Rocío Borrego-Varillas

Diffractive optics for spectral control of the supercontinuum generated in sapphire with femtosecond pulses

We propose the use of kinoform diffractive lenses to focus near infrared femtosecond pulses in sapphire crystals for supercontinuum generation. It is shown that a strongly peaked structure appears in the blue region of the supercontinuum spectra. The central wavelength of this peak can be easily controlled by simply changing the lens-crystal distance. Moreover, when compared with the supercontinuum generated with a refractive lens in analogous conditions, the spectral extension of the so-generated continuum is larger. Our results were corroborated for sapphire plates with different thicknesses as well as in other transparent dielectrics such as fused silica.

Frequency resolved wavefront retrieval and dynamics of diffractive focused ultrashort pulses

In this work we demonstrate the ability of the spatiotemporal characterization technique STARFISH to retrieve the wavelength dependent wavefront of focused ultrashort laser pulses. The high resolution achievable with this technique allows measuring the wavefront at the focal spot. In particular, the method is applied to study the effects of focusing with a kinoform diffractive lens. The evolution from converging to diverging wavefronts as the pulse propagates along the focal region is analyzed for each wavelength. The spatiotemporal intensity and spatially resolved spectrum structure of the pulses, as well as their profiles on axis, are also presented. Numerical simulations of the propagation of such pulses confirm the experimental results.

Wavelength tuning of femtosecond pulses generated in nonlinear crystals by using diffractive lenses.

We demonstrate that diffractive lenses (DLs) can be used as a simple method to tune the central wavelength of femtosecond pulses generated from second-order nonlinear optical processes in birefringent crystals. The wavelength tunability is achieved by changing the relative distance between the nonlinear crystal and the DL, which acts in a focusing configuration. Besides the many practical applications of the so-generated pulses, the proposed method might be extended to other wavelength ranges by demonstrated similar effects on other nonlinear processes, such as high-order harmonic generation.

On-axis non-linear effects with programmable Dammann lenses under femtosecond illumination

We demonstrate the utilization of Dammann lenses codified onto a spatial light modulator (SLM) for triggering non-linear effects. With continuous wave illumination Dammann lenses are binary phase optical elements that generate a set of equal intensity foci. We theoretically calculate the influence of ultrashort pulse illumination on the uniformity of the generated pattern, which is affected by chromatic aberration for pulses with temporal widths lower than 100 fs. The simulations also indicate that acceptable uniformity can be achieved for pulses of several fs by shortening the distance among foci which can be easily modified with the SLM. Multifocal second-harmonic generation (SHG) and on-axis multiple filamentation are produced and actively controlled in β-BaB2O4 (BBO) and fused silica samples, respectively, with an amplified Ti: Sapphire femtosecond laser of 30 fs pulse duration. Experimental results are in very good agreement with theoretical calculations.

Wavefront retrieval of amplified femtosecond beams by second-harmonic generation

We present a new approach for wavefront characterization of near transform-limited intense femtosecond beams using the angular and spectral dependences of the second-harmonic generation conversion efficiency in uniaxial crystals. The method is applied to different aberrated beams and results are compared with the measurements performed with a commercial sensor, finding very good agreement. The phase retrieval dependence with different parameters (e.g. crystal thickness) is discussed. Successful application to sharpen intensity profiles is also demonstrated.

Enhancement of filamentation postcompression by astigmatic focusing

The energy scaling up of pulse postcompression is still an open issue. In this work we analyze the use of astigmatic focusing to improve the output pulses in a filamentation based postcompression setup. Unlike spherical conditions, astigmatic focusing enhances the output energy and the spectral broadening of the filament. This is due to the increase of critical power, allowing a considerable improvement of the postcompression energy and stability in a simple way. We demonstrated compression from FWHM 100 fs, 10 nm, 3 mJ input pulses to 13 fs, 142 nm, near 1 mJ pulses.

Diffractive optics for spectral tuning of second harmonic and supercontinuum generated in nonlinear crystals

It is shown that diffractive lenses can tune the spectrum of femtosecond pulses after nonlinear optical processes. We focus on spectra of second-order pulses generated in birefringent crystals and supercontinuum in sapphire crystals. The tunability is achieved by changing the relative distance between the nonlinear crystal and the diffractive lens.

Controlled multi-beam supercontinuum generation in fused silica by means of spatial light modulators

We report on deterministic femtosecond multi-filamentation in fused silica by encoding a diffractive microlens array into a spatial light modulator. The efficiency and focal length of each microlens are electronically controlled. Even for an input inhomogeneous irradiance distribution, this allows for a precise control of the energy coupled to the filaments and the spectra of the generated supercontinuum beams. This method may be useful in different applications such as filamentation-based tumor treatment, laser processing, or spectroscopy.

Spatiotemporal Dynamics of Femtosecond Pulses Shaped by Diffractive Optical Elements

We present a complete experimental characterization and simulation of the spatiotemporal and spatio-spectral effects taking place when a femtosecond pulse is shaped by a diffractive optical element.

Generation of on-axis optical filaments by means of Dammann lenses

Summary form only given. The generation of filaments with ultrashort laser pulses is based on the sustained balance between self-focusing (Kerr effect) and ionization. When the peak power of the laser pulse is close to a critical self-focusing power value (Pcr) a single filament develops. If the beam power exceeds about more than 10 times Pcr, modulation instability can cause several co-propagating filaments formed from a single laser beam spontaneously, thus entering into the multiple filaments (MFs) regime. To avoid the random space-time localization of MFs, the initial beam is modified by methods that include, among others, changes in its ellipticity or wavefront shaping. However, although the generation of controlled off-axis MFs has been extensively studied, less attention has been paid to the generation of sequential on-axis MFs. This is the key point of this work.In summary, by using a phase-only SLM programmable, on-axis multiple MFs have been generated in a fused silica glass with femtosecond laser pulses. By changing the Dammann lenses parameters we demonstrated complete control over the position, width and peak intensity of the MFs. Several applications are expected to benefit for these results, such as in-depth parallel processing of transparent dielectrics [2] or the creation of long filaments by the concatenation of shorter ones [3].