Browsing by Author "Hentschel, Mario"

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Open Access
Adaptive method for quantitative estimation of glucose and fructose concentrations in aqueous solutions based on infrared nanoantenna optics
(2019) Schuler, Benjamin; Kühner, Lucca; Hentschel, Mario; Giessen, Harald; Tarín, Cristina
In life science and health research one observes a continuous need for new concepts and methods to detect and quantify the presence and concentration of certain biomolecules-preferably even in vivo or aqueous solutions. One prominent example, among many others, is the blood glucose level, which is highly important in the treatment of, e.g., diabetes mellitus. Detecting and, in particular, quantifying the amount of such molecular species in a complex sensing environment, such as human body fluids, constitutes a significant challenge. Surface-enhanced infrared absorption (SEIRA) spectroscopy has proven to be uniquely able to differentiate even very similar molecular species in very small concentrations. We are thus employing SEIRA to gather the vibrational response of aqueous glucose and fructose solutions in the mid-infrared spectral range with varying concentration levels down to 10 g/l. In contrast to previous work, we further demonstrate that it is possible to not only extract the presence of the analyte molecules but to determine the quantitative concentrations in a reliable and automated way. For this, a baseline correction method is applied to pre-process the measurement data in order to extract the characteristic vibrational information. Afterwards, a set of basis functions is fitted to capture the characteristic features of the two examined monosaccharides and a potential contribution of the solvent itself. The reconstruction of the actual concentration levels is then performed by superposition of the different basis functions to approximate the measured data. This software-based enhancement of the employed optical sensors leads to an accurate quantitative estimate of glucose and fructose concentrations in aqueous solutions.
Open Access
Complex 2D & 3D plasmonic nanostructures : Fano resonances, chirality, and nonlinearities
(2013) Hentschel, Mario; Giessen, Harald (Prof. Dr.)
This thesis covers two topics of the still emergent field of plasmonics. On the one hand we make use of the interaction of particle plasmon resonances to create 2D as well as 3D complex plasmonic structures which show radically different optical properties than the individual building blocks do. On the other hand we utilize the strongly enhanced local electric field associated with plasmonic nanostructures for nonlinear optical processes. In particular, we study the formation of Fano resonances in complex nanoparticles arrangements. So-called plasmonic oligomers, that are highly symmetric arrangements of metallic nanoparticles, are discussed in detail. These clusters support dark modes which lead to pronounced scattering minima in their otherwise broad dipolar scatting peaks. We demonstrate the amazing tunability of these clusters and the formation of higher order dark modes. Moreover, we discuss the plasmonic analogue of electromagnetically induced transparency (EIT) in 2D as well as 3D arrangements of metallic nano-bars. We show that such 3D particle groupings are capable of encoding their 3D arrangement in well pronounced and unique optical spectra. We thus envision that our structure can serve as a three-dimensional plasmon ruler enabling the optically determination of three-dimensional arrangements on the nanoscale. Taking the concept of plasmonic EIT one step further, we demonstrate that the destructive interference between normal plasmonic modes, which leads to plasmonic EIT and decreased absorbance in the structure, can be switch to constructive interference and thus enhanced absorbance. It can be argued that this phenomenon is the plasmonic analogue of electromagnetically induced absorbance (EIA). What is more, we discuss the formation of optical chirality in 3D arrangements of metallic nanoparticles which vastly outperform any naturally occurring chiral substances in the strength of their interaction with an external light field. We deduce the prerequisites for this strong response and demonstrate that only configurational chirality, that is a handed arrangement of equally sized particles, leads to a strong plasmonic chiral optical response. Compositional chirality, that is the use of different sized particles in an unhanded arrangement, is not favourable. This finding is in contrast to chemistry and molecular physics where a so-called chiral center, a carbon atom dressed with four different ligands, is the archetype chiral building block. Moreover, we show that it is possible to optically deduce the spatial arrangements of individual particles in these structures, as chirality is an inherently 3D property. Furthermore, we will demonstrate the formation of a strong and broadband chiral optical response upon the formation of charge transfer modes, that is, due to ohmic contact of the clusters constituents. Finally, we demonstrate the plasmonic analogue of diastereomers, structures possessing several chiral centers. We thus construct plasmonic composite structures consisting of two different handed sub-units. We show that the optical response, in striking contrast to their molecular counterparts, can be described in terms of fundamental building blocks. The chiral optical response of such complex structures can thus be traced back to the optical properties of the constituting elements. Finally, we investiagte nonlinear optical processes in plasmonic and plasmonic-dielectric-hybrid systems. In particular, we investigate third harmonic generation from dimer nanoantennas and show that the nonlinear optical response, in contrast to common belief, is not governed by gap nonlinearities but fully described by the linear optical properties of the antenna. A simple nonlinear harmonic oscillator model is shown to reproduce all experimental features. Moreover, we will discuss the selective filling of bowtie nanoantennas with the chi2 active material LiNbO3 and the nonlinear optical response of this hybrid system. As an outlook we discuss the role of symmetries in nonlinear optics and the perceived implications for nonlinear plasmon optics.
Open Access
Dielectric Mie voids : confining light in air
(2023) Hentschel, Mario; Koshelev, Kirill; Sterl, Florian; Both, Steffen; Karst, Julian; Shamsafar, Lida; Weiss, Thomas; Kivshar, Yuri; Giessen, Harald
Manipulating light on the nanoscale has become a central challenge in metadevices, resonant surfaces, nanoscale optical sensors, and many more, and it is largely based on resonant light confinement in dispersive and lossy metals and dielectrics. Here, we experimentally implement a novel strategy for dielectric nanophotonics: Resonant subwavelength localized confinement of light in air. We demonstrate that voids created in high-index dielectric host materials support localized resonant modes with exceptional optical properties. Due to the confinement in air, the modes do not suffer from the loss and dispersion of the dielectric host medium. We experimentally realize these resonant Mie voids by focused ion beam milling into bulk silicon wafers and experimentally demonstrate resonant light confinement down to the UV spectral range at 265 nm (4.68 eV). Furthermore, we utilize the bright, intense, and naturalistic colours for nanoscale colour printing. Mie voids will thus push the operation of functional high-index metasurfaces into the blue and UV spectral range. The combination of resonant dielectric Mie voids with dielectric nanoparticles will more than double the parameter space for the future design of metasurfaces and other micro- and nanoscale optical elements. In particular, this extension will enable novel antenna and structure designs which benefit from the full access to the modal field inside the void as well as the nearly free choice of the high-index material for novel sensing and active manipulation strategies.
Open Access
Electro-active metaobjective from metalenses-on-demand
(2022) Karst, Julian; Lee, Yohan; Floess, Moritz; Ubl, Monika; Ludwigs, Sabine; Hentschel, Mario; Giessen, Harald
Switchable metasurfaces can actively control the functionality of integrated metadevices with high efficiency and on ultra-small length scales. Such metadevices include active lenses, dynamic diffractive optical elements, or switchable holograms. Especially, for applications in emerging technologies such as AR (augmented reality) and VR (virtual reality) devices, sophisticated metaoptics with unique functionalities are crucially important. In particular, metaoptics which can be switched electrically on or off will allow to change the routing, focusing, or functionality in general of miniaturized optical components on demand. Here, we demonstrate metalenses-on-demand made from metallic polymer plasmonic nanoantennas which are electrically switchable at CMOS (complementary metal-oxide-semiconductor) compatible voltages of ±1 V. The nanoantennas exhibit plasmonic resonances which can be reversibly switched ON and OFF via the applied voltage, utilizing the optical metal-to-insulator transition of the metallic polymer. Ultimately, we realize an electro-active non-volatile multi-functional metaobjective composed of two metalenses, whose unique optical states can be set on demand. Overall, our work opens up the possibility for a new level of electro-optical elements for ultra-compact photonic integration.
Open Access
Hybrid fiber-solid-state laser with 3D-printed intracavity lenses
(2023) Angstenberger, Simon; Ruchka, Pavel; Hentschel, Mario; Steinle, Tobias; Giessen, Harald
Open Access
Interaction of edge exciton polaritons with engineered defects in the hyperbolic material Bi2Se3
(2021) Lingstädt, Robin; Talebi, Nahid; Hentschel, Mario; Mashhadi, Soudabeh; Gompf, Bruno; Burghard, Marko; Giessen, Harald; Aken, Peter A. van
Hyperbolic materials exhibit unique properties that enable intriguing applications in nanophotonics. The topological insulator Bi2Se3 represents a natural hyperbolic optical medium, both in the THz and visible range. Here, using cathodoluminescence spectroscopy and electron energy-loss spectroscopy, we demonstrate that Bi2Se3 supports room-temperature exciton polaritons and explore the behavior of hyperbolic edge exciton polaritons, which are hybrid modes resulting from the coupling of the polaritons bound to the upper and lower edges of Bi2Se3 nanoplatelets. We compare Fabry-Pérot-like resonances emerging in edge polariton propagation along pristine and artificially structured edges and experimentally demonstrate the possibility to steer edge polaritons by means of grooves and nanocavities. The observed scattering of edge polaritons by defect structures is found to be in good agreement with finite-difference time-domain simulations. Our findings reveal the extraordinary capability of hyperbolic polariton propagation to cope with the presence of defects, providing an excellent basis for applications such as nanooptical circuitry, nanoscale cloaking and nanoscopic quantum technology.
Open Access
Machine learning methods of regression for plasmonic nanoantenna glucose sensing
(2021) Corcione, Emilio; Pfezer, Diana; Hentschel, Mario; Giessen, Harald; Tarín, Cristina
The measurement and quantification of glucose concentrations is a field of major interest, whether motivated by potential clinical applications or as a prime example of biosensing in basic research. In recent years, optical sensing methods have emerged as promising glucose measurement techniques in the literature, with surface-enhanced infrared absorption (SEIRA) spectroscopy combining the sensitivity of plasmonic systems and the specificity of standard infrared spectroscopy. The challenge addressed in this paper is to determine the best method to estimate the glucose concentration in aqueous solutions in the presence of fructose from the measured reflectance spectra. This is referred to as the inverse problem of sensing and usually solved via linear regression. Here, instead, several advanced machine learning regression algorithms are proposed and compared, while the sensor data are subject to a pre-processing routine aiming to isolate key patterns from which to extract the relevant information. The most accurate and reliable predictions were finally made by a Gaussian process regression model which improves by more than 60% on previous approaches. Our findings give insight into the applicability of machine learning methods of regression for sensor calibration and explore the limitations of SEIRA glucose sensing.
Open Access
Mass-producible micro-optical elements by injection compression molding and focused ion beam structured titanium molding tools
(2020) Ristok, Simon; Roeder, Marcel; Thiele, Simon; Hentschel, Mario; Guenther, Thomas; Zimmermann, André; Herkommer, Alois; Giessen, Harald
Open Access
Predicting concentrations of mixed sugar solutions with a combination of resonant plasmon-enhanced SEIRA and principal component analysis
(2022) Pfezer, Diana; Karst, Julian; Hentschel, Mario; Giessen, Harald
The detection and quantification of glucose concentrations in human blood or in the ocular fluid gain importance due to the increasing number of diabetes patients. A reliable determination of these low concentrations is hindered by the complex aqueous environments in which various biomolecules are present. In this study, we push the detection limit as well as the discriminative power of plasmonic nanoantenna-based sensors towards the physiological limit. We utilize plasmonic surface-enhanced infrared absorption spectroscopy (SEIRA) to study aqueous solutions of mixtures of up to five different physiologically relevant saccharides, namely the monosaccharides glucose, fructose, and galactose, as well as the disaccharides maltose and lactose. Resonantly tuned plasmonic nanoantennas in a reflection flow cell geometry allow us to enhance the specific vibrational fingerprints of the mono- and disaccharides. The obtained spectra are analyzed via principal component analysis (PCA) using a machine learning algorithm. The high performance of the sensor together with the strength of PCA allows us to detect concentrations of aqueous mono- and disaccharides solutions down to the physiological levels of 1 g/L. Furthermore, we demonstrate the reliable discrimination of the saccharide concentrations, as well as compositions in mixed solutions, which contain all five mono- and disaccharides simultaneously. These results underline the excellent discriminative power of plasmonic SEIRA spectroscopy in combination with the PCA. This unique combination and the insights gained will improve the detection of biomolecules in different complex environments.
Open Access
Tailored optical functionality by combining electron‐beam and focused gold‐ion beam lithography for solid and inverse coupled plasmonic nanostructures
(2020) Hentschel, Mario; Karst, Julian; Giessen, Harald
Plasmonics is a field uniquely driven by advances in micro‐ and nanofabrication. Many design ideas pose significant challenges in their experimental realization and test the limits of modern fabrication techniques. Here, the combination of electron‐beam and gold ion‐beam lithography is introduced as an alternative and highly versatile route for the fabrication of complex and high fidelity plasmonic nanostructures. The capability of this strategy is demonstrated on a selection of planar as well as 3D nanostructures. Large area and extremely accurate structures are presented with little to no defects and errors. These structures exhibit exceptional quality in shape fidelity and alignment precision. The combination of the two techniques makes full use of their complementary capabilities for the realization of complex plasmonic structures with superior optical properties and functionalities as well as ultra‐distinct spectral features which will find wide application in plasmonics, nanooptics, metasurfaces, plasmonic sensing, and similar areas.
Open Access
Tailoring enhanced optical chirality : design principles for chiral plasmonic nanostructures
(2012) Schäferling, Martin; Dregely, Daniel; Hentschel, Mario; Giessen, Harald
Electromagnetic fields with strong optical chirality can be formed in the near-field of chiral plasmonic nanostructures. We calculate and visualize the degree of chirality to identify regions with relatively high values. This leads to design principles for a simple utilization of chiral fields. We investigate planar geometries which offer a convenient way to access the designated fields as well as three-dimensional nanostructures which show a very high local optical chirality.
Open Access
Towards fiber-coupled plasmonic perfect absorber superconducting nanowire photodetectors for the near- and mid-infrared
(2023) Mennle, Sandra; Karl, Philipp; Ubl, Monika; Ruchka, Pavel; Weber, Ksenia; Hentschel, Mario; Flad, Philipp; Giessen, Harald