Femtosecond Laser-material Interaction Research Articles

Super-regular femtosecond laser nanolithography based on dual-interface plasmons coupling

Abstract Advances in femtosecond laser-material interaction facilitate the extension of maskless optical processing to the high efficiency and deep-subwavelength scale. Here, a hybrid plasmon lithography technique has been demonstrated by irradiating near-infrared femtosecond laser pulses onto the Si material coated with thin Cr films in a vacuum chamber, and superior nanograting structures are found to deeply penetrate through the thin Cr film into the underlying Si substrate. In stark contrast to the common ripple structures formed on the Si surface, the Cr-layer mediated Si nanograting structures not only exhibit the spatially super-regular arrangements with a deep-subwavelength period of 355 nm but also present the nonsinusoidal sharp-edged groove geometry with a large depth-to-width aspect ratio of 2.1. Theoretical analyses and calculations reveal that the anomalous structure characterizations are physically ascribed to the excitation of dual-interface coupled plasmons in the thin metal layer, which possess the squeezed spatial wavelength and the periodic columnar intensity distributions. Moreover, the further deepening of periodic nanostructures into the Si substrate is also elucidated by the simulation of electric field enhancements at the bottom of shallow grooves under irradiation of subsequent laser pulses. In combination with a wet etching process, the Si nanograting structures can be modified into the smooth and narrow-mouthed V-profiles, whose optical measurements show a near omnidirectional antireflection especially in the visible range of 565–750 nm, which is expected for the design of advanced photonic devices.

Control of supercontinuum generation from filamentation of shaped femtosecond laser pulses

Supercontinuum (SC), as one of the most spectacular phenomena occurring in the nonlinear process of intense femtosecond laser-material interaction, has attracted considerable interest. The broadband SC sources have a variety of applications including the spectroscopy, fluorescence microscopy, remote sensing, and generation of few-cycle pulses. Over the last few decades, the SC has been extensively investigated in various optical media, including liquid, gas, and solid. Especially, ultrabroadband SC sources have achieved remarkable development in the photonic crystal and micro-structured fibers. Even so, the generation of the SC with high brightness, high spatiotemporal coherence and good maneuverability, is still a challenging topic. The SC generation from femtosecond filamentation is a unique white light source with high pulse energy, high brightness and high spatiotemporal coherence, whose spectral range spans from ultraviolet to mid-infrared. In recent years, numerous studies have been conducted to optimize the filamentation and SC. The control of filamentation such as the filament length, number and position, as well as the generation of the ultra-broadband spectrum with high spectral energy density has been realized. To date, the optimal control of SC has been realized by the spatial modulation or time-domain shaping of the femtosecond laser pulse. However, there is no report on the control of SC generation and filamentation by spatiotemporally modulating the femtosecond laser pulses as far as we know. In this work, a spatiotemporal modulation for the femtosecond laser pulse is proposed, which combines the spatial modulation by using microlens array (MLA) and the laser pulse shaping based on liquid crystal spatial light modulator. We investigate the control of the SC generation from the filamentation of the spatiotemporally modulated femtosecond laser pulses in fused silica by using the feedback optimal control based on genetic algorithm. In our experiments, with the increase of the iterative generation, the cut-off wavelength in the blue-side extension of the SC becomes shorter gradually, and the spectral intensity of the SC increases significantly. After the eighth iteration, the increase of the spectral intensity slows. With the number of iterations increasing further, the intensity and broadening of SC spectrum will no longer apparently change. Hence, the feedback optimization control of spectral intensity of SC is realized, and the SC with controllable spectral intensity in a certain range is obtained. The maximum intensity variation of SC is more than three times. By integrating the spectral intensities of SC for different iterative generations, we characterize the increase trend of SC conversion efficiency. During the first few iterations, the conversion efficiency increases rapidly. Then it increases slowly after eighth generations and reaches its maximum after several generations (10th generation). The conversion efficiency has a similar evolution to the spectral intensity of the SC. To explain the physical mechanism, the initial envelope of the shaping pulse with typical iteration generation is calculated. It can be concluded that the spatial modulation of MLA allows for higher incident laser energy and for more filaments’ generation, which increases the energy of SC radiation directly. The peak intensity and envelope distribution of time domain pulse are the main factors affecting the spectral intensity and broadening the SC.

Integrating two-temperature and classical heat accumulation models to predict femtosecond laser processing of silicon

A robust, computationally efficient modeling method describing multi-pulse femtosecond laser-material interaction is required to determine the optimal laser parameters to control and differentiate non-thermal ablation and heat accumulation processes for surface structuring and laser welding applications. We establish a three-dimensional, two-temperature model (TTM) and a heat-accumulation model based on classical heat generation and conduction equations to evaluate their efficacy and efficiency in simulating non-thermal ablation and heat accumulation during multi-pulse femtosecond laser processing of silicon. Only the TTM is capable of accurately predicting the laser fluences required to achieve non-thermal ablation, which is experimentally validated. Both the TTM and the classical heat accumulation model can predict heat accumulation. The TTM can accurately predict heat accumulation, but requires lengthy simulation times on the order of several hours. The classical heat accumulation model consistently predicts heat accumulation with the TTM and is time efficient, but is case specific to interaction parameters, requiring input of an experimentally-determined absorption coefficient. For the first time to our knowledge, an integrated modeling method is devised to accurately and efficiently simulate laser-processing-induced heat accumulation by using the TTM to determine an absorption coefficient to feed back to the heat accumulation model to extend it to the general case. This integrated modeling method enables the accurate prediction of heat accumulation with simulation times on the order of a minute per pulse, defining a path to determine laser parameters to control heat accumulation for specific processing applications.

Direct visualization of the complete evolution of femtosecond laser-induced surface structural dynamics of metals

Despite extensive studies of femtosecond laser-material interactions, even the simplest morphological responses following femtosecond pulse irradiation have not been fully resolved. Past studies have revealed only partial dynamics. Here we develop a zero-background and high-contrast scattered-light-based optical imaging technique through which we capture, for the first time, the complete temporal and spatial evolution of the femtosecond laser-induced morphological surface structural dynamics of metals from start to finish, that is, from the initial transient surface fluctuations, through melting and ablation, to the end of resolidification. We find that transient surface structures first appear at a delay time on the order of 100 ps, which is attributed to ablation driven by pressure relaxation in the surface layer. The formation dynamics of the surface structures at different length scales are individually resolved, and the sequence of their appearance changes with laser fluence is found. Cooling and complete resolidification, observed here for the first time, are shown to occur more slowly than previously predicted by two orders of magnitude. We examine and identify the mechanisms driving each of these dynamic steps. The visualization and control of morphological surface structural dynamics not only are of fundamental importance for understanding femtosecond laser-induced material responses but also pave the way for the design of new material functionalities through surface structuring.

Computation of the expansion parameters of femto-waveguides using a two dimensional μ-Raman map and guided modes

The expansion parameters of a numerical elastic model that simulates femtosecond laser-material interactions are computed. To achieve this, two dimensional micro-Raman mapping, an elastic-optic finite element model, and the experimental intensity profile of the guided modes were used. To complement this data, the Raman deformation potential constants of the lithium niobate A1(TO4) phonon were obtained and compared with those reported in previous works.

Rapid bulk analysis using femtosecond laser ablation inductively coupled plasma time-of-flight mass spectrometry

Laser Ablation (LA) for direct solid sampling has advanced to provide accuracy and precision similar to that obtained using SN. Analytical performance metrics such as accuracy, precision, and limits of detection obtained when using femtosecond lasers are similar to those by SN. The reason for this improved performance is that the femtosecond laser produces essentially mono-disperse nanometer sized particles that are relatively easily digested in the ICP. In addition, the femtosecond laser material interaction is a photophysical process significantly reducing melting and elemental fractionation. A remaining aspect of ablation sampling is representative sampling; how much mass must be ablated to accurately analyze an inhomogeneous sample. The quantity of sample needed for bulk analysis is dependent on the inhomogeneity (or heterogeneity). Obviously, this same issue exists with sample digestion, and milligrams to grams of material are used to address this requirement. Laser ablation is beneficial in that only micrograms or less of sample are needed for the analysis. However, for bulk analysis of an inhomogeneous sample, the question is how much mass is required to adequately represent the bulk sample, and maintain the benefits of no sample digestion. The goal of this study was to demonstrate bulk analysis of a homogeneous and inhomogeneous sample by ablating large sample volumes using a high repetition rate (20 kHz) femtosecond pulsed laser. This approach is beneficial for bulk analysis of heterogeneous samples, for rapid analysis of difficult to dissolve samples, and for analyzing impurities over a large surface of homogeneous samples.

飞秒激光-物质相互作用产生Kα X射线

飞秒激光-物质相互作用产生Kα X射线

Plasma density inside a femtosecond laser filament in air: Strong dependence on external focusing

Our experiment shows that external focusing strongly influences the plasma density and the diameter of femtosecond Ti-sapphire laser filaments generated in air. The control of plasma filament parameters is suitable for many applications such as remote spectroscopy, laser induced electrical discharge, and femtosecond laser material interactions. The measurements of the filament showed the plasma density increases from 10(15)cm(-3) to 2 x 10(18)cm(-3) when the focal length decreases from 380 cm to 10 cm while the diameter of the plasma column varies from 30 microm to 90 microm. The experimental results are in good qualitative agreement with the results of numerical simulations.

Analysis of Optical Emission towrards Optimisation of Femtosecond Laser Processing

Recent research and development on the femtosecond laser has shown that it is a powerful tool for high preci- sion micromachining. However, femtosecond laser-material interaction is fast and complex due to its ultrashort pulse dura- tion. As the laser-induced plasma possesses useful information to understand the physical mechanism of laser processing, we investigate the optical emission during femtosecond laser material processing. Laser-induced plasma spectra of brass, silicon and tin were captured using a fibre optic spectrometer. Key parameters of the plasma such as plasma temperature and electron density were estimated based on the spectral details. These results may serve as a necessary basis of further re- search into the development of on-line monitoring techniques for the optimisation of the femtosecond laser processing.

IN-PROCESS MONITORING OF FEMTOSECOND LASER MATERIAL PROCESSING

Recent research and development on the femtosecond laser has shown that it is a powerful tool for high precision micromachining. However, the femtosecond laser-material interaction is fast and complex due to its ultrashort pulse duration. Consequently, it is not an easy task to optimize process parameters and to ensure high-quality fabrication. In this context, we propose an in-line monitoring technique based on plasma plume diagnostics. By analyzing the spectra of the femtosecond laser-induced plasma, which was detected and analyzed using a spectrometer, the plasma temperature of the copper sample was estimated. As drilling holes and cutting lines are two common applications in industry, the relationship between the emission light and the geometry of the fabricated structure was investigated based on the time-varying signals from the photodetector. These results will help us to understand the mechanism of femtosecond laser ablation so as to achieve high quality micromachining.

Measurements of intense femtosecond laser pulse propagation in air

The nonlinear self-focusing of an intense femtosecond pulse propagating in air can be balanced by the plasma defocusing as the laser intensity is increased above the threshold for multiphoton ionization. The resultant laser∕plasma filament can extend many meters, suitable for many applications such as remote atmospheric breakdown, laser induced electrical discharge, and femtosecond laser material interactions. Direct (bore-sight) measurements of filament size and fluence over 4 m showed a preservation of the total energy in the filament during propagation. This indicates the energy lost in creating the central plasma column through multiphoton ionization was continuously being replenished from the surrounding radiation. Electrical measurement of the filament conductivity estimated the plasma density to be 1×1016cm−3 and electrical discharges triggered by a femtosecond laser filament were found to occur at substantially reduced breakdown fields.

Imaging femtosecond laser-induced electronic excitation in glass

While substantial progress has been achieved in understanding laser ablation on the nanosecond and picosecond time scales, it remains a considerable challenge to elucidate the underlying mechanisms during femtosecond laser material interactions. We present experimental observations of electronic excitation inside a wide band gap glass during single femtosecond laser pulse (100 fs, 800 nm) irradiation. Using a femtosecond time-resolved imaging technique, we measured the evolution of a laser-induced electronic plasma inside the glass and calculated the electron number density to be on the order of 1019 cm−3.