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Currently, the nonlinear optical properties of 2D materials are attracting the attention of an ever-increasing number of research groups due to their large potential for applications in a broad range of scientific disciplines. Here, we investigate the interplay between nonlinear photoluminescence (PL) and several degenerate and nondegenerate nonlinear optical processes of a WS2 monolayer at room temperature. We illuminate the sample using two femtosecond laser pulses at frequencies ω1 and ω2 with photon energies below the optical bandgap. As a result, the sample emits light that shows characteristic spectral peaks of the second-harmonic generation, sum-frequency generation, and four-wave mixing. In addition, we find that both resonant and off-resonant nonlinear excitation via frequency mixing contributes to the (nonlinear) PL emission at the A-exciton frequency. The PL exhibits a clear correlation with the observed nonlinear effects, which we attribute to the generation of excitons via degenerate and nondegenerate multiphoton absorption. Our work illustrates a further step toward understanding the fundamental relation between parametric and nonparametric nondegenerate optical mechanisms in transition-metal dichalcogenides. In turn, such understanding has great potential to expand the range of applicability of nonlinear optical processes of 2D materials in different fields of science and technology, where nonlinear mechanisms are typically limited to degenerate processes.

Nanostructured gratings in a metal surface can highly enhance nonlinear optical processes. The geometrical parameters that characterize a grating can be optimized to achieve intense near-fields, which in turn enhance the nonlinear optical signals. For a nonlinear process that involves multiple frequencies, like four-wave mixing (FWM), the optimization of grating parameters necessary to enhance the radiation in-coupling for both frequencies is not trivial. Here, we propose, compute, and experimentally demonstrate a grating design that is resonant to two excitation frequencies and thus enhances the frequency mixing processes more efficiently. Second- and third-order nonlinear mechanisms are studied using two spatially and temporally overlapped laser pulses with different frequencies. Using our grating design, we achieve an unprecedented nonlinear FWM enhancement factor of 7 × 10 3.

The light–energy coupling during femtosecond laser processing of glass is mediated by non-linear ionization mechanisms through the formation of an electron plasma. Its transient optical properties provide information about the density, temperature and scattering rate of the excited electrons. In turn, these properties strongly condition the features and size of the permanent optical modification near the focal volume that are desirable to fabricate photonic devices, such as optical waveguides inside transparent materials. Here, we report on the spectral response of a fs-laser-induced electron plasma inside fused silica by measuring its transient transmission using a broadband probe. We model the interaction of the probe beam with the plasma by combining Drude–Sommerfeld model with Gaussian optics. In this manner, we take into account both the laser–plasma interaction and the influence of the chromatic aberration inherent to a broadband-based system. We find good agreement between experiments at several processing energies and simulations and provide an estimate of the dielectric function of the excited material.

We experimentally study the mass distribution of a sheet of liquid tin formed by the impact of a ns-laser pulse on a spherical microdroplet. The mass distribution is obtained using a low-intensity, second ns-laser pulse, which induces vaporization of the stretching thin tin sheet. This careful vaporization enables the investigation of the thickness profile of the sheet, and its mass, at early times after laser pulse impact on a droplet, which have remained inaccessible by the methods used in recent work [B. Liu et al., Phys. Rev. Appl. 13, 024035 (2020)]. The vaporization method, moreover, allows the visualization of the thick rim that bounds the thin sheet. Our results unambiguously demonstrate that increasing the energy of the ns-laser pulse incident on the droplet, which enables reaching a predetermined target radius more quickly, results in a larger mass fraction remaining in the sheet. Specifically, our studies show a doubling of the sheet mass fraction by reducing the required expansion time. As a corollary, less tin will end up in other channels of the mass distribution, such as fragments surrounding the sheet. Accordingly, more mass would be available in the target sheet for interaction with the more energetic, main laser pulse that is used in the industry to produce a hot and dense plasma from tin sheet targets in order to create extreme ultraviolet light for nanolithography.

Nonlinear processing of glass using femtosecond laser pulses has been shown as a reliable method for producing photonic devices. To optimize the writing of optical waveguides and harness the true power of fs-laser modification, it is imperative that we gain a greater understanding of the physical mechanisms behind it. This requires experimental and theoretical studies into the dynamical processes during ultrashort laser–matter interaction. In this study, plasma emission spectroscopy and microscopy are shown as a reliable monitor for predicting the smoothness or roughness of the final laser-induced modification. Spectrally and temporally resolved optical transmission measurements using a supercontinuum probe beam are presented as experimental approach for investigating the transient optical properties of fs-laser-processed glass. The description of the light–plasma interaction is given by a simple model, which is used for inferring the order of magnitude of the electron density under such experimental conditions.

We have investigated the effect of the laser wavelength on the fabrication of optical waveguides and tracks of modified material via direct laser writing inside fused silica. The size of the laserinscribed tracks, the material modification thresholds, the structural changes, and the waveguide writing energy range show a strong dependence on laser wavelengths ranging from ultraviolet to near-infrared. We used numerical simulations that consider the laser-excited electron plasma dynamics (via multiple rate equations) along with Gaussian beams theory to calculate the size of the laser-affected volume that has been further compared with the experimental results. This study yields insight into how to predict and design the spatial features of laser-inscribed lines and also aids of understanding the underlying physical mechanisms linked to laser-glass interaction when using different laser wavelengths.

8 págs.; 5 figs.; 1 tab., Self-assembly (SA) of molecular units to form regular, periodic extended structures is a powerful bottom-up technique for nanopatterning, inspired by nature. SA can be triggered in all classes of solid materials, for instance, by femtosecond laser pulses leading to the formation of laser-induced periodic surface structures (LIPSS) with a period slightly shorter than the laser wavelength. This approach, though, typically involves considerable material ablation, which leads to an unwanted increase of the surface roughness. We present a new strategy to fabricate high-precision nanograting structures in silicon, consisting of alternating amorphous and crystalline lines, with almost no material removal. The strategy can be applied to static irradiation experiments and can be extended into one and two dimensions by scanning the laser beam over the sample surface. We demonstrate that lines and areas with parallel nanofringe patterns can be written by an adequate choice of spot size, repetition rate and scan velocity, keeping a constant effective pulse number (N ) per area for a given laser wavelength. A deviation from this pulse number leads either to inhomogeneous or ablative structures. Furthermore, we demonstrate that this approach can be used with different laser systems having widely different wavelengths (1030 nm, 800 nm, 400 nm), pulse durations (370 fs, 100 fs) and repetition rates (500 kHz, 100 Hz, single pulse) and that the grating period can also be tuned by changing the angle of laser beam incidence. The grating structures can be erased by irradiation with a single nanosecond laser pulse, triggering recrystallization of the amorphous stripes. Given the large differences in electrical conductivity between the two phases, our structures could find new applications in nanoelectronics., This work has been supported by the LiNaBioFluid project of the H2020 program of the European Commission (FETOPEN- 665337) as well as by the Spanish TEC2014-52642-C2- 1-R. MG-L and JH-R acknowledge the grants respectively awarded by the Spanish Ministry of Education and the Spanish Ministry of Economy and Competiveness.

7 pags.; 4 figs.; 1 tab., © 2015 American Chemical Society. We have investigated the use of tightly focused, temporally shaped femtosecond (fs)-laser pulses for producing nanostructures in two dielectric materials (sapphire and phosphate glass) with different characteristics in their response to pulsed laser radiation. For this purpose, laser pulses shaped by third-order dispersion (TOD) were used to generate temporally asymmetric excitation pulses, leading to the single-step production of subwavelength ablative and subablative surface structures. When compared to previous works on the interaction of tightly focused TOD-shaped pulses with fused silica, we show here that this approach leads to very different nanostructure morphologies, namely, clean nanopits without debris surrounding the crater in sapphire and well-outlined nanobumps and nanovolcanoes in phosphate glass. Although in sapphire the debris-free processing is associated with the much lower viscosity of the melt compared to fused silica, nanobump formation in phosphate glass is caused by material network expansion (swelling) upon resolidification below the ablation threshold. The formation of nanovolcanoes is a consequence of the combined effect of material network expansion and ablation occurring in the periphery and central part of the irradiated region, respectively. It is shown that the induced morphologies can be efficiently controlled by modulating the TOD coefficient of the temporally shaped pulses., This work has been partially supported by the Spanish MINECO projects TEC2011-22422 and MAT2013-33517. J.H.-R. acknowledges the FPI grant and the FPI-EEBB travel grant awarded by the Spanish Ministry of Science and Innovation. N.G., B.Z., C.S., M.W., and T.B. acknowledge support from the DFG priority program no. 1327.

The permanent refractive index change induced by ultrashort laser pulses in zinc phosphate glasses has been investigated both at the surface and in bulk. At the sample surface, irradiations have been performed by using loosely focused single fs-laser pulses at different energies. Optical microscopy images of the irradiations illustrate an interferometric pattern in form of concentric Newton rings due to the laser induced multilayer system (unmodified glass, thin laser-modified layer, air). This experimental reflectivity modulation along with simulations based on Abeles theory for multilayer optical systems allows retrieving laser-induced refractive index changes on the order of Δns= -10 -3 . In bulk, fs-laser written waveguides have been generated by translating the sample with respect to a tightly focused laser beam. The so-produced waveguides have been characterized by studying the optical near field of the TEM 00 guided mode at 660 nm and using white light microscopy. The optical changes linked to the inscribed waveguides have been characterized by measuring the far field output profiles yielding values of approximately Δn b = +3·10 -4 . The laser-modified optical properties in bulk and at the surface will be linked to the glass structural changes as well as discussed in terms of the role of the incubation effects for multi-pulse processing.

The fs-laser induced refractive index change in zinc aluminum phosphate glasses and its underneath structural modifications have been investigated both at the surface and in bulk.

7 págs.; 5 figs.; 1 tab.; OCIS codes: (140.3390) Laser materials processing; (320.5540) Pulse shaping; (320.7120) Ultrafast phenomena; (140.3440) Laser-induced breakdown; (240.6648) Surface dynamics; (300.2140) Emission., © 2015 Optical Society of America. We have analyzed the influence of the temporal pulse shape on femtosecond (fs) laser-induced surface ablation processes in sapphire. To this end, single transform-limited (TL), stretched, and third-order-dispersion (TOD) shaped fs pulses have been used, while the dynamics of the interaction were analyzed by fs-resolved microscopy and correlated with plasma emission intensity and crater morphology. The modification of the pulse shape enables changing the ablation mechanism from a strong, thermally mediated ablation process to a gentle ablation process mediated by Coulomb explosion (CE), with respective ablation depths of 100-200 nm and 5-10 nm. Analysis of the transient optical response allows direct comparison of the transient plasma carrier densities involved, observing comparable peak values for both processes. For strong ablation induced by TL pulses, a direct relation between plasma density and local ablation depth is found, but this does not hold for the CE-mediated process observed for TOD-shaped pulses. For TOD-shaped pulses at very high fluence, a different ablation mechanism involving explosive boiling is identified. This mechanism leads to the formation of deep craters with reduced lateral extension and steep walls. This amount of control over the ablation mechanisms by a simple selection of the pulse shape should be of interest for new surface structuring approaches., This work has been partially supported by the Spanish TEC2011-22422 project. J. H.-R. and M. G.-L. acknowledge the grants, respectively, awarded by the Spanish Ministry of Science and Innovation and the Spanish Ministry of Education. We are grateful to Prof. S. Marcos and M. Sun from IO-CSIC for providing access and assistance with the topography measurements.

We report the appearance of transient Newton rings in dielectrics (sapphire and lead-oxide glass) during ablation with single fs laser pulses. Employing femtosecond microscopy with 800 nm excitation and 400 nm illumination, we observe a characteristic ring pattern that dynamically changes for increasing delay times between pump and probe pulse. Such transient Newton rings have been previously observed in metals and semiconductors at fluences above the ablation threshold and were related to optical interference of the probe beam reflected at the front surface of the ablating layer and at the interface of the non-ablating substrate. Yet, it had been generally assumed that this phenomenon cannot be (and has not been) observed in dielectrics due to the different ablation mechanism and optical properties of dielectrics. The fact that we are able to observe them has important consequences for the comprehension of the ablation mechanisms in dielectrics and provides a new method for investigating these mechanisms in more detail., Work presented at High Power Laser Ablation Conference (HPLA 2014), 21-25 April 2014, Santa Fe (USA)

Femtosecond laser pulse temporal shaping techniques have led to important advances in different research fields like photochemistry, laser physics, non-linear optics, biology, or materials processing. This success is partly related to the use of optimal control algorithms. Due to the high dimensionality of the solution and control spaces, evolutionary algorithms are extensively applied and, among them, genetic ones have reached the status of a standard adaptive strategy. Still, their use is normally accompanied by a reduction of the problem complexity by different modalities of parameterization of the spectral phase. Exploiting Rabitz and co-authors' ideas about the topology of quantum landscapes, in this work we analyze the optimization of two different problems under a deterministic approach, using a multiple one-dimensional search (MODS) algorithm. In the first case we explore the determination of the optimal phase mask required for generating arbitrary temporal pulse shapes and compare the performance of the MODS algorithm to the standard iterative Gerchberg-Saxton algorithm. Based on the good performance achieved, the same method has been applied for optimizing two-photon absorption starting from temporally broadened laser pulses, or from laser pulses temporally and spectrally distorted by non-linear absorption in air, obtaining similarly good results which confirm the validity of the deterministic search approach. © 2013 Springer-Verlag Berlin Heidelberg., This work has been partially supported by the Spanish TEC2011-22422 project funded by the Spanish Ministry of Economy and Competitiveness. M.G.-S. and J.H.-R. acknowledge the funding of their contracts by the same ministry.

8 pags.; 7 figs.; 1 tab.; OCIS codes: (140.3300) Laser beam shaping; (160.4760) Optical properties; (320.7120) Ultrafast phenomena; (350.3390) Laser materials processing., We report on reflectivity and transmission patterns resembling Newton rings at the surface of a broad range of dielectric materials upon irradiation with single femtosecond laser pulses. We demonstrate that the patterns are due to the formation of a submicrometer layer of modified material underneath the laser-irradiated region. This permanent layer acts as a low-finesse micro Fabry-Perot etalon, producing a system of dark and bright rings upon illumination with narrowband light, whose number and optical contrast are related to thickness and optical constants of the layer. We find that the appearance of Newton rings is a universal phenomenon in fs-laser irradiated inorganic dielectrics (amorphous and crystalline), polymers, and semiconductors above the ablation threshold. We demonstrate how this phenomenon can be exploited for characterization of the layer by studying in detail three different dielectric materials as model systems, namely, fused silica, quartz, and phosphate glass, at fluences above and below the ablation threshold. An analysis of the Newton rings allows quantifying in a simple way the sign and amount of the changes in the complex refractive index as well as the thickness of the laser-modified layer. This technique greatly helps in characterizing the often problematic residual surface layer produced in laser structuring applications and has the ability to serve as an in situ, real-time monitor for minimizing its thickness and optical changes. © 2014 Optical Society of America., This work has been partially supported by the Spanish TEC2011-22422 project. J. H.-R. and M G.-L. acknowledge the grants respectively awarded by the Spanish Ministry of Science and Innovation and the Spanish Ministry of Education.

The use of temporally shaped femtosecond laser pulses is shown to enable the control of ablation mechanisms in Sapphire ranging from gentle over strong ablation to explosive boiling.

Fused silica surface structuring has been performed using temporally shaped femtosecond laser pulses. For this purpose we have designed pulse bursts with a triangular intensity envelope and different slope sign and interpulse separation that were experimentally generated using a home-made temporal pulse shaper. We have found that pulse bursts with decreasing intensity envelopes are remarkably more efficient in terms of surface ablation than bursts with increasing intensity envelopes. The results reveal that laser energy coupling in the material is enhanced as the interpulse spacing decreases. A study of the ablation depth using stretched single pulses was carried out and compared to results obtained for pulse bursts with different interpulse spacing. We find that the deepest crater was achieved with bursts of 0.5 ps interpulse separation and decreasing envelope. This pulse form also induced the largest change of the surface reflectivity after irradiation. The results are discussed in terms of how the laser energy coupling efficiency is linked to the temporal pulse shape. © 2013 Optical Society of America., This work has been partially supported by the Spanish TEC2011-22422 project. J. H.-R. acknowledges a grant awarded by the Spanish Ministry of Science and Innovation.

We have analyzed the ablation depth yield of fused silica irradiated with shaped pulse trains with a separation of 500 fs and increasing or decreasing intensity envelopes. This temporal separation value is extracted from previous studies on ablation dynamics upon irradiation with transform-limited 100 fs laser pulses. The use of decreasing intensity pulse trains leads to a strong increase of the induced ablation depth when compared to the behavior, at the same pulse fluence, of intensity increasing pulse trains. In addition, we have studied the material response under stretched (500 fs, FWHM) and transform-limited (100 fs, FWHM) pulses, for which avalanche or multiphoton ionization respectively dominates the carrier generation process. The comparison of the corresponding evolution of the ablated depth vs. fluence suggests that the use of pulse trains with decreasing intensity at high fluences should lead to enhanced single exposure ablation depths, beyond the limits corresponding to MPI- or AI-alone dominated processes. © Springer-Verlag 2012, This work has been partially supported by the Spanish TEC2008-01183 and TEC2011-22422 projects.

We have investigated plasma formation and relaxation dynamics induced by single femtosecond laser pulses at the surface of crystalline SiO2 (quartz) along with the corresponding topography modifications. The use of fs-resolved pump-probe microscopy allows combining spatial and temporal resolution and simultaneous access to phenomena occurring in adjacent regions excited with different local fluences. The results show the formation of a transient free-electron plasma ring surrounding the location of the inner ablation crater. Optical microscopy measurements reveal a 30% reflectivity decrease in this region, consistent with local amorphization. The accompanying weak depression of ≈15 nm in this region is explained by gentle material removal via Coulomb explosion. Finally, we discuss the timescales of the plasma dynamics and its role in the modifications produced, by comparing the results with previous studies obtained in amorphous SiO2 (fused silica). For this purpose, we have conceived a new representation concept of time-resolved microscopy image stacks in a single graph, which allows visualizing quickly suble differences of the overall similar dynamic response of both materials. © 2011 Elsevier B.V. All rights reserved., This work has been partially supported by the Spanish TEC2008- 01183 project. J.H.-R. acknowledges a grant under the same project. D.P. acknowledges a grant of the Spanish Ministry of Science and Education under TEC 2005-00074.

The transient reflectivity response of phase-change Ge2 Sb 2 Te5 films to intense femtosecond laser pulses is studied by ultrafast coherent phonon spectroscopy. The three different phases (amorphous, fcc-, and hcp-crystalline), as well as laser-crystallized films, are investigated, featuring different photoexcited carrier and coherent optical phonon dynamics. At least two main phonon frequencies are identified for each phase/material and their evolution for increasing pump fluences is investigated for the fcc-crystalline phase and the laser-crystallized material, revealing strong differences. We find evidence that a considerable fraction of amorphous phase remains in the laser-crystallized material, which features a different phonon frequency, not related to other phases. These results are important for emerging strategies aimed at driving ultrafast phase transitions via coherent phonon excitation for applications in data storage. © 2011 American Institute of Physics.

5 pags.; 4 figs., We demonstrate that the broad surface plasmon resonance (SPR) of a single layer of near-coalescence silver nanoparticles (NPs), embedded in a dielectric matrix can be tailored by irradiation with a single nanosecond laser pulse into a distribution featuring a sharp resonance at 435 nm. Scanning electron microscopy studies reveal the underlying mechanism to be a transformation into a distribution of well-separated spherical particles. Additional exposure to multiple femtosecond laser pulses at 400 nm or 800 nm wavelength induces polarization anisotropy of the SPR, with a peak shift that increases with laser wavelength. The spectral changes are measured in-situ, employing reflection and transmission micro-spectroscopy with a lateral resolution of 4 μ m. Spectral maps as a continuous function of local fluence can be readily produced from a single spot. The results open exciting perspectives for dynamically tuning and switching the optical response of NP systems, paving the way for next-generation applications. © 2014 AIP Publishing LLC., This work has been supported by the Spanish Ministry of Economy and Competitiveness (MAT2009-14282-C02-01, TEC2011-22422, and TEC2012-38901-C02-01). J.D. acknowledges support from the ERASMUS placement program, G.B. a grant from the CSIC-JAE predoctoral program, co-funded by the European Social Fund, and J.H.-R. a grant from the Spanish Ministry of Science and Innovation

Zinc-phosphate glass surface ablation was investigated using tightly focused fs-laser pulses. Evidences of self-scattering effects due to the interaction between the laser light and a transient plasma-generated during surface ablation have been investigated.