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A new type of optical sensing device based on artificial metamaterials
(Nanowerk Spotlight) Over the past decade, electromagnetic metamaterials have become an extremely active field of research in both the physics and the engineering communities. Metamaterials gain their properties from their structure rather than directly from their composition and show the peculiarity of having an index of refraction at optical frequencies from negative to very high positive values.
In our recent paper in Advanced Materials ("Topological Darkness in Self-Assembled Plasmonic Metamaterials") we suggested a new type of optical sensing device based on artificial metamaterials with topological darkness. Topological darkness from an optical point of view is a singularity with a zero of an optical field (no reflected, scattered, transmitted light) while the phase in the optical field can circulate around these points of zero intensities giving jump up to π.
We demonstrated that topological darkness can be realized in bulk plasmonic metamaterials produced by inexpensive self-assembly methods which could make its application to phase-sensitive detection practical. The 3D plasmonic structures suggested in our work take advantage of the enhanced phase sensitivity which occurs when light intensity drops sharply. Although our work was specific to core-shell Ag@SiO2 nanoparticle layer by layer designs, the findings carry much broader implications of plasmonic nanostructures with regard to understanding why and how 3D design is better and more cost-efficient compared to 2D structures fabricated by electron beam lithography.
Self-assembled core-shell metamaterials
Self-assembled core-shell metamaterials. a) Schematics of one layer. b) Three-layer core–shell metamaterial. c) Cross-section of the structure. d) TEM image of six-layer metamaterial shown at two magnifications ×21333 and ×42667 in inset (TEM operated at 10 kV). (Reprinted with permission by Wiley-VCH Verlag)
Topological darkness was achieved with the help of localized surface plasmon resonances (LSPR) in nanoparticles made of noble metals like gold or silver cores and covered by SiO2 shells.
We examined three-dimension (3D) samples which contain the 4-7 layers of core-shell Ag@SiO2 nanoparticles. These samples were prepared by repeating 4-7 times a single-layer procedure based on the inverse Langmuir-Shaefer method which is very attractive for applications due to relatively low fabrications costs.
Such bulk plasmonic metamaterials contain the assembly of a huge number of nano-sized resonators (of order 1012 or more per mm3) and can cover a large area (we investigated sample with size of 1 cm × 1 cm).
Every resonator exhibits localized surface plasmon resonances when irradiated by visible light. We have experimentally and theoretically studied the effect of the morphology of the core-shell nanoparticles on the frequency and width of the LSPR and their optical behaviors. We found that the dielectric resonances, due to excitation of LSPS, can be obtained from simple core-shell 1-3 layers, whereas more complex morphologies like 4-7 layers of mutually coupled Ag@SiO2 nanoparticles are needed to generate topological darkness effects.
Fundamentally, the intensity of transmission/reflection light and trajectory of a light beam are governed by the refractive indices of the media with which light interacts and through which propagates. To achieve the topological darkness, it needs to carefully control the refractive index of the nanostructures.
A simple examination of the Fresnel expressions shows that for any fixed thickness of covering film there exists a range of the indices of refraction which would guarantee the maximal absorption. However, such indices of refraction are impossible to find in natural materials. While we cannot hope to find such materials in nature, the field of artificial metamaterials allows one to engineer the optical materials with desired index of refraction.
We have properly designed plasmonic metamaterials which exhibit topologically protected zero reflection yielding to sharp phase changes nearby. A simple optical scheme based on ellipsometric method is used to detect changes of the intensity and phase of the light reflected from/transmitted through 3D-plasmonic system.
We have shown experimentally that the ellipsometric reflection (polarized reflection) for 4-7 layer of plasmonic metamaterials can demonstrate the occurrence of topological darkness for the fixed angle of incidence and light wavelength.
Our experimental results confirm that all changes of the reflection along the dispersion curve – the curve of the effective complex index of refraction – happen within the range of LSPRs of the silver core (375-425 nm). This implies that the plasmonic effects are responsible for the observed darkness and plasmonic enhancement of electromagnetic fields in blue region should contribute to enhanced phase sensitivity.
We found that the phase of the reflected light displays jump when a reflection curve goes through the point of rapid change and exhibits total darkness. The observed topological darkness can be used to improve the phase-sensitive plasmonic sensing of the materials for chemical, biological and medical applications.
The concept of topological darkness has previously been considered in many papers. Possible first in historical context, the absence of reflection was observed for light reflected from a single interface at the Brewster angle (due to the absence of radiation in the direction of an oscillating dipole). Also, light reflection in the Kretschmann configuration (usually for a thin noble metal film deposited on a dielectric substrate) should in theory provide zero reflection.
For both above mentioned cases, zero reflection can be achieved for very narrow wavelength bands and at fixed angle of incidence. The next generation of this kind of devices were based on coupling of incident light waves with propagating and localized surface plasmon modes.
In our previous work we have experimentally demonstrated blackbody-like behaviors in a thin nanostructured gold film with thickness of just 90 nm ("Plasmonic blackbody: Almost complete absorption of light in nanostructured metallic coatings"). The topological darkness in plasmonic blackbody films was observed over a wide optical wavelength range and for broad range of angles of light incidence. Later similar properties was recorded in nanostructures known as a coherent absorber ("Time-Reversed Lasing and Interferometric Control of Absorption"). Such nanostructures may find applications in future generation of solar cells.
In order to test our devices, we covered them with graphene. Graphene is one of the best materials to measure the sensitivity of such nanostructures. Then we introduced hydrogen onto the graphene, which allowed us to calibrate the devices with far superior sensitivity than with any other material.
By using reversible hydrogenation of graphene and binding of streptavidin-biotin, we demonstrated an areal mass sensitivity at a level of fgmm-2 and detection of individual biomolecules, respectively. This high sensitivity relied on the topological properties of the light phase.
In this research, we intended to integrate previous knowledge about topological darkness to further improve the performance of plasmonic nanostructures, especially in terms of how large sharp phase changes can be achieved near zero reflection. We found that a 3D -plasmonic system exhibits sensitivity of an order of magnitude better than existing 2D nanostructures due to phase jump larger than π.
Our technology also paves the way, using electron-beam lithography manufacturing, for low-cost fabrication of conventional plasmonic nanosensors based on light phase changes.
We hope that these devices will provide the capability to reveal a single molecule through an optical system, and to analyze its components very quickly. For example, devices based on plasmonic effect can be used for continuously monitoring drug-abusing athletes, finding drug traffickers and terrorists concealing explosives, while it could also detect the presence of human viruses with an accuracy greater than ever before.
By Dr. Vasyl G. Kravets, Research Associate, University of Manchester
 

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