Single Nanosecond Laser Pulse Research Articles

Raman spectroscopy of thermo- and laser-induced transformations of gouache paint layer of copper phthalocyanine blue

Methods to control polymorphic modifications of phthalocyanines using optical (laser) radiation and possible photoinduced transformations of polymorphs are of practical interest in problems of identification and attribution of paintings, laser (micro)sampling, and the development of phthalocyanine structures for technical applications in optics, optoelectronics, and medicine. In this work, we compare the thermal and laser-induced changes of a gouache paint layer based on copper phthalocyanine (CuPc) PB15. The thermally induced color changes of the paint layer are quantified using the CIE Lab D65/10 color space. (Nano)rods formed in the paint layer when the sample is heated to 450°C at normal pressure without humidity control are studied using absorption spectroscopy, Raman microspectroscopy, and scanning electron microscopy. It is shown that the formation of (nano)rods is related to the α→β polymorph transition of CuPc. Low-frequency markers of the CuPc β-polymorph are revealed in the Raman spectra. For the sample containing (nano)rods, the a* color coordinate substantially increases (by about 30 units), whereas the L* and b* coordinates remain almost unchanged. Irradiation with a single nanosecond laser pulse at a wavelength of 532 nm leads to the laser ablation of the paint layer at fluences exceeding a threshold level of about 3 J/cm2. Irradiation at fluences of greater than 0.5 J/cm2, but lower than the ablation threshold leads to color change of the paint layer due to the α→ε transition of CuPc. Similar transformations are observed at the periphery of and inside ablation crater.

Extracellular ATP release predominantly mediates Ca2+ communication locally in highly organised, stellate-Like patterned networks of adult human astrocytes.

The 'Astrocyte Network' and the understanding of its communication has been posed as a new grand challenge to be investigated by contemporary science. However, communication studies in astrocyte networks have investigated traditional petri-dish in vitro culture models where cells are closely packed and can deviate from the stellate form observed in the brain. Using novel cell patterning approaches, highly organised, regular grid networks of astrocytes on chip, to single-cell fidelity are constructed, permitting a stellate-like in vitro network model to be realised. By stimulating the central cell with a single UV nanosecond laser pulse, the initiation/propagation pathways of stellate-like networks are re-explored. The authors investigate the mechanisms of intercellular Ca2+ communication and discover that stellate-like networks of adult human astrocytes in vitro actually exploit extracellular ATP release as their dominant propagation pathway to cells in the network locally; being observed even down to the nearest neighbour and next nearest neighbouring cells-contrary to the reported gap junction. This discovery has significant ramifications to many neurological conditions such as epilepsy, stroke and aggressive astrocytomas where gap junctions can be targeted. In cases where such gap junction targeting has failed, this new finding suggests that these conditions should be re-visited and the ATP transmission pathway targeted instead.

Laser ablation microsampling of copper phthalocyanine blue by the second-harmonic radiation of Nd:YAG laser

An original nonlinear optical method employing laser ablation with single nanosecond laser pulses at a wavelength of λ = 532 nm is used to study organic paint layer (copper phthalocyanine blue (PB15)). Two processes (photochemical reaction and formation of craters), which appear to be nonlinear result from the pulsed radiation. It is shown that the nonlinear optical procedure (laser ablation microsampling (LAMS)) can be used to obtain microparticles of PB15 sufficient for the further analysis. The Raman spectra of the surface of the paint layer prior to and after LAMS and microparticles are obtained. Spectral data, including those in the low-frequency region, are discussed.

Diffusion dynamics and characterization of attogram masses in optically trapped single nanoparticles using laser-induced plasma imaging

In the present work, a wavelength-selected plasma imaging analysis system is presented and used to track photons emitted from single-trapped nanoparticles in air at atmospheric pressure. The isolated nanoentities were atomized and excited into plasma state using single nanosecond laser pulses. The use of appropriate wavelength filters alongside time-optimized acquisition settings enabled the detection of molecular and atomic emissions in the plasma. The photon detection efficiency of the imaging line resulted in a signal > 400 times larger than the simultaneously-acquired dispersive spectroscopy data. The increase in sensitivity outlined the evolution of diverse physicochemical processes at the single particle scale which included heat and momentum transfer from the plasma into the particle as wells as chemical reactions. The imaging detection of excited fragments evidenced different diffusion kinetics and time frames for atoms and molecules and their influence upon both the spectroscopic emission readout and fabrication processes using the plasma as a reactor. Moreover, the origin of molecular species, whether naturally-occurring or derived from a chemical reaction in the plasma, could also be studied on the basis of compositional gradients found on the images. Limits of detection for the inspected species ranged from tens to hundreds attograms, thus leading to an exceptional sensing principle for single nanoentities that may impact several areas of science and technology.

Single-pulse real-time billion-frames-per-second planar imaging of ultrafast nanoparticle-laser dynamics and temperature in flames

Unburnt hydrocarbon flames produce soot, which is the second biggest contributor to global warming and harmful to human health. The state-of-the-art high-speed imaging techniques, developed to study non-repeatable turbulent flames, are limited to million-frames-per-second imaging rates, falling short in capturing the dynamics of critical species. Unfortunately, these techniques do not provide a complete picture of flame-laser interactions, important for understanding soot formation. Furthermore, thermal effects induced by multiple consecutive pulses modify the optical properties of soot nanoparticles, thus making single-pulse imaging essential. Here, we report single-shot laser-sheet compressed ultrafast photography (LS-CUP) for billion-frames-per-second planar imaging of flame-laser dynamics. We observed laser-induced incandescence, elastic light scattering, and fluorescence of soot precursors - polycyclic aromatic hydrocarbons (PAHs) in real-time using a single nanosecond laser pulse. The spatiotemporal maps of the PAHs emission, soot temperature, primary nanoparticle size, soot aggregate size, and the number of monomers, present strong experimental evidence in support of the theory and modeling of soot inception and growth mechanism in flames. LS-CUP represents a generic and indispensable tool that combines a portfolio of ultrafast combustion diagnostic techniques, covering the entire lifecycle of soot nanoparticles, for probing extremely short-lived (picoseconds to nanoseconds) species in the spatiotemporal domain in non-repeatable turbulent environments. Finally, LS-CUP’s unparalleled capability of ultrafast wide-field temperature imaging in real-time is envisioned to unravel mysteries in modern physics such as hot plasma, sonoluminescence, and nuclear fusion.

Nonvolatile reconfigurable dynamic Janus metasurfaces in the terahertz regime

Metasurfaces, especially tunable ones, have played a major role in controlling the amplitude, phase, and polarization of electromagnetic waves and attracted growing interest, with a view toward a new generation of miniaturized devices. However, to date, most existing reconfigurable devices are bounded in volatile nature with sustained external energy to maintain and single functionality, which restrict their further applications. Here, we demonstrate for the first time, to our knowledge, nonvolatile, reconfigurable, and dynamic Janus metasurfaces by incorporating phase-change material Ge 2 Se 2 Te 5 (GST) in the terahertz (THz) regime. First, we experimentally show the reversible switching characteristic of GST on large areas by applying a single nanosecond laser pulse, which exhibits excellent contrast of THz properties in both states. Then, we present a multiplex metasurface scheme. In each metasurface, three sets of structures are adopted, in which two sets integrate GST. The effective structures can be reversely modulated by the amorphization and crystallization of GST. As a proof of concept, the dynamic beam splitter, bifocal metalens, dual-mode focusing optical vortex generators, and switchable metalens/focusing optical vortex generators are designed, fabricated, and experimentally characterized, and can be switched reversibly and repeatedly with the help of optical and thermal stimuli. Our scheme will pave the way toward the development of multifunctional and compact THz devices and may find use for applications in THz imaging, sensing, and communications.

Rayleigh wave and super-shear evanescent wave excited by laser-induced shock at a soft solid–liquid interface observed by photoelasticity imaging technique

We investigated laser-induced shock excitation of elastic surface waves at a free surface and a soft solid–liquid interface using a custom-designed photoelasticity imaging technique. Epoxy-resin and pure water were selected as the solid and liquid media. The elastic surface waves were excited via a shock process induced by focusing a single nanosecond laser pulse on the solid surface. To confirm the experimental observations, the roots of the Rayleigh and Stoneley equations were calculated. For a free surface, we present an entire-field observation of elastic surface waves, which includes a super-shear evanescent wave (SEW) that propagates faster than the shear wave but slower than the longitudinal wave. For a soft solid–liquid interface, we demonstrate the presence of a non-leaky Rayleigh wave that corresponds to a real root of the Stoneley equation. We also evidence the existence of a SEW that propagates 1.7 times faster than the shear speed in the solid and corresponds to a complex conjugate root of the Stoneley equation. These results correct the previously accepted notion that the Scholte wave is the only surface wave that can be generated at a soft solid–liquid interface.

Single-Pulsed SERS with Density-Based Clustering Analysis.

We developed a new method for obtaining surface-enhanced Raman scattering (SERS) spectra with extremely high sensitivity and spectral resolution. In this method, thousands of SERS spectra are acquired, followed by a data selection procedure based on density-based spatial clustering of applications with noise (DBSCAN). Each spectrum is recorded by exposure to a single nanosecond laser pulse to avoid the effect of time averaging. The reconstructed spectrum consists of the data that belong to the clusters. The method was applied to a crystal violet aqueous solution with a concentration of 10-7 mol/L. The results suggest that several minor Raman peaks were successfully recovered, which cannot be detected in conventional SERS measurements. Moreover, the method is also effective for separately observing Raman peaks that overlap with other neighboring peaks. This method extends the possibilities of SERS and will contribute to future high-resolution spectroscopy in condensed phases.

Low-cost camera-based pulsed and modulated continuous wave laser detection for laser-based optical communications

A novel, low-cost, camera-based method of detecting a single nanosecond (ns) laser pulse and kHz modulated continuous wave and pulsed lasers has been developed. The detector uses a simple optical modification to a standard rolling shutter color camera combined with image processing techniques to distinguish lasers from other illumination sources and extract a laser’s wavelength and pulse repetition frequency. In addition, the detector is also capable of detecting a single ns laser pulse at any given time. Such a detector has applications in free-space optical communications (FSO comms) as a low-cost broadband method of extracting information from multiple sources and as a detector of laser range finders. A low-cost prototype (≈£600) has been developed using entirely off-the-shelf components and assessed in laboratory and field trial conditions, with the ability to measure laser wavelengths to ±5 nm and pulse repetition frequencies to within ±5 % at a distance of 660 m. In the laboratory, the prototype was also able to detect each of the 1000 pulses generated by a 10 Hz 10 ns 532-nm pulsed laser, as well as 100 pulses sent at random intervals, highlighting its capability to detect a ns pulse at any given time. This novel technology offers a low-cost method of detecting lasers and extracting their pulse repetition frequencies, with a wide field of view and high spatial resolution. Ultimately, this technology has applications in FSO comms for between vehicles or platforms of interest, with a capability of communicating with multiple targets simultaneously.

Wavelength-specific optoacoustic-induced vibrations of the guinea pig tympanic membrane.

.Significance: Optoacoustic-induced vibrations of the hearing organ can potentially be used for a hearing device. To increase the efficiency of such a hearing device, the conversion of the light energy into vibration energy within each type of irradiated tissue has to be optimized.Aim: To analyze the wavelength-dependency of optoacoustic-induced vibrations within the tympanic membrane (TM), and to define the most efficient and best-suited optical stimulation parameters for a novel auditory prosthesis.Approach: Single nanosecond laser pulses, continuously tunable in a range of visible to near-infrared, were used to excite the guinea pig TM. The induced vibrations of the hearing organ were recorded at the malleus using a laser Doppler vibrometer.Results: Our results indicate a strong wavelength-dependency of the vibration’s amplitude correlating with the superposition of the absorption spectra of the different specific tissue components.Conclusions: We investigated the spectrum of the vibrations of the hearing organ that were induced optoacoustically within a biological membrane embedded in air, in an animal model. First applications for these results can be envisioned for the optical stimulation of the peripheral hearing organ as well as for research purposes.

Displacement of energy deposition during formation of nanosecond laser plasmas by self-defocusing

The long-term hydrodynamics of laser generated plasma regions are dictated by the spatial distribution of the deposited laser energy. Using an integrated chemical-optical solver to track energy deposition from single and dual nanosecond laser pulses, we show that self-defocusing results in a biased growth of the plasma region towards the focusing lens. This displacement of energy deposition scales with the Rayleigh range and can be reduced by maximizing the beam divergence.

Expansion dynamics and radiation absorption in plumes induced by irradiation of a copper target by single and multiple nanosecond laser pulses in the doughnut beam mode

The two-dimensional kinetic simulations of the plume expansion process induced by irradiation of a copper target in argon background gas with nanosecond laser pulses in the doughnut beam mode are performed. The target is irradiated by single and double laser pulses, as well as by bursts consisting of up to 35 individual pulses. The simulations are based on a hybrid computational model that includes the direct simulation Monte Carlo method, where the ionization and absorption of laser radiation are considered. The numerical results are systematically compared with the results obtained for a Gaussian beam at the same pulse energy, peak fluence, and peak intensity. The structure of the plumes induced by a doughnut beam is found to drastically differ from the structure of the plumes induced by a Gaussian beam. The doughnut beam initially induces a toroidal shock wave. The self-interaction of this shock at the axis of symmetry results in the formation of multiple internal shock waves that strongly increase the plume temperature and density. The interaction between plumes induced by a double pulse in the doughnut beam mode occurs in two distinct regimes depending on the beam size and inter-pulse separation time. At a relatively small peak laser fluence, the effect of plasma shielding in the plumes induced by a burst of pulses in the doughnut beam mode is found to be smaller compared to the case of a Gaussian beam. At a relatively large fluence, the simulations reveal the opposite trend, but the amount of vaporized material is found to be always larger for a doughnut beam than for a Gaussian one. The obtained results suggest that the absorption of laser radiation in plasma plumes induced by nanosecond laser pulses can be tuned by changing the radial distribution of energy in the incident laser beam.

Self-aligned local contact opening and n+ diffusion by single-step laser doping from POx/Al2O3 passivation stacks

Laser doping is a promising route to realise industrially compatible processing of local contacts for high-efficiency solar cells, especially when the same film acts as both dopant source and passivation layer. In this work we demonstrate simultaneous local contact opening and n+ laser doping of silicon from positively charged POx/Al2O3 thin-film stacks, which also provide outstanding passivation of n-type silicon surfaces. Local n+ doped regions with sheet resistance ranging from 35 to ~540 Ω/□ are formed using single nanosecond laser pulses with varying fluence. ECV profiling shows net n-type doping in all cases, confirmed by SIMS profiling to be due to phosphorus from the POx layer. J0 of metallised laser-doped regions is consistent with values achieved for state-of-the-art furnace diffusions with similar sheet resistance, confirming that laser-induced recombination-active defects are avoided. A minimum J0 of 540 fA cm−2 is obtained for metallised laser-doped regions formed from POx/Al2O3 passivation stacks having J0 of 2.5 fA cm−2. The combination of outstanding passivation of uncontacted n-type regions offered by POx/Al2O3, with self-aligned formation of locally-diffused contact openings via single-step laser processing, opens up exciting possibilities for simplified fabrication of high-efficiency cell structures.

Prediction of water droplet behavior on aluminum alloy surfaces modified by nanosecond laser pulses

Metal processing by laser radiation is shown to be a promising way to obtain surfaces with controllable wetting and evaporation characteristics. Such surfaces can be used in designing the modern cooling systems based on phase transitions. However, large-scale applications of metal processing by laser radiation are hampered by the lack of a theory that allows predicting the wetting and evaporation of liquid droplets on modified surfaces. In this work, we study a water droplet behavior during its wetting and evaporation on hydrophilic and hydrophobic aluminum alloy surfaces modified by single nanosecond laser pulses. To predict the wettability of the laser-processed surfaces, we proposed to use the empirical correlation of the contact angle on the surface free energy obtained in terms of dynamic contact angles. We analyzed the applicability of two well-known theoretical models to determine the evaporation rates of droplets on hydrophilic and hydrophobic laser-processed surfaces. The Spalding and diffusive evaporation models can be used for water droplet evaporating from the hydrophilic aluminum alloy surface at a temperature difference between the surface and air in the chamber of 0–6.5 K. At higher temperature difference from 6.5 K to 9 K, only the Spalding model is applicable. In the case of the hydrophobic laser-processed aluminum alloy surface, the mass loss rate of evaporating droplet at the temperature difference range of 0–9 K can be predicted by the diffusive model. In addition, this model was found to be appropriate for predicting the geometric dimensions of a droplet evaporating in a pinned contact line mode on hydrophilic surfaces without heating at any stage of its evaporation. The results of this study can be used to predict the wetting and evaporation characteristics of water droplets on aluminum alloy surfaces modified by single nanosecond laser pulses.

Reversible optical tuning of GeSbTe phase-change metasurface spectral filters for mid-wave infrared imaging.

Tunable narrowband spectral filtering across arbitrary optical wavebands is highly desirable in a plethora of applications, from chemical sensing and hyperspectral imaging to infrared astronomy. Yet, the ability to reconfigure the optical properties, with full reversibility, of a solid-state large-area narrowband filter remains elusive. Existing solutions require either moving parts, have slow response times, or provide limited spectral coverage. Here, we demonstrate a 1-inch diameter continuously tunable, fully reversible, all-solid-state, narrowband phase-change metasurface filter based on a GeSbTe-225 (GST)-embedded plasmonic nanohole array. The passband of the presented device is with transmittance and operates across the 3–5 µm thermal imaging waveband. Continuous, reconfigurable tuning is achieved by exploiting intermediate GST phases via optical switching with a single nanosecond laser pulse, and material stability is verified through multiple switching cycles. We further demonstrate multispectral thermal imaging in the mid-wave infrared using our active phase-change metasurfaces. Our results pave the way for highly functional, reduced power, compact hyperspectral imaging systems and customizable optical filters for real-world system integration.

Excitation probability of the hyperfine ground state of Doppler-broadened muonic hydrogen through dipole-forbidden transition

We study the magnetic dipole induced excitation probability of the hyperfine ground state of Doppler-broadened muonic hydrogen (pμ−) by a single nanosecond laser pulse with pulse durations of 20, 2, and 0.2 ns pulses with a Gaussian temporal shape such that the pulse bandwidth is, respectively, narrower than, comparable to, and broader than the Doppler width at 300 K. We numerically solve a set of density matrix equations to obtain the excitation probability at the various laser intensities and detunings. For the resonant 20 ns pulse, the excitation probability increases very slowly as the laser intensity increases. The width of the excitation probability lineshape mainly comes from the Doppler broadening and hardly increases with intensity. For the resonant 0.2 ns pulse a complete population inversion occurs at a certain laser intensity, and the excitation probability oscillates between 0 and unity as the intensity further increases. Owing to the inherent pulse bandwidth the width of the excitation probability lineshape is very broad even at the low intensity, and it increases further as the intensity increases. For the resonant 2 ns pulse at a moderate intensity (∼1011 W/cm2) the excitation probability is higher than those by the 20 and 0.2 ns pulses with a relatively narrow width. Our study can serve as a guideline for the development of the relevant laser source, depending on the purpose to excite muonic hydrogen, i.e., precise measurement of the ground state hyperfine splitting of muonic hydrogen or production of spin-polarized muonic hydrogen.

Modeling of silicon surface topographies induced by single nanosecond laser pulse induced melt-flows

Irradiation with a single nanosecond laser pulse in the melting regime can result in a characteristic change in the surface morphology of crystalline silicon. This has been verified experimentally in a variety of situations, where dimple-shaped surface topographies are produced. In this work, the dimple height, depth, and width are modeled following and extending in a more rigorous manner the approach of Wood and Giles [Phys. Rev. B 23, 2923–2942 (1981)] and that of Schwarz-Selinger and coworkers [Phys. Rev. B 64, 155323 (2001)], upon varying the laser irradiation parameters such as peak energy density, pulse duration, and wavelength. This is achieved with numerical simulations of one-dimensional heat flow as input to the analytical fluid-flow equations.

Use of colloidal monolayers of glass spheres for the improvement of the optoacoustic ultrasound generation

New type of optoacoustic transducer containing monolayer of dielectric spheres is presented. Experiments of the ultrasound generation by the single nanosecond laser pulses (2nd harmonic of Nd:YAG) in a liquid absorbing ink were carried out. It was found that the irradiation of the liquid through a mask containing the monolayer of glass spheres result in the greater ultrasound intensity and the broader spectrum compared to the same setup without the mask. The effect is explained by the focusing of the laser light by the spheres. The laser intensity is being concentrated into the multitude of the ‘hot spots’ where the high-frequency ultrasound is generated.

Photoacoustic Resonance Imaging

In this paper, a new imaging modality, named photoacoustic resonance imaging (PARI), is proposed and experimentally demonstrated. Being distinct from conventional single nanosecond laser pulse induced wideband PA signal, the proposed PARI method utilizes multiburst modulated laser source to induce PA resonant signal with enhanced signal strength and narrower bandwidth. Moreover, imaging contrast could be clearly improved than conventional single-pulse laser based PA imaging by selecting optimum modulation frequency of the laser source, which originates from physical properties of different materials beyond the optical absorption coefficient. Experimental validation on phantoms and ex vivo tissues have been performed to show the merits of the proposed PARI method with much improved image contrast.

Generation of laser-induced periodic surface structures (LIPSS) in fused silica by single NIR nanosecond laser pulse irradiation in confinement

High-quality submicron pattern formation in dielectrics by lasers is challenging, in particular with regard to large areas. Self-assembly and self-organization mechanisms, which arise from the interaction of the laser photons with the material of the sample, can provide conditions for efficient micron patterning of large surface areas. This study demonstrates the formation of submicron ripples in fused silica (SiO2) by nanosecond laser irradiation, which extends the category of lasers capable of the controllable manufacturing of laser-induced surface structures (LIPSS). A newly designed layer system was deposited on the surface of samples, comprising, on top of the dielectric substrate, an interlayer, an absorber, and a confinement layer. The samples were irradiated though the dielectric substrate with single nanosecond (λ = 1064) nm laser pulses. It was found that the period of the LIPSS can be tuned by changing the nanosecond laser energy, the confinement strength, and the laser pulse length from 5 ns to 25 ns. LIPSS with periods in the range of 400–600 nm, with a depth of up to 140 nm and with an orientation perpendicular to the polarization direction of the laser light, were found experimentally. The design of the sample finally enables an intra-pulse modification of the dielectric surface that provides temporal conditions near the dielectric substrate surface for surface plasmon polaritron (SPP) generation. The proposed mechanism of LIPSS formation thus comprises the ablation of the absorber layer and a plasma formation confined between the substrate surface and the confinement layer. This process causes a modification of the substrate surface. In consequence, temporary SPP generation occurs at the modified SiO2 surface, resulting in a lateral intensity modulation of the nanosecond laser beam within the pulse duration. This lateral intensity modulation of the laser pulse causes localized variation of the ablation, which finally results in the LIPSS found.The experimentally observed dependencies of the LIPSS period on the pulse energy, the confinement layer thickness, and the pulse length can be explained with the proposed model. The findings of this work provide a method of enabling local adjustment of the LIPSS period, which can be used in applications such as writing hierarchical structures of variable sizes.