Nanosystems Initiative Munich

Abstract This essay demonstrates how Thomas Mann, in resuming work on The Magic Mountain , incorporated political and religious views encountered not only in contemporary written sources but also in the context of the events following the First World War. These views served him both to construct the antithetical positions of the progressive Settembrini and the reactionary Naphta, the two self-appointed tutors of Hans Castorp, and to formulate the synthesis of these positions, which the protagonist arrives at in the dream of the famous „Snow Chapter.“ This synthesis, the message of The Magic Mountain that transcends the novel itself, expresses the political and religious stance that Thomas Mann developed during the course of writing the novel and which represents his commitment to Weimar democracy. This stance constitutes a synthesis of the German spirit, shaped by Romanticism, with the Western spirit, a cultural synthesis, for which he explicitly invokes Ernst Troeltsch.

Flat optical metasurfaces are transforming photonics research by enabling new ways to control light in ultrathin, versatile photonic devices. The rise of quasi-bound states in the continuum (qBIC) metasurfaces has enabled tailored high-quality (Q) factor resonances in subwavelength nanostructured thin films, analogous to traditional optical cavities. In this perspective, we explore the emergence of cavity quantum electrodynamics (QED) in optical qBIC metasurfaces, specifically those constructed from van der Waals (vdW) layered materials. Because of their remarkable properties, vdW metasurfaces can support intrinsic optical resonances within the same active material hosting luminescent species, such as excitons or defects, leading to optimal light-matter coupling. This approach of self-hybridizing the cavity-emitter system into a single platform effectively overcomes limitations in on-chip integration of conventional cavities. Combining vdW materials with optically engineered qBIC metasurfaces opens exciting possibilities for exploring nanoscale light-matter interactions. Moreover, the distinctive features of vdW materials, from vertical heterostructures to twist-angle-dependent properties, offer a unique platform bridging the condensed matter physics of 2D materials and engineered nanophotonics. We propose that harnessing strong light-matter coupling in vdW-integrated qBIC metasurfaces will pave the way for next-generation nanoscale polaritonic devices.

A platform is introduced to achieve ultra-strong coupling (USC) between light and matter using widely available materials. USC is a light-matter interaction regime characterized by coupling strengths exceeding 10% of the ground state energy. It gives rise to novel physical phenomena, such as efficient single-photon coupling and quantum gates, with applications in quantum sensing, nonlinear optics, and low-threshold lasing. Although early demonstrations in plasmonic systems have been realized, achieving USC in dielectric platforms, which offer lower losses and high Q-factors, remains challenging due to typically low mode overlap between the photonic field and the material resonance. Here, dielectric dual gradient metasurfaces supporting quasi-bound-states-in-the-continuum are leveraged to spatially encode both the spectral and coupling parameter space and demonstrate USC to an epsilon-near-zero (ENZ) mode in an ultra-thin SiO2 layer. The strong out-of-plane electric fields in tapered bar structure overlap exceptionally well with those of the ENZ mode, resulting in a normalized coupling strength of η = 0.10 and a mode splitting equivalent to 20% of the ENZ mode energy; a four-to-five-fold increase compared to previous approaches. The strong field confinement of the approach opens new possibilities for compact and scalable polaritonic devices, such as tunable frequency converters and low-energy optical modulators.

Quasi-bound states in the continuum (qBICs) achieved through symmetry breaking in photonic metasurfaces are a powerful approach for engineering resonances with high quality factors and tunability. However, miniaturization of these devices is limited as the in-plane unit-cell size typically scales linearly with the resonant wavelength. By contrast, polariton resonators can be deeply subwavelength, offering a promising solution for achieving compact devices. Here we demonstrate that low-loss mid-infrared surface phonon polaritons enable metasurfaces supporting qBICs with unit-cell volumes up to 105 times smaller than the free-space volume λ03. Using 100-nm-thick free-standing silicon carbide membranes, we achieve highly confined qBIC states with exceptional robustness against incident-angle variations, a feature unique among qBIC systems. This absence of angular dispersion enables mid-infrared vibrational sensing of thin, weakly absorbing molecular layers using a reflective objective, a method that typically degrades resonance quality in standard qBIC metasurfaces. We introduce surface-phonon-polariton-based qBICs as a platform for ultraconfined nanophotonic systems, advancing the miniaturization of mid-infrared sensors and devices for thermal radiation engineering.

We present a doubly resonant nanocavity for second harmonic generation (SHG) in the visible range, consisting of a 590 nm crystalline gallium phosphide (c-GaP) layer, a gold mirror, and a silica spacer to achieve optimal overlap of pump and harmonic resonances. Experimental results show a conversion efficiency spanning 4 orders of magnitude, with a maximum η̃max = 0.014% at λSHG = 600 nm, comparable to best-performing semiconductor metasurfaces, and surpassing a bulk 400 μm GaP crystal by over an order of magnitude. Using nonlinear scattering theory, we explain the spectral response and demonstrate wavelength tunability via angle of incidence or spacer thickness adjustments. We extend this result to a closed-cavity design with an ultrathin gold top layer which we predict can achieve efficiencies up to η̃max ∼ 0.4% when the thickness of the c-GaP layer approaches the coherence length. This design can be scaled up to planar nanocavities with higher Q-factor and used to enhance the performance of metasurfaces, advancing nonlinear conversion efficiency toward practical applications.

This work presents a D-band fully differential four-stage common-source low noise amplifier (LNA). The design operates in 110 GHz-140 GHz frequency region, with a 22.4 GHz 3-dB bandwidth, achieving a peak gain of 23.5 dB at 115 GHz with a DC power consumption of 52 mW. It has a noise figure of 7.5dB at 140 GHz and 8.2 dB at 120 GHz. To allow broadband simultaneous matching and biasing, transformer-based interstage matching has been utilized. The design also utilizes back-gate biasing to further increase the linearity and gain of the amplifier. Keywords—CMOS, D-band, FDSOI, LNA, mmW, 6G

The optical response of metal/dielectric interfaces is largely influenced by surface plasmon-polariton (SPP) modes. In the mid-infrared (IR) range, SPPs can probe the inner physicochemical properties of metal/dielectric systems, including interfaces with mid-IR polaritonic 2D crystals. Using advanced nanoscopy techniques, we characterize mid-IR SPP modes at air/gold and hexagonal boron nitride (hBN) 2D crystal/gold interfaces via synchrotron infrared nanospectroscopy (SINS) and scattering-scanning near-field optical microscopy (s-SNOM) imaging. SPPs in these systems show micrometer-sized wavelengths and propagation lengths over 20 micrometers at room temperature. In hBN/Au, both SPPs and hyperbolic phonon polaritons (HPhPs) coexist, creating SPP-HPhP wave superposition. The experimental momentum and damping of the SPP waves are determined from the s-SNOM imaging and the SINS spatio-spectral linescan. Thereby, we retrieve the experimental frequency-momentum dispersion relation, presenting excellent agreement with theory. Furthermore, we characterize an anti-crossing of the SPP dispersion near the in-plane transverse optical phonon frequency of hBN, indicating that SPP modes and phonon form a coupled system interacting in the strong coupling regime. Such an interaction of SPPs with phonons can be further explored to enhance the sensibility of mid-IR nanospectroscopy techniques.

Optical vortices promise enhanced optical and quantum information processing via the orbital angular momentum multiplexing. Nanophotonics opens the possibility of realizing subwavelength optical vortices through coupling optical beams into subwavelength surface polaritons in the near-field, although the topological charge is always fixed. We report dispersion-driven topological charge multiplication by leveraging the strong sublinear dispersion of low-loss surface phonon polaritons on silicon carbide membranes, capable of switching the topological charges within a small ∼3% frequency range in the mid-infrared frequency range. This offers the possibility of all-optical ultrafast switching of optical vortices at mid-infrared frequencies.

Abstract Semiconductor‐based surface‐enhanced Raman spectroscopy (SERS) substrates, as a new frontier in the field of SERS, are hindered by their poor electromagnetic field confinement and weak light‐matter interaction. Metasurfaces, a class of 2D artificial materials based on the electromagnetic design of nanophotonic resonators, enable strong electromagnetic field enhancement and optical absorption engineering for a wide range of semiconductors. However, the engineering of semiconductor substrates into metasurfaces for improving SERS activity remains underexplored. Here, an improved SERS metasurface platform is developed that leverages the combination of titanium oxide (TiO2) and the emerging physical concept of optical bound states in the continuum (BICs) to boost the Raman emission. Moreover, fine‐tuning of BIC‐assisted resonant absorption offers a pathway for maximizing the photoinduced charge transfer effect (PICT) in SERS. High values of BIC‐assisted electric field enhancement (|E/E0|2 ≈103) are achieved, challenging the preconception of weak electromagnetic (EM) field enhancement on semiconductor SERS substrates. The BIC‐assisted TiO2 metasurface platform offers a new dimension in spectrally‐tunable SERS with earth‐abundant and bio‐compatible semiconductor materials, beyond the traditional plasmonic ones.