The unique characteristics of ultrafast lasers, such as picosecond and femtosecond lasers, have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. This review describes the characteristics of ultrafast laser processing and the recent advancements and applications of both surface and volume processing. Surface processing includes micromachining, micro- and nanostructuring, and nanoablation, while volume processing includes two-photon polymerization and three-dimensional (3D) processing within transparent materials. Commercial and industrial applications of ultrafast laser processing are also introduced, and a summary of the technology with future outlooks are also given. Scientists in Asia have reviewed the role of ultrafast lasers in materials processing. Koji Sugioka from RIKEN in Japan and Ya Cheng from the Shanghai Institute of Optics and Fine Mechanics in China describe how femtosecond and picosecond lasers can be used to perform useful tasks in both surface and volume processing. Such lasers can cut, drill and ablate a variety of materials with high precision, including metals, semiconductors, ceramics and glasses. They can also polymerize organic materials that contain a suitable photosensitizer and can three-dimensionally process inside transparent materials such as glass, and are already being used to fabricate medical stents, repair photomasks, drill ink-jet nozzles and pattern solar cells. The researchers also explain the characteristics of such lasers and the interaction of ultrashort, intense pulses of light with matter.

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Ultrafast lasers—reliable tools for advanced materials processing

We show that short-pulse laser-induced classical ripples on dielectrics, semiconductors, and conductors exhibit a prominent "non-classical" characteristic-in normal incidence the periods are definitely smaller than laser wavelengths, which indicates that the simplified scattering model should be revised. Taking into account the surface plasmons (SPs), we consider that the ripples result from the initial direct SP-laser interference and the subsequent grating-assisted SP-laser coupling. With the model, the period-decreasing phenomenon originates in the admixture of the field-distribution effect and the grating-coupling effect. Further, we propose an approach for obtaining the dielectric constant, electron density, and electron collision time of the high-excited surface. With the derived parameters, the numerical simulations are in good agreement with the experimental results. On the other hand, our results confirm that the surface irradiated by short-pulse laser with damage-threshold fluence should behave metallic, no matter for metal, semiconductor, or dielectric, and the short-pulse laser-induced subwavelength structures should be ascribed to a phenomenon of nano-optics.

Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident laser

The formation of laser-induced periodic surface structures (LIPSS) in different materials (metals, semiconductors, and dielectrics) upon irradiation with linearly polarized fs-laser pulses (τ ∼ 30–150 fs, λ ∼ 800 nm) in air environment is studied experimentally and theoretically. In metals, predominantly low-spatial-frequency-LIPSS with periods close to the laser wavelength λ are observed perpendicular to the polarization. Under specific irradiation conditions, high-spatial-frequency-LIPSS with sub-100-nm spatial periods (∼λ/10) can be generated. For semiconductors, the impact of transient changes of the optical properties to the LIPSS periods is analyzed theoretically and experimentally. In dielectrics, the importance of transient excitation stages in the LIPSS formation is demonstrated experimentally using (multiple) double-fs-laser-pulse irradiation sequences. A characteristic decrease of the LIPSS periods is observed for double-pulse delays of less than 2 ps.

Femtosecond laser-induced periodic surface structures

Laser-induced periodic surface structures (LIPSS, ripples) are a universal phenomenon and can be generated on almost any material upon irradiation with linearly polarized radiation. With the availability of ultrashort laser pulses, LIPSS have gained an increasing attraction during the past decade, since these structures can be generated in a simple single-step process, which allows a surface nanostructuring for tailoring optical, mechanical, and chemical surface properties. In this study, the current state in the field of LIPSS is reviewed. Their formation mechanisms are analyzed in ultrafast time-resolved scattering, diffraction, and polarization constrained double-pulse experiments. These experiments allow us to address the question whether the LIPSS are seeded via ultrafast energy deposition mechanisms acting during the absorption of optical radiation or via self-organization after the irradiation process. Relevant control parameters of LIPSS are identified, and technological applications featuring surface functionalization in the fields of optics, fluidics, medicine, and tribology are discussed.

Laser-Induced Periodic Surface Structures— A Scientific Evergreen

Laser-induced periodic surface structures (LIPSS) (ripples) with different spatial characteristics have been observed after irradiation of single-crystalline zinc oxide surfaces with multiple linearly polarized femtosecond pulses (150–200 fs, 800 nm) in air. For normal incident laser radiation, low spatial frequency LIPSS (LSFL) with a period (630–730 nm) close to the wavelength and an orientation perpendicular to the laser polarization have been found in the fluence range between ∼0.7 and ∼0.8 J/cm2 and predominantly for pulse numbers up to N=100. For lower fluences (0.5–0.7 J/cm2), a sharp transition from the LSFL features toward the formation of high spatial frequency LIPSS (HSFL) appears at any given pulse number below N=100. The HSFL are always parallel to the LSFL, exhibit spatial periods between 200 and 280 nm, and completely substitute the LSFL for pulse numbers N>100. Additionally, the influence of the angle of incidence has been studied experimentally for both LIPSS types revealing a different b...

Femtosecond laser-induced periodic surface structures revisited: A comparative study on ZnO

We report the laser-induced periodic surface structure (LIPSS) with periodicity about a quarter of the laser wavelength on unpolished diamond film treated by a P-polarized femtosecond laser. The short period LIPSS is parallel to the laser polarization and independent on the incidence angle. The LIPSS perpendicular to the laser polarization with periodicity shorter than a third of the laser wavelength slightly dependent on the incidence angle is also observed as well as the LIPSS perpendicular to the laser polarization with periodicity dependent on the incidence angle. The results are explained by interference of the incident laser and surface scattered wave related to the excited electrons during laser interactions with diamond, being in excellent agreement with a previously developed theory.

Femtosecond laser-induced periodic surface structure on diamond film

Two-peak photoluminescence (PL), blue and red, is obtained in the long-time aged porous silicon samples that were initially anodized under Ar+ 488 nm laser illumination Static and dynamic photoluminescence, photoluminescence excitation, and Fourier transformation infrared absorption and reflection spectra are studied It is found that the weight of blue PL increases with increasing the power intensity of the laser used during sample preparation A good monoexponential microsecond decay over three decades with lifetime of about 53 CLS is reported Infrared spectra are quantitatively studied to reveal the influence of the illuminating laser during anodization on the samples and the relationship between the oxidation states of the porous silicon and the blue and/or red photoluminescence It is suggested that the blue PL in these aged samples comes from the nanometer c-Si core, while the red PL originates from the oxide surface

Origin of the blue and red photoluminescence from aged porous silicon

Photoluminescence (PL) and its decay in the composite system of dye/porous silicon, consisting of oxidized porous silicon (Psi) and impregnated dyes, have been studied. We find that the PL spectra of the dyes are similar to that of dyes in ethanol solution while being quite different from those of dye films absorbed on the surface of c‐Si. The line shape of the spectra is more symmetric for impregnated dyes than that in solution. The PL decay of the impregnated dyes has been measured. The lifetime is about 100 ps for rhodamine B impregnated in as‐prepared Psi and it becomes much shorter for that in the annealed Psi layers. The influence of absorbed dyes on the PL decay of Psi has also been measured. It shows that impregnated dyes provide new nonradiative processes for the deactivation of the excited Psi.

Photoluminescence and its decay of the dye/porous‐silicon composite system

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at ${T}_{s}=121$ K and a magnetic one to a spin density wave (SDW) state at ${T}_{N}=110$ K. In the low-temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around $300{\mathrm{cm}}^{\ensuremath{-}1}$ that separates two regions of quasilinear conductivity. We outline that these characteristic absorption features are signatures of Dirac fermions, similar to what was previously reported for the ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ system [Z.-G. Chen et al., Phys. Rev. Lett. 119, 096401 (2017)]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at $250{\mathrm{cm}}^{\ensuremath{-}1})$ develops a strongly asymmetric line shape in the SDW state and note that this behavior can be explained in terms of a strong coupling with the Dirac fermions.

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Optical study of Dirac fermions and related phonon anomalies in the antiferromagnetic compound CaFeAsF

Micro-Raman spectroscopy was employed to investigate the structural changes of diamond films prepared by hot filament chemical vapor deposition and treated by femtosecond (fs) laser and nanosecond (ns) lasers. Breit–Wigner–Fano and Lorentzian line shape simulations were used to fit the spectra. For 266 nm ns laser treated samples, increasing laser power density results in the transformation of amorphous carbons in diamond films into nanocarbon clusters whose size increases and saturates rapidly at around 5.1 nm. At the same time, the Raman G peak position considerably shifts upwardly with increasing laser power density. The different change behavior of the nanocarbons and G peak is interpreted in light of the charge transfer from the graphite π bands to the localized edge states. As the 266 nm laser power density is high enough, a Raman peak in the range of 1150–1200 cm−1 appears, which is attributed to the presence of amorphous diamond. In the case of fs laser treated samples, much more power density (>1...

Yurong Ma

Papers

Femtosecond laser-induced periodic surface structure on diamond film

Origin of the blue and red photoluminescence from aged porous silicon

Photoluminescence and its decay of the dye/porous‐silicon composite system

Optical study of Dirac fermions and related phonon anomalies in the antiferromagnetic compound CaFeAsF

Raman investigation of amorphous carbon in diamond film treated by laser