Pulsed UV Laser Irradiation Research Articles

Laser crystallization of amorphous TiO2 on polymer

The deposition of crystalline TiO2 on polymers can boost its use in a large plethora of applications. In this work, we deposited, through the low-temperature atomic layer deposition (LT-ALD) technique, a thin layer of amorphous TiO2 on polymethyl-methacrylate (PMMA), and afterwards, we induced a phase transition from amorphous to crystalline anatase by pulsed UV-laser irradiation. A pulsed UV laser with a low penetration length was used to avoid the heating and damaging of the polymeric substrate. The diffusion of the heat and the temperature behaviour were simulated and discussed. We studied experimentally the effect of the laser fluence and pulse number on the amorphous-crystal transition. We observed the presence of two thresholds for the formation of the crystalline phase: on the fluence and the number of laser shots. Moreover, widening of pre-existing cracks is observed, as soon as fluence and the number of pulses increase. To improve further the quality of the deposited layer, we introduced a ZnO interlayer between TiO2 and the PMMA substrate. The effect of this interlayer was also discussed. Lastly, wettability, as a measure of the overall quality of the layer, was measured and interpreted by using the Cassie model.

Ultrafast Deep-Ultraviolet Laser-Induced Voltage Response of Pyrite.

Ultrafast, high-sensitivity deep-ultraviolet (UV) photodetectors are crucial for practical applications, including optical communication, ozone layer monitoring, flame detection, etc. However, fast-response UV photodetectors based on traditional materials suffer from issues of expensive production processes. Here, we focused on pyrite with simultaneously cheap production processes and ultrafast response speed. Nanoseconds photovoltaic response was observed under UV pulsed laser irradiation without an applied bias at room temperature. In addition, the response time of the laser-induced voltage (LIV) signals was ~20 ns, which was the same as the UV laser pulse width. The maximum value of the responsivity is 0.52 V/mJ and the minimum value of detectivity was about to ~1.4 × 1013 Jones. When there exists nonuniform illumination, a process of diffusion occurs by which the carriers migrate from the region of high concentration toward the region of low concentration. The response speed is limited by a factor of the diffusion of the carriers. With an increment in laser energy, the response speed of LIV is greatly improved. The high response speed combined with low-cost fabrication makes these UV photodetectors highly attractive for applications in ultrafast detection.

Optical Quality Laser Polishing of CVD Diamond by UV Pulsed Laser Irradiation

AbstractIt is a great technical challenge to precisely polish diamond substrates due to their extremely high hardness, especially for micro‐optics fabrication. Here, a simple laser polishing process is demonstrated for the optical quality surface finish of chemical vapor deposition diamond using a 355 nm nanosecond laser. Raman spectroscopy and surface profile analyses reveal that the laser polishing is an ablation‐based process that consists of laser graphitization and the subsequent laser ablation of the graphitized layer. An optimized strategy is proposed to realize the high‐quality polishing by combining the ablation effect and defocusing laser irradiation. The polishing strategy can effectively reduce the peak‐to‐valley height difference of a rough surface and automatically enables the laser fluence at a low level close to the ablation threshold. Laser polishing at such critical laser fluence can greatly avoid harmful effects caused by high laser fluence and achieve precision materials’ removal while maintaining optical surface quality. The approach is capable of delivering an average roughness Ra down to 8.02 nm and a high transmittance up to 80% of mechanically polished diamond in the visible spectrum. High optical performances make it possible to directly fabricate micro‐optical components on diamond substrates using this novel laser polishing approach.

Evaluation of Local Mechanical and Chemical Properties via AFM as a Tool for Understanding the Formation Mechanism of Pulsed UV Laser-Nanoinduced Patterns on Azo-Naphthalene-Based Polyimide Films.

Aromatic polyimides containing side azo-naphthalene groups have been investigated regarding their capacity of generating surface relief gratings (SRGs) under pulsed UV laser irradiation through phase masks, using different fluencies and pulse numbers. The process of the material photo-fluidization and the supramolecular re-organization of the surface were investigated using atomic force microscopy (AFM). At first, an AFM nanoscale topographical analysis of the induced SRGs was performed in terms of morphology and tridimensional amplitude, spatial, hybrid, and functional parameters. Afterward, a nanomechanical characterization of SRGs using an advanced method, namely, AFM PinPoint mode, was performed, where the quantitative nanomechanical properties (i.e., modulus, adhesion, deformation) of the nanostructured azo-polyimide surfaces were acquired with a highly correlated topographic registration. This method proved to be very effective in understanding the formation mechanism of the surface modulations during pulsed UV laser irradiation. Additionally to AFM investigations, confocal Raman measurements and molecular simulations were performed to provide information about structured azo-polyimide chemical composition and macromolecular conformation induced by laser irradiation.

Combined reactive plasma jet-laser etching method for technical optical glass containing metal oxides

The machining of optical glass materials with beam technologies enables novel fabrication processes for innovative optical elements with freeform surfaces. However, the complex material composition of optical glass such as N-BK7® is challenging for non-abrasive, pure chemical machining. N-BK7 can be machined utilizing reactive atmospheric plasma jet (RAPJ) etching. However, the etching mechanism results in process-inherent surface composition changes that limit potential applications of RAPJ-machining. A residues layer is formed which causes a temporally changing etching behavior and an increased roughness. Therefore, a process sequence of utilizing RAPJ etching and laser cleaning is proposed and demonstrated that enables to overcome the stated limitations. In this cyclic sequence, N-BK7 optical glass is etched first with a fluorine-containing reactive atmospheric plasma jet resulting in an etched but also residues-modified surface. Thereafter, the residues are removed by pulsed UV laser irradiation without ablation of the glass. This combined laser-enhanced reactive plasma jet etching allows etching of optical glass with a higher average etching rate and smoother machined surfaces.

Atmosphere-sensitive photoluminescence of Co x Fe3-x O4 metal oxide nanoparticles.

In this work the photoluminescence (PL) of CoxFe3−xO4 spinel oxide nanoparticles under pulsed UV laser irradiation (λexc = 270 nm) is investigated for varying Co/Fe ratios (x = 0.4⋯2.5). A broad emission in the green spectral range is observed, exhibiting two maxima at around 506 nm, which is dominant for Fe-rich nanoparticles (x = 0.4, 0.9), and at around 530 nm, that is more pronounced for Co-rich nanoparticles (x > 1.6). As examinations in different atmospheres show that the observed emission reacts sensitively to the presence of water, it is proposed that the emission is mainly caused by OH groups with terminal or bridging metal–O bonds on the CoxFe3−xO4 surface. Raman spectroscopy supports that the emission maximum at 506 nm corresponds to terminal OH groups bound to metal cations on tetrahedral sites (i.e., Fe3+), while the maximum around 530 nm corresponds to terminal OH groups bound to metal cations on octahedral sites (i.e., Co3+). Photoinduced dehydroxylation shows that OH groups can be removed on Fe-rich nanoparticles more easily, leading to a conversion process and the formation of new OH groups with different bonds to the surface. As such behavior is not observed for CoxFe3−xO4 with x > 1.6, we conclude that the OH groups are more stable against dehydroxylation on Co-rich nanoparticles. The higher OH stability is expected to lead to a higher catalytic activity of Co-rich cobalt ferrites in the electrochemical generation of oxygen.

Synthesis of manganese (IV) oxide at activated carbon on reduced graphene oxide sheets via laser irradiation technique for organic binder-free electrodes in flexible supercapacitors

We report a novel, green, scalable technique to synthesize binder-free, high-purity conductive composite comprising activated carbon (AC), manganese dioxide nanorods (MnO2), and reduced graphene oxide sheets (rGO) for flexible supercapacitors with outstanding electrochemical performance. UV pulsed laser irradiation of GO-based composite dispersion (AC/GO or MnO2@AC/GO) in ethanol aqueous medium was used to induce a photocatalytic reduction of GO and simultaneous anchor AC particles or AC loaded MnO2 nanorods (MnO2@AC) on the reduced GO sheets (rGO) at room temperature and atmospheric pressure. rGO sheets serve as a large surface area, conductive binder to enhance the ion adsorption, electrical conductivity, and mechanical flexibility of supercapacitor electrodes. This laser-induced photocatalytic reduction method was used to prepare two different rGO-based colloidal composites AC/rGO (CG) and MnO2@AC/rGO (MCG). The prepared rGO-based colloidal composites were used to fabricate symmetric supercapacitors (CG//CG and MCG//MCG) and asymmetric supercapacitors (MCG//CG) in which MCG is the positive electrode and CG is the negative one. All prepared rGO-based supercapacitors demonstrated significant improvement in their electrochemical performance compared with rGO-free AC based supercapacitors. The enhancement in the electrochemical properties of rGO-based supercapacitors could be attributed to the intrinsic characteristics of rGO, such as high surface area, excellent electrical conductivity, and super mechanical flexibility. Our approach is a one-step, scalable, cost-effective synthesis technique to produce all binder-free AC/rGO based composites for flexible energy-storage devices.

Study of UV repetition laser-induced absorption on fused silica surface using a surface thermal lensing technique.

A surface thermal lensing technique is used to study the time evolution of the absorption of a fused silica surface when exposed to UV pulsed laser irradiation in the nanosecond regime. The time evolution of the absorption is characterized as a saturated exponential increase, and the disruptive change indicates that the macrodamage occurs. A time-dependent absorption defect model is used to fit the experimental data, and a single-photon process is proposed to explain the results. This method can be used to estimate the operation lifetime of optical components.

Effect of pulsed UV laser irradiation on 4H-SiC MOS with thermal gate oxide

The impact of 193 nm ArF pulsed UV laser irradiation on thermally grown SiO2/4H-SiC structures was investigated. The effect of pulsed UV light on the near-interface oxide trap of SiO2/SiC structure is greater than that on the interface and the near-interface SiC substrate. The defects near the interface do not change monotonously with the increase in irradiation power, which indicates that UV irradiation has at least two effects on SiO2/SiC. When the irradiation power is too high, new defects will be introduced into the oxide layer. This article reveals a potential mechanism for high-energy photon irradiation to change device performance in semiconductor plasma technology.

Operation of thin-film thermoelectric generator of Ge-rich poly-Ge1-xSnx on SiO2 fabricated by a low thermal budget process

A thin-film thermoelectric generator composed of p- and n-type poly-Ge1−xSnx (x ∼ 0.02) on a Si(001) covered with SiO2 has been successfully fabricated by low thermal budget processes (under 300 °C) and demonstrated for the first time. Both the crystallization and dopant activation were simultaneously performed using pulsed UV laser irradiation in flowing water. A recorded activation ratio of Sb in the poly-Ge1−xSnx enabled a relatively high power factor (9.2 μ Wcm−1 K−2 at RT), which is comparable to the counterparts of n-type Ge1−xSnx layers epitaxially grown on InP(001).

Amplified spontaneous emission from a surface-modified GaN film fabricated under pulsed intense UV laser irradiation

We propose a simple method for fabricating random structures directly on Mg-doped GaN thin films. The process is relatively simple, involving only irradiation with strong UV pulses from a fabrication laser on the thin-film surface. After intense UV laser pulses (>400 MW/cm2) are irradiated on the flat GaN film, the surface is roughened and quasi-periodic structures form. When the roughened surface is excited with laser light of intensity about 10 times smaller than the fabrication laser intensity, emission increases around 367 nm, and spectral narrowing and threshold behavior are observed. Because such behaviors are not observed without the application of intense laser irradiation, we conclude that amplified spontaneous emission is induced in the modified GaN surface by the intense UV laser pulse irradiation. This method offers the possibility of easy and direct fabrication of microscale random-laser devices on semiconductor substrates.

β-Bi2O3 reduction by laser irradiation in a liquid environment.

This study reports the structural and stoichiometric modifications of bismuth oxide nanoparticles in the β phase (β-Bi2O3) by UV pulsed laser irradiation in water or ethanol solutions. Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance and high-resolution transmission electron microscopy were used to characterize the synthesized nanomaterials. The various analyses demonstrate that the laser irradiation of β-Bi2O3 nanospheres is a green, fast and effective method to produce Bi2O2CO3 nanosheets or metallic Bi nanoparticles depending on the liquid environment used. Bi subcarbonate is obtained by laser irradiation in water, whereas metallic Bi is produced by laser irradiation in ethanol, and in particular the relative amount of metallic Bi is found to depend on the laser fluence. These typologies of materials find application in several fields, such as photocatalytic processes, light filters and environmental sensors.

Controlled formation of gold nanostructures on biopolymer films upon electromagnetic radiation

The localized formation of gold nanostructures with controlled size and shape on chitosan films doped with gold precursor upon electromagnetic irradiation of various types is demonstrated here. Such controlled formation is achieved by tuning the wavelength, the energy and the interaction time of the radiation with the composite films. In particular, the use of a single UV nanosecond laser pulse results in the formation of gold sub-micron platelets with specific crystal structure, while increasing the number of pulses, further precursor reduction and photofragmentation induce the formation of gold nanoparticles. Using x-ray radiation as an alternative energy source, the reduction of the gold precursor and the subsequent formation of particles follow a different pathway. Specifically, x-ray-induced photo-reduction triggers the selective formation of gold sub-micron platelets with a very well defined {111} crystal phase. In this case, the density of crystal platelets increases by increasing the irradiation time of the films, while no photofragmentation process is observed. The gold structures pre-formed by x-ray radiation can be fragmented by subsequent pulsed UV laser irradiation forming nanoparticles with much narrower size distribution compared to that obtained via exclusive UV irradiation. Thanks to the perfect coupling between the natural polymeric matrix and gold nanostructures, the bionanocomposite systems developed could find various applications in biomaterial science and in biosensors field.

Laser-driven coating of vertically aligned carbon nanotubes with manganese oxide from metal organic precursors for energy storage

Carbon nanotubes-transition metal oxide systems are intensively studied due to their excellent properties for electrochemical applications. In this work, an innovative procedure is developed for the synthesis of vertically aligned multi-walled carbon nanotubes (VACNTs) coated with transition metal oxide nanostructures. VACNTs are grown by plasma enhanced chemical vapor deposition and coated with a manganese-based metal organic precursor (MOP) film based on manganese acetate solution. Subsequent UV pulsed laser irradiation induces the effective heating-decomposition of the MOP leading to the crystallization of manganese oxide nanostructures on the VACNT surface. The study of the morphology, structure and composition of the synthesized materials shows the formation of randomly oriented MnO2 crystals, with few nanometers in size, and to their alignment in hundreds of nm long filament-like structures, parallel to the CNT’s long axis. Electrochemical measurements reveal a significant increase of the specific capacitance of the MnO2-VACNT system (100 F g−1) as compared to the initial VACNT one (21 F g−1).

One-Pot Hybrid SnO2 /Poly(methyl methacrylate) Nanocomposite Formation through Pulsed Laser Irradiation.

The localized in situ formation of tin dioxide (SnO2 ) nanoparticles embedded in poly(methyl methacrylate) (PMMA) films is presented. This is achieved by the photoinduced conversion of the tin acetate precursor included in polymeric films, through controlled UV or visible pulsed laser irradiation at λ=355 and 532 nm, respectively. The evolution of the formation of nanoparticles is followed by UV/Vis spectroscopy and shows that their growth is affected in different ways by the laser pulses at the two applied wavelengths. This, in combination with electron microscopy analysis, reveals that, depending on the irradiation wavelength, the size of the nanoparticles in the final nanocomposites differs. This difference is attributed to distinct mechanistic pathways that lead to the synthesis of small nanoparticles (from 1.5 to 4.5 nm) at λ=355 nm, whereas bigger ones (from 5 to 16 nm) are formed at λ=532 nm. At the same time, structural studies with both X-ray and electron diffraction measurements demonstrate the crystallinity of SnO2 nanoparticles in both cases, whereas XPS analysis confirms the light-induced oxidation of tin acetate into SnO2 . Taken all together, it is demonstrated that the pulsed laser irradiation at λ=355 and 532 nm leads to the formation of SnO2 nanoparticles with defined features highly dispersed in PMMA solid matrices.

Gradual modification of ITO particle's crystal structure and optical properties by pulsed UV laser irradiation in a free liquid jet.

Indium tin oxide (ITO) particle coatings are known for high transparency in the visible, good conductive properties and near-infrared absorption. These properties depend on ITO particle's stoichiometric composition, defects and size. Here we present a method to gradually change ITO particle's optical properties by a simple and controlled laser irradiation process. The defined irradiation process and controlled energy dose input allows one to engineer the absorption and transmission of coatings made from these particles. We investigate the role of the surrounding solvent, influence of laser fluence and the specific energy dose targeting modification of the ITO particle's morphology and chemistry by stepwise laser irradiation in a free liquid jet. TEM, SEM, EDX, XPS, XRD and Raman are used to elucidate the structural, morphological and chemical changes of the laser-induced ITO particles. On the basis of these results the observed modification of the optical properties is tentatively attributed to chemical changes, e.g. laser-induced defects or partial reduction.

Detection of Neutral Species in the MALDI Plume Using Femtosecond Laser Ionization: Quantitative Analysis of MALDI-MS Signals Based on a Semiequilibrium Proton Transfer Model

We investigated neutral species in the matrix-assisted laser desorption and ionization (MALDI) plume using femtosecond laser ionization spectrometry with simultaneous measurement of the standard MALDI spectrum of the identical MALDI event induced by pulsed UV laser irradiation. The ratio of neutral species in the plume [A]p/[M]p (A = phenylalanine (Phe) or alanine (Ala), M = 2,5-dihydroxybenzoic acid (DHB)) was confirmed to be the same as that of the sample mixture in the range of [A]0/[M]0 = 4 × 10-4-1, indicating the validity of the widely adopted approximation [A]p/[M]p = [A]0/[M]0 in the reaction quotient of the proton transfer reaction MH+ + A ⇄ M + AH+. An effective parameter representing the extent of thermal equilibrium in the thermal proton transfer model is introduced for the first time. Numerical simulation based on this semiequilibrium model successfully reproduced variations of MALDI signal intensities AH+ and MH+ with two parameters: the fraction of ionized matrix a ≤ 10-5 and an effective temperature T = 1200 and 1100 K for Phe/DHB and Ala/DHB systems, respectively. These values show good agreement with those determined previously by different experimental approaches. The extent of thermal equilibrium was determined to be 95% and 98% for Phe/DHB and Ala/DHB systems, respectively, suggesting that the proton transfer reactions almost proceed to their thermal equilibrium.

Laser-induced nanostructuration of vertically aligned carbon nanotubes coated with nickel oxide nanoparticles

A versatile method is explored to decorate vertically aligned multi-walled carbon nanotubes (VACNTs) with NiO nanostructures. Multi-walled VACNTs are grown by plasma-enhanced chemical vapor deposition and coated with NiO nanoparticles (NPs) by drop casting. After that, the system is submitted to nanosecond pulsed UV laser irradiation in atmospheric environment. Laser irradiation provokes rapid heating–melting–cooling processes which lead to the recrystallization of NiO NPs on the outer walls of VACNTs. In this way, and depending on the laser fluence and the number of accumulated pulses, different nano-architectures such as continuous NiO coatings and spiny features on VACNTs are obtained. High-resolution scanning and transmission electron microscopies and Raman spectroscopy, corroborated with photothermal simulations, suggest that the grown nanostructures are mainly created by the laser-induced high temperatures (photothermal mechanisms). However, the observed reconstruction of the outer graphitic shells of VACNTs point to the catalytic action of NiO NPs, probably induced by the direct action of laser radiation.

Laser-induced in situ synthesis of Pd and Pt nanoparticles on polymer films

We present the localized in situ formation of Pd and Pt nanoparticles embedded in chitosan solid films. This is achieved by the photo-induced reduction of metallic precursors, previously incorporated in chitosan films, through controlled UV pulsed laser irradiation. Interestingly, at high number of laser pulses, Pd and Pt follow different formation pathways, contrary to their common photoreduction mechanism occurring at low irradiation pulses. Specifically, in the case of the Pd, a photofragmentation process takes place fracturing the previously formed nanoparticles into smaller ones; whereas in the case of Pt, the prolonged irradiation promotes the nanoparticles agglomeration. The combination of both precursors in a binary solid system results in the combined formation of both Pd and Pt nanoparticles in the polymer film upon laser irradiation. The herein reported approach is an efficient and precise tool to generate size- and density-controlled Pd and Pt nanoparticles in desired areas of polymeric films, rendering this method a potential candidate for the fabrication of flexible polymeric devices for gas-sensing or electro-catalysis applications. Schematic representation of Pd, Pt and Pd–Pt combined NPs formation mechanisms under pulsed laser irradiation

Microstructure variation in fused silica irradiated by different fluence of UV laser pulses with positron annihilation lifetime and Raman scattering spectroscopy

We present an original study on the non-destructive evaluation of the microstructure evolution of fused silica induced by pulsed UV laser irradiation at low fluence (less than 50% Fth). Positron annihilation spectroscopy discloses that the spatial size of the vacancy cluster is increased exponentially with the linearly elevated laser fluence. Particularly, the vacancy cluster size in bulk silica is significantly increased by 14.5% after irradiated by pulsed 355nm laser at F=14J/cm2 (50% Fth), while the void size varies only ∼2%. UV laser-excited Raman results suggest that the bond length and average bond angle of SiOSi bridging bond are both slightly reduced. Results reveals that the rearrangement process of (SiO)n fold rings and breakage of the SiO bridging bond in bulk silica occurred during pulsed UV laser irradiation. The micro-structural changes were taken together to clarify the effect of sub-threshold laser fluence on material stability of silica glass. The obtained data provide important information for studying material stability and controlling the lifetime of fused silica optics for high power laser system.