Threshold Pulse Energy Research Articles

High efficiency laser ablation of gold thin film by 2.8 GHz intraburst repetition rate pulses

This work initially presents our progress in developing a 14 W all-polarization-maintaining (PM) Yb-doped fiber laser system. This system can generate bursts with a 2.8 GHz intra-burst repetition rate and offers flexibility in burst repetition rates, starting from 100 kHz. The laser produces pulse energies up to 210 nJ, which can be dechirped to approximately 650 fs. Subsequently, we utilized the developed laser in thin film gold processing. We sent up to 6.4 W and employed three different intraburst pulse configurations on a 100 nm-thick gold film coated on glass. We achieved a record gold removal efficiency of 125 μg/W.s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \ ext{g}/\ ext {W.s}$$\\end{document} using ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document}5 nJ pulses. Moreover, we determined an ablation pulse energy threshold of approximately 1 nJ, representing the lowest pulse energy to the best of our knowledge.

Single femtosecond laser pulse-induced valence state conversion in BaFCl: Sm3+ nanocrystals for low-threshold optical storage.

Femtosecond laser-induced valence state conversion (VC) in solid materials has attracted significant research attention due to its potential application in ultra-high density optical storage, boasting advantages such as ultra-high recording speed, easy reading, and high signal-to-noise ratio. However, identifying appropriate materials and technological solutions conducive to efficient single-laser-shot recording remains a pivotal challenge for practical applications. In this work, we report single femtosecond laser pulse-induced VC in BaFCl: Sm3+ nanocrystals utilizing a 4F-configuration optical imaging system comprising two-dimensional scan galvo mirrors. For the first time, we experimentally reveal the luminescence mechanisms and channels of multiphoton absorption-induced Sm2+ ions under both single and multiple 800 nm fs laser pulses. Leveraging the highly efficient single femtosecond laser pulse induced VC, we demonstrate a prototype optical storage experiment by sweeping the recording laser pulse. Remarkably, a threshold pulse energy as low as ∼100 nJ for effective single-laser-shot recording in BaFCl: Sm3+ nanocrystals is obtained under the current experimental conditions. Our investigations offer profound insights into the physical mechanisms underlying femtosecond laser induced VC in solid materials, thereby promoting the prospects of VC based optical storage toward practical applications.

Raman study of the soft modification of graphene by femtosecond laser irradiation

AbstractProcessing a graphene surface by irradiation with a fs‐pulsed laser at energies below the ablation threshold allows the production of non‐thermally modified regions with slightly different properties than pristine graphene. Oxidized nanoislands, optical forging, and nanopore networks are modifications of the graphene structure recently reported using a wide range of fs‐laser energies. In this work, we first determined the pulse energy threshold of a fs‐laser for the ablation of a commercial CVD graphene sample. Then we irradiated the sample in an extended range of pulse energies relative to the ablation threshold value. The irradiation process was simultaneously monitored by Raman spectroscopy, and the evolution of each Raman parameter (central position, bandwidth, and intensity) was analyzed. Special attention was given to the pulse energy that produced linear relationships between most Raman parameter pairs. These linear correlations evidence the interrelation between multiple Raman parameters and a soft processing method applicable to the controlled modification of the graphene lattice.

Experimental investigation into ultrafast laser ablation of polypropylene by burst and single pulse modes

Experimental investigation into ultrashort laser ablation by burst and Single Pulse (SP) modes on Polypropylene was performed. Isolated craters with different set of laser parameters were produced on flat polymer samples to assess the influence of the temporal separation between pulses on surface quality, removal rate (time needed to remove a certain volume) and ablation efficiency (pulse energy needed to remove a certain volume). The laser parameters that allow the development of structures with minimum redeposited material splashed around them and maximum removal rates were identified. Pulse energies of 2.5 μJ and 6 μJ were identified as the removal pulse energy threshold and the value from which a structure depth saturation starts, respectively.The use of bursts provides improved quality and higher removal rates compared to the Single Pulse mode when the same number of pulses of identical pulse energies were emitted in both cases, even though the total eliminated volume is higher in the SP mode. The advantages of using bursts of 3 pulses are reported. Increasing the number of pulses per burst to 4 do not increase the removal rate when compared to bursts of 3 pulses.

Passively Q-switched operation based on Sb2Se3 and self-power near infrared photodetector

In this paper, Antimony selenide （Sb2Se3） nanorods are prepared by the hot solvent method, the Sb2Se3 is detected by near-infrared detectors with good photoelectric properties. Afterwards its nonlinear optical performance was tested, compared with metal chalcogen compounds, the prepared Sb2Se3 has great bandwidth saturation absorption response, higher modulation depth (43.27%) and lower saturation intensity (2.13 MW/cm2), which is very suitable for making saturable absorber (SA) devices. In addition, we integrate Sb2Se3 SA into a 1.5 μm erbium-doped fiber laser and realize Q-switching for the first time. Our results show the obtained Q-switched laser has high single pulse energy and low self-starting threshold and good stability. Moreover, by adjusting the polarization state, a stable double wavelength spectrum appears. This indicates that Sb2Se3 has outstanding application potential in optical switching devices and pulse laser engineering.

Optical dosimeter for selective retinal therapy based on multi-port fiber-optic interferometry.

Selective retinal therapy (SRT) employs a micro-second short-pulse lasers to induce localized destruction of the targeted retinal structures with a pulse duration and power aimed at minimal damage to other healthy retinal cells. SRT has demonstrated a great promise in the treatment of retinal diseases, but pulse energy thresholds for effective SRT procedures should be determined precisely and in real time, as the thresholds could vary with disease status and patients. In this study, we present the use of a multi-port fiber-based interferometer (MFI) for highly sensitive real-time SRT monitoring. We exploit distinct phase differences among the fiber ports in the MFI to quantitatively measure localized fluctuations of complex-valued information during the SRT procedure. We evaluate several metrics that can be computed from the full complex-valued information and demonstrate that the complex contour integration is highly sensitive and most correlative to pulse energies, acoustic outputs, and cell deaths. The validity of our method was demonstrated on excised porcine retinas, with a sensitivity and specificity of 0.92 and 0.88, respectively, as compared with the results from a cell viability assay.

Energy limit for linear-to-nonlinear femtosecond laser pulse focusing in air

Propagation of a tightly focused high-power ultrashort laser pulse in an optical medium is usually substantially influenced by the medium optical nonlinearity that can noticeably affect the laser pulse parameters around the nonlinear focus and lead to unavoidable and often undesirable spatial distortions of the focal waist. We present the results of our experimental study and numerical simulations on a femtosecond Ti:Sapphire laser pulse propagation in air under different spatial focusing. We concentrated our study on spectral-angular and spatial pulse transformations under different focusing regimes - from linear to nonlinear one, when pulse filamentation occurs. For the first time to the best of our knowledge, we found the laser pulse numerical apertures range – namely, from NA = 2·10-3 to 5 10-3(for the laser pulse energy of 1 mJ), where the laser pulse distortions both in frequency-angular spectrum and pulse spatial shape are minimal. By means of the numerical simulations, we found the threshold pulse energy and peak power in a wide range of focusing conditions, within which a transition between the linear and strongly nonlinear laser pulse focusing in air takes place. This energy limit is shown to decrease with pulse numerical aperture enhancement. Our findings identify the laser pulse numerical apertures and energes adequate for getting a maximum laser intensity with a good beam quality around the focal point suitable for various laser micropatterning and micromachining technologies.

A simple strategy for increasing optical waveguide performance using spherical aberration

Femtosecond Laser Writing (FLW) has some advantages over competing techniques; it is a direct, highly flexible, and maskless process that can inscribe waveguides in almost any type of transparent material. FLW has proven to produce photonic integrated circuits with innovative design in a deterministic way for several applications, including quantum photonics and microfluidics. Although direct laser writing of waveguides has been extensively studied, there is still an intense search for methods to produce waveguides with lower losses and higher optical quality, because improvements in this sense will impact photonic research and technology. In this work, straight waveguides were directly inscribed into Eagle XG glass samples in a single process step at fabrication depths up to 370 µm. The propagation loss and the mode profile for single-mode waveguides were obtained. The laser pulse energy threshold that triggers the optical breakdown as a function of the depth was measured and used to study its relationship with the resulting waveguide optical properties. A trend between the waveguide losses and the observed pulse energy threshold for optical breakdown was found, which is connected with the microscope objective spherical aberration. It can be useful for finding the best conditions for producing low-loss waveguides. Compared to the waveguides written with the corrected pulses, propagation losses up to 2.5 times smaller were observed in our experiments when applying the strategy presented here. Therefore, by taking advantage of the inherent spherical aberration, the FLW technique can be made capable of much more at even higher intensities than previously possible.

Ultrafast Laser Inducing Continuous Periodic Crystallization in the Glass Activated via Laser‐Prepared Crystallite‐Seeds

AbstractTailoring ultrafast light–matter interaction as an enabler to create functional periodic crystalline patterns inside transparent dielectrics is of great scientific and technological significance but remains a tremendous challenge because of plenty of unclarified process details and physical mechanisms. Here, a dynamic process of ultrafast laser‐induced continuous periodic crystallization (CPC) to generate self‐organized crystal arrays (CAs) inside the glass is reported. The crystallite‐seeds induced by ultrafast laser are revealed to offer an auxiliary effect that can initiate and maintain the CPC process, which promises the high‐efficiency writing of CAs. The auxiliary effect originates from photo‐induced defects in the crystallite‐seeds, functions by drastically decreasing the pulse energy threshold for writing CAs, and can be manipulated by tuning the glass compositions and laser parameters. The created CAs are erasable and rewritable, which allows for potential applications in optical information encryption and data storage. This work not only opens up an avenue for creating embedded periodic crystalline patterns with extremely high efficiency but also helps to clarify the dynamics of ultrafast laser nanostructuring inside transparent dielectrics.

Random laser emission from neodymium doped zinc tellurite glass-powder presenting luminescence concentration quenching

Random Laser (RL) operation is reported for the first time in neodymium ions (Nd3+) doped zinc-tellurite (TZO) glass-powders. TZO glass was prepared by conventional melt-quenching technique and polydisperse micrometric grains with varying Nd3+ concentrations were obtained after milling. The samples were excited by a pulsed (5 ns, 5 Hz) source at 585 nm, in resonance with transition 4I9/2 → {4G5/2, 2G7/2}. For low excitation intensities all powder samples showed a broad emission band centered at 1068 nm (corresponding to transition 4F3/2 → 4I11/2) with microseconds decay time. Random lasing was only achieved for samples with Nd2O3 concentrations higher than 2.0 wt. %, despite the occurrence of Luminescence Concentration Quenching (LCQ). Excitation pulse energy thresholds for laser action were observed at 8 and 5 μJ/mm2 for samples doped with 5.0 and 10.0 wt. % of Nd2O3, respectively. The glassy-grains act simultaneously as gain medium and scatterers and the RL feedback mechanism is attributed to the light reflections in the grains-air interfaces. Above the laser threshold, we observed RL emission at 1068 nm, in the nanosecond range. The experimental data are supported by a rate-equation model that corroborates the main results. The results herein reported show that TZO glass may be employed as promising disordered medium for operation of RL sources.

Femtosecond-Laser Assisted Surgery of the Eye: Overview and Impact of the Low-Energy Concept.

This article provides an overview of both established and innovative applications of femtosecond (fs)-laser-assisted surgical techniques in ophthalmology. Fs-laser technology is unique because it allows cutting tissue at very high precision inside the eye. Fs lasers are mainly used for surgery of the human cornea and lens. New areas of application in ophthalmology are on the horizon. The latest improvement is the high pulse frequency, low-energy concept; by enlarging the numerical aperture of the focusing optics, the pulse energy threshold for optical breakdown decreases, and cutting with practically no side effects is enabled.

Theoretical fundamentals of short pulse laser–metal interaction: A review

Short and ultrashort pulse lasers offer excellent advantages in laser precision machining mainly because of their high pulse energy and low ablation threshold. The complex process of laser interaction with metals limits the in-depth investigation into laser ablation. Numerical simulation is important in the study of fundamental mechanisms. This review explores the start-of-the-art methods for the theoretical simulation of the laser ablation of metals, including plasma formation and expansion. Laser-induced period surface structures are also studied.

The effect of elliptical focusing on the threshold of a nanosecond BBO crystal optical parametric oscillator

ABSTRACTIn this paper, we report the results of our numerical model developed to determine the optimum elliptical focusing parameters of the high repetition rate input UV (355 nm) pump beam for achieving reliable low threshold operation of a singly resonant nanosecond β-BaB2O4 (BBO) based optical parametric oscillator (OPO). The analysis predicts that it is possible to achieve a substantial reduction in the threshold of the OPO by optimizing the 355 nm pump beam in an elliptical focus. We have used our model calculations to find out the maximum size of the elliptical focus for which the threshold pulse energy of the Type-I BBO crystal based OPO is lowered to ∼0.7 mJ. This provides the opportunity for a multi-kHz repetition rate, nanosecond BBO based OPO, pumped with the third harmonic of a DPSSL system operating with a pulse energy of ∼2–3 mJ, while maintaining high average power.

An automatic variable laser attenuator for IRMPD spectroscopy and analysis of power-dependence in fragmentation spectra

Infrared ion spectroscopy (IRIS) requires reproducibility and standard protocols in order to be fully appreciated as an analytical tool. An important parameter in IRIS measurements is the infrared laser power (energy per pulse), which is often difficult to control precisely on a day-to-day basis. We have developed an automatic variable attenuator to keep the laser power at a well-defined value, independent of IR frequency and time, while maintaining full overlap of the laser beam with the ion cloud in our quadrupole ion trap mass spectrometer. The device has been used to record IR spectra of the metabolite prostaglandin D2 at different laser pulse energies, allowing us to assess the effects on the IRMPD spectra of excessive ion depletion, a threshold pulse energy below which dissociation is not observed, and unobserved fragments due to electron detachment or fragmentation to below the low mass cut-off of the trap.

Optical emission of graphite plasma generated in ambient air using low-irradiance carbon dioxide laser pulses

Optical emission studies of graphite plasma induced by infrared (IR) Transversely Excited Atmospheric carbon dioxide (TEA CO2) laser pulses in ambient air at atmospheric pressure are reported. The plasma was induced at relatively low-irradiance, up to 40 MW cm−2, and the plasma emission was recorded using time-integrated LIBS measurements. The time profile of the 160 mJ laser pulse is composed of a short, 100 ns long initial spike, and a long 2 μs tail. About 60 mJ is contained in the initial pulse, and the 100-mJ tail contribution is favorable for extended plasma absorption that promotes creation of long-lasting highly-excited plasma. With laser pulse focused behind the target surface, recorded spectra consisted of intensive, sharp atomic and single charged ionic spectral lines of carbon, and trace elements, e.g. Ca, Cu, V, Si, and Ti. Good signal to background ratios obtained indicate potential application in the analysis of impurities in graphite, and also elemental analysis of other materials with high carbon content. The average electron number density was determined from Stark-broadened emission profile of C I 247.9 nm line, and the line intensity ratio of CII 250.9 nm/C I 247.9 nm line pair was used for estimation of ionization temperature. Depending on the applied fluence, electron density was in the range 2.6–4.8 × 1017 cm−3, and ionization temperature between 19,000 and 22,000 K. Beside line spectra, intensive and well-developed band spectra of diatomic molecules C2 (Swan system), and CN (violet system) were obtained. Pulse energy threshold for observation of molecular emission was 50 mJ. From the spectroscopic studies of the emission bands, the rotational and vibrational temperatures were estimated by comparing the experimental and simulated emission spectra. Vibrational and rotational temperatures deduced from Δν = 0 sequences of the Swan system of C2 were 3100 K and 3850 K, respectively. The most intense band of the CN violet system showed strong self-absorption and led to overestimated temperature values, Tvib = Trot = 4900 K.

Optimal Generation of Ten Individual Green-to-Red Raman Source for Wavelength-Dependent Real-Time OR-PAM Images

In this study, ten individual green-to-red stimulated Raman scattering (SRS) peaks are optimally generated for wavelength-dependent real-time optical-resolution photoacoustic microscopy (OR-PAM) with contrast agents. Exogenous contrast agents, such as dyes, semiconducting polymers, and metallic nanoparticles, are frequently used in OR-PAM to provide structural, functional, and molecular images of biological samples with a high spatial resolution. For an effective use of various exogenous contrast agents for functional or molecular OR-PAM, selectable visible and near-infrared wavelengths are needed to select the absorption wavelengths of the contrast agents. Therefore, we optimized an SRS source, which can selectively generate ten narrow-band SRS peaks with a bandwidth smaller than 11 nm and individually produce sufficient threshold pulse energies (116–756 nJ) at a high pulse repetition rate (PRR) of 300 kHz. The wavelength-dependent photoacoustic signal was easily obtained in a methylene blue phantom and in vivo mouse studies using the optimal generation conditions of the ten-wavelength green-to-red SRS source, from 545 to 695 nm. In particular, for the first time, wavelength-dependent real-time in vivo OR-PAM images at a PRR of hundreds of kilohertz were obtained to distinguish gold nanoparticles from distributed blood vessels of a mouse ear.

Selective retina therapy: toward an optically controlled automatic dosing.

Selective retina therapy (SRT) targets the retinal pigment epithelium (RPE) with pulsed laser irradiation by inducing microbubble formation (MBF) at the intracellular melanin granula, which leads to selective cell disruption. The following wound healing process rejuvenates the chorio-retinal junction. Pulse energy thresholds for selective RPE effects vary intra- and interindividually. We present the evaluation of an algorithm that processes backscattered treatment light to detect MBF as an indicator of RPE cell damage since these RPE lesions are invisible during treatment. Eleven patients with central serous chorioretinopathy and four with diabetic macula edema were treated with a SRT system, which uses a wavelength of 527nm, a repetition rate of 100Hz, and a pulse duration of 1.7 μs. Fifteen laser pulses with stepwise increasing pulse energy were applied per treatment spot. Overall, 4626 pulses were used for algorithm parameter optimization and testing. Sensitivity and specificity were the metrics maximized through an automatic optimization process. Data were verified by fluorescein angiography. A sensitivity of 1 and a specificity of 0.93 were achieved. The method introduced in this paper can be used for guidance or automatization of microbubble-related treatments like SRT or selective laser trabeculoplasty.

Femtosecond laser induced selective etching in fused silica: optimization of the inscription conditions with a high-repetition-rate laser source.

Femtosecond laser induced selective etching (FLISE) of dielectric materials is a promising technique for fabricating various microfluidic devices. Here we experimentally studied the dependence of the selective etching speed in fused silica glass on laser pulse energy, repetition rate, and inscription speed using a 1030 nm femtosecond laser. The evolution of micromorphology of the laser inscribed lines was revealed with optical microscopy, scanning electron microscopy, as well as anisotropic diffraction of the optical gratings formed by these inscribed lines. A single pulse energy threshold is required to initiate the FLISE. Further, a laser repetition rate window between an upper threshold and a lower threshold was observed, which were limited by the thermal-induced disruption of the nanogratings and by the disconnection of successive pulses modified spots respectively. The synergetic influences of the above factors were evaluated by the exposure laser energy density, which shows a common threshold for different inscription conditions and demonstrates itself to be an excellent criterion for choosing appropriate parameters in FLISE. The formation of continuous nanogratings is confirmed to be the major mechanism of FLISE in fused silica. Our observations not only help one to understand the micro mechanism in FLISE of fused silica, but also are of great use for fabricating large-scale microfluidic circuits.

Interferometric autocorrelation of ultrafast optical pulses in silicon sub-micrometer p-i-n waveguides.

We investigated the high-sensitivity interferometric autocorrelation of ultrafast optical pulses utilizing two-photon absorption in sub-micrometer silicon p-i-n waveguides. The autocorrelation sensitivities were evaluated to be about 0.5 and 4.5 × 10-8 W2 for 1- and 0.5-mm devices, respectively. Such sensitivities are about 100 times higher than the traditional two-photon conductivity photodetectors in commercial autocorrelators; thus favor weak pulse characterization. We comprehensively studied the interferometric autocorrelation performances by the experiment and FDTD (finite-difference time-domain) simulation. The pulse energy dependences of measured autocorrelation photocurrents and pulse widths were well explained by the simulation with the free carrier absorption and free carrier plasma effect considered. The autocorrelation error tends to occur if the pulse energy is high enough to cause strong free carrier effects and the threshold pulse energy for error occurrence is increased for shorter devices, but accurate autocorrelation measurement was achieved for sub-Watts pulses at which the influences of free carrier effects on interferometric autocorrelation was negligible. The minimum applicable range of pulse widths was estimated from waveguide dispersion analysis to be ~0.09 and 0.13 ps with a 10% target error for 0.5-mm and 1-mm devices, respectively. The interferometric autocorrelation in sub-micrometer silicon p-i-n waveguides is promising as a monolithic photonic device for on-chip monitor and diagnostics of weak ultrafast pulses.

Spatially resolved laser-induced breakdown spectroscopy in laminar premixed methane–air flames

Laser-induced breakdown spectroscopy was evaluated for the analysis of the structure of laminar premixed methane–air flames. Firstly, breakdown threshold pulse energy and plasma energy in different areas of the flame were measured simultaneously, and an approximate linear relation between them was detected. Secondly, a new approach was proposed to qualitatively characterize the flame temperature distributions based on the plasma energy distributions. Finally, combination of the spatial analysis of the spectrum intensity, plasma energy and equivalence ratio, the laminar premixed flames structure was investigated deeply, including the distribution of the flame temperature, the width and distribution of different flame region (e.g. premixed combustion regions, high temperature regions.),as well as the location of the flame front.