Anisha Gopalakrishnan

3D nanostar dimers with a sub-10-nm gap for single-/few-molecule surface-enhanced raman scattering.

Plasmonic nanostar-dimers, decoupled from the substrate, have been fabricated by combining electron-beam lithography and reactive-ion etching techniques. The 3D architecture, the sharp tips of the nanostars and the sub-10 nm gap size promote the formation of giant electric-field in highly localized hot-spots. The single/few molecule detection capability of the 3D nanostar-dimers has been demonstrated by Surface-Enhanced Raman Scattering.

Plasmon based biosensor for distinguishing different peptides mutation states

Periodic and reproducible gold nanocuboids with various matrix dimensions and with different inter-particle gaps were fabricated by means of top-down technique. Rhodamine 6G was used as a probe molecule to optimize the design and the fabrication of the cuboid nanostructures. The electric field distribution for the nanocuboids with varying matrix dimensions/inter-particle gap was also investigated. These SERS devices were employed as biosensors through the investigation of both myoglobin and wild/mutated peptides. The results demonstrate the probing and the screening of wild/mutated BRCA1 peptides, thus opening a path for the fabrication of simple and cheap SERS device capable of early detection of several diseases.

Bimetallic 3D nanostar dimers in ring cavities: recyclable and robust surface-enhanced Raman scattering substrates for signal detection from few molecules.

Top-down fabrication of electron-beam lithography (EBL)-defined metallic nanostructures is a successful route to obtain extremely high electromagnetic field enhancement via plasmonic effects in well-defined regions. To this aim, various geometries have been introduced such as disks, triangles, dimers, rings, self-similar lenses, and more. In particular, metallic dimers are highly efficient for surface-enhanced Raman spectroscopy (SERS), and their decoupling from the substrate in a three-dimensional design has proven to further improve their performance. However, the large fabrication time and cost has hindered EBL-defined structures from playing a role in practical applications. Here we present three-dimensional nanostar dimer devices that can be recycled via maskless metal etching and deposition processes, due to conservation of the nanostructure pattern in the 3D geometry of the underlying Si substrate. Furthermore, our 3D-nanostar-dimer-in-ring structures (3D-NSDiRs) incorporate several advantageous aspects for SERS by enhancing the performance of plasmonic dimers via an external ring cavity, by efficient decoupling from the substrate through an elevated 3D design, and by bimetallic AuAg layers that exploit the increased performance of Ag while maintaining the biocompatibility of Au. We demonstrate SERS detection on rhodamine and adenine at extremely low density up to the limit of few molecules and analyze the field enhancement of the 3D-NSDiRs with respect to the exciting wavelength and metal composition.

Plasmon resonance tuning in metal nanostars for surface enhanced Raman scattering

We report the fabrication of Au nanostar arrays by means of electron beam lithography, in which the plasmon resonance energy can be tuned via the nanostar size from the visible into the near-infrared region. The spectral response of the nanostar arrays was investigated by optical extinction (transmittance) experiments, and their surface enhanced Raman scattering performance has been tested at two different excitation wavelengths, 633 nm and 830 nm, using chemisorbed Cresyl violet molecules as analyte. The experimental results are supported by numerical simulations of the spatial and spectral electric field enhancement.

A new route to produce efficient surface-enhanced Raman spectroscopy substrates: gold-decorated CdSe nanowires

Surface-enhanced Raman spectroscopy is a popular tool for the detection of extremely small quantities of target molecules. Au nanoparticles have been very successful in this respect due to local enhancement of the light intensity caused by their plasmon resonance. Furthermore, Au nanoparticles are biocompatible, and target substances can be easily attached to their surface. Here, we demonstrate that Au-decorated CdSe nanowires when employed as SERS substrates lead to an enhancement as large as 105 with respect to the flat Au surfaces. In the case of hybrid metal–CdSe nanowires, the Au nucleates preferably on lattice defects at the lateral facets of the nanowires, which leads to a homogeneous distribution of Au nanoparticles on the nanowire, and to an efficient quenching of the nanowire luminescence. Moreover, the size of the Au nanoparticles can be well controlled via the AuCl3 concentration in the fabrication process. We demonstrate the effectiveness of our SERS substrates with two target substances, namely, cresyl-violet and rhodamine-6G. Au-decorated nanowires can be easily fabricated in large quantities at low cost by wet-chemical synthesis. Furthermore, their deposition onto various substrates, as well as the functionalization of these wires with the target substances, is as straightforward as with the traditional markers.

Photonic Crystals for Plasmonics: From Fundamentals to Superhydrophobic Devices

In the last couple of decades we have been witnessing an enormous technological advancement in the field of micro-technology to the extent that nowadays we talk about nanotechnology. Faster computers, LCD based mobiles, nanoparticles for UV absorption in suntan lotions are just few of many examples where nanotechnology plays a fundamental role. The merit of this is mainly in the advance of the fabrication methods. Present techniques such as Focused Ion Beam (FIB) lithography guarantee a resolution of less than 10 nanometers which is about five times more precise than ten years before. Also Photonic Crystals (PhCs), among the others, take advantage from this extremely high resolution level allowing a downscale that permits the realization of structures which in principle can work at vey high energy. Historically PhCs were known as Bragg mirrors and only in 1987 (Yablonovitch, 1987; Sajeev, 1987) with the works of Yablonovitch and Sajeev the term Photonic Crystals was introduced. Nowadays, besides their natural application as filters in particular under full band gap conditions, PhCs see a number of applications: optical fibers (Birks et al., 1997; Zhao et al., 2010), vertical cavity surface emitting lasers (Yokouchi et al., 2003), high reflection coatings, temperature sensors (Song et al., 2006), high efficiency solar cells (Bermel et al., 2007), electric field detectors (Song & Proietti Zaccaria, 2007), non-linear analysis (Malvezzi et al., 2002; Malvezzi et al., 2003), just to name a few. Many are the techniques for the fabrication of PhCs, for example by means of focused-ion beam (Cabrini et al., 2005), two-photon fabrication (Deubel et al., 2004), laser-interference (Proietti Zaccaria et al., 2008a) or waver-fusion techniques (Takahashi et al., 2006). Here we shall focus on the role that PhCs can play for another exciting discipline known as Plasmonics. It refers to the capability of some devices of sustaining an optical surface mode, namely an electromagnetic wave travelling at the interface between two different materials such as a dielectric and a metal. Such a wave originates from the coupling of incident photons on the interface with

3D plasmonic nanostructures as building blocks for ultrasensitive Raman spectroscopy

The fabrication of complex 3D plasmonic nanostructures opens new scenarios towards the realization of high electric field confinement and enhancement. We exploit the unique properties of these nanostructures for performing Raman spectroscopy in the single/few molecules detection limit.