Fluence Density Research Articles

Polymer optical fiber amplifier based on all-inorganic perovskite quantum dots

The proliferation of polymer optical fibers (POFs) has opened multiple avenues of optical-based networks and sensing applications in the visible spectral range. However, the lack of efficient amplifiers significantly hinders their utilization in practical scenarios. As emerging gain media, halide perovskites have attracted considerable attention in exploring their practical applications. In this Letter, we investigated the optical gain properties of cesium lead bromide quantum dots (CsPbBr3 QDs), and by facet dip-coating, we realized a polymer optical fiber amplifier. Under a 400-nm, 163-fs Ti:Sapphire laser pumping, the amplified spontaneous emission (ASE) thresholds of 1.6 and 20.1 μJ/cm2 were achieved through stripe pumping of a QDs-thin-film and end-pumping of a polymer-fiber facet dip-coated with the QDs, respectively. A gain coefficient of 232.2 ± 22.8 cm−1 was obtained using the variable stripe length method. By coupling a broadband continuous-wave light source into the POF as the signal, the optical gain behavior was studied with varying pump fluence and signal power density. More than 20 dB optical gain was achieved within the ASE wavelength region of 530–540 nm with a predicted theoretical maximum gain of 33.6 dB. The research verifies the feasibility of amplifying continuous-wave signals in the visible spectrum and potentially closing the research gap in visible-light optical-to-optical amplifiers. This opens the avenue for further research and innovations in practical polymer-based optical amplification for a plethora of applications, including all optical processing chips and short-range interconnects, as well as visible-light and underwater communications.

A theoretical study of the signal enhancement mechanism of coaxial DP-LIBS

In the field of dual-pulse laser-induced breakdown spectroscopy (DP-LIBS) research, the pursuit of methods for determining pulse intervals and other parameters quickly and conveniently in order to achieve optimal spectral signal enhancement is paramount. To aid researchers in identification of optimal signal enhancement conditions and more accurate interpretation of the underlying signal enhancement mechanisms, theoretical simulations of the spatiotemporal processes of coaxial DP-LIBS-induced plasma have been established in this work. Using a model based on laser ablation and two-dimensional axisymmetric fluid dynamics, plasma evolutions during aluminum–magnesium alloy laser ablation under single-pulse and coaxial dual-pulse excitations have been simulated. The influences of factors, such as delay time, laser fluence, plasma temperature, and particle number density, on the DP-LIBS spectral signals are investigated. Under pulse intervals ranging from 50 to 1500 ns, the time evolutions of spectral line intensity, dual-pulse emission enhancement relative to the single-pulse results, laser irradiance, spatial distribution of plasma temperature and species number density, as well as laser irradiance shielded by plasma have been obtained. The study indicates that the main reason behind the radiation signal enhancement in coaxial DP-LIBS-induced plasma is attributed to the increased species number density and plasma temperature caused by the second laser, and it is inferred that the shielding effect of the plasma mainly occurs in the boundary layer of the stagnation point flow over the target surface. This research provides a theoretical basis for experimental research, parameter optimization, and signal enhancement tracing in DP-LIBS.

A Study of 10 Rotating Radio Transients Using Parkes Radio Telescope

Rotating Radio Transients (RRATs) are a relatively new subclass of pulsars that emit detectable radio bursts sporadically. We analyzed 10 RRATs observed using the Parkes telescope, with eight of these observed via the ultra-wide-bandwidth low-frequency (UWL) receiver. We measured the burst rate and produced integrated profiles spanning multiple frequency bands for three RRATs. We also conducted a spectral analysis on both integrated pulses and individual pulses of three RRATs. All of their integrated pulses follow a simple power law, consistent with the known range of pulsar spectral indices. Their average spectral indices of single pulses are −0.9, −1.2, and −1.0 respectively, which are within the known range of pulsar spectral indices. Additionally, we find that the spreads of single-pulse spectral indices for these RRATs (ranging from −3.5 to +0.5) are narrower compared to what has been observed in other RRATs. Notably, the average spectral index and scatter of single pulses are both relatively small. For the remaining five RRATs observed at the UWL receiver, we also provide the upper limits on fluence and flux density. In addition, we obtain the timing solution of PSR J1709-43. Our analysis shows that PSRs J1919+1745, J1709-43, and J1649-4653 are potentially nulling pulsars or weak pulsars with sparse strong pulses.

Fluence dependent dynamics of excitons in monolayer MoSi2Z4 (Z = pnictogen)

Reduced dielectric screening in two-dimensional materials enables bound excitons, which modifies their optical absorption and optoelectronic response. Here, we demonstrate the existence of excitons in the bandgap of the monolayer family of the newly discovered synthetic (Z=N, P, and As) series of materials. All three monolayers support several bright and strongly bound excitons with binding energies varying from 1 eV to 1.35 eV for the lowest energy exciton resonances. We show that on increasing the pump fluence or photo-excited carrier density, the lowest energy exciton first undergoes a redshift followed by a blueshift, due to the renormalized exciton binding energies. The exciton binding energy varies as a Lennard-Jones-like potential as a function of the inter-exciton spacing. This establishes an atom-like attractive and repulsive interaction between excitons depending on the inter-exciton separation. Our study shows that the series of monolayers offer an exciting test-bed for exploring the physics of strongly bound excitons and their non-equilibrium dynamics.

Analysis of molten pool dynamics and surface smoothing time scale in laser polishing alloy materials

Laser polishing (LP) is a promising technology for improving surface quality, while the understanding of mechanism of the material redistribution dynamics in LP process remains incomplete. In this study, from the aspect of molten pool dynamics, we demonstrate that there is only one optimal laser-material interaction time for surface smoothing, which is defined as the surface smoothing time scale (SSTS). Through the multi-physics model and experimental research, we found that the typical SSTS value for alloy materials ranges from tens to hundreds of microseconds, which mainly depends on liquid phase viscosity of the material determined by the temporal and spatial variations of temperature field in the LP process. The evolution of molten pool flowing and surface roughness over time under non-uniform temperature field caused by different processing parameters, such as laser fluence and power density, are analyzed in detail. We found that the mechanism of existence of SSTS lies in the Marangoni effect caused by non-uniform temperature field. The Marangoni effect shortens the SSTS in the initial process, and it is also the main mechanism for boundary bulging when the radiation time exceeds the SSTS. Finally, we designed a series of experiments and found a fluctuation of the roughness with the processing parameters such as scanning speed, which can be well explained according to the SSTS theory. This study can provide a theoretical basis for acquiring the process window of LP.

Energy-based devices in the treatment of acne scars in skin of color.

Acne scarring is disfiguring and psychologically taxing on patients. Many energy-based modalities have emerged and been studied for the treatment of acne scarring; however, there is a paucity of these studies in skin phototypes IV-VI. To review the medical literature and discuss the most significant studies regarding safety and efficacy of energy-based devices (ablative lasers, non-ablative lasers, and radiofrequency microneedling) in the treatment of ethnic skin (skin phototypes IV-VI). A literature search was conducted using the PubMed database and bibliographies of relevant articles. Ablative and non-ablative lasers have proven to be effective for treatment of acne scars in ethnic skin. The risk of developing adverse effects such as post-inflammatory hyperpigmentation is contingent upon several factors including skin phototype, laser device, fluence, and moreso density settings. Non-ablative fractional lasers have been considered first line for the treatment of acne scars in skin of color due to their better safety profile; however, they are less efficacious and require more treatments compared to ablative lasers. Studies regarding efficacy and safety of radiofrequency microneedling for treatment of acne scarring in skin of color are limited, but are promising. Ablative lasers, non-ablative lasers, and radiofrequency microneedling are all useful treatments for acne scarring in ethnic skin when appropriate settings are used. Further head-to-head studies are needed to evaluate their efficacy and safety in darker skin phototypes V-VI.

Long and Short Fast Radio Bursts Are Different from Repeating and Nonrepeating Transients

Abstract We collect 133 fast radio bursts (FRBs), including 110 nonrepeating and 23 repeating ones, and systematically investigate their observational properties. To check the frequency dependence of FRB classifications, we define our samples with a central frequency below/above 1 GHz as subsample I/II. First, we find that there is a clear bimodal distribution of pulse width for subsample I. If we classify FRBs into short FRBs (sFRBs; <100 ms) and long FRBs (lFRBs; >100 ms) as done for short and long gamma-ray bursts (GRBs), the sFRBs at higher central frequency are commonly shorter than those at lower central frequency not only for nonrepeating but also repeating sFRBs. Second, we find that fluence and peak flux density are correlated with a power-law relation of F ∝ S p , obs γ for both sFRBs and lFRBs whose distributions are obviously different. Third, the lFRBs with isotropic energies ranging from 1042 to 1044 erg are more energetic than the sFRBs in the F–DM EX plane, indicating that they are two representative types. Finally, it is interesting to note that the peak flux density behaves independently on the redshift when the distance of the FRBs becomes far enough, which is similar to the scenario of the peak flux evolving with redshift in the field of GRBs. We predict that fainter FRBs at a higher redshift of z > 2 can be successfully detected by FAST and the Square Kilometre Array in the near future.

Evaluation and measurement of laser induced Zr-plasma parameters along with self-generated electric and magnetic fields under various pressures of Ar environment

Nd:YAG (532 nm, 6 ns) laser generated Zr plasma parameters under various pressure of Ar at irradiance of 17 GWcm−2 have been investigated and their correlation with the estimated Self-Generated Electric and Magnetic fields (SGEMF’s) has been established. Faraday Cup is used for measurements of Kinetic Energy (K.E) and fluence, whereas, electric and magnetic probes have been employed for the measurements of SGEMFs of Zr plasma. Under various Ar pressures ranging from 1 Torr to 200 Torr, an increasing trend of K.E and decreasing trend for the fluence of Zr plasma ions have been observed. The K.E of Zr ions increases from 15.6 keV to 140.8 keV, whereas, fluence decreases from 1.2×1012 cm−2 to 6.8×1011 cm−2 by increasing the Ar pressures. The electron fluence increases up to 4.25×1013 cm−2 and then decreases from 3.4×1013 cm−2 to 9.1×1012 cm−2 by increasing Ar gas pressure. Similarly, the self-generated electric field (SGEF) of Zr plasma decreases form 95 Vm−1 to 0 and self-generated -magnetic-field (SGMF) initially increases from 100 G to 340 G and then decreases to 145 G. The monotonic decrease in electric field with increasing the Ar gas pressure is attributed to the monotonic decrease of ion fluence due to ambipolar diffusion and decrease in charge separation by recombination losses. Similarly, the initial increase and then a decrease of magnetic field along with reversal is related to initial increase and then decrease of electron fluence and non-parallel density and temperature gradients (∇ne×∇Te≠ 0) of Zr plasma.

Low-fluence and low-density CO2 laser: histological analysis of collagen fiber changes in skin and its clinical repercussions in photorejuvenation.

This study refers to clinical and histologic analysis of effects on photorejuvenation after one single treatment of fractional CO2 laser with low fluence and low density. To analyze histologically the quantitative variation of collagen fibers type I and III, elastic fibers, and epidermal thickness on D84, besides clinical evaluation of amount, length, thickness, and depth of periocular wrinkles during the same period. This is an open, prospective, interventional study. There were 40 healthy female with age between 35 and 65years. Twenty-six participants were randomly selected for D0 and D84 biopsy. A single session of fractional CO2 laser was done in the hole face, using a 800-µm tip, 5% density, and 10mJ fluence with a single pass. On D0, D42, and D84, a clinical comparative analysis of amount, length, depth, and thickness of periocular wrinkles has been done. On histological analysis, a comparative quantitative evaluation of collagen fibers type I and III, elastic fibers, and epidermal thickness has been done on D0 and D84. The results of this study denoted a significant clinical improvement of amount (- 32.17%; p < 0.0001), thickness (- 33%; p < 0.0001), lenght (- 35.84%; p < 0.0001), and depth of periocular wrinkles (- 32.46%; p < 0.0001). A significant increase in the amount of collagen fibers type III was observed on D84 (+ 60.67%; p = 0.0013). Collagen fibers type I and elastic fibers did not have the same result, with a nonsignificant increase (+ 8.31%; p = 0.3820) and a decrease (- 12.4%; p = 0.0585) respectively. Epidermal thickness has a tendency to significant variation (p = 0.05553). The results demonstrate that fractional CO2 laser with low fluence and low density is a safe and efficient option for photorejuvenation of the face.

Effect of wavefront rotation on the photoionization process by ultrafast laser spatiotemporal focusing

In this paper, we theoretically studied the propagation of the first-order spatiotemporal coupled ultrafast pulse in fused silica glass by using the Cartesian nonlinear Schrödinger equation coupled with the electron density rate equation. We found that wavefront rotation, one parameter of the shaped pulse, may play an integral role to adjust the asymmetrical modification under different initial coupling conditions. During the focusing process, the pulse wavefront generates or rotates the pulse front through angular dispersion, and this process directly causes the change of inclined direction of the pulse front tilt at the focal plane. The spatiotemporal coupling change could make a difference in the asymmetric distributions of intensity, fluence, and electron density at the plane of interaction, and further affect the photoionization process. Therefore, the research on the influence of wavefront rotation on the pulse front tilt under the initial pulse incidence condition helps us to understand the temporal and spatial evolution of ultrafast laser pulses. Based on our numerical simulation, the possible mechanism of nonreciprocal direct-writing phenomena is revisited by taking into account the effect of wavefront rotation.

Laser irradiation of ocular tissues to enhance drug delivery

Scleral and corneal membranes represent substantial barriers against drug delivery to the eye. Conventional hypodermic needles-based intraocular injections are clinically employed to overcome these barriers. This study, for the first time, investigated a non-invasive alternative to intraocular injections by laser irradiation of ocular tissues. The P.L.E.A.S.E.® laser device was applied on excised porcine scleral and corneal tissues, which showed linear relationships between depths of laser-created micropores and laser fluences at range 8.9–444.4 J/cm2. Deeper and wider micropores were observed in scleral relative to corneal tissues. The permeation of rhodamine B and fluorescein isothiocyanate (FITC)-dextran were investigated through ocular tissues at different laser parameters (laser fluences 0–44.4 J/cm2 and micropore densities 7.5 and 15%). Both molecules showed enhanced permeation through ocular tissues on laser irradiation. Maximum transscleral permeation of the molecules was attained at laser fluence 8.9 J/cm2 and micropore density 15%. Transcorneal permeation of rhodamine B increased with increasing either laser fluence or micropore density, while that of FITC-dextran was not affected by either parameter. The transscleral water loss increased significantly after laser irradiation then returned to the baseline values within 24 h, indicating healing of the laser-created micropores. Laser irradiation is a promising technique to enhance intraocular delivery of both small and large molecule drugs.

Neutronic analyses of Upper port ECRH antenna system for CFETR

• 3D full neutronic model of CFETR ECRH was built to deliver the nuclear analysis. • The maximum neutron damage on ECRH antenna appear at the steering mirrors. • Irradiation of VV and TFC can meet the requirements after shielding optimization. • TBR of the whole reactor reduced to 1.137 after 3 ECRH antenna introduced. Electron Cyclotron Resonance Heating (ECRH) is one of the auxiliary heating systems designed for CFETR. The antenna of ECRH system is planned to be plugged into CFETR through 3 upper ports and 2 blanket modules of each port will be penetrated for the transmission of wave. In this study, three dimensional neutronic analysis has been performed to evaluate the shielding effect and conduct the optimization of ECRH antenna design. The simulation has been done on the integration of ECRH antenna system with 22.5-degree CFETR model. The particle transport has been done by MCNP with the support of FENDL-2.1 nuclear data library. An accurate neutron flux distribution has been obtained by employing the ‘on-the-fly’ global variance reduction technique. The neutron damage of TFC, VV and port was analysed to assess the shielding performance of ECRH antenna system. Initial results demonstrated that the fast neutron fluence and nuclear heating density of TFC and helium production rate of VV port were beyond the design limit. By adding shielding blocks, these parameters didn’t exceed the limits. The irradiation damage on antenna components were calculated for estimating its life after neutron irradiation and providing the support of cooling system design. The TBR decreased due to the blanket opening from original 1.149 to 1.137 in the whole reactor.

Charge Carrier Relaxation in Colloidal FAPbI3 Nanostructures Using Global Analysis

We study the hot charge carrier relaxation process in weakly confined hybrid lead iodide perovskite colloidal nanostructures, FAPbI3 (FA = formaminidium), using femtosecond transient absorption (TA). We compare the conventional analysis method based on the extraction of the carrier temperature (Tc) by fitting the high-energy tail of the band-edge bleach with a global analysis method modeling the continuous evolution of the spectral lineshape in time using a simple sequential kinetic model. This practical approach results in a more accurate way to determine the charge carrier relaxation dynamics. At high excitation fluence (density of charge carriers above 1018 cm−3), the cooling time increases up to almost 1 ps in thick nanoplates (NPs) and cubic nanocrystals (NCs), indicating the hot phonon bottleneck effect. Furthermore, Auger heating resulting from the multi-charge carrier recombination process slows down the relaxation even further to tens and hundreds of picoseconds. These two processes could only be well disentangled by analyzing simultaneously the spectral lineshape and amplitude evolution.

Fractional carbon dioxide laser for facial scarring due to windscreen glass shatter injury.

Facial injuries due to flying windscreen glass pieces are common in road accident and require surgical intervention by plastic surgeons to debride, suture, and dress the injuries. Posthealing patients visit dermatologists for further treatment of residual scar tissue. Interestingly, tiny papules or small nodular scars seen on otherwise level background which persists for beyond 3months (scar modeling time) may actually harbor small fragments of glass. Such lesions can only be treated by fractional CO2 therapy. Two patients who had papules and small nodular lesions on the accident scarred areas were subjected to fractional CO2 laser (ultraPulse, Lumenis)with moderate fluence and high density, once and observed over a period of 12weeks. On targeting the papules and small nodules, tiny particles of glass appeared out of the broken lesions. Irrespective of the surgery done on accident victims involving windshield glass injuries, fractional CO2 may be considered at a later stage, especially with presence of papules interspersed in the area of facial scars.

Direct measurement of radial fluence distribution inside a femtosecond laser filament core

Modulation and direct measurement of the radial fluence distribution inside a single filament core (especially less than 100 μm in diameter) is crucial to filament-based applications. We report direct measurements of the radial fluence distribution inside a femtosecond laser filament core and its evolution via the filament-induced ablation method. The radial fluence distributions were modulated by manipulating the input pulse diffraction through an iris. Compared with using a traditionally circular iris, a stellate iris substantially suppressed the diffraction effect, and laser fluence, intensity and plasma density inside the filament core were considerably increased. The radial fluence inside filament cores was also quantitatively measured via the filament drilling diaphragms approach. Furthermore, numerical simulations were performed to support the experimental results by solving nonlinear Schrödinger equations. The effects of the tooth size of the stellate iris were numerically investigated, which indicated that bigger tooth favors higher fluence and longer filament. In addition to being beneficial in understanding the filamentation process and its control, the results of this study can also be valuable for filament-based applications.

Frequency Effects on Helium–Oxygen Dielectric Barrier Discharges from Multibreakdowns to Single Breakdown per Half-Cycle

In this work, a one-dimensional fluid model is established to study the effects of frequency on (He + O2) dielectric barrier discharge. As the frequency increases from 20 to 30 kHz, the first pulse becomes more intensive, while the second pulse gradually decreases and finally disappears. The plasma transforms from multibreakdowns to single breakdown per half-cycle. In order to study the production of reactive oxygen species (ROS) during this transformation, the spatiotemporal distributions of ROS densities are presented and the average ROS densities, as well as their wall fluences, are statistically analyzed. It is suggested that the production of each species increases along with the frequency, but when the second pulse disappears, the productions of some species, such as $${{{\text{O}}}^{ - }}$$ and $${\text{O}}_{2}^{ * }$$, decline slightly. Moreover, the wall fluence and average density of a specific species vary with similar trends.

Damages on pure tungsten irradiated by compression plasma flows

Since the plasma pulse parameters of CPFs (compression plasma flows) are similar to the ones of ELMs (edge localized modes) in TOKAMAK, it can be used to study the ELMs induced transient heat damage on plasma facing materials (PFMs) experimentally. However, it is difficult to directly measure the CPFs energy deposition on the target. We adopted an indirect measuring method and with digital calculation corrections. We observed the damage on pure tungsten samples induced by CPFs with the corrected average energy densities ε from 87 J/cm2 to 158 J/cm2, and the pulse duration about 0.1 ms. The results showed that cracking, melting, ablation damages appeared even at the lowest energy fluence density (87 J/cm2). Two kinds of cracks were observed: primary cracks caused by the brittleness of the W and thermal stress from temperature gradient, and secondary crack caused by thermal stress from resolidification. Many kinds of melting damages including molten flow traces, reticulate pattern, nano sized pinholes at the intersection points of the pattern, micro particles, and columnar crystal grain on the tungsten surface were observed. The ablation damage including droplet ejection and mass loss after irradiation were observed and mass loss increased as the ε increasing.

Fractional Carbon Dioxide Laser: Optimizing Treatment Outcomes for Pigmented Atrophic Acne Scars in Skin of Color.

Dark skin type has high propensity to acne scarring and is often complicated by persistent erythema or pigmentation at the base. Fractional lasers are available for the longest period and are able to improve most atrophic acne scars. Often pigmented scar bases and dark skin types limit the use of aggressive laser parameters. Long pulse mode is preferred over short pulse to prevent epidermal damage; low fluence is chosen versus high fluence and low density versus high density. Repeated treatments are needed to minimize complications and optimize results; all these must be achieved through a controlled stage of inflammation. Interventional priming with chemical peels and laser toning before ablative fractional carbon dioxide laser helps to reduce photodamage, recent tan, and pigment at scar base, thus minimizing the risk of post-inflammatory hyperpigmentation. Multiple recent literature evidence validates the combinations to optimize outcomes in atrophic acne scars as discussed in this review article.

Femtosecond laser reshaping yields gold nanorods with ultranarrow surface plasmon resonances.

The irradiation of gold nanorod colloids with a femtosecond laser can be tuned to induce controlled nanorod reshaping, yielding colloids with exceptionally narrow localized surface plasmon resonance bands. The process relies on a regime characterized by a gentle multishot reduction of the aspect ratio, whereas the rod shape and volume are barely affected. Successful reshaping can only occur within a narrow window of the heat dissipation rate: Low cooling rates lead to drastic morphological changes, and fast cooling has nearly no effect. Hence, a delicate balance must be achieved between irradiation fluence and surface density of the surfactant on the nanorods. This perfection process is appealing because it provides a simple, fast, reproducible, and scalable route toward gold nanorods with an optical response of exceptional quality, near the theoretical limit.

Temporal scaling law and intrinsic characteristic of laser induced damage on the dielectric coating

Abstract High power laser is essential for optical manipulation and fabrication. When the laser travels through optics and to the target finally, irreversible damage on the dielectric coating is always accompanied without knowing the law and principle of laser induced damage. Here, an experimental study of laser induced damage threshold (LIDT) F th of the dielectric coating under different pulse duration t is implemented. We observe that the temporal scaling law of square pulse for high-reflectivity (HR) coating and anti-reflectivity (AR) coating are F th = 9.53 t 0.47 and F th = 6.43 t 0.28 at 1064 nm, respectively. Moreover, the intrinsic LIDT of HR coating is 62.7 J/cm 2 where the coating is just 100% damaged by gradually increasing the fluence densities of a 5ns-duration pulse, which is much higher than the actual LIDT of 18.6 J/cm 2 . Thus, a more robust and reliable high power laser system will be a reality, even working at very high fluence, if measures are taken to improve the actual LIDT to a considerable level near the intrinsic value.