Simple analytical expression for the peak-frequency shifts of plasmonic resonances for sensing

We derive a closed-form expression that accurately predicts the peak frequency-shift and broadening induced by tiny perturbations of plasmonic nanoresonators without critically relying on repeated electrodynamic simulations of the spectral response of nanoresonator for various locations, sizes or shapes of the perturbing objects. The force of the present approach, in comparison with other approaches of the same kind, is that the derivation is supported by a mathematical formalism based on a rigorous normalization of the resonance modes of nanoresonators consisting of lossy and dispersive materials. Accordingly, accurate predictions are obtained for a large range of nanoparticle shapes and sizes, used in various plasmonic nanosensors, even beyond the quasistatic limit. The expression gives quantitative insight, and combined with an open-source code, provides accurate and fast predictions that are ideally suited for preliminary designs or for interpretation of experimental data. It is also valid for photonic resonators with large mode volumes.Comment: 24 pages, 9 figures, journal pape

Unbiased All-Optical Random-Number Generator

The generation of random bits is of enormous importance in modern information science. Cryptographic security is based on random numbers which require a physical process for their generation. This is commonly performed by hardware random number generators. These exhibit often a number of problems, namely experimental bias, memory in the system, and other technical subtleties, which reduce the reliability in the entropy estimation. Further, the generated outcome has to be post-processed to "iron out" such spurious effects. Here, we present a purely optical randomness generator, based on the bi-stable output of an optical parametric oscillator. Detector noise plays no role and no further post-processing is required. Upon entering the bi-stable regime, initially the resulting output phase depends on vacuum fluctuations. Later, the phase is rigidly locked and can be well determined versus a pulse train, which is derived from the pump laser. This delivers an ambiguity-free output, which is reliably detected and associated with a binary outcome. The resulting random bit stream resembles a perfect coin toss and passes all relevant randomness measures. The random nature of the generated binary outcome is furthermore confirmed by an analysis of resulting conditional entropies.Comment: 10 pages, 4 figure

Coupling strength of complex plasmonic structures in the multiple dipole approximation

We present a simple model to calculate the spatial dependence of the interaction strength between two plasmonic objects. Our approach is based on a multiple dipole approximation and utilizes the current distributions at the resonances in single objects. To obtain the interaction strength, we compute the potential energy of discrete weighted dipoles associated with the current distributions of the plasmonic modes in the scattered fields of their mutual partners. We investigate in detail coupled stacked plasmonic wires, stereometamaterials and plasmon-induced transparency materials. Our calculation scheme includes retardation and can be carried out in seconds on a standard PC

Enhancing the optical excitation efficiency of a single self-assembled quantum dot with a plasmonic nanoantenna

We demonstrate how the controlled positioning of a plasmonic nanoparticle modifies the photoluminescence of a single epitaxial GaAs quantum dot. The antenna particle leads to an increase of the luminescence intensity by about a factor of eight. Spectrally and temporally resolved photoluminescence measurements prove an increase of the quantum dot's excitation rate. The combination of stable epitaxial quantum emitters and plasmonic nanostructures promises to be highly beneficial for nanoscience and quantum optics.Comment: 5 pages, 4 figure

Phase-locked photon-electron interaction without a laser

Ultrafast electron-photon spectroscopy in electron microscopes commonly requires ultrafast laser setups. Photoemission from an engineered electron source is used to generate pulsed electrons, interacting with a sample that is excited by the ultrafast laser pulse at a specified time delay. Thus, developing an ultrafast electron microscope demands the exploitation of extrinsic laser excitations and complex synchronization schemes. Here, we present an inverse approach based on cathodoluminescence spectroscopy to introduce internal radiation sources in an electron microscope. Our method is based on a sequential interaction of the electron beam with an electron-driven photon source (EDPHS) and the investigated sample. An electron-driven photon source in an electron microscope generates phase-locked photons that are mutually coherent with the near-field distribution of the swift electron. Due to their different velocities, one can readily change the delay between the photons and electrons arriving at the sample by changing the distance between the EDPHS and the sample. We demonstrate the mutual coherence between the radiations from the EDPHS and the sample by performing interferometry with a combined system of an EDPHS and a WSe2 flake. We assert the mutual frequency and momentum-dependent correlation of the EDPHS and sample radiation, and determine experimentally the degree of mutual coherence of up to 27%. This level of mutual coherence allows us to perform spectral interferometry with an electron microscope. Our method has the advantage of being simple, compact and operating with continuous electron beams. It will open the door to local electron-photon correlation spectroscopy of quantum materials, single photon systems, and coherent exciton-polaritonic samples with nanometric resolution

Long-term stability of capped and buffered palladium-nickel thin films and nanostructures for plasmonic hydrogen sensing applications

One of the main challenges in optical hydrogen sensing is the stability of the sensor material. We found and studied an optimized material combination for fast and reliable optical palladium-based hydrogen sensing devices. It consists of a palladium-nickel alloy that is buffered by calcium fluoride and capped with a very thin layer of platinum. Our system shows response times below 10 s and almost no short-term aging effects. Furthermore, we successfully incorporated this optimized material system into plasmonic nanostructures, laying the foundation for a stable and sensitive hydrogen detector

Optical properties of metallic photonic crystal structures

Gegenstand dieser Arbeit ist die Untersuchung der linearen optischen Eigenschaften von periodisch angeordneten Metall-Nanodrähten, die auf dielektrischen oder metallischen Substraten aufgebracht sind. Die verwendeten Nanostrukturen können dabei der neuen Materialklasse der sogenannten polaritonischen photonischen Kristalle zugeordnet werden. Im Gegensatz zu rein dielektrischen Nanostrukturen, deren Transmissionsspektren durch die Anregung geometrischer Resonanzen gekennzeichnet sind, können die Spektren der verwendeten metallischen photonischen Kristalle zusätzliche elektronische Resonanzen im sichtbaren Spektralbereich aufweisen. Diese als Partikelplasmon bezeichneten Resonanzen sind auf die Kollektivschwingung der Leitungsbandelektronen in den einzelnen Nanodrähten zurückzuführen. Eine sehr wichtige Fragestellung ist in diesem Zusammenhang, in wie weit mögliche elektromagnetische Kopplungsphänomene die optischen Eigenschaften der untersuchten metallischen Nanostrukturen zusätzlich beinflussen können. Sowohl die direkte elektromagnetische Wechselwirkung zwischen den einzelnen Nanodrähten, als auch die mögliche Wechselwirkung mit verschiedenen Oberflächenmoden sollen im Rahmen dieser Arbeit eingehender betrachtet werden. Neben der Verwendung verschiedener dielektrischer Substrate wird vor allem der Einfluss metallischer Substratschichten demonstriert. Ziel der Untersuchungen ist es dabei, generell ein besseres und genaueres Verständnis der physikalischen Ursachen der beobachteten Kopplungsphänomene zu erhalten. Im Rahmen dieser Arbeit kommen sowohl experimentelle Methoden als auch theoretische Simulationen zur Anwendung, um einen tieferen Einblick in die komplexen Zusammenhänge zu ermöglichen. Die Arbeit belegt eindeutig, dass sowohl die Entstehung von Polaritonmoden als auch so genannte Hybridisierungseffekte die optischen Transmissionseigenschaften sehr stark beeinflussen. Die beobachteten grundlegenden Kopplungsphänomene können möglicherweise zur Entwicklung oder Optimierung neuartiger optischer Bauelemente genutzt werden