arXiv subjects

Alfred J. Meixner

Publications and source records attributed to Alfred J. Meixner.

14 recordsLinked to original sources

arXiv2024

Nonlinear optical response and spontaneous polarization in layer-stacked gallenene using second harmonic generation

Gallenene is a promising low-dimensional material with a structure down to the thickness of a single atom, similar to graphene. However, van der Waals stacking of two-dimensional (2D) gallenene under confinement remain poorly understood. In this study, we present evidence of the formation of parallel-stacked hexagonal gallenene (a100) structures in liquid gallium. The present study demonstrates the AB stacking of 2D gallenene a100 crystals in liquid gallium sandwiched between two graphene layers, as observed through transmission electron microscopy. A nonlinear optical response of the confined hexagonal gallenene was investigated through second harmonic generation (SHG) microscopy. The SHG signal exhibits periodic peak intensity shifts upon angular rotation up to 90 degrees and intensity dampening at elevated temperatures. These findings offer insights on device applications of 2D gallenene.

cond-mat.mtrl-sci↗

arXiv2024

Interferometric Mass Photometry at the Quantum Limit of Sensitivity

We present an innovative optical imaging system for measuring parameters of a small particle such as a macromolecule or nanoparticle at the quantum limit of sensitivity. In comparison to the conventional confocal interferometric scattering (iSCAT) approach, our setup adds a second arm to form a Michelson interferometer that allows us to tune a relative phase. We evaluate the quantum Cram\'er-Rao bound (QCRB) for different quantum states, including single-mode coherent states, multi-frequency coherent states, and phase-averaged coherent states. Our results show that the proposed setup can achieve the QCRB of sensitivity and outperform iSCAT for all considered quantum states for mass and phase estimation of a particle.

quant-ph↗

arXiv2023

Defect emission and its dipole orientation in layered ternary Znln2S4 semiconductor

Defect engineering is promising to tailor the physical properties of two-dimensional (2D) semiconductors for function-oriented electronics and optoelectronics. Compared with the extensively studied 2D binary materials, the origin of defects and their influence on physical properties of 2D ternary semiconductors have not been clarified. In this work, we thoroughly studied the effect of defects on the electronic structure and optical properties of few-layer hexagonal Znln2S4 via versatile spectroscopic tools in combination with theoretical calculations. It has been demonstrated that the Zn-In anti-structural defects induce the formation of a series of donor and acceptor levels inside the bandgap, leading to rich recombination paths for defect emission and extrinsic absorption. Impressively, the emission of donor-acceptor pair (DAP) in Znln2S4 can be significantly tailored by electrostatic gating due to efficient tunability of Fermi level (Ef). Furthermore, the layer-dependent dipole orientation of defect emission in Znln2S4 was directly revealed by back focal plane (BFP) imagining, where it presents obviously in-plane dipole orientation within a dozen layers thickness of Znln2S4. These unique features of defects in Znln2S4 including extrinsic absorption, rich recombination paths, gate tunability and in-plane dipole orientation will definitely benefit to the advanced orientation-functional optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2022

Substrate Effects on the Speed Limiting Factor of WSe$_2$ Photodetectors

We investigate the time-resolved photoelectric response of WSe$_2$ crystals on common glass and flexible polyimide substrates to determine the effect of the dielectric environment on the speed of the photodetectors. We show that varying the substrate material can alter the speed-limiting mechanism: while the detectors on polyimide are RC limited, those on glass are limited by slower excitonic diffusion processes. We attribute this to a shortening of the depletion layer at the metal electrode/WSe2 interface caused by the higher dielectric screening of glass compared to polyimide. The photodetectors on glass show a tunable bandwidth which can be increased to 2.6 MHz with increasing the electric field.

cond-mat.mtrl-sci↗

arXiv2021

Spatially resolved fluorescence of caesium lead halide perovskite supercrystals reveals quasi-atomic behavior of nanocrystals

We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffraction to reveal surface defects in supercrystals of self-assembled caesium lead halide perovskite nanocrystals and study their effect on the fluorescence properties. Upon comparison with density functional modelling, we show that a loss in structural coherence, an increasing atomic misalignment between adjacent nanocrystals, and growing compressive strain near the surface of the supercrystal are responsible for the observed fluorescence blueshift and decreased fluorescence lifetimes. Such surface defect-related optical properties extend the frequently assumed analogy between atoms and nanocrystals as so-called quasi-atoms. Our results emphasize the importance of minimizing strain during the self-assembly of perovskite nanocrystals into supercrystals for lighting application such as superfluorescent emitters.

cond-mat.mtrl-sci↗

arXiv2020

Direct observation of structural heterogeneity and tautomerization of single hypericin molecules

Tautomerization is a fast chemical reaction where structures of the reactants differ only in the position of a proton and a double bond. Tautomerization often occurs in natural substances and is a fundamental process in organic- and biochemistry. However, studying the optical properties of tautomeric species is challenging due to ensemble averaging. Many molecules, such as porphines, porphycenes or phenanthroperylene quinones, exhibit a reorientation of the transition dipole moment (TDM) during tautomerization, which can be directly observed in a single molecule experiment. A prominent phenanthroperylene quinone is hypericin showing antiviral, antidepressive, and photodynamical properties. Here, we study single hypericin molecules by using confocal microscopy combined with higher order laser modes. Observing abrupt flipping of the image pattern allows to draw conclusions about the coexistence of different tautomers and their conversion path. Time-dependent density functional theory calculations show that hypericin is cycling between the four most stable tautomers. This approach allows to unambiguously assign a TDM orientation to a specific tautomer and enables to determine the chemical structure in situ. Additionally, tautomerization can not only be observed by the image pattern orientation, but also as intermittency in the fluorescence emission of a single molecule. Time correlated single photon counting enables to determine the excited state lifetimes of the hypericin tautomers. Our approach is not only limited to hypericin, but can be applied to other molecules showing a TDM reorientation during tautomerization, helping to get a deeper understanding of this important process.

physics.chem-ph↗

arXiv2020

Periodic Fluorescence Variations of CdSe Quantum Dots Coupled to Aryleneethynylenes with Aggregation Induced Emission

CdSe nanocrystals and aggregates of an aryleneethynylene derivative are assembled into a hybrid thin film with dual fluorescence from both fluorophores. Under continuous excitation, the nanocrystals and the molecules exhibit anti-correlated fluorescence intensity variations, which become periodic at low temperature. We attribute this to a structure-dependent aggregation induced emission of the aryleneethynylene derivative, which impacts the rate of excitation energy transfer between the molecules and nanocrystals. Energy transfer also affects the electric transport properties of the hybrid material under optical excitation. This work highlights that combining semiconductor nanocrystals with molecular aggregates, which exhibit aggregation induced emission, can result in unprecedented emerging optical properties.

physics.app-ph↗

arXiv2020

Structural order matters: Enhanced electronic coupling in self assembled micro-crystals of Au-nanoclusters

We report an easy and broadly applicable method for the controlled self-assembly of atomically precise Au32(nBu3P)12Cl8 nanoclusters into micro-crystals. This enables the determination of emergent optoelectronic properties resulting from long-range order in such assemblies. Compared to the same nanoclusters in glassy, polycrystalline ensembles, we find a 100-fold increase in the electric conductivity and charge carrier mobility as well as additional optical transitions. We show that these effects are due to a vanishing energetic disorder and a drastically reduced activation energy to charge transport in the highly ordered assemblies. This first structure-transport correlation on self-assembled superstructures of atomically precise gold nanoclusters paves the way towards functional materials with novel collective optoelectronic properties.

cond-mat.mes-hall↗

arXiv2020

Multimode Vibrational Strong Coupling of Methyl Salicylate to a Fabry-Perot Microcavity

The strong coupling of an IR-active molecular transition with an optical mode of the cavity results in vibrational polaritons, which opens a new way to control chemical reactivity via confined electromagnetic fields of the cavity. In this study, we design a voltage-tunable open microcavity and we show the formation of multiple vibrational polaritons in methyl salicylate. A Rabi splitting and polariton anticrossing behaviour is observed when the cavity mode hybridizes with the C=O stretching vibration of methyl salicylate. As this vibration contributes to the reaction coordinate of the photoinduced proton transfer process in methyl salicylate, we suggest the coupling might be used to modulate the photophysical properties of the molecule. Furthermore, the proposed theoretical model based on coupled harmonic oscillator reveals that the absorption of uncoupled molecules must also be considered to model the experimental spectra properly and that simultaneous coupling of multiple molecular vibrations to the same cavity mode has a significant influence on the Rabi splitting.

physics.optics↗

arXiv2019

Hypericin: Single molecule spectroscopy of an active natural ingredient

Hypericin can be found in nature in Hypericum perforatum (St. John's Wort) and has become subject of intense biochemical research. Studies report of antidepressive, antineoplastic, antitumor and antiviral activity of hypericin. Among the variety of potential applications hypericin can be used as photosensitizer in photodynamic therapy (PDT), where it is brought into cancer cells and produces singlet oxygen upon irradiation with a suitable light source. Therefore, the photophysical properties of hypericin are crucial for a successful application in a medical treatment. Here, we present the first single molecule optical spectroscopy study of hypericin. Its photostability is large enough to obtain single molecule fluorescence, surface enhanced Raman spectra (SERS), fluorescence lifetime, antibunching and blinking dynamics. Embedding hypericin in a PVA matrix changes the blinking dynamics, reduces the fluorescence lifetime and increases the photostability. Single molecule SERS spectra show both the neutral and deprotonated form of hypericin and exhibit sudden spectral changes, which can be associated with a reorientation of the single molecule with respect to the surface.

physics.chem-ph↗

arXiv2019

Direct phase mapping of the light scattered by single plasmonic nanoparticles

In this work, we present a novel technique to directly measure the phase shift of the optical signal scattered by single plasmonic nanoparticles in a diffraction-limited laser focus. We accomplish this by equipping an inverted confocal microscope with a Michelson interferometer and scanning single nanoparticles through the focal volume while recording interferograms of the scattered and a reference wave for each pixel. For the experiments, lithographically prepared gold nanorods where used, since their plasmon resonances can be controlled via their aspect ratio. We have developed a theoretical model for image formation in confocal scattering microscopy for nanoparticles considerably smaller than the diffraction limited focus We show that the phase shift observed for particles with different longitudinal particle plasmon resonances can be well explained by the harmonic oscillator model. The direct measurement of the phase shift can further improve the understanding of the elastic scattering of individual gold nanoparticles with respect to their plasmonic properties.

physics.optics↗

arXiv2019

Tunable strong coupling of two adjacent optical λ/2 Fabry-Pérot microresonators

Optical half-wave microresonators enable to control the optical mode density around a quantum system and thus to modify the temporal emission properties. If the coupling rate exceeds the damping rate, strong coupling between a microresonator and a quantum system can be achieved, leading to a coherent energy exchange and the creation of new hybrid modes. Here, we investigate strong coupling between two adjacent lambda/2 Fabry-Pérot microresonators, where the resonance of one microresonator can be actively tuned across the resonance of the other microresonator. The transmission spectra of the coupled microresonators show a clear anticrossing behavior, which proves that the two cavity modes are strongly coupled. Additionally, we can vary the coupling rate by changing the resonator geometry and thereby investigate the basic principles of strong coupling with a well-defined model system. Finally, we will show that such a coupled system can theoretically be modelled by coupled damped harmonic oscillators.

physics.optics↗

arXiv2018

Two and three photon excited luminescence of single gold nanoparticles: Switching between plasmon- and electron-hole-pair emission by ultrashort laser pulses

In this work we use femtosecond laser pulses of 800 nm wavelength to excite and characterize the multiphoton luminescence emission of single gold nanoparticles. For excitation with 100 fs laser pulses we observe a two and three photon emission dominated by radiative electron hole pair recombination, while the emission is caused by radiative plasmon decay for excitation with 500 fs pulses. For single gold nanorods with different aspect ratios, we study the interplay between the particle plasmon and electron hole pairs, which enables us to develop a quantitative model to fully describe the two and three photon luminescence emission of single gold nanoparticles.

physics.optics↗

arXiv2012

Electrodynamic coupling of electric dipole emitters to a fluctuating mode density within a nano-cavity

We investigate the impact of rotational diffusion on the electrodynamic coupling of fluorescent dye molecules (oscillating electric dipoles) to a tunable planar metallic nanocavity. Fast rotational diffusion of the molecules leads to a rapidly fluctuating mode density of the electromagnetic field along the molecules' dipole axis, which significantly changes their coupling to the field as compared to the opposite limit of fixed dipole orientation. We derive a theoretical treatment of the problem and present experimental results for rhodamine 6G molecules in cavities filled with low and high viscosity liquids. The derived theory and presented experimental method is a powerful tool for determining absolute quantum yield values of fluorescence.

physics.optics↗