J. Oberlé

Wide-field optical coherence tomography: imaging of biological tissues

We describe a two-dimensional optical coherence tomography technique with which we were able to obtain multiple longitudinal slices of a biological sample directly in a single Z scan. The system is based on a femtosecond Cr4+:forsterite laser and an infrared camera for wide-field imaging of the sample with a depth resolution of 5 microm. With this imaging apparatus we were able to investigate human skin and mouse ear samples and to observe the different constitutive tissues.

Photophysics of 4-dimethylamino 4′-cyanostilbene and model compounds: dual excited states revealed by sub-picosecond transient absorption and Kerr ellipsometry

4-Dimethylamino 4′-cyanostilbene (DCS) and two selectively bridged compounds are investigated using sub-picosecond time-resolved absorption and Kerr ellipsometry experiments. The latter technique makes it possible to work at low excitation energy and low concentrations, thereby avoiding intensity and concentration effects which exist in DCS derivatives. Using this technique, in a non-polar solvent (cyclohexane), only the presence of a single excited state is observed for all studied compounds. In polar solvents, the bridged derivative where twisting of the anilino moiety is prevented but double bond twisting is allowed also reveals the presence of only one excited state. On the other hand, in polar solvents, for the DCS compound and the related bridged derivative where the anilino moiety is still able to twist, a precursor-successor relationship is clearly observed between two different excited states. These results can be understood within a four excited states model derived from a previously suggested diagram: the DE state (delocalized excited state), the ICT state (internal charge transfer, highly polar, nearly planar configuration, formed quasi-instantaneously from the DE state by electronic reorganization), the CRICT state (conformational relaxed ICT state, highly polar and fluorescent, involves conformational geometric changes such as twisting of the anilino group which enhances charge transfer) and the “phantom” state P∗ on the trans⇄cis isomerization pathway (twisted double bond, low polar, non-fluorescent). In this study, we observed the formation of the CRICT excited state on a time scale ranging from 2 to 20 ps depending on the solvent characteristics (polarity, viscosity and hydrogen bonding ability).

Real-time two-dimensional imaging in scattering media by use of a femtosecond Cr 4+ :forsterite laser

An original femtosecond Cr(4+):forsterite laser source associated with a nonlinear optical correlator was used for imaging through scattering media with 1220-nm light. The system, which operates as an ultrafast optical gate by sum-frequency generation in a nonlinear crystal, was able to detect the light reflected from a resolution chart hidden in a turbid medium, at an attenuation of as much as 15 mean free paths. When the object was illuminated with a collimated beam, real-time two-dimensional images were obtained, with a maximum transverse resolution of ~20 microm.

Dual excited states in 4- dimethylamino 4′-cyanostilbene (DCS) revealed by sub-picosecond transient absorption and Kerr ellipsometry

4-dimethylamino 4′-cyanostilbene (DCS) and p-dimethylamino, p′-cyano, 1-1′-bi-indanylidene (substituted “stiff stilbene” DCS-1324) are investigated using sub-picosecond time-resolved absorption and Kerr ellipsometry experiments. The latter technique makes it possible to work at low excitation energy and low concentration, thereby avoiding intensity and concentration effects which exist in DCS derivatives. Using this technique, in non-polar solvent (cyclohexane), only the presence of a single excited state is observed for the two studied compounds. In polar solvents, DCS-1324 derivative where twisting of the anilino moiety is prevented but double-bond twisting is allowed also reveals the presence of only one excited state. On the other hand, in polar solvents, for the DCS a precursor-successor relationship is clearly observed between two different excited states. These results can be understood within a four excited states model derived from a previously suggested diagram: the delocalized excited (DE) state, the internal charge transfer (ICT) state (highly polar, nearly planar configuration, formed quasi instantaneously from the DE state by electronic reorganization), the conformational relaxed ICT (CRICT) state (highly polar and fluorescent, involves conformational geometric changes such as twisting of the anilino group which enhances charge transfer) and the “phantom” state P∗ on the trans ↔ cis isomerization pathway (twisted double bond, lowly polar, non-fluorescent).

Influence of Cr4+ ion concentration on cw operation of forsterite laser and its relation to thermal problems

Abstract The output power characteristics of a cw chromium-doped forsterite laser were measured at 270 K for crystals with different concentrations of Cr 4+ ions. A maximum output power of 900 mW was obtained with the lowest concentration of ions in the laser crystal, when it absorbed 7.6 W of the Nd:YAG pump laser at a wavelength of 1064 nm. It is shown that thermal equilibrium inside the crystal strictly controls the output power.

Structural modelling of optical and electrochemical properties of 4-aminodiphenylamines – optoelectronic studies on a polyaniline repeating unit

Amino-diphenylanilines and their planarized and twisted model compounds have been investigated by steady state and time-resolved absorption and emission, as well as by spectroelectrochemistry. These polyaniline model compounds show that the observation of excited states with full charge separation is linked to molecular twisting where the diaminobenzene is the donor and the phenyl group the acceptor. The observable charge transfer fluorescence shows the characteristic features of twisted intramolecular charge transfer (TICT) excited states, i.e. forbidden emissive properties and strong solvatochromic red shift. The transient absorption spectrum of the TICT state matches the ground state absorption spectrum of the electrochemically produced radical cation of the molecule. This is the first example where excited-state properties of the neutral and ground state properties of the radical cation are directly linked.

Femtosecond optical pulse shaping for tunable terahertz pulse generation

We report on the generation of phase-locked terahertz pulse pairs and tunable narrow-band terahertz pulses by means of optical pulse shaping combining a liquid crystal spatial light filter and optical rectification in ZnTe. They are generated through simple sinusoidal and/or triangular phase modulations and do not require any optimization algorithm. The corresponding spectra are tunable between 0.5 and 2.5 THz with a spectral bandwidth as narrow as 140 GHz.

Optical-limiting properties of a push–pull diphenyl-butadiene

Abstract The optical-limiting behavior of 1-( p - N , N -dimethylamino)-4-( p -cyanophenyl)-1,3-butadiene (DCB) solution in dichloromethane was experimentally and theoretically studied at 532 nm with nanosecond and picosecond pulses. Energy-dependent transmissivity measurements were fitted using a three-level molecular model including two-photon absorption. The best fits were obtained with very high two-photon absorption cross-sections, indicating that two-photon absorption is the predominant mechanism of the strong optical limiting behavior observed in a DCB solution.

Impact of dispersion, free carriers, and two-photon absorption on the generation of intense terahertz pulses in ZnTe crystals

We study the evolution of the energy and the spectrum of a terahertz wave generated by optical rectification of an ultrashort laser pulse in ZnTe crystals with different thicknesses. As the pump intensity increases, we observe a shift in the terahertz spectrum toward lower frequencies. Moreover, at high pump intensities, in disagreement with common sense, thin crystals have a better conversion efficiency than thicker ones. These phenomena are accounted for by the pump depletion induced by two-photon absorption, pulse broadening and the impact of the photoinduced free carriers on the complex refractive index of the crystal in the terahertz range.We study the evolution of the energy and the spectrum of a terahertz wave generated by optical rectification of an ultrashort laser pulse in ZnTe crystals with different thicknesses. As the pump intensity increases, we observe a shift in the terahertz spectrum toward lower frequencies. Moreover, at high pump intensities, in disagreement with common sense, thin crystals have a better conversion efficiency than thicker ones. These phenomena are accounted for by the pump depletion induced by two-photon absorption, pulse broadening and the impact of the photoinduced free carriers on the complex refractive index of the crystal in the terahertz range.

Experimental determination of the nonlinear refractive index in an operating Cr:forsterite femtosecond laser

Abstract The characteristics of a femtosecond forsterite Kerr-lens mode-locked laser pumped synchronously by an Nd:YAG laser are reported. Transform-limited femtosecond pulses of 60-fs (FWHM) duration with average TEM00 output powers of 200 mW at 1.25 μm were generated. Experimental studies of power stability and self-starting regime are described and a new experimental method to determine the nonlinear refractive index of the chromium-doped forsterite crystal (which is either the laser rod) is reported. We infer a value of 6 × 10−16 cm2/W at 1.25 μm, making possible Kerr-lens mode-locking.