Diego Rativa

Third-order nonlinear optical properties of bismuth-borate glasses measured by conventional and thermally managed eclipse Z scan

Third-order nonlinearity one order of magnitude larger than silica is measured in bismuth-borate glasses presenting a fast response (<200 fs). The results for the sign and magnitude of the nonlinearity were obtained using a combination of the eclipse Z scan with thermal nonlinearity managed Z scan, whereas the Kerr shutter technique was employed to obtain the electronic time response of the nonlinearity, all performed with 76 MHz repetition rate 150 fs pulses at 800 nm. Conventional Z scans in the picosecond regime at 532 and 1064 nm were also independently performed, yielding the values of the third-order nonlinear susceptibilities at those wavelengths. The results obtained for the femtosecond response, enhanced third-order nonlinearity of this glass (with respect to silica), place this glass system as an important tool in the development of photonics devices. Electro-optical modulators, optical switches, and frequency converters are some of the applications using second-order nonlinear properties of the Bi-glass based on the rectification model.

Thermally managed eclipse Z-scan

We report a new variation of the conventional Z-scan method to characterize the third-order optical nonlinearity of photonic materials. By exploiting the combination of the eclipse Z-scan with a thermal nonlinearity management technique, we demonstrate an improvement in sensitivity and flexibility of the method to simultaneously characterize the thermal and nonthermal nonlinearity of optical materials. The method is demonstrated by measuring the nonlinear refractive index in CS(2), SiO(2) and H(2)O, standard materials, and also in a biomaterial, the amino acid Tryptophan in water solution, using a femtosecond Ti-Sapphire laser operating at 76MHz repetition rate.

One photon nonresonant high-order nonlinear optical properties of silver nanoparticles in aqueous solution

In this work we determine the third, fifth- and seventh-order nonresonant nonlinear optical properties of silver nanoparticles (9 nm average diameter) colloids in aqueous solution under high intensity excitation. The nonlinear optical response and its dependence with the nanoparticles filling factor was measured and theoretically described. We show that for low inclusion concentration, the third order nonlinearity of the colloid can be described by the generalized Maxwell-Garnett model. With the increase of the nanoparticle concentration, changes in the medium nonlinearities was observed leading to high order effects. The fifth- and seventh- order susceptibilities were obtained for highly concentrated silver nanoparticle colloid and the data was supported by a theoretical model. The conventional Z-scan technique was employed, using 80 f s laser pulses at 800 nm, in a regime of high pulse energy (microJ) and low repetition rate (1 kHz).

Ultrasmall spot size scanning laser ophthalmoscopy

An ultrasmall spot size scanning laser ophthalmoscope has been developed that employs an annular aberration-corrected incident beam to increase the effective numerical aperture of the eye thereby reducing the width of the probing light spot. Parafovea and foveal cone photoreceptor visibility determined from small area retinal image scans are discussed from the perspective of mode matching between the focused incident beam and the waveguide modes of individual cones. The cone visibility near the fovea centralis can be increased with the annular illumination scheme whereas the visibility of larger parafovea cones drops significantly as a consequence of poorer mode match. With further improvements of the implemented wavefront correction technology it holds promise for individual cone-photoreceptor imaging at the fovea centralis and for optical targeting of the retina with increased resolution.

Wavefront sensing with an axicon

The use of a large apex-angle axicon for common-path interferometric wavefront sensing is proposed. The approach is a variant of point-diffraction interferometry bearing similarities to pyramidal wavefront sensing. A theoretical basis for wavefront sensing with an axicon is developed, and the outcomes of numerical simulations are compared to experimental results obtained with spherical and cylindrical ophthalmic trial lenses. It is confirmed that the axicon can be used for wavefront sensing, although its refraction may ultimately complicate and limit its operational range.

Absence of an integrated Stiles-Crawford function for coherent light.

The Stiles-Crawford effect that relates visibility to pupil point is typically expressed by a Gaussian function at any given wavelength of illumination. The pupil location of the maximum and the width of this function refer, respectively, to the pointing and waveguide properties of individual cone photoreceptors. In vision simulations, the function is integrated across the pupil when estimating effective retinal images, but the validity of this approach has still not been unequivocally confirmed. Indeed, aberrations and coherence properties may significantly alter not only the amplitude but also the phase distribution of the light at the retina in a way that differs fundamentally from that of the Maxwellian illumination configuration used when characterizing the effect. Here, we report on an experimental comparison of the traditionally determined Stiles-Crawford function and the equivalent for annular and half-annular apertures using extended highly coherent and incoherent sources. We show that an integrated Stiles-Crawford function is absent for coherent light but remains valid for highly incoherent light at the pupil. The results are supported by numerical evidence for coherent light propagation and are in agreement with a light-coupling understanding of retina photoreceptor waveguides.

Analysis of individual cone-photoreceptor directionality using scanning laser ophthalmoscopy

Scanning laser ophthalmoscopy has been used to measure individual cone-photoreceptor directionalities in the living human eye. The directionality is determined at different retinal eccentricities where it is expected that cones have diameters ranging between 5–10μm, comparable to the spot size of the incident beam. Individual cone directionality values are compared with the predicted directionalities obtained by using the waveguide model of light coupling to and from photoreceptors for the case of a focused incident beam.

Spatial and spectral characterisation of the first and second Stiles–Crawford effects using tuneable liquid-crystal filters

A semi-automated bipartite Maxwellian illumination system was used to examine the Stiles–Crawford effects of the first and second kind for the fovea of the authors’ eyes. The illumination was realised with a tungsten–halogen source in combination with tuneable liquid-crystal spectral filters. Visibility and hueshift dependencies were examined spatially and spectrally. They have been found to be in fair agreement with former studies in terms of overall appearance but with a notable reduction in directionality for the blue spectral range. The observations are discussed in relation to recent theory that relates the Stiles–Crawford effect of the second kind to the Stiles–Crawford effect of the first kind via a pigment-specific colour visibility function. The results confirm a direct relationship between the two effects and a reduction in hueshift for narrowband illumination. Differences in the green spectral range remain, however, that may be a consequence of a different directionality of S-cones as compared to the M- and L-cones.

Nonlinear optical properties of aromatic amino acids in the femtosecond regime

We report experimental and theoretical investigations of the third-order optical nonlinearities of aromatic amino acids (Phenylalanine, Histidine, Tryptophan, and Tyrosine) in aqueous solutions. The Z-scan technique with femtosecond laser pulses at 800 nm was explored for the determination of the nonlinear refractive index, nonlinear absorption coefficient, and the second-order hyperpolarizability of each amino acid. Experimental results were compared with theoretical analysis based on post-Hartree Fock MP2/6-311+G**.

Hartmann-Shack wavefront sensing for nonlinear materials characterization

We present two new techniques exploiting a Hartmann-Shack wavefront sensor to characterize the optical self-focusing effect of nonlinear materials. We demonstrate that the defocus Zernike coefficient (C5) can be used to quantify nonlinear optical properties of materials. In the first technique proposed, the wavefront of a collimated laser beam transmitted through a nonlinear sample is analyzed with different irradiance values. In the second technique,instead of conventional detectors, a Hartmann- Shack sensor is used in a Z-scan setup. The methods are demonstrated by measuring the nonlinear refractive indices of CS2 and Quartz, using femtosecond Ti:sapphire lasers at 76 MHz and 1 KHz repetition rate.