Colocalization of gold nanoparticle-conjugated DNA hybridization for enhanced surface plasmon detection using nanograting antennas

We have investigated enhanced surface plasmon resonance detection through colocalization of gold nanoparticle (GNP)-conjugated target molecules and near-fields established by nanograting-based antennas. The target colocalization was implemented by angled dielectric thin-film deposition on the nanograting structure. The concept was tested by detecting DNA hybridization and shows that the colocalization produces an additional 60%-80% increase of resonance shifts. The colocalization involves a much smaller number of target molecules, so that the measured enhancement per molecule by the colocalization of GNP-conjugated DNA oligomers was estimated to be by more than 2 orders of magnitude relative to that of thin-film-based conventional detection.     

Reflectivity in femtosecond-laser-induced structural changes of diamond-like carbon film

We report characteristic changes in reflectivity of diamond-like-carbon (DLC) films following irradiation of femtosecond laser pulses at laser fluence around the ablation threshold, where we have observed the formation of periodic nanostructure on the film surface and the modification of bonding structure from DLC to glassy carbon in our recent experiments. The experimental results have shown that the reflectivity change should be a reliable measure to monitor the experimental condition to produce the structural changes, being closely associated with the fundamental physical process to induce these phenomena. PACS 79.20.Ds; 81.40.Tv; 42.62.Cf     

Fluence dependence of femtosecond-laser-induced nanostructure formed on TiN and CrN

The fluence dependence of the nanostructure formation, which has been observed in our recent experiments on femtosecond (fs)-laser ablation, was examined in detail for hard thin films of TiN and CrN. The size D of the periodic fine structure formed with fs-laser pulses can be divided into two regions that depend on the laser fluence F, i.e. the region I where D increases rapidly with increasing F near the ablation threshold, and the region II where D increases slowly with an increase in F and almost saturates. The nanostructure has been observed only in the region I with a narrow width of F. The region II produces a periodic ripple structure whose size is 1/2–4/5 of the wavelength used. The effects of the thermal process and material composition on the nanostructure formation are discussed. PACS 61.80.Ba; 79.20.Ds; 42.62.Cf     

Optical near-fields of triangular nanostructures

We compare simulations of optical near-fields of single triangular nanostructures with experimental results from a near-field ablation technique on a periodic arrangement of triangles. We find good agreement of the lateral near-field distributions; nevertheless their dependency on the polarization of the incident light differs by 90°. Upon increasing the lateral distances of the nanotriangle arrangement in the experiment, the polarization dependence agrees with the simulation. We conclude that this at first sight unexpected behaviour stems from the coupling of near-fields by scattered surface waves and their interaction with the incoming beam. PACS 78.67.Bf; 81.07.-b     

Nanoscale cavitation instability of the surface melt along the grooves of one-dimensional nanorelief gratings on an aluminum surface

Femtosecond laser nanostructuring at low fluences produces a one-dimensional quasiperiodic grating of grooves on an aluminum surface with a period (≈0.5 μm) that is determined by the length of a surface electromagnetic wave. The structure of the grooves of the surface nanograting is formed by regular nanopeaks following with a period of about 200 nm. Some nanopeaks manifest craters at their tops. It is suggested that nanopeaks are formed due to the frozen nanoscale spallative ablation of a nanolayer of an aluminum melt in quasiperiodic regions corresponding to interference maxima of the laser radiation with the surface electromagnetic wave. The periodicity of the appearance of nanopeaks along grooves is due to the previously predicted mechanism of cavitation deformation of the melt surface in the process of macroscopic spallation ablation. However, in this case, cavitation is coherent (similar to a near-critical spinodal decay) rather than spontaneous.     

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。     

Femtosecond pulse laser-induced self-organized nanostructures on the surface of ZnO crystal

This paper reports self-organized nanostructures observed on the surface of ZnO crystal after irradiation by a focused beam of a femtosecond Ti：sapphire laser with a repetition rate of 250kHz. For a linearly polarized femtosecond laser, the periodic nanograting structure on the ablation crater surface was promoted. The period of self-organization structures is about 180 nm. The grating orientation is adjusted by the laser polarization direction. A long range Bragglike grating is formed by moving the sample at a speed of 10μm/s. For a circularly polarized laser beam, uniform spherical nanoparticles were formed as a result of Coulomb explosion during the interaction of near-infrared laser with ZnO crystal.     

Picosecond and femtosecond pulsed laser ablation and deposition of quasicrystals

A Nd:glass laser with pulse duration of 250 fs and 1.3 ps has been used to evaporate a Al₆₅Cu₂₃Fe₁₂ quasicrystalline target. The gaseous phase obtained from the ablation process has been characterised by several techniques such as emission spectroscopy, quadrupole mass spectrometry and ICCD imaging, used to study the plume composition, energy and morphology. The results show that the ablation processes in the short-pulse regimes are very different to the nanosecond one. In particular the plume angular distribution shows a characteristic high cosine exponent and the composition is completely stoichiometric and independent from the laser fluence. Furthermore the mass spectra indicate the presence of clusters, both neutral and ionised and the emission from the target suggest a rapid thermalisation leading to the melting of the surface. To clarify the ablation process some films have been deposited, on oriented silicon, at different experimental conditions and analysed by scanning electron microscopy, atomic force microscopy, energy dispersive X-ray analysis and X-ray diffraction. The analyses show the presence of nanostructured films retaining the target stoichiometry but consisting of different crystalline and non crystalline phases. In particular the nanostructure supports the hypothesis of the melting of the target during the ablation and a mechanism of material ejection is proposed for both picosecond and femtosecond regimes.     

Ultra fine carbon nitride nanocrystals synthesized by laser ablation in liquid solution

Crystalline carbon nitride nanopowders and nanorods have been successfully synthesized at room temperature and pressure using the novel technique of pulsed laser ablation of a graphite target in liquid ammonia solution. High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and Fourier transform infrared spectroscopy (FTIR) were used to systematically study the morphology, nanostructure and chemical bonding. The experimental composition and structure of the nanoparticles are consistent with the theoretical calculations for α-C₃N₄. After 2 h ablation the particles had a size distribution ∼8–12 nm, whereas after 5 h ablation the particles had grown into nanorod-like structures with a crystalline C₃N₄ tip. A formation mechanism for these nanorods is proposed whereby nanoparticles are first synthesized via rapid formation of an embryonic particle, followed by a slow growth, eventually leading to a one-dimensional nanorod structure.     

Multipulse feedback in self-organized ripples formation upon femtosecond laser ablation from silicon

The influence of positive feedback on self-organized nanostructure (ripples) formation is investigated for multipulse femtosecond laser ablation from silicon surface. We find an increase of the modified surface area and of complexity and feature size with accumulated dose, confirming the previously postulated feedback effect of dose accumulation. More interestingly, a variation of temporal pulse-to-pulse separation, at constant total incident irradiation dose, strongly affects the structure formation. Though the feedback becomes weaker with increasing time intervals between successive pulses, pulses do not act independently even for separations of up to one second. To account for this observation, a model of perturbation decay and outdiffusion from the excited volume is suggested and compared to the experimental results. Inspection by surface sensitive microscopy (AFM, SEM) and conventional and high-resolution transmission electron microscopy reveal complex structural modification upon the laser interaction: even well outside the irradiated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. This is confirmed by photoluminescence studies on the band–band and the dislocation-related D1-line.     

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.     

多脉冲飞秒激光对锗材料的烧蚀效应

 开展了脉宽为40 fs的不同数量激光脉冲对锗材料的烧蚀效应实验,采用扫描电镜、激光共聚焦显微镜等方法对不同数量的飞秒激光脉冲作用下锗材料表面烧蚀区进行了检测,并对作用后材料烧蚀形貌演化规律进行了分析,初步分析了锗材料烧蚀区周围形成的不同环区的形貌特征及成因,对各环区烧蚀形貌特征随激光作用脉冲数的增加而产生的形貌演化过程进行了观测。并给出单脉冲飞秒激光对锗材料的烧蚀阈值为1.2 J·cm^-2,采用激光共聚焦显微镜测得该阈值条件下单个飞秒激光脉冲对锗材料的烧蚀深度约为150 nm。     

Coherent linking of periodic nano-ripples on a ZnO crystal surface induced by femtosecond laser pulses

We demonstrate the coherent linking of periodic nano-ripples formed on the surface of ZnO crystals induced by femtosecond laser pulses. By adjusting the distance between two laser scanning zones, the periodic nano-ripples induced by two separated laser writing processes can be coherently linked and the ZnO nanograting with much longer grooves is therefore produced. The length limitation of this kind of nanograting previously set by the laser focus size is thus overcome. The micro-Raman mapping technique is used to evaluate the quality of coherent linking, and the underlying physics is discussed. The demonstrated scheme is promising for producing large-size self-organized nanogratings induced by femtosecond laser pulses.     

Plasmonics properties of trimetallic Al@Al2O3@Ag@Au and Al@Al2O3@AuAg nanostructures

Bimetallic and trimetallic nanoparticles have attracted significant attention in recent times due to their enhanced electrochemical and catalytic properties compared to monometallic nanoparticles. The numerical calculations using Mie theory has been carried out for three-layered metal nanoshell dielectric–metal–metal (DMM) system consisting of a particle with a dielectric core (Al@Al₂O₃), a middle metal Ag (Au) layer and an outer metal Au (Ag) shell. The results have been interpreted using plasmon hybridization theory. We have also prepared Al@Al₂O₃@Ag@Au and Al@Al₂O₃@AgAu triple-layered core–shell or alloy nanostructure by two-step laser ablation method and compared with calculated results. The synthesis involves temporal separations of Al, Ag, and Au deposition for step-by-step formation of triple-layered core–shell structure. To form Al@Ag nanoparticles, we ablated silver for 40 min in aluminium nanoparticle colloidal solution. As aluminium oxidizes easily in water to form alumina, the resulting structure is core–shell Al@Al₂O₃. The Al@Al₂O₃ particle acts as a seed for the incoming energetic silver particles for multilayered Al@Al₂O₃@Ag nanoparticles is formed. The silver target was then replaced by gold target and ablation was carried out for different ablation time using different laser energy for generation of Al@Al₂O₃@Ag@Au core–shell or Al@Al₂O₃@AgAu alloy. The formation of core–shell and alloy nanostructure was confirmed by UV–visible spectroscopy. The absorption spectra show shift in plasmon resonance peak of silver to gold in the range 400–520 nm with increasing ablation time suggesting formation of Ag–Au alloy in the presence of alumina particles in the solution.     

Effect of irradiation history on the preparation of laser induced periodic microstructure on polyimide surface

Although UV laser is proved to be an effective tool to prepare microstructure on polymer surface, laser ablation accompanied by the formation of laser induced periodic surface structure (LIPSS) limits its application in many fields. The purpose of this report is to investigate the effect of pre-irradiation in advance, using a low-fluence laser, on the LIPSS formation. The properties of pre-irradiated PI films were characterized by X-ray photoelectron spectroscopy (XPS), surface tension based on the contact angle measurements and UV-vis spectra. It was found that pre-irradiation at low fluence led to the changes in surface property such as chemical components though no LIPSS was formed. As a result, threshold of LIPSS formation on such pre-irradiated PI film decreased and fine LIPSS with deeper amplitude was obtained.     

飞秒激光诱导ZnO :Al薄膜周期结构及其光致发光特性

利用单束800 run飞秒激光在掺杂了Al的ZnO薄膜中制备了纳米周期条纹结构.研究了不同能流密度的飞秒激光在照射不同的时间后,表面纳米周期结构的变化规律及其形成机制.利用He-Ge激光器作为激发光源,研究了ZnO:Al薄膜的光致发光特性及其与纳米周期结构的关系.结果表明,近带隙发光增强的主要原因是800 nm飞秒激光...     

基于双相延迟模型的飞秒激光烧蚀金属模型

为了分析飞秒激光烧蚀过程,在双相延迟模型的基础上建立了双曲型热传导模型.模型中考虑了靶材的加热、蒸发和相爆炸,还考虑了等离子体羽流的形成和膨胀及其与入射激光的相互作用,以及光学和热物性参数随温度的变化.研究结果表明:等离子体屏蔽对飞秒激光烧蚀过程有重要的影响,特别是在激光能量密度较高时;两个延迟时间的比值对飞秒激光烧蚀过程中靶材的温度特性和烧蚀深度有较大的影响;飞秒激光烧蚀机制主要以相爆炸为主.飞秒激光烧蚀的热影响区域较小,而且热影响区域的大小受激光能量密度的影响较小.计算结果与文献中实验结果的对比表明基于双相延迟模型的飞秒激光烧蚀模型能有效对飞秒激光烧蚀过程进行模拟.     

Periodic nanostructures on Si(100) surfaces generated by high-repetition rate sub-15 fs pulsed near-infrared laser light

Nanoscale rifts and ripples at a periodicity of 130 nm were generated on Si(100) surfaces immersed in water using tightly focused 800 nm 12 fs pulsed 85 MHz laser light at subnanojoule pulse energies. At radiant exposure close to the ablation threshold rifts were typically 20-50 nm in width and 70 nm in depth running perpendicular to the laser polarization. On increase of the irradiance, the rifts broadened and formed periodic ripples, whereas at highest exposure, a random nanoporous surface topology emerged. Rift and ripple formation is explained by laser-induced standing surface plasma waves, which result in periodic variation of dissipation and ablation.     

Nanosecond pulsed laser ablation of silicon in liquids

Laser fluence and laser shot number are important parameters for pulse laser based micromachining of silicon in liquids. This paper presents laser-induced ablation of silicon in liquids of the dimethyl sulfoxide (DMSO) and the water at different applied laser fluence levels and laser shot numbers. The experimental results are conducted using 15 ns pulsed laser irradiation at 532 nm. The silicon surface morphology of the irradiated spots has an appearance as one can see in porous formation. The surface morphology exhibits a large number of cavities which indicates as bubble nucleation sites. The observed surface morphology shows that the explosive melt expulsion could be a dominant process for the laser ablation of silicon in liquids using nanosecond pulsed laser irradiation at 532 nm. Silicon surface’s ablated diameter growth was measured at different applied laser fluences and shot numbers in both liquid interfaces. A theoretical analysis suggested investigating silicon surface etching in liquid by intense multiple nanosecond laser pulses. It has been assumed that the nanosecond pulsed laser-induced silicon surface modification is due to the process of explosive melt expulsion under the action of the confined plasma-induced pressure or shock wave trapped between the silicon target and the overlying liquid. This analysis allows us to determine the effective lateral interaction zone of ablated solid target related to nanosecond pulsed laser illumination. The theoretical analysis is found in excellent agreement with the experimental measurements of silicon ablated diameter growth in the DMSO and the water interfaces. Multiple-shot laser ablation threshold of silicon is determined. Pulsed energy accumulation model is used to obtain the single-shot ablation threshold of silicon. The smaller ablation threshold value is found in the DMSO, and the incubation effect is also found to be absent.