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Micro/Nano Lithography

Shaping gold nanorods for plasmonic applications

The restructuring of metallic nanorod arrays using femtosecond laser writing may lead to novel plasmonic devices based on localized assemblies of interacting metallic nanorods.
27 February 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0633

Nanostructuring is a key element for the development of optics, optoelectronics, and photonics capable of operating on subwavelength scales. However, the realization of highly integrated optical devices and sensors with nanolocal electromagnetic field control requires structural elements comparable to and smaller than a wavelength of light. One widely investigated approach is to use appropriately designed metallic and metallodielectric nanostructures to manipulate light in the form of various surface plasmon excitations, such as surface plasmon polaritons and localized surface plasmons.1 These electromagnetic surface modes are associated with the coherent excitation of the free-electron density charge waves at the interface of a metal and a dielectric material. Various plasmonic circuit elements based on nanostructured metal films, such as waveguides, mirrors, lenses, resonators, and plasmonic crystals, have been designed and tested. Plasmonic nanostructures can provide both passive and active all-optical elements, the latter capable of operating at low-control light intensities due to the electromagnetic field enhancement resulting from plasmonic excitations.2

Alternative plasmonics can be developed based on discrete sets of interacting metallic particles. We have recently developed an approach to inexpensively fabricate large arrays (1cm2 with up to 1 billion nanorods) of aligned nanoscale metallic rods attached to a substrate.3 Typically, the nanorods have a diameter of some 20nm with length controllable between 50 and 450nm. These are required to tailor the linear and nonlinear behavior of plasmonic excitations. However, to develop their functionalities and integrate them in complex plasmonic circuits for sensing and telecommunication applications, the nanorod assemblies must be configured in various shapes.

An attractive solution is to use femtosecond laser-based direct writing technology to restructure free-standing nanorods and fabricate nanorod assemblies in the required shapes. This approach is uniquely versatile because of its ability not only to ablate nanorods from the arrays but also to melt adjacent ones, thus yielding arrays with continuous metallic nanostructures. It also offers the possibility to create polymer structures with embedded nanorods.4 The application of femtosecond laser technology (see Figure 1) enables the precise adjustment of the laser power delivered to the target material while allowing high-resolution restructuring of the nanorod arrays. By scanning the laser beam over the sample surface, structures of different geometries can be written. Plasmonic elements including simple lines, splitters, and double splitters have been fabricated in this manner. Examination of the specific elements of these structures reveals that nanorod ablation can be successfully achieved with high precision. At optimal laser parameters, the ablation process can create a sharp boundary with single-nanorod sharpness (see Figure 2). However, the accuracy strongly depends on laser power and on the properties of the nanorod array that may cause melting to occur at the boundary.

Figure 1. Schematic setup for restructuring gold nanowires by femtosecond laser irradiation.

Figure 2. SEM images of (a) 2×2 splitter created on gold nanorods by two-photon polymerization of a negative photoresist using femtosecond laser radiation and (b) the boundary of an ablated nanorod structure (nanorod length: 250nm, diameter: 25nm, spacing 40nm). The femtosecond pulse energy was 80nJ.

The possibility to locally restructure metallic nanorod arrays using femtosecond laser writing may lead to the development of novel plasmonic devices based on localized assemblies of interacting metallic nanorods, refractive index sensors, extreme nanoscale waveguides, and splitters/combiners for optical chips. Combined with nonlinear polymers, these structures provide an opportunity to develop integratable nonlinear optical nanodevices.

Carsten Reinhardt
Nanotechnology Department, Laser Zentrum
Hannover, Germany
Wayne Dickson, Robert Pollard
Centre for Nanostructured Media,
Queens University Belfast
Belfast, Northern Ireland

1. A. V. Zayats, I. I. Smolyaninov, A. A. Maradudin, Nano-optics of surface plasmon polaritons,
Phys. Rep.
 408, no. 3–4, pp. 131-314, 2005. doi: 10.1016/j.physrep.2004.11.001.
2. G. A. Wurtz, R. Pollard, A. V. Zayats, Optical bistability in nonlinear surface-plasmon polaritonic crystals,
Phys. Rev. Lett.
 97, no. 5, pp. 057402, 2006. doi:10.1103/PhysRevLett.97.057402.
3. R. Atkinson, W. R. Hendren, G. A. Wurtz, W. Dickson, P. Evans, A. V. Zayats, R. J. Pollard, Anisotropic optical properties of arrays of gold nanorods embedded in alumina,
Phys. Rev. B
 73, no. 23, pp. 235402, 2006. doi:10.1103/PhysRevB.73.235402.