Since its discovery, the asymmetric Fano resonance has been a characteristic feature of interacting quantum systems. The shape of this resonance is distinctively different from that of conventional symmetric resonance curves. Recently, the Fano resonance has been found in plasmonic nanoparticles, photonic crystals, and electromagnetic metamaterials. The steep dispersion of the Fano resonance profile promises applications in sensors, lasing, switching, and nonlinear and slow-light devices.

The Fano resonance in plasmonic nanostructures and metamaterials

Localized Surface Plasmon Resonance Sensors

Plasmons in strongly coupled metallic nanostructures.

We experimentally demonstrate a perfect plasmonic absorber at λ = 1.6 μm. Its polarization-independent absorbance is 99% at normal incidence and remains very high over a wide angular range of incidence around ±80°. We introduce a novel concept to utilize this perfect absorber as plasmonic sensor for refractive index sensing. This sensing strategy offers great potential to maintain the performance of localized surface plasmon sensors even in nonlaboratory environments due to its simple and robust measurement scheme.

Infrared Perfect Absorber and Its Application As Plasmonic Sensor

Photonic metamaterials are man-made structures composed of tailored micro- or nanostructured metallodielectric subwavelength building blocks. This deceptively simple yet powerful concept allows the realization of many new and unusual optical properties, such as magnetism at optical frequencies, negative refractive index, large positive refractive index, zero reflection through impedance matching, perfect absorption, giant circular dichroism and enhanced nonlinear optical properties. Possible applications of metamaterials include ultrahigh-resolution imaging systems, compact polarization optics and cloaking devices. This Review describes recent progress in the fabrication of three-dimensional metamaterial structures and discusses some of the remaining challenges.

Past achievements and future challenges in the development of three-dimensional photonic metamaterials

We experimentally demonstrate a planar metamaterial analogue of electromagnetically induced transparency at optical frequencies. The structure consists of an optically bright dipole antenna and an optically dark quadrupole antenna, which are cut-out structures in a thin gold film. A pronounced coupling-induced reflectance peak is observed within a broad resonance spectrum. A metamaterial sensor based on these coupling effects is experimentally demonstrated and yields a sensitivity of 588 nm/RIU and a figure of merit of 3.8.

Planar metamaterial analogue of electromagnetically induced transparency for plasmonic sensing.

We present a method to enhance the sensing properties of a localized plasmon sensor. The concept is based on the combination of localized plasmons in nanostructures and a photonic microcavity. Metal nanorods that are placed at Bragg distance above a metal mirror form a Fabry–Perot microcavity and constitute a coupled photonic-plasmonic system. The localized plasmon resonances of the nanorods and the phase shifts upon plasmon excitation are extremely sensitive to changes in the refractive index of the material surrounding the nanorods. Compared to the plasmonic nanorods alone, the coupled photonic-plasmonic system allows for a much more sensitive detection of small refractive index changes.

Cavity-enhanced localized plasmon resonance sensing

We introduce the concept of nonlinear plasmonic sensing, relying on third harmonic generation from simple plasmonic nanoantennas. Because of the nonlinear conversion process we observe a larger sensitivity to a local change in the refractive index as compared to the commonly used linear localized surface plasmon resonance sensing. Refractive index changes as small as 10–3 can be detected. In order to determine the spectral position of highest sensitivity, we perform linear and third harmonic spectroscopy on plasmonic nanoantenna arrays, which are the fundamental building blocks of our sensor. Furthermore, simultaneous detection of linear and nonlinear signals allows quantitative comparison of both methods, providing further insight into the working principle of our sensor. While the signal-to-noise ratio is comparable, nonlinear sensing gives about seven times higher relative signal changes.

Nonlinear Plasmonic Sensing.

Plasmonic nanostructures are intensely investigated as sensors due to their high sensitivity to changes in their nearest dielectric environment. However, additional steps have to be taken to provide specificity for a particular analyte. Here, gold nanoantennas are coated with a thin layer of a boronic acid functionalized hydrogel, which reversibly swells in the presence of glucose. This process is especially sensitive to low glucose concentrations and allows for measurement of values in the physiological millimolar range. The boronic acid is highly specific to glucose, and large molecules, such as proteins, which could cause possible disturbances to the measurements, are successfully blocked from the plasmonic sensing volume by the hydrogel film. Our concept is thus suited to detect physiological glucose levels in the tear liquid under the presence of ambient proteins using functionalized plasmonic sensors on contact lenses at eyesafe wavelengths for optical readout.

Martin Mesch

Papers

Infrared Perfect Absorber and Its Application As Plasmonic Sensor

Planar metamaterial analogue of electromagnetically induced transparency for plasmonic sensing.

Cavity-enhanced localized plasmon resonance sensing

Nonlinear Plasmonic Sensing.

Functionalized Hydrogel on Plasmonic Nanoantennas for Noninvasive Glucose Sensing