UV Laser Research Articles

Dense plasma diagnostics with a Nomarski interferometer using a frequency-tripled Ti:sapphire laser

An overdense plasma is considered as a new tool for high-power laser pulse compression. For this purpose, a high density plasma was generated by illuminating an intense fs Ti:sapphire laser pulse onto a surface of an aluminum disk and its spatio-time-resolved dynamics was investigated by a Nomarski interferometer. To probe the laser-produced high density plasma, the original Ti:sapphire laser pulse with a wavelength of 800 nm was frequency-tripled and the UV laser pulse of 266 nm in wavelength was used in the interferometer. It turned out that the fs UV laser interferometer can provide a fairly high density information up to 8.5 × 1020 cm-3. Our experimental result shows that the initial high density plasma near the solid target surface expands rapidly and its density profile in space is gradually reduced to exponential in the ns time scale. This result will provide an important information towards the next step for experimental demonstration of the laser pulse compression using a plasma.

Research and integrate laser marking system on plastic materials using 355 nm UV laser

This article presents the results of research, design and integration of a laser equipment system on plastic materials using a UV laser of wavelength 355 nm, power of 5 W, integrated from semi-finished modules available on the market. The research team investigated the influence of technical parameters on the UV laser engraving process on plastic materials such as wavelength, output laser power, pulse width, engraving speed, engraving area size, electricity source... As a result, the research team has come up with a set of parameters for the manufacturing equipment system for a number of specific plastic materials. Some experimental UV laser engraving products on plastic materials of the manufactured equipment system show images with high sharpness and contrast, small heat-affected areas, can engrave sizes as small as micrometers, with a high speed. Research results showed that UV laser equipment is a good choice for laser applications in the semiconductor industry, micro-mechanics, medicine and peripheral equipment...

Electro-optic solution to overcome the transient regime of a cavity dumped UV laser source?

Laser marking is one of the main industrial applications of lasers. To mark many materials or to obtain patterns of small dimensions, triggered pulsed solid laser sources are increasingly used emitting in the fundamental in the infrared and to which two nonlinear (NL) stages are added to obtain an emission in the UV. The conversion efficiency as well as that of the marking then depends mainly on the peak power of the UV pulses generated. One of the solutions for obtaining a higher peak power is to reduce the duration of the laser pulses. To do this, it is possible to change the architecture of the laser cavity by switching from a Q-Switch operating mode (QS) to a cavity dumped mode (CD). With this architecture it is thus possible to reduce the duration of the pulses by an order of magnitude. This peak power gain is unfortunately achieved at the cost of a very long transient regime, up to a few tens of milliseconds, compared to the classic QS which generally only occurs for the first laser pulse. Unfortunately, laser marking is by nature based on chopped operation for the laser, which corresponds to working permanently in a transient state. Marking with a CD architecture laser is then often full of defects and therefore unacceptable in terms of quality. After a study of the problem specific to the use of a UV source with CD architecture in the context of marking, we propose an original solution to dissociate the chopped regime, specific to marking, from the transient regime of the laser. This solution is based on the use of an RTP (Rubidium Titanyle Phosphate RTiOPO4) electro-optical crystal placed between the laser output and the NL stages. This crystal is driven like a half-wave plate by high voltage square pulses of a duration corresponding to the duration of the marking laser pulse trains. This thus makes it possible to obtain laser operation in a stabilized regime, therefore outside the transient regime, and simultaneously chopped operation characteristic of laser marking. The new operating mode obtained is characterized in detail with the new marking performances which prove to be optimal and compatible with the requirements of a quality marking. In addition to making the CD architecture compatible with laser marking, this solution also makes it possible to adjust the average power, and therefore the energy of the laser pulses to adapt it according to the materials and this without changing the thermal behavior of the laser. Last advantage, by adjusting the extinction voltage applied to the RTP outside the cavity, it is also possible to keep a few milliwatts of the marking laser beam as the aiming beam, with the certainty of perfect alignment with the full power beam.

A TDLAS-based photofragmentation method for spatially resolved measurement of KOH and KCl as well as its application in biomass combustion processes

The release of gas-phase potassium species, mainly KOH, KCl, and K atoms, from burning biomass fuels can introduce severe problems to boilers, such as fouling, slagging, and corrosion. In the present work, an optical technique combining laser-induced photofragmentation and tunable diode laser absorption spectroscopy is developed for simultaneously measuring the concentration of KOH, KCl and K atoms with a high temporal and spatial resolution. Two laser sheets with a thickness of about 1 mm at wavelengths of 266 and 355 nm, respectively, were adopted to photodissociate KOH and KCl molecules into K atoms. A continuous wave laser at 766 nm generated by a tunable diode laser passed perpendicularly through the laser sheet to detect the K atom. The measured fragmentation-induced K-atom absorbance was correlated to KOH and KCl concertation through a calibration process in a homogenous combustion environment where the concentrations of KOH and KCl were monitored by UV absorption spectroscopy. The calibration curves were verified to be independent of temperature. A typical spatial resolution of 1 mm3 was realized where the value depended on the overlap volume of the UV laser sheet and the 766 nm laser beam. Finally, this technique was applied to measure the release behavior of KOH, KCl and K atoms from burning wood and straw pellets.

A cell-free multi-enzyme cascade for the synthesis of CDP-glycerol.

CDP-glycerol is a nucleotide-diphosphate-activated version of glycerol. In nature, it is required for the biosynthesis of teichoic acid in Gram-positive bacteria, which is an appealing target epitope for the development of new vaccines. Here, a cell-free multi-enzyme cascade was developed to synthetize nucleotide-activated glycerol from the inexpensive and readily available substrates cytidine and glycerol. The cascade comprises five recombinant enzymes expressed in E. coli that were purified by immobilized metal affinity chromatography. As part of the cascade, ATP is in-situ regenerated from polyphosphate to reduce synthesis costs. The enzymatic cascade was characterized at laboratory scale and the products analyzed by high performance anion exchange chromatography (HPAEC)-UV and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). After the successful synthesis was confirmed, a design of experiments approach was used to screen for optimal operation conditions (temperature, pH value and MgCl2 concentration). Overall, a substrate conversion of 89% was achieved with respect to the substrate cytidine.

UV laser formation of complex topologies for sensitive elements of navigation sensors based on surface acoustic waves

A method utilizing a UV laser to form complex electrode structures on the surface of the sensitive element prototype for a solid-state gyroscope (SSG) based on surface acoustic waves (SAW) was tested. The prospects of using the method are demonstrated in terms of the flexibility of changing the topology, the speed of navigation sensors prototypes manufacturing, and reducing the cost. Methods for forming surface structures on piezoelectric materials using an IR and UV laser are compared. The parameters of laser formation of a surface pattern for configuring the topology of a sensitive element by evaporation of a thin-film coating 5000 nm and 200 nm thick deposited on lithium niobate substrates are determined. The amplitude-frequency characteristics of the manufactured samples of SAW-based SSG sensing elements are obtained. Additionally a finite-element modeling was performed to obtain theoretical prediction of dependencies of the sensing element characteristics on the thickness of the metallization forming the electrodes. It was found that when using a fiber UV laser with a wavelength of 355 nm, it is necessary to impose more stringent requirements on the quality of metallization and on the calculation of laser radiation regimes for processing coatings with a thickness of 200 nm. It is shown that when using a UV laser, it is possible to manufacture more compact sensitive elements of navigation sensors on surface acoustic waves than when using IR lasers.

Analysis of polyisoprene oligomers via in situ silver nanoparticle formation on thin-layer chromatography plate using matrix-assisted laser-induced desorption/ionization mass spectrometry.

The direct mass spectrometry (MS) detection of polyisoprene (PI) oligomers on a thin-layer chromatography (TLC) plate using matrix-assisted laser-induced desorption/ionization (MALDI) with silver trifluoroacetate as the cationization reagent was investigated. The spots of PI oligomers and silver trifluoroacetate on the TLC plate resulted in brown materials after UV laser irradiation. It was suggested that silver trifluoroacetate yielded Ag nanoparticles as brown materials after heating via laser irradiation. The nanoparticles behaved as an inorganic matrix and a source of Ag+ adduct in the analysis of PI oligomers. The use of organic matrices together with silver trifluoroacetate reduced the signal intensity of PI oligomers on MALDI-MS on a TLC plate. The separation of PI oligomers (polymerization degree, n = 5-11) by TLC resulted in a single elliptical spot without a clear separation after the chromatographic procedure. However, in MS imaging, differences in migration lengths based on degrees of polymerization were clearly observed and the degrees of polymerization were identified without comparison with standards.

Mass-selected ion-molecule cluster beam apparatus for ultrafast photofragmentation studies.

We describe an apparatus for investigating the excited-state dissociation dynamics of mass-selected ion-molecule clusters by mass-resolving and detecting photofragment-ions and neutrals, in coincidence, using an ultrafast laser operating at high repetition rates. The apparatus comprises a source that generates ion-molecule clusters, a time-of-flight spectrometer, and a mass filter that selects the desired anions, and a linear-plus-quadratic reflectron mass spectrometer that discriminates the fragment anions after the femtosecond laser excites the clusters. The fragment neutrals and anions are then captured by two channeltron detectors. The apparatus performance is tested by measuring the photofragments: I-, CF3I-, and neutrals from photoexcitation of the ion-molecule cluster CF3I·I- using femtosecond UV laser pulses with a wavelength of 266nm. The experimental results are compared with our ground state and excited state electronic structure calculations as well as the existing results and calculations, with particular attention to the generation mechanism of the anion fragments and dissociation channels of the ion-molecule cluster CF3I·I- in the charge-transfer excited state.

Student Competition (Technology Innovation) ID 1984861

Backgrounds/Objective Intraspinal microstimulation (ISMS) is a neuromodulation technique for restoring walking after spinal cord injury. The objective of this study was to fabricate a stretchable ISMS device suitable for pigs, a clinically-relevant animal model. Methods Polyimide-insulated microwires (50µm, Pt-Ir, 80%/20%) were used for fabrication of electrodes. Their tips were de-insulated (∼0.15mm2) and sharpened using nanosecond and femtosecond UV lasers. Microcoils were fabricated from 25μm microwires (Pt-Ir, 80%/20%) to add stretchability to the lead wires. Sixteen microelectrode-leads were connected to a custom, wirelessly controlled stimulator using Medtronic extension cables (Model 37081). The implants were tested in seven domestic pigs and current pulse trains were delivered to various rostro-caudal regions of the lumbar spinal cord (1s, 40Hz, 50µA-300µA) to activate locomotor-related muscle synergies. The kinematics and isometric joint forces of the evoked hindlimb responses were recorded. Results Graded joint movements were evoked with increasing stimulus amplitude. Changes in the hip, knee, and ankle joints angles evoked by ISMS at 300µA were 17.9±1˚, 28.1±1˚, and 21.6±2˚, respectively. Isometric joint forces evoked by ISMS at 300µA were 12.21±0.91N, 7.4±0.71N, and 1.7±0.15N for knee extension, hip flexion, and ankle flexion, respectively. Conclusion The movements evoked using the developed ISMS implant could generate full ranges of motion in the joints. The graded responses imply a near-physiological recruitment order of motoneurons, which is necessary for achieving long walking distances without muscle fatigue. The results show the capability of the developed ISMS device in generating movements in pigs, and the implants’ potential for future use in humans.

Enhanced Photoluminescence and Prolonged Carrier Lifetime through Laser Radiation Hardening and Self-Healing in Aged MAPbBr3 Perovskites Encapsulated in NiO Nanotubes.

Organic-inorganic perovskites hold great promise as optoelectronic semiconductors for pure color light emitting and photovoltaic devices. However, challenges persist regarding their photostability and chemical stability, which limit their extensive applications. This paper investigates the laser radiation hardening and self-healing-induced properties of aged MAPbBr3 perovskites encapsulated in NiO nanotubes (MAPbBr3@NiO) using photoluminescence (PL) and fluorescence lifetime imaging (FLIM). After deliberately subjecting the MAPbBr3@ NiO to atmospheric conditions for two years, the sample remains remarkably stable. It exhibits no changes in PL wavelength during UV laser irradiation and self-healing. Furthermore, exposure to UV light at 375 nm enhances the PL of the self-healed MAPbBr3@NiO. FLIM analysis sheds light on the mechanism behind photodegradation, self-healing, and PL enhancement. The results indicate the involvement of many carrier-trapping states with low lifetime events and an increase in peak lifetime after self-healing. The formation of trapping states at the perovskite/nanotube interface is discussed and tested. This study provides new insights into the dynamics of photo-carriers during photodegradation and self-healing in organic-inorganic perovskites.

Dimethicone-assisted interlaced scanning compound cutting for regulating thermal damage in CFRP laser cutting by synergistic suppression of coupled heat effect

Carbon fiber reinforced polymer (CFRP) is prone to produce significant thermal damage such as heat-affected zone (HAZ) during laser cutting, which seriously affects the processing quality. In this paper, a new method of dimethicone-assisted interlaced scanning mode (DISM) compound cutting is proposed to regulate the surface damage of CFRP nanosecond UV laser hole cutting. Based on the simulation and experimental testing analysis, the coupling effect of heat conduction and heat accumulation during the laser cutting process, as well as the suppression mechanism of heat effects by compound cutting methods have been described. Compared to the default cutting method, DISM can effectively reduce the damage of CFRP laser hole cutting. The average matrix recession (MR) width of the hole entrance edge is reduced by 47.54% for DISM, while eliminating the heat-damaged matrix (HDM) zone, resulting in a comprehensive reduction of HAZ by 85.2%. The most obvious parameter combination for the reduction of MR width is 50 kHz, 1000 mm/s, and 0.06 mm, where the MR is reduced by 61.31% (37.61 μm→14.55 μm). Interlaced scanning mainly reduces MR width, while dimethicone-assisted processing exhibits suppression of HDM. The compound cutting method significantly reduces thermal damage and improves CFRP mechanical properties.

Laser damage properties of LiB3O5 crystal surface under UV laser irradiation.

LiB3O5 (LBO) crystal has a very high bulk laser damage threshold. Laser damage often occurs on the surfaces with a large number of processing defects during application. In this paper, the surface laser damage threshold, damage growth threshold, and damage growth curve of LBO crystal and fused silica under the same processing process have been comparatively studied by using a 355 nm pulsed laser. The surface laser damage performance of LBO crystal has been comprehensive evaluated. The results show that the laser damage threshold and damage growth threshold of LBO are about twice that of fused silica, and the damage growth coefficient is about 0.7 times that of fused silica. The detection and analysis of impurity defects and photothermal weak absorption defects show that the subsurface defects of LBO crystal are less than that of fused silica. Laser damage morphologies show that the damage process is related to strongly bonded chemical structure and anisotropic physical characteristics of LBO crystal. These characteristics together determine the high threshold damage performance of LBO crystal. The results of this study are of great guidance for the application of LBO crystal in high-power laser systems.

(Invited) The Importance of Characterizing and Calibrating Light Engines in Vat Photopolymerization Additive Manufacturing

Vat photopolymerization (VP) is an additive manufacturing technique that uses spatially patterned light to cure liquid resins into solid materials. As an additive manufacturing technique, parts with complex geometries can be manufactured that would not be possible with conventional techniques. Large scale VP has found wide adoption by a range of industries including automotive, dental, consumer goods, and prototyping. On a smaller scale, hobbyist level users have found innumerable uses for VP and this market, in particular, has fueled the development of a plethora of low-cost printers. Common to all VP printers is the need for a light engine which illuminates the resin and initiates the photopolymerization process. Historically, this light engine was a UV laser with relatively simple optical properties including low divergence, narrow spectral bandwidth, and fixed wavelength – spatial extent of the photopolymerization was controlled by rastering the focused beam across the build plate. More recently, digital micromirror device (DMD) and liquid crystal display (LCD) based light engines have been developed which expose the resin in a parallel fashion by controlling the exposure in a 2-dimensional pixelated manner. These more recent light engine designs tend to rely upon light emitting diodes (LEDs) as the photon source and these have more complex properties including high divergence (i.e. Lambertian emitter), broad spectral bandwidth, and less well-defined wavelength. Many printer manufacturers have developed multi-emitter light engine designs that compound these characteristics. As VP manufacturing continues to mature, the reproducibility of parts, both in terms of dimensional and functional properties, is becoming more and more critical. Here, we present our work on characterizing existing VP printer light engines. We show the types of heterogeneity that are present in these light engines and how they can directly impact printed parts. This then leads us to a discussion about what optical properties are important to control in VP light engines for improved print reproducibility. Finally, we discuss our latest efforts to develop a fully calibrated and characterized uniform light engine for performing careful photopolymerization studies that inform on the underlying photopolymerization physics without convolving light engine heterogeneity with the underlying photophysics. Figure 1

Photoacoustic real-time monitoring of UV laser ablation of aged varnish coatings on heritage objects

A novel, simple and non-invasive diagnostic methodology is presented herein, as an effective approach to monitor in real-time the UV laser-assisted thinning of aged varnished coatings from heritage objects, using the acoustic signals generated upon light interaction with matter. Detection of the photoacoustic (PA) signals is realized by means of a non-contact air-coupled ultrasonic transducer with a frequency response in the MHz regime. This proposed statistical approach utilizes the information the PA signals provide during the laser ablation process, embedded in their temporal delay and amplitude reduction during varnish removal. Based on the cross-correlation of the PA signals, this monitoring technique determines the critical laser pulse, responsible for complete varnish removal. The efficiency of our methodology has been examined on a series of aged varnish mock-ups on different substrates. The laser treated surfaces were further assessed using various techniques, in order to confirm the results of our PA monitoring strategy. Application of this technique is aimed at validating its potential and suitability in addressing different varnish cleaning challenges. Based on previous encouraging results on encrusted stonework, primary objective of this study is to establish the PA monitoring methodology as an essential, real-time monitoring tool for controlled laser ablation of aged varnish coatings from heritage surfaces (paintings, metallic objects, wall paintings etc.), ensuring their efficient removal while preserving the integrity of the objects. Implementation of the PA technique may pave the way for numerous applications as regards the reliable monitoring of removing unwanted layers from various surfaces of interest.

High-efficiency non-ablative UV laser nano-scale processing of fused silica by stable filamentation.

Over the last decades, three-dimensional micro-manufacturing of fused silica via near-infrared ultrafast laser exposure combined with an etching step has become an established technique for producing complex three-dimensional components. Here, we explore the effect of ultraviolet exposure on process efficiency. Specifically, we demonstrate that shorter wavelengths not only enable enhanced resolution but also yield higher etching selectivity, with an order of magnitude lower pulse energy and significantly higher repetition rates than current practice. This result is obtained using an exposure regime where the laser beam alternates between regimes of self-focusing and defocusing in a stable manner, forming a localized filament. Using this principle, we demonstrate the fabrication of self-organized nano-channels with diameters as small as 120 nm after etching, reaching extreme aspect ratios, exceeding 1500.

Nonlinear processes and laser damage induced by a temporally modulated nanosecond UV laser beam. Part I: stimulated Brillouin scattering

Phase modulation is currently applied on pump laser beams propagating in thick optical elements to avoid optical damage induced by Brillouin backscattering. The experimental measurements show that adding a 2 GHz amplitude modulation to such a phase-modulated laser beam can compromise this optical preservation by increasing the Brillouin backscattering signal. These results are confirmed and explained with a theoretical analysis, developed in the perturbative regime, and with the numerical results obtained by solving three-wave coupling equations. Particularly, we show that when the frequency of the amplitude modulation is a harmonic of the phase modulation frequency, the Brillouin gain value is increased and can exceed the one obtained without amplitude modulation.

Nonlinear processes and laser damage induced by a temporally modulated nanosecond UV laser beam. Part II: Kerr self-focusing

In this paper, we study the impact of Kerr self-focusing on laser-induced damage of fused silica with temporal modulation. In our experiments, temporal modulations are generated with high bandwidth amplitude modulators between 2 and 10 GHz. Measurement of the spatial profile after propagation is made possible due to a dedicated imaging setup. First results without temporal modulation experimentally and numerically show that the nonlinear gain increases exponentially with intensity. Complementary experiments with temporal modulation at different frequencies show an increase in the nonlinear gain. Eventually, we show that laser-induced damage is enhanced with amplitude modulation, with a dependence on the frequency of the amplitude modulation. Optimization of phase modulation allowed us to mitigate backwards stimulated Brillouin scattering and to focus on the sole impact of Kerr self-focusing on laser-induced damage.

Investigations on terthiophene as an electrically conductive polymer for UV laser lithography

Abstract Polymers hold great potential for the use in microsensors and organic electronics. They are highly adaptable, easy to process and can contribute new or improved capabilities compared to semiconductors. Direct UV laser lithography also gains increasing attention. Because it avoids expensive photomasks, it is especially attractive where small numbers of specialized microcomponents are needed, like in prototyping. Lithography necessitates materials, which can be shaped by UV radiation. For many microsensor applications, there is the additional requirement of electric conductivity, preferably in the same material. We approached this demand by combining a Novolak and terthiophene doped with copper(II) perchlorate to form an interpenetrating polymer network, which possesses properties of both of its constituents. From this, we manufactured test structures with the UV laser of a micro pattern generator. In previous conference contributions, we showed a first proof of principle. In this publication, we present results of new experiments that demonstrate the characteristics in more detail. We improved our electrical setup to conduct four-terminal measuring. We used it to first verify previous results and investigated the material’s response to alternating currents up to 10 kHz. We then compared the electrical resistivity of differently sized structures for temperatures between 20 and 90 °C and examined long-term stability of their resistance by subjecting samples to temperatures of up to 60 °C for several hours. Additionally, we tested the influence of UV radiation on the resistance. Our samples exhibited good lithographic qualities. Resistivities were around 2 Ω mm and temperature sensitivity up to −407 Ω K−1. UV radiation induced a partially reversible increase of the electric resistance. The long-term stability of the material was temperature-dependent.

Instant identification of dental white spot using K-means algorithm via laser-induced fluorescence and associated hyperspectral imaging.

Dental caries (DC) is a chronic illness that affects 2 billion individuals worldwide, with 520 million of those suffering from it in their primary teeth. It is apparent that early DC detection and subsequent minimally invasive therapy are crucial clinical requirements. The first reversible clinical indication of demineralization is dental white spot (DWS) lesions. However, diagnosing DWS poses extreme challenges for practitioners. In this investigation, a customized laser-induced fluorescence system with a hyperspectral imaging (HI) camera and a non-ionization laser light supply was created for DWS localization and early DC detection. A UV laser diode source with a wavelength of 395nm was used for light stimulation for the 10 test samples of teeth. The emitted signature of the main tooth components, including dentin, DWS, enamel, and DC, was recorded. An attempt was made to increase the system's sensitivity to the fluorescent signal by applying a logarithmic scale to the spectral signature. Moreover, further discrimination may be achieved by signal strength. We identified that the fluorescent signal's peak intensity at 771nm works best for discriminating DWS from normal areas, or DC. For characterizing dentin, the re-emitted frequency at 500nm has the maximum intensity. Next, we presented our imaging grouping strategy that combines visual enhancement through a moving average, MA, filtering and segmenting an image using K-means clustering (K-mc) (K=8) for instant and precise DWS grouping for the constructed HI images at (500nm and 771nm). Despite the tiny structure and its DWS white appearance, our approach could successfully demarcate the DWS on the tested teeth. Dental examiners might benefit from our simple, non-invasive, non-ionizing optical diagnosis approach to help them make their first assessments and experience accurate and exact delineation of the DWS to obtain immediate and higher rates of early-stage DC detection.

All-solid-state CW Pr3+:YLF green laser at 522 nm end-pumped by a high-power fiber-coupled 444 nm blue LD module

Continuous wave (CW) green lasers have a lot of important applications in many fields, including holography, interferometry, atom cooling and trapping, and quantum optics, and they are usually achieved by frequency-doubling 1 µm lasers based on the Nd3+ gain media. In this paper, we present an all-solid-state CW green laser with an output wavelength of 522 nm, which was directly attained by employing a Pr3+:YLF crystal pumped with a high-power fiber-coupled blue laser diode (LD) module as the gain medium. Due to the negative thermal lens effect of the Pr3+:YLF crystal, the designed laser resonator had to be lengthened with the increase in the incident pump power. As a result, when a 0.5% doped Pr3+:YLF crystal was employed as the gain medium and the incident pump power was 12 W, the length of the resonator was optimized to 311.3 mm and the maximum output power of 522 nm green laser was up to 886 mW. The obtained conversion efficiency and beam quality M2 were 11.25% and 1.15, respectively. The long-term power stability within 4.5 h was better than ±1.5% at an output power of 700 mW. The obtained watt-level green laser can also be used to generate high power CW deep UV laser for laser processing of silicon and organic materials, inspection, etc.