes through a maximum at an optimum frequency which depends on the magnitude and the sign of the applied dc electric field. The phase shift associated with the moving grating adds or subtracts from the static phase shift induced by hot-electron transport in the semiconductor quantum wells, depending on the sign of the field and the sign of the dominant photocarriers. We observe a linear relationship between the roll-off frequency and the power of the writing beams. Received: 26 November 1998 / Revised version: 22 January 1999 / Published online: 12 April 1999     

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     

Harmonic generation in an ionized gas medium with a 100-femtosecond, high repetition rate laser source at intermediate intensities

 We report the realization of a vacuum-ultraviolet radiation source based on high-order harmonic generation in noble-gas samples, operating at high repetition rate. In particular, we observed up to the 13th harmonic (λ=61 nm) of the fundamental frequency of a short pulse, high repetition rate titanium–sapphire laser after its interaction with a Xe gas jet. The effects of the propagation of the fundamental and harmonic beams through an ionized medium are studied by analysing the spectral profile of the 9th and 7th harmonics. Finally, we report a study of the dependence of the harmonic conversion efficiency on relative position of the focus and the gas target. Received: 29 March 1996/Revised version: 25 July 1996     

On the two-wave mixing in BaTiO3 with a self-bent pump beam

Received: 25 February 1997/Revised version: 20 June 1997     

The characterization of plasma-polymerized C60 thin films

Plasma-polymerized C₆₀ thin films were investigated in the form of a field effect transistor (FET) structure. In the FET device, the C₆₀ polymer acts as a p-type semiconductor, whereas C₆₀ is an n-type semiconductor. Its conduction mechanism can be described as due to variable range hopping. As a sensing device, the C₆₀ polymer can behave as a gas sensor for electrophoric gases and can also be operated as a photo-sensing device in air. Received: 21 April 2001 / Accepted: 23 July 2001 / Published online: 2 October 2001     

High-dynamic-range pulse-front steepening of amplified femtosecond pulses by third-order dispersion

Received: 9 June 1998/Revised version: 4 November 1998     

Effects of electron relaxation on multiple harmonic generation from metal surfaces with femtosecond laser pulses

Calculations are presented for the first four (odd and even) harmonics of an 800 nm laser from a gold surface, with pulse widths ranging from 100 down to 14 fs. For peak laser intensities above 1 GW/cm² the harmonics are enhanced because of a partial depletion of the initial electron states. At 10¹¹ W/cm² of peak laser intensity the calculated conversion efficiency for 2nd-harmonic generation is 3 × 10⁻⁹, while for the 5th-harmonic it is 10⁻¹⁰. The generated harmonic pulses are broadened and delayed relative to the laser pulse because of the finite relaxation times of the excited electronic states. The finite electron relaxation times cause also the broadening of the autocorrelations of the laser pulses obtained from surface harmonic generation by two time-delayed identical pulses. Comparison with recent experimental results shows that the response time of an autocorrelator using nonlinear optical processes in a gold surface is shorter than the electron relaxation times. This seems to indicate that for laser pulses shorter than ∼30 fs, the fast nonresonant channel for multiphoton excitation via continuum-continuum transitions in metals becomes important as the resonant channel becomes slow (relative to the laser pulse) and less efficient.     

On the theory of relaxation of electrons excited by femtosecond laser pulses in thin metallic films

Received: 14 October 1998 / Revised version: 21 December 1998     

Ionization spectrum broadening and frequency blue-shift of high-intensity femtosecond laser pulses in gas-filled capillary tubes

. We report on the experimental and theoretical study of spectrum transformation and frequency blue-shift of femtosecond laser pulses with intensities up to 2×10¹⁶ W/cm², propagating in glass capillary tubes under gas ionization. Monomode optical guiding with 45% transmission efficiency is demonstrated in a 100-μm-diameter, 20-cm length capillary. A broadening of the initial spectrum as much as several initial spectrum widths is achieved. Besides the broadening, the mean frequency of the output radiation in the spectrum experiences a blue-shift of up to several initial spectrum widths, caused by the non-stationary, non-linear process of gas ionization. Our numerical simulations, in the form of a simple one-dimensional model for the propagation of intense laser pulses in gas-filled capillaries, are in good qualitative agreement with the experimental results. These simulations show the possibility of significant compression of an output pulse in a simple compression scheme (e.g. a piece of silica glass with normal dispersion), which is very important for obtaining laser pulses with few optical cycles at the millijoule energy level. Received: 25 September 2001 / Revised version: 6 December 2001 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +7-8312/363-792, E-mail: dekart@ufp.appl.sci-nnov.ru     

Rotational Raman scattering dependence on pump polarization

The influence of the ellipticity degree of the pump radiation polarization on threshold energies and energy conversion efficiencies of stimulated rotational Raman scattering (SRRS) are investigated in H₂ with a XeCl excimer laser, by using different focusing geometries. For the first time, it is shown that the dependence of SRRS on pump radiation polarization is also affected by the focusing geometry, i.e. whether low- or high-angle focusing is applied. The experimentally determined threshold energies as a function of the pump radiation polarization are in accordance with theory mainly at high-angle focusing geometries. It is also shown for the first time that gain suppression effects on vibrational Raman scattering have to be considered in understanding the experimental results. Received: 5 January 1999 / Final version: 10 September 1999 / Published online: 8 March 2000     

The effects of annealing on the structure and magnetic properties of CoNi patterned nanowire arrays

NiCo alloy nanowire arrays were fabricated by using an electrochemical method in a porous anodic aluminum oxide (PAO) template. X-ray-diffraction results showed that there was a preferred orientation in the CoNi nanowire arrays with Ni content ranging from 0.2 to 0.8 in the as-electro-deposited state, while a random orientation was observed after the sample was heat treated in an argon atmosphere. When the shape anisotropy is very high in the sample, it is found that the squareness (Mr/Ms) of the hysteresis loops for the samples is only about 0.6 before annealing, but increases to about 0.9 after annealing. All the results are discussed qualitatively and an explanation for the magnetic reversal mechanism in terms of a localized magnetization model is given. As the CoNi alloy nanowire arrays have high bit densities, they can be used as perpendicular magnetic recording media of high-density quantum magnetic disks. Received: 8 January 2001 / Accepted: 6 June 2001 / Published online: 30 August 2001     

A study of some optical properties of hafnium dioxide (HfO2) thin films and their applications

2 ), a series of films ranging in thickness from 50 to 10000 nm was prepared by using an electron beam gun inside an evacuated coating chamber of pressure 1×10^-5 mbar. The films were obtained on optical glass substrate by using oxygen with a backfill pressure of 2.4×10^-4 mbar during the deposition processes. The optical constants of the films were computed in the spectral wavelength region (350–2000 nm) from the transmission, reflection and thickness measurements. A computer program was created to determine two optical parameters n and k of the films, and this was achieved by entering the practical results into the computer program, which solved a series of equations for each wavelength. The effects created by changing various evaporation conditions (thickness, substrate temperature and evaporation rate) were studied in the spectral wavelength range, and the optimum values of the various conditions were obtained while achieving the best optical performance. According to the investigations of the HfO₂ material, two applications of the anti-reflection (AR) multi-layer coatings were achieved in two different spectral wavelength ranges. The first application was measured in the visible and near infra-red (VIS/NIR) range from 500 nm to 850 nm deposited on the glass substrate. The second application was measured in the infrared (IR) range from 7500 nm to 11500 nm deposited on germanium substrate. Computer modelling for designing the optical multi-layer system has been presented. The theoretical formulation and experimental results with the same specification were achieved. The correlation between the theoretical and the experimental results reveals a close agreement that offers a convenient method for predicting and controlling the multi-layer coating. By continuous measurement of the optical and mechanical (durability) performances of the coating process, high-quality films were produced in the manufacture of various optical devices. Received: 16 April 1997/Accepted: 5 August 1997     

Excimer laser ablation of thin gold films on a quartz crystal microbalance at various argon background pressures

Received: 21 June 1996/Accepted: 9 December 1996     

Molecular orientations and photorefractive effects in C60-doped liquid crystals with capillary-flow-induced alignment

Molecular orientations and photorefractive effects of C₆₀-doped nematic liquid crystals were investigated with layer-structured liquid crystal cells where a nematic phase was injected. Regardless of the surface treatments of the layers, liquid crystals injected with nematic phase showed a higher degree of director orientation and photorefractivity than those injected with an isotropic phase. The nematic alignment of liquid crystals in an untreated cell is generated by a capillary flow of liquid crystal during the nematic injection. However, it was observed that the grating formation was delayed a little longer with injection of nematic phase than with injection of isotropic phase. Received: 11 March 2002 / Revised version: 22 April 2002 / Published online: 12 July 2002     

Influence of thermal treatment on second-harmonic generation in silver islands films

A reduction in second-harmonic generation (SHG) efficiency at silver islands films is observed when such submicroscopic structures are tempered at approx. 410K. This behavior can be attributed to two facts: an increase in symmetry in film morphology and changes in the particles' electron plasma resonances causing field enhancements inside the particles. The observed correspondence between submicroscopic surface morphology and obtained second-harmonic signal suggests a SHG microscope for the maplike distinguishing between differently structured surface areas.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Nonlinear refraction of undoped and Fe-doped KTiOAsO4 crystals in the femtosecond regime

The third-order optical nonlinearities in undoped and Fe-doped KTA crystals have been measured using the Z-scan technique with femtosecond pulses at 780-nm wavelength. The nonlinear refractive index is determined to be 1.7×10⁻¹⁵ cm²/W and 0,9×10⁻¹⁵ cm²/W for undoped and Fe-doped KTA, respectively. No two-photon absorption occurs in these crystals. It is shown that doping with Fe₂O₃ could weaken refractive nonlinearity of KTA, suggesting that Fe:KTA will improve the performance of KTA in high-intensity femtosecond laser applications. In addition, the measured nonlinear index of refraction in KTA crystals is about five times lower than that predicted by the two-band theory. One of the reasons for the discrepancy is given as the applicable limit of the simple theory in which a two-parabolic band model has been assumed in the analysis. Received: 3 June 1999 / Published online: 20 October 1999     

Spontaneous transverse pattern formation in a microchip laser excited by a doughnut pump profile

A transition from a pure Laguerre–Gaussian (LG) mode to a pattern of optical vortex lattices in a large-Fresnel-number microchip laser is experimentally demonstrated by controlling the cavity Q-factor. The cooperative frequency locking of nearly degenerate modes is found to be a primary process for the generation of the optical vortex lattices in a class-B laser. When the cavity Q-factor is high enough, a LG-like mode and a structure of optical vortex lattices are found to coexist. Competition between coexisting transverse patterns of different symmetry gives rise to chaotic fluctuations. Received: 20 March 2002 / Revised version: 20 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +886-35/729134, E-mail: yfchen@cc.nctu.edu.tw     

Generation of more than 40 rotational Raman lines by picosecond and femtosecond Ti:sapphire laser for Fourier synthesis

Received: 14 November 1996