Christophe Cassagne

Diode-pumped 99 fs Yb : CaF2 oscillator

We demonstrate the generation of 99 fs pulses by a mode-locked laser oscillator built around a Yb:CaF(2) crystal. An average power of 380 mW for a 13 nm bandwidth spectrum centered at 1053 nm is obtained. The short-pulse operation is achieved thanks to a saturable absorber mirror and is stabilized by the Kerr lens effect. We investigated the limits of the stabilization process and observed a regime slowly oscillating between mode locking and Q switching.

Structure, nonlinear properties, and photosensitivity of (GeSe_2)_100-x(Sb_2Se_3)_x glasses

Chalcogenide glasses from (GeSe2)​100-​x(Sb2Se3)​x system were synthesized, with x varying from 5 to 70, in order to evaluate the influence of antimony selenide addn. on nonlinear optical properties and photosensitivity. Nonlinear refractive index and two photon absorption coeffs. were measured both at 1064 nm in picosecond regime using the Z-​scan technique and at 1.55 μm in femtosecond regime using an original method based on direct anal. of beam profile change while propagating in the chalcogenide glasses. The study of their photosensitivity at 1.55 μm revealed highly glass compn. dependent behavior and quasi-​photostable compns. have been identified in femtosecond regime. To better understand these characteristics, the evolution of the glass transition temp., d. and structure with the chem. compn. were detd.

Nonlinear characterization of materials using the D4σ method inside a Z-scan 4f-system

We show that direct measurement of the beam radius in Z-scan experiments using a CCD camera at the output of a 4f-imaging system allows higher sensitivity and better accuracy than Baryscan. One of the advantages is to be insensitive to pointing instability of pulsed lasers because no hard (physical) aperture is employed as in the usual Z-scan. In addition, the numerical calculations involved here and the measurement of the beam radius are simplified since we do not measure the transmittance through an aperture and it is not subject to mathematical artifacts related to a normalization process, especially when the diffracted light intensity is very low.

Measurements of the third- and fifth-order optical nonlinearities of water at 532 and 1064 nm using the D4σ method

The nonlinear response of liquid water was investigated at 1064 and 532 nm using a Nd:YAG laser delivering pulses of 17 ps and its second harmonic. The experiments were performed using the D4σ method combined with the Z-scan technique. Nonlinear refractive indices of third- and fifth-order were determined, as well as the three-photon absorption coefficient, for both wavelengths. A good agreement was found between theory and experiment.

Degenerate multiwave mixing using Z-scan technique

This study deals with a multiwave mixing experiment using Z-scan technique. We show that it is possible to simplify the complex degenerate four-wave mixing(DFWM) experimental setup in order to characterize nonlinear materials. One object composed of three circular apertures is used as an input in a 4 f coherent imaging system. When the nonlinear material is moved around the focus, we observe the variations of the forward fourth wave at the output as well as other diffracted waves. Experimental and simulated images are presented here to validate our approach. We show also that this method increases significantly the sensitivity of the measurement compared to DFWM. We provide a simple quadratic relation that allows the characterization of the cubic optical nonlinearity.

Nonlinear properties of unfilled d shell metal porphyrins of 5,10,15,20-tetraphenyl-21H, 23H-porphine cobalt(II) et 5,10,15,20-tetrakis(4-methoxyphenyl)-21H, 23H-porphine cobalt(II) using D4 σ-Z-scan

We study the nonlinear (NL) absorption and refraction of 5,10,15,20-tetraphenyl-21H, 23H-porphin cobalt(II) (CoTPP) and 5,10,15,20-tetrakis(4-methoxyphenyl)-21H, 23H-porphin cobalt(II) (MCoTPP). The measurements were carried out in solutions prepared with chlorobenzene applying the D4σ-Z-scan technique with a laser delivering single pulses in the picosecond regime at 532nm. For each compounds, the NL coefficients were measured for three concentrations. The results show that these materials with unfilled d shell exhibit a saturable absorption (SA), where they have been compared to the basic structure tetraphenylporphyrin (TPP) and zinc tetraphenylporphyrin (ZnTPP) with filled d shell metal in order to see the influence of the central metal ion on the NL absorption (NLA) behavior.

Investigation of the third-order nonlinear optical properties in porphyrin solutions in the picosecond regime

The D4σ-Z-scan technique is used to evaluate the sign and the magnitude of the nonlinear (NL) refractive index and the NL absorption (NLA) coefficient with a laser delivering single pulses in the picosecond regime at 355 nm, 532 nm and 1064 nm. The NL optical response of the zinc porphyrins (Pph) has been found to be significantly enhanced. The study of the NLA and refraction is performed, taking into account the time dependence and the spectral width of the incident laser. The variations of the NL coefficients as a function of the intensity and the concentration are provided and are found to be linear.

Dark-field Z-scan technique with highly nonlinear absorbing materials: Application to porphyrins

The newly introduced imaging Dark-field Z-scan (DFZ-scan) technique is applied here to demonstrate its ability to measure the optical third-order nonlinear (NL) index coefficient in the presence of nonlinear absorption (NLA). We present our experimental results for two compounds of Porphyrin molecules, known to have relatively high NLA in the picosecond regime, at 532nm, allowing to test both cases of positive and negative nonlinear refraction (NLR). Moreover, we show that, it is possible to increase the sensitivity of the method by introducing a circular aperture in the image plane. Also, limitations are discussed to process optically a pure NL refractive signal, not subject to the influence of the NLA one, at the output of the imaging setup.

Nonlinear optical characterization of tetraphenylporphyrin in the picosecond regime

Applying the D4σ-Z-scan technique with a laser delivering single pulses in the picosecond regime at 1064 nm, 532 nm, and 355 nm we report on the nonlinear (NL) optical properties of 5,10,15,20-Tetraphenyl-21H,23H-porphine (TPP) and 5,10,15,20-Tetraphenyl-21H,23H-porphine zinc (ZnTPP) solutions prepared in chlorobenzene. Accurate measurements of the NL coefficients for 4 concentrations in both compounds are provided. The results show no response in the IR for both materials, negative NL refraction for TPP in the green changing sign to positive for ZnTPP and positive NL refraction in the UV for both compositions. The NL absorption of these materials is quite huge in the visible range where applications for optical limiting are needed.

Synthesis and nonlinear optical properties of zirconia-protected gold nanoparticles embedded in sol–gel derived silica glass

A new approach to dope a silica glass with gold nanoparticles (GNPs) is presented. It consisted in embedding zirconia-coated GNPs in a silica sol to form a doped silica gel. Then, the sol-doped nanoporous silica xerogel is densified leading to the formation of a glass monolith. The spectral position and shape of the surface plasmon resonance (SPR) reported around 520 nm remain compatible with small spherical GNPs in a silica matrix. The saturable absorption behavior of this gold/zirconia-doped silica glass has been evidenced by Z-scan technique. A second-order nonlinear absorption coefficient β of about −13.7 cm GW−1 has been obtained at a wavelength near the SPR of the GNPs.