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We report on femtosecond laser ablation experiments on percolating gold layers deposited on a glass substrate. In our experiments, we measure changes in optical transmission and reflection induced by single, high-intensity infrared laser pulses as a function of the time delay between the pump and the probe. For the highest pump intensities we find that on a time scale of about 150 ps after excitation, the transmission and reflection approach values close to the substrate values. We attribute this rapid ablation to vaporization of the entire layer when the injected energy exceeds the cohesive energy of the material. This vaporization results in the rapid transformation of the gold layer into a sufficiently dilute mist of atoms and nano-particles which renders the material almost optically transparent to the probe pulse. SEM images of the surfaces show how the morphology of the films changes at relatively low excitation intensities and show the complete removal of the gold at high intensities. We find that the ablation threshold for percolating Au on glass is 2.3 × 1011 W/cm2, which is two orders of magnitude lower than the damage threshold for continuous gold layers as reported in the literature.

The precise characterization of ultrashort laser pulses has been of interest to the scientific community for many years. Frequency-resolved optical gating (FROG) has been extensively used to retrieve the temporal and spectral field distributions of ultrashort laser pulses. In this work, we exploit the high, broad-band nonlinear optical response of a WS2monolayer to simultaneously characterize two ultrashort laser pulses with different frequencies. The relaxed phase-matching conditions in a WS2monolayer enable the simultaneous acquisition of the spectra resulting from both four-wave mixing (FWM) and sum-frequency generation (SFG) nonlinear processes while varying the time delay between the two ultrashort pulses. Next, we introduce an adjusted double-blind FROG algorithm, based on iterative fast Fourier transforms between two FROG traces, to extract the intensity distribution and phase of two ultrashort pulses from the combination of their FWM and SFG FROG traces. Using this algorithm, we find an agreement between the computed and observed FROG traces for both the FWM and SFG processes. Exploiting the broad-band nonlinear response of a WS2monolayer, we additionally characterize one of the pulses using a second-harmonic generation (SHG) FROG trace to validate the pulse shapes extracted from the combination of the FWM and SFG FROG traces. The retrieved pulse shape from the SHG FROG agrees well with the pulse shape retrieved from our nondegenerate cross-correlation FROG measurement. In addition to the nonlinear parametric processes, we also observe a nonlinearly generated photoluminescence (PL) signal emitted from the WS2monolayer. Because of its nonlinear origin, the PL signal can also be used to obtain complementary autocorrelation and cross-correlation traces.

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.

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)

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.