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We present a detailed analysis of the mode-locker consisting of a nonlinear crystal and a dichroic mirror, introduced by Stankov and known as the frequency-doubling nonlinear mirror (FDNLM). Our aim is to determine optimal values for two physical parameters: the conversion efficiency η 0 of the nonlinear crystal, and the reflection coefficient R ω of the output mirror. The optimization is based on a set of three figures of merit: the reflected intensity , the pulse shortening ratio and the Gaussian shape factor , which are combined to yield a final decision factor . Experimental investigations of η 0 and R ω carried out using a FDNLM mode-locked Nd:YAG laser show good agreement with the theoretical predictions. In addition, a comparative study with other available experimental results is presented. This work demonstrates the capacity of this method to evaluate the best performance of this mode-locking not only for the steady-state pulse domain but also in the transient one.

The vibrational response of p-nitrothiophenol (p-NTP) self-assembled monolayers (SAMs), on Pt and on Au, has been investigated by combining theoretical methods with vibrational spectroscopies. Experimentally, the vibrational signatures have been measured using infrared spectroscopy (IR), polarization-modulation reflection absorption infrared spectroscopy (RAIRS), and with sum frequency generation spectroscopy (SFG). Theoretically, density functional theory calculations (DFT) have predicted the molecular vibrations and have estimated their IR vectors and Raman tensors, necessary to simulate the spectra and therefore to interpret the vibrational signatures as well as to retrieve the molecular orientation. So, a tilt angle (v) of 60° for the molecular 1,4-axis of p-NTP has been directly estimated for both Pt and Au SAMs by the polarizationdependent SFG data. Then, combining these results with RAIRS measurements has allowed to determine rotation angles (ξ) of 56° and 66° for p-NTP SAMs on Pt and Au, respectively.

Polarisation-dependent sum frequency generation (SFG) spectroscopy is used to investigate the orientation of molecules on metallic surfaces. In particular, self-assembled monolayers (SAMs) of dodecanethiol (DDT) and of p-nitrothiophenol (p- NTP), grown on Pt and on Au, have been chosen as models to highlight the ability of combining ppp and ssp polarisations sets (representing the polarisation of the involved beams in the conventional order of SFG, Vis and IR beam) to infer orientational information at metallic interfaces. Indeed, using only the ppp set of data, as it is usually done for metallic surfaces, is not sufficient to determine the full molecular orientation. We show here that simply combining ppp and ssp polarisations enables both the tilt and rotation angles of methyl groups in DDT SAMs to be determined. Moreover, for p-NTP, while the SFG active vibrations detected with the ppp polarisation alone provide no orientational information, however, the combination with ssp spectra enables to retrieve the tilt angle of the p- NTP 1,4 axis. Though orientational information obtained by polarisation-dependent measurements has been extensively used at insulating interfaces, we report here their first application to metallic surfaces. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.

The ability of microcontact printing to build highly ordered alkanethiol self-assembled monolayers (SAMs) on Pt substrates within a short time is studied by sum frequency generation (SFG) spectroscopy and contact angle measurements. The deposition of ordered hexadecanethiol and dodecanethiol monolayers onto platinum substrates is achieved in less than 1 s. The film order and the alkane chain orientation are deduced from the SFG fingerprint acquired under different sets of laser polarization. Comparisons between the SAMs prepared by printing or by immersion demonstrate that both methods lead to the same high quality organization. Patterning effects within printed films are also investigated with respect to the layer conformation. Finally, wetting properties of printed layers are correlated with the printing duration, corroborating the spectroscopic results.

International audience; The anchoring of thiolated single-stranded DNA (HS-ssDNA) monolayers onto platinum substrates was investigated by sum-frequency generation spectroscopy. Different buffer solutions were used for the preparation of the adlayers. Vibrational fingerprints in the 2700–3100 cm−1 spectral range showed the intercalation of Tris/EDTA (TE) buffer molecules within the HS-ssDNA self-assembled monolayer. Buffer contribution to SFG can be quenched either by using SFG inactive molecules like KH2PO4/K2HPO4/NaCl (PBS) or by repeated rinsing of the DNA layer with pure water. Comparing the SFG spectra of HS-ssDNA and mercaptohexanol (MCH), which had been self-assembled onto the same substrate, enabled us to infer ordering of the anchor arms and strong disordering of the DNA strands of HS-ssDNA monolayers self-assembled on platinum.

Optical parametric oscillators (OPOs) are attractive tools for research on tissue ablation upon infrared irradiation. Here, we report on the performance of several mid-infrared nonlinear crystals, namely type I and type II AgGaS2 (AGS) and type I CdSiP2 (CSP), used in synchronously-pumped OPOs tuned to a wavelength of 6.45 µm, coinciding with the amide II absorption band of proteins. CSP-based OPOs clearly exhibit better performance in comparison to AGS: First, the oscillation threshold with CSP is three (five) times lower than type II (type I) AGS. Second, the idler conversion efficiency is more favourable for CSP and allows reaching 27.5 mW of idler average power, while 13 and 6 mW are obtained with type II and type I AGS, respectively. Such performance makes CSP suitable for high power 6.45 µm surgical applications. Preliminary ablation experiments on liver tissues with our CSP-based OPO highlight the promising future of CSP in medical applications.

The molecular orientation of adsorbed molecules forming selfassembled monolayers can be determined by combining vibrational sum-frequency generation (SFG) measurements with quantum chemical calculations. Herein, we present a theoretical methodology used to simulate the SFG spectra for different combinations of polarizations. These simulations are based on calculations of the IR vectors and Raman tensors, which are obtained from density functional theory computations. The dependency of the SFG vibrational signature with respect to the molecular orientation is presented for the molecules p-nitrothiophenol and 2,4-dinitroaniline. It is found that a suitable choice of basis set as well as of exchange-correlation (XC) functional is mandatory to correctly simulate the SFG intensities and consequently provide an accurate estimation of the adsor bed molecule orientation. Comparison with experimental data shows that calculations performed at the B3LYP/6-311 + + G(d,p) level of approximation provide good agreement with experimental frequencies, and with IR and Raman intensities. In particular, it is demonstrated that polarization and diffuse functions are compulsory for reproducing the IR and Raman spectra, and consequently vibrational SFG spectra, of systems such as p-nitrothiophenol. Moreover, the investigated XC functional reveal their influence on the relative intensities, which show rather systematic variations with the amount of Hartree-Fock exchange. Finally, further aspects of the modeling are revealed by considering the frequency dependence of the Raman tensors.

International audience; For the first time, the sum-frequency generation (SFG) spectra of surface species have been measured in porous silicon (PS) layers. The range of the vibrations 2000-2300 cm-1 for the native species Si-H x has been explored. Furthermore for the studied mesoporous materials, it has been found that, as opposed to the infrared absorption, the SFG signal increases with porosity. Finally, the angular dependence and the enhancement of the SFG signal were measured in a one-dimensional PS photonic crystal structure with a defect producing an energy level in the photonic band gap.

Infrared/visible sum-frequency generation (SFG) spectroscopy is used to study the recognition of a protein (avidin) by a derived vitamin (biocytin) adsorbed on a calcium fluoride substrate. The specificity of the process is tested by replacing avidin with bovine serum albumin or presaturated avidin. The SFG spectroscopy shows drastic modifications in the CH and NH spectral ranges only upon exposure of the biocytin film to avidin. The comparison of the SFG data with Fourier transform infrared reflection absorption spectra (FT-IRRAS) in the same spectral ranges illustrates the advantages of nonlinear spectroscopy for studying and detecting recognition between biomolecules.

Vibrational information recorded by infrared-visible sum frequency generation spectroscopy was used to study the adsorption of a derivated vitamin (biocytin) on different substrates and its subsequent reaction with a protein (avidin). No reaction is observed on metallic substrates. When the experiments are carried out with a CaF2 substrate in the total internal reflection configuration, significant changes of the CH and NH vibrations can be related to the specific bonding of avidin to biocytin.

Two-color sum-frequency generation spectroscopy (2C-SFG) is used to probe the molecular and electronic properties of an adsorbed layer of the green fluorescent protein mutant 2 (GFPmut2) on a platinum (111) substrate. First, the spectroscopic measurements, performed under different polarization combinations, and atomic force microscopy (AFM) show that the GFPmut2 proteins form a fairly ordered monolayer on the platinum surface. Next, the nonlinear spectroscopic data provide evidence of particular coupling phenomena between the GFPmut2 vibrational and electronic properties. This is revealed by the occurrence of two doubly resonant sum-frequency generation processes for molecules having both their Raman and infrared transition moments in a direction perpendicular to the sample plane. Finally, our 2C-SFG analysis reveals two electronic transitions corresponding to the absorption and fluorescence energy levels which are related to two different GFPmut2 conformations: the B (anionic) and I forms, respectively. Their observation and wavelength positions attest the keeping of the GFPmut2 electronic properties upon adsorption on the metallic surface.

We demonstrate the ability of doubly resonant sum–frequency generation (DR-SFG) to investigate electron–phonon couplings at C 60 –metal interfaces. Due to its coupling to electronic transitions, the totally symmetric A g (2) vibration of C 60 exhibits a huge enhancement of its nonlinear response for sum–frequency energies above the molecular electronic gap. We attribute this resonance to the coupling of the pentagonal pinch mode with the t 1u lowest unoccupied molecular orbital (LUMO) of C 60 .

Laser tunability from 10 to 21 microm is obtained by use of an optical parametric oscillator based on a KTP crystal followed by a difference-frequency stage with a CdSe crystal. An all-solid-state picosecond Nd:YAG oscillator mode locked by a frequency-doubling nonlinear mirror is used for synchronous pumping.