arXiv subjects

Harald Giessen

Publications and source records attributed to Harald Giessen.

53 records · Page 3Linked to original sources

arXiv2021

Electrically switchable metasurface for beam steering using PEDOT

Switchable and active metasurfaces allow for the realization of beam steering, zoomable metalenses, or dynamic holography. To achieve this goal, one has to combine high-performance metasurfaces with switchable materials that exhibit high refractive index contrast and high switching speeds. In this work, we present an electrochemically switchable metasurface for beam steering where we use the conducting polymer poly(3,4-ethylene-dioxythiophene) (PEDOT) as an active material. We show beam diffraction with angles up to 10° and change of the intensities of the diffracted and primary beams employing an externally applied cyclic voltage between -1 V and +0.5 V. With this unique combination, we realize switching speeds in the range of 1 Hz while the extension to typical display frequencies in the tens of Hz region is possible. Our findings have immediate implications on the design and fabrication of future electronically switchable and display nanotechnologies, such as dynamic holograms.

physics.optics↗

arXiv2020

Highly Confined In-plane Exciton-Polaritons in Monolayer Semiconductors

2D materials support unique excitations of quasi-particles that consist of a material excitation and photons called polaritons. Especially interesting are in-plane propagating polaritons which can be confined to a single monolayer and carry large momentum. In this work, we report the existence of a new type of in-plane propagating polariton, supported on monolayer transition-metal-dicalcogonide (TMD) in the visible spectrum, which has not yet been observed. This 2D in-plane exciton-polariton (2DEP) is described by the coupling of an electromagnetic light field with the collective oscillations of the excitons supported by monolayer TMDs. We expose the specific experimental conditions required for the excitation of the 2DEP and show that these can be created if the TMD is encapsulated with hexagonal-boron-nitride (hBN) and cooled to cryogenic temperatures. In addition, we compare the properties of the 2DEPs with those of surface-plasmons-polaritons (SPPs) at the same spectral range, and find that the 2DEP exhibit over two orders-of-magnitude larger wavelength confinement. Finally, we propose two configurations for the possible experimental observation of 2DEPs.

physics.optics↗

arXiv2020

Interaction of edge exciton polaritons with engineered defects in the van der Waals material Bi2Se3

Hyperbolic materials exhibit unique properties that enable a variety of intriguing applications in nanophotonics. The topological insulator Bi2Se3 represents a natural hyperbolic optical medium, both in the THz and visible range. Here, using cathodoluminescence spectroscopy and electron energy-loss spectroscopy, we demonstrate that Bi2Se3, in addition to being a hyperbolic material, supports room-temperature exciton polaritons. Moreover, we explore the behavior of hyperbolic edge exciton polaritons in Bi2Se3. Edge polaritons are hybrid modes that result from the coupling of the polaritons bound to the upper and lower edges of Bi2Se3 nanoplatelets. In particular, we use electron energy-loss spectroscopy to compare Fabry-Pérot-like resonances emerging in edge polariton propagation along pristine and artificially structured edges of the nanoplatelets. The experimentally observed scattering of edge polaritons by defect structures was found to be in good agreement with finite-difference time-domain simulations. Moreover, we experimentally proved coupling of localized polaritons in identical open and closed circular nanocavities to the propagating edge polaritons. Our findings are testimony to the extraordinary capability of the hyperbolic polariton propagation to cope with the presence of defects. This provides an excellent basis for applications such as nanooptical circuitry, cloaking at the nanometer scale, as well as nanoscopic quantum technology on the nanoscale.

physics.app-ph↗

arXiv2020

3D printed micro-optics for quantum technology: Optimized coupling of single quantum dot emission into a single mode fiber

Future quantum technology relies crucially on building quantum networks with high fidelity. To achieve this challenging goal, it is of utmost importance to connect single quantum systems in a way such that their emitted single-photons overlap with the highest possible degree of coherence. This requires perfect mode overlap of the emitted light of different emitters, which necessitates the use of single mode fibers. Here we present an advanced manufacturing approach to accomplish this task: we combine 3D printed complex micro-optics such as hemispherical and Weierstrass solid immersion lenses as well as total internal reflection solid immersion lenses on top of single InAs quantum dots with 3D printed optics on single mode fibers and compare their key features. Interestingly, the use of hemispherical solid immersion lenses further increases the localization accuracy of the emitters to below 1 nm when acquiring micro-photoluminescence maps. The system can be joined together and permanently fixed. This integrated system can be cooled by dipping into liquid helium, by a Stirling cryocooler or by a closed-cycle helium cryostat without the necessity for optical windows, as all access is through the integrated single mode fiber. We identify the ideal optical designs and present experiments that prove excellent high-rate single-photon emission by high-contrast Hanbury Brown and Twiss experiments.

physics.app-ph↗

arXiv2020

Quantum dot single-photon emission coupled into single-mode fibers with 3D printed micro-objectives

User-friendly single-photon sources with high photon-extraction efficiency are crucial building blocks for photonic quantum applications. For many of these applications, such as long-distance quantum key distribution, the use of single-mode optical fibers is mandatory, which leads to stringent requirements regarding the device design and fabrication. We report on the on-chip integration of a quantum dot microlens with a 3D-printed micro-objective in combination with a single-mode on-chip fiber coupler. The practical quantum device is realized by deterministic fabrication of the QD-microlens via in-situ electron-beam lithography and 3D two-photon laser writing of the on-chip micro-objective and fiber-holder. The QD with microlens is an efficient single-photon source, whose emission is collimated by the on-chip micro-objective. A second polymer microlens is located at the end facet of the single-mode fiber and ensures that the collimated light is efficiently coupled into the fiber core. For this purpose, the fiber is placed in the on-chip fiber chuck, which is precisely aligned to the QD-microlens thanks to the sub-$μ$m processing accuracy of high-resolution two-photon direct laser writing. This way, we obtain a fully integrated high-quality quantum device with broadband photon extraction efficiency, a single-mode fiber-coupling efficiency of 26%, a single-photon flux of 1.5 MHz at single-mode fibre output and a multi-photon probability of 13 % under pulsed optical excitation. In addition, the stable design of the developed fiber-coupled quantum device makes it highly attractive for integration into user-friendly plug-and-play quantum applications.

physics.app-ph↗

arXiv2020

Arrays of individually controllable optical tweezers based on 3D-printed microlens arrays

We present a novel platform of optical tweezers which combines rapid prototyping of user-definable microlens arrays with spatial light modulation (SLM) for dynamical control of each associated tweezer spot. Applying femtosecond direct laser writing, we manufacture a microlens array of 97 lenslets exhibiting quadratic and hexagonal packing and a transition region between the two. We use a digital micromirror device (DMD) to adapt the light field illuminating the individual lenslets and present a detailed characterization of the full optical system. In an unprecedented fashion, this novel platform combines the stability given by prefabricated solid optical elements, fast reengineering by rapid optical prototyping, DMD-based real-time control of each focal spot, and extensive scalability of the tweezer pattern. The accessible tweezer properties are adaptable within a wide range of parameters in a straightforward way.

physics.optics↗

arXiv2019

Microwave probing of bulk dielectrics using superconducting coplanar resonators in distant-flip-chip geometry

Dielectric measurements on insulating materials at cryogenic temperatures can be challenging, depending on the frequency and temperature ranges of interest. We present a technique to study the dielectric properties of bulk dielectrics at GHz frequencies. A superconducting coplanar Nb resonator is deposited directly on the material of interest, and this resonator is then probed in distant-flip-chip geometry with a microwave feedline on a separate chip. Evaluating several harmonics of the resonator gives access to various probing frequencies, in the present studies up to 20 GHz. We demonstrate the technique on three different materials (MgO, LaAlO3, and TiO2), at temperatures between 1.4 K and 7 K.

cond-mat.supr-con↗

arXiv2019

Near-unity light absorption in a monolayer WS2 van der Waals heterostructure cavity

Excitons in monolayer transition-metal-dichalcogenides (TMDs) dominate their optical response and exhibit strong light-matter interactions with lifetime-limited emission. While various approaches have been applied to enhance light-exciton interactions in TMDs, the achieved strength have been far below unity, and a complete picture of its underlying physical mechanisms and fundamental limits has not been provided. Here, we introduce a TMD-based van der Waals heterostructure cavity that provides near-unity excitonic absorption, and emission of excitonic complexes that are observed at ultra-low excitation powers. Our results are in full agreement with a quantum theoretical framework introduced to describe the light-exciton-cavity interaction. We find that the subtle interplay between the radiative, non-radiative and dephasing decay rates plays a crucial role, and unveil a universal absorption law for excitons in 2D systems. This enhanced light-exciton interaction provides a platform for studying excitonic phase-transitions and quantum nonlinearities and enables new possibilities for 2D semiconductor-based optoelectronic devices.

physics.optics↗

arXiv2019

Interaction of OAM light with Rydberg excitons: Modifying dipole selection rules

Orbital angular momentum (OAM) light possesses in addition to its usual helicity ($s=\pm \hbar$, depending on its circular polarization) an orbital angular momentum $l$. This means that in principle one can transfer more than a single quantum of $\hbar$ during an optical transition from light to a quantum system. However, quantum objects are usually so small (typically in the nm range) that they only locally probe the dipolar character of the local electric field. In order to sense the complete macroscopic electric field, we utilize Rydberg excitons in the semiconductor cuprite ($\text{Cu}_2\text{O}$), which are single quantum objects of up to $μm$ size. Their interaction with focused OAM light, allows for matching the focal spot size and the wavefunction diameter. Here, the common dipole selection rules ($Δj=\pm 1$) should be broken, and transitions of higher $Δj$ with higher order OAM states should become more probable. Based on group theory, we analyze in detail the optical selection rules governing this process. Then we are able to predict what kind of new exciton transitions (quantum number $n$ and $l_{\text{exc}}$) one would expect in absorption spectroscopy on $\text{Cu}_2\text{O}$ using different kinds of OAM light.

physics.optics↗

arXiv2018

Hydrogen-regulated chiral nanoplasmonics

Chirality is a highly important topic in modern chemistry, given the dramatically different pharmacological effects that enantiomers can have on the body. Chirality of natural molecules can be controlled by reconfiguration of molecular structures through external stimuli. Despite the rapid progress in plasmonics, active regulation of plasmonic chirality, particularly in the visible spectral range, still faces significant challenges. In this Letter, we demonstrate a new class of hybrid plasmonic metamolecules composed of magnesium and gold nanoparticles. The plasmonic chirality from such plasmonic metamolecules can be dynamically controlled by hydrogen in real time without introducing macroscopic structural reconfiguration. We experimentally investigate the switching dynamics of the hydrogen-regulated chiroptical response in the visible spectral range using circular dichroism spectroscopy. In addition, energy dispersive X-ray spectroscopy is used to examine the morphology changes of the magnesium particles through hydrogenation and dehydrogenation processes. Our study can enable plasmonic chiral platforms for a variety of gas detection schemes by exploiting the high sensitivity of circular dichroism spectroscopy.

physics.optics↗

arXiv2018

Coupling a single solid state quantum emitter to an array of resonant plasmonic antennas

Plasmon resonant arrays or meta-surfaces shape both the incoming optical field and the local density of states for emission processes. They provide large regions of enhanced emission from emitters and greater design flexibility than single nanoantennas. This makes them of great interest for engineering optical absorption and emission. Here we study the coupling of a single quantum emitter, a self-assembled semiconductor quantum dot, to a plasmonic meta-surface. We investigate the influence of the spectral properties of the nanoantennas and the position of the emitter in the unit cell of the structure. We observe a resonant enhancement due to emitter-array coupling in the far-field regime and find a clear difference from the interaction of an emitter with a single antenna.

physics.optics↗

arXiv2017

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.

quant-ph↗

arXiv2015

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.

physics.optics↗

arXiv2011

Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle

Optical nanoantennas are a novel tool to investigate previously unattainable dimensions in the nanocosmos. Just like their radio-frequency equivalents, nanoantennas enhance the light-matter interaction in their feed gap. Antenna enhancement of small signals promises to open a new regime in linear and nonlinear spectroscopy on the nanoscale. Without antennas especially the nonlinear spectroscopy of single nanoobjects is very demanding. Here, we present for the first time antenna-enhanced ultrafast nonlinear optical spectroscopy. In particular, we utilize the antenna to determine the nonlinear transient absorption signal of a single gold nanoparticle caused by mechanical breathing oscillations. We increase the signal amplitude by an order of magnitude which is in good agreement with our analytical and numerical models. Our method will find applications in linear and nonlinear spectroscopy of nanoobjects, ranging from single protein binding events via nonlinear tensor elements to the limits of continuum mechanics.

cond-mat.mes-hall↗

arXiv2011

Excitonic Fano Resonance in Freestanding Graphene

We investigate the role of electron-hole correlations in the absorption of freestanding monolayer and bilayer graphene using optical transmission spectroscopy from 1.5 to 5.5 eV. Line shape analysis demonstrates that the ultraviolet region is dominated by an asymmetric Fano resonance. We attribute this to an excitonic resonance that forms near the van-Hove singularity at the saddle point of the band structure and couples to the Dirac continuum. The Fano model quantitatively describes the experimental data all the way down to the infrared. In contrast, the common non-interacting particle picture cannot describe our data. These results suggest a profound connection between the absorption properties and the topology of the graphene band structure.

cond-mat.mes-hall↗

arXiv2010

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.

cond-mat.mes-hall↗

arXiv2009

Ultrafast time-resolved spectroscopy of 1D metal-dielectric photonic crystals

We study the all-optical switching behavior of one-dimensional metal-dielectric photonic crystals due to the nonlinearity of the free metal electrons. A polychromatic pump-probe setup is used to determine the wavelength and pump intensity dependence of the ultrafast transmission suppression as well as the dynamics of the process on a subpicosecond timescale. We find ultrafast (sub-picosecond) as well as a slow (millisecond) behavior. We present a model of the ultrafast dynamics and nonlinear response which can fit the measured data well and allows us to separate the thermal and the electronic response of the system.

cond-mat.other↗