This work presents the initial activation of the Mobile Ultrafast High-Energy Laser Facility (MU-HELF) located on a 1 km test range at the Townes Institute Science and Technology Experimentation Facility (TISTEF). The MU-HELF was designed to study nonlinear laser propagation effects including filamentation and produces pulses at 800 nm with current peak powers as high as 5 TW. The pulse width, energy, size, and focusing conditions of the launched beams are all readily adjustable. Several data collection techniques have been implemented that enable high-resolution, single-shot beam profiles, spectra, and energy measurements at any point along the range. Atmospheric conditions are also continuously measured during laser propagation using the array of monitoring equipment available at TISTEF. The newly active test facilities and data collection procedures demonstrated here will drive future in-depth high-intensity laser propagation studies and development of field-deployable applications.
High intensity ultrafast near-IR laser induced filaments in air possess very precise characteristics. Each filament is controlled in spatial extent by the non-linear optical processes that are responsible for their formation. The spatial extent of the filament has a Townesian profile comprising a central high intensity region of ~ 400 μm full width, surrounded by a lower intensity peripheral field extending out several millimeters that maintains the long term propagation stability of the filament. The energy content within each filament is clamped by the threshold power needed for its establishment, for ~100 fs pulses this is ~3GW. Energy greater than this results in either the formation of additional filaments or is dispersed into the peripheral field and is diffracted out of the beam. Thus each filament carries a finite energy. Nonetheless, light filaments are an effective way of propagating over large distances extremely high power densities (< 1013 W/cm2), several orders of magnitude higher than the ablation threshold of nearly all materials. The level of ablation of solid surfaces is however limited by the maximum energy (few mJ) carried in each filament. In the present study we make detailed measurements of the ablation of GaAs, examining both the plasma interaction and the resulting material ablation. In addition we probe the use of additional nanosecond infrared laser light focused on the surface concurrently with the filament at intensities. We observe significantly increased filament initiated ablation when followed by lower intensity nanosecond radiation. Ultrafast radiometric studies of the plasma evolution provides new understandings of this augmented ablation process.
This work compares the effects of ablation on GaAs, Al, and Ti samples exposed to two different regimes: a focused 800nm, 14.13mJ pulse of 55fs duration from a Ti: Sapphire laser with an intensity of 2×1016 W cm-2 and a focused 1064nm, 14.61mJ pulse of 10ns duration from a Nd: YAG laser with an intensity of 2×1010 W cm-2. The craters are examined using optical microscopy, white light interferometry, and scanning electron microscopy. Among the effects examined in this paper are the conduction of energy throughout the material, formation of nanodroplets outside of the crater, nanopits in the center of the crater, and the effects of phase explosion inside the crater.
Single-shot ablation of GaAs samples by a collinear femtosecond-nanosecond (fs-ns) dual-pulse is investigated. Significantly enhanced material removal is achieved by optimally combining a single 8 ns pulse at 1064 nm and a single 50 fs pulse at 800 nm in time. The resulting ablation craters are examined for inter-pulse delays ranging from -50 ns (ns first) to +1 μs (fs first) as well as very long delays of ±30 s. Crater profilometry is conducted with white light interferometry and optical microscopy to determine the volume of ablated material and identify surface features that reveal information about the physical mechanism of material removal during fs-ns dual-pulse ablation.
High-power femtosecond filaments—laser-light beams capable of kilometer-long propagation—attract interest of nonlinear-optics community due to their numerous applications in remote sensing, lightning protection, virtual antennas, and waveguiding. Specific arrangements of filaments, into waveguides or hyperbolic metamaterials, allow for efficient control and guiding of electromagnetic radiation, radar-beam manipulation, and resolution enhancement. These applications require spatially uniform distribution of densely packed filaments.
In order to address this challenge, we investigate the dynamic properties of large rectangular filament arrays propagating in air depending on four parameters: the phase difference between the neighboring beams, the size of the array, separation between the beams, and excitation power. We demonstrate that, as a result of the mutual interaction between the filaments, the arrays where the nearest neighbor beams are out-of-phase are more robust than the arrays with all the beams in phase.
Our analysis of the array stability reveals that there exist certain trade-offs between the stability of a single filament and the stability of the entire array. We show that in the design of the experiment, the input parameters have to be chosen in such a way that they ensure a sufficiently high filling fraction, but caution has to be used in order not to compromise the overall array stability.
In addition, we show the possibility of filament formation by combining multiple beams with energies below the filamentation threshold. This approach offers additional control over filament formation and allows one to avoid the surface damage of external optics used for filamentation.