Absorption Of Femtosecond Laser Pulses Research Articles

Predicting and probing the local temperature rise around plasmonic core-shell nanoparticles to study thermally activated processes.

Ultrafast spectroscopy can be used to study dynamic processes on femtosecond to nanosecond timescales, but is typically used for photoinduced processes. Several materials can induce ultrafast temperature rises upon absorption of femtosecond laser pulses, in principle allowing to study thermally activated processes, such as (catalytic) reactions, phase transitions, and conformational changes. Gold-silica core-shell nanoparticles are particularly interesting for this, as they can be used in a wide range of media and are chemically inert. Here we computationally model the temporal and spatial temperature profiles of gold nanoparticles with and without silica shell in liquid and gas media. Fast rises in temperature within tens of picoseconds are always observed. This is fast enough to study many of the aforementioned processes. We also validate our results experimentally using a poly(urethane-urea) exhibiting a temperature-dependent hydrogen bonding network, which shows local temperatures above 90 ◦C are reached on this timescale. Moreover, this experimentally shows the hydrogen bond breaking in such polymers occurs within tens of picoseconds.

Ultrathin Langmuir–Schaefer films of slipped-cofacial J-type phthalocyanine dimer: Supramolecular organization, UV/Vis/NIR study and nonlinear absorbance of femtosecond laser radiation

Based on a comprehensive theoretical and experimental approach, a low-defect thin film of stable dimeric phthalocyanine zinc complex (bis-[2-hydroxy-9(10),16(17), 23(24)-tri-tert-butylphthalocyanine]zinc – J-[OHPctZn]2) was prepared. It is shown that the dye forms stable amorphous edge-on monolayers at the air/water interface with an insignificant content of cylindrical 3D aggregates. These floating layers keep their structure when transferred by the Langmuir-Schaefer method onto a solid substrate, demonstrating a tendency towards ordered H-aggregation with preserving their linear molecular properties. Mixed two- and three-photon absorption of femtosecond laser pulses (280 fs) was firstly detected on this sample, as well as the ability of a material to the nonlinear attenuation of the intense irradiation, which is important for optical limiting.

Femtosecond-Laser-Induced Nanoscale Blisters in Polyimide Thin Films through Nonlinear Absorption

Nonlinear absorption of femtosecond laser pulses provides a unique opportunity to confine energy deposition in any medium to a region that is below the focal diameter of a pulse. Illumination of a polymer film through a transparent high-band-gap material such as glass, followed by nonlinear absorption of 800-nm light in polymers, allows us to further restrict absorption to a very thin layer along the propagation direction. We demonstrate this confinement by simulating femtosecond-laser-induced polymer modification by linear, two-photon, and three-photon absorption, and discuss the control over energy absorption in polymers that multiphoton processes offer. Energy deposited in a thin polymer film induces a protruding blister. We present experimental results for blister diameter and height scaling with variation of pulse energy. Using pulse energies of 20--200 nJ and 0.4-NA focusing, we fabricate blisters with diameters of $1--5.5\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ and heights of $75\phantom{\rule{0.2em}{0ex}}\mathrm{nm}$ to $2\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Using 0.95-NA focusing, we obtain laser-induced blisters with diameters as small as 700 nm, suggesting blister-based laser-induced forward transfer is possible on and below the 1-$\ensuremath{\mu}\mathrm{m}$ scale. Submicrometer blister formation with use of femtosecond lasers also offers a method of direct, precise laser writing of microstructures on films with single laser pulses. This method is a possible alternative to lithography, laser milling, and laser-based additive machining.

Femtosecond Laser Synthesis of Luminescent Carbon Dots from Toluene

Products of reactions in toluene induced by nonlinear absorption of femtosecond laser pulses are studied. Laser synthesis of carbon dots with characteristic sizes of several nanometers and luminescence in the visible spectrum is demonstrated. A relation between synthesis conditions, morphology, and composition of carbon dots and their optical properties is established. The possibility of single-stage laser synthesis of polyethylene glycol passivated carbon dots having an enhanced quantum yield and improved spectral characteristics of luminescence is demonstrated. The results of the work open up new opportunities for obtaining promising carbon nanomaterials and optimizing their optical properties through the use of femtosecond laser pulses.

MHz-ultrasound generation by chirped femtosecond laser pulses from gold nano-colloidal suspensions.

Strong absorption of femtosecond laser pulses in Au nano-colloidal suspensions was used to generate coherent ultrasound signals at 1-20 MHz frequency range. The most efficient ultrasound generation was observed at negative chirp values and was proportional to the pulse duration. Maximization of a dimensionless factor A ≡ αc0tp defined as the ratio of pulse duration tp and the time required for sound at speed c0 to cross the optical energy deposition length (an inverse of the absorption coefficient α) given by 1/(αc0). Chirp controlled pulse duration allows effective enhancement of ultrasound generation at higher frequencies (shorter wavelengths) and is promising for a high spatial resolution acoustic imaging.

Quantitative characterization of energy absorption in femtosecond laser micro-modification of fused silica.

We present the results of experimental and theoretical study of an energy absorption of femtosecond laser pulse in fused silica. Fundamental and second harmonics of ytterbium laser were used in experiment while general case was considered theoretically and numerically. More efficient absorption at the second harmonics is confirmed both experimentally and numerically. Quantitative characterization of the theoretical model is performed by fitting key parameters of the absorption process such as cross-section of multi-photon absorption and effective electronic collision and recombination times.

Heating of a metal nanofilm during femtosecond laser pulse absorption

We have studied the temperature evolution of electrons and the lattice of a metal nanofilm interacting with a femtosecond s- or p-polarised pulse. It is shown that even if the film thickness is greater than the skin-layer depth, the temperature distribution during the pulse action may be close to the uniform one because of the high electron thermal conductivity, which leads to a rapid redistribution of energy over the film thickness.

Nonlinear absorption and optical damage threshold of carbon-based nanostructured material embedded in a protein

Physical processes in laser–matter interaction used to be determined by generation of fast electrons resulting from efficient conversion of the absorbed laser radiation. Composite materials offer the possibility to control the absorption by choice of the host material and dopants. Reported here strong absorption of ultrashort laser pulse in a composite carbon-based nanomaterial including single-walled carbon nanotubes (SWCNTs) or multilayer graphene was measured in the intensity range between 1012 and 1016 W cm−2. A protein (lysozyme) was used as the host. The maximum absorption of femtosecond laser pulse has reached 92–96 %. The optical damage thresholds of the coatings were registered at an intensity of (1.1 ± 0.5) × 1013 W cm−2 for the embedded SWCNTs and at (3.4 ± 0.3) × 1013 W cm−2 for the embedded graphene. Encapsulated variant of the dispersed nanomaterial was investigated as well. It was found that supernatant protein in the coating material tends to dominate the absorption process, independently of the embedded nanomaterial. The opposite was observed for the encapsulated material.

Modification of a-Si: H Films via Non-linear Femtosecond Laser Pulse Absorption

The modification of a-Si:H via non-linear femtosecond laser pulse absorption was studied and the characteristic thresholds for hydrogen diffusion/effusion, crystallization and material ablation were determined. To consider the impact of the hydrogen content on laser materials processing, a-Si:H was deposited at different temperatures (25°C, 200°C, 520°C) resulting in different hydrogen contents (30%, 13%, and <1%). Essential information for device applications such as the degree of crystallization and the Si-H dissociation are obtained from micro-Raman spectroscopy of the laser treated areas. The prospects of a flexible non-linear fs laser material processing of a-Si:H for bulk and surface modification will be discussed.

Non-linear absorption of femtosecond laser pulses in a SiN layer—influence of silicon doping type

Abstract In this article, silicon nitride (SiN x ) layers deposited on planar silicon wafers with two different doped areas (emitter: highly phosphorous doped and bulk: lightly boron doped) were locally irradiated by laser pulses. Our investigation is focused on the ablation mechanisms. For that purpose, an ultra-short laser source (pulse duration 0.3–12 ps, wavelength 1025 nm, Gaussian profile) was used. For high pulse durations and low fluences the SiN x layer is removed completely by lift-off. However, for lower pulse durations and higher fluences, the SiN x layer is not completely removed, due to direct ablation. By using an emitter, direct ablation of SiN x layers were observed also for higher pulse durations. Thus, the absorption process in the SiN x layer can be described as avalanche ionization with seed electrons from silicon.

Laser microstructured biodegradable scaffolds

The two-photon polymerization technique (2PP) uses non-linear absorption of femtosecond laser pulses to selectively polymerize photosensitive materials. 2PP has the ability to fabricate structures with a resolution from tens of micrometers down to hundreds of nanometers. Three-dimensional microstructuring by the 2PP technique provides many interesting possibilities for biomedical applications. This microstructuring technique is suitable with many biocompatible polymeric materials, such as polyethylene glycol, polylactic acid, polycaprolactone, gelatin, zirconium-based hybrids, and others. The process of fabrication does not require clean room conditions and does not use hazard chemicals or high temperatures. The most beneficial property of 2PP is that it is capable of producing especially complex three-dimensional (3-D) structures, including devices with overhangs, without using any supportive structure. The flexibility in controlling geometries and feature sizes and the possibility to fabricate structures without the addition of new material layers makes this technique particularly appealing for fabrication of 3-D scaffolds for tissue engineering.

Signals of the femtosecond photon echo in inorganic films and their recording

We report the results of the first experiment on recording of photon echo and self-diffraction signals in a zinc oxide semiconductor film of nanoscale thickness, which are excited by two-photon absorption of femtosecond laser pulses with wavelengths of 800 to 840nm for the exciton transition at room temperature.

Nonlinear absorption of femtosecond laser pulses (800 nm) by atmospheric air and water vapour

Quantitative data on the nonlinear absorption cross sections of femtosecond Ti : Sapphire laser pulses in air and water vapour have been obtained. A photoacoustic spectrometer calibrated based on the calculated value of linear absorption of laser pulses with a wavelength of 800 nm and a spectral width of 17.7 nm is used to find the nonlinear absorption cross sections of water vapour and air: σ2w = (2.6±0.4) × 10-55 cm4 s and σ2a = (8.7±1.0) × 10-56 cm4 s, respectively. Based on measuring the absorption of femtosecond Ti : Sapphire laser pulses with a photoacoustic detector calibrated with the known linear absorption of ruby laser radiation by water vapour in air, the air nonlinear absorption cross section is found to be (8.2±0.9) × 10-56 cm4 s.

Simulation of absorption of femtosecond laser pulses in solid-density copper

We present a simulation of absorption of femtosecond laser pulses by a copper target. The modeling involved thermodynamic functions calculated by using a first-principles full-potential linear muffin-tin orbital method and chemical-picture-based model of dense plasma utilizing a superconfiguration approach. The results of the simulation are compared to experimental and other theoretical data. The role of the electron-ion energy exchange is analyzed and further work on detailed improvement of the presented theoretical model is discussed.

Ultrafast Laser Induced Structural Modification of Fused Silica—Part II: Spatially Resolved and Decomposed Raman Spectral Analysis

Nonlinear absorption of femtosecond laser pulses enables the induction of structural changes in the interior of bulk transparent materials without affecting their surface. In the present study, femtosecond laser pulses were tightly focused within the interior of bulk fused silica specimen. Localized plasma was formed, initiating rearrangement of the random network structure. Cross sections of the induced features were examined via decomposition of spatially resolved Raman spectra and a new method for the quantitative characterization of the structure of amorphous fused silica was developed. The proposed method identifies the volume fraction distribution of ring structures within the continuous random network of the probed volume of the target material and changes of the distribution with laser process conditions. Effects of the different process conditions and the material response to different mechanisms of feature generation were discussed as well.

Cascaded nonlinear absorption of femtosecond laser pulses in dielectrics

We investigated the energy deposition process leading to the waveguide inscription in transparent dielectrics both experimentally and theoretically. Parameters of multiphoton absorption process and inscription thresholds were measured in a range of materials including YAG, ZnSe, RbPb2Cl5 crystals, and in fused silica and BK7 glasses.

Experimental and theoretical study of absorption of femtosecond laser pulses in interaction with solid copper targets

The reflectivity of near-infrared 150 fs laser pulses from copper targets has been measured in the intensity range of $6\ifmmode\times\else\texttimes\fi{}{10}^{11}\char21{}1.6\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{W}\text{ }{\text{cm}}^{\ensuremath{-}2}$, showing a drop in reflectivity versus intensity as the target is heated. A simple semianalytical model of femtosecond electron heating and resultant optical absorption is developed to describe the time dependent and integral reflectivity covering the range from cold metal response to hot plasma response. The model is in good agreement with experimental ultrafast reflectivity measurements over the intensity range studied.

Two-temperature relaxation and melting after absorption of femtosecond laser pulse

The theory and experiments concerned with the electron–ion thermal relaxation and melting of overheated crystal lattice constitute the subject of this paper. The physical model includes two-temperature (2T) equation of state, many-body interatomic potential, the electron–ion energy exchange, electron thermal conductivity, and optical properties of solid, liquid, and two phase solid–liquid mixture. Two-temperature hydrodynamics and molecular dynamics codes are used. An experimental setup with pump–probe technique is used to follow evolution of an irradiated target with a short time step 100 fs between the probe femtosecond laser pulses. Accuracy of measurements of reflection coefficient and phase of reflected probe light are 1% and ∼ 1 nm, respectively. It is found that, firstly, the electron–electron collisions make a minor contribution to a light absorption in solid Al at moderate intensities; secondly, the phase shift of a reflected probe results from heating of ion subsystem and kinetics of melting of Al crystal during 0 ps < t < 4 ps, where t is time delay between the pump and probe pulses measured from the maximum of the pump; thirdly, the optical response of Au to a pump shows a marked contrast to that of Al on account of excitation of d-electrons.

INTRACELLULAR MANIPULATION BY FEMTOSECOND LASERS: REVIEW

Multiphoton absorption of femtosecond laser pulses focused through an objective with high numerical aperture (NA) can be used to image and manipulate cellular and intracellular objects. This review highlights recent advances in intracellular manipulation, including nanosurgery and labeling in living cells with femtosecond lasers.

Buried channel waveguides in Yb-doped KY(WO 4) 2 crystals fabricated by femtosecond laser irradiation

We report on the fabrication of optical channel waveguides inside KY(WO 4) 2 crystals, both undoped and Yb-doped, for applications in compact solid-state lasers and three-dimensional photonic devices. Nonlinear absorption of femtosecond laser pulses has been employed in order to induce refractive-index changes in these crystals. The irradiation damage results in a decrease of the material density compared to the surrounding bulk. In this work, two types of buried channel waveguides have been fabricated in the KY(WO 4) 2 crystals. First ones (type I) were formed in the vicinity of the irradiated regions due to the presence of an induced compressive lattice strain, which causes an increase of the local refractive index. Light can be better confined in the second (type II) of channel waveguides that are created between pairs of damaged regions. For the best confined channels (type II), the propagation loss value measured at 1 μm amounted to 2–2.5 dB/cm.