Ultraviolet Femtosecond Research Articles

Characterization of Micro-Holes Drilled Using a UV Femtosecond Laser in Modified Polyimide Flexible Circuit Boards.

Modified polyimide (MPI) flexible printed circuits (FPCs) are used as chip carrier boards. The quality of the FPC directly affects the reliability of the integrated circuit. Furthermore, micro-holes are critical components of FPCs. In this study, an ultraviolet (UV) femtosecond laser is used to drill micro-holes in double-layer flexible circuit boards with MPI as the substrate. The morphology of the micro-hole wall in the copper foil and MPI layer is observed, and the effects of the laser processing parameters on the diameter and depth of the micro-holes are analyzed. The drilling process and mechanism of micro-holes obtained using a UV femtosecond laser in MPI FPCs are discussed. The results show that the morphology of femtosecond laser-machined copper is closely related to the laser energy, and a periodic structure is observed during the machining process. Copper, MPI, and copper oxides are the most common molten deposits in micro-holes during drilling. The depth of the micro-holes increases with an increase in the energy of a single pulse, scanning time, and scanning overlap rate of the laser beam. However, the diameter exhibits no discernible alteration. The material removal rate increased significantly when laser processing was applied to the MPI resin layer.

Evaluation of femtosecond-LA-ICP-TOFMS for multi-elemental mapping at cellular resolution of human-tissue from cancer patients

The characterization of the multi-elemental distributions within a thin tissue section of a liver metastasis from colorectal cancer patient after oxaliplatin-based chemotherapy, is critically evaluated using Ultraviolet Femtosecond Laser Ablation Inductively Coupled Plasma-Time of Flight Mass Spectrometry (UV-fs-LA-ICP-TOFMS). Different femtosecond laser ablation conditions, in terms of spot size and energy per pulse, were investigated to achieve proper ablation of the formalin-fixed paraffin-embedded tumor sections, while also trying to minimize the ablation of the glass substrate. Moreover, histological images of the tissues before and after the ablation processes were combined with UV-fs-LA-ICP-TOFMS elemental mapping to achieve multi-elemental spatial distributions at cellular level. For instance, spatial distributions of endogenous 31P, 56Fe, 63Cu and 64Zn analytes were determined within the thin tissue section. Furthermore, spatial distribution of elements such as 24Mg, 28Si, and 40Ca were employed to study glass substrate ablation during tissue analysis. Additional analytes, which might be partially associated to compounds employed on hematoxylin and eosin tissue staining processes, such as 23Na, 27Al, 32S, 39K, 79Br and 127I, were also detected. Finally, the accumulation of the oxaliplatin-based chemotherapy products in the stroma was confirmed from the spatial distribution of 196Pt ion signals within the tissue.

Evaluation of Ultraviolet-Femtosecond-Laser Ablation-ICP-TOFMS for fast, sensitive and high-spatially resolved multi-elemental mapping of laser-deposited Hastelloy coated carbon steel

Ultraviolet Femtosecond Laser Ablation-Inductively Coupled Plasma-Time of Flight Mass Spectrometry (UV-fs-LA-ICP-TOFMS) is critically evaluated for fast multi-elemental mapping of Hastelloy coated carbon steel that was immersed in a liquid corrosive environment for two months. The Hastelloy coating, which is a multi-elemental Cr-Fe-Ni-Mo-W matrix, is produced using laser cladding technology. A UV-fs laser is employed to reduce the ablation thermal effects improving the spatial resolution in the elemental analysis of the coating, interface and substrate. Moreover, the TOFMS is equipped with ion beam attenuation grids to expand its linear dynamic range, allowing the determination of major, minor and trace elements. In particular, spatial distribution of major (e.g., Co, Ni, Fe), minor (e.g., Mn, Si) and trace (e.g., Cu, In, Zn) elements from two sequential mapping analysis were determined. Quantification is achieved through Relative Sensitivity Factors calculated using matrix-matched external reference materials and making use of measurements carried out by SEM-EDX to determine the concentration of internal standard elements. Qualitative and quantitative elemental maps highlight diffusion processes at the coating-substrate interface. Moreover, correlated spatial distributions of ion signals from external corrosive elements (e.g., 32S, 138Ba, 202Hg, 205Tl, and 209Bi) were detected in localized regions within the material and on the external surface.

Effect of Femtosecond Ultraviolet and Infrared Laser Wavelength on Plasma Characteristics of Metals, Ceramics and Glass Samples Using Femtosecond Laser-Induced Breakdown Spectroscopy.

In this work, we present studies on the effect of laser wavelengths on the laser-induced plasma characterization using a femtosecond (fs) ytterbium-doped potassium-gadolinium tungstate (Yb:KGW) laser. Plasma plumes of copper, steel, ceramics, and glass samples were induced using a multiple shot of 200 fs laser pulses with 1030 nm and 343 nm wavelengths at fixed laser fluence () and analyzed using the laser-induced breakdown spectroscopy (LIBS) technique. Time-resolved fs-LIBS measurements were performed on the same set of samples and under the same experimental conditions. For the calculation of plasma parameters, the set of spectral lines of Cu(I) (for copper sample), Fe(I) (for steel sample), and Ca(I), K(I) (for glass and ceramics samples) were observed. The plasma electron temperature and density were evaluated from the Boltzmann plots and Stark-broadening profiles of the plasma spectral lines, assuming the local thermodynamic equilibrium condition. Time-resolved plasma temperature and electron density for fs-LIBS using ultraviolet (UV) and infrared (IR) laser wavelengths were analyzed and no significant dependence on fs laser wavelength was observed for any of the samples. However, for all samples the signal-to-noise ratio (SNR) significantly increased using UV laser radiation: copper (), steel (), glass (), and ceramics (). Therefore, by using a fs UV laser wavelength for laser-induced breakdown spectroscopy experiments, for certain materials the SNR and at the same time the limit of detection can be significantly enhanced.

Maximizing the Response of a Helium Atom at the Third Harmonic of an Intense Femtosecond Ultraviolet Pulse

The optimal regime of three-photon resonant excitation of a helium atom via a femtosecond ultraviolet (UV) pulse was discovered and numerically studied, at which the maximum power of the third harmonic of the UV field is achieved in the spectrum of dipole acceleration (the second time derivative of the induced dipole moment) of the atom. It is shown that the optimal frequency of the UV field nearly coincides with the frequency of the three-photon transition |1s2⟩–|1s2p⟩, taking into account its shift as a result of the dynamic Stark effect, and the intensity of the UV field is dictated by the condition of maximizing the product of the populations of the |1s2⟩ and |1s2p⟩ states, averaged over the time interval during which the UV field is non-zero. For the considered UV field durations, from 10 to 100 cycles of the carrier frequency (from units to tens of femtoseconds), the optimal intensity lies in the range from 1014 W/cm2 to several units of 1014 W/cm2. It is shown that with an optimal choice of the frequency and intensity of the UV field, the dynamics of excitation of bound and continuum states, as well as the shape of the time envelope of the dipole acceleration of the atom, weakly depend on the duration of the UV field envelope; only their time scale changes significantly. In addition, under optimal conditions, the average power of the third harmonic signal in the dipole acceleration spectrum is practically independent of the duration of the UV field envelope.

Nonlinear Response of Diluted Gases to an Ultraviolet Femtosecond Pulse

Quantum-mechanical simulations of the nonlinear response of a one-dimensional quantum system with the energy structure close to that of the xenon atom to an ultraviolet femtosecond pulse with an intensity of 1–100 TW/cm2 reveal the dispersion of the cubic nonlinearity coefficient in the range of 266–400 nm and its intensity dependence. This excludes the description of the response of bound electrons as {{chi }^{{(3)}}}{{E}^{3}}. The calculation of the polarization with this one-dimensional quantum model can be used to simulate the propagation of ultraviolet femtosecond radiation in a gas.

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.

In-Situ Imaging of a Light-Induced Modification Process in Organo-Silica Films via Time-Domain Brillouin Scattering.

We applied time-domain Brillouin scattering (TDBS) for the characterization of porogen-based organosilicate glass (OGS) films deposited by spin-on-glass technology and cured under different conditions. Although the chemical composition and porosity measured by Fourier-transform infrared (FTIR) spectroscopy and ellipsometric porosimetry (EP) did not show significant differences between the films, remarkable differences between them were revealed by the temporal evolution of the Brillouin frequency (BF) shift of the probe light in the TDBS. The observed modification of the BF was a signature of the light-induced modification of the films in the process of the TDBS experiments. It correlated to the different amount of carbon residue in the samples, the use of ultraviolet (UV) femtosecond probe laser pulses in our optical setup, and their intensity. In fact, probe radiation with an optical wavelength of 356 nm appeared to be effective in removing carbon residue through single-photon absorption processes, while its two-photon absorption might have led to the breaking of Si-CH3 bonds in the OSG matrix. The quantum chemical calculations confirmed the latter possibility. This discovery demonstrates the possibility of local modifications of OSG films with a nanometric resolution via nonlinear optical processes, which could be important, among other applications, for the creation of active surface sites in the area-selective deposition of atomic layers.

Dynamics of Photoreactions in (CF3I)nXem Mixed Clusters Exposed to Femtosecond Ultraviolet Radiation Inducing Multiphoton Ionization

Dynamics of Photoreactions in (CF3I)nXem Mixed Clusters Exposed to Femtosecond Ultraviolet Radiation Inducing Multiphoton Ionization

High power green picosecond laser and high efficiency ultraviolet femtosecond laser through second harmonic generation using K3B6O10Br crystal

We demonstrated high power green picosecond (ps) laser and high efficiency ultraviolet (UV) femtosecond (fs) laser through second harmonic generation (SHG) using type I phase-matched K3B6O10Br (KBOB) nonlinear optical crystals. The green ps laser was generated by using a homemade high power 1064 nm ps laser as the pump source. When the pump power was 28.2 W with a repetition rate of 800 kHz, 14.2 W ps green laser was obtained with the maximum optical conversion efficiency of 50.4%. When the pump power was 85.2 W with a repetition rate of 800 kHz, the maximum output power of 41.8 W of ps green laser was achieved with an optical conversion efficiency of 49.1%. During the UV fs laser generation, a 710 nm fs laser with a repetition rate of 1 kHz was used to pump the KBOB crystal. When the pump power of 710 nm laser was 54 mW, the 355 nm UV fs laser was obtained with output power of 18 mW and optical conversion efficiency of 33.3%. The experimental results show that the KBOB crystal is a highly qualified candidate for the application in generating high power ultrafast green and UV lasers.

Phase-Type Fresnel Zone Plate with Multi-Wavelength Imaging Embedded in Fluoroaluminate Glass Fabricated via Ultraviolet Femtosecond Laser Lithography.

Herein, we report a novel optical glass material, fluoroaluminate (AlF3) glass, with excellent optical transmittance from ultraviolet to infrared wavelength ranges, which provides more options for application in optical devices. Based on its performance, the phase-type Fresnel zone plate (FZP) by ultraviolet femtosecond (fs) laser-inscribed lithography is achieved, which induces the refractive index change by fs-laser tailoring. The realization of ultraviolet fs-laser fabrication inside glass can benefit from the excellent optical performance of the AlF3 glass. Compared with traditional surface-etching micro-optical elements, the phase-type FZP based on AlF3 glass exhibits a clear and well-defined geometry and presents perfect environmental suitability without surface roughness problems. Additionally, optical focusing and multi-wavelength imaging can be easily obtained. Phase-type FZP embedded in AlF3 glass has great potential applications in the imaging and focusing in glass-integrated photonics, especially for the ultraviolet wavelength range.

Skeletal Structure of the Chromophore of Photoactive Yellow Protein in the Excited State Investigated by Ultraviolet Femtosecond Stimulated Raman Spectroscopy.

We studied ultrafast structural dynamics of photoactive yellow protein (PYP) using ultraviolet femtosecond stimulated Raman spectroscopy. By employing the Raman pump and probe pulses in the ultraviolet region, resonantly enhanced, rich vibrational features of the excited-state chromophore were observed in the fingerprint region. In contrast to the marked spectral change reported for the excited-state chromophore in solution, in the protein, all of the observed Raman bands in the fingerprint region did not show any noticeable spectral shifts nor band shape changes during the excited-state lifetime of PYP. This indicates that the significant skeletal change does not occur on the chromophore in the excited state of PYP and that the trans conformation is retained in its lifetime. Based on the femtosecond Raman data of PYP obtained so far, we discuss a comprehensive picture of the excited-state structural dynamics of PYP.

Improvement of absorption light of laser texturing on silicon surface for optoelectronic application

This work aim to achieving laser nano-textured Si structures by means of an ultra-fast ultra-violet femtosecond (fs) laser texturing technique. The evidences of the impact of the duration of irradiation of the femtosecond laser pulses on Silicon (Si) have been investigated experimentally. A Ti:Sapphire laser at 800 nm (130 fs pulse duration with 1 kHz rate of repetition) has been employed to create nano-textured surfaces in an n-type (111) Si wafer of 0.015 Ω.cm resistivity and 508 ± 5 μm thickness. The average sizes of diameters were estimated to be in the 25−45 nm. From PL measurements of Si nanostructures, size dependent shifts in the direction of the blue spectral region have been emphasized. A highly symmetrical band shape was recognized from the samples' Raman spectra after the irradiation, the phonon frequency (phonon peak shift) shifted below 520 cm−1. The surface nanostructured Si showed lower reflectance values compared with bulk Si (un-irradiated). The observed enhancement of the fabricated Si nanostructures broad spectral response, which is in the range between the visible to the near infrared (NIR) 400−1150 nm, was attributed to increasing light absorption within sub-nanostructures. Thus, the proposed technique could be a promising approach to enhance the performance of Si-based photo-detector.

Suppression of Fragmentation in Mass Spectrometry.

Suppressing fragmentation is a constant challenge in mass spectrometry because a molecular ion can readily be identified and provides information concerning the molecular weight of an analyte. Several techniques such as charge exchange chemical ionization (CECI) and vacuum ultraviolet emission ionization (VUVEI) have been developed to date for achieving this purpose. In this study, we report on the use of tunable ultraviolet (UV) and near-infrared (NIR) femtosecond (fs) lasers (35 fs) for the multiphoton ionization (MPI) of cis- and trans-4-methylcyclohexanols, the reference molecules that are currently used to examine fragmentation suppression. The results obtained here were compared with those obtained by CECI and VUVEI because they were reported as the best techniques for suppressing fragmentation. A molecular ion was strongly enhanced by carefully minimizing the excess energy in the ionic state using tunable UV and NIR fs-lasers. The ratio of the intensities for molecular and fragment ions, [M]+/[M-H2O]+, increased significantly (9.5-fold and 8.5-fold for cis- and trans-isomers, respectively, in UV fs-MPI) compared to the values obtained by CECI and VUVEI, respectively.

Surface Roughness and Crystallinity of Silicon Solar Cells Irradiated by Ultraviolet Femtosecond Laser Pulses

Surface Roughness and Crystallinity of Silicon Solar Cells Irradiated by Ultraviolet Femtosecond Laser Pulses

A 355 nm ultraviolet femtosecond laser through second harmonic generation using K3B6O10Br nonlinear optical crystal

We demonstrate the 355 nm ultraviolet (UV) femtosecond (fs) laser through second harmonic generation using a type-I phase-matched nonlinear optical crystal of K3B6O10Br (KBOB). The effects of the length of nonlinear optical crystals and the focal length of lenses on the output power and conversion efficiency were investigated systematically. The maximum output power of 265 mW UV fs laser at 355 nm was obtained with a pump of 1.408 W fs laser at 710 nm with a repetition rate of 80 MHz. The corresponding optical conversion efficiency was 18.8%. The experimental results show that KBOB crystal is a promising crystal in generating high-power UV fs laser because of its excellent nonlinear optical properties.

Numerical Investigation on the Influence of Water Vapor Ionization on the Dynamic and Energy Deposition of Femtosecond Ultraviolet Laser Filamentation in Air

The effects of water vapor ionization on the nonlinear propagation of femtosecond laser pulses with a 248 nm wavelength are numerically investigated in this paper. It is found that ionization of H2O molecules plays a significant role in air ionization, which seriously affects the dynamic and energy deposition of filamentation. The propagation of femtosecond pulses in air with different humidity levels are compared. The total number of electrons and total deposited pulse energy increase with the humidity increases. However, they tend to be saturated in high humidity conditions. Results presented here are conducive to characterizing the long-range propagation of filaments under atmospheric conditions.

Electronic and vibrational relaxation dynamics of NH3 Rydberg states probed by vacuum-ultraviolet time-resolved photoelectron imaging.

Time-resolved dynamics of high-lying Rydberg states of ammonia (NH3) prepared by using a vacuum ultraviolet (VUV) pump (∼9.3 eV) and an ultraviolet (UV) probe (∼4.7 eV) pulse are reported using photoelectron imaging detection. After photoexcitation, two main features appear in the photoelectron spectrum with vertical binding energies of ∼1.8 eV and ∼3.2 eV and with distinctly different anisotropy parameters β of ∼1.3 and ∼0.7, respectively. This information allows the unambiguous assignment of the respective Rydberg states and disentangles the induced electronic and vibrational dynamics. The combination of velocity-map imaging with femtosecond VUV and UV pulses is shown to offer an attractive approach for studying the dynamics of high-lying Rydberg states of small molecules.

Interface morphology and gas production by a refractive 347 nm ultraviolet femtosecond laser: Comparison with established laser systems

To evaluate gas production and interface quality after refractive lenticule creation. A new ultraviolet (UV) 347nm femtosecond laser was compared with the FS200 and the Visumax lasers. Department of Anatomy II, University of Erlangen-Nürnberg, Erlangen, Germany. Experimental study. Refractive lenticules were created in 52 porcine corneas with the UV laser (n=20), the Visumax laser (n=20), and a modified FS200 laser (n=12). Twelve corneas in each laser group were fixed immediately after lenticule creation to entrap the gas produced. Lenticule extraction was omitted. From 16 other corneas (UV and Visumax lasers; n=8 each) -5.0 diopter (D) and -10.0 D lenticules were extracted. On lenticule extraction, a flap was created and lifted. In 12 additional corneas, a flap was cut using the FS200 laser and lifted afterward; laser insitu keratomileusis was performed on 8 of these samples. The corneas were processed for light microscopy (gas bubble 2-dimensional morphometry) and scanning electron microscopy (interface quality). The Visumax generated approximately one half as much gas as the FS200. The UV laser undercut the Visumax by another 50%. The interfaces created by the FS200 appeared smooth and without irregularities. The optical zone margin was always conspicuous in the Visumax samples but hardly discernible in the UV specimens. Slight irregularities and very small lenticule pieces were found occasionally in both groups. The UV interfaces appeared slightly rougher than the Visumax interfaces. Ultraviolet laser lenticule extraction seems promising. The interface quality was similar to that of the clinical Visumax system while gas production was reduced significantly.

Angular streaking and sideband formation in rotating terahertz and far-infrared fields

Electron spectra in atomic ionization by extreme ultraviolet (XUV) or soft x-ray femtosecond pulses, in the presence of a strong circularly polarized far-infrared or terahertz (THz) field are theoretically investigated. When the XUV pulse duration is much shorter than the period of the THz field, the conditions for circular streaking are fulfilled. The peculiarities of the circular streaking are discussed. In the other extreme case when the duration of the XUV pulse is longer than the THz field period, an analog of the sidebands is formed. Special attention is paid to the intermediate case when the duration of the XUV pulse is similar to the THz period. In this case, a well-developed interference structure appears in the photoelectron spectra. A quasiclassical description of this interference structure is presented and compared with numerical calculations based on a quantum-mechanical approach.