Gladys Mínguez-Vega

Scale-tunable optical correlation with natural light

We describe two different scale-tunable optical correlators working under totally incoherent light. They behave as spatially incoherent wavelength-independent imaging systems with an achromatic point-spread function (PSF). In both cases it is possible to adapt the scale of the achromatic PSF, i.e., to modify the scaling factor of the PSF and preserve the chromatic compensation, by ones shifting the input along the optical axis. The remarkable properties of these systems allow us to carry out a scale-tunable color pattern-recognition experiment with natural light.

Diffraction efficiency achromatization by random change of the blaze angle

Diffraction efficiency of blazed diffractive elements turns out to be more resistant to changes of the illuminating wavelength if the blaze angle of their periods varies randomly within a certain range.

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.

Diffractive optics for processing ultrashort light pulses

In this work we combine, in principle, two disjoint optical fields, diffractive optics and ultrashort light radiation. This combination allows us to manipulate in a very unconventional manner femtosecond pulses and, on the other hand, to implement a set of novel applications. In our case we have focused our attention on material processing and biophotonics applications.

High-contrast Lau fringes with white light

A novel optical set-up that allows a totally incoherent Lau effect is demonstrated. It is based on dispersion-compensated techniques that employ strong dispersive elements. In this way, three commercially available diffractive lenses and a refractive objective are used for achromatic Lau fringes production with spatial and temporally incoherent illumination.

In situ fabrication of a solar nanofluid by a femtosecond laser ablation of tin in ethylene glycol

Harvesting solar radiation should be improved to make this type of renewable energy more competitive in economic terms. Furthermore, to develop “green” nanotechnology is a prerequisite for crossing the boundary between research and industrial applications. In this contribution we will show the synthesis of a solar nanofluid of tin with ethylene glycol by pulsed laser ablation in liquids, a green and sustainable method for nanoparticles production. We will demonstrate that the nanofluid obtained with this technology is more stable that the one synthesized by conventional chemistry and have a reduced solar irradiance transmission through the time.

Generation of spectrally shaped UV-vis supercontinuum femtosecond pulses by means of diffractive lenses

When a high-power infrared femtosecond pulse is focused into a transparent solid, different non-linear effects such as self-phase modulation, self-steepening, plasma generation or filamentation take place. These effects usually give rise to an extreme spectral broadening of the pulse that is termed supercontinuum (SC) generation. This contribution demonstrates the possibility to tailor the spectral features of SC pulses by using diffractive lenses (DLs), which is of great interest for applications in spectroscopy demanding spectrally shaped pulses.

Alleviating the mechanical tolerances in femtosecond laser micromachining by diffractive focusing

To meet the ever-increasing demands of high resolution and versatility of microprocessing technology, diffractive optical elements (DOEs) are used to get the desired focal pattern [1]. The simplest phase DOE is a diffractive lens (DL). The focal length of a DL is given by f=fo λo/λ being fo the focal length for the central wavelength of the laser λo. Within the paraxial approximation, the focal irradiance distribution of a long pulse focused by a refractive lens (RL) or a DL of the same numerical aperture (NA) and focal length is expected to show similar behavior [2]. However, for sub 100 fs pulses, important differences appear owing to the large spectral bandwidth of ultrashort pulsed radiation [3]. Recently, measurements of the intensity ablation threshold by defocusing a RL have been carried out for 100 fs pulses [4]. In this contribution, we compare the ablation region obtained under defocusing conditions for both RL and DL with sub 100 fs pulses.

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].

Scanless two-photon microscopy with a 30 fs laser by means of a diffractive dispersion compensation module

Summary form only given. Scanless two-photon microscopy uses a spatial light modulator (SLM) to shape the incoming laser beam into any user-defined light pattern. These microscopes, that do not contain mechanically moving parts, improve the temporal resolution of conventional scanning multiphoton microscopy since they highly mitigate the severe temporal limitations inherent to sequential scanning of the sample [1]. However, diffractive optical elements encoded into a SLM for multiphoton microscopy prevent the use of ultrafast sources (say, pulses shorter than 100 fs) due to huge dispersion (both spatial and temporal), which limits high resolution nonlinear excitation [2]. We have engineered a simple dispersion-compensated module (DCM) based on the diffractive lens-pair configuration shown in Ref. [3] that permits to extend the range of pulsed sources available for scanless multiphoton microscopes to ultrashort femtosecond pulses. In this way, we strongly alleviate some unwanted effects (specifically, spatial chirp and pulse-front tilt).Our experimental setup is shown in Fig. 1(a). A mode-locked Ti: sapphire laser that emits pulses of 30 fs temporal width, 800 nm central wavelength, at 1 kHz repetition rate is used as pulsed source. The pulsed laser beam impinges by means of a beam splitter onto a Fourier computer generated hologram (CGH) encoded into a phase-only SLM. The dashed box in Fig. 1(a) is the DCM. It is made up of a lens achromat L1 (focal length f1 300 =) mm coupled to a diffractive lens pair, DL1 and DL2. The focal lengths of DL1 and DL2, for the central wavelength of the laser are f01 = -150 mm and f02 150 = , mm respectively. The distances between the optical elements are l=300 mm and d=d=150 mm. After the DCM, in order to properly excite the fluorescence signal in Rhodamine B (RB), we use a telescope with a refractive lens L2 (focal length f2 100 =) mm and a 20X microscope objective MO1 with focal distance 10 mm. To observe the fluorescence signal, the RB plane is imaged onto a conventional CCD sensor by means of a 50X microscope objective MO2. We place a suited filter F before the CCD camera to prevent from propagation of the infrared signal.In the experiment we encoded the outline of a bicycle on the CGH. The theoretical reconstruction is illustrated in Fig. 1(b). The average power was adjusted to 3mW to ensure wide-field fluorescence signalling. When we do not use the DCM, we realize that spatial and temporal broadening of the pulse at the sample plane makes useless the diffractive SLM, and in fact no signal is recorded. The fluorescence signal was subsequently recovered when we employed the DCM (see Fig. 1(d)). The dispersion compensation abilities of the DCM preserve the temporal width of the laser pulse at the sample plane, and consequently the fluorescence signal increases with respect to the uncompensated situation. Note that the number of emitted photons at the sample depends inversely on the pulse width. Furthermore, transverse spatial resolution for the CGH reconstruction is also maintained due to the spatial chirp compensation capacity of the module. Finally, we check that diffractiondriven fluorescence signalling is possible without the DCM, but at the expense of using an extra 2mW average power to compensate for temporal broadening, see Fig. 1(c). However, the uncompensated spatial chirp leads to a blurred signal that prevents correct hologram reconstruction and thus irradiance patterning.