Increasing Laser Intensity Research Articles

The interfacial features in photoluminescence of In0.52Al0.48As/InP distinguished with selective excitation

The photoluminescence experiment was performed at 77 K on two types of heterostructures of In0.52Al0.48As/InP grown by molecular beam epitaxy. One type has the so-called direct interface formed with In0.52Al0.48As grown on the InP (100) substrate, and another is a double heterostructure type, i.e., InP/In0.52Al0.48As/InP, including the inverse interface formed with the InP cap. The thickness of both the as-grown InAlAs and InP layers is 2 µm. Under the excitation of 852 nm semiconductor laser, a single peak at 1.198 eV (P0) was observed for the direct interface whereas the inverse interface showed two peaks at 1.240 eV (P1) and 1.117 eV (P2). However, P2 manifests differently from P1 with two interesting features. First, it disappears when the excitation was replaced with the 532 nm laser. Second, when the cap layer of InP/In0.52Al0.48As/InP is thinned to 200 nm, P2 displays a large blueshift effect with the increase in laser intensity, but the extent of blueshift drops when the excitation is increased further into a higher power range. The phenomenon of P2 was attributed to the interface excitation-induced interplay between the luminescence with a carrier localization effect and that caused by the recombination of 2-dimensional electrons with the light-injected holes in the potential well at the interface.

Change of dominant material properties in laser perforation process with high-energy lasers up to 120 kilowatt

In laser materials processing, energy losses due to reflection, heat conduction and thermal radiation play an important role. In this publication, we show that with increasing laser intensity, the energy lost within the sample becomes less important for metal perforation processes. We compare the laser-matter interaction of aluminum and steel plates. Material parameters such as density, melting point and especially thermal conductivity differ strongly, leading to much longer perforation times for aluminum in comparison to steel at laser powers of 20 kW. However, this behavior changes at laser powers of more than 80 kW where the perforation times of aluminum become shorter than the corresponding times for steel. By comparing experimental data and simulations, we conclude that thermal conduction is the dominant factor of energy loss at low powers, but is reduced at high laser powers.

3D Printing of Personalised Carvedilol Tablets Using Selective Laser Sintering.

Selective laser sintering (SLS) has drawn attention for the fabrication of three-dimensional oral dosage forms due to the plurality of drug formulations that can be processed. The aim of this work was to employ SLS with a CO2 laser for the manufacturing of carvedilol personalised dosage forms of various strengths. Carvedilol (CVD) and vinylpyrrolidone-vinyl acetate copolymer (Kollidon VA64) blends of various ratios were sintered to produce CVD tablets of 3.125, 6.25, and 12.5 mg. The tuning of the SLS processing laser intensity parameter improved printability and impacted the tablet hardness, friability, CVD dissolution rate, and the total amount of drug released. Physicochemical characterization showed the presence of CVD in the amorphous state. X-ray micro-CT analysis demonstrated that the applied CO2 intensity affected the total tablet porosity, which was reduced with increased laser intensity. The study demonstrated that SLS is a suitable technology for the development of personalised medicines that meet the required specifications and patient needs.

Effect of using reflective target materials on enhancing UV nanosecond laser machining of sapphire

The machining of sapphire, a hard and transparent material, is of significant interest in various industries due to its exceptional properties. Traditional machining methods for sapphire suffer from drawbacks such as low efficiency and poor processing quality. Laser machining, particularly using ultraviolet (UV) nanosecond lasers, has emerged as a promising alternative for machining sapphire. In this article, we explored the use of reflective target materials to enhance laser energy absorption during the machining of sapphire. The effect of different reflective target materials, including aluminum and silicon, on the laser machining process was examined. Parametric calculations and finite-difference time-domain (FDTD) simulation were performed to evaluate the laser intensity at the top surface of sapphire when a reflective target material was employed. The results demonstrated that the use of reflective target materials leads to increased laser intensity due to the interference of incident and reflected laser beams, thus improving material removal efficiency. Moreover, experimental observations confirmed the effectiveness of using reflective target materials in enhancing the machined depth of sapphire. For example, the machined depth of 21.6 µm with a power density of 1.7 × 108 W/cm2 increased to 23.3 µm and 26.5 µm by using silicon and aluminum target materials. It is found that using aluminum target led to higher machined depth than silicon owing to its higher reflectivity.

Radiation intensity dependence of CdSe photothermal and photocarrier radiometry response evidenced by step-sine modulation method

The radiation intensity dependence of photothermal (PT) and photocarrier (PC) signals from n-type CdSe single crystals was investigated by modulated infrared radiometry (MIRR) in the mid-IR range (5–11.3 μm) with superbandgap photoexcitation. The influence of dc temperature increase of the sample was avoided by a new step-sine modulation method that combines the advantages of transient and periodical modulation. With increasing laser intensity I, the amplitude of the PC component shows a sub-linear dependence (|SPC| ∝ I0.5), while the PT one has the expected linear dependence (|SPT| ∝ I). As a result, the transition frequency ft between the two components is shifted to higher frequencies, which is explained in the frame of a simple model. The origin of the observed effects is the decrease of the effective photocarrier lifetime τ ∝ I−0.5 over three laser intensity decades. In contrast, previous studies on nonlinear PC response in semiconductors performed in the near-IR range (0.7–1.8 μm) have found supra-linear |SPC| dependence with exponent between 1 and 2. This difference is attributed to the fact that the near-IR radiometric signal features characteristics of a photoluminescence (PL) signal that are different from those of the mid-IR PC signal, as shown in our previous study [J. Appl. Phys. 119, 125108 (2016)] on the same CdSe samples.

Ultrashort and high-collimation X/γ-rays generated by nonlinear inverse Thomson scattering between off-axis electrons and circularly polarized intense laser pulses.

The properties of nonlinear inverse Thomson scattering (NITS) are investigated in the collision between a circularly polarized tightly focused intense laser pulse and a relativistic off-axis electron with numerical simulations. Due to the asymmetric effect of the laser field on the off-axis electrons, the electron trajectory is torqued to the off-axis direction, and the symmetry of the spatial radiation is also destroyed, which causes the concentrations of the radiation in the off-axis direction. With the increase of laser intensity, the torsion effect is more obvious, the radiation collimation improves, the direction turns to sideways. With the increase of electron's initial energy, the direction turns back to backwards and the degree of off-axis effect decreases. In both cases, the power exponentially enhances, the pulse width shortens, the spectrum broadens and super-continuity appears. With the laser intensity, the duration of sideways X-ray pulse from the low-energy (2.61MeV) electron is only 0.2 as, and the normalized intensity reaches 109. While using ultra-high-energy (100MeV) electrons, the duration of backwards γ-ray pulse reaches 1.22 zs, and the normalized intensity reaches 1017. These results help the understanding of nonlinear Thomson scattering and provide important numerical references for the research of NITS as high-quality X-ray and γ-ray sources.

Reversible and Irreversible Layer Edge Relaxation in Laser‐Radiation‐Hardened 2D Organic–Inorganic Perovskite Crystals

The layer edge states or low energy state (LES) in 2D hybrid organic–inorganic perovskites demonstrate a prolonged carrier lifetime for better performance of optoelectronic devices. However, the fundamental understanding of LES in 2D perovskites is still inconclusive. Herein, a photoluminescence (PL) study of LES in 2D Ruddlesden–Popper perovskites is presented with n = 2 and n = 3 from their cleaved cross sections that are more stable than the natural edge. The PL measurements clearly observe reversible, and irreversible surface relaxations (case I and case II) in three laser intensity ranges, further supported by a PL excitation cycle from low to high laser intensity, and vice versa. The PL wavelength of LES is tunable with laser intensity and blueshifts with increasing laser intensity during irreversible surface relaxation process (case I). Fluorescence lifetime imaging (FLIM) shows that the LES has a longer lifetime than the band‐edge emission in the sample without a photodegradation, while the BE lifetime becomes relatively longer in the area with a photodegradation. The presented laser tunable LES and the related irreversible relaxation process provide a new insight that can help improve the photostability in 2D perovskites and understand roles of LESs in optoelectronic device performance.

Thomson scattering measurements of electron temperature and electron density in laser-driven Gd plasmas

In this study, we demonstrate the laser intensity scaling of electron temperature in Gd laser-produced plasmas (LPPs) through experiment and simulation. The spatial and temporal profiles of electron density ne and temperature Te in Gd LPPs were directly measured during a drive laser pulse duration of 7 ns at a laser wavelength of 1064 nm using collective Thomson scattering. We found that the measured maximum Te value in Gd LPPs increases with increasing laser intensity IL , with the dependence , in the IL range of 1010–. Radiation-hydrodynamic simulation code of the STAR-2D was performed under identical conditions to those used in the experiment, and extended further to higher laser intensities of up to ; the simulated Te was found to be in good agreement with the experimental data over the used IL range. The simulation indicates that higher overall maximum Te typically exists 50–70 above the target and modifies the dependence as . Experiments reveal that a laser intensity of is required to achieve the optimum condition ( 100 eV and ion charge states , at –) for efficient beyond extreme ultraviolet (EUV) light source. In addition, two spot sizes (150 and 250 in diameter) were used to study the effect of the spot size on the Te profile. Our experimental findings, supported by the simulations, show that larger spots can create uniform Te profiles, higher maximum Te , and larger high-Te regions in LPPs. The results and scaling laws presented in this study provide important information for developing more powerful and energy-efficient EUV light sources.

Excitonic Floquet states in quantum wire

We use Floquet theory to study the effects of laser light coupling to an electron–hole pair confined in a quantum wire. Fast oscillating electric field directed along the wire continuously displaces spatially the electron and hole in opposite directions, that influences on effective time-averaged electrostatic interaction by shallowing its minimum. Renormalization of binding energy leaves distinctive stamp in Floquet energy spectra because both the ponderomotive and confining energies may be neglected in considered perturbative regime. Due to renormalization of binding energy the blueshifted dressed exciton’s energy states form crossings and avoided crossings in energy spectra while their oscillator strengths are gradually reduced for increasing laser intensity, these features are strongly dependent on spatial sizes of wire. Discussed properties of Floquet exciton confined in QWr could potentially be used to build the fast terahertz optical bright-dark state switcher or to realize Floquet-Landau-Zener transition.

Laser parameters affecting the asymmetric radiation of the electron in tightly focused intense laser pulses

The nonlinear radiation of the electron is a distinctive feature of the action of tightly focused linearly polarized lasers. In this paper, from the perspective of radiation symmetry, the effect of laser parameters on the electron radiation power in the time domain is studied systematically. An asymmetric bimodal structure is found in the time domain in the direction of the maximum radiation. For this special structure, an explanation is given based on the electron dynamics perspective. The structure is compared with the symmetric bimodal structure in the classical theory. The increase in laser intensity, while significantly increasing the radiated power of the electron, exacerbates the asymmetry of the electron radiation. The variation in the initial phase of the laser leads to a periodic variation in the electron motion, which results in a periodic extension of the electron spatial radiation with a period of π. Moreover, the existence of jump points with a phase difference of π in the range of 0–2π is found. The increase in pulse width reduces the radiated power, extends the radiation range, and alleviates the radiation asymmetry. The results in this paper contribute to the study of electron radiation characteristics in intense laser fields.

Effects of power density on residual stress and microstructural behavior of Ti-2.5Cu alloy by laser shock peening without coating

The present study concerns the micro structural changes of Ti-2.5 Cu after LPwC at various laser intensities (i.e., ≈3 GW cm−2, 6 GW cm−2 and 9 GW cm−2). X-ray diffraction (XRD), electron backscattered diffraction (EBSD), transmission electron microscope (TEM) were used to analyze the microstructure. X-ray photoelectron spectroscopy (XPS) was used to characterize the oxidation behavior of the samples after peening. The depth profile of residual stress was obtained using the sin2ѱ X-ray diffraction technique, and the compressive residual stress increased for all the peened conditions in the surface and sub-surface regions, and the maximum obtained was −402.44 MPa at 50 μm depth for the 9 GW cm−2. Furthermore, the cross-sectional micro hardness of the peened samples improved at a depth of 50 μm with increasing laser intensity. For the highest energy, the maximum hardness at a depth of 50 μm was 439.40 HV (0.1), which was higher than the unpeened sample (315.56 HV0.1). TEM studies showed nano precipitates (50–170 nm), mechanical twins with a width ranging from (27.20–118.51 nm) and cell size dislocation structure. EBSD results conformed the high angle grain boundaries increased to 43% with the formation of mechanical twins at the higher energy.

Simulations of plasmas driven by laser wavelengths in the 1.064—10.6 μ m range for their characterization as future extreme ultraviolet light sources

We characterize the properties of extreme ultraviolet (EUV) light source plasmas driven by laser wavelengths in the λ laser = 1.064 − 10.6 μ m range and laser intensities of I laser = 0.5 − 5 × 10 11 W cm−2 for λ laser = 1.064 μ m. Detailed numerical simulations of laser-irradiated spherical tin microdroplet targets reveal a strong laser-wavelength dependence on laser absorptivity and the conversion efficiency of generating in-band EUV radiation. For λ laser = 1.064 μ m irradiation, the increase in in-band radiation with increasing laser intensity is offset by only a minor reduction in conversion efficiency. Radiative losses are found to dominate the power balance for all laser wavelengths and intensities, and a clear shift from kinetic to in-band radiative losses with increasing laser wavelength is identified. Yet, with increasing laser intensity, such a shift is absent. We find that the existence of a maximum conversion efficiency, near λ laser = 4 μ m, originates from the interplay between the optical depths of the laser light and the in-band EUV photons for this specific droplet-target geometry.

Laser-induced Fano asymmetry, electron-phonon coupling, and phase transition in lanthanide sesquioxide (Ln2O3; Ln = Eu, Gd, Dy) nanoparticles: A Raman spectroscopic investigation

Laser power-dependent Raman spectroscopy is deployed to probe Fano interference in asymmetrically broadened Tg modes and the associated line shift in three technologically sound, meticulously characterized rare-earth sesquioxide systems. Group theoretical analysis is accompanied to introspect the Raman-active optic modes in cubic, monoclinic, and trigonal phases and identify the laser heating-induced local phase transitions. With increasing laser intensity, a regular redshift and larger negative asymmetry in the Raman peaks are detected, which is attributed to moderations in Fano scattering by enhanced electron–phonon coupling amid the focussed photoexcited electron plasma and is illustrated using a Feynman diagram. A quantitative study is thereby performed to unveil the intrinsic nature of discrete-continuum Fano resonance in the nanoparticles of interest emphasizing the high sensitivity of Raman spectra to the excitation strength that perturbs the generic vibrational features at the Brillouin zone center by influencing the interference conditions, force constant, and length of the associated bonds compelled by tensile stress. A rising trend of the charge–phonon coupling constant (λ) with laser power validates a stronger particle–quasiparticle coupling, whereas a shorter anharmonic phonon lifetime (τanh) indicates faster interactions. Using Allen's formalism, the charge density of states [N(εF)] at the Fermi level per spin and molecule is calculated, which pertains to a negative regression dependence in the λN(εF)−τanh dynamics.

Core-resonance line-shape analysis of atoms undergoing strong-field ionization

Using attosecond transient absorption spectroscopy for time delays where the near-infrared pump and the extreme ultraviolet (XUV) probe pulses overlap, sub-cycle structures in the build-up of absorption lines in xenon ions are investigated as a function of the pump intensity during strong-field ionization. We observe a half-cycle-periodic change in the line-shape asymmetry of the ionic 4d–5p resonances. Analyzing the line shapes, we find that in particular the phase of the induced dipole emission is modified, and the magnitude of this phase modulation decreases with increasing laser intensity. We discuss the influence of ground state depletion on interfering pathways involved in XUV-assisted strong-field ionization.

Experimental study of dynamics and soft X-ray emission of laser produced plasma from low-density foam Ti targets

Low-density foam targets are expected to be good candidates for efficient laser produced plasma (LPP) and soft X-ray (SXR) sources due to the less energetic debris compared with solid targets. Three Ti targets with different densities were irradiated by a pulsed Nd:YAG laser in vacuum. The average ion energy and the time-of-flight (TOF) peak voltage decreased as the Ti target density decreased. Furthermore, the differences among the three Ti targets increased with increasing laser intensity. The average ion energy of low-density foam Ti targets had a slightly weaker angular dependence compared with solid Ti targets. The plasma plume optical images illustrated that solid Ti plasma expanded faster in the direction of laser incidence with stronger anisotropy compared to foam Ti plasma. In addition, the optical radiation intensity integrated from a wavelength region of 200–850 nm of plasma plume increased with decreasing initial target density, which indicated that more absorbed laser energy was converted to plasma optical radiation energy (internal energy) instead of ion kinetic energy for targets with lower initial densities. The SXR radiation had no significant difference among the three Ti targets. These results demonstrate that appropriately reducing the initial target density can significantly reduce kinetic energy of ion debris without affecting the SXR radiation output.

Hot electron and x-ray generation by sub-ns kJ-class laser-produced tantalum plasma

This paper presents experimental investigation of temperature scaling and threshold of instability in hot electron and bremsstrahlung radiation from the interaction of sub-nanosecond and kilo-joule class laser pulses with a tantalum foil target at Prague Asterix Laser System. The laser intensity was varied between and W · cm−2 at the target focus. The energy distribution functions of electrons were measured by an angular array of magnetic spectrometers indicating the electron temperature in the range between 30 keV and 70 keV. The bremsstrahlung spectrum was characterized using a scintillator-based calorimeter. In particular, we show the laser-energy scaling of the total flux of hot electrons in the forward and backward directions with respect to the laser vector, the conversion efficiency of the laser energy to the energy carried by hot electrons, and the temperature of hot electrons as well as the unfolded bremsstrahlung temperature using a Monte Carlo code consistent with signals of the scintillator detector. The scaling shows that the electron flux increases discontinuously with increasing laser intensity from W · cm−2 with consequent instability in the production of hot electrons and bremsstrahlung radiation.

Multichannel dissociative ionization of ethanol in intense ultraviolet laser fields: Energy correlation between photoelectron emission and fragment recoil

We investigate multichannel dissociative ionization of ethanol in intense ultraviolet laser fields using a photoelectron-photoion coincidence momentum imaging technique. Product channels are clearly separated with detected photoions, and channel-specific photoelectron spectra exhibit multiple spectral components unambiguously assigned to four- and five-photon ionization to the electronic ground state and the electronically excited state. We measure kinetic energy distributions of fragment ions as a function of the energy of a correlated photoelectron to reveal how much energy is provided to the fragment recoil of fragment ions in the course of photoelectron emission and subsequent electronic excitation. Subsequent electronic excitation rather than photoelectron emission governs the internal energy of ethanol cations. The role of subsequent electronic excitation becomes more decisive as the laser intensity increases.

Anomalous time of flight behavior of fast ions in laser produced aluminum plasma

In this work, dynamics of multi-charged ions emitted from an aluminum plasma produced by Q switched Nd: Yag laser is studied using time of flight (TOF) measurements from Langmuir probe and spectroscopy under Ar ambient of 0.02 mbar. The temporal evolution of multi-charged ions, background neutrals, and ions is systematically studied for varying laser intensities. The temporal evolution shows all species have double-peak structure for all laser intensities considered in the study. The fast peak is sharp whereas the slow peak is broadly similar to that observed in previous studies. Moreover, higher charged ions have higher velocity, indicating acceleration from the transient electric field produced at the very initial temporal stages of expansion. Interestingly, the fast peak gets delayed, whereas the slow peak advances in time with increased laser intensity, which has not been reported in earlier studies. The observations point toward the possible role of ambipolar electric fields in the unexpected observed behavior of the TOF profiles.

Probing the effect of orbital deformation on the atomic tunneling-ionization-time distribution by phase-of-the-phase spectroscopy

We theoretically studied the tunneling ionization time of atomic ${p}_{\ifmmode\pm\else\textpm\fi{}}$ orbitals by introducing a weak linearly polarized second-harmonic field to the attoclock frame. In our scheme, the ionization time is retrieved by analyzing the relative phase dependence of ionization yield, where one can be free from handling the Coulomb interaction on the emitted electron. By solving the time-dependent Schr\"odinger equation, we verified that at low laser intensity the ionization time delays are both close to zero for the $2{p}_{\ifmmode\pm\else\textpm\fi{}}$ orbitals, though their offset angles in the photoelectron angular distributions are different. As the laser intensity increases, the atomic orbitals are deformed significantly, which affects the tunneling-ionization-time distribution. With our scheme, the orbital deformation-induced ionization time delay is unambiguously determined. Furthermore, the depletion of the ground state at the high laser intensity enhances the relative contribution of early ionization events and thus the ionization peak shifts to the earlier time. This effect is also directly revealed by our scheme.

Control of the single-order harmonic generation by changing the laser parameters of two-color pulse

Abstract We theoretically investigate the control of the single-order harmonic generation by changing the laser parameters of two-color pulse. Four important results have been found. For instance, (1) as the controlling chirp changes from the down-chirp to the up-chirp, the generation of the single-order harmonic can move to the lower harmonic order. (2) As the main pulse duration increases, the larger order single harmonic can be found. While, as the controlling pulse duration increases, the multi-orders harmonic enhancements can be found instead of the single-order harmonic enhancement. (3) As the main laser intensity increases, the larger order single harmonic can be obtained. While, as the controlling laser intensity increases, the multi-orders harmonic enhancements are obtained. (4) With the increase of the wavelength of the main pulse, the lower order single harmonic can be found. While, as the controlling laser wavelength increases, the single-order harmonic is disappeared. Finally, through analyzing the harmonic emission process, the reasonable explanation of the single-order harmonic control is given.