Femtosecond Irradiation Research Articles

Fabrication of ZnSe Microlens Array for a Wide Infrared Spectral Region

The infrared (IR) is a strategically important band for numerous applications from environment to biochemical imaging. Here a novel IR optical component integrates more than 2000 plano-concave microlenses on the zinc selenide (ZnSe) surface within a footprint of 5×5mm 2 is proposed by a combined method of femtosecond laser direct irradiation and chemical wet etching. Microlenses with diameter from 15 to 30 μm are realized by controlling the laser power and etching time. The uniform close-packed microlens arrays (MLAs) with minimized volume and high transparency at wavelengths of 0.76~22μm are realized. The MLAs have relatively good topography and surface quality. The modulation transfer function (MTF), imaging test and optical-focusing test confirm the outstanding IR imaging. Owing to the merits of micro-size, light weight, high integration and excellent IR imaging performance, the as-fabricated MLA is promising in cutting-edge mid-IR and far-IR application, such as IR sensing system, imaging system, and so on.

Shear bond strength of a self-adhesive resin cement to dentin surface treated with Nd:YAG and femtosecond lasers.

This study aims to evaluate the effect of Nd:YAG and femtosecond lasers irradiation on the shear bond strength (SBS) of a self-adhesive resin cement to the human dentin surface. One hundred extracted third molar teeth were randomly divided into 10 experimental groups according to dentin surface treatments; with and without the bonding agent, Nd:YAG 302J/cm2 and 440J/cm2, femtosecond 4J/cm2 and 7J/cm2, and control groups were prepared. After surface treatments, a self-adhesive resin cement was luted by using a bonding jig (Ultradent Products Inc.). The specimens were then subjected to shear test at a crosshead speed of 0.5mm/min, and failure loads were recorded as megapascal (MPa). Two-way analysis of variance and Tukey HSD tests were performed (p˂0.05). Representative specimens from each experimental subgroup were examined by means of SEM. The highest SBS values were obtained in Group 302J/cm2 Nd:YAG with bonding agent, and there is no statistical difference between Group 440J/cm2 Nd:YAG with bonding and Group 7J/cm2 femtosecond with bonding (p > 0.05). The lowest SBS values were observed in Group control without bonding agent. Nd:YAG and femtosecond laser treatments improved the adhesion between the dentin surface and the self-adhesive resin cement.

Quantitative investigation of laser-induced damage fatigue in HfO2 and ZrO2 single layer coatings.

The decrease of laser-induced damage threshold (LIDT) of optical materials when irradiated with multiple laser pulses is an important phenomenon commonly known as the optical fatigue effect. In case of pulsed femtosecond irradiation fatigue is usually attributed to incubation of laser-induced lattice defects. In this study, standard S-on-1 LIDT test was complimented with in situ time-resolved digital holographic microscopy (TRDHM) to quantitatively investigate fatigue of catastrophic damage for HfO2 and ZrO2 single layer ion-beam-sputtered optical coatings. It was identified that ablation (critical damage) was preceded by exponential increase in optical path length visible as positive phase shift (subcritical damage). Atomic force microscopy was used to show that physical damage originates as localized 100 nm wide nanogrooves perpendicular to laser polarization. A novel link was established between LIDT fatigue and mechanical fatigue crack growth from cyclic loads which allowed construction of a unified numerical fatigue model that reproduced both S-on-1 and TRDHM experimental data.

Two-level ablation and damage morphology of Ru films under femtosecond extreme UV irradiation

The dynamics of a thin ruthenium film irradiated by femtosecond extreme UV laser pulses is studied with a hybrid computational approach, which includes Monte Carlo, two-temperature hydrodynamics and molecular dynamics models. This approach is capable of accurate simulations of all stages of material evolution induced by extreme UV or X-ray photons: from nonequilibrium electron kinetics till complete lattice relaxation. We found that fast energy deposition in a subsurface layer leads to a two-level ablation: the top thin layer is ablated as a gas–liquid mixture due to expansion of overheated material at near and above critical conditions, whereas a thicker liquid layer below is ablated via a cavitation process. The latter occurs due to a thermo-mechanically induced tensile pressure wave. The liquid ablating layer exhibits unstable behaviour and disintegrates into droplets soon after detachment from the rest of the target. Our simulations reveal basic processes leading to formation of specific surface morphologies outside and inside the damage craters. The calculated ablation threshold, crater depth and morphological features are in quantitative agreement with the experimental data, which justifies the applicability of our hybrid model to study laser-induced material damage.

Spectroscopy of Rubidium Atoms in a Femtosecond Pulsed Optical Dipole Trap

The spectral properties of atoms localized in an optical dipole trap formed by femtosecond pulsed radiation are studied experimentally. It is shown that the shift of the absorption line of an atom due to the interaction of an atom with the field of localizing radiation coincides with the shift of the line of an atom localized in the field of cw laser radiation. Theoretical analysis shows that pulsed radiation with experimentally achievable average intensity and duration of a pulse makes it possible to implement an atom localization mode in which the line shift is absent.

Femtosecond laser irradiation of titanium oxide thin films: accumulation effect under IR beam

This paper discusses the mechanisms of laser-induced periodic surface structures (LIPSS) formation using a high repetition rate femtosecond laser beam irradiation of magnetron-sputtered titanium oxide thin films (TiO1.8) grown onto SiO2/Si substrates. An Yb:YKW 500 fs linearly polarized laser emitting at a wavelength, λ, of 1030 nm, was used to irradiate the films (300 nm thickness) at a repetition rate of 100 kHz under both static and dynamic (scanning) conditions. Under static beam conditions, an incubation behavior related to materials in thin film form was established with a damage threshold of 72 mJ/cm2. Close to this fluence value and increasing the number of laser shots from 1 to 1000, micro-cracking occurred and propagated inside the beam waist diameter zone estimated close to 60 µm. In addition, using a higher fluence value of 280 mJ/cm2, i.e., well above the damage threshold, a melting occurred in an intermediate zone within the irradiated area, with a surprising ‘cure effect’ that contributes to the micro-cracks stabilization. Simultaneously, at the center of the Gaussian laser beam spot, the entire film ablation was observed. Furthermore, irradiation under dynamic mode with a scanning speed of 4 mm/s and a repetition rate of 100 kHz were achieved for the large-scale processing of the TiO1.8 films up to surface area of 25 × 25 mm2. For these irradiation conditions case that correspond to a fluence of 110 mJ/cm2 and a cumulative number of shots of 3000, 2D-LIPSS nano-cracks (200 nm length and λ/8 to λ/9 period) are obtained over the whole irradiated surface, a phenomenon that is mainly attributed to a thermo-mechanical ablation mechanism.

Improvement of crystallization in CVD diamond coating induced by femtosecond laser irradiation

Herein, structural modification of a chemical vapor deposit (CVD) diamond film by a low fluence femtosecond laser irradiation has been studied with Raman spectroscopy, X-ray adsorption fine structure (XAFS) spectroscopy, and X-ray diffraction (XRD). Polycrystalline CVD diamond films of approximately 20 μm thickness were irradiated with femtosecond laser of varied defocus distances corresponding to the laser fluence range of 0.1–127.3 J/cm2. At fluences below 1.27 J/cm2, although there was no observable change in the appearance of the film surface, however, the FWHM of the diamond peak in the Raman spectra was observed to reduce considerably from 17 to 7 cm−1. Furthermore, the higher fraction of diamond phase with the increase in laser fluence strongly suggests that the microstructure of the diamond film has been modified by the femtosecond laser. By employing XAFS measurement, it can be found that after irradiation, the weak π* fine structure in the pre-edge almost vanished in response to the significant increase in σ* adsorption; further, a distinct diamond crystal characteristic peak was observed, which further confirms the femtosecond laser-induced modification, and its occurrence inside the bulk film. XRD results also pointed out that the improved diamond crystallinity with the integral intensities increased by approximately 20% after the femtosecond irradiation. Based on the above-mentioned results, an improvement in the crystallinity of CVD diamond under the low-fluence femtosecond laser irradiation has been confirmed in this study; this is attributed to a photo-induced phase transition from amorphous carbon to diamond.

Copper, gold, and platinum under femtosecond irradiation: Results of first-principles calculations

The paper investigates the interaction of femtosecond laser pulses with the thin films of copper, gold, and platinum. It considers electron-phonon relaxation processes and melting in the metal system nonequilibrium heated by laser radiation. Instead of the approximated formula by Wang et al. [Phys. Rev. B 50, 8016 (1994)], which is widely used to determine the temperature dependence of the electron-phonon coupling factor, we propose an improved approach for its more accurate calculation from first principles. Comparison with experiments and other calculations shows our approach to provide good calculation accuracy. Melting time versus absorbed energy density was estimated for the films and shown to be markedly sensitive to latent heat at low absorbed energy densities $(l1$ MJ/kg). Our calculations taken to study the temporal evolution of the (220) diffraction peak intensity after femtosecond irradiation show good agreement between experimental and theoretical data, which was attained due to higher accuracy in our determination of the temperature dependence electron-phonon coupling factor.

Enhanced optical emission in laser-induced breakdown spectroscopy by combining femtosecond and nanosecond laser pulses

The role of inter-pulse delay time in laser-induced Cu plasma with femtosecond and nanosecond double-pulse irradiation had been investigated by optical emission spectroscopy. The experimental results showed that a significant enhancement effect in the atomic line emission was observed by changing the inter-pulse delay time. The spectral intensity of Cu plasma produced by a femtosecond + nanosecond double-pulse was higher than that produced by a nanosecond + femtosecond dual-pulse. That is to say, the shorter laser pulse (femtosecond) was efficient in coupling to the sample and the longer laser pulse (nanosecond) was suitable for re-exciting the produced plasma. In addition, the corresponding plasma temperature and electron density were also calculated. The plasma temperature in the case of femtosecond + nanosecond was lower than that in the case of nanosecond + femtosecond, while the electron density in the case of femtosecond + nanosecond was higher than that in the case of nanosecond + femtosecond. This study is helpful for the research on sample detection using femtosecond and nanosecond double-pulse laser-induced breakdown spectroscopy.

Influences of nanosecond pulse pre-irradiation on femtosecond laser damage resistance of gold pulse compression grating

The gold pulse compression gratings (PCG) used in the Bidirectional Grating Compressor (BGC) suffers a nano- and femtosecond dual pulses irradiation. In the case that the ns pre-pulse comes 10 nanoseconds early before the fs pulse, experiments were conducted to investigate damage behaviors under the dual pulse irradiation. 1-on-1 laser induced damage threshold (LIDT) of the ns–fs dual pulses and fs single pulse were tested and compared. Scanning Electron Microscope (SEM) was employed to capture damage onset and exam the damage macro- and micro-morphologies. The mono-shot method, comparison between the damage zone and spatial distribution of laser pulses, was employed to analyze the difference of the damage onset fluences for the two cases thoroughly. It was found that there is no degradation of laser damage resistance due to the ns pre-pulse. Instead, a slightly increase (∼10%) of fs LIDT caused by the ns pre-pulse was found.

Photocontrolled Release of the Anticancer Drug Chlorambucil with Caged Harmonic Nanoparticles

AbstractWhile chemotherapy is one of the most used treatments in oncology, the systemic administration of chemotherapeutics generally results in undesired damages to healthy tissues and cells, side effects such as severe nausea and leukopenia, and reduced efficacy due to multidrug resistance and poor target accessibility. The limitations of conventional chemotherapy formulation have prompted the development of alternative nanomaterials‐based strategies to achieve targeted and stimuli sensitive payload delivery to reach optimal local drug concentration at tumor sites. In this study, the anticancer drug chlorambucil (Clb) was conjugated to the surface of silica coated lithium niobate (LNO) harmonic nanoparticles (HNPs) using a photocaging tether based on coumarin‐4‐yl methyl derivative. Upon laser pulsed femtosecond irradiation at 790 nm, the second harmonic emission from the metal oxide core induced the efficient release of Clb, with concomitant contribution from the nonlinear absorption of the coumarin (CM)‐based moiety.

Non-thermal fluence threshold for femtosecond pulsed x-ray radiation damage in perovskite complex oxide epitaxial heterostructures

Intense hard x-ray pulses from a free-electron laser induce irreversible structural damage in a perovskite oxide epitaxial heterostructure when pulse fluences exceed a threshold value. The intensity of x-ray diffraction from a 25-nm thick epitaxial BiFeO3 layer on a SrTiO3 (STO) substrate measured using a series of pulses decreases abruptly with a per-pulse fluence of 2.7 × 106 photons μm−2 at a photon energy of 9.7 keV but remains constant for 1.3 × 106 photons μm−2 or less. The damage resulted in the destruction of the BiFeO3 thin film within the focal spot area and the formation of a deep cavity penetrating into the STO substrate via the removal of tens of nanometers of material per pulse. The damage threshold occurs at a fluence that is insufficient to heat the absorption volume to the melting point. The morphology of the ablated sample is consistent with fracture rather than melting. Together, these results indicate that the damage occurs via a nonthermal process consistent with ultrafast ionization of the absorption volume.

Target properties – Plasma dynamics relationship in laser ablation of metals: Common trends for fs, ps and ns irradiation regimes

Better understanding of complex relationships between laser-produced plasma dynamics and ablation target material properties is key for optimization of numerous technological and analytical applications. Continuing a previous work [Irimiciuc et al., Appl. Surf. Sci. 417 (2017) 108–118] which proposed empirical laws for some of these relationships (in femtosecond irradiation regime), we present here a systematic study on several metallic targets (Al, Cu, Mn, Ni, Ti, Zn) irradiated by nanosecond, picosecond and femtosecond laser pulses. The experimental approach is based on complementary diagnostics, with the implementation of both electrical and optical techniques for transient plasma characterization. The empirical equations previously reported for femtosecond ablation pulses are now extended to the picosecond and nanosecond regimes.

Optical Activity Effect on Planar Chiral Metamaterials

Metamaterial is an artificial material which has a negative refractive index. Both of permitivitty and permeability have a negative value to possess negative refraction. The breakthrough of metamaterial development was present as the invention of planar chiral metamaterials. This research was focussed on the development of a new design of planar chiral metamaterial with different depth. The developed metamaterial design was analyzed by chirality analysis in relation to the optical activity characteristics.The planar chiral metamaterial development was following a design, a structure examination, and a fabrication process using a focused ion beam system. Chirality characterization were measured using a femtosecond Ti:Sapphire pumped terahertz emission spectroscopy system. THz signal was occurred due to the optical rectification process as a result of an intense femtosecond laser pulses radiation on planar chiral metamaterial sample. The samples were capable to generate a THz emission over a frequency range of 0 − 2.24 THz, while the THz signal was measured as ∼5 ps. The effect of optical activity was able to rotate the polarization plane on a frequency range of 0 – 1.4 THz for both of the clockwise and the counter-clockwise samples. The maximum polarization plane rotation were +13° and −18° at a frequency 1.35 THz. It can be concluded that the new design planar chiral metamaterial with different depth has been successfully obtained with its elliptic polarized THz emission characteristic.

Temporal field and frequency spectrum of intense femtosecond radiation dynamics in the process of plasma formation in a dielectric medium

We consider the propagation of intense femtosecond pulses in a dielectric medium with an induced plasma. This plasma is excited by the strong electric field of the pulses, stimulating sequential electron transition to the conductivity band through excited discrete energy states. It is demonstrated that this simple dynamic model of ultrashort optical pulse propagation in a dielectric medium with self-induced plasma can be applied to experiments on the superbroadening of the pulse spectrum into the long-wavelength range, including terahertz generation, as well as the generation of high-order harmonics.

Micro-structuring, ablation, and defect generation in graphene with femtosecond pulses

Femtosecond micromachining offers a contact-free and mask-less technique for material patterning. Femtosecond pulse irradiation of single layer graphene by low energy pulses around 12 nJ over a 15 second illumination period is studied. The ablated graphene holes are surrounded by a radially symmetric region characterized by generated defects, whose extension can be modified based on the laser fluence. The femtosecond induced structural modifications can be attractive for graphene or carbon-based device fabrication as well as sensor and transistor applications, where regions of varying carrier concentrations and different electrical, optical, or physical properties are desired.

Ultrafast Broadband Diagnostics of the Filling of the s Band at the Two-Photon Femtosecond Laser Excitation of a Gold Film

The transmission of near infrared femtosecond laser pulses (wavelength of 800 nm) and supercontinuum radiation (300–750 nm) generated by them through a 50-nm gold film immersed in water has been experimentally studied at the power of focused laser radiation in the range of 1–10 GW. An increase in the excitation intensity is accompanied by the blueshift of the edge of the extinction band of the film, which is attributed to the filling of electronic states of the s band with a low density of states caused by two-photon interband transitions from d bands with a high density of states.

Ag Nanoparticles/\u03b1-Ag2WO4 Composite Formed by Electron Beam and Femtosecond Irradiation as Potent Antifungal and Antitumor Agents

The ability to manipulate the structure and function of promising systems via external stimuli is emerging with the development of reconfigurable and programmable multifunctional materials. Increasing antifungal and antitumor activity requires novel, effective treatments to be diligently sought. In this work, the synthesis, characterization, and in vitro biological screening of pure α-Ag2WO4, irradiated with electrons and with non-focused and focused femtosecond laser beams are reported. We demonstrate, for the first time, that Ag nanoparticles/α-Ag2WO4 composite displays potent antifungal and antitumor activity. This composite had an extreme low inhibition concentration against Candida albicans, cause the modulation of α-Ag2WO4 perform the fungicidal activity more efficient. For tumor activity, it was found that the composite showed a high selectivity against the cancer cells (MB49), thus depleting the populations of cancer cells by necrosis and apoptosis, without the healthy cells (BALB/3T3) being affected.

Free radical production by femtosecond laser lens irradiation in porcine eyes

To investigate the relationship between the generation of free radicals and femtosecond laser lens irradiation. AMO Laboratory, Tokyo, Japan. Exvivo studies. Hydroxyphenyl fluorescein (HPF) was injected into the anterior chamber of fresh 6-month-old porcine cadaver eyes (N=31). After laser irradiation, a plate reader was used to measure the fluorescence of the aqueous humor samples. Relative fluorescence units (RFU) were calculated by subtracting the average control value from the measured values of all the samples. Experiment 1: After determining the RFU in the 7 laser-irradiated eyes, the relationship with the amount of laser energy was then assessed. Experiment 2: To clarify the issue regarding the degree of attenuation of the fluorescence intensity, HPF was simultaneously injected into 2 eyes, with 1 eye irradiated and the other eye used as a control. After dividing the RFU of the laser-irradiated eye by the control RFU, the laser irradiation-control RFU ratio was calculated, and the relationship with the laser irradiation energy amount then assessed. There was a significant correlation between the femtosecond laser lens irradiation energy and the RFU in the aqueous humor (P<.05, analysis of variance). Femtosecond laser lens irradiation increases the quantity of free radicals in the aqueous humor, with the extent of the increase dependent on the amount of laser energy. These results suggest that excessive laser irradiation during femtosecond laser-assisted cataract surgery might cause corneal endothelial damage because of the free radicals created during the procedure.

Polymorph-Controlled Crystallization of Acetaminophen through Femtosecond Laser Irradiation

Polymorph-controlled crystallization of acetaminophen was conducted through femtosecond laser double-pulse irradiation. Dependence of the crystal form of acetaminophen on the pulse energy and doubl...