Absorption Of Laser Pulses Research Articles

Modeling resonant energy absorption of finite laser pulses in a doped porous dielectric slab

Modeling resonant energy absorption of finite laser pulses in a doped porous dielectric slab

Transient absorption of warm dense matter created by an X-ray free-electron laser

Warm dense matter is at the boundary between a plasma and a condensed phase and plays a role in astrophysics, planetary science and inertial confinement fusion research. However, its electronic structure and ionic structure upon irradiation with strong laser pulses remain poorly understood. Here, we use an intense and ultrafast X-ray free-electron laser pulse to simultaneously create and characterize warm dense copper using L-edge X-ray absorption spectroscopy over a large irradiation intensity range. Below a pulse intensity of 1015 W cm−2, an absorption peak below the L edge appears, originating from transient depletion of the 3d band. This peak shifts to lower energy with increasing intensity, indicating the movement of the 3d band upon strong X-ray excitation. At higher intensities, substantial ionization and collisions lead to the transition from reverse saturable absorption to saturable absorption of the X-ray free-electron laser pulse, two nonlinear effects that hold promise for X-ray pulse-shaping. We employ theoretical calculations that combine a model based on kinetic Boltzmann equations with finite-temperature real-space density-functional theory to interpret these observations. The results can be used to benchmark non-equilibrium models of electronic structure in warm dense matter.

Absorption of ultrashort laser pulses in medium with account for propagation effects

The absorption of ultrashort laser pulses in an optically dense medium is theoretically studied based on the total absorption coefficient over the entire duration of the pulse, taking into account the effects of propagation and reflection of pulse from vacuum/medium interface. The approach is the generalization of our previous work (available at: www.nobelprize.org/prizes/physics/2023/summary), which traced a ‘bridge’ between the micro- and macro-description of radiation processes in a medium induced by electromagnetic pulses of arbitrary duration. The expression for the total absorption coefficient is obtained in terms the complex refractive index of the medium. Various limits of this expression are considered. Numerical calculations of the dependence of the total absorption coefficient on the parameters of the laser pulses and the thickness of the medium layer were carried out and the results obtained were discussed.

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 experiment shows the hydrogen bond breaking in such polymers occurs within tens of picoseconds.

Coupled atomistic spin-lattice simulations of ultrafast demagnetization in 3d ferromagnets.

Despite decades of research, the role of the lattice and its coupling to the magnetisation during ultrafast demagnetisation processes is still not fully understood. Here we report on studies of both explicit and implicit lattice effects on laser induced ultrafast demagnetisation of bcc Fe and fcc Co. We do this using atomistic spin- and lattice dynamics simulations following a heat-conserving three-temperature model. We show that this type of Langevin-based simulation is able to reproduce observed trends of the ultrafast magnetization dynamics of fcc Co and bcc Fe. The parameters used in our models are all obtained from electronic structure theory, with the exception of the lattice dynamics damping term, where a range of parameters were investigated. It was found that while the explicit spin-lattice coupling in the studied systems does not impact the demagnetisation process notably, the lattice damping has a large influence on the details of the magnetization dynamics. The dynamics of Fe and Co following the absorption of a femtosecond laser pulse are compared with previous results for Ni and similarities and differences in the materials' behavior are analysed. For all elements investigated so far with this model, we obtain a linear relationship between the value of the maximally demagnetized state and the fluence of the laser pulse , which is in agreement with experiments. Moreover, we demonstrate that the demagnetization amplitude is largest for Ni and smallest for Co. This holds over a wide range of the reported electron-phonon couplings, and this demagnetization trend is in agreement with recent experiments.

Direct Multiphoton Femtosecond Infrared Laser Excitation of a Diamond Lattice in the Two-Phonon Region and Modification of Color Centers

The nonlinear absorption of ultrashort laser pulses with intensities of 0.17–1.7 TW/cm2 at an intrinsic two-photon absorption wavelength of 4673 nm in type IIb diamond has been studied experimentally. It has been shown that the main absorption mechanism in the studied sample is two-photon absorption with a coefficient of β2 = (72 ± 7) cm/TW. Transmission microspectroscopy, visible photoluminescence, and infrared Fourier-transform microspectroscopy have demonstrated the possibility of laser-induced transformation of nitrogen impurity centers in synthetic type Ib diamond at higher radiation intensities.

Optical generation and detection of GHz longitudinal and transverse acoustic waves in solids assisted by two-dimensional metallic grating structure

Absorption of laser pulses with picosecond temporal width in a two-dimensional nano-scale metallic grating deposited on a transparent substrate launches both longitudinal and transverse acoustic waves in to the sample. The periodic metallic structure makes the generated acoustic waves propagate several different directions. The propagating acoustic waves are monitored optically by the delayed light pulses (probe light) as the transient reflectivity change which is known as the Brillouin oscillation. The probe light is also diffracted by the metallic grating, allowing the detection of the acoustic waves propagating in the multiple directions efficiently. The analysis of the Brillouin oscillation frequencies brings the longitudinal and transverse sound velocities and the optical refractive index simultaneously from a single measurement. We verify the above mentioned scheme using a sample with two-dimensional square lattice (period 380 nm) of aluminum square islands of lateral dimension 200 × 200 nm2 and thickness of 50 nm on a fused silica substrate of thickness 1 mm. From the obtained Brillouin oscillation frequencies in several tens GHz, the acoustic and optical properties of the substrate material are successfully retrieved. The technique would be applicable to investigate the acoustic and optical properties of more complicated systems, such as anisotropic and/or inhomogeneous medium.

Detection of molecular oxygen using nanosecond-laser-induced plasma.

Molecular oxygen (O2) concentration is measured by employing nanosecond laser-induced plasmas (ns-LIP) over a broad temperature spectrum ranging from 300 K to 1000 K, in the presence of an additional oxygen-containing molecule, CO2. Typically, emission spectra emanating from ns-LIP are devoid of molecular information, as the ns-LIP causes the dissociation of molecular species within the plasma. However, atomic oxygen absorption lines that momentarily appear at 777 nm in the broadband emission from the early-stage plasma are determined to be highly sensitive to the O2 mole fraction but negligibly affected by the CO2 mole fraction. The atomic O absorbing the plasma emission originates from the O2 adjacent to the plasma: robust UV radiation from the early-stage plasma selectively dissociates adjacent O2, exhibiting a relatively low photodissociation threshold, thus generating the specific meta-stable oxygen capable of absorbing photons at 777 nm. A theoretical model is introduced, explicating the formation of the meta-stable O atom from adjacent O2. To sustain the UV radiation from the plasma under high-temperature and low-density ambient conditions, a preceding breakdown is triggered by a split laser pulse (532 nm). This breakdown acts as a precursor, seeding electrons to intensify the inverse-Bremsstrahlung photon absorption of the subsequent laser pulse (1064 nm). Techniques such as proper orthogonal decomposition (POD) and support vector regression (SVR) are employed to precisely evaluate the O2 mole fraction (<1% uncertainty), by analyzing the short-lived (<10 ns) O2-indicator depicted in the early-stage plasma.

Influence of plasma inhomogeneity and ohmic heating on the nonlinear absorption of intense laser pulse in collisional magnetized plasma

AbstractThe nonlinear absorption in the interaction of an intense laser pulse with a collisional magnetized plasma is studied by considering the effects of plasma inhomogeneity, ohmic heating, and ponderomotive force. For this purpose, we obtain the plasma electron density, the effective dielectric permittivity, and the wave equation using the Maxwell and hydrodynamic equations and solve this equation by the Runge–Kutta numerical method. The results are shown that by increasing the plasma inhomogeneity, the inverse bremsstrahlung absorption coefficient is increased, and also the density ramp parameter and its sign can affect more the absorption coefficient than the temperature ramp parameter . However, when the initial electron density and temperature increase, the laser field amplitude and the absorption coefficient are increased and the spatial damping rate of the laser pulse becomes highly peaked inside the plasma. It is shown by increasing laser pulse energy, the nonlinear bremsstrahlung absorption coefficient is decreased significantly. The results also indicate that by increasing the external magnetic field, the dielectric permittivity is decreased while the laser energy spatial damping, and the absorption coefficient are increased. Moreover, it is found that by considering the ohmic heating effect, the electrons absorb further energy from the fields and consequently the nonlinear absorption is increased.

Formation of plasmon-exciton nanostructures based on quantum dots and metal nanoparticles with a nonlinear optical response

The establishment of the conditions for the formation of nanostructures with plasmon-exciton interaction based onquantum dots and plasmonic nanoparticles that provide unique nonlinear optical properties is an urgent task. The study demonstrates the formation of plasmon-exciton nanostructures based on hydrophilic colloidal Zn0.5Cd0.5S, Ag2S quantum dots and metal nanoparticles. Transmission electron microscopy and optical absorption and luminescence spectroscopy were used to substantiate the formation of plasmon-exciton hybrid nanostructures. The phase composition of the studied samples was determined by X-ray diffraction. The results obtained using ARLX’TRA diffractometer (Switzerland) indicated a cubic crystal structure (F43m) of synthesised Zn0.5Cd0.5S quantum dots and monoclinic (P21/C) crystal lattice of Ag2S. Transmission electron microscopy revealed that plasmonic nanoparticles are adsorption centres for quantum dots. The average sizes of the studied samples were determined: colloidal Ag2S quantum dots (2.6 nm), Zn0.5Cd0.5S(2.0 nm) and metal nanoparticles: silvernanospheres (10 nm) and gold nanorods (4x25 nm). The transformation of the extinction spectra of the light and the luminescence quenching of quantum dots have been established in mixtures of quantum dots and plasmonic nanoparticles. The nonlinear optical parameters of the studied samples were determined using the Z-scanning method at wavelengths of 355 and 532 nm in the field of nanosecond laser pulses. The conditions for the formation of hybrid nanostructures that provide an increase of the coefficient of nonlinear absorption of laser pulses (355 and 532 nm) up to 9 times with a duration of 10 ns due to the reverse saturable absorption occurring due to cascade two-quantum transitions in the intrinsic and local states of colloidal quantum dots and the suppression of nonlinear refraction, were determined. The observed changes were explained by the manifestation of the Purcell effect on the states of quantum dots in the presence of nanoresonators (gold nanorods and silver nanospheres). The results of these studies create new opportunities for the development of original systems for controlling the intensity of laser radiation, as well as quantum sensors of a new generation

Three-dimensional printing of silica glass with sub-micrometer resolution

Silica glass is a high-performance material used in many applications such as lenses, glassware, and fibers. However, modern additive manufacturing of micro-scale silica glass structures requires sintering of 3D-printed silica-nanoparticle-loaded composites at ~1200 °C, which causes substantial structural shrinkage and limits the choice of substrate materials. Here, 3D printing of solid silica glass with sub-micrometer resolution is demonstrated without the need of a sintering step. This is achieved by locally crosslinking hydrogen silsesquioxane to silica glass using nonlinear absorption of sub-picosecond laser pulses. The as-printed glass is optically transparent but shows a high ratio of 4-membered silicon-oxygen rings and photoluminescence. Optional annealing at 900 °C makes the glass indistinguishable from fused silica. The utility of the approach is demonstrated by 3D printing an optical microtoroid resonator, a luminescence source, and a suspended plate on an optical-fiber tip. This approach enables promising applications in fields such as photonics, medicine, and quantum-optics.

Volumetric absorption illumination induced by laser radiation in a 2D thermoelastic microelongated semiconductor body with temperature-dependent properties

In this paper, we construct a new model based on the coupling of thermoelasticity, plasma, and microelongation effect under volumetric absorption of laser pulse. Three different thermoelasticity theories are applied to construct the new model in a 2D thermoelastic semiconducting medium whose properties are temperature-dependent. The medium surface is exposed to laser radiation having spatial and temporal Gaussian distributions; in addition, the surface is considered traction-free. The general solutions were obtained analytically via Laplace and Fourier transformations; for Laplace inverse, we use the well-known Riemann sum approximation. As an application and consistency validation, silicon material is used.

Localized absorption of laser energy in X-mode configuration of magnetized plasma

The heating of ions via lower hybrid (LH) waves has been observed in several astrophysical as well as laboratory plasmas. We have conducted particle-in-cell simulations to demonstrate absorption of the incident laser pulse at a chosen localized point in the plasma target by manipulating its density profile. We show that a part of the incident laser propagates inside the plasma target when its frequency lies below the LH resonance frequency. Thereafter, as it experiences a negative density gradient, it approaches the resonance point where its group velocity approaches zero. This is where the electromagnetic (EM) energy prominently gets converted into the electrostatic and eventually the kinetic energy of ions. Thus, by tailoring the plasma density profile one can have the absorption of incident EM wave energy at a designated location inside the plasma. This may be important in various applications where energy deposition/heating of plasma in a localized region is desirable.

Nanodroplet vaporization with pulsed-laser excitation repeatedly amplifies photoacoustic signals at low vaporization thresholds.

Nanodroplets' explosive vaporization triggered by absorption of laser pulses produces very large volume changes. These volume changes are two orders of magnitude higher than those of thermoelastic expansion generated by equivalent laser pulses, and should generate correspondingly higher photoacoustic waves (PAW). The generation of intense PAWs is desirable in photoacoustic tomography (PAT) to increase sensitivity. The biocompatibility and simplicity of nanodroplets obtained by sonication of perfluoropentane (PFP) in an aqueous solution of bovine serum albumin (BSA) containing a dye make them particularly appealing for use as contrast agents in clinical applications of PAT. Their usefulness depends on stability and reproducible vaporization of nanodroplets (liquid PFP inside) to microbubbles (gaseous PFP inside), and reversible condensation to nanodroplets. This work incorporates porphyrins with fluorinated chains and BSA labelled with fluorescent probes in PFP nanodroplets to investigate the structure and properties of such nanodroplets. Droplets prepared with average diameters in the 400-1000 nm range vaporize when exposed to nanosecond laser pulses with fluences above 3 mJ cm-2 and resist coalescence. The fluorinated chains are likely responsible for the low vaporization threshold, ∼2.5 mJ cm-2, which was obtained from the laser fluence dependence of the photoacoustic wave amplitudes. Only ca. 10% of the droplets incorporate fluorinated porphyrins. Nevertheless, PAWs generated with nanodroplets are ten times higher than those generated by aqueous BSA solutions containing an equivalent amount of porphyrin. Remarkably, successive laser pulses result in similar amplification, indicating that the microbubbles revert back to nanodroplets at a rate faster than the laser repetition rate (10 Hz). PFP nanodroplets are promising contrast agents for PAT and their performance increases with properly designed dyes.

High refractive index change in Type A laser modification using a multi-scan approach

The focusing of femtosecond laser pulses is an efficient and robust way to fabricate integrated optical components in glasses. However, the induced refractive index changes are weak if compared with those obtained in lithography. Several solutions have been found to overcome such problem, the multi-scan technique being one of these. The present work implements the multi-scan approach on the femtosecond inscription of Type A laser modifications in high repetition rate regime. Therefore, a study on the effects of the absorption of subsequent laser pulses on the increase of the induced refractive index is conducted. As a result, the achievement of high and smooth refractive index change, up to 2 × 10−2, in a commercial silver-glass is reported. Moreover, such high and controlled index contrast is exploited for the fabrication of low-propagation-losses and engineered waveguides exhibiting single-mode operation in the VIS-NIR range.

Assessment of the thermal effect of femtosecond and millisecond laser pulses in microsurgery of mammalian embryos

We compare the thermal effect exerted by femtosecond and millisecond pulses during microsurgical procedures on embryos. The linear and nonlinear absorption of laser pulses in an aqueous medium, as well as heat transfer from the focusing region of the laser beam to adjacent regions, are considered.

Energy deposition and incubation effects of nonlinear absorption of ultrashort laser pulses in dielectrics

Although ultrashort laser has been widely employed in micromachining thanks to its excellent processing precision, one of the main challenges it faces when applied to 3D modification inside dielectrics is its processing efficiency. Many applications require multiple pulses to achieve significant modification to create structure such as microlenses. We report incubation experiments on energy deposition and the control of material modification in fused silica. This allows us to develop a practical incubation model by taking account different ionization mechanisms, in which coefficients relating to multiphoton and avalanche ionization change with laser shots due to accumulating defects. We then extend our study to the scheme where a pre-pulse is used to limit the absorption volume through pre-seeding. Both experiments and simulations show that the efficiency of laser processing can be significantly improved without sacrificing the spatial resolution with this method, especially for longer pulses.

Study on the polarization dependence of nonlinear absorption of ultrafast laser pulses in bulk fused silica.

By studying the nonlinear absorption of ultrafast laser pulses in fused silica, we examine, both with experiments and numerical simulations, the different polarization dependence of multiphoton ionization and avalanche ionization. Results show multiphoton ionization and avalanche ionization play different roles in femtosecond and picosecond laser micromachining, and the contribution via avalanche ionization increases with pulse duration. Meanwhile, the spatial distribution of the free carriers generated by circularly polarized pulses is more concentrated than those generated by linear polarization for picosecond laser pulses. These properties make the circular polarized ultrafast laser a possible way to improve the ultrafast laser micromachining efficiency and spatial quality, and can help to reduce some problematic nonlinear effects in ultrafast laser micromachining of low energy band materials.

The study of the optical nonlinear properties of bulk ZnSe for immersion applications

In this work, we study the nonlinear absorption of ultrashort laser pulses of variable duration (0.3-10 ps) in a bulk polycrystalline ZnSe dielectric. The transmission of the sample was measured for two wavelengths, 515 and 1030 nm, in a wide range of intensities. A nonlinear behavior of the transmission is found when the sample is irradiated with laser radiation at a wavelength of 515 nm with absorption saturation in the region of 3-5 TW/cm2. Surface damage thresholds were obtained for a wavelength of 1030 nm for pulse durations of 0.3-10 ps. Keywords: nonlinear ZnSe absorption, solid-state immersion, two-photon absorption, chalcogenide glass surface ablation, ZnSe ablation thresholds.

Исследование оптических нелинейных свойств объемного ZnSe для иммерсионных применений

In this work, we study the nonlinear absorption of ultrashort laser pulses of variable duration (0.3–10 ps) in a bulk polycrystalline ZnSe dielectric. The transmission of the sample was measured for two wavelengths, 515 and 1030 nm, in a wide range of intensities. A nonlinear behavior of the transmission is found when the sample is irradiated with laser radiation at a wavelength of 515 nm with absorption saturation in the region of 3–5 TW/cm2. Surface damage thresholds were obtained for a wavelength of 1030 nm for pulse durations of 0.3–10 ps.