Showing papers by "Mario Hentschel published in 2020"

Reconfigurable Plasmonic Chirality: Fundamentals and Applications.

Abstract: Molecular chirality is a geometric property that is of great importance in chemistry, biology, and medicine. Recently, plasmonic nanostructures that exhibit distinct chiroptical responses have attracted tremendous interest, given their ability to emulate the properties of chiral molecules with tailored and pronounced optical characteristics. However, the optical chirality of such human-made structures is in general static and cannot be manipulated postfabrication. Herein, different concepts to reconfigure the chiroptical responses of plasmonic nano- and micro-objects are outlined. Depending on the utilized strategies and stimuli, the chiroptical signature, the 3D structural conformation, or both can be reconfigured. Optical devices based on plasmonic nanostructures with reconfigurable chirality possess great potential in practical applications, ranging from polarization conversion elements to enantioselective analysis, chiral sensing, and catalysis.

Watching in situ the hydrogen diffusion dynamics in magnesium on the nanoscale.

Abstract: Active plasmonic and nanophotonic systems require switchable materials with extreme material contrast, short switching times, and negligible degradation. On the quest for these supreme properties, an in-depth understanding of the nanoscopic processes is essential. Here, we unravel the nanoscopic details of the phase transition dynamics of metallic magnesium (Mg) to dielectric magnesium hydride (MgH2) using free-standing films for in situ nanoimaging. A characteristic MgH2 phonon resonance is used to achieve unprecedented chemical specificity between the material states. Our results reveal that the hydride phase nucleates at grain boundaries, from where the hydrogenation progresses into the adjoining nanocrystallites. We measure a much faster nanoscopic hydride phase propagation in comparison to the macroscopic propagation dynamics. Our innovative method offers an engineering strategy to overcome the hitherto limited diffusion coefficients and has substantial impact on the further design, development, and analysis of switchable phase transition as well as hydrogen storage and generation materials.

Electron-driven photon sources for correlative electron-photon spectroscopy with electron microscopes

Abstract: Abstract Electron beams in electron microscopes are efficient probes of optical near-fields, thanks to spectroscopy tools like electron energy-loss spectroscopy and cathodoluminescence spectroscopy. Nowadays, we can acquire multitudes of information about nanophotonic systems by applying space-resolved diffraction and time-resolved spectroscopy techniques. In addition, moving electrons interacting with metallic materials and optical gratings appear as coherent sources of radiation. A swift electron traversing metallic nanostructures induces polarization density waves in the form of electronic collective excitations, i.e., the so-called plasmon polariton. Propagating plasmon polariton waves normally do not contribute to the radiation; nevertheless, they diffract from natural and engineered defects and cause radiation. Additionally, electrons can emit coherent light waves due to transition radiation, diffraction radiation, and Smith-Purcell radiation. Some of the mechanisms of radiation from electron beams have so far been employed for designing tunable radiation sources, particularly in those energy ranges not easily accessible by the state-of-the-art laser technology, such as the THz regime. Here, we review various approaches for the design of coherent electron-driven photon sources. In particular, we introduce the theory and nanofabrication techniques and discuss the possibilities for designing and realizing electron-driven photon sources for on-demand radiation beam shaping in an ultrabroadband spectral range to be able to realize ultrafast few-photon sources. We also discuss our recent attempts for generating structured light from precisely fabricated nanostructures. Our outlook for the realization of a correlative electron-photon microscope/spectroscope, which utilizes the above-mentioned radiation sources, is also described.

Electrons Generate Self-Complementary Broadband Vortex Light Beams Using Chiral Photon Sieves.

Abstract: Planar electron-driven photon sources have been recently proposed as miniaturized light sources, with prospects for ultrafast conjugate electron-photon microscopy and spectral interferometry. Such ...

Interaction of edge exciton polaritons with engineered defects in the van der Waals material Bi2Se3

Abstract: Hyperbolic materials exhibit unique properties that enable a variety of 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, in addition to being a hyperbolic material, supports room-temperature exciton polaritons. Moreover, we explore the behavior of hyperbolic edge exciton polaritons in Bi2Se3. Edge polaritons are hybrid modes that result from the coupling of the polaritons bound to the upper and lower edges of Bi2Se3 nanoplatelets. In particular, we use electron energy-loss spectroscopy to compare Fabry-Perot-like resonances emerging in edge polariton propagation along pristine and artificially structured edges of the nanoplatelets. The experimentally observed scattering of edge polaritons by defect structures was found to be in good agreement with finite-difference time-domain simulations. Moreover, we experimentally proved coupling of localized polaritons in identical open and closed circular nanocavities to the propagating edge polaritons. Our findings are testimony to the extraordinary capability of the hyperbolic polariton propagation to cope with the presence of defects. This provides an excellent basis for applications such as nanooptical circuitry, cloaking at the nanometer scale, as well as nanoscopic quantum technology on the nanoscale.

Optical properties of niobium nitride plasmonic nanoantennas for the near- and mid-infrared spectral range

Abstract: Investigating new materials plays a very important role for advancing the field of nanofabrication and nanoplasmonics. Even though niobium nitride (NbN) is mainly known for its superconducting properties when fabricating superconducting nanowire single-photon detectors, we demonstrate that it is also a material for plasmonic nanoantenna applications. In this work we measure physical properties of thin NbN films, such as permittivity and superconductivity, and demonstrate the feasibility and tuning of the plasmonic nanoantenna resonance throughout the near- and mid-infrared spectral range. Therefore, we fabricate NbN structures, using electron beam lithography in combination with Ar ion-beam etching. Additionally, we determine the refractory properties of the NbN nanoantennas, namely their high temperature stability. We find that they are stable up to 500°C under ambient conditions. These aspects make them attractive for a multitude of plasmonic applications ranging from refractory emitters/absorbers to candidates for plasmonically improved superconducting single-photon detectors.

Optimizing magnesium thin films for optical switching applications: rules and recipes

Abstract: While magnesium holds great potential as hydrogen storage material, interest has recently shifted to its use in optical switching applications. The hydrogen-induced phase transition from metallic magnesium to dielectric magnesium hydride is a promising candidate for switchable and active plasmonic systems. Most studies in the past have been performed on magnesium thin films and were directed to the investigation and optimization of hydrogen storage rather than to the optical properties. While these studies found a strong influence of the material morphology and crystallinity on the bulk and thin film properties, no in-depth study has revealed rules and recipes to tune and control the nanoscale morphology. Here, we demonstrate that the nanocrystallinity, that is, the crystallite size and morphology on the nanoscale, as well as the surface roughness of magnesium thin films in an optically switchable geometry, can be tuned and adjusted by a comprehensive set of evaporation parameters. The required film geometries, optical properties, and the applications at hand determine the deposition parameters and need to be chosen accordingly. Further, we find that the surface roughness changes drastically upon hydrogenation. Our results have an immediate impact on the understanding as well as the fabrication of optically active devices where magnesium is being used.

Tailored Optical Functionality by Combining Electron-Beam and Focused Gold-Ion Beam Lithography for Solid and Inverse Coupled Plasmonic Nanostructures

Abstract: DOI: 10.1002/adom.202000879 plasmonic structures, they struggle as soon as complex arrangements of different nanoparticles are needed, possibly even distributed on large areas as well as in periodic arrangements. Only recently, with the advent of sophisticated DNA-guided nanotechnology, also in combination with other techniques, it became possible to create such structures.[8–11] In contrast, top-down techniques such as optical lithography and electron-beam lithography straightforwardly offer the required periodic and well defined arrangement on surfaces. Taking these techniques a step further, they are also able to realize 3D arrangements by virtue of mark recognition and layered fabrication.[12] Additionally, these top down techniques allow for a larger variety of structures and can be used to fabricate solid particles of almost any metal or dielectric as well as inverse structures, such as slits and holes in metallic and dielectric films employing Babinet’s principle.[13] Electron-beam lithography (EBL) has been utilized for many years and is a prime candidate for highest resolution structuring and layered processes. Using positive and negative tone resists, lift-off, reactive and nonreactive ion (beam) etching, a plethora of solid and inverse structures can be fabricated.[14,15] The technique, however, faces a few limitations: The fabrication of “sculptured” structures, that is, of structures with varying thickness or shape or of particles with complex 3D shapes, is extremely difficult if not impossible. While EBL gray-scale lithography can be applied in some cases, this technique is intrinsically process-instable and difficult to control. Another limitation lies with the EBL resists as not all samples or structures tolerate resists, either due to contamination issues, due to the fragility of the substrate, for example, in case of transmission electron microscopy (TEM) membranes, or due to very small sample sizes which do not allow for homogeneous resist spin coating. In some of these cases the use of a focused ion-beam (FIB) tool has proven successful.[16,17] Here, a beam of focused ions sputters and allows to structure the material of interest directly.[18–21] The main use of these so-called cross beam tools, consisting of a (crossed) scanning electron column and a focused ion beam column, lies with the fabrication of TEM cuts and lamellas for material inspection. Consequently, the resolution of these tools is limited in two aspects: For so-called FIB cuts as well as TEM lamella production large amounts of material have to be removed which Plasmonics is a field uniquely driven by advances in microand 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.

Electrically switchable metasurface for beam steering using PEDOT polymers

Abstract: We present an electrically switchable metasurface for beam steering where we use the conducting polymer PEDOT as an active material. We show intensity-tunable beam diffraction with angles up to 10°, employing an externally applied voltage.

Mass-producible micro-optical elements by injection compression molding and focused ion beam structured titanium molding tools.

Abstract: We demonstrate mass production compatible fabrication of polymer-based micro Fresnel lenses by injection compression molding. The extremely robust titanium-molding tool is structured with high precision by focused ion beam milling. In order to achieve optimal shape accuracy in the titanium we use an iterative design optimization. The inverse Fresnel lens structured into the titanium is transferred to polymers by injection compression molding, enabling rapid mass replication. We show that the optical performance of the molded diffractive Fresnel lenses is in good agreement with simulations, rendering our approach suitable for applications that require compact and high-quality optical elements in large numbers.

Switchable Optical Nonlinearity at the Metal to Insulator Transition in Magnesium Thin Films

Abstract: We report resonantly enhanced and switchable third order nonlinearity in magnesium thin films. Utilizing an optical parametric oscillator as a tunable broadband light source, we find a highly wavel...

Electrostatic potential mapping at ferroelectric domain walls by low-temperature photoemission electron microscopy

Abstract: Low-temperature X-ray photoemission electron microscopy (X-PEEM) is used to measure the electric potential at domain walls in improper ferroelectric Er0.99Ca0.01MnO3. By combining X-PEEM with scanning probe microscopy and theory, we develop a model that relates the detected X-PEEM contrast to the emergence of uncompensated bound charges, explaining the image formation based on intrinsic electronic domain-wall properties. In contrast to previously applied low-temperature electrostatic force microscopy (EFM), X-PEEM readily distinguishes between positive and negative bound charges at domain walls. Our study introduces an X-PEEM based approach for low-temperature electrostatic potential mapping, facilitating nanoscale spatial resolution and data acquisition times in the order of 0.1-1 sec.