arXiv subjects

Malte C. Gather

Publications and source records attributed to Malte C. Gather.

At least 19 recordsLinked to original sources

arXiv2026

Sky-blue Solution-processed TADF and Hyperfluorescent OLEDs via a High-Work-Function PEDOT:PSS:PFI Hole Injection Layer

Solution-processed OLEDs offer a scalable route to large-area devices, but achieving competitive efficiency in multilayer stacks remains a challenge due to difficulties in hole injection as well as solvent orthogonality between layers. A hole injection layer (HIL) based on a blend of PEDOT:PSS and a perfluorinated ionomer (PFI) addresses both challenges simultaneously: surface segregation of PFI during film drying creates a high-work-function interface improving hole injection into high-ionisation potential host materials, while the insolubility of the blend in aromatic solvents prevents intermixing with subsequently deposited layers. Using this HIL with a solution-processed mCPCN:SpiroAC-TRZ emissive layer, sky-blue OLEDs with a peak external quantum efficiency (EQEmax) of 21.7%, an external quantum efficiency (EQE) of 20.2% at 100 cd m-2, a peak luminance of 29,000 cd m-2 and a full width at half maximum (FWHM) emission linewidth of 69 nm are demonstrated. The PFI-based HIL approach is extended to the photochemically stable TADF emitter 5TCzBN (EQEmax 18.4%, FWHM 73 nm) and to sensitized / hyperfluorescent OLEDs incorporating the multi-resonance TADF terminal emitters n-DABNA (EQEmax 22.8%, FWHM 26 nm) and the dendrimer TCzBN-BMes (EQEmax 20.4%, FWHM 29 nm). These results establish the PEDOT:PSS:PFI HIL as a broadly compatible platform for high-performance solution-processed TADF OLEDs.

physics.app-ph↗

arXiv2026

Round-Robin Test of a Light-Emitting Electrochemical Cell: Establishing a Reference Protocol for Quality Research

Emerging technologies benefit from a jointly established reference protocol, which can lower the bar of entry for new researchers while serving as a calibration standard for established actors. The light-emitting electrochemical cell (LEC) combines electrochemistry and optoelectronics in an intricate manner, and it can by that enable sustainable and commercially relevant printing fabrication of emissive thin-film devices. However, LEC performance is sensitive to a range of material and processing parameters, which frequently results in inadequate, or even erroneous, device evaluation. With this in mind, we present herein a LEC reference protocol, which details the sourcing of materials and the procedures and parameters for robust device fabrication and operation. The protocol has been tested across nine international research groups, and the collected results from this interlaboratory round-robin test confirm that good LEC performance can be reproducibly obtained following our protocol. We also identify common pitfalls that can arise during LEC development, and present practical steps for attaining optimum LEC performance. We hope this reference protocol will improve the quality of future LEC research and serve as a guide for future researchers entering this vibrant field.

cond-mat.soft↗

arXiv2025

Monitoring microplastics in live reef-building corals with microscopic laser particles

Micro- and nanoplastics pose a growing threat to marine organisms, such as reef-building corals. Yet, our understanding of microplastic uptake, interaction with coral tissue, and incorporation into coral skeletons remains limited, mainly due to the invasiveness of existing methods for detecting microplastics. Here, we exploit optical resonances in polymer spheres to transform microplastic particles into microscopic lasers. The bright, distinctive, and stable spectral signatures emitted by the microscopic laser particles function as optical barcodes, allowing extended tracking of microplastics transport through optically opaque coral tissue. Simultaneously, the lasers provide real-time sensing of dynamic changes at the microplastic surface with nanoscale resolution. Using confocal hyperspectral imaging, we establish the technical and analytical framework to capture coral anatomy and combine tracking and surface sensing into an integrated, non-invasive approach. With this, we explore the transport and internalization of individual microplastics in live corals, opening new avenues for understanding their ecological impact.

physics.bio-ph↗

arXiv2025

Tunable photogating in a molecular aggregate coupled graphene phototransistor

We present a graphene photodetector coupled to a layer of aggregated organic semiconductor. A graphene phototransistor is covered with a thin film of merocyanine molecules. The aggregation of the molecular layer can be controlled by the deposition parameters and post-deposition annealing to obtain films ranging from amorphous to a highly aggregated state. The molecular layer has a uniaxial structure with excitonic transitions whose transition dipole moments are well defined. The presence of the molecular layer results in an enormous increase in the response of the phototransistor. We further demonstrate that the signal-enhancement is due to p-photodoping of the graphene. The spectroscopic photoresponse suggests that the photodoping via monomers and molecular aggregates takes place differently. Our photodetector is a platform to study the influence of molecular aggregation and order on charge transport processes between aggregated organic semiconductors and two-dimensional materials.

cond-mat.mes-hall↗

arXiv2025

Electrochemically induced hyperfluorescence based on the formation of charge-transfer excimers

Used extensively in sensing applications, the application of solution-state electrochemiluminescent devices (ECLDs) in lighting and displays has been constrained by their low luminance and short operational lifetime. Here, we report a record improvement in the luminance, efficiency, and operational longevity of ECLDs by introducing electrochemically induced hyperfluorescence (ECiHF) and demonstrate its use in a calligraphic display. We use the double-decker arrangement assumed by the electron donor and acceptor segments of the molecule TpAT-tFFO to realize thermally activated delayed fluorescence from an electrogenerated charge-transfer (CT) excimer state and a subsequent energy transfer to the rubrene emitter TBRb. ECLDs based on this strategy achieve an unprecedented luminance of >6,200 cd/m2 and their operational lifetime is more than 10-fold longer than all previous ECLDs with meaningful efficiency or brightness. We identify energy level alignment between excimer and emitter as a crucial factor for efficient ECiHF; spectroelectrochemical analysis reveals that devices with energy gaps < 0.4 eV operate on a pure excimer mechanism across a wide range of frequencies. Our findings highlight the potential of ECiHF for improving ECLDs and pave the way to commercial applications of this form of fluid light.

physics.optics↗

arXiv2025

Narrowband, angle-stable, and highly efficient polariton organic light emitting diodes employing thermally activated delayed fluorescence

Narrowband emission is crucial for next generation optoelectronic devices to satisfy demands for high color brilliance. Microcavities can narrow emission spectra of organic light-emitting diodes (OLEDs) through the creation of resonant standing waves, independent of emitter material, enabling flexibility in molecular and device design. However, they induce a strong angle-dependence of the perceived emission color. Utilizing strong light-matter coupling of cavity photons with the virtually angle-independent exciton, leads to exciton-polariton emission showing reduced linewidths and suppressed angular dispersion if tuned correctly. Creating polaritons in highly efficient materials such as thermally activated delayed fluorescence (TADF) molecules is however difficult as their low oscillator strengths intrinsically disfavor light-matter interaction. Here, we present the successful combination of a highly efficient but intrinsically broadband TADF emitter with a strongly absorbing assistant strong coupling material in a modified microcavity structure. Through optimizing the assistant strong coupling layer architecture, we demonstrate polariton OLEDs with exceptionally narrowband, angle stable emission at external quantum efficiencies above 20% for both bottom- and top-emitting designs, more than doubling the performance of previous record devices. The results pave the way for utilizing polaritonic emission at practical device efficiencies for display applications.

physics.optics↗

arXiv2025

DNA Sensing with Whispering Gallery Mode Microlasers

Nucleic acid sensing is crucial for advancing diagnostics, therapeutic monitoring and molecu-lar biology research, by enabling the precise identification of DNA and RNA interactions. Here, we present an innovative sensing platform based on DNA-functionalized whispering gallery mode (WGM) microlasers. By correlating spectral shifts in laser emission to changes in refractive index, we demonstrate real-time detection of DNA hybridization and structural changes. The addition of gold nanoparticles to the DNA strands significantly enhances sensi-tivity, and labeling exclusively the sensing strand or a hairpin strand eliminates the need for secondary labeling of the target strand. We further show that ionic strength influences DNA compactness, and we introduce a hairpin-based system as a dual-purpose sensor and con-trolled release mechanism for potential drug delivery. This versatile WGM-based platform of-fers promise for sequence-specific nucleic acid sensing, multiplexed detection, and in vivo ap-plications in diagnostics and cellular research.

physics.optics↗

arXiv2024

High-power pulsed electrochemiluminescence for optogenetic manipulation of Drosophila larval behaviour

Electrochemiluminescence (ECL) produces light through electrochemical reactions and has shown promise for various analytic applications in biomedicine. However, the use of ECL devices (ECLDs) as light sources has been limited due to insufficient light output and low operational stability. In this study, we present a high-power pulsed operation strategy for ECLDs to address these limitations and demonstrate their effectiveness in optogenetic manipulation. By applying a biphasic voltage sequence with short opposing phases, we achieve intense and efficient ECL through an exciplex-formation reaction pathway. This approach results in an exceptionally high optical power density, exceeding 100 microW/mm2, for several thousand pulses. Balancing the ion concentration by optimizing the voltage waveform further improves device stability. By incorporating multiple optimized pulses into a burst signal separated by short rest periods, extended light pulses of high brightness and with minimal power loss over time were obtained. These strategies were leveraged to elicit a robust optogenetic response in fruit fly (Drosophila melanogaster) larvae expressing the optogenetic effector CsChrimson. The semi-transparent nature of ECLDs facilitates simultaneous imaging of larval behaviour from underneath, through the device. These findings highlight the potential of ECLDs as versatile optical tools in biomedical and neurophotonics research.

physics.optics↗

arXiv2024

Breaking the angular dispersion limit in thin film optics by ultra-strong light-matter coupling

Thin film interference is integral to modern photonics and optoelectronics, e.g. allowing for precise design of high performance optical filters, efficiency enhancements in photovoltaics and light-emitting devices, as well as the realization of microlasers and high-performance photodetectors. However, interference inevitably leads to a change of spectral characteristics with angle, which is generally undesired and can limit the usefulness of thin-film coatings and devices. Here, we introduce a strategy to overcome this fundamental limit in optics by utilizing and tuning the exciton-polariton modes arising in ultra-strongly coupled microcavities. We demonstrate optical filters with narrow pass bands that shift by less than their half width (<15 nm) even at extreme angles. Our filters cover the entire visible range and surpass comparable metal-dielectric-metal filters in all relevant metrics. By expanding this strategy to strong coupling with the photonic sidebands of dielectric multilayer stacks, we also obtain filters with high extinction ratios and up to 98% peak transmission. Based on these findings, we realize ultrathin and flexible narrowband filter films, monolithically integrate our filters with organic photodiodes, and demonstrate polarization-sensitive polariton filters. These results illustrate how strong coupling provides additional degrees of freedom in thin film optics that will enable a multitude of exciting new applications in micro-optics, sensing, and biophotonics.

physics.optics↗

arXiv2023

Optically manipulated micromirrors for precise excitation of WGM microlasers

Whispering gallery mode microlasers are highly sensitive refractive index sensors widely explored for biophotonic and biomedical applications. Microlaser excitation and collection of the emitted light typically utilize microscope objectives at normal incidence, limiting the choice of the oscillation plane of the modes. Here, we present a platform that enables the excitation of microlasers from various directions using an optically manipulated micromirror. The scheme enables precise sensing of the environment surrounding the microlasers along different well-controlled planes. We further demonstrate the capability of the platform to perform a time-resolved experiment of dynamic sensing using a polystyrene probe bead orbiting the microlaser.

physics.optics↗

arXiv2023

Snapshot hyperspectral imaging of intracellular lasers

Intracellular lasers are emerging as powerful biosensors for multiplexed tracking and precision sensing of cells and their microenvironment. This sensing capacity is enabled by quantifying their narrow-linewidth emission spectra, which is presently challenging to do at high speeds. In this work, we demonstrate rapid snapshot hyperspectral imaging of intracellular lasers. Using integral field mapping with a microlens array and a diffraction grating, we obtain images of the spatial and spectral intensity distribution from a single camera acquisition. We demonstrate widefield hyperspectral imaging over a 3$\times$3 mm$^2$ field of view and volumetric imaging over 250$\times$250$\times$800 $μ$m$^3$ volumes with a spatial resolution of 5 $μ$m and a spectral resolution of less than 0.8 nm. We evaluate the performance and outline the challenges and strengths of snapshot methods in the context of characterising the emission from intracellular lasers. This method offers new opportunities for a diverse range of applications, including high-throughput and long-term biosensing with intracellular lasers.

physics.optics↗

arXiv2023

Orientation distributions of vacuum-deposited organic emitters revealed by single-molecule microscopy

The orientation of luminescent molecules in organic light-emitting diodes (OLEDs) strongly influences device performance. However, our understanding of the factors controlling emitter orientation is limited as current measurements only provide ensemble-averaged orientation values. Here, we use single-molecule imaging to measure the transition dipole orientation of individual molecules in a state-of-the-art thermally evaporated host and thereby obtain complete orientation distributions of the hyperfluorescence-terminal emitter C545T. We achieve this by realizing ultra-low doping concentrations (10-6 wt%) of C545T and minimising background levels to reliably measure the photoluminescence of the emitter. This approach yields the orientation distributions of >1000 individual emitter molecules in a system relevant to vacuum-processed OLEDs. Analysis of solution- and vacuum-processed systems reveals that the orientation distributions strongly depend on the nanoscale environment of the emitter. This work opens the door to attaining unprecedented information on the factors that determine emitter orientation in current and future material systems for OLEDs.

physics.optics↗

arXiv2023

Schlieren texture induced Anderson localization in an organic exciton-polariton laser

Non-linearities in organic exciton-polariton microcavities represent an attractive platform for second-generation quantum devices. However, progress in this area hinges on the development of material platforms for high-performance polariton lasing, scalable and sustainable fabrication, and ultimately strategies for electrical pumping. Here, we show how introducing Schlieren textures in a liquid crystalline conjugated polymer and the associated microdomains of distinct chain orientation enable in-plane Anderson localization of polaritons. In high-Q distributed Bragg reflector microcavities, this strong localization facilitated polariton lasing at unprecedented thresholds of 136 fJ per pulse, thus providing a pathway to the study of fundamental effects at low polariton numbers. Anderson localization further permitted polariton lasing in more lossy metallic microcavities while maintaining a competitive lasing threshold. The facile fabrication of these cavities will drastically reduce the complexity of integrating polariton laser with other structures and the high conductivity of metallic mirrors provides a route to electrical pumping.

physics.optics↗

arXiv2022

Highly efficient polaritonic light emitting diodes with angle-independent narrowband emission

Angle-independent, narrowband emission is required for many optoelectronic devices, ranging from high-definition displays to sensors. However, emerging materials for electroluminescent devices, such as organics and perovskites, show spectrally broad emission due to intrinsic disorder. Coupling this emission to an optical resonance reduces the linewidth, but at the cost of inheriting the severe angular dispersion of the resonator. Strongly coupling a dispersionless exciton state to a narrowband optical microcavity could overcome this issue; however, electrically pumped emission from the resulting polaritons has been hampered by poor efficiencies. Here, we present a universal concept for polariton-based emission from state-of-the-art organic LEDs (OLEDs) by introducing an assistant strong coupling layer, thus avoiding quenching-induced efficiency losses. We realize red- and green-emitting, narrowband (FWHM <20 nm) and spectrally tuneable polaritonic OLEDs with up to 10% external quantum efficiency and high luminance (>20,000 cd m$^{-2}$ at 5 V). Optimizing cavity detuning and coupling strength allows to achieve emission with ultralow-dispersion (<10 nm spectral shift at 60° tilt). These results have significant implications for on-demand polariton emission and demonstrate the practical relevance of strong light-matter coupling for next-generation optoelectronics, particularly display technology.

physics.optics↗

arXiv2022

Local sensing of absolute refractive index during protein-binding using microlasers with spectral encoding

Multiplexed, specific and sensitive detection of antigens is critical for the rapid and accurate diagnosis of disease and the informed development of personalized treatment plans. Here, we show that polymer microsphere lasers can be used as photonic sensors to monitor and quantify direct surface binding of biomolecules via changes in the refractive index. The unique spectral signature of each individual laser can be used to find their size and effective refractive index which adds a new encoding dimension when compared to conventional fluorescent beads. We utilize antibody-functionalized microlasers to selectively detect protein binding. Different stages of the multilayer surface modification can be resolved, and protein binding is demonstrated for two different proteins, IgG and CRP. Moreover, by continuously monitoring single lasers, we demonstrate the possibility of real-time monitoring of binding dynamics between antigens in solution phase and the immobilized antibodies. For multiplexed detection, the microlasers are employed in a flow cytometer configuration, with fast spectral detection and identification of microlasers with and without antigen binding. We envision that by combining microlasers with well-established surface modification chemistries and flow geometries, the multiplexing ability of microbead immunoassays can be strongly increased while also opening avenues for single cell profiling within heterogenous cell populations.

physics.optics↗

arXiv2020

A substrateless, flexible, and water-resistant organic light-emitting diode

Despite widespread interest, ultrathin and highly flexible light-emitting devices that can be seamlessly integrated and used for flexible displays, wearables, and as bioimplants remain elusive. Organic light-emitting diodes (OLEDs) with $μ$m-scale thickness and exceptional flexibility have been demonstrated but show insufficient stability in air and moist environments due to a lack of suitable encapsulation barriers. Here, we demonstrate an efficient and stable OLED with a total thickness of $\approx$12 $μ$m that can be fully immersed in water or cell nutrient media for weeks without suffering substantial degradation. The active layers of the device are embedded between conformal barriers formed by alternating layers of parylene-C and metal oxides that are deposited through a low temperature chemical vapour process. These barriers also confer stability of the OLED to repeated bending and to extensive postprocessing, e.g. via reactive gas plasmas, organic solvents, and photolithography. This unprecedented robustness opens up a wide range of novel possibilities for ultrathin OLEDs.

physics.app-ph↗

arXiv2018

Non-obstructive intracellular nanolasers

Nanophotonic objects like plasmonic nanoparticles and colloidal quantum dots can complement the functionality of molecular dyes in biomedical optics. However, their operation is usually governed by spontaneous processes, which results in broad spectral features and limited signal-to-noise ratio, thus restricting opportunities for spectral multiplexing and sensing. Lasers provide the ultimate spectral definition and background suppression, and their integration with cells has recently been demonstrated. However, laser size and threshold remain problematic. Here, we report on the design, high-throughput fabrication and intracellular integration of semiconductor nanodisk lasers. By exploiting the large optical gain and high refractive index of GaInP/AlGaInP quantum wells, we obtain lasers with volumes 1000-fold smaller than the eukaryotic nucleus ($V_{laser}$<0.1 $μ$m$^3$), lasing thresholds 500-fold below the pulse energies typically used in two-photon microscopy ($E_{th} \approx $0.13 pJ), and excellent spectral stability (<50 pm wavelength shift). Multiplexed labelling with these lasers allows cells-tracking through micro-pores, thus providing a powerful tool to study cell migration and cancer invasion.

physics.optics↗

arXiv2016

An exciton-polariton laser based on biologically produced fluorescent protein

Under adequate conditions, cavity-polaritons form a macroscopic coherent quantum state, known as Bose-Einstein condensate (BEC). Compared to Wannier-Mott excitons in inorganic semiconductors, the localized Frenkel excitons in organic emitter materials show weaker interaction but stronger coupling, which recently enabled the first realization of BEC at room temperature. However, this required ultrafast optical pumping which limits the applications of organic BECs. Here, we demonstrate room-temperature BEC of cavity-polaritons in simple laminated microcavities filled with the biologically produced enhanced green fluorescent protein (eGFP). The unique molecular structure of eGFP prevents exciton annihilation even at high excitation densities, thus facilitating BEC under conventional nanosecond pumping. BEC is clearly evidenced by a distinct threshold, an interaction-induced blueshift of the condensate, long-range coherence and the presence of a second threshold at higher excitation density which is associated with the onset of photon lasing and results from thermalization of the exciton reservoir.

cond-mat.mes-hall↗