Μs Laser Pulse Research Articles

Laser lithotripsy for pancreatic duct stones

Aim. To confirm the possibility of minimally-invasive laser procedure for fragmentation of stones in the pancreatic duct following the results of laboratory experiments and medical practice. Materials and methods. A fully functional hardware complex consisting of a pulsed laser unit, video endoscopic system and cooling system was created for experimental stone fragmentation in laboratory. The experiments involved microscopic video registration of the fragmentation process, pressure and temperature measurement in the stone area. Pancreatic stones, human and artificial gallstones as well as porcine pancreas were used in the experiments; the gastrointestinal tract was made of silicone, the pancreas was made of gelatin aqueous solution jelly. In clinical practice, a patient with lithiasis of Wirsung's duct was assigned to ultrasound and CT, then endoscopic papillosphincterotomy, endoscopic pancreatoscopy, laser lithotripsy and lithoextraction, pancreatic duct stenting. The procedures involved a laser unit, videoduodenoscope, catheter and their video systems. Results. The laboratory experiments established that effective and safe laser fragmentation of stones from 4 × 7 mm into 0.1–2 mm occurs in the flowing fluid in front of the stone at 1 μs laser pulse duration, 5–15 Hz frequency and 120–150 mJ laser energy. After laser fragmentation of the stone in a patient with virsungolithiasis and removal of fragments from the duct, the patient was discharged from the hospital on the 12th day due to improvement of healthcondition. Conclusion. The possibility of minimally-invasive fragmentation of stones in the pancreatic duct by laser lithotripsy has been proved.

Development of a high-average-power microsecond erbium–ytterbium-doped fiber laser system

We report a 200 kHz master oscillator power amplifier erbium–ytterbium-doped fiber laser system at a wavelength of 1548.7 nm with a constant pulse energy of ∼190 μJ providing a tuneable pulse duration of 2 to 5μs. The all-fiber laser system consists of a gain-modulated laser diode used as a seeder, two pre-amplifiers, and a booster amplifier. The output power of the laser system after collimation was 38.24 W, with a root mean square instability of 0.19% providing an excellent performance under environmental perturbations. The beam quality factor was estimated to be M2(x) = 1.08 in the horizontal and M2(y) = 1.09 in the vertical planes indicating a near-diffraction-limited laser beam. After synchronization to femtosecond pulses, the obtained μs laser pulses are expected to contribute to novel microprocessing of semiconductors.

Prediction of Shock Wave Velocity Induced by a Combined Millisecond and Nanosecond Laser Based on Convolution Neural Network

The variation of shock-wave velocity with time induced by a millisecond-nanosecond combined pulse laser (CPL) on silicon is investigated. The convolution neural network (CNN) is used to predict the shock-wave velocity induced by a single ns laser and CPL with a ns laser energy density of 6, 12 and 24 J/cm2, ms laser energy density of 0 and 226.13 J/cm2, and pulse delay of 0, 0.4 and 0.8 ms. The four-layer CNN model was applied, ns laser energy density, ms laser energy density, pulse delay and time were set as the input parameter, while the shock-wave velocity was set as the output parameter. The correlation coefficient (R2), mean absolute error (MAE) and root mean square error (RMSE) of the CNN model on the test data set was 0.9865, 3.54 and 3.01, respectively. This indicated that the CNN model shows a high reliability in the prediction of CPL-induced shock-wave velocity with limited experimental data.

Empirical mode decomposition of local field potential data from optogenetic experiments.

This study investigated the effects of cocaine administration and parvalbumin-type interneuron stimulation on local field potentials (LFPs) recorded in vivo from the medial prefrontal cortex (mPFC) of six mice using optogenetic tools. The local network was subject to a brief 10 ms laser pulse, and the response was recorded for 2 s over 100 trials for each of the six subjects who showed stable coupling between the mPFC and the optrode. Due to the strong non-stationary and nonlinearity of the LFP, we used the adaptive, data-driven, Empirical Mode Decomposition (EMD) method to decompose the signal into orthogonal Intrinsic Mode Functions (IMFs). Through trial and error, we found that seven is the optimum number of orthogonal IMFs that overlaps with known frequency bands of brain activity. We found that the Index of Orthogonality (IO) of IMF amplitudes was close to zero. The Index of Energy Conservation (IEC) for each decomposition was close to unity, as expected for orthogonal decompositions. We found that the power density distribution vs. frequency follows a power law with an average scaling exponent of ~1.4 over the entire range of IMF frequencies 2-2,000 Hz. The scaling exponent is slightly smaller for cocaine than the control, suggesting that neural activity avalanches under cocaine have longer life spans and sizes.

Enhancement of surface properties of polyetheretherketone implant material by fractional laser texturing

Background: Polyetheretherketone (PEEK) is a promising implant material due to its superior biomechanical strength. However, due to its hydrophobic nature and lack of cellular adhesion properties, it has poor integration with bone tissue. Methods: A fractional CO2 laser was used with various parameters for surface texturing of PEEK substrate to enhance its surface properties. An optical microscope and field-emission scanning electron microscope (FESEM) were used to examine the surface morphology of untextured and laser-textured samples. Energy dispersive X-ray spectroscopy (EDX) was performed to determine the effect of the laser on the microstructure of PEEK. Surface microroughness, atomic force microscopy (AFM), and wettability were investigated. Results: There were significant increases in microroughness, nanoroughness, surface area ratio, and wettability after laser texturing with no change in the elemental composition. The best results were obtained by using 400 µs laser pulse duration with a dot separation distance of 0.2 mm and a 60° staggered dots pattern. Conclusions: Laser surface texturing of PEEK implant material by fractional CO2 laser is an easy and fast method of introducing patterned topographical features with no need for additional devices. With further investigations, this method of PEEK modification might have the potential to be used in the implant field.

High-speed photothermally triggered bidirectional current switching in VO2/AlN/Si-based heterostructure device

The bidirectional current switching (BCS) of a two-terminal device based on a vanadium dioxide (VO2) thin film can be realized by harnessing the photothermally induced phase transition of VO2, triggered by laser pulses. Although this photothermally controlled BCS can achieve a large on-state current and high on/off current ratio compared to those obtained by other implementation approaches, a low repetition rate (< 10 Hz) and slow response time (tens of milliseconds) still restrict its practical use. To tackle these issues, we analyzed the fundamental cause of the performance limit of the BCS through simulations using COMSOL Multiphysics and found that the thermal conductivity of the substrate and the critical temperature of the VO2 film were mainly responsible for the switching speed of the BCS. On the basis of this analysis, we fabricated a VO2-based heterostructure device on a silicon substrate with a buffer layer of aluminum nitride. We measured the transient responses of the device current switched by directly irradiating the device with near-infrared laser pulses under various conditions. The highest-ever repetition rate (100 Hz) could be attained in a 50 mA BCS with 5 ms laser pulses. The response time was also remarkably improved to < 1 ms.

Upconversion FRET quantitation: the role of donor photoexcitation mode and compositional architecture on the decay and intensity based responses

Lanthanide-doped colloidal nanoparticles capable of photon upconversion (UC) offer long luminescence lifetimes, narrowband absorption and emission spectra, and efficient anti-Stokes emission. These features are highly advantageous for Förster Resonance Energy Transfer (FRET) based detection. Upconverting nanoparticles (UCNPs) as donors may solve the existing problems of molecular FRET systems, such as photobleaching and limitations in quantitative analysis, but these new labels also bring new challenges. Here we have studied the impact of the core-shell compositional architecture of upconverting nanoparticle donors and the mode of photoexcitation on the performance of UC-FRET from UCNPs to Rose Bengal (RB) molecular acceptor. We have quantitatively compared luminescence rise and decay kinetics of Er3+ emission using core-only NaYF4: 20% Yb, 2% Er and core-shell NaYF4: 20% Yb @ NaYF4: 20% Yb, 5% Er donor UCNPs under three photoexcitation schemes: (1) direct short-pulse photoexcitation of Er3+ at 520 nm; indirect photoexcitation of Er3+ through Yb3+ sensitizer with (2) 980 nm short (5–7 ns) or (3) 980 nm long (4 ms) laser pulses. The donor luminescence kinetics and steady-state emission spectra differed between the UCNP architectures and excitation schemes. Aiming for highly sensitive kinetic upconversion FRET-based biomolecular assays, the experimental results underline the complexity of the excitation and energy-migration mechanisms affecting the Er3+ donor responses and suggest ways to optimize the photoexcitation scheme and the architecture of the UCNPs used as luminescent donors.

Optical breakdown in holmium:YAG laser lithotripsy

Optical breakdown produces shock waves and cavitation and is typically observed with the Q-switched lasers. In contrast, the free-running mode holmium:YAG laser lithotripters produce long (&amp;gt;150 μs) laser pulses that were reported to break urinary stones via thermal decomposition of stones and explosive vaporization of interstitial water without optical breakdown. Here, we show evidence of optical breakdown in holmium:YAG laser lithotripsy using fine temporal and spatial resolution of ultrahigh-speed video microscopy. Images were collected with a Shimadzu HPV-X2 camera at rate up to 10 million frames per second. Infrared- and visible-light photodetectors (10-ns rise time) resolved temporal profiles of infrared laser pulses and visible sparks indicative of optical breakdown. Hydrophone measurements showed shock waves produced by optical breakdown and subsequent collapses of vapor bubbles, capable of breaking glass microscope slides with a single laser pulse. Optical breakdown was observed with the laser fiber tip in contact with synthetic and whole surgically retrieved urinary stones. These observations suggest that the previously unappreciated optical breakdown with holmium:YAG lasers can be an additional acoustic mechanism of action in laser lithotripsy, currently the most used treatment modality for urinary stones, and in other laser procedures. [Work supported by NIDDK of NIH under Award No. R43DK129104.]

Numerical analysis and semi-analytical prediction of the depth of holes drilled with combined ms and ns laser pulses

The evolution of the depth of holes drilled with a single ms laser pulse combined with a series of ns laser pulses for different processing parameters was studied numerically. The results show that the achievable hole depth can be increased by increasing the intensity of the ms pulses and by saving the absorbed energy that would be lost in overheating of the melt. The melt includes the one generated between subsequent ns pulses and the un-expelled melt after the effect of shock waves induced by the previous ns pulse. The overheating of the un-expelled melt is avoided by increasing the intensity of the ns pulses. Additionally, increasing the repetition rate of the ns pulses during the processing with a single ms pulse reduces the energy wasted for overheating the generated melt and the energy transferred radially to the rim of the hole. On the basis of the numerically obtained relations between the intensity of the ns pulse and the changed depth of the hole per pulse, a semi-analytical method was developed to predict the depth of the hole drilled with combined laser pulses. The applicability of the method is discussed and verified by comparing the semi-analytical results with the numerical and experimental results.

Investigating the Thermal Stress of Millisecond Pulsed Laser Irradiation on Charge-Coupled Devices

In this study, a two-dimensional model describing thermal stress on a charge-coupled device (CCD) induced by ms laser pulses was examined. Considering the nonlinearity of the CCD’s material parameters and the melting phase transition process of aluminum electrode materials was considered by using equivalent specific heat capacity method, the physical process where a laser pulse irradiating a CCD pixel array was simulated using COMSOL Multiphysics software. The temperature field and thermal stress field were calculated and analyzed. In order to clarify the mechanism producing damage on the CCD detector, Raman spectra from silicon were measured with a micro-Raman spectrometer to determine stress change in the CCD chip. The procedure presented herein illustrates a method for evaluating strain in a CCD after laser irradiation.

Laser-Induced Desorption of co-deposited Deuterium in Beryllium Layers on Tungsten

Abstract For the development of the tritium monitoring system in ITER the hydrogen isotope release by Laser-Induced Desorption (LID) from Be layers is studied to determine the laser parameters for a high desorption efficiency while minimising dust production and surface modifications is also pursued. Be layers of 1 µm thickness with 25–30 at% D and 3 × 1022 D/m2 comparable to JET-ILW areal concentrations [1] have been produced by High Power Impulse Magnetron Sputtering (HiPIMS) on ITER grade W. Laser pulses of 1, 5 and 10 ms duration heat the layer in vacuum in the Fuel REtention DIagnostic Setup (FREDIS) and release the retained D thermally. By mass spectrometry in FREDIS and subsequent Nuclear Reaction Analysis (NRA) inside the laser spot the desorbed and remaining D is quantified. While a pulse duration of 1 ms cannot fully desorb the deuterium, it is found that a single 5 or 10 ms laser pulse with an absorbed energy density of ca. 1.5 MJ/m2 corresponding to a heat flux factor around 20 MW√s/m2 leads to nearly complete desorption of the retained D. This encourages the development of a useful tritium monitoring system, although the present layers produce some dust due to local delamination of the layer on at least 11% of the heated surface (at 1.4 MJ/m2 absorbed energy within 5 ms) and lead to unavoidable crack formation.

Few-photon detection using InAs avalanche photodiodes.

An avalanche photodiode with a ratio of hole-to-electron ionization coefficients, k = 0, is known to produce negligible excess noise irrespective of the avalanche gain. The low noise amplification process can be utilized to detect very low light levels. In this work, we demonstrated InAs avalanche photodiodes with high external quantum efficiency of 60% (achieved without antireflection coating) at the peak wavelength of 3.48 µm. At 77 K, our InAs avalanche photodiodes show low dark current (limited by 300 K blackbody background radiation), high avalanche gain and negligible excess noise, as InAs exhibits k = 0. They were therefore able to detect very low levels of light, at 15-31 photons per 50 µs laser pulse at 1550 nm wavelength. These correspond to the lowest detected average power by InAs avalanche photodiodes, ranging from 19 to 40 fW. The measurement system's noise floor was dominated by the pre-amplifier. Our results suggest that, with a lower system noise, InAs avalanche photodiodes have high potential for optical detection of single or few-photon signal levels at wavelengths of 1550 nm or longer.

Synthesis, electronic structure and physical properties of polycrystalline Ba2FePnSe5 (Pn = Sb, Bi)

We report a new synthesis route towards polycrystalline Ba2FePnSe5 (Pn = Sb, Bi) chalcogenides based on ball milling and subsequent spark plasma sintering. The polycrystalline samples were characterized by transmission electron microscopy and powder X-ray diffraction. We discovered that both compounds undergo rapid, radiation induced phase changes between a crystalline and an amorphous phase. Thin layers of amorphous Ba2FePnSe5 (Pn = Sb, Bi) were obtained by applying 0.3 ms laser pulses (λ = 1064 nm) on the surface of polycrystalline samples. By using a series of subsequent laser pulses at room temperature the surface of Ba2FeBiSe5 can be reversibly changed between amorphous and crystalline states. The thermal conductivity can be used to follow the amorphization and recrystallization processes as a function of laser pulses. Resistivity measurements and calculations of the electronic structure revealed band gaps of 1.04 eV (Pn = Bi) and 1.14 eV (Pn = Sb). We found anomalies in the thermal conductivity of Ba2FePnSe5 (Pn = Sb, Bi) at the Neel temperature due to the antiferromagnetic ordering of the Fe spins. We also show that the distortion of the PnSe6 octahedra is caused by stereoactive 5s2 and 6s2 lone pairs of Sb and Bi.

Bidirectional current triggering in planar devices based on serially connected VO2 thin films using 965 nm laser diode.

By incorporating a 965 nm laser diode, the bidirectional current triggering of up to 30 mA was demonstrated in a two-terminal planar device based on serially connected vanadium dioxide (VO2) thin films grown by pulsed laser deposition. The bidirectional current triggering was realized by using the focused beams of laser pulses through the photo-thermally induced phase transition of VO2. The transient responses of laser-triggered currents were also investigated when laser pulses excited the device at a variety of pulse widths and repetition rates of up to 4.0 Hz. A switching contrast between off- and on-state currents was obtained as ~8333, and rising and falling times were measured as ~39 and ~29 ms, respectively, for 50 ms laser pulses.

Laser induced mortality of Anopheles stephensi mosquitoes.

Small, flying insects continue to pose great risks to both human health and agricultural production throughout the world, so there remains a compelling need to develop new vector and pest control approaches. Here, we examined the use of short (<25 ms) laser pulses to kill or disable anesthetized female Anopheles stephensi mosquitoes, which were chosen as a representative species. The mortality of mosquitoes exposed to laser pulses of various wavelength, power, pulse duration, and spot size combinations was assessed 24 hours after exposure. For otherwise comparable conditions, green and far-infrared wavelengths were found to be more effective than near- and mid-infrared wavelengths. Pulses with larger laser spot sizes required lower lethal energy densities, or fluence, but more pulse energy than for smaller spot sizes with greater fluence. Pulse duration had to be reduced by several orders of magnitude to significantly lower the lethal pulse energy or fluence required. These results identified the most promising candidates for the lethal laser component in a system being designed to identify, track, and shoot down flying insects in the wild.

Automated 3D bone ablation with 1,070 nm ytterbium-doped fiber laser enabled by inline coherent imaging.

Laser osteotomy bears well-identified advantages over conventional techniques. However, lack of depth control and collateral thermal damage are barriers to wide clinical implementation. Flexible fiber delivery and economical benefits of ytterbium-doped fiber lasers make them desirable for laser osteotomy. In this work, we demonstrate automated bone ablation with a 1,070 nm industrial-scale fiber laser to create 3D target structures with minimal thermal side-effects. Fresh and dry ex vivo cortical bone samples are ablated using 50-100 µs laser pulses of 15-30 mJ. In situ inline coherent imaging monitors ablation dynamics with micron precision and on microsecond timescales. Ablation depth is extracted by on-the-fly processing of ICI data, enabling feedback control of depth (via laser pulse number). Final ablated morphology, measured by an ex situ stylus profiler, is compared to the target shape. Histological examination is performed to quantify the thermal side-effects of laser ablation. Percussion drilled hole depth is highly variable for fixed laser parameters (880 ± 151 µm on fresh bone and 1038 ± 148 µm on dry bone) due to nondeterministic ablation. ICI-enabled depth control is implemented to achieve precise ablation of complex 3D features. The RMS deviation between target and ablated morphology is 12.6 µm. The heat-affected zone is found to be 5-10 µm on fresh and dry bone. An ytterbium-doped fiber laser is utilized for cortical bone ablation with limited thermal side-effects. In situ real-time ICI measurement enables characterization of bone ablation dynamics. Furthermore, ICI closed-loop feedback realizes depth-controlled ablation on heterogeneous bone. This proof-of-principle study shows great promise for ICI-guided laser osteotomy.

Using magnetic permeability bits to store information

Steps are described in the development of a new magnetic memory technology, based on states with different magnetic permeability, with the capability to reliably store large amounts of information in a high-density form for decades. The advantages of using the permeability to store information include an insensitivity to accidental exposure to magnetic fields or temperature changes, both of which are known to corrupt memory approaches that rely on remanent magnetization. The high permeability media investigated consists of either films of Metglas 2826 MB (Fe40Ni38Mo4B18) or bilayers of permalloy (Ni78Fe22)/Cu. Regions of films of the high permeability media were converted thermally to low permeability regions by laser or ohmic heating. The permeability of the bits was read by detecting changes of an external 32 Oe probe field using a magnetic tunnel junction 10 μm away from the media. Metglas bits were written with 100 μs laser pulses and arrays of 300 nm diameter bits were read. The high and low permeability bits written using bilayers of permalloy/Cu are not affected by 10 Mrad(Si) of gamma radiation from a 60Co source. An economical route for writing and reading bits as small at 20 nm using a variation of heat assisted magnetic recording is discussed.

20 mA bidirectional laser triggering in planar devices based on vanadium dioxide thin films using CO(2) laser.

By utilizing a CO2 laser centered at ~10.6 μm as an optical stimulus, we demonstrated bidirectional laser triggering in a two-terminal planar device based on a highly resistive vanadium dioxide (VO2) thin film. The break-over voltage of the VO2-based device was measured as large as ~294.8 V, which resulted from the high resistivity of insulating VO2 grains comprising the thin film and the large electrode separation of the device. The bidirectional current switching of up to 20 mA was achieved by harnessing the dramatic resistance variation of the device photo-thermally induced by the laser illumination. The transient responses of laser-triggered currents were also analyzed when laser pulses excited the device at a variety of pulse widths and repetition rates. In the transient responses, a maximum switching contrast between off- and on-state currents was measured as ~7067 with an off-state current of ~2.83 μA, and rising and falling times were measured as ~30 and ~16 ms, respectively, for 100 ms laser pulses.

Biocompatible Ferrofluids With Iron Oxide Nanoparticles Fabricated by Laser Target Evaporation

Iron oxide magnetic nanoparticles were obtained by a highly productive laser target evaporation technique while varying type and pressure of the working gas and laser pulse duration and average output power. Water-based suspensions stabilized with sodium citrate were prepared. Synthesis in air, 100 μs laser pulse duration, and 200 W average power resulted in disaggregated superparamagnetic nanoparticles with an average diameter of 9.9 nm.

Laser microwelding of thin Al layers for interconnection of crystalline Si solar cells: analysis of process limits for ns and μs lasers

We investigated a laser welding process for contacting aluminum-metallized crystalline silicon solar cells to a 10-μm-thick aluminum layer on a glass substrate. We analyzed the threshold for laser-induced damage in dependence on the solar cell metallization thickness by applying the process on SiNx passivated lifetime samples. In addition, we measured the mechanical failure stress of the laser welds by perpendicular tear-off. We applied two types of laser processes; one used single or multiple 20-ns-laser pulses at 355 nm with fluences between 12 and 40 J∕cm2, and the other used single 1.2-μs-laser pulses at 1064 nm with 33 to 73 J∕cm2. The ns-laser pulses were able to contact down to 1-μm-thick aluminum layers on silicon without inducing laser damage to the silicon and led to a sufficiently strong mechanical contact. In the case of μs-laser pulses, the limiting thickness was 2 μm. Both laser systems were compared and discussed with respect to the theory of laser-induced forward transfer. The application was shown by contacting solar cells with 2-μm Al metallization on the rear side using μs-laser pulses proving a damage-free contacting.