Mechanism for spherical dome and microvoid formation in polycarbonate using nanojoule femtosecond laser pulses

Spherical domes are created on the surface of polycarbonate samples, and microvoids are formed within the bulk using only a femtosecond oscillator with pulse energy of just 0.47?nJ. Size of spherical domes and shape of microvoids are controlled by changing the laser focus inside the material. Their formation is explained by a combination of heat accumulation and dome formation dynamics, where the dome acts as a microlens shifting the laser focus within the sample. The technique described here provides a simple avenue for fabricating smooth microlens arrays of various sizes and opens the possibility for direct fabrication of complex three-dimensional microfluidic channels in transparent materials.     

Thermal melting and ablation of silicon by femtosecond laser radiation

The space-time dynamics of thermal melting, subsurface cavitation, spallative ablation, and fragmentation ablation of the silicon surface excited by single IR femtosecond laser pulses is studied by timeresolved optical reflection microscopy. This dynamics is revealed by monitoring picosecond and (sub)nanosecond oscillations of probe pulse reflection, which is modulated by picosecond acoustic reverberations in the dynamically growing surface melt subjected to ablation and having another acoustic impedance, and by optical interference between the probe pulse replicas reflected by the spalled layer surface and the layer retained on the target surface. The acoustic reverberation periods change during the growth and ablation of the surface melt film, which makes it possible to quantitatively estimate the contributions of these processes to the thermal dynamics of the material surface. The results on the thermal dynamics of laser excitation are supported by dynamic measurements of the ablation parameters using noncontact ultrasonic diagnostics, scanning electron microscopy, atomic force microscopy, and optical interference microscopy of the modified regions appearing on the silicon surface after ablation.     

Laser removal of mold growth from paper

Femtosecond laser nano-processing by enhanced light scattered from nanospheres has received much attention. Enhanced scattered near field enables us to ablate nanoholes at nanometer scales below the diffraction limit. In addition, the interference between the scattered far field and the irradiated laser enables us to fabricate spatially controlled periodic surface structures. In this paper, we simulated the time evolution of scattered near field and far field during the free electron excitation in silicon (Si) by femtosecond laser irradiation. The optical property of Si changes from dielectric to metal-like Si due to the increase of the free electron number density excited by femtosecond laser pulse. It is elucidated that the scattered field of Si shifts from Mie scattering to plasmonic scattering during laser irradiation. We achieved the optimal free electron density and laser intensity for precisely controlled periodic surface structures fabrication. We explained the temporal behavior of the scattering near field and far field from the standpoint of dielectric function of the materials.     

Fabrication of Micro -Optical Devices by a Femtosecond Laser

Femtosecond laser is a perfect laser source for materials processing when high accuracy and small structure size are required. Due to the ultra short interaction time and the high peak power, the process is generally characterized by the absence of heat diffusion and, consequently molten layers. Various induced structures have been observed in materials after the femtosecond laser irradiation. Here, we report on fabrication of micro-optical devices by the femtosecond laser. 1) formation of optical waveguide with internal loss less than 0.5dB/cm in the wavelength region from 1.2 to 1.6 mm, by translating a silica glass perpendicular to the axis of the focused femtosecond laser beam; 2) nano-scale valence state manipulation of active ions inside transparent materials; 3) space-selective precipitation and control of metal nanoparticles inside transparent materials; The mechanisms and applications of the femtosecond laser induced phenomena were also discussed.     

Optical properties of silicon nanoparticles synthesized via electrical spark discharge in water

In this paper, we report a simple and low-cost technique for fabrication of silicon nanoparticles via electrical spark discharge between two plane silicon electrodes immersed in deionized water (DI). The pulsed spark discharge with the peak current of 60 A and a duration of a single discharge pulse of 60 μs was used in our experiment. The structure, morphology, and average size of the resulting nanoparticles were characterized by means of X-Ray Diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM). TEM images illustrated nearly spherical and isolated Si nanoparticles with diameters in the 3–8 nm range. The Raman peaks of the samples were shifted to the lower wave numbers in comparison to this of bulk crystalline silicon indicating the existence of tiny particles. The optical absorption spectrum of the nanoparticles was measured in the violet–visible (UV–Vis) spectral region. By measuring of the band gap we could estimate the average size of the prepared particles. The silicon nanoparticles synthesized exhibited a photoluminescence (PL) band in the violet-blue region with a double peak at around 417 and 439 nm. It can be attributed to oxide-related defects on the surface of silicon nanoparticles, which can act as the radiative centers for the electron-hole pair recombination.     

Parallel fabrication of silicon concave microlens array by femtosecond laser irradiation and mixed acid etching

A method of multi-beam femtosecond laser irradiation combined with modified HF-HNO_3-CH_3 COOH etching is used for the parallel fabrication of all-silicon plano-concave microlens arrays(MLAs). The laser beam is split by a diffractive optical element and focused by a lens to drill microholes parallely on silicon. An HF-HNO_3-H_2SO_4-CH_3 COOH solution is used to expand and polish laser-ablated microholes to form microlenses. Compared with the HF-HNO_3-CH_3 COOH solution,the solution with H_2SO_4 can effectively reduce the etched surface roughness. The morphologies of MLAs at different laser powers and pulse numbers are observed. The image array formed by the silicon microlenses is also demonstrated.     

Study of irradiation conditions and thermodynamics of optical glass in the problem of modification of materials by femtosecond laser pulses

The local heating of glass by a focused femtosecond laser pulse and cooling of an irradiated region are numerically modeled. The structural modifications that change the optical properties of glass are assumed to occur within a bulk region whose temperature after irradiation exceeds the glass transition temperature. The shape of the modified region obtained from the calculations coincides with that known from experimental data available. The size of this region is determined by the spatiotemporal dynamics of the laser beam under multiphoton absorption conditions. The heating of glass is maximal in front of a thin lens used for the beam focusing.     

Femtosecond laser drilling of crystalline and multicrystalline silicon for advanced solar cell fabrication

The rear contact solar cell concept has been implemented to increase the solar cell efficiency. Practically, it necessitates rapid fabrication of a large number of via holes to form low-loss current paths. It is not a trivial task to drill a number of microscopic holes through a typical Si wafer of ??200???m thickness at reasonable processing throughput and yield. In this research, a femtosecond laser is employed to drill via holes in both crystalline silicon (c-Si) and multicrystalline silicon (mc-Si) thin wafers of ??170???m thickness with various laser parameters such as number of laser shots and pulse energy. Since a significantly high pulse energy compared to ablation threshold is mainly applied, aiming to achieve a rapid drilling process, the femtosecond laser beam is subjected to complex non-linear characteristics. Therefore, the relative placement of the sample with respect to the laser focal position is also rigorously examined. While the non-linear effect at high pulse energy regime is complex, it also facilitates the drilling process in terms of achieving high-aspect ratio, for example, by extending the effective depth of focus by non-linear effect. Cross-sectional morphological analysis in conjunction with on-line emission and shadowgraph imaging are carried out in order to elucidate the drilling mechanism.     

Laser Fabrication of Nanostructures on a Pure Aluminum Surface in Air

We report on the fabrication of surface nanoparticles and micro/nanograting structures on bulk pure aluminum in air using a 150 fs, 775 nm femtosecond laser. We investigate the size of the generated surface nanoparticles under irradiation with different femtosecond laser pulses. Smaller nanoparticles can be induced by a larger number of laser pulses and a lower laser fluence. In addition, we observe the formation of micro/nanogratings when the laser focus is scanned across a pure aluminum surface in air. We obtain micro- and nano-grating composite structures on a pure aluminum surface by adjusting the laser fluence and scan velocity. Femtosecond laser surface ablation of bulk pure aluminum in air is potentially a promising technique for efficient fabrication of surface nanostructures.     

纳秒激光单脉冲烧蚀硅表面的超快成像研究

利用ICCD可以在纳秒时间尺度下成像的特点,以飞秒准连续激光产生的超短脉冲光为探测光,对纳秒激光单脉冲烧蚀硅靶表面的演化过程进行动态监测。在能量密度为50J／cm^2时,捕获了纳秒单脉冲激光烧蚀硅靶面过程中等离子体演化的时间分辨图像。图像表明,纳秒激光烧蚀硅靶产生的等离子体开始时密度大,膨胀速度快,当纳秒激光脉冲过后,等离子体不再产生,并且其膨胀速度不再增加,直至完全消失。     

Applications of excimer laser in nanofabrication

This paper addresses novel applications of an excimer laser (308 nm wavelength, 20 ns pulse duration) in nanofabrication. Specifically, laser assisted nanoimprint lithography (LAN), self-perfection by liquefaction (SPEL), fabrication of metal nanoparticle arrays, and the fabrication of sub-10-nm nanofluidic channels are covered. In LAN, a polymeric resist is melted by the laser pulse, and then imprinted with a fused silica mold within 200 ns. LAN has been demonstrated in patterning various polymer nanostructures on different substrates with high fidelity and uniformity, and negligible heat effect on both the mold and the substrate. SPEL is a novel technology that uses selective melting to remove fabrication defects in nanostructures post fabrication. Depending on the boundary conditions, SPEL is categorized into three basic types: Open-SPEL that takes place with surface open, Capped-SPEL where a cap plate holds the top surface of the nanostructures and Guided-SPEL where a plate held a distance above the structure guides the molten materials to rise and form a new structure with better profile. Using SPEL (in less than 200 ns), we have achieved a reduction of line edge roughness (LER) of Cr lines to 1.5 nm (3σ) (560% improvement from the original), which is well below what the previous technologies permit, and a dramatic increase of the aspect ratio of a nanostructure. We have used SPEL to make sub-25-nm smooth cylindrical NIL pillar molds and smoothing Si waveguides. Excimer laser is also used to make metal nanoparticles. Monolayers of particles are fabricated on various substrates (silicon, fused silica and plastics) by exposing thin metal films to a single laser pulse. Periodic nanoparticle arrays have been fabricated by fragmentation of metal grating lines. The periodicity of these nanoparticles can be regulated by surface topography such as shallow trenches. Finally, an excimer laser pulse has been used to melt the top portion of 1D and 2D Si gratings to seal off the top surface, forming enclosed nanofluidic channel arrays. The channel width has been further reduced to 9 nm using self-limited thermal oxidation. DNA stretching using 20 nm wide self-sealed channels is also demonstrated.     

Picosecond laser ablation of nano-sized WTi thin film

Interaction of an Nd:YAG laser, operating at 532 nm wavelength and pulse duration of 40 ps, with tungsten-titanium (WTi) thin film (thickness, 190 nm) deposited on single silicon (100) substrate was studied. Laser fluences of 10.5 and 13.4 J/cm² were found to be sufficient for modification of the WTi/silicon target system. The energy absorbed from the Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following WTi/silicon surface morphological changes were observed: (i) ablation of the thin film during the first laser pulse. The boundary of damage area was relatively sharp after action of one pulse whereas it was quite diffuse after irradiation with more than 10 pulses; (ii) appearance of some nano-structures (e.g., nano-ripples) in the irradiated region; (iii) appearance of the micro-cracking. The process of the laser interaction with WTi/silicon target was accompanied by formation of plasma.     

3-Dimensional simulation and footprint of optical breakdown in dielectrics induced by femtosecond laser pulse

In this paper dynamics of optical breakdown process in dielectrics induced by femtosecond laser pulses have been simulated numerically. Using rate equations, the dynamics of free electron density in the focusing region during a few laser pulse duration time was studied numerically. Sources of free electron production as well as sinks of electron reduction in the interacting region were considered in the calculations. In the simulation, the propagation of the laser beam through the focusing volume was also taken into account and the footprint of the breakdown process (as an estimation for medium bulk damage) was simulated 3-dimensionally. The temporal and spatial evolution of free electron density has been used for simulating the 3-dimensional footprint of optical breakdown region. The results show that the dynamics of the breakdown footprint (and volume) strongly depends on the laser and medium characteristics.     

Intensity of optical field inside a weakly absorbing spherical particle irradiated by a femtosecond laser pulse

Using the Fourier technique in combination with the Mie theory, we study numerically the spatiotemporal evolution of the intensity of the internal optical field inside micron-sized weakly absorbing spherical particles upon diffraction by these particles of a femtosecond laser field. A number of specific features of the dynamics of the spatial intensity distribution of the femtosecond pulses inside the particles are found to depend on the pulse width, the shape of the laser beam, the size of the particles, and the geometry of their irradiation. It is shown that, under conditions of nonstationary diffraction, the internal optical field is usually excited in a resonance way, with the eigenfrequencies of one or several high-Q resonance modes of the particle falling into the central part of the original pulse spectrum. This causes a time delay of the light in the particle and a reduction of the absolute maximum in the time dependence of the internal field intensity as compared with a stationary regime. The greatest reduction of the peak occurs at exact resonance. In this case, the decrease in the peak intensity may reach several orders of magnitude. Irradiation of a particle by a narrow Gaussian beam of femtosecond duration directed toward the particle center enhances the internal field intensity as compared with the case of near-edge incidence.     

Direct fabrication of periodic patterns with hierarchical sub-wavelength structures on poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) thin films using femtosecond laser interference patterning

A simple optical interference method for the fabrication of simply periodic and periodic with a substructure on poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) using femtosecond laser interference patterns is demonstrated. The femtosecond laser pulse was split by a diffractive beam splitter and overlapped with two lenses. Homogeneous periodic arrays could be fabricated even using a single laser pulse. In addition, multipulse irradiation resulted in reproducible sub-wavelength ripples oriented perpendicularly to the laser polarization with spatial period from 170 to 220 nm (around one-fourth of the laser wavelength). In addition, the observed size of the spatial period was not affected by the number of incident laser pulses or accumulated energy density. Using high energy pulses it was possible to completely remove the PEDOT:PSS layer without inducing damage to the underneath substrate.     

Control of femtosecond laser written waveguides in silica glass

Writing conditions for the fabrication of optical waveguides in bulk fused silica glass by use of 1 kHz focused femtosecond laser pulses at 800 nm were systematically determined for different focusing geometries. The results demonstrate that waveguides can be formed based on optical breakdown, filamentation (single or multiple), or a combination of both processes, when using pulse energies lower than the threshold of structural damage. The mechanisms of laser-induced index change are also discussed. PACS 42.65.Jx; 42.70.Ce; 42.79.Gn     

Dynamics of exciton formation at the Si(100) c(4 x 2) surface

Carrier recombination at the Si(100) c(4 x 2) surface and the underlying surface electronic structure is unraveled by a combination of two-photon photoemission and many-body perturbation theory: An electron excited to the silicon conduction band by a femtosecond infrared laser pulse scatters within 220 ps to the unoccupied surface band, needs 1.5 ps to jump to the band bottom via emission of optical phonons, and finally relaxes within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds.     

Formation of conical microstructures upon laser evaporation of solids

The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (∼10⁴) at laser fluence of 1–2 J/cm² below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 10⁵ Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20–30 μm. It is suggested that the spatial period of the structures (10–20 μm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40–70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti. Received: 12 February 2000 / Accepted: 28 March 2000 / Published online: 20 June 2001     

Study of the plasma plume generated during near IR femtosecond laser irradiation of silicon targets

Ti:sapphire femtosecond laser ablation of silicon has been investigated by Langmuir probe and time-gated optical emission spectroscopy. The measured spectra show the presence of a fast ion population preceding the main plume core of slow ions and neutrals produced by a thermal ablation mechanism. By analyzing the fluence thresholds for the emission of the two ion populations, we provide clear experimental evidence that fast ions are ejected non-thermally from the sample surface as a result of the Si surface supercritical state induced by the intense ultrashort laser pulse irradiation. PACS 52.50.Jm; 52.38.Mf; 79.20.Ds; 61.82.Fh     

飞秒激光诱导硒化锌晶体表面自组织生长纳米结构

 以250 kHz高重复频率钛宝石飞秒激光聚焦到硒化锌晶体表面,利用扫描电子显微镜观测飞秒激光辐照后晶体的表面结构。发现线偏振激光辐照的区域形成了自组织周期性纳米结构,其周期为160 nm左右,并且可以通过改变激光的偏振方向调节纳米光栅结构的取向;当晶体相对于激光光束以10 mm/s速度移动,经激光扫描后,在晶体表面形成了长程类布拉格光栅。当飞秒激光光束为圆偏振光时,辐照区域形成均匀的纳米颗粒。