Michael Tripepi

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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\sim }{{{\boldsymbol{\lambda }}}{{{\boldsymbol{/}}}{{{\boldsymbol{80}}}$$\end{document} ~ λ / 80 . 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. Laser machining can modify and reshape materials on the scale comparable to light’s wavelength. Here, authors use tailored microstructures to push the limit of laser machining to a scale that is almost 100 times smaller than a wavelength of light.

As in all physical models, the response of a system to external perturbations is only approximately linear and with greater driving force, becomes nonlinear. The study of light interactions in solids is no different and with the development of laser technology, this can be explored with great detail. The use of femtosecond, and even few-cycle pulse lasers, allows for the study of very intense, localized interactions, which can range from frequency generation and spectral broadening of the incident pulse to photoionization of electrons and optical damage of the material itself. This work presents a trio of experiments that investigate the interaction of strong laser pulses with crystalline solids using femtosecond pulses. The first experiment characterizes the supercontinuum generation (SCG) in undoped, single-crystal yttrium aluminum garnet (YAG) optical fibers. Pump wavelengths near the zero-dispersion wavelength of YAG (1600 nm) are used to study SCG in both the normal and anomalous group-velocity dispersion (GVD) regimes. The output spectra of the fiber over many input pulse energies are collected for each pump wavelength and presented in 2D "energy scans." Three different criteria are developed to help characterize the SCG in each energy scan. The second experiment looks at laser damage of gallium nitride (GaN) and gallium oxide (Ga2O3) using few-cycle pulses (FCPs) with a central wavelength near 760 nm. These experiments report on the damage thresholds as well as examine the crater morphology of single and multi-shot damage. Simulations using the Keldysh photoionization model and finite-difference time-domain method coupled to ionization and plasma effects are also employed to understand the carrier dynamics for single-shot exposure and possible mechanisms that differentiate FCP from longer femtosecond mechanisms. The final experiment returns to YAG in order to study damage under FCPs using a pump-probe technique known as time-resolved surface microscopy. The dynamics of the damage process are imaged from < 85 fs to 2 ns and analyzed using a Python code to extract the relative change in transmission along a vertical trace of the image. Using the Drude-Lorentz model, the change in transmission can be converted to the free electron concentration in the conduction band, which is used to estimate the relaxation time(s) for the carriers in YAG. An algorithm is also developed to overlay images of the damage crater and focal spot to quickly and robustly determine the damage threshold. Atomic force microscopy images of the damage craters revealing unusual characteristics for short pulse damage are also discussed.

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/cm(2). An ultrashort pulsed laser (FWHM = 53 fs, lambda = 1.9 mu m) 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. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Yttrium aluminum garnet (YAG) is a common host material for both bulk and single-crystal fiber lasers. With increasing interest in developing optical technologies in the short-wave infrared and mid-infrared wavelength range, YAG may be a potential supercontinuum medium for these applications. Here, we characterize femtosecond laser pumped supercontinuum generation with 1200-2000 nm pump wavelengths (lambda(p)) for undoped, single-crystal YAG fibers, which are representative of the normal, zero, and anomalous-dispersion regimes. Supercontinuum was observed over the spectral region of about 0.2 to 1.6 lambda(p). Z-scan measurements were also performed of bulk YAG, which revealed little dispersion of the nonlinear index of refraction (n(2)) in the region of interest. The measured values of n(2) (similar to 1 x 10(-6) cm(2)/GW) indicate a regime in which the nonlinear length, L-NL, is less than the dispersion length, L-D, (L-NL << L-D). We report intensity clamping of the generated filament in the normal group velocity dispersion (GVD) regime and an isolated anti-Stokes peak in the anomalous GVD regime, suggesting further consideration is needed to optimize supercontinuum generation in this fiber medium. (C) 2021 Optical Society of America

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.

Polycrystalline materials can mediate efficient frequency up-conversion for mid-infrared light. Motivated by the need to understand the properties of the harmonic and supercontinuum radiation from such media, we utilize realistic numerical simulations to reveal its complex temporal and spatial structure. We show that the generated radiation propagates in the form of long-duration pulse trains that can be difficult to compress and that optical filamentation in high-energy pulses gives rise to fine-structured beam profiles. We identify trends concerning pulse energy, sample length, and the microstructure of the material that can inform optimization for different applications. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Conference Title: 2021 Conference on Lasers and Electro-Optics (CLEO) Conference Start Date: 2021, May 9 Conference End Date: 2021, May 14 Conference Location: San Jose, CA, USAUltrafast dynamics of ultrafast single pulse induced micro-explosions in bulk fused silica was captured using.time resolved shadowgraphy. Experimental and theoretical considerations identify such micro-explosions creating warm dense matter (WDM) states.

We present a new analysis of the profile data from the 47 millisecond pulsars comprising the 12.5 yr data set of the North American Nanohertz Observatory for Gravitational Waves, which is presented in a parallel paper (Alam et al., hereafter NG12.5). Our reprocessing is performed using "wideband" timing methods, which use frequency-dependent template profiles, simultaneous time-of-arrival (TOA) and dispersion measure (DM) measurements from broadband observations, and novel analysis techniques. In particular, the wideband DM measurements are used to constrain the DM portion of the timing model. We compare the ensemble timing results to those in NG12.5 by examining the timing residuals, timing models, and noise-model components. There is a remarkable level of agreement across all metrics considered. Our best-timed pulsars produce encouragingly similar results to those from NG12.5. In certain cases, such as high-DM pulsars with profile broadening or sources that are weak and scintillating, wideband timing techniques prove to be beneficial, leading to more precise timing model parameters by 10%-15%. The high-precision, multiband measurements of several pulsars indicate frequency-dependent DMs. Compared to the narrowband analysis in NG12.5, the TOA volume is reduced by a factor of 33, which may ultimately facilitate computational speed-ups for complex pulsar timing array analyses. This first wideband pulsar timing data set is a stepping stone, and its consistent results with NG12.5 assure us that such data sets are appropriate for gravitational wave analyses.

We present time-of-arrival (TOA) measurements and timing models of 47 millisecond pulsars observed from 2004 to 2017 at the Arecibo Observatory and the Green Bank Telescope by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). The observing cadence was three to four weeks for most pulsars over most of this time span, with weekly observations of six sources. These data were collected for use in low-frequency gravitational wave searches and for other astrophysical purposes. We detail our observational methods and present a set of TOA measurements, based on "narrowband" analysis, in which many TOAs are calculated within narrow radio-frequency bands for data collected simultaneously across a wide bandwidth. A separate set of "wideband" TOAs will be presented in a companion paper. We detail a number of methodological changes, compared to our previous work, which yield a cleaner and more uniformly processed data set. Our timing models include several new astrometric and binary pulsar measurements, including previously unpublished values for the parallaxes of PSRs J1832−0836 and J2322+2057, the secular derivatives of the projected semimajor orbital axes of PSRs J0613−0200 and J2229+2643, and the first detection of the Shapiro delay in PSR J2145−0750. We report detectable levels of red noise in the time series for 14 pulsars. As a check on timing model reliability, we investigate the stability of astrometric parameters across data sets of different lengths. We also report flux density measurements for all pulsars observed. Searches for stochastic and continuous gravitational waves using these data will be subjects of forthcoming publications.

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). (C) 2020 Optical Society of America.