Efficiently shaping femtosecond, transverse Gaussian laser beams to flat-top beams with flat wavefronts is critical for large-scale material processing and manufacturing. Existing beam shaping devices fall short either in final beam homogeneity or efficiency. We present an approach that uses refractive optics to perform the majority of the beam shaping and then uses a fine-tune device (spatial light modulator) to refine the intensity profile. For the beam that we selected, circularly asymmetric with intensity fluctuations, our method achieved a uniformity of 0.055 within 90% of the beam area at 92% efficiency. The optimization involved an iterative beam shaping process that converged to optimum within 10 iterations.
We demonstrated laser-induced broadband emitters (LIBEs) with spectral emissivity higher than 0.96 from 0.3 um to 15 um wavelength to increase thermal radiative energy transport. Localized material removal induced by ultrafast femtosecond laser irradiation results in the hierarchical formation of microstructures decorated with micro-/nano- particles, leading to an exceptional enhancement in a spectral absorptivity on different types of substrates. Finite-difference time-domain simulations validated the effects of surface topology on the experimentally measured absorptivity. Moreover, LIBEs maintain their enhanced spectral absorptivity of 0.92 after heating at elevated temperatures for over 100 hours. Our results provide new insights into the use of ultrafast laser-matter interactions in cutting-edge energy harvesting and thermal management applications.
We study the effects of controlled atmospheric turbulence on femtosecond filament-induced plasmas and ablation characteristics upon its interaction with solids at standoff distances. We evaluated shot-to-shot optical emission signal stability, electron number density, and plasma temperature as a function of turbulence strength. The surface topography of the ablation craters resulting from the filament-matter interaction was studied in detail and was correlated to the corresponding optical emission signals under different levels of turbulence. These findings provide first insights into filament-induced plasma optical emission signals and filament target-interaction under controlled turbulence conditions for remote elemental and isotopic sensing applications.
Remote laser detection requires laser beam propagation through often unfavorable real-world atmospheric conditions. Turbulence is one of the main factors causing the beam to get distorted and lose its integrity and has direct implications on our ability to detect spectroscopic signatures remotely. One method to suppress turbulence effects is the use of vortex beams which have a spiral phase structure and carry the Orbital Angular Momentum (OAM). Vortex beams are generally considered to be more robust against the turbulence effects compared to conventional Gaussian beams. However, the actual vortex beam performance in remote sensing is also heavily dependent on the exact mode of vortex beam family it belongs to. In this work, we simulate the propagation of beams to a distance of 100 meters under controlled turbulence and compared the performance of a conventional Gaussian beam to two different modes of vortex beams: Hypergeometric Gaussian (HyGG) and Laguerre Gaussian (LG) beams. Through detailed evaluation of the beam circularity as a quantitative metric for beam propagation under turbulence, we demonstrate that HyGG vortex beams exhibit improved resistance against turbulence when compared to conventional Gaussian beams and LG vortex beams. This work provides insights toward a more comprehensive understanding of how vortex beams benefit long-range remote detection and identifying new strategies for long propagation-range propagation of tailored laser beams.
In this work, we study the ablation dynamics of copper (Cu) induced by single fs pulse and fs GHz bursts using in situ multimodal diagnostics; time-resolved scattering imaging, emission imaging, and optical emission spectroscopy. Multimodal probing techniques reveal that fs GHz bursts rapidly remove molten liquid Cu from the irradiated spot due to the recoil pressure exerted by following fs pulses. Material ejection stops after burst irradiation due to the limited amount of remnant matter, combined with the suppressed heat conduction into the target material. Our work provides insights into the complex ablation mechanisms of GHz fs bursts, which are critical in selecting optimal laser conditions in cross-cutting processing and micro/nano-fabrication applications.
Materials processing by ultrafast lasers offers several attractive possibilities for micro/nano fabrication applications.
Several exciting prospects arise in the context of surface and bulk laser induced modifications. These form the basis for
diverse applications, including the development and functionalization of laser engineered surfaces, the laser transfer of
biomolecules and the functionalization of 3D structures constructed by multiphoton stereolithography. In particular, two
examples will be discussed in the following, namely a new approach for the development of superhydrophobic, self
cleaning surfaces and the fabrication of functionalized scaffolds, for tissue engineering applications.
We report on the reproducible growth of stoichiometric thin films of ferromagnetic intermetallic compound NiMnSb by pulsed laser deposition (PLD) on various substrates. The films are grown at moderate temperature (around 200 degree(s)C) using polycrystalline targets. Two different substrates were employed - single crystalline silicon and InAs polycrystalline - to investigate the influence of the thin layer/substrate lattice mismatch on the quality of the grown film. XRD and EDX analyses indicate that the layers are of high crystalline quality and their stoichiometry is very close to that of the corresponding targets, respectively. SEM images show that there are droplets on the surface of the films and their composition is similar to that of the targets. Magnetic measurements performed at both room temperature and 5K find that the investigated samples have small Hc values.
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