Emerson C. Barbano

Femtosecond third-order nonlinear spectra of lead-germanium oxide glasses containing silver nanoparticles.

This work reports on the spectral dependence of both nonlinear refraction and absorption in lead-germanium oxide glasses (PbO-GeO₂) containing silver nanoparticles. We have found that this material is suitable for all-optical switching at telecom wavelengths but at the visible range it behaves either as a saturable absorber or as an optical limiter.

Accurate measurement of nonlinear ellipse rotation using a phase-sensitive method

We report on the accurate measurement of nonlinear ellipse rotation (NER) by means of a phase-sensitive method employing a dual-phase lock-in. The magnitudes and signs of pure refractive electronic nonlinearities of silica and BK7 were determined with this new method using 150 femtosecond (fs) laser pulses at 775 nm. Experimental and theoretical analyses of the NER signal were carried out and the results were compared to those obtained with the Z-scan technique.

Nonlinear optical waveguides in As 2 S 3 -Ag 2 S chalcogenide glass thin films

Chalcogenide glasses (ChGs) are promising materials for nonlinear optical applications due to their elevated third-order optical susceptibility. Nonetheless, oxidation and photorefractive effects make the characterization and processing of this class of materials an intricate process. Employing ultrashort laser pulses, we were able to investigate the optical nonlinearities of As2S3 thin films doped with Ag2S nanocrystals, as well as produce channel waveguides. The results showed that the nonlinear refractive indexes of these films are in the order of 2.3-5.2 x 10−18 m2/W at wavelengths in the range of 775-1600 nm. Such film revealed to be proper for the development of nonlinear optical devices, since we have demonstrated the functionality of an 8.7 mm long waveguide for spectral broadening femtosecond IR pulses.

Influence of self-focusing of ultrashort laser pulses on optical third-harmonic generation at interfaces

We report on the third-harmonic generation of femtosecond laser pulses at interfaces. We measured slabs of different types of optical glasses and demonstrated that the asymmetric intensity profile observed for a tightly focused beam can be explained by self-focusing effects.

Broadband third-harmonic generation on interfaces using femtosecond pulses

We report on third-harmonic generation (THG) in optical materials using femtosecond pulses and Z-scan method. Here we have played with beam focusing parameters and, in this way, we could track the THG signal at function of Rayleigh ranges. We observed that the femtosecond pulse has broadband spectrum and such property also affects the thirdharmonic (TH) spectrum. In this experiment we were able to distinguish the contribution of bulk and interface on the THG by measuring the intensity and spectral profile of the TH signal.

Comparative study of electronic and orientational nonlinear refractive indices with nonlinear ellipse rotation measurements

We investigated the contribution of molecular orientation nonlinearity to the nonlinear ellipse rotation (NER) effect. There is an enhancement of 2.25 when this process is compared to pure electronic third-order effects. NER measurements were performed with picosecond laser pulses in SF6 glass and CS2 that respectively present mostly electronic and orientational nonlinearities. A Z-scan was also performed to probe the Kerr-lens signal. The enhancement effect can be understood by taking into account the tensor nature of the third-order susceptibility.

Measurement of third-order nonlinearities in selected solvents as a function of the pulse width

We investigated the magnitude and origin of the nonlinear refraction in several solvents with the nonlinear ellipse rotation measurements as a function of the pulse duration in the range from 60fs to 2ps. Due to the presence of non-instantaneous nuclear contributions concurrently with the nearly instantaneous electronic nonlinearity, solvents present effective refractive nonlinearities that depend on the pulse duration. By proposing an empirical model where the nonlinearity grows exponentially with the pulse duration normalized to the response time, we could separate contributions from fast isotropic and slow nuclear reorientational nonlinearities. Z-scan measurements were also carried out to support our model.

Third-harmonic generation at the interfaces of a cuvette filled with selected organic solvents.

We report on the third-harmonic generation (THG) of tightly focused femtosecond laser pulses at the interfaces of a cuvette filled with organic solvents. Such a system presents four interfaces separating two materials of different refractive indices and third-order nonlinear susceptibilities where the THG takes place because the symmetry around the focus is broken. We selected two cuvettes (silica and B270 crown glass) filled with different organic solvents (acetone, chloroform, and dimethyl sulfoxide) in order to have a variety of interfaces with different linear and nonlinear optical properties. For some of the peaks, the self-focusing modifies the expected cubic power law dependence for THG and as a consequence the four peak profiles may be quite uneven. Although the THG is due to the electronic part of the nonlinear susceptibility, it can suffer from the influence of the self-focusing effect, a Kerr nonlinearity that can have both instantaneous electronic and slow nuclear contributions. This mixture of two distinct third-order nonlinear processes was never considered for such interfaces. All the THG signals could be understood by taking into account the self-focusing effect. Furthermore, the nonlinear refractive indices, n(2), and third-order nonlinear susceptibilities of the solvents, χ((3)), could be determined simultaneously by the THG signals using the cuvette walls as a reference.

New simple method for measuring nonlinear polarization ellipse rotation with high precision using a dual-phase lock-in

The measurements of nonlinear ellipse rotation (NER) can be very helpful to determine the magnitude, as well, the origin of the nonlinearity. As it is known, NER is related to particular component of the third-order nonlinear susceptibility which can be different from other nonlinear effects. In this way, here we propose a new method to improve the measuring precision of the NER angle using a dual phase lock-in. We also did a well known Z-scan measurement which provides the nonlinear refractive index to give support to our results. Using these two measurements, we could study several materials nonlinearity with different origin and we could reveal the tensor nature of the refractive nonlinearities. Material with thermal, molecular orientation and nonresonant electronic origins could be easily distinguished by those techniques.

Spatial Frequency Modulated Imaging (SPIFI) with amplitude or phase grating from a spatial light modulator

Spatial Frequency Modulated Imaging (SPIFI) with single element detection has previously been demonstrated with a time varying amplitude spatial frequency. This has been shown in a variety of modalities (linear, TPEF, SHG) and also with variations on the base design to provide additional dimensions of information. SPIFI is also capable of providing enhanced resolution images. However, the signal-to-noise is a limiting factor in the quality of the resolution enhancement. We present a microscope design which uses a nematic spatial light modulator to provide a time varying amplitude from an amplitude or phase grating. Twophoton excitation fluorescence images of 10-µm fluorescent polystyrene beads are presented using a phase grating. Additionally, the microscope can provide spatial gratings in polarization which provide an alternative means of imaging in third harmonic generation (THG). THG images are provided using an amplitude and polarization-grating modulation pattern.