Multi-resonant compact nanoaperture with accessible large nearfields

We present a compact and multi-resonant aperture system supporting large local electromagnetic fields, which is highly advantageous for applications demanding high signal-to-noise ratio. The system is composed of subwavelength H-shaped apertures with extended arms fabricated through thin metal films on a free-standing dielectric membranes. Through finite-difference time-domain calculations, we numerically investigate the multi-resonant spectral nature of the compact system in detail, which is associated with the individual optical response of the constituting aperture elements. In order to realize the aperture systems working in the spectral region of interest, we introduce a fine-tuning mechanism of the optical responses through geometrical device parameters. Furthermore, we present a circuit analog of the system demonstrating the impedance characteristics of the plasmonic modes supported by the aperture arrays. The highly accessible large electromagnetic fields extending over a large volume improve the sensitivity of the nanostructured platform to the surface conditions by enhancing the overlap between biomolecules in the vicinity and the local optical fields. We also experimentally demonstrate this highly advantageous field generation capability through large absorption signal enhancements in the surface enhanced infrared absorption spectroscopy of protein bilayers.

Chemically resolved imaging of biological cells and thin films by infrared scanning near-field optical microscopy

Giorgio Margaritondo

The infrared (IR) absorption of a biological system can potentially report on fundamentally important microchemical properties. For example, molecular IR profiles are known to change during increases in metabolic flux, protein phosphorylation, or proteolytic cleavage. However, practical implementation of intracellular IR imaging has been problematic because the diffraction limit of conventional infrared microscopy results in low spatial resolution. We have overcome this limitation by using an IR spectroscopic version of scanning near-field optical microscopy (SNOM), in conjunction with a tunable free-electron laser source. The results presented here clearly reveal different chemical constituents in thin films and biological cells. The space distribution of specific chemical species was obtained by taking SNOM images at IR wavelengths ( l) corresponding to stretch absorption bands of common biochemical bonds, such as the amide bond. In our SNOM implementation, this chemical sensitivity is combined with a lateral resolution of 0.1 mum (approximate tolambda1/70), well below the diffraction limit of standard infrared microscopy. The potential applications of this approach touch virtually every aspect of the life sciences and medical research, as well as problems in materials science, chemistry, physics, and environmental research.

2003

Nanostencil lithography for high-throughput fabrication of infrared plasmonic sensors

Serap Aksu, Hatice Altug, Min Huang

We demonstrate a novel fabrication approach for high-throughput fabrication of engineered infrared plasmonic nanorod antenna arrays with Nanostencil Lithography (NSL). NSL technique, relying on deposition of materials through a shadow mask, offers the flexibility and the resolution to fabricate radiatively engineer nanoantenna arrays for excitation of collective plasmonic resonances. Overlapping these collective plasmonic resonances with molecular specific absorption bands can enable ultrasensitive vibrational spectroscopy. First, nanorod antenna arrays fabricated using NSL are investigated using SEM and optical spectroscopy, and compared against the nanorods with the same dimensions fabricated using EBL. No irregularities on the periodicity or the physical dimensions are detected for NSL fabricated nanorods. We also confirmed that the antenna arrays fabricated by NSL shows high optical quality similar to EBL fabricated ones. Furthermore, we show nanostencils can be reused multiple times to fabricate selfsame structures with identical optical responses repeatedly and reliably. This capability is particularly useful when high-throughput replication of the optimized nanoparticle arrays is desired. In addition to its high-throughput capability, NSL permits fabrication of plasmonic devices on surfaces that are difficult to work with electron/ion beam techniques. Nanostencil lithography is a resist free process thus allows the transfer of the nanopatterns to any planar substrate whether it is conductive, insulating or magnetic. As proof of the versatility of the NSL technique, we show fabrication of plasmonic structures in variety of geometries. We also demonstrate that nanostencil lithography can be used to achieve functional plasmonic devices in a single fabrication step, on variety of substrates. We introduced NSL for fabrication of nanoplasmonic structures including antenna arrays on rigid surfaces such as silicon, CaF2 and glass. In conclusion, Nanostencil Lithography enables plasmonic substrates supporting spectrally narrow far-field resonances with enhanced near-field intensities which are very useful for vibrational spectroscopy. We believe this nanofabrication scheme, enabling the reusability of stencil and offering flexibility on the substrate choice and nano-pattern design could significantly enhance wide-use of plasmonics in sensing technologies.

SPIE-INT SOC OPTICAL ENGINEERING2011

Nanostencil lithography for high-throughput fabrication of infrared plasmonic sensors

Serap Aksu, Hatice Altug, Min Huang

SPIE-INT SOC OPTICAL ENGINEERING2011