Pulse Fluence Research Articles

Waveguiding and Lasing in 2D Organic Semiconductor Znq2

Organic semiconductors have been proven as emerging platforms for exploring strong light–matter interactions and quantum optics so as to develop coherent and quantum photonic devices. Though there has been abundant research on organic semiconductors, lasing action is rarely reported in individual 2D organic semiconductor waveguides or nanocavities. Herein, a strong optical waveguiding phenomenon is observed not only in the individual 2D organic semiconductor Znq2 nanosheet with micrometer size area, but also in the interconnecting or neighboring Znq2 nanosheets. The optical propagation loss coefficient of the interconnecting Znq2 sheets is ≈800 dB cm−1. More importantly, the phenomenon of optically pumped lasing in 2D Znq2 nanosheets is demonstrated, as evidenced by a sudden increase in emission intensity at a threshold pump pulse fluence of 49.9 W cm−2 at 480 nm. As verified by numerical FDTD simulations, the Znq2 nanosheet constitutes a microcavity that supports whisper gallery mode, providing suffcient feedback for laser oscillations. Herein, the coexistence of low‐loss waveguiding and excellent lasing in 2D organic semiconductor nanosheets are revealed, which open up a new pathway for photonics and quantum optics based on 2D organic semiconductors.

Ultrafast laser processing of ceramics: Comprehensive survey of laser parameters

The productivity and quality of laser micromachining depend on multiple laser parameters that are intricately correlated. For these optimizations, a quick survey of laser parameters is vital. Recently, the authors developed a Yb-doped fiber chirped-pulse amplification system that can control various laser parameters in a wide range (pulse duration: 0.4–400 ps, repetition: single shot to 1 MHz, etc.). In this work, using this laser system, percussion microdrilling of three types of advanced ceramics, AlN, Al2O3, and Y2O3-doped ZrO2, was explored. In the case of the microdrilling of the Al2O3 ceramic, the ablation volume increased about 2–3 times as the pulse repetition increased from 100 Hz to 1 MHz. This suggests a different mechanism because the volume removal became dominant at 1 MHz. Scanning electron microscope observation confirmed a drastic melt formation at 1 MHz. From these, there is an additional volume removal due to the heat accumulation by multipulse irradiation on the Al2O3 ceramic at a higher repetition rate. It was also found that the variation of ablation volume with the pulse duration and fluence exhibited a big difference among these ceramics. A comprehensive survey of ultrafast laser ablation of ceramics was demonstrated.

Estimating fast transient detection pipeline efficiencies at UTMOST via real-time injection of mock FRBs

ABSTRACT Dedicated surveys using different detection pipelines are being carried out at multiple observatories to find more fast radio bursts (FRBs). Understanding the efficiency of detection algorithms and the survey completeness function is important to enable unbiased estimation of the underlying FRB population properties. One method to achieve end-to-end testing of the system is by injecting mock FRBs in the live data stream and searching for them blindly. Mock FRB injection is particularly effective for machine-learning-based classifiers, for which analytic characterization is impractical. We describe a first-of-its-kind implementation of a real-time mock FRB injection system at the upgraded Molonglo Observatory Synthesis Telescope (UTMOST) and present our results for a set of 20 000 mock FRB injections. The injections have yielded clear insight into the detection efficiencies and have provided a survey completeness function for pulse width, fluence, and dispersion measure (DM). Mock FRBs are recovered with uniform efficiency over the full range of injected DMs; however, the recovery fraction is found to be a strong function of the width and signal-to-noise ratio (SNR). For low widths (≲20 ms) and high SNR (≳9) the recovery is highly effective with recovery fractions exceeding 90 per cent. We find that the presence of radio-frequency interference causes the recovered SNR values to be systematically lower by up to 20 per cent compared to the injected values. We find that wider FRBs become increasingly hard to recover for the machine-learning-based classifier employed at UTMOST. We encourage other observatories to implement live injection set-ups for similar testing of their surveys.

Femtosecond Laser Ablation in Burst Mode as a Function of Pulse Fluence and Intra-Burst Repetition Rate

Femtosecond Laser Ablation in Burst Mode as a Function of Pulse Fluence and Intra-Burst Repetition Rate

THz generation by optical rectification of intense near-infrared pulses in organic crystal BNA.

Generation of terahertz radiation by optical rectification of intense near-infrared laser pulses in N-benzyl-2-methyl-4-nitroaniline (BNA) is investigated in detail by carrying out a complete characterization of the terahertz radiation. We studied the scaling of THz yield with pump pulse repetition rate and fluence which enabled us to predict the optimal operating conditions for BNA crystals at room temperature for 800 nm pump wavelength. Furthermore, recording the transmitted laser spectrum allowed us to calculate the nonlinear refractive index of BNA at 800 nm.

Effects of femtosecond laser micromachining on the surface and substrate properties of poly-lactic acid (PLA)

The effects of femtosecond laser ablation on the surface morphology, surface chemical composition, and matrix properties of poly-lactic acid (PLA) were explored in this paper, since the analysis of the different energy density range is crucial in evaluating to what extent material properties change during the micromachining. Scanning electron microscopy (SEM) analysis found that different surface microstructures, i.e., the convex microstructure and the through hole, were generated by single and multiple pulses femtosecond laser ablations, respectively. The photo-thermal features appeared on the PLA surface displayed a dependence on the femtosecond laser pulse number or fluence. The surface affected by femtosecond laser ablation was also analyzed using Fourier transforms infrared (FTIR) as well as X-ray photoelectron spectroscopy (XPS), which indicated that an oxidation reaction had occurred during the micromachining. The ablation of PLA surface resulted in a decreased molecular weight and an increase of polydispersity, which was consistent with a drop in transition temperature (Tm). Therefore, the substrate also degraded although the micromachining process was only performed on the surface. The variation in PLA surface microstructures, chemistry, and matrix performance resulted from different laser parameters were related to the laser-matter interaction effects.

Intersubband Relaxation in CdSe Colloidal Quantum Wells.

The dynamics of intersubband relaxation are critical to quantum well technologies such as quantum cascade lasers and quantum well infrared photodetectors. Here, intersubband relaxation in CdSe colloidal quantum wells, or nanoplatelets, is studied via pump-push-probe transient spectroscopy. An initial interband pump pulse is followed by a secondary infrared push excitation, resonant with intersubband absorption, which promotes electrons from the first conduction band of the quantum well to the second conduction band. A probe pulse monitors subsequent electron cooling to the band edge of the quantum well. Using this technique, intersubband relaxation is studied as a function of critical variables such as colloidal quantum well size and thickness, surface ligand chemistry, temperature, and excitation pulse intensity. Larger quantum well sizes, judicious selection of surface ligand chemistry (e.g., thiolates), low temperatures, and elevated push pulse fluences slow intersubband relaxation. However, compared to resonant intraband relaxation in colloidal quantum dots (up to hundreds of picoseconds), intersubband relaxation in colloidal quantum wells is rapid (<1 ps) under all examined conditions. These experiments indicate that rapid relaxation is driven by both LO phonon and surface scattering. The short time scale of relaxation observed in these materials may hinder intersubband technologies such as mid-infrared detectors, although such rapid relaxation may prove valuable in optical switching.

Analysis of laser focusing effect on quantification of LII images

Laser-induced incandescence (LII) is a widely used technique for measuring soot concentrations. For flame applications LII is frequently deployed as a planar diagnostic to measure the two-dimensional soot field. However, when the laser sheet is focused, as is typical to reach the requisite laser fluence level and achieve good spatial resolution, the complex laser power dependence of the LII signal generation process can introduce a large variation in LII signal sensitivity across an LII image. In this work, this effect is quantified for the first time as a function of laser pulse fluence, using a typical planar LII excitation scheme with a clipped Gaussian YAG laser beam focused with a 1 m focal length lens. Furthermore, the cross-sectional energy distribution in the laser sheet was measured across the image plane, to relate the details of the laser sheet focal properties with the resultant LII behavior. The results show that a unique laser fluence level (referenced to the focal plane) exists whereby there is essentially no dependence of LII signal on position relative to the focal plane. However, at lower or higher fluences, the radial signals either decrease (low fluence) or increase (high fluence) rapidly with increasing distance away from the focal point. For measurements using an LII ‘plateau’ laser fluence level, as is usual in environments with significant optical depth (i.e. sufficiently strong soot levels), the LII signals are found to be 2.5X larger 40 mm away from the focal point. An analysis conducted by combining a previously measured LII fluence dependence for a top-hat laser profile with the laser sheet cross-sections measured in this work shows general agreement with the measured results for LII signal variation. Further, the sensitivity of LII signals at high fluences to the laser beam spatial profile, particularly away from the sheet focus, is highlighted.

Femtosecond Plasmonic Laser Nanosurgery (fs-PLN) mediated by molecularly targeted gold nanospheres at ultra-low pulse fluences

Plasmonic Laser Nanosurgery (PLN) is a novel photomodification technique that exploits the near-field enhancement of femtosecond (fs) laser pulses in the vicinity of gold nanoparticles. While prior studies have shown the advantages of fs-PLN to modify cells, further reduction in the pulse fluence needed to initiate photomodification is crucial to facilitate deep–tissue treatments. This work presents an in-depth study of fs-PLN at ultra-low pulse fluences using 47 nm gold nanoparticles, conjugated to antibodies that target the epithelial growth factor receptor and excited off-resonance using 760 nm, 270 fs laser pulses at 80 MHz repetition rate. We find that fs-PLN can optoporate cellular membranes with pulse fluences as low as 1.3 mJ/cm2, up to two orders of magnitude lower than those used at lower repetition rates. Our results, corroborated by simulations of free-electron generation by particle photoemission and photoionization of the surrounding water, shed light on the off-resonance fs-PLN mechanism. We suggest that photo-chemical pathways likely drive cellular optoporation and cell damage at these off-resonance, low fluence, and high repetition rate fs-laser pulses, with clusters acting as local concentrators of ROS generation. We believe that the low fluence and highly localized ROS-mediated fs-PLN approach will enable targeted therapeutics and cancer treatment.

Formation of sub-wavelength laser induced periodic surface structure and wettability transformation of CFRP laminates using ultra-fast laser

Present work focuses on wettability transformation of carbon fiber reinforced plastic (CFRP) surface by generating micro/nano-structures using a femtosecond laser system. Laser induced periodic surface structures (LIPSS) were realized on carbon fiber (CF) surface at different defocus distances and pulse energies. The hydrophilic CFRP surface obtained just after the laser treatment progressively depicts hydrophobic behavior and further transforming to near superhydrophobic after cleaning followed by vacuum process for 4 h. Scanning electron microscopy (SEM) analysis was performed to capture the generated structure on the carbon fiber surface. Pulse fluence was effective parameters for transition of LSFL to HSFL. The lowest spatial period Λ ≈ 95 ± 6 nm and Λ ≈ 387 ± 15 nm was achieved for HSFL and LSFL, respectively, at 0.3 mJ.

Histopathological and biomechanical changes in soft palate in response to non-ablative 9.3-μm CO2 laser irradiation: an in vivo study.

The purpose of this study was to investigate in vivo the biomechanical and morphological changes in soft palates of Wistar rats from non-ablative irradiation with a 9.3-μm CO2 laser. A blinded, randomized, controlled study was designed with 45 Wistar rats categorized into treated and control sets. The treated set was exposed to 9.3-μm CO2 laser irradiation at an average power of 1.0W and a single pulse fluence of 0.16J/cm2 scanned using an automated system at a repetition rate of 315Hz in a patterned area covering 0.4cm2 in 6s. The tissue of each animal was excised and divided into two halves. One-half was sectioned for histopathology, and the other half was used to measure tissue stiffness, which was reported as the effective Young's modulus. Measurements for both sets were taken at three time points: days 1, 21, and 35. There were no significant adverse events or changes in the behavior of the rats over the duration of the study. The treated set exhibited an order of magnitude increase in stiffness relative to the controls, which was maintained over the three time points. Histopathology showed a moderate contraction/disruption of the lamina propria collagen observed at day 1 and collagen accumulation observed at days 21 and 35 in the tissue remodeling phase. Non-ablative 9.3-μm CO2 laser irradiation can safely increase oral mucosal stiffness and can be used as an effective treatment to reduce tissue vibrations that are associated with snoring.

Enhanced selective solar absorption of surface nanotextured semi-insulating 6H–SiC

Surface treatments were performed on single-crystal semi-insulating 6H–SiC by femtosecond pulsed laser irradiation, aimed at analyzing the effect of the laser-induced periodic surface structures (LIPSSs) on the films’ optical properties. The surface morphology study of the laser-induced nanostructures allows determining the modification threshold fluence of about 0.7 J cm−2 as well as detecting fine (160 nm) and coarse (450–550 nm) ripples according to different values of the laser pulse fluence (ΦP) released to the material. Micro-Raman spectroscopy allows determining the presence of undesired amorphous structural phases when ΦP exceeds 2.15 J cm−2, whereas no compositional variations occur for lower values of ΦP. Samples treated on the entire surface with the pulse fluence conditions to obtain fine ripples were optically tested. Although the long-range order is progressively lost as the accumulated laser fluence increases, the heaviest treated samples show solar absorptance values > 75% and spectral selectivity up to 1.7 projected at the operating temperature of 1000 K, thus pointing out the suitability of fs-laser surface textured 6H–SiC to act as a selective solar absorber for energy conversion devices operating at high temperature.

Effect of laser pulse duration and fluence on DKDP crystal laser conditioning

The impact of laser conditioning (LC) fluence and pulse duration on nanosecond (ns) laser damage performance of deuterated potassium dihydrogen phosphate (DKDP) crystal is studied. The result shows that higher LC fluence leads to a better damage resistance. In general, the sub-nanosecond LC effect is better than the nanosecond LC. However, in the range of 0.3 ns to 0.8 ns, the pulse duration has no obvious impact on the LC effect. An ultra-fast process characterization technology is employed to demonstrate that the cleaning effect of the protuberance defects on the surface is one of sub-ns LC mechanism. Eventually, a couple of optimized LC parameters that doubled the maximum damage threshold of DKDP crystal is proposed.

A comprehensive approach to the characterization of the deposited energy density during laser–matter interactions in liquids and solids

We present a novel approach to the characterization of ultrafast laser–matter interaction processes in solids and liquids under extreme conditions of microplasma generation. Through the combination of three-dimensional propagation imaging, absorption measurements, shadowgraphy and photoacoustic imaging we can restore plasma electron density distribution, laser pulse fluence profile and the value of deposited energy density inside the bulk of the material and characterize the regime of the laser pulse propagation. The developped concept is important for understanding the physics of ultrafast laser–matter interactions with strong implications for precision control of laser micromachining, bioprocessing and biotreatment.

Uniform LIPSS on titanium irradiated by two-color double-pulse beam of femtosecond laser

The authors have investigated the uniformity of laser-induced periodic surface structures (LIPSSs) generated on titanium surfaces irradiated with a two-color double-pulse cross-polarized beam with a time delay of Δt = 0–200 ps. The double-pulse beam consisted of 800 nm pulses with a duration of 150 fs and 400 nm pulses with a duration of &amp;gt;150 fs. The fundamental-pulse fluence F800 and the second-harmonic pulse fluence F400 were set to be near the corresponding ablation thresholds of F800th = 0.108 J/cm2 and F400th = 0.090 J/cm2, respectively. The authors found that uniform LIPSSs could be produced on titanium surfaces using laser fluences of 1.5F400th + 0.9F800th and a delay of Δt = 0–2 ps. The periodicity and direction of the LIPSSs were characterized by the wavelength and electric field of the fundamental (800 nm) pulse. The results suggest that the longer-wavelength pulse influences surface plasma wave generation and improves uniformity by the second harmonic pulse even though laser plasma is produced on the surface.

Residual stresses in Ti6Al4V alloy after surface texturing by femtosecond laser pulses

Surface topography and residual stresses in surface layers of α + β titanium alloy Ti6Al4V textured by 1030-nm, 320-fs-laser pulses were studied by scanning electron microscopy and X-ray diffraction analysis. It was found that multipulse laser processing leads to the formation of laser-induced periodic surface structures (LIPSS) on the surface of Ti6Al4V alloy. XRD studies showed that depending on the laser pulse fluence, both tensile and compressive residual stresses are formed in thin near-surface layers.

Investigation on the effects of laser parameters on the plasma profile of copper using picosecond laser induced plasma spectroscopy

Laser induced plasma generation and characterization, affected by laser parameters and sample physical properties, represent important phenomena in many fields of applications. In this work, we present new studies on the effects of an appropriate combination of laser wavelengths and pulse energies on the generated plasma characterization using a single shot picosecond Nd:YAG laser. The plasma plume of a pure copper sample has been generated by laser induced plasma spectroscopy (LIPS) using a single shot 170 ps laser pulse with wavelengths (266, 355, 532 and 1064 nm) and varying laser fluence (10–41 J/cm2). The spectral intensities of Cu I 324.75, 327.39, 515.32 and 521.82 nm have been observed. The plasma electron temperature and density have been determined from the Boltzmann plots and Stark-broadening profiles of the plasma spectral lines, respectively, assuming the local thermodynamic equilibrium (LTE) condition. It has been found that the electron temperature and electron density values increase from around 10,000 to 20,000 K and around 2 × 1017 to 1.5 × 1018 cm−3, respectively, with the increase in the laser wavelength and pulse fluence gradually. These observations can be understood due to the variations of mass-ablation rates, inverse-Bremsstrahlung, and photo-ionization with the studied pulse wavelength and pulse energy. The obtained results explore the opportunity to control specific generated plasma parameters by applying proper picosecond pulse parameters which can be considered in many fields of material science spectroscopic analysis and control the plasma interaction dynamics.

Temporal characterization of laser-induced plasma of tungsten in air

AbstractIn this manuscript, the time-resolved laser-induced breakdown spectroscopy (LIBS) on tungsten target in air and the coexistence of LTE among atoms and ions as well as the fulfillment of optically thin plasma condition are reported. The laser-induced plasma (LIP) of tungsten is generated by focusing the second harmonic of a Q-switched Nd:YAG laser of pulse width ~7 ns and repetition rate of 1 Hz on the tungsten target. The temporal evolution of LIP of tungsten is recorded at four different incident laser fluences of 60, 120, 180, and 270 J/cm2. The several atomic and singly ionized lines of tungsten are identified in LIP. For the estimation of plasma temperature via the Boltzmann plot, the transitions at 430.7, 449.4, 468.0, 484.3, 505.3, and 524.2 nm of Atomic transition of tungsten (WI) and that of the ionic transitions, First Ionic transition of Tungsten (WII) at 251.0, 272.9, and 357.2 nm are selected. The electron density is estimated using the Stark-broadened profile of WI line at 430.2 nm. The McWhirter criteria for the local thermodynamic equilibrium (LTE) condition is verified in present experimental conditions as well as the relaxation time and diffusion length are estimated to take into account the transient and inhomogeneous nature of the plasma. The optically thin plasma condition is studied by assessing the experimental intensity ratio of atomic lines and compared with that of the theoretical intensity ratio (branching ratio). The signal to noise ratio (SNR) is also obtained as a function of time with respect to laser pulse and incident laser fluence. All these observations indicate that the spectra should be recorded within the temporal window of 1–3.5 µs with respect to laser pulse where the plasma can be treated as optically thin as well as under LTE simultaneously along with the large SNR.

Formation of Hollow Gold Nanocrystals by Nanosecond Laser Irradiation

The irradiation of spherical gold nanoparticles (AuNPs) with nanosecond laser pulses induces shape transformations yielding nanocrystals with an inner cavity. The concentration of the stabilizing surfactant, the use of moderate pulse fluences, and the size of the irradiated AuNPs determine the efficiency of the process and the nature of the void. Hollow nanocrystals are obtained when molecules from the surrounding medium (e.g., water and organic matter derived from the surfactant) are trapped during laser pulse irradiation. These experimental observations suggest the existence of a subtle balance between the heating and cooling processes experienced by the nanocrystals, which induce their expansion and subsequent recrystallization keeping exogenous matter inside. The described approach provides valuable insight into the mechanism of interaction of a pulsed nanosecond laser with AuNPs, along with interesting prospects for the development of hollow plasmonic nanoparticles with potential applications related to gas and liquid storage at the nanoscale.

Transient Thermal Responses of Skin to Pulsed Millimeter Waves

This study examines thermal responses of skin to pulsed millimeter wave (mm-wave) and radiofrequency (RF) radiation. We review limits for pulse fluence in the IEEE Std. C95.1-2019 and the 2020 guideline of the International Commission on Nonionizing Radiation Protection (ICNIRP), as well as the recently re-affirmed guidelines of the U.S. Federal Communications Commission (FCC). The focus of the study is on millimeter-wave frequencies (30-300 GHz) where energy is absorbed close to the body surface and intense pulses could potentially cause high temperature gradients at the skin, but the model is extended to lower frequencies as well. The study employs a simple one-dimensional baseline thermal model for skin and Pennes' bioheat equation (BHTE), together with a baseline model for thermal damage to skin based on a standard model. The predicted temperature increases produced by 3-sec pulses at 94 GHz are consistent with previous experimental results with no adjustable parameters in the model. The few reported data on thermal damage to the skin from pulsed 94 GHz energy are insufficient to enable a conventional analysis of damage thresholds and the data may be affected by errors in dosimetry. The baseline model suggests that the implicit limits on pulse fluence in the present FCC guidelines might allow, in extreme (but in practice unrealistic) cases, transient increases in skin temperature that approach thresholds for thermal pain but which remain well below levels anticipated to cause thermal damage. Limits on pulse fluence in the current IEEE and ICNIRP exposure guidelines would preclude such effects. Such extreme pulses are far above those that are emitted by wireless and other technologies but may be emitted by some nonlethal weapons systems. FCC's proposed “device-based time averaging” rules will restrict thermal transients in skin from mm-wave transmitters to levels that are roughly an order of magnitude below the slower temperature increases produced by the low-frequency components of the modulation waveform and appear to be excessively conservative. An appendix discusses the applicability of two approximations to the analytical solutions to the bioheat equation that can be used to estimate temperature increases in skin from exposure to mm-waves.