Femtosecond Laser Irradiation Research Articles

Crystallinity in periodic nanostructure surface on Si substrates induced by near- and mid-infrared femtosecond laser irradiation

Laser-induced periodic surface structure (LIPSS), which has a period smaller than the laser wavelength, is expected to become a potential technique for fine surface processing. We report the microscopic and macroscopic observations of the crystallinity of LIPSSs, where the characteristics such as defects generation and residual strain were analyzed, respectively. The LIPSSs were formed on a Si substrate using two different femtosecond pulses from Ti:Sapphire laser with near-infrared wavelength (0.8 μm) and free-electron laser (FEL) with mid-infrared wavelength (11.4 μm). The photon energies of the former and latter lasers used here are higher and lower than the Si bandgap energies, respectively. These LIPSSs exhibit different crystalline states, where LIPSS induced by Ti:Sapphire laser show residual strain while having a stable crystallinity; in contrast, FEL-LIPSS generates defects without residual strain. This multiple analysis (microscopic and macroscopic observations) provides such previously-unknown structural characteristics with high spatial resolution. To obtain LIPSS with suitable properties and characteristics based on each application it is paramount to identify the laser sources that can achieve such properties. Therefore, identifying the structural information of the LIPSS generated by each specific laser is of great importance.

Electroplating Cobalt Films on Silicon Nanostructures for Sensing Molecules

In this study, we electroplated Co and Cu on nano-spiked silicon substrates that were treated with femtosecond laser irradiations. With energy-dispersive X-ray (EDX) analysis by a scanning electron microscope (SEM), it was found that both Co and Cu are primarily coated on the spike surfaces without changing the morphology of the nanospikes. We also found that nanoscale bridges were formed, connecting the Co-coated silicon spikes. The formation of these bridges was studied and optimized through a series of time-controlled electroplating and oxidizing processes. The bridges are related to the oxidation of Co in the air. When it is irradiated with visible light, this special structure has shown a capability of interactions with carbon monoxide and carbon dioxide molecules. The electroplated cobalt may be used for gas sensors.

Physicochemical Modifications on Thin Films of Poly(Ethylene Terephthalate) and Its Nanocomposite with Expanded Graphite Nanostructured by Ultraviolet and Infrared Femtosecond Laser Irradiation

In this work, the formation of laser-induced periodic surface structures (LIPSS) on the surfaces of thin films of poly(ethylene terephthalate) (PET) and PET reinforced with expanded graphite (EG) was studied. Laser irradiation was carried out by ultraviolet (265 nm) and near-infrared (795 nm) femtosecond laser pulses, and LIPSS were formed in both materials. In all cases, LIPSS had a period close to the irradiation wavelength and were formed parallel to the polarization of the laser beam, although, in the case of UV irradiation, differences in the formation range were observed due to the different thermal properties of the neat polymer in comparison to the composite. To monitor the modification of the physicochemical properties of the surfaces after irradiation as a function of the laser wavelength and of the presence of the filler, different techniques were used. Contact angle measurements were carried out using different reference liquids to measure the wettability and the solid surface free energies. The initially hydrophilic surfaces became more hydrophilic after ultraviolet irradiation, while they evolved to become hydrophobic under near-infrared laser irradiation. The values of the surface free energy components showed changes after nanostructuring, mainly in the polar component. Additionally, for UV-irradiated surfaces, adhesion, determined by the colloidal probe technique, increased, while, for NIR irradiation, adhesion decreased. Finally, nanomechanical properties were measured by the PeakForce Quantitative Nanomechanical Mapping method, obtaining maps of elastic modulus, adhesion, and deformation. The results showed an increase in the elastic modulus in the PET/EG, confirming the reinforcing action of the EG in the polymer matrix. Additionally, an increase in the elastic modulus was observed after LIPSS formation.

The Motion of a Charged Particle in the Electromagnetic Field of a Polarization-Modulated Wave

This article presents an exact solution of the equation of motion of a charged particle in the electromagnetic field of a high-intensity polarization-modulated wave. Expressions for the average kinetic energy of a particle without regard to its rest energy in the case of circular and linear polarization of a modulated wave are obtained. The motion of a charged particle in the field was analyzed and expressed in terms of dependences of its average kinetic energy on the electromagnetic wave intensity and on various types of modulation depths. The contribution of each type of modulation to the energy characteristics of a charged particle was demonstrated. Solving the equation of motion of a charged particle in the electromagnetic field of a plane wave opens up possibilities for various applications related, in particular, to various developments of multi-frequency lasers and laser modulation technology. This study was proposed due to the growing interest in experiments using high-intensity femtosecond laser radiation and high-temperature plasma

Statistical parameters of femtosecond laser pulse post-filament propagation on a 65 m air path with localized optical turbulence

High-power femtosecond laser radiation propagates nonlinearly in air, exhibiting pulse self-focusing and strong multiphoton medium ionization, which leads to the spatial fragmentation of laser pulses into highly localized light channels commonly called filaments. Filaments are characterized by high optical intensity and reduced (even zero) angular spreading and can contain laser plasma or be plasmaless (postfilaments). The presence of optical turbulence on the propagation path dramatically changes pulse filamentation dynamics and in some cases causes pulse fragmentation enhancement and collapse arrest. For the first time, to the best of our knowledge, we experimentally and theoretically investigate the transverse profile of Ti:sapphire femtosecond laser radiation nonlinearly propagating a 65 m air path to the region of postfilament evolution after passing through an artificial localized air turbulence. We show that when a turbulence layer is placed before the filamentation region, the average number of high-intensity local fluence maxima (“hot points”) in the beam profile and their sizes grow as the turbulence strength increases, and then saturates at some levels. On the contrary, the deposition of a turbulence screen within the filamentation region has almost no effect on either the number or the average diameter of postfilaments.

Impact of pre-annealing process on electrical properties and stability of indium zinc oxide thin-film transistors

This paper examined the effects of no treatment versus plasma treatment, and femtosecond laser irradiation as pre-annealing processes on indium zinc oxide (IZO) films and annealing at high temperatures. The plasma pre-annealed multilayer stacked IZO TFTs showed better electrical properties with mobility enhancement from 2.45 to 7.81 cm2/Vs, but exhibited diminished on–off current ratio (Ion/Ioff). The IZO thin-film transistor (TFT) prepared with femtosecond laser pre-annealing with low pulse energy generation (power of 3 W at 700 nm wavelength) for 100 s has also exhibited significantly improved electrical performance, the saturation mobility increased to 4.91 cm2/Vs, the Ion/Ioff ratio was enhanced from 4.5 × 105 to 2.1 × 106, the threshold voltage improved from − 1.44 to − 0.25 V, and the subthreshold swing was reduced from 1.21 to 0.61 V/dec. In conclusion, IZO TFTs with improved performance can be prepared using a femtosecond laser pre-annealing process, which has great potential for fabricating low-cost, high-performance devices.

Structure-terahertz property relationship and femtosecond laser irradiation effects in chalcogenide glasses

We report structure-terahertz (THz) property relationship for various non-oxide chalcogenide glasses including unary (vitreous selenium (Se)), binary (arsenic sulfide (As-S), arsenic selenide (As-Se), and germanium selenide (Ge-Se)), and ternary (germanium arsenic selenide (Ge-As-Se)), systems along with commercially available AMTIR-1, IRG 22, and IRG 24 Ge-As-Se glasses. This comprehensive study is the first of its kind to combine Raman spectroscopy to examine structural units, connectivity, and glass network and terahertz time-domain spectroscopy (THz-TDS) to record the THz refractive index, n(THz), across a broad THz bandwidth. THz-TDS was carried out at Alfred University (AU) and National Institute of Standards and Technology (NIST), ultimately providing confidence in n(THz) values measured at AU. Vitreous Se, <r> = 2.0, record the minimum THz refractive index value of all Se-containing glasses. As-S and As-Se binary glasses have the highest measurable THz refractive index value at <r> = 2.4. Ge-Se binary glasses measure increased THz refractive index as <r> increases, with the maximum at <r> = 2.8. Ternary Ge-As-Se glasses record the maximum THz refractive index value at <r> = 2.5 for Ge10As30Se60. Low-repetition rate femtosecond laser irradiation (≈1 KHz, ≈40 fs, and ≈70 mW) was used to modify As-S and As-Se glass systems, where Raman and THz-TDS were used to observe minimal structural and THz refractive index values changes, respectively. Long-wave infrared (LWIR) (e.g., 10 μm)-THz (e.g., 1.0 THz) refractive index correlation is presented for all binary and ternary studied chalcogenide glasses. Such a correlation is valuable for predicting and designing chalcogenide glasses for integrated optical applications across THz and IR regions.

Polaron-Enhanced Polariton Nonlinearity in Lead Halide Perovskites.

Exciton-polaritons offer a versatile platform for realization of all-optical integrated logic gates due to the strong effective optical nonlinearity resulting from the exciton-exciton interactions. In most of the current excitonic materials there exists a direct connection between the exciton robustness to thermal fluctuations and the strength of the exciton-exciton interaction, making materials with the highest levels of exciton nonlinearity applicable at cryogenic temperatures only. Here, we show that strong polaronic effects, characteristic for perovskite materials, allow overcoming this limitation. Namely, we demonstrate a record-high value of the nonlinear optical response in the nanostructured organic-inorganic halide perovskite MAPbI3, experimentally detected as a 19.7 meV blueshift of the polariton branch under femtosecond laser irradiation. This is substantially higher than characteristic values for the samples based on conventional semiconductors and monolayers of transition-metal dichalcogenides. The observed strong polaron-enhanced nonlinearity exists for both tetragonal and orthorhombic phases of MAPbI3 and remains stable at elevated temperatures.

The Effect of Femtosecond Laser Irradiation and Plasmon Field on the Degree of Conversion of a UDMA-TEGDMA Copolymer Nanocomposite Doped with Gold Nanorods.

In this work, the effects of femtosecond laser irradiation and doping with plasmonic gold nanorods on the degree of conversion (DC) of a urethane dimethacrylate (UDMA)-triethylene glycol dimethacrylate (TEGDMA) nanocomposite were investigated. The UDMA-TEGDMA photopolymer was prepared in a 3:1 weight ratio and doped with dodecanethiol- (DDT) capped gold nanorods of 25 × 75 or 25 × 85 nm nominal diameter and length. It was found that the presence of the gold nanorods alone (without direct plasmonic excitation) can increase the DC of the photopolymer by 6-15%. This increase was found to be similar to what could be achieved with a control heat treatment of 30 min at 180 °C. It was also shown that femtosecond laser impulses (795 nm, 5 mJ pulse energy, 50 fs pulse length, 2.83 Jcm-2 fluence), applied after the photopolymerization under a standard dental curing lamp, can cause a 2-7% increase in the DC of undoped samples, even after thermal pre-treatment. The best DC values (12-15% increase) were obtained with combined nanorod doping and subsequent laser irradiation close to the plasmon resonance peak of the nanorods (760-800 nm), which proves that the excited plasmon field can directly facilitate double bond breakage (without thermoplasmonic effects due to the short pulse length) and increase the crosslink density independently from the initial photopolymerization process.

Effect of the Length and Pressure of a Gas Jet on Optical Harmonics Generation by 4.5-μm Femtosecond Laser Radiation of a Fe:ZnSe Laser System

The effect of the length and pressure of an argon gas jet on low-order harmonics (5th, 7th, 9th, and 11th) generation by 4.5-μm femtosecond laser radiation of a Fe:ZnSe laser system has been studied experimentally. It has been shown that an increase in the length of the generation medium up to the waist length allows one to increase the generation efficiency by a factor of 12. It has also been demonstrated that a change in the length of the gas medium changes the pressure dependence of the energy of the generated radiation because of change in the phase matching conditions, whose correct simulation requires the inclusion of nonlinear propagation effects for pump pulse and generated harmonics.

Optical and Thermal Behavior of Germanium Thin Films under Femtosecond Laser Irradiation.

In this study, we theoretically investigate the response of a germanium thin film under femtosecond pulsed laser irradiation. Electron and lattice temperatures, as well as material-specific optical properties such as dielectric function and reflectivity, were calculated during the irradiation using an extended two-temperature model coupled with the carrier density rate equation and the Drude model. Melting and ablation fluence thresholds were also predicted, resulting in 0.14 J cm-2 and 0.35 J cm-2, respectively. An ultrafast change in both optical and thermal properties was detected upon laser irradiation. Results also indicate that thermal melting occurs after germanium takes on a metallic character during irradiation, and that the impact ionization process may have a critical role in the laser-induced thermal effect. Therefore, we suggest that the origin of the thermal modification of germanium surface under femtosecond laser irradiation is mostly due the impact ionization process and that its effect becomes more important when increasing the laser fluence.

The Antimicrobial Effect of Gold Quantum Dots and Femtosecond Laser Irradiation on the Growth Kinetics of Common Infectious Eye Pathogens: An In Vitro Study.

We studied the antimicrobial effect of gold quantum dots (AuQDs), femtosecond laser irradiation, and the combined effect of laser irradiation and AuQD treatment against common infectious eye pathogens. The INSPIRE HF100 laser system (Spectra Physics) provided a femtosecond laser, which was pumped by a mode-locked femtosecond Ti: sapphire laser MAI TAI HP (Spectra Physics), while a Quanta-Ray nanosecond Nd: YAG laser (Spectra-Physics) was used to precisely synthesize 7.8, 8.7, and 11.6 nm spherical AuQDs. Then, the in vitro growth kinetics and growth rate analysis of E. coli, methicillin-resistant Staphylococcus aureus, Enterococcus faecalis, Listeria monocytogenes, and Candida albicans (treated with the AuQDs, femtosecond laser irradiation, or combined laser and AuQDs treatment) was measured. The biocompatibility of the AuQDs with the retinal epithelial cell lines (ARPE-19) and their toxicity to the cells was assayed. The results showed that (1) in vitro irradiation using a 159 J/cm2 energy density obtained from the 400 nm femtosecond laser suppressed the growth of each of the five pathogens. (2) Similarly, treatment with the AuQDs was antimicrobial against the four bacteria. The AuQDs with an average size of 7.8 nm were more highly antimicrobial and biocompatible and were less cytotoxic than the larger AuQD sizes. (3) The combined femtosecond laser irradiation and AuQD treatment was more highly antimicrobial than each treatment alone. (4) The AuQD treatment did not impair the rate of wound closure in vitro. These findings suggest that combined femtosecond laser irradiation and AuQD treatment is significantly antimicrobial against Candida albicans, Gram-positive L. monocytogenes, S. aureus, and E. faecalis, as well as Gram-negative E. coli. The nontoxicity and biocompatibility of the AuQD particles tested suggest that this form of treatment may be clinically viable.

Sub-40 nm nanogratings self-organized in PVP-based polymer composite film by photoexcitation and two sequent splitting under femtosecond laser irradiation

Laser-induced periodic surface structures (LIPSSs) on various materials have been extensively investigated because of their wide applications. The combination of different materials allows for greater freedom in tailoring their functions and achieving responses not possible in a homogeneous material. By utilizing a femtosecond (fs) laser to irradiate the Fe-doped Polyvinyl Pyrrolidone (PVP) composite film, highly regular ultrafine nanogratings (U-nanogratings) with a period as small as 35.0 (±2.0) nm can be self-organized on the surface with extremely high efficiency. The period of the U-nanogratings can be controlled by varying the scanning speed of the laser beam (deposited energy) and the thickness of the composite film. Based on the experimental, theoretical, and simulation results, we propose a two-step formation mechanism: composite film excitation and two sequent grating-splitting. The high photosensitivity and low glass transition temperature of the composite film facilitate the fabrication of the ultrafine nanostructures. The proposed design method for the composite material and fabrication process could not only provide a strategy for obtaining highly regular U-nanogratings, but also offer a platform to explore the interaction physics between ultra-short pulses and matter under extreme conditions.

Ultrafast Laser-Induced Formation of Hollow Gold Nanorods and Their Optical Properties.

Laser irradiation has been shown to be an efficient means to modify structures and shapes of plasmonic nanoparticles for tuning their properties. Thermomechanical deformations of single-crystal and penta-twinned gold nanorods by femtosecond laser irradiations have been studied by classical molecular dynamics simulations. It is demonstrated that hollow gold nanorods could be formed by femtosecond laser irradiations under certain conditions of maximum temperatures in nanorods by laser heating and cooling rates due to the extrinsic solvent. For a given maximum temperature and cooling rate, a larger cavity is induced in the irradiated single-crystal nanorod. The results also indicate that at the same cooling rate a higher threshold of maximum temperature can be required for producing the cavity in the twinned nanorod. The optical spectra of the laser-irradiated gold nanorods are calculated, and the shifts in the surface plasmon resonance peak of the nanorods are illustrated due to the thermal reshaping and the plasmon hybridization mechanism. Moreover, we show the formation of the hollow gold nanorod possessing the surface plasmon resonance peak in the second near-infrared window and a relatively small aspect ratio (∼2.8), which is highly desirable and suitable for serving as agents in biomedical imaging and photothermal therapy applications.

Two-stage conductivity switching of GST thin films induced by femtosecond laser radiation

Crystallization of as-deposited amorphous Ge2Sb2Te5 films (180 nm) by femtosecond pulsed laser radiation have been studied using in-situ electrical resistance measurements with temporal resolution. The results show that a double pulses impact is optimal for the controllability of the crystallization process. In this case, the switching time of the film resistance is less than 100 ns and is associated with the formation of conducting crystalline paths in the material after the pulse laser impact. We have proposed a model of the electrical conductivity dynamics caused by impact of the ultrashort laser radiation, which is well related to experimental data.

Comparison study of the femtosecond laser-induced surface structures on silicon at an elevated temperature.

The temperature dependency of femtosecond laser induced surface structures opens up a new scenario for studying ultrafast laser-mater interaction on the surface and a novel method for controlling the features of these structures. The shape and crystallinity of micro/nano surface structures created by femtosecond laser irradiation of n-type silicon (100) at elevated temperatures were compared in this study. Low spatial frequency laser induced periodic ripples structures (LSFL), micrometer-sized grooves, and spikes occur at room temperature as the number of pulses increases. At 400 °C, however, the grooves parallel to the polarization are the dominant structures, notwithstanding the presence of LSFL. As the temperature rises, the periodicities of LSFL increase, which we believe is due to a reduction in the oscillation of the surface plasmon polaritons due to the increased damping rate at higher temperatures. Furthermore, Raman spectra reveal that surface structures generated at 400 °C have higher crystallinity than those formed at 25 °C. Our simulations show that the better crystallinity at high temperatures is due to a slower resolidification velocity which is caused by a smaller temperature gradient and higher energy absorption. Our findings demonstrate that the features of femtosecond laser induced surface structures, such as periodicity and crystallinity, can be controlled by adjusting the substrate temperature simultaneously, paving the way for high crystallinity surface micro/nano-structures.

Europium spectroscopic properties impacted by femtosecond laser 3D nanostructuring in lithium niobium borosilicate glasses

We report in this article a study of luminescence properties of rare earth (herein Eu3+) embedded in oriented nanostructures photo-induced by femtosecond laser irradiation in a lithium niobium borosilicate glass. A systematic comparison is made between the Eu3+ luminescence properties in the irradiated area and the glass matrix, including the effect of Eu3+ concentration on luminescence spectra, lifetimes and absorption/emission cross sections. Luminescence intensity is enhanced in the irradiated area compared to the glass matrix, which can be proved useful in optical devices engineering.

Features of Femtosecond Laser Radiation Filamentation in Air under Aberration Focusing

Features of Femtosecond Laser Radiation Filamentation in Air under Aberration Focusing

Superhydrophobic and Antibacterial Hierarchical Surface Fabricated by Femtosecond Laser

Superhydrophobic surfaces are important in many applications owing to their special properties such as self-cleaning, anti-icing, antibacterial, and anti-fogging. In this paper, a micro/nano hierarchical superhydrophobic surface with a low roll-off angle was created on 304 stainless steel. The water contact angle was measured to be 152° with a roll-off angle of 7.3°. Firstly, microscale bumps were created by femtosecond laser irradiation. Secondly, zinc oxide (ZnO) nanowires were fabricated on the laser-induced bumps using a hydrothermal synthesis method. Results show that after laser treatment and ZnO nanostructuring, the stainless steel surface became superhydrophobic. However, the roll-off angle of this hierarchical structure surface was larger than 90°. To reduce the surface activity, trimethoxy silane hydrophobic coating was applied. A 7.3° roll-off angle was achieved on the coated surface. The underlying mechanism was discussed. The hydrophobic ZnO structured surface can help prevent bacterial contamination from water, which is important for implants. Thus, for biomedical applications, the antibacterial property of this hierarchical surface was examined. It was found that the antibacterial property of sample surfaces with ZnO nanowires were significantly increased. The optical density (OD) of Escherichia coli (E. coli) attached to the original surface was 0.93. For the micro-structured surface (with bumps), the OD was 0.9, and for the hierarchical surface (with bump &amp; nanowires), it was 0.54. For nanostructured ZnO nanowire surface, the OD was only 0.09. It demonstrates good antibacterial properties of ZnO nanowires.

Molybdenum Blues with Tunable Light Absorption Synthesized by Femtosecond Laser Irradiation of Molybdenum Trioxide in Water/Ethanol Mixtures

AbstractMolybdenum blues (MBs) have attracted increasing attention due to their tunable structures and properties, which make them applicable in numerous fields. In this work, a novel strategy to synthesize MB nanorings by irradiating MoO3 suspended in water/ethanol mixtures with intense femtosecond laser pulses is demonstrated. It is found that the MoO3 can be dissolved in the water during laser irradiation to form molybdic acid, which provides an acidic environment for the formation of MB. Concentrations of ethanol as low as 1% result in the formation of MB and by adjusting the concentration of ethanol in the solvent, the absorption band can be tuned due to the modification of MoVOMoVI entities. The MB synthesized in 30% ethanol appears the darkest blue and assembly of vesicles 115 nm in size is observed. At high concentrations of ethanol (&gt;70%), HXMoO3 and MoO3‐X are preferentially formed instead of MB. The photothermal conversion efficiency of the MBs synthesized in 1% and 30% ethanol is above 40%.