arXiv subjects

Maximilian Rödel

Publications and source records attributed to Maximilian Rödel.

8 recordsLinked to original sources

arXiv2026

Time-of-flight photon spectroscopy for scanning tunneling microscopy luminescence

We build and commission a time-of-flight photon spectrometer (TOFS) for scanning tunneling microscopy luminescence (STML). We obtain the spectrum by exploiting the wavelength dependent refractive index of a long dispersive optical fiber that converts photon arrival times into wavelength information; blue photons are delayed more than red photons. The setup uses a pulsed excitation source, either laser flashes or voltage pulses, to launch the photons from the junction into a single photon detector. The TOFS calibration can be performed and transferred to the STML setup from a separate benchtop experiment using pulsed light sources with known wavelength. We verify the TOFS during STML operation by simultaneously recording luminescence from the Ag-Ag(111) plasmon using a conventional grating spectrometer. Our experiments show that the TOFS is a straightforward and cost-effective addition to existing STML setups with good performance in the near-infrared range. The TOFS is compatible with a drop-in replacement of the single photon detector such as a superconducting nanowire single photon detector or even bolometers that may expand the useful spectral range beyond the abilities of current STML setups.

cond-mat.other↗

arXiv2026

Locally-Induced Stark Shifts of Collective Excitonic Modes in Polyradical Aggregates

Active control of dark long-lived excitonic states in molecular aggregates using local electric fields is a pivotal challenge for advancing nanoscale optoelectronics and quantum device engineering. This experimental study investigates the collective excitonic states in aggregates composed of radical chromophores. With the strong optical enhancement provided by tip-enhanced photoluminescence (TEPL) spectroscopy, bright and dark excitonic modes are observed emerging due to interexciton coupling and induce changes in their spectra with the electric field locally applied within the nanocavity gap. Proportionally scaling Stark shifts are revealed as well as the emission peak sharpening of the dark states and a divergent behavior of the bright states in asymmetric measurement positions of the nanocavity above the aggregates. The observed complex behavior is discussed in terms of influence of the field, molecule arrangement, nanocavity coupling, dark mode lifetimes and electrostatic charge inhomogeneities in the clusters. This sensitivity to the external parameters demonstrates an effective means of control over radical excitonic aggregates.

physics.atm-clus↗

arXiv2025

Probing plexciton dynamics with higher-order spectroscopy

Coupling molecular transition dipole moments to surface-plasmon polaritons (SPPs) results in the formation of new optical quasiparticles, i.e., plexcitons. Mixing the specific properties of matter excitations and light modes has proven to be an efficient strategy to alter a variety of molecular processes ranging from chemical reactions to exciton transport. Here, we investigate energy transfer in a plexcitonic system of zinc phthalocyanine (ZnPc) molecules aggregated in the crystalline α-phase and an SPP on a planar gold surface. By tuning the angle of incidence, we vary the degree of mixing between excitonic and SPP character of the excited state. We apply our recently developed higher-order pump-probe spectroscopy to separate the system's fifth-order signal describing the dynamics of two-particle interactions. The time it takes for two quasiparticles to meet and annihilate is a measure of their movement and thus the transport of excitation energy in the system. We find that the transport extracted from the fifth-order signal is surprisingly unaffected by the mixing ratio of exciton and SPP contributions of the plexciton. Using a rate equation model, we explain this behavior by fast transition from the plexcitonic states to many localized excitonic dark states that do not have an SPP contribution. Our results give an indication of how hybrid exciton-plasmon systems should be designed to exploit the delocalization of the involved plasmon modes for improved transport.

physics.chem-ph↗

arXiv2025

How to Fix Silver for Plasmonics

Silver (Ag) is considered an ideal material for plasmonic applications in the visible wavelength regime due to its superior optical properties, but its use is limited by the poor chemical stability and structural quality of thermally evaporated thin films and resulting nanostructures. In this study, we present a simple approach to enhance the structural and optical quality as well as the chemical stability of Ag thin films by alloying with gold (Au) through thermal co-evaporation. We investigate Ag$_{100-x}$Au$_x$ thin films with Au contents ranging from 5 to 20 at% analyzing their surface morphology, crystallite structure, optical properties, and chemical stability. Our results show that low Au concentrations significantly reduce the roughness of co-evaporated thin films (down to 0.4 nm RMS), and significantly enhance the resistance to oxidation, while maintaining a defined crystallite growth. Importantly, these improvements are achieved without the need for template stripping, metallic wetting layers, or epitaxial substrates, enabling direct deposition on glass. Among the compositions studied, Ag$_{95}$Au$_5$ thin films exhibit the highest chemical stability, lowest optical losses in the visible spectral range, and excellent plasmonic properties even outcompeting pure Ag. As a proof-of-concept, we fabricate high-quality Ag$_{95}$Au$_5$ optical antennas that exhibit long-term durability under ambient conditions. Our approach provides a practical solution to overcome the limitations of Ag for plasmonic device applications.

cond-mat.mtrl-sci↗

arXiv2024

Individually Addressable Nanoscale OLEDs

Augmented Reality (AR) and Virtual Reality (VR), require miniaturized displays with ultrahigh pixel densities. Here, we demonstrate an individually addressable subwavelength OLED pixel based on a nanoscale electrode capable of supporting plasmonic modes. Our approach is based on the notion that when scaling down pixel size, the 2D planar geometry of conventional organic light-emitting diodes (OLEDs) evolves into a significantly more complex 3D geometry governed by sharp nanoelectrode contours. These cause (i) spatially imbalanced charge carrier transport and recombination, resulting in a low quantum efficiency, and (ii) filament growth, leading to rapid device failure. Here, we circumvent such effects by selectively covering sharp electrode contours with an insulating layer, while utilizing a nano-aperture in flat areas of the electrode. We thereby ensure controlled charge carrier injection and recombination at the nanoscale and suppress filament growth. As a proof of principle, we first demonstrate stable and efficient hole injection from Au nanoelectrodes in hole-only devices with above 90 % pixel yield and longtime operation stability and then a complete vertical OLED pixel with an individually addressable nanoelectrode (300 x 300 nm$^{2}$), highlighting the potential to further leverage plasmonic nanoantenna effects to enhance the performance and functionality of nano-OLEDs.

physics.optics↗

arXiv2024

Anisotropic Photo-Physical Properties of Plexcitons in Strongly Coupled Metal-Organic Thin Films

Exciton plasmon polaritons have gained increasing interests over recent years due to their versatile properties emerging by the underlying light-matter coupling and making them potential candidates for new photonic applications. We have advanced this concept by studying thin films of laterally aligned J-type aggregates of self-assembled tetra-bay phenoxy-dendronized perylene bisimide (PBI) molecules, arranged in a helical manner of three strains on a silver surface. As a result of the interaction between the uniformly aligned dipole moments and the surface plasmons of a thin silver layer underneath, the excitonic state at 1.94 eV evolves into dispersions in absorption and emission, both characterized by a distinct anisotropy. The coupling constant defined by the scalar product of the transition dipole moment $\vecμ$ and the surface plasmon wavevector $\vec{k}_x$ shows a pronounced two-fold rotational symmetry with values between almost 0 to 28 meV. Complementary TD-DFT calculations of the angular dependent absorption and photoluminescence provide insights in the coherent energy exchange between the excitonic and plasmonic sub-systems. Additionally, power dependent PL studies yield first evidence that the diffusion length of the coupled exciton-plasmon polaritons exceeds that of the mere Frenkel state in neat PBI by at least one order of magnitude. Our results not only demonstrate the possibility to control the photo-physical properties of strongly coupled states by their spatially anisotropic light-matter interaction but also reveal innovative strategies to influence opto-electronic device operation by the directional transport of hybrid state energy.

physics.optics↗

arXiv2024

Excimer Formation in Zinc-phthalocyanine Revealed using Ultrafast Electron Diffraction

The formation of excited dimer states, so called excimers, is an important phenomenon in many organic molecular semiconductors. In contrast to Frenkel exciton-polaron excited states, an excimer is long-lived and energetically low-lying due to stabilization resulting from a substantial reorganization of the inter-molecular geometry. In this letter, we show that ultrafast electron diffraction can follow the dynamics of solid-state excimer formation in polycrystalline thin films of a molecular semiconductor, revealing both the key reaction modes and the eventual structure of the emitting state. We study the prototypical organic semiconductor zinc-phthalocyanine (ZnPc) in its crystallographic $α$-phase as a model excimeric system. We show, that the excimer forms in a two-step process starting with a fast dimerization ($\lesssim 0.4\, \text{ps}$) followed by a subsequent slow shear-twist motion ($14\, \text{ps}$) leading to an alignment of the $π$-systems of the involved monomers. Furthermore, we show that while the same excimer geomtry is present in partially fluorinated derivatives of ZnPc, the formation kinematics slow down with increasing level of fluorination.

physics.chem-ph↗

arXiv2021

The Role of Molecular Arrangement on the Dispersion in Strongly Coupled Metal-Organic Hybrid Structures

Metal-organic hybrid structures have been demonstrated a versatile platform to study primary aspects of light-matter interaction by means of emerging states comprising excitonic and plasmonic properties. Here we are studying the wave-vector dependent photo-excitations in gold layers covered by molecular films of zinc-phthalocyanine and its fluorinated derivatives (FnZnPc, with n = 0,4,8,16). These layered metal-organic samples show up to four anti-crossings in their dispersions correlating in energy with the respective degree of ZnPc fluorination. By means of complementary structural and theoretical data, we attribute the observed anti-crossings to three main scenarios of surface plasmon coupling: i) to aggregated $α$-phase regions within the FnZnPc layers at 1.75 eV and 1.85 eV , ii) to a coexisting F16ZnPc $β$-polymorph at 1.51 eV, and iii) to monomers, preferentially located at the metal interface, at 2.15 eV. Whereas energy and splitting of the monomer anti-crossings depend on strength and average tilting of the molecular dipole moments, the aggregate related anti-crossings show a distinct variation with degree of fluorination. These observations can be consistently explained by a change in FnZnPc dipole density induced by an increased lattice spacing due to the larger molecular van der Waals radii upon fluorination. The reported results prove Au/FnZnPc bilayers a model system to demonstrate the high sensitivity of exciton-plasmon coupling on the molecular alignment at microscopic length scales.

physics.optics↗