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Nowadays, the development of information technology demands miniaturization and increased functionality in integrated photonic circuits and the design of new materials with extraordinary optical properties. It has steadily evolved towards the use of nanophotonics that search for new approaches for manipulating light at the nanoscale.

Linking nano- and micro-worlds
One of the avenues in nanophotonics research is plasmonics, or metal optics, based on the controlled excitation of surface plasmons (SPs). Metals that support SPs—collective oscillations of free electrons—can concentrate electromagnetic fields on the nanoscale, while enhancing local field strengths by several orders of magnitude. Plasmonic structures supporting different SP modes—propagating or localized—can serve both as nanophotonic components and as a link between the nano- and micro-worlds, coupling light into the nanoscale through plasmonic nanoantennae and dramatically enhancing light–matter interaction on the nanoscale.

Plasmonic structures exhibit a variety of novel effects, including extraordinary light transmission, collimation of light through a subwavelength aperture, giant field enhancement and surface plasmon waveguiding. In addition to subwavelength confinement, plasmonics brings another attractive feature into the area of integrated optical waveguides — the unique possibility of using the same metal circuitry for guiding SPs and carrying electrical signals.

Light interaction with surface plasmons in specially designed metallic structures has also resulted in demonstrations of effects unattainable with naturally occurring materials including, for example, negative permeability (optical magnetism), negative refractive index at visible wavelengths and nonlinear effects in magnetic metamaterials. These observations led to a new area of photonics called optical metamaterials that has been exponentially growing over the last few years.

Massive signal enhancement
Plasmonics does not only aim at developing optical devices for information technology. Surface plasmons are of interest to a wide community of researchers working in the areas of chemical and biological sensing, microscopy, nanolithography, light sources, data storage and energy conversion. Tight field confinement to the metal surface (typically on the order of or smaller than the wavelength in the corresponding media) makes SPs very sensitive to surface irregularities, so that SPs have long been used for surface analysis, including bio-sensing. Giant field enhancement achievable in metal nanostructures gives rise to massive signal enhancement in surface-enhanced Raman spectroscopy (SERS), enabling single molecule detection.

The recent boom in plasmonics comes from advances in nanofabrication that allow metals to be structured on the nanoscale. Designing and nanostructuring metal surfaces in a controllable way opens up the possibility of controlling the SP properties and tailoring them for specific applications.

Typically, to fabricate plasmonic structures one should deal with small (subwavelength) periodicities and feature sizes of the order of 100 nm. Such feature sizes are smaller than the resolution of state-of-the-art photolithography (due to the diffraction limit), thus requiring nanofabrication processes with 100 or sub-100 nm resolution. One of the widely used techniques for making nanostructures is electronbeam lithography (EBL). In addition to high resolution (below 100 nm), EBL offers almost complete pattern flexibility and can be used for creating a large variety of nanostructures. The majority of plasmonic structures ranging from subwavelength metal nanowires and chains of metal nanoparticles for SP guiding to twodimensional plasmonic nanostructures for SP manipulation, focusing and guiding are made using electronbeam lithography. However, plasmonic arrays consisting of subwavelength holes in a metal film or metal nanoparticles should often be patterned on a large-scale (multiple length scales) to be used as plasmonic metamaterials or substrates for sensing applications. Due to the low throughput of the serial point-by-point electron-beam writing and its high fabrication costs only small areas (of the order of 100 µm × 100 µm) can normally be structured with EBL. Hence, fabrication of plasmonic nanostructured surfaces requires the development of high precision and high throughput manufacturing processes. Examples of such techniques are microcontact printing (soft lithography) and nanoimprint lithography (NIL).

Pattern transfer
Being a next generation lithography candidate, NIL accomplishes pattern transfer by the mechanical deformation of the resist via a stamp rather than a photo- or electro-induced reaction in the resist, as in most lithographic methods. Thus the resolution of the technique is not limited by the wavelength of the light source, and the smallest attainable features are given solely by stamp fabrication. Moreover, NIL provides parallel processing with high throughput, being therefore suitable for large-scale patterning of plasmonic structures. Nowadays NIL provides high resolution wafer-scale processing using standard cleanroom procedures offering simplicity and low cost.

In addition to the ability of creating high resolution, large scale patterns in a resist, in a similar way to other standard lithography techniques, NIL can imprint complex, non-planar profiles in various polymers. This feature is crucial for the imprinting of functional devices for a wide range of applications in electronics, photonics, data storage, and biotechnology. In plasmonics, imprinting complex surfaces in polymers are used to make profiled metal surfaces, such as metal V-grooves for SP guiding.

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Reference
Plasmonic components fabrication via nanoimprint, Alexandra Boltasseva, 2009, J. Opt. A: Pure Appl. Opt. 11 114001