A highly automated apparatus for ultra-fast laser ablation studies.

We present a novel experimental apparatus that can be used for extensive systematic studies of (single- and multi-shot) ultra-short laser pulse ablation. It is fully automated and generates a large number of ablation sites in a short time on a small sample surface area. For each site, the apparatus takes four in situ images: an image of the incident ablation beam (to determine pulse energy), a white light reference image of the pristine sample site, an image of the reflected ablation spot, and a white light image of the ablated sample site. The setup can perform ablation experiments as a function of many parameters, including pulse energy, pulse duration, number of pulses, time between pulses, and focus size. As a proof of concept, we present example results on single-shot ablation off crystalline silicon. Using only data acquired in situ in the presented setup, we determine the single-shot ablation threshold as a function of pulse duration and verify the threshold value using optical interferometric profilometry. The values we found agree well with literature values.

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.

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.

Efficient and versatile surface integral approach to light scattering in stratified media.

Recent advances in nano-optics elicit a growing need for more efficient numerical treatments of light-matter interaction in stratified backgrounds. While being known for its many favorable properties, usage of the surface integral approach is hindered by numerical difficulties associated with layered-medium Green's functions. We present an efficient and robust implementation of this approach, addressing the limiting issues. The singularity extraction method is generalized to account for the occurring secondary-term singularities. The resulting scheme thus allows for arbitrary positioning of the scatterers. Further, the laborious matrix-filling process is dramatically accelerated through a simple and robustly-devised, spatial interpolation scheme, completely devoid of integral evaluations. The accuracy and versatility of the method are demonstrated by treating several representative plasmonic problems.

Modal symmetries at the nanoscale: a route toward a complete vectorial near-field mapping.

We use symmetry considerations to understand and unravel near-field measurements, ultimately showing that we can spatially map three distinct fields using only two detectors. As an example, we create 2D field maps of the out-of-plane magnetic field and two in-plane fields for a silicon ridge waveguide. Furthermore, we are able to identify and remove polarization mixing of less than 1/30 of our experimental signals. Since symmetries are prevalent in nanophotonic structures and their near-fields, our method can have an impact on many future near-field measurements.

Self-scattering effects in femtosecond laser nanoablation.

We experimentally investigate the self-reflectivity of intense strongly focused femtosecond laser pulses used for single-shot femtosecond laser ablation of silicon-on-insulator (SOI). We model the self-reflectivity using 2D finite-difference time-domain simulations of a single femtosecond laser pulse interacting with a submicrometer-sized time- and space-dependent plasma induced by the incident pulse itself and find excellent agreement with our experimental results. The simulation shows that the laser-induced plasma scatters the incident pulse into the guided modes of the device layer of the SOI.

Characterization of bending losses for curved plasmonic nanowire waveguides.

We characterize bending losses of curved plasmonic nanowire waveguides for radii of curvature ranging from 1 to 12 microm and widths down to 40 nm. We use near-field measurements to separate bending losses from propagation losses. The attenuation due to bending loss is found to be as low as 0.1 microm(-1) for a curved waveguide with a width of 70 nm and a radius of curvature of 2 microm. Experimental results are supported by Finite Difference Time Domain simulations. An analytical model developed for dielectric waveguides is used to predict the trend of rising bending losses with decreasing radius of curvature in plasmonic nanowires.

Negative-index metamaterials: looking into the unit cell.

With their potential for spectacular applications, like superlensing and cloaking, metamaterials are a powerful class of nanostructured materials. All these applications rely on the metamaterials acting as a homogeneous material. We investigate a negative index metamaterial with a phase-sensitive near-field microscope and measure the optical phase as a function of distance. Close to the metamaterial we observe extremely large spatial phase variations within a single unit cell which vanish on a 200 nm length scale from the sample. These deviations of a state-of-the-art metamaterial from a homogeneous medium can be important for nanoscale applications.