Femtosecond laser damage of germanium from near- to mid-infrared wavelengths

Nanoscale reshaping of resonant dielectric microstructures by light-driven explosions

Femtosecond-laser-assisted material restructuring employs extreme optical intensities to localize the ablation regions. To overcome the minimum feature size limit set by the wave nature of photons, there is a need for new approaches to tailored material processing at the nanoscale. Here, we report the formation of deeply-subwavelength features in silicon, enabled by localized laser-induced phase explosions in prefabricated silicon resonators. Using short trains of mid-infrared laser pulses, we demonstrate the controllable formation of high aspect ratio (>10:1) nanotrenches as narrow as [Formula: see text]. The trench geometry is shown to be scalable with wavelength, and controlled by multiple parameters of the laser pulse train, such as the intensity and polarization of each laser pulse and their total number. Particle-in-cell simulations reveal localized heating of silicon beyond its boiling point and suggest its subsequent phase explosion on the nanoscale commensurate with the experimental data. The observed femtosecond-laser assisted nanostructuring of engineered microstructures (FLANEM) expands the nanofabrication toolbox and opens exciting opportunities for high-throughput optical methods of nanoscale structuring of solid materials.

Femtosecond Laser Fabrication of Submillimeter Microlens Arrays with Tunable Numerical Apertures

In recent years, the demand for optical components such as microlenses has been increasing, and various methods have been developed. However, fabrication of submillimeter microlenses with tunable numerical aperture (NA) on hard and brittle materials remains a great challenge using the current methods. In this work, we fabricated a variable NA microlens array with submillimeter size on a silica substrate, using a femtosecond laser-based linear scanning-assisted wet etching method. At the same time, the influence of various processing parameters on the microlens morphology and NA was studied. The NA of the microlenses could be flexibly adjusted in the range of 0.2 to 0.45 by changing the scanning distance of the laser and assisted wet etching. In addition, the imaging and focusing performance tests demonstrated the good optical performance and controllability of the fabricated microlenses. Finally, the optical performance simulation of the prepared microlens array was carried out. The result was consistent with the actual situation, indicating the potential of the submillimeter-scale microlens array prepared by this method for applications in imaging and detection.

Ultrafast Laser Material Damage Simulation—A New Look at an Old Problem

The chirped pulse amplification technique has enabled the generation of pulses of a few femtosecond duration with peak powers multi-Tera and Peta–Watt in the near infrared. Its implementation to realize even shorter pulse duration, higher energy, and higher repetition rate laser systems relies on overcoming the limitations imposed by laser damage of critical components. In particular, the laser damage of coatings in the amplifiers and in post-compression optics have become a bottleneck. The robustness of optical coatings is typically evaluated numerically through steady-state simulations of electric field enhancement in multilayer stacks. However, this approach cannot capture crucial characteristics of femtosecond laser induced damage (LID), as it only considers the geometry of the multilayer stack and the optical properties of the materials composing the stack. This approach neglects that in the interaction of an ultrashort pulse and the materials there is plasma generation and associated material modifications. Here, we present a numerical approach to estimate the LID threshold of dielectric multilayer coatings based on strong field electronic dynamics. In this dynamic scheme, the electric field propagation, photoionization, impact ionization, and electron heating are incorporated through a finite-difference time-domain algorithm. We applied our method to simulate the LID threshold of bulk fused silica, and of multilayer dielectric mirrors and gratings. The results are then compared with experimental measurements. The salient aspects of our model, such as the implementation of the Keldysh photoionization model, the impact ionization model, the electron collision model for ‘low’-temperature, dense plasma, and the LID threshold criterion for few-cycle pulses are discussed.

Generation of even and odd high harmonics in resonant metasurfaces using single and multiple ultra-intense laser pulses

High harmonic generation (HHG) opens a window on the fundamental science of strong-field light-mater interaction and serves as a key building block for attosecond optics and metrology. Resonantly enhanced HHG from hot spots in nanostructures is an attractive route to overcoming the well-known limitations of gases and bulk solids. Here, we demonstrate a nanoscale platform for highly efficient HHG driven by intense mid-infrared laser pulses: an ultra-thin resonant gallium phosphide (GaP) metasurface. The wide bandgap and the lack of inversion symmetry of the GaP crystal enable the generation of even and odd harmonics covering a wide range of photon energies between 1.3 and 3 eV with minimal reabsorption. The resonantly enhanced conversion efficiency facilitates single-shot measurements that avoid material damage and pave the way to study the controllable transition between perturbative and non-perturbative regimes of light-matter interactions at the nanoscale.

Strong nonlinearities, like high harmonic generation in optical systems, can lead to interesting applications in photonics. Here the authors fabricate a thin resonant gallium phosphide metasurface capable of avoiding the laser-induced damage and demonstrate efficient even and odd high harmonic generation from it when driven by mid-infrared laser pulses.

Surface structure evolution and Raman response for multipulse, few-cycle, laser damaged ZnSe

Multiple 11-fs infrared, few-cycle laser pulses were applied to a polycrystal ZnSe surface to study the evolution of surface damage morphologies. The polycrystalline grain boundaries seem to be the initiation site of surface damage and formation of ripples, which evolve as the result of many laser pulses at the same site. Scanning electron microscopy and atomic force microscopy (AFM) were applied to characterize the surface. The crystalline change and material phase transition were examined by confocal Raman spectroscopy. The thermal expansion coefficient increased slightly in the ablated zone compared to the non-ablated zone according to an AFM thermal tip test. The results show the growth and organization of surface ripples and the change of thermal properties as the number of irradiations at each site increases.

Comparison of damage and ablation dynamics of multilayer dielectric films initiated by few-cycle pulses versus longer femtosecond pulses

The importance of high intensity few- to single-cycle laser pulses for applications such as intense isolated attosecond pulse generation is constantly growing, and with the breakdown of the monochromatic approximation in field ionization models, the few-cycle pulse (FCP) interaction with solids near the damage threshold has ushered a new paradigm of nonperturbative light-matter interaction. In this Letter, we systematically study and contrast how femtosecond laser-induced damage and ablation behaviors of SiO₂/HfO₂-based reflective multilayer dielectric thin film systems vary between FCP and 110 fs pulses. With time-resolved surface microscopy and ex situ analysis, we show that there are distinct differences in the interaction depending on the pulse duration, specifically in the "blister" morphology formation at lower fluences (damage) as well as in the dynamics of debris formation at higher fluences (ablation).

Single-Shot Multi-Stage Damage and Ablation of Silicon by Femtosecond Mid-infrared Laser Pulses

Although ultrafast laser materials processing has advanced at a breakneck pace over the last two decades, most applications have been developed with laser pulses at near-IR or visible wavelengths. Recent progress in mid-infrared (MIR) femtosecond laser source development may create novel capabilities for material processing. This is because, at high intensities required for such processing, wavelength tuning to longer wavelengths opens the pathway to a special regime of laser-solid interactions. Under these conditions, due to the λ² scaling, the ponderomotive energy of laser-driven electrons may significantly exceed photon energy, band gap and electron affinity and can dominantly drive absorption, resulting in a paradigm shift in the traditional concepts of ultrafast laser-solid interactions. Irreversible high-intensity ultrafast MIR laser-solid interactions are of primary interest in this connection, but they have not been systematically studied so far. To address this fundamental gap, we performed a detailed experimental investigation of high-intensity ultrafast modifications of silicon by single femtosecond MIR pulses (λ = 2.7–4.2 μm). Ultrafast melting, interaction with silicon-oxide surface layer, and ablation of the oxide and crystal surfaces were ex-situ characterized by scanning electron, atomic-force, and transmission electron microscopy combined with focused ion-beam milling, electron diffractometry, and μ-Raman spectroscopy. Laser induced damage and ablation thresholds were measured as functions of laser wavelength. The traditional theoretical models did not reproduce the wavelength scaling of the damage thresholds. To address the disagreement, we discuss possible novel pathways of energy deposition driven by the ponderomotive energy and field effects characteristic of the MIR wavelength regime.

Temporally Resolved Intensity Contouring (TRIC) for characterization of the absolute spatio-temporal intensity distribution of a relativistic, femtosecond laser pulse

Today’s high-power laser systems are capable of reaching photon intensities up to 10²² W cm⁻², generating plasmas when interacting with material. The high intensity and ultrashort laser pulse duration (fs) make direct observation of plasma dynamics a challenging task. In the field of laser-plasma physics and especially for the acceleration of ions, the spatio-temporal intensity distribution is one of the most critical aspects. We describe a novel method based on a single-shot (i.e. single laser pulse) chirped probing scheme, taking nine sequential frames at frame rates up to THz. This technique, to which we refer as temporally resolved intensity contouring (TRIC) enables single-shot measurement of laser-plasma dynamics. Using TRIC, we demonstrate the reconstruction of the complete spatio-temporal intensity distribution of a high-power laser pulse in the focal plane at full pulse energy with sub-picosecond resolution.

Ultrafast mid-infrared high harmonic and supercontinuum generation with n2 characterization in zinc selenide

Polycrystalline ZnSe is an exciting source of broadband supercontinuum and high-harmonic generation via random quasi phase matching, exhibiting broad transparency in the mid-infrared (0.5-20 μm). In this work, the effects of wavelength, pulse power, intensity, propagation length, and crystallinity on supercontinuum and high harmonic generation are investigated experimentally using ultrafast mid-infrared pulses. Observed harmonic conversion efficiency scales linearly in propagation length, reaching as high as 36%. For the first time to our knowledge, n₂ is measured for mid-infrared wavelengths in ZnSe: n₂(λ=3.9 μm)=(1.2±0.3)×10^-14 cm²/W. Measured n₂ is applied to simulations modeling high-harmonic generation in polycrystalline ZnSe as an effective medium.

Few cycle pulse laser ablation study of single layer TiO2 thin films using time resolved surface microscopy

The few cycle optical pulse induced strong field laser solid interaction is a rich area of fundamental and applied research that spans from the study of extreme non-linearities in solids and next generation ultra-broad band high damage threshold optics design and fabrication to peta-Hertz optoelectronics of the future. Our understanding of the extremely non-pertubative phenomena of few cycle pulse (FCP) laser damage and ablation of bulk solids and thin films is still limited. In this work, we present a systematic study of the dynamics of the FCP laser ablation process of single layer TiO₂ thin films from 1 ps to 10 ns after a single 9 fs pulse with nominal wavelength of 760 nm interacts with the surface using time-resolved surface microscopy (TRSM) technique. It is observed that FCP ablation craters for certain films exhibit markedly different features when compared to those created by 50 - 150 fs pulses with similar fluences. TRSM measurements also reveal that FCP ablation dynamics strongly depend on the thickness-dependent E-field distribution inside the films (nominally λ/2 vs λ/4), in which the dynamics of free carrier generation due to strong field ionization may play an important role as well. A one-dimensional finite-difference time-domain (FDTD) simulation that takes into account strong field ionization and free carrier absorption is used in conjunction with the TRSM measurements to estimate the excited free carrier density prior to ablation. We also propose a mechanism for the differences in ablation craters between the films based on the FDTD simulation results.