L. Misoguti

Shaped-pulse optimization of coherent emission of high-harmonic soft X-rays

When an intense laser pulse is focused into a gas, the light–atom interaction that occurs as atoms are ionized results in an extremely nonlinear optical process—the generation of high harmonics of the driving laser frequency. Harmonics that extend up to orders of about 300 have been reported, some corresponding to photon energies in excess of 500 eV. Because this technique is simple to implement and generates coherent, laser-like, soft X-ray beams, it is currently being developed for applications in science and technology; these include probing the dynamics in chemical and materials systems and imaging. Here we report that by carefully tailoring the shape of intense light pulses, we can control the interaction of light with an atom during ionization, improving the efficiency of X-ray generation by an order of magnitude. We demonstrate that it is possible to tune the spectral characteristics of the emitted radiation, and to steer the interaction between different orders of nonlinear processes.

Optical properties of L-alanine Organic Crystals

This work reports on the optical properties of L-alanine crystals grown from aqueous solution using the well-known slow evaporation method. Linear properties including absorption spectra and refractive indices are determined, as well as nonlinear properties of second harmonic generation (SHG), such as the phase-matching loci and efficiency. Although the SHG efficiency is smaller than those of other known materials, the study of L-alanine crystals is very important since it is part of more complex aminoacids including L-arginine and asparagine, with much higher SHG efficiency. Some mechanical properties are also presented, namely hardness and thermal expansion coefficients.

Optical properties of L-threonine crystals

Abstract Second- and third-order optical nonlinearities of L-threonine amino acid crystals grown from aqueous solution are investigated in this work. The absorption spectra indicate a good transparency between 250 and 1500 nm. Refractive indices were measured as a function of temperature and wavelength, allowing the determination of the thermo-optic and Sellmeier equation coefficients. Phase-matching curves for second harmonic generation from radiation at 1.064 μm were obtained. The conversion efficiency was measured along the entire phase-matching loci and its maximum value was found to be comparable to that of KDP. The nonlinear refractive index n 2 , measured through the Z-scan technique with femtosecond pulses, is of the order of 10 −20 m 2 /W.

Z-scan measurements using femtosecond continuum generation

We present a single beam Z-scan technique using an intense, broadband, white-light continuum (WLC) beam for the direct measurement of nonlinear absorption spectra. In order to demonstrate the validity of our technique, we compared the results of tetraaniline and Sudan 3 solutions obtained with WLC and conventional single wavelength light sources. Both approaches lead to the same nonlinear spectrum, indicating that the association of the Z-scan technique and the WLC source results in an useful method for the measurement of nonlinear spectra of both absorbing (saturable absorption or reverse saturable absorption) and transparent (two-photon absorption) samples.

Two-photon absorption in azoaromatic compounds

Abstract We report on the two-photon absorption (2PA) cross-section for eight azoaromatic compounds in dimethyl sulfoxide solutions, using the Z-scan technique with femtosecond laser pulses at 775 nm. The 2PA cross-sections of these molecules are of the order of (1–6)×10 −50 cm 4 s photon −1 . A correlation was established between the molecular structure and the two-photon absorption process, which makes it possible to develop molecular design strategies to create structures with suitable two-photon absorption coefficients.

Characterization of dynamic optical nonlinearities with pulse trains

We report a simple extension of the Z-scan technique where pulse trains characteristic of a Q-switched and mode-locked Nd:YAG laser are employed to investigate the dynamics of third-order optical nonlinearities. The method allows discriminating between fast and accumulative nonlinearities, and was applied to the investigation of liquid samples, namely chloroform, benzene, and a solution of Disperse Red 1 dye.

Two-photon absorption cross-section spectrum of a π-conjugated polymer obtained using the white-light continuum Z-scan technique

We have used the white-light continuum (WLC) Z-scan technique to determine the degenerate two-photon absorption (2PA) cross-section spectrum of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene). The results obtained show good agreement with those employing a single-wavelength source, although much faster because of the wavelength multiplexing introduced by the use of a broadband source. The improved spectral resolution of the WLC Z-scan allowed the observation of a 2PA peak around 675nm, a subtle feature in the nonlinear spectrum that could be fitted using a sum-over-states model. The WLC Z-scan technique has proved to be an important tool to determine the 2PA spectrum of polymers.

Characterization of dynamic optical nonlinearities in ytterbium bis-phthalocyanine solution

Abstract Dynamic optical nonlinearities of ytterbium bis-phthalocyanine (YbPc 2 ) dissolved in chloroform were characterized using the Z-scan technique with pulse trains. Two contributions of opposite signs were observed for the nonlinear refraction: a fast process related to the singlet population, and a slow accumulative contribution arising from the triplet population and thermal lensing. As the fluence increases, the nonlinear absorption first presents a weak saturation, followed by a reverse saturable absorption process. We used a six-energy-level diagram to explain these results and to obtain spectroscopic parameters such as the excited state cross-sections and the intersystem crossing lifetime.

Two-photon absorption in perylene derivatives

This work reports on the two-photon absorption (2PA) cross-section for bis (n-butylimido) perylene (BuPTCD), bis (benzimidazo) perylene (AzoPTCD), bis (benzimidazo) thioperylene (Monothio BZP), bis (phenethylimido) perylene (PhPTCD), bis (benzylimido) perylene (BePTCD) and n-pentylimido-benzimidazo perylene (PAzoPTCD) solutions measured using the Z-scan technique with femtosecond laser pulses at 775 nm. All perylene derivatives studied present large 2PA cross-sections, only comparable to the best ones reported in the literature. The experimental 2PA cross-sections obtained for each PTCD compound are in good agreement with a simplified sum-over-state calculation. The results of the present work indicates perylene derivatives as promising materials for two-photon applications.

Phase matching in cascaded third-order processes

We present an analytical, computational, and experimental study of frequency mixing and propagation that takes place in the guided-wave nonlinear frequency conversion of ultrashort pulses from the near infrared to the far and the vacuum ultraviolet. In addition to the standard phase-matching conditions for frequency mixing, we observe that cascaded frequency conversion, in which one of the input frequency components is generated simultaneously, is subject to two additional phase-matching conditions. In cascaded quasi-phase matching, one of the input fields oscillates in strength with a period that matches the phase mismatch. In gain-assisted phase matching, the growth of one of the input fields results in growth of the signal, in spite of a phase mismatch for the final mixing process.