Simultaneous Characterization of Two Ultrashort Optical Pulses at Different Frequencies Using a WS2 Monolayer.

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 WS2 monolayer to simultaneously characterize two ultrashort laser pulses with different frequencies. The relaxed phase-matching conditions in a WS2 monolayer 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 WS2 monolayer, 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 WS2 monolayer. Because of its nonlinear origin, the PL signal can also be used to obtain complementary autocorrelation and cross-correlation traces.

Nonlinear Optical Response of a WS2 Monolayer at Room Temperature upon Multicolor Laser Excitation.

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.

Femtosecond laser-ablation of gel and water.

We study the expansion dynamics of super-heated material during ultra-fast laser ablation of water and gel, using transient-reflectivity microscopy. We find that the expansion dynamics of water and gel, as observed during the first few nanoseconds, are extremely similar over a large range of ablation energies. We measure the crater topography of the gel after irradiation with a single laser shot, using optical interferometric microscopy, and estimate the mass that is ejected during the ablation. We calculate the laser energy deposited during irradiation by simulating the precise spatial distribution of the electron plasma density and temperature. We link the amount of removed mass obtained experimentally with the simulations of the deposited laser energy.

Optical method for micrometer-scale tracerless visualization of ultrafast laser induced gas flow at a water/air interface.

We study femtosecond-laser-induced flows of air at a water/air interface, at micrometer length scales. To visualize the flow velocity field, we simultaneously induce two flow fronts using two adjacent laser pump spots. Where the flows meet, a stationary shockwave is produced, the length of which is a measure of the local flow velocity at a given radial position. By changing the distance between the spots using a spatial light modulator, we map out the flow velocity around the pump spots. We find gas front velocities near the speed of sound in air vs for two laser excitation energies. We find an energy scaling that is inconsistent with the Sedov-Taylor model. Due to the flexibility offered by spatial beam shaping, our method can be applied to study subsonic laser-induced gas flow fronts in more complicated geometries.

Ultrafast laser ablation of trapped gold nanoparticles.

We investigate the interaction of femtosecond (fs) laser pulses with single gold nanoparticles, trapped in a linear Paul trap. We study the scattering response of the particles as a function of the polarization angle of a cw laser at three different wavelengths. These measurements provide a value of the visibility that we compare with Mie theory calculations in order to obtain an estimate of the particle radius. We monitor the particle size during ultrafast laser ablation, obtaining an accurate figure for the mass loss as a function of the fs-laser dose. We discuss the particle mass loss induced by a single fs-laser shot and its relation with the number of absorbed photons.

Transient scattering effects and electron plasma dynamics during ultrafast laser ablation of water.

We study the dynamics of single-shot ultrafast laser ablation of a water-gas interface. We model the extremely nonlinear light-water interaction during the first picosecond by simulating the laser pulse propagation while dynamically calculating the spatial distribution of the dielectric function. We make use of a finite-difference time-domain algorithm to solve Maxwell's equations and Rethfeld's multiple rate equation model to consider the local excitation of a dense electron plasma. We validate our model by comparing the simulated transient reflectivity with experimental results and find excellent agreement.

Dynamics of femtosecond laser-induced shockwaves at a water/air interface using multiple excitation beams.

We investigate the formation, propagation, and interaction of femtosecond laser-induced flows of compressed air at a water/air interface by recording the transient reflectivity of shockwaves. Subsonic fronts of compressed air and weak shockwaves can be hard to detect due to their inherently subtle change of refractive index. Therefore, we study these weak flows by looking at the interaction dynamics of two and four shockwaves simultaneously produced at adjacent locations. An analytic model is used to retrieve the velocity and position of the shockwave from the experimental results. The use of multi-spot excitation opens up a versatile method to further investigate and understand the physical mechanisms contributing to photomechanical tissue damage during femtosecond-laser-based surgery and to study the fluid dynamics of complex systems.

Femtosecond laser-controlled self-assembly of amorphous-crystalline nanogratings in silicon.

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 eff) 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.

Nanofabrication of tailored surface structures in dielectrics using temporally shaped femtosecond-laser pulses.

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.

White and full color upconversion film-on-glass displays driven by a single 978 nm laser.

White and full-color displays based on upconversion (UC) processes in multilayered NaLu1-x-yYbxTmy(WO4)2/NaLu1-x-zYbxHoz(WO4)2 films deposited on 20 × 20 mm² Pyrex glass substrates are demonstrated by scanning with a 978 nm focused beam from a diode laser. Moreover, spatially resolved red, green and blue pixels are selected by focusing the excitation light at different depths on three stacked films with compositions individually optimized for UC emission of each fundamental color. The highest temperature used in synthesis/deposition process was 580 °C allowing the use of glass substrates.