Ion Cohanoschi

Three- and four-photon absorption of a multiphoton absorbing fluorescent probe

The utility of multiphoton excitation processes has been the subject of increased attention due to their potential applications in biophotonics, biology, and medicine through three-dimensional fluorescence imaging and photodynamic therapy. Evidence of this are the multiple applications of two-photon absorption (2PA) in fluorescence spectroscopy and 3D imaging over the last several years because of its large effective Stoke¿s Shift and high spatial resolution.[l,2] However, because the irradiation penetration depth of 2PA is limited in medical and biological applications due to the unwanted absorption and scattering when two red photons are used, the scientific communib recently started to explore higher order absorption processes at longer wavelengths such as three- (3PA) and four-photon absorption (IPA) that minimize the scattered light losses, and reduce the unwanted linear absorption in the living organism transparency window.

Volume-chirped Bragg gratings: monolithic components for stretching and compression of ultrashort laser pulses

Abstract. An innovative type of optical component—a volume Bragg grating—has recently become available commercially and has found wide applications in optics and photonics due to its unusually fine spectral and angular filtering capability. Reflecting volume Bragg gratings, with the grating period gradually changing along the beam propagation direction (chirped Bragg gratings—CBGs) provide stretching and recompression of ultrashort laser pulses. CBGs, being monolithic, are robust devices that have a footprint three orders of magnitude smaller than that of a conventional Treacy compressor. CBGs recorded in photo-thermo-refractive glass can be used in the spectral range from 0.8 to 2.5 μm with the diffraction efficiency exceeding 90%, and provide stretching up to 1 ns and compression down to 200 fs for pulses with energies and average powers exceeding 1 mJ and 250 W, respectively, while keeping the recompressed beam quality M2<1.4, and possibly as low as 1.1. This paper discusses fundamentals of stretching and compression by CBGs, the main parameters of the gratings including the CBG effects on the laser beam quality, and currently achievable CBG specifications.

Viscosity dependence of optical limiting in carbon black suspensions

We measure the optical limiting behavior of carbon black suspensions in various viscosity solvents by using a 10-Hz repetition rate, 532-nm, 5-ns pulsed laser. We found that, for common solvents used in the past such as water and ethanol, the limiting behavior ceases after a few laser firings and a turnover in the limiting curve appears. This can be explained by depletion of the carbon black within the focal volume. This turnover shifts to lower energies as the viscosity of the solvent becomes greater. However, for low viscosity liquids, such as carbon disulfide or pentane, the limiting is unaffected by the repetition rate, at least for frequencies up to 10 Hz, because of diffusion of the carbon black particles. This diffusion allows fresh material to replace the irradiated volume within the time between pulses.

Effect of the concentration of organic dyes on their surface plasmon enhanced two-photon absorption cross section using activated Au nanoparticles

In this article we present the study of the surface plasmon enhanced two-photon absorption of a hydrophilic stilbene derivative (trans-4,4’-diaminostilbene) in aqueous solution at different concentrations. The observed exponential growth of the effective two-photon absorption cross section [σ2′(Au)] is attributed to the electric-field augmentation via surface plasmon resonance between nanoparticles, i.e., hot spots, and the molecular density on Au nanospheres. An unprecedented σ2′(Au)=550 000 GM has been measured. This result opens a new universe of applications in multiphoton imaging, photodynamic therapy, telecommunications, optical limiting, and multidimensional data storage using hybrid systems.

Thermal tuning of volume Bragg gratings for high power spectral beam combining

A tabletop kW-level spectral beam combining (SBC) system using volume Bragg gratings (VBGs) recorded in photothermo- refractive (PTR) glass was presented at the last meeting [1]. Diffraction efficiency of VBGs close to 100% was demonstrated. However, when using VBGs for spectral beam combining, it is important to ensure high diffraction efficiency for the diffracted beam and low diffraction efficiency for the transmitted beams simultaneously. The unique, unmatched properties of VBGs allow spectral beam combining achieving this condition at wavelengths with less than 0.25 nm separation. We present modeling of reflecting VBGs for high power SBC that takes into account laser spectral bandwidth, beam divergence, PTR-glass scattering losses, and grating non-uniformity. A method for optimization of VBG parameters for high-efficiency SBC with an arbitrary number of channels is developed. Another important aspect of spectral beam combiner design is maintaining high diffraction efficiency as the temperature of beam-combining VBGs changes during operation due to absorption of high power radiation. A new technique of thermal tuning of large aperture VBGs, designed to maintain high efficiency of beam combining without mechanical adjustment over a wide range of laser power, is developed. Finally, these tools are used to demonstrate a robust and portable 5-channel SBC system with near diffraction limited spectrally-combined output beam.

Cross-correlation technique for dispersion characterization of chirped volume Bragg gratings

We propose using cross-correlation frequency-resolved optical gating for dispersion characterization of optical elements with high dispersion, such as ultrashort pulse stretchers and compressors. The technique is based on spectrally resolved second-order cross correlation (sum frequency generation) of a stretched pulse with a reference short pulse. Dispersion of optical elements with a high pulse stretching ratio can be completely characterized using this method, even with moderate resolution of spectral measurements of the cross-correlation signal. The proposed technique is used to measure dispersion of a chirped Bragg grating recorded in photo-thermo-refractive glass. It was found that dispersion of these gratings is almost linear with wavelength and has approximately the same slope with opposite signs for two orientations of the grating with opposite faces of the grating used as the input face. Analysis of higher-order dispersion of the grating shows some variations of dispersion across the aperture of the grating, mostly in the amount of third-order dispersion contribution.

Efficient chirped Bragg gratings for stretching and compression of high energy ultrashort laser pulses at 800 nm spectral region

This paper reports on new generation of recorded in PTR glass chirped gratings with enhanced beam quality and efficiency for stretching and compression of ultrashort laser pulses in 800 nm spectral region.

The impact of the π-conjugation length on the three-photon absorption cross-section of fluorene derivatives

We report the effect of pi-conjugation length on three-photon absorption cross-section of fluorene derivatives with different structural motifs. We demonstrate that three-photon absorption is enhanced by extending the conjugated bridge of those systems.

Structure-property relationship for three-photon absorbing fluorene derivatives: a systematic study

We report the first systematic three-photon absorption structure/property study on a series of three fluorene derivatives with different structural motifs. We demonstrate that symmetric intramolecular charge transfer enhances three-photon absorption on conjugated systems.

Two-photon-induced excited-state absorption in high-index fibers

We have studied the nonlinear optical properties of a high-index (n = 1.82) glass that is used as the core material in a commercially available fiber optic inverter, which is a coherent fiber bundle twisted 180 degrees to produce an inverted image. We have determined through open aperture Z-scan the two-photon absorption coefficient of the glass to be 0.8 cm/GW using 23 ps pulses (FWHM) at 532 nm, far from the linear absorption edge of 320 nm. For 5 ns (FWHM) pulses the nonlinear absorption is much larger, and is dominated by two-photon induced excited-state absorption. These effects contribute to the nanosecond optical limiting response that we have observed for the inverter using an F/5 focusing geometry.