Nanoporous antireflection coating for high-temperature applications in the infrared.

Antireflection (AR) coatings are essential to the performance of optical systems; without them, surface reflections increase significantly at steep angles and become detrimental to the functionality. AR coatings apply to a wide range of applications from solar cells and laser optics to optical windows. Many times, operational conditions include high temperatures and steep angles of incidence (AOIs). The implementation of AR coatings is extremely challenging in these conditions. Nanoporous coatings made from high-temperature-tolerant materials offer a solution to this problem. The careful selection of materials is needed to prevent delamination when exposed to high temperatures, and an optimal optical design is needed to lower surface reflections at both the normal incidence and steep AOIs. This paper presents nanoporous silicon dioxide and hafnium dioxide coatings deposited on a sapphire substrate using oblique angle deposition by electron beam evaporation, a highly accurate deposition technique for thin films. Developed coatings were tested in a controlled temperature environment and demonstrated thermal stability at temperatures up to 800°C. Additional testing at room temperature demonstrated the reduction of power reflections near optimal for AOIs up to 70° for a design wavelength of 1550 nm. These findings are promising to help extend the operation of technology at extreme temperatures and steep angles.

Femtosecond damage experiments and modeling of broadband mid-infrared dielectric diffraction gratings.

High peak and average power lasers with high wall-plug efficiency, like the Big Aperture Thulium (BAT) laser, have garnered tremendous attention in laser technology. To meet the requirements of the BAT laser, we have developed low-dispersion reflection multilayer dielectric (MLD) gratings suitable for compression of high-energy pulses for operations at 2 micron wavelength. We carried out 10000-on-1 damage tests to investigate the fluence damage thresholds of the designed MLD gratings and mirrors, which were found between 100-230 mJ/cm2. An ultrashort pulsed laser (FWHM = 53 fs, λ = 1.9 µm) operating at 500 Hz was used in the serpentine raster scans. The atomic force microscope images of the damage sites show blister formation of the underlying layers at lower fluences but ablation of the grating pillars at higher fluences. We simulated the dynamic electronic excitation in the MLD optics with a finite-difference in the time domain approach in 2D. The simulation results agree well with the LIDT measurements and the observed blister formation. This model is able to evaluate the absolute LIDT of MLD gratings.

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).

Few-cycle pulse laser induced damage threshold determination of ultra-broadband optics.

A systematic study of few-cycle pulse laser induced damage threshold (LIDT) determination was performed for commercially-available ultra-broadband optics, (i.e. chirped mirrors, silver mirrors, beamsplitters, etc.) in vacuum and in air, for single and multi-pulse regime (S-on-1). Multi-pulse damage morphology at fluences below the single-pulse LIDT was studied in order to investigate the mechanisms leading to the onset of damage. Stark morphological contrast was observed between multi-pulse damage sites formed in air versus those in vacuum. One effect of vacuum testing compared to air included suppression of laser-induced periodic surface structures (LIPSS) formation, possibly influenced by a reduced presence of damage debris. Another effect of vacuum was occasional lowering of LIDT, which appears to be due to the stress-strain performance of the coating design during laser irradiation and under the external stress of vacuum ambience. A fused silica substrate is also examined, and a non-LIPSS nanostructuring is observed on the surface. Possible mechanisms are discussed.

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.

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.

Photon acceleration and tunable broadband harmonics generation in nonlinear time-dependent metasurfaces.

Time-dependent nonlinear media, such as rapidly generated plasmas produced via laser ionization of gases, can increase the energy of individual laser photons and generate tunable high-order harmonic pulses. This phenomenon, known as photon acceleration, has traditionally required extreme-intensity laser pulses and macroscopic propagation lengths. Here, we report on a novel nonlinear material-an ultrathin semiconductor metasurface-that exhibits efficient photon acceleration at low intensities. We observe a signature nonlinear manifestation of photon acceleration: third-harmonic generation of near-infrared photons with tunable frequencies reaching up to ≈3.1ω. A simple time-dependent coupled-mode theory, found to be in good agreement with experimental results, is utilized to predict a new path towards nonlinear radiation sources that combine resonant upconversion with broadband operation.

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.