Absorption Of Pulses Research Articles

Measurement of ultrafast dynamics of photoexcited carriers in β-Ga2O3 by two-color optical pump-probe spectroscopy

We report results from ultrafast two-color optical pump-probe spectroscopy on bulk β-Ga2O3. A two-photon absorption scheme is used to photoexcite carriers with the pump pulse and free-carrier absorption of the probe pulse is used to record the subsequent dynamics of the photoexcited carriers. Our results are consistent with carrier recombination via defect-assisted processes. We also observe transient polarization-selective optical absorption of the probe pulse by defect states under nonequilibrium conditions. A rate equation model for electron and hole capture by defects is proposed and used to explain the data. Whereas the rate constants for electron capture by defects are found to be temperature-independent, they are measured to be strongly temperature-dependent for hole capture and point to a lattice deformation/relaxation process accompanying hole capture. Our results shed light on the mechanisms and rates associated with carrier capture by defects in β-Ga2O3.

Localised opto-thermal response of nematic liquid crystal to laser light

ABSTRACTWe report on the thermal response of a thin nematic liquid crystal layer to a strong laser pulse, which is partially absorbed by the Indium Tin Oxide electrodes of the measuring LC cell. We measure local increase and temperature profile of the heated region and we measure the time response of the birefringence of this nematic layer. It is found that local increase of the temperature due to absorption of a 90 Mw laser light of 10 MS duration can easily exceed °C and depends on the localisation and focusing of the laser beam. Local thermal response of the heated nematic cell is measured by monitoring the time dependence of birefringence during the cooling and we found that nematic layer can be cooled down at an extremely fast rate of 10000 K/s. Optical retardation of 200 nm can be switched in 100 μs by laser pulse absorption and heating/cooling.

Nonlinear absorption of ultrashort ultrahigh intensity laser pulses in fullerene

Non linear absorption of intense few cycle laser pulses in fullerenes is studied by treating it as a laser driven nonlinear oscillator, using a rigid shell model. Efficient collisionless absorption of intense (Intensity I < 1018W/cm2) ultrashort laser pulses in fullerene (C60) is estimated owing to its extraordinary anharmonicty which is quite different than the conventionally used solid gas clusters. Compared to gas clusters nonlinear resonance in C60 has unusual characteristics. Factors other than nonlinear resonance which contribute in energy absorption in C60 are outlined. Strong dephasing of electrons with the laser field both at onset and during the dynamical oscillations leads to the efficient energy gain by the electrons. It is found that fullerene has a distinct threshold intensity requirement depending on its charge for efficient absorption. These properties make fullerene unique for efficient absorption of high intensity few cycle laser pulses.

Absorption of Ultrashort Laser Pulses by Diatomic Molecules

The absorption of ultrashort laser pulses by diatomic CO and HF molecules is studied theoretically. The probability of absorption in the total pulse action time and its dependence on the pulse duration and carrier frequency are calculated. In contrast to atomic systems, where the finite pulse duration effects are noticeable on time scales of the order of femtoseconds, these effects in molecular structures manifest themselves on time scales of the order of picoseconds. In this case, the vibrational–rotational structure of molecular transitions plays a significant role. The results of our calculations of the vibrational–rotational spectra for the absorption of ultrashort laser pulses by diatomic CO and HF molecules are presented. The transition from the nonlinear dependence of the absorption probability for short pulse durations to the standard dependence at long pulse durations corresponding to the transition probabilities per unit time is investigated.

Toughness, elasticity and physical properties for the evaluation of foamed concrete reinforced with hybrid fibers

This study has been undertaken to investigate the evaluation of Fiber-reinforced Foamed Concrete (FRFC) performance by the use of toughness and non-destructive tests. These tests cover the workability, density, static modulus of elasticity, toughness, ultrasonic pulse velocity and absorption tests. Different FRFC mixes using carbon fibers in the order of 0.5, 1 and 1.5% carbon fibers were used. Also, the combinations of carbon fibers (C) and polypropylene fibers (PP) as 1% C+ 0.5% PP, and 0.5% C+1% PP were prepared. Lastly, the inclusion of polypropylene fibers with the order of 1.5% PP was used to strengthen the foamed concrete mix.The results showed that the use of 1.5% of C has affected the modulus of elasticity and flexural toughness of foamed concrete.On the other hand, a strong relationship is found between compressive strength and ultrasonic pulse velocity for FRFC.

Ultrahigh Conductivity and Superior Interfacial Adhesion of a Nanostructured, Photonic-Sintered Copper Membrane for Printed Flexible Hybrid Electronics.

Inkjet-printed electronics using metal particles typically lack electrical conductivity and interfacial adhesion with an underlying substrate. To address the inherent issues of printed materials, this Research Article introduces advanced materials and processing methodologies. Enhanced adhesion of the inkjet-printed copper (Cu) on a flexible polyimide film is achieved by using a new surface modification technique, a nanostructured self-assembled monolayer (SAM) of (3-mercaptopropyl)trimethoxysilane. A standardized adhesion test reveals the superior adhesion strength (1192.27 N/m) of printed Cu on the polymer film, while maintaining extreme mechanical flexibility proven by 100 000 bending cycles. In addition to the increased adhesion, the nanostructured SAM treatment on printed Cu prevents formation of native oxide layers. The combination of the newly synthesized Cu ink and associated sintering technique with an intense pulsed ultraviolet and visible light absorption enables ultrahigh conductivity of printed Cu (2.3 × 10-6 Ω·cm), which is the highest electrical conductivity reported to date. The comprehensive materials engineering technologies offer highly reliable printing of Cu patterns for immediate use in wearable flexible hybrid electronics. In vivo demonstration of printed, skin-conformal Cu electrodes indicates a very low skin-electrode impedance (<50 kΩ) without a conductive gel and successfully measures three types of biopotentials, including electrocardiograms, electromyograms, and electrooculograms.

Resonance Absorption of a Train of Ultrawideband Electromagnetic Pulses in the Ionosphere

The problem of collisionless absorption of ultrawideband electromagnetic pulses in the ionosphere is studied analytically. It is shown that absorption has a resonance character if the repetition rate of a train of pulses is equal to the frequency of the excited plasma waves.

MEMS Fabrication of Silicon Microwire Targets.

Micro- and nano-structured surfaces can lead to increased absorption of incident laser pulses in high power laser experiments. Here we describe how MEMS (Micro-Electro-Mechanical System) manufacturing techniques (in this case, optical lithography, and deep silicon etching), have been used to create surface structured (“microwire”) laser targets consisting of 15um tall, regular arrays of 2um or 3um wide silicon microcolumns for a recent experimental campaign using the Vulcan laser at Rutherford Appleton Laboratory (RAL)

Generation of keV hot near-solid density plasma states at high contrast laser-matter interaction

We present experimental evidence of ultra-high energy density plasma states with the keV bulk electron temperatures and near-solid electron densities generated during the interaction of high contrast, relativistically intense laser pulses with planar metallic foils. Experiments were carried out with the Ti:Sapphire laser system where a picosecond pre-pulse was strongly reduced by the conversion of the fundamental laser frequency into 2ω. A complex diagnostics setup was used for evaluation of the electron energy distribution in a wide energy range. The bulk electron temperature and density have been measured using x-ray spectroscopy tools; the temperature of supra-thermal electrons traversing the target was determined from measured bremsstrahlung spectra; run-away electrons were detected using magnet spectrometers. Analysis of the bremsstrahlung spectra and results on measurements of the run-away electrons showed a suppression of the hot electron production in the case of the high laser contrast. Characteristic x-ray radiation has been used for evaluation of the bulk electron temperature and density. The measured Ti line radiation was simulated both in steady-state and transient approaches using the code FLYCHK that accounts for the atomic multi-level population kinetics. The best agreement between the measured and the synthetic spectrum of Ti was achieved at 1.8 keV electron temperature and 2 × 1023 cm−3 electron density. By application of Ti-foils covered with nm-thin Fe-layers, we have demonstrated that the thickness of the created keV hot dense plasma does not exceed 150 nm. Results of the pilot hydro-dynamic simulations that are based on a wide-range two-temperature Equation of States, wide-range description of all transport and optical properties, ionization, electron, and radiative heating, plasma expansion, and Maxwell equations (with a wide-range permittivity) for description of the laser absorption are in excellent agreement with experimental results. According to these simulations, the generation of keV-hot bulk electrons is caused by the collisional mechanism of the laser pulse absorption in plasmas with a near solid step-like electron density profile. The laser energy, first deposited into the nm-thin skin-layer, is then transported into 150 nm depth by the electron heat conductivity. This scenario is opposite to the volumetric character of the energy deposition produced by supra-thermal electrons.

Interaction of Ultrashort Laser Pulses with Atoms in Plasmas

The paper is devoted to the investigation of the absorption of ultrashort laser pulses on atoms in plasmas, accounting for the different broadening mechanisms of atomic resonant transitions. The analysis is made in terms of the absorption probability during the entire interaction between the laser pulse and atom. Attention is mainly given to dependence of probability upon the pulse duration and the carrier frequency of the pulse. The results are presented via dimensionless parameters and functions describing the effect of finite pulse duration on atomic spectra for different broadening mechanisms, namely Doppler, Voigt, Holtsmark and their combinations, as well as the Stark line broadening of Rydberg atomic lines.

Enhanced energy absorption of high intensity laser pulses by targets of modulated surface

Investigations of energy transfer of high intensity (I = 5 × 1019 W/cm2), ultrashort (&amp;lt;30 fs) Ti:Sa laser pulses to solid targets with a randomly rough surface have been performed. We investigated the influence of the target surface morphology on the efficiency of energy transfer of p- polarized laser pulses characterized by a very high contrast. Targets with a roughness σ larger than ∼20% of the laser wavelength proved to absorb a remarkably large fraction of energy reaching up to 70%, almost independent of the incidence angle. Numerical simulations of various interaction conditions are in agreement with the experimental data and confirm the effect of the target morphology and its surface parameters on the enhanced energy absorbed fraction.

Mid-infrared quantum cascade laser spectroscopy probing of the kinetics of an atmospherically significant radical reaction, $$\hbox {CH}_{3}\hbox {O}_{2}+\hbox {NO}_{2}+\hbox {M}\rightarrow \hbox {CH}_{3}\hbox {O}_{2}\hbox {NO}_{2}+\hbox {M}$$ CH 3 O 2 + NO 2 + M → CH 3 O 2 NO 2 + M , in the gas phase

The kinetic parameters of an important atmospheric reaction, $$\hbox {CH}_{3}\hbox {O}_{2} + \hbox {NO}_{2} + \hbox {M}\rightarrow \hbox {CH}_{3}\hbox {O}_{2}\hbox {NO}_{2} + \hbox {M}$$ , have been recorded by monitoring directly the changes in concentrations of methylperoxy radicals $$(\hbox {CH}_{3}\hbox {O}_{2})$$ in the gas phase employing a new mid-infrared quantum cascade laser (QCL)-based apparatus. $$\hbox {CH}_{3}\hbox {O}_{2 }$$ radicals in our apparatus have been generated by pulsed UV laser (266 nm) photolysis of $$\hbox {CH}_{3}\hbox {I}$$ in a gaseous mixture with oxygen. The absorption band corresponding to the mid-infrared O-O stretching fundamental of the peroxy radical, within a narrow spectral range, 1070–1120 $$\hbox {cm}^{-1}$$ , has been recorded by tuning the wavelength of the QCL operated in CW mode. The kinetics of the aforementioned reaction of $$\hbox {CH}_{3}\hbox {O}_{2}$$ with $$\hbox {NO}_{2}$$ has been followed by analyzing the changes of the infrared (QCL) decay profile of $$\hbox {CH}_{3}\hbox {O}_{2}$$ at $$9.1~\upmu \hbox {m}$$ $$(1098.9~\hbox {cm}^{-1})$$ maintaining a pseudo first order reaction condition. We noticed that the rate constant of the reaction at 298 K varies in the range of (1.21–3.08) $$\times 10^{-12}$$ $$\hbox {cm}^{3 }~\hbox {molecule}^{-1 }\,\hbox {s}^{-1 }$$ for changing the total pressure in the range of 75–730 mbar. The absorption cross-section of $$\hbox {CH}_{3}\hbox {O}_{2}$$ at the probe wavelength $$(1098.9~\hbox {cm}^{-1})$$ , has been estimated for the first time to be $$8.3 \pm 0.4 \times 10^{-20}~\hbox {cm}^{2}$$ . Kinetics measurement of an atmospherically important gas phase reaction, $$\hbox {CH}_{3}\hbox {O}_{2}+\hbox {NO}_{2}+\hbox {M}\rightarrow \hbox {CH}_{3}\hbox {O}_{2}\hbox {NO}_{2}+\hbox {M}$$ , is studied using a newly developed apparatus based on pulsed UV laser photolysis time-resolved mid-infrared absorption spectroscopy. $$\hbox {CH}_{3}\hbox {O}_{2}$$ radicals are probed in the mid-infrared by a CW quantum cascade laser (QCL).

Time-domain Brillouin scattering assisted by diffraction gratings

Absorption of ultrashort laser pulses in a metallic grating deposited on a transparent sample launches coherent compression/dilatation acoustic pulses in directions of different orders of acoustic diffraction. Their propagation is detected by the delayed laser pulses, which are also diffracted by the metallic grating, through the measurement of the transient intensity change of the first order diffracted light. The obtained data contain multiple frequency components which are interpreted by considering all possible angles for the Brillouin scattering of light achieved through the multiplexing of the propagation directions of light and coherent sound by the metallic grating. The emitted acoustic field can be equivalently presented as a superposition of the plane inhomogeneous acoustic waves, which constitute an acoustic diffraction grating for the probe light. Thus, the obtained results can also be interpreted as a consequence of probe light diffraction by both metallic and acoustic gratings. The realized scheme of time-domain Brillouin scattering with metallic grating operating in reflection mode provides access to acoustic frequencies from the minimal to the maximal possible in a single experimental configuration for the directions of probe light incidence and scattered light detection. This is achieved by monitoring of the backward and forward Brillouin scattering processes in parallel. Applications include measurements of the acoustic dispersion, simultaneous determination of sound velocity and optical refractive index, and evaluation of the samples with a single direction of possible optical access.

Rogue waves for a generalized nonlinear Schrödinger equation with distributed coefficients in a monomode optical fiber

Investigated in this paper is the generalized nonlinear Schrödinger equation with distributed coefficients, which describes the amplification or absorption of pulses propagating in a monomode optical fiber with distributed group-velocity dispersion and self-focusing Kerr nonlinearity. By virtue of the Kadomtsev-Petviashvili hierarchy reduction, we obtain the rogue waves based on rogue-wave solutions in terms of the Gramian under certain constraint. We study the effects of group-velocity dispersion, nonlinearity and amplification/absorption coefficients on the rogue waves with the help of figures. Amplitudes of the rogue waves are independent with the group-velocity dispersion and nonlinearity coefficients. The first-order rogue wave with an eye-shaped distribution density and the second-order rogue waves with the highest-peak amplitude and with the triple-peak structure are presented. Both the intermingled or separated composite rogue waves are derived. Periodic rogue waves are obtained and period of the periodic rogue wave increases with the period of the group-velocity dispersion. Furthermore, nonlinear tunneling of the rogue waves is observed: rogue waves get amplified when they reach to the dispersion barriers and recover their original shapes after passing through the barriers, while amplitudes of the rogue waves decrease inside the dispersion wells. Amplification/absorption coefficient influence the background and amplitude of the rogue wave, and three types of the backgrounds are discussed due to different amplification/absorption coefficients.

Darboux transformations, solitons, breathers and rogue waves for the modified Hirota equation with variable coefficients in an inhomogeneous fiber

Investigated in this paper is the modified Hirota equation with variable coefficients, which can describe the amplification or absorption of pulses propagating in an inhomogeneous optical fiber. We construct a couple of the Darboux transformations, and obtain the one-soliton, two-soliton, breather and rogue-wave solutions. Effects of the group velocity dispersion, third-order dispersion, nonlinear focus length and velocity of propagation are graphically analyzed. Bell-, parabolic-, cubic- and periodic-type solitons are obtained. Bound state and head-on interactions for the two solitons are presented, with those interactions shown to be elastic. We can see two types of the breathers there, including the Akhmediev breathers and Kuznetsov–Ma solitons, and the rogue-wave pair consisting of two rogue waves is obtained.

Directivity of GHz shear acoustic waves launched by the absorption of short laser pulses at the interface between a transparent and an absorbing material

When a short laser pulse is focused at the interface between a transparent and an absorbing material, an expansion source is created that generates mainly longitudinal acoustic pulses in the absorbing material. However, the interaction of these waves with the interface favours the excitation of shear acoustic pulses. In this paper, we present an analytical prediction of the directivity of shear waves launched in a semi-infinite absorbing medium, and we introduce corrections in order to account for detection in an absorbing layer. Experiments were performed with a picosecond ultrasonic set-up focusing femtosecond laser pulses to a micron spot size along the interface between sapphire and titanium. In this configuration, acoustic wave-fronts were reflected at the free surface of the micron titanium layer, and then optically detected at the interface. We also present an approach to filter and enhance the shear waves by processing measured wave-fronts recorded at multiple locations along the interface. In the resulting signals, the contribution of GHz shear acoustic waves, propagating mainly in directions that are oblique with respect to the interface, is enhanced.

国外差分吸收激光雷达探测大气CO 2 研究综述

Differential absorption lidar is an indispensable tool to measure atmospheric CO2 for temporal and spatial distribution. The 2.0 μm wavelength Ho:Tm:YLF/Ho:Tm:LuLiF lidar were used for remote sensing atmospheric CO2 with heterodyne receiver. The KTP parametric oscillator was injected into the seeds, and a differential absorption lidar (DIAL) system of 1.6 μm by using the photon counting technique to profile the atmospheric CO2 in the troposphere under 7 km. Modulation continuous wave seed laser intensity and the integral path differential absorption lidar, common 1.6 μm optical fiber amplifier, and the use of correlation detection technique, have unique advantages and characteristics in the detection of atmospheric CO2 column concentrations. The space program for detecting atmospheric CO2 column concentrations is NASA's ASENDS (active sensing of CO2 emission over nights, days, and seasons) mission which adopts the method of pulse and integral path differential absorption. The absorption of CO2 gas cell as a reference to stabilize seed light frequency and the precise control of cavity length lock the on-line wavelength of transmitter, is the key technique for DIAL to measure atmospheric CO2.

Atmospheric CO2 measurements with a 2-μm DIAL instrument

We report on ground-based atmospheric concentration measurements of carbon dioxide, using a pulsed direct detection differential absorption lidar operating at 2051 nm. The transmitter is based on a tunable parametric source emitting 10-mJ energy, 10-ns duration Fourier-limited pulses. Range resolved concentration measurements have been carried out on the aerosol back-scattered signal. Cloud signals have been used to get long range integrated-path measurements.

Absorption of Ultrashort Electromagnetic Pulses by ITO Nanoparticles

The paper investigates the cross-section and probability of absorption of ultrashort electromagnetic pulses by ITO nanoparticles in a dielectric matrix. It discusses the suitability of using the total probability of photoabsorption instead of the probability per unit time for such pulses. It is shown that in the case of negative detuning of the pulse carrier frequency from the plasmon resonance frequency, the photoabsorption probability is a nonlinear function of the pulse duration. For detuning with sufficiently large magnitude, the formation of a maximum is possible in this functional relationship, the position of which depends on the refractive index of the matrix. A method is proposed for measurement of environmental parameters by the analysis of time dependence of this photoprocess. Calculations are performed for a corrected Gaussian pulse and ITO nanoparticles with different doping levels.

The Kubo-Greenwood formula as a result of the random phase approximation for the electrons of the metal

For calculation of short laser pulse absorption in metal the imaginary part of permittivity, which is simply related to the conductivity, is required. Currently to find the static and dynamic conductivity the Kubo-Greenwood formula is most commonly used. It describes the electromagnetic energy absorption in the one-electron approach. In the present study, this formula is derived directly from the expression for the permittivity expression in the random phase approximation, which in fact is equivalent to the method of the mean field. The detailed analysis of the role of electron-electron interaction in the calculation of the matrix elements of the velocity operator is given. It is shown that in the one-electron random phase approximation the single-particle conductive electron wave functions in the field of fixed ions should be used. The possibility of considering the exchange and correlation effects by means of an amendment to a local function field is discussed.