Terahertz magnetic response of plasmonic metasurface resonators: origin and orientation dependence.

The increasing miniaturization of everyday devices necessitates advancements in surface-sensitive techniques to access phenomena more effectively. Magnetic resonance methods, such as nuclear or electron paramagnetic resonance, play a crucial role due to their unique analytical capabilities. Recently, the development of a novel plasmonic metasurface resonator aimed at boosting the THz electron magnetic response in 2D materials resulted in a significant magnetic field enhancement, confirmed by both numerical simulations and experimental data. Yet, the mechanisms driving this resonance were not explored in detail. In this study, we elucidate these mechanisms using two semi-analytical models: one addressing the resonant behaviour and the other examining the orientation-dependent response, considering the anisotropy of the antennas and experimental framework. Our findings contribute to advancing magnetic spectroscopic techniques, broadening their applicability to 2D systems.

Direct electron beam patterning of electro-optically active PEDOT:PSS.

The optical and electronic tunability of the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has enabled emerging applications as diverse as bioelectronics, flexible electronics, and micro- and nano-photonics. High-resolution spatial patterning of PEDOT:PSS opens up opportunities for novel active devices in a range of fields. However, typical lithographic processes require tedious indirect patterning and dry etch processes, while solution-processing methods such as ink-jet printing have limited spatial resolution. Here, we report a method for direct write nano-patterning of commercially available PEDOT:PSS through electron-beam induced solubility modulation. The written structures are water stable and maintain the conductivity as well as electrochemical and optical properties of PEDOT:PSS, highlighting the broad utility of our method. We demonstrate the potential of our strategy by preparing prototypical nano-wire structures with feature sizes down to 250 nm, an order of magnitude finer than previously reported direct write methods, opening the possibility of writing chip-scale microelectronic and optical devices. We finally use the high-resolution writing capabilities to fabricate electrically-switchable optical diffraction gratings. We show active switching in this archetypal system with >95 % contrast at CMOS-compatible voltages of +2 V and -3 V, offering a route towards highly-miniaturized dynamic optoelectronic devices.

Ultracompact wavefront characterization of femtosecond 3D printed microlenses using double-frequency Ronchi interferometry.

3D printed microoptics have become important tools for miniature endoscopy, novel CMOS-based on-chip sensors, OCT-fibers, among others. Until now, only image quality and spot diagrams were available for optical characterization. Here, we introduce Ronchi interferometry as ultracompact and quick quantitative analysis method for measuring the wavefront aberrations after propagating coherent light through the 3D printed miniature optics. We compare surface shapes by 3D confocal microscopy with optical characterizations by Ronchi interferograms. Phase retrieval gives us the transversal wave front aberration map, which indicates that the aberrations of our microlenses that have been printed with a Nanoscribe GT or Quantum X printer exhibit RMS wavefront aberrations as small as λ/20, Strehl ratios larger than 0.91, and near-diffraction limited modulation transfer functions. Our method will be crucial for future developments of 3D printed microoptics, as the method is ultracompact, ultra-stable, and very fast regarding measurement and evaluation. It could fit directly into a 3D printer and allows for in-situ measurements right after printing as well as fast iterations for improving the shape of the optical surface.

Hybrid fiber-solid-state laser with 3D-printed intracavity lenses.

Microscale 3D-printing has revolutionized micro-optical applications ranging from endoscopy, imaging, to quantum technologies. In all these applications, miniaturization is key, and in combination with the nearly unlimited design space, it is opening novel, to the best of our knowledge, avenues. Here, we push the limits of miniaturization and durability by realizing the first fiber laser system with intra-cavity on-fiber 3D-printed optics. We demonstrate stable laser operation at over 20 mW output power at 1063.4 nm with a full width half maximum (FWHM) bandwidth of 0.11 nm and a maximum output power of 37 mW. Furthermore, we investigate the power stability and degradation of 3D-printed optics at Watt power levels. The intriguing possibilities afforded by free-form microscale 3D-printed optics allow us to combine the gain in a solid-state crystal with fiber guidance in a hybrid laser concept. Therefore, our novel ansatz enables the compact integration of a bulk active media in fiber platforms at substantial power levels.

Spatially Resolved Nonlinear Plasmonics.

Nonlinear optical plasmonics investigates the emission of plasmonic nanoantennas with the aid of nonlinear spectroscopy. Here we introduce nonlinear spatially resolved spectroscopy (NSRS) which is capable of imaging the k-space as well as spatially resolving the THG signal of gold nanoantennas and investigating the emission of individual antennas by wide-field illumination of entire arrays. Hand in hand with theoretical simulations, we demonstrate our ability of imaging various oscillation modes inside the nanostructures and therefore spatial emission hotspots. Upon increasing intensity of the femtosecond excitation, an individual destruction threshold can be observed. We find certain antennas becoming exceptionally bright. By investigating those samples taking structural SEM images of the nanoantenna arrays afterward, our spatially resolved nonlinear image can be correlated with this data proving that antennas had deformed into a peanut-like shape. Thus, our NSRS setup enables the investigation of a nonlinear self-enhancement process of nanoantennas under critical laser excitation.

Electrically switchable metallic polymer metasurface device with gel polymer electrolyte.

We present an electrically switchable, compact metasurface device based on the metallic polymer PEDOT:PSS in combination with a gel polymer electrolyte. Applying square-wave voltages, we can reversibly switch the PEDOT:PSS from dielectric to metallic. Using this concept, we demonstrate a compact, standalone, and CMOS compatible metadevice. It allows for electrically controlled ON and OFF switching of plasmonic resonances in the 2-3 µm wavelength range, as well as electrically controlled beam switching at angles up to 10°. Furthermore, switching frequencies of up to 10 Hz, with oxidation times as fast as 42 ms and reduction times of 57 ms, are demonstrated. Our work provides the basis towards solid state switchable metasurfaces, ultimately leading to submicrometer-pixel spatial light modulators and hence switchable holographic devices.

Dielectric Mie voids: confining light in air.

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.

Electro-active metaobjective from metalenses-on-demand.

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.

Launching Coherent Acoustic Phonon Wave Packets with Local Femtosecond Coulomb Forces.

Generation and manipulation of coherent acoustic phonons enables ultrafast control of solids and has been exploited for applications in various acoustic devices. We show that localized coherent acoustic phonon wave packets can be launched by ultrafast Coulomb forces in a scanning tunneling microscope (STM) using tip-enhanced terahertz electric fields. The wave packets propagate at the speed of the longitudinal acoustic phonon, creating standing waves up to 0.26 THz for a 6.4 nm thin Au film on mica. The ultrafast lattice displacement can be as large as 5 pm and is precisely controlled by varying the tip-sample distance. This nonthermal femtosecond Coulomb-force-based excitation mechanism is applicable in nano-optomechanics for advanced terahertz engineering and opens new perspectives in exploiting coherent phonons at the atomic scale.

Predicting Concentrations of Mixed Sugar Solutions with a Combination of Resonant Plasmon-Enhanced SEIRA and Principal Component Analysis.

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.

Fabrication and nanophotonic waveguide integration of silicon carbide colour centres with preserved spin-optical coherence.

Optically addressable spin defects in silicon carbide (SiC) are an emerging platform for quantum information processing compatible with nanofabrication processes and device control used by the semiconductor industry. System scalability towards large-scale quantum networks demands integration into nanophotonic structures with efficient spin-photon interfaces. However, degradation of the spin-optical coherence after integration in nanophotonic structures has hindered the potential of most colour centre platforms. Here, we demonstrate the implantation of silicon vacancy centres (VSi) in SiC without deterioration of their intrinsic spin-optical properties. In particular, we show nearly lifetime-limited photon emission and high spin-coherence times for single defects implanted in bulk as well as in nanophotonic waveguides created by reactive ion etching. Furthermore, we take advantage of the high spin-optical coherences of VSi centres in waveguides to demonstrate controlled operations on nearby nuclear spin qubits, which is a crucial step towards fault-tolerant quantum information distribution based on cavity quantum electrodynamics.

Plasmonic Metasurface Resonators to Enhance Terahertz Magnetic Fields for High-Frequency Electron Paramagnetic Resonance.

Nanoscale magnetic systems play a decisive role in areas ranging from biology to spintronics. Although, in principle, THz electron paramagnetic resonance (EPR) provides high-resolution access to their properties, lack of sensitivity has precluded realizing this potential. To resolve this issue, the principle of plasmonic enhancement of electromagnetic fields that is used in electric dipole spectroscopies with great success is exploited, and a new type of resonators for the enhancement of THz magnetic fields in a microscopic volume is proposed. A resonator composed of an array of diabolo antennas with a back-reflecting mirror is designed and fabricated. Simulations and THz EPR measurements demonstrate a 30-fold signal increase for thin film samples. This enhancement factor increases to a theoretical value of 7500 for samples confined to the active region of the antennas. These findings open the door to the elucidation of fundamental processes in nanoscale samples, including junctions in spintronic devices or biological membranes.

Electrically switchable metallic polymer nanoantennas.

Electrical switching of a metal-to-insulator transition would provide a building block for integrated electro-optically active plasmonics. In this work, we realize plasmonic nanoantennas from metallic polymers, which show well-pronounced localized plasmon resonances in their metallic state. As a result of the electrochemically driven optical metal-to-insulator transition of the polymer, the plasmonic resonances can be electrically switched fully off and back on at video-rate frequencies of up to 30 hertz by applying alternating voltages of only ±1 volt. With the use of this concept, we demonstrate electrically switchable beam-steering metasurfaces with a 100% contrast ratio in transmission. Our approach will help to realize ultrahigh efficiency plasmonic-based integrated active optical devices, including high-resolution augmented and virtual reality technologies.

Machine Learning Methods of Regression for Plasmonic Nanoantenna Glucose Sensing.

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.

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

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 sources usually follow the symmetry of the electron-induced polarization: transition-radiation-based sources, for example, only generate p-polarized light. Here we demonstrate that the polarization, the bandwidth, and the directionality of photons can be tailored by utilizing photon-sieve-based structures. We design, fabricate, and characterize self-complementary chiral structures made of holes in an Au film and generate light vortex beams with specified angular momentum orders. The incoming electron interacting with the structure generates chiral surface plasmon polaritons on the structured Au surface that scatter into the far field. The outcoupled radiation interferes with transition radiation creating TE- and TM-polarized Laguerre-Gauss light beams with a chiral intensity distribution. The generated vortex light and its unique spatiotemporal features can form the basis for the generation of structured-light electron-driven photon sources.

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

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.

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

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.

Reconfigurable Plasmonic Chirality: Fundamentals and Applications.

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.

Purcell-Enhanced Spontaneous Emission of Molecular Vibrations.

Infrared (IR) spectroscopy of molecular vibrations provides insight into molecular structure, coupling, and dynamics. However, picosecond scale intermolecular and intramolecular many-body interactions, nonradiative relaxation, absorption, and thermalization typically dominate over IR spontaneous emission. We demonstrate how coupling to a resonant IR antenna can enhance spontaneous emission of molecular vibrations. Using time-domain nanoprobe spectroscopy we observe an up to 50% decrease in vibrational dephasing time T_{2,vib}, based on the coupling-induced population decay with T_{κ}≃550 fs and an associated Purcell factor of >10^{6}. This rate enhancement of the spontaneous emission of antenna-coupled molecular vibrations opens new avenues for IR coherent control, quantum information processing, and quantum chemistry.

Utilizing niobium plasmonic perfect absorbers for tunable near- and mid-IR photodetection.

With the fast development of single photon-based technologies such as quantum computing and quantum cryptography, conventional avalanche photodiodes as single photon detectors are not the optimum tools anymore. They are currently replaced by Superconducting Nanowire Single Photon Detectors (SNSPDs) based on the superconducting to normal conducting phase transition. The current challenge with SNSPDs lies in overcoming the trade-off between detection efficiency and recovery time. While a large active area will lead to high detection efficiency, the associated high kinetic inductance causes a long recovery time. Plasmonic effects can play an important role in the absorption enhancement of SNSPDs. Nanostructuring with a suitable geometry can provide a high-absorption cross-section at the intrinsic nanowire surface plasmon resonance, which can be significantly larger than their geometric cross-section. We present a photodetector based on the intrinsic localized surface plasmon resonance of a niobium nanowire, which is one of the common superconductors with low kinetic inductance. Additionally, we are increasing the absorption of our nanostructures even further using a plasmonic perfect absorber scheme. We fabricated a plasmonic perfect absorber superconducting photodetector, investigated its response to external light at resonance, and proved its plasmonic behavior as evidenced by its polarization dependence.