By Anming Hu and Y. Norman Zhou

The interaction of light with metallic nanomaterials has led to a new branch of photonics called plasmonics. It has been found that the external electromagnetic waves can excite the collective electron displacements, known as surface plasmons (SP), density waves of electrons propagate along the conductor surface like ripples that spread across the water surface after you throw a stone into a pond. Plasmonic devices offer the potential to transmit optical signals and electric currents through the same thin metallic circuitry, thereby creating the possibilities to combine electronic and photonic components on the same chip. ^{1, 2}This will bring the revolution in high speed carrying of large scale digital data and optical computing.

To date the fabrication of such a plasmonic device is technically challenged since the welding at a nanoscale is required for the permanent integration of individual functional components. Femtosecond laser induced non-thermal melting as a promising method to realize a nanowelding without dramatically changing the shape and crystallinity of nanoparticles. One femtosecond is only 10^-15 second. Nanowelding with ultrashort laser pulses (< 10^-12 sec) is distinct from other fusion nanowelding processes because the interaction between ultrafast laser pulses and materials is a non-thermal phenomenon at surface.³ As the electron-lattice thermal coupling time (typically several picoseconds, 1 ps = 1000 fs) is much longer than the laser pulse width, the electrons do not have enough time to transfer the excitation energy to the lattice. Instead, electrons are excited leading to direct emission. This is accompanied by a reduction in the bond energy between lattice atoms, resulting in “melting” of surface atoms. This melting is non-thermal because it is not due to vigorously thermal vibrations of lattices at an elevated temperature. Furthermore, due to the surface emission the melting is a surface effect. Hence, in this process, melting occurs only on a nanoscale at the surface without damaging the bulk, making it ideal for welding of nanoparticles.

Fig. 1 presents welded Ag nanoparticles with120 fs laser pulses at an intensity of 10¹² W/cm². Four nanoparticles are welded together and form a chain structure. It is obvious that the nanoparticles kept their original shapes and the necks are formed by fusion. This is different from previous laser brazing with a nanosecond laser pulse where Au nanoparticles are totally melted and brazed individual Pt nanoparticles.⁴ Certainly the surface melting is beneficial to keep the original function since the surface plasmonic properties can be dramatically changed by shapes. It is important to point out that the irradiation at a higher energy leads to split nanoparticles into tiny particles. This is consistent with widely reported results that laser bombardment results in the further refinement of nanoparticles. Thus, the key to femtosecond laser induced nanowelding is the proper choice of the energy range. It is expected that the proper energy is dependent on materials.

Fig. 1 Welded Ag nanoparticles with 120 femtosecond pulses at an intensity of 10¹² W/cm².

Plasmonic properties of welded Ag nanoparticles have been simulated numerically using a 3D finite element method and a Drude-Lorentz model. ⁵The complex permittivity of silver was obtained from experimental data.  Fig. 2 shows a comparison of the electrical potential distribution between two and four adjacent particles and between welded nanoparticles in air. The diameter of each nanoparticle is 50 nm and the gap is 5 nm for adjacent pairs and -5 nm for welded pairs (corresponding to an overlapping central distance of 5 nm). The results show that welded nanoparticles possess a ring-shaped hot spot in the neck area compared to a central hot-spot in pairs that are together, but not joined. This clearly indicates that the hot-spot area is enhanced in laser welded pairs.  Unlike adjacent dimmers or trimers, which are bonded by a weak van der Waals interaction, welded nanoparticles form strong and permanent joints. These are suitable for use as stable, repeatable Raman probes.

 
Fig. 2 The cross-sectional views of normalized electric field distributed at the surface of the nanostructures in air, top panels: adjacent two and four Ag spheres with a diameter of 50 nm and a central gap of 5 nm; low panels: welded two and four Ag spheres at the same diameter and a overlapped central distance of 5 nm (i.e., the central gap of -5 nm).

References:

¹J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White and M. J. Brongersma, Nature Nanomater. 9 (2010) 193

²E. Ozbay, Science 311 (2006) 189

³A. Hu, M. Rybachuk, Q. B. Lu and W. W. Duley, Appl. Phys. Lett. 91 (2007) 131906

⁴F. Mafune, J. Kohno, Y. Takeda, T. Kondow, J. Am. Chem. Soc. 125 (2003) 1686

⁵X. Y. Zhang, A. Hu, T. Zhang, W. Lei, X. J. Xue, Y. Zhou and W. W. Duley, ACS Nano (DOI: 10.1021/nn203336m)

Michael Schmidt, Maik Zimmermann, Martin Hohmann and Jan Paulus

Introduction
Optics –seen through the glasses of scientific disciplines – is one of the most widespread topics ranging from basic physics over engineering to medicine. The field of optics therefore is not only basic technology but also enabling technology for a lot of new techniques. To further enhance the innovation level in optics, especially the interdisciplinary research has to be strengthened. To reach this goal, young and motivated scientists are needed that do not hesitate to look beyond their own discipline. Real interdisciplinary innovation needs people that feel at home in more than one community, e.g. in optics engineering and in medicine.

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David L. Bourell and Joseph J. Beaman, Jr.

Additive manufacturing is a collection of computer-controlled processes that create parts in a layerwise fashion without part-specific tooling. Applications share the common characteristics of part production with complex geometry in relatively small production runs. Historically, applications were limited to production of prototypes and casting inserts, since part mechanical properties and surface finish were inadequate for actual structural applications. More recently, coupled with post processing, additive manufacturing has been used to produce a variety of production tooling, short-run structural parts, customized bio-engineered parts, mass-customized parts, architectural designs, parts for automotive and aerospace applications, archaeological replicas and artwork [1]. The demand for products and services from additive-manufacturing technology has been strong over its 23-year history (1988-2010). The compound annual growth rate of revenues produced by all products and services over this period is an impressive 25.1% [2].  This article describes an historical context of additive manufacturing technology based largely on US patent literature.

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