Martin Mesch

Infrared perfect absorber and its application as plasmonic sensor.

We experimentally demonstrate a perfect plasmonic absorber at lambda = 1.6 microm. Its polarization-independent absorbance is 99% at normal incidence and remains very high over a wide angular range of incidence around +/-80 degrees. 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.

Cavity-enhanced localized plasmon resonance 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.

Nonlinear Plasmonic 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.

From dark to bright: first-order perturbation theory with analytical mode normalization for plasmonic nanoantenna arrays applied to refractive index sensing

We present a first-order perturbation theory to calculate the frequency shift and linewidth change of photonic resonances in one- and two-dimensional periodic structures under modifications of the surrounding refractive index. Our method is based on the resonant state expansion, for which we extend the analytical mode normalization to periodic structures. We apply this theory to calculate the sensitivity of bright dipolar and much darker quadrupolar plasmonic modes by determining the maximum shift and optimal sensing volume.

Plasmon hybridization in stacked metallic nanocups

We investigate hybridized electric and magnetic plasmon modes in stacked nanocups. To elucidate the coupling mechanism we demonstrate the analogy between split-ring-resonators and nanocups in the case of dipolar excitation and compare the behavior of stacked nanocups to stacked split-ring-resonators. The interplay of electric coupling with the symmetric and antisymmetric coupling of magnetic moments in effective split-ring-resonator resonances in the nanocups leads to experimentally observed hybridized modes in the coupled nanocup system. Our stacked nanocups are easily manufacturable at low cost, they cover a large-area, and can serve as SERS or SEIRA substrates. They might also serve as novel plasmonic nanoantennas, as templates for nonlinear plasmonics, and as stacked meander surfaces for metamaterial-assisted imaging.

Regression methods for ophthalmic glucose sensing using metamaterials

We present a novel concept for in vivo sensing of glucose using metamaterials in combination with automatic learning systems. In detail, we use the plasmonic analogue of electromagnetically induced transparency (EIT) as sensor and evaluate the acquired data with support vector machines. The metamaterial can be integrated into a contact lens. This sensor changes its optical properties such as reflectivity upon the ambient glucose concentration, which allows for in situ measurements in the eye. We demonstrate that estimation errors below 2% at physiological concentrations are possible using simulations of the optical properties of the metamaterial in combination with an appropriate electrical circuitry and signal processing scheme. In the future, functionalization of our sensor with hydrogel will allow for a glucose-specific detection which is insensitive to other tear liquid substances providing both excellent selectivity and sensitivity.

Highly Sensitive Refractive Index Sensors with Plasmonic Nanoantennas−Utilization of Optimal Spectral Detuning of Fano Resonances

We analyze and optimize the performance of coupled plasmonic nanoantennas for refractive index sensing. The investigated structure supports a sub- and super-radiant mode that originates from the weak coupling of a dipolar and quadrupolar mode, resulting in a Fano-type spectral line shape. In our study, we vary the near-field coupling of the two modes and particularly examine the influence of the spectral detuning between them on the sensing performance. Surprisingly, the case of matched resonance frequencies does not provide the best sensor. Instead, we find that the right amount of coupling strength and spectral detuning allows for achieving the ideal combination of narrow line width and sufficient excitation strength of the subradiant mode, and therefore results in optimized sensor performance. Our findings are confirmed by experimental results and first-order perturbation theory. The latter is based on the resonant state expansion and provides direct access to resonance frequency shifts and line width changes as well as the excitation strength of the modes. Based on these parameters, we define a figure of merit that can be easily calculated for different sensing geometries and agrees well with the numerical and experimental results.

Dynamic modeling of the hydrogel molecular filter in a metamaterial biosensing system for glucose concentration estimation.

We present a novel concept for ophthalmic glucose sensing using a biosensing system that consists of plasmonic dipole metamaterial covered by a layer of functionalized hydrogel. The metamaterial together with the hydrogel can be integrated into a contact lens. This optical sensor changes its properties such as reflectivity upon the ambient glucose concentration, which allows in situ measurements in the eye. The functionalization of the sensor with hydrogel allows for a glucose-specific detection, providing both selectivity and sensitivity. As a result of the presented work we derive a dynamic model of the hydrogel that can be used for further simulation studies.

Discrete wavelength selection for the optical readout of a metamaterial biosensing system for glucose concentration estimation via a support vector regression model

In this contribution, a method to select discrete wavelengths that allow an accurate estimation of the glucose concentration in a biosensing system based on metamaterials is presented. The sensing concept is adapted to the particular application of ophthalmic glucose sensing by covering the metamaterial with a glucose-sensitive hydrogel and the sensor readout is performed optically. Due to the fact that in a mobile context a spectrometer is not suitable, few discrete wavelengths must be selected to estimate the glucose concentration. The developed selection methods are based on nonlinear support vector regression (SVR) models. Two selection methods are compared and it is shown that wavelengths selected by a sequential forward feature selection algorithm achieves an estimation improvement. The presented method can be easily applied to different metamaterial layouts and hydrogel configurations.

Noninvasive optical glucose monitoring at physiological levels using a functionalized plasmonic sensor

Summary form only given. Optical glucose sensing is crucial for noninvasive diabetes monitoring. One possibility is to analyze the tear liquid in the eye. We utilize complex plasmonic nanostructures and the slopes of their resonance structures in combination with a hydrogel layer that was functionalized with aminophenylboronic acid to selectively detect glucose in an aqueous solution at the millimolar level, which represents physiological concentrations in the blood as well as in the tear liquid. We observe spectral shifts in the range of a few nanometers that are linear for not too high concentrations due to local refractive index variation at the near fields of the gold nanostructures.The transmittance curves show good enough reproducibility and low spectral shift variation upon repeated experiments, which can be easily compensated using data processing techniques. We also demonstrate that our structure does not show hysteresis upon exposure and withdrawal of glucose even at levels of 50 mM. We integrated the functionalized nanostructure into a demonstrator that was able to record and display the glucose level on a tablet computer in real time. Our sensor concept could be utilized on a soft contact lens that could be worn for a few days, read out either by a laser/photodiode combination in reflection, in particular as we work at eye-safe wavelengths around 1.5 μm.