Monolithic OLED-Microwire Devices for Ultrastrong Magnetic Resonant Excitation

Sub-micron spin-based magnetic field imaging with an organic light emitting diode

Quantum sensing and imaging of magnetic fields has attracted broad interests due to its potential for high sensitivity and spatial resolution. Common systems used for quantum sensing require either optical excitation (e.g., nitrogen-vacancy centres in diamond, atomic vapor magnetometers), or cryogenic temperatures (e.g., SQUIDs, superconducting qubits), which pose challenges for chip-scale integration and commercial scalability. Here, we demonstrate an integrated organic light emitting diode (OLED) based solid-state sensor for magnetic field imaging, which employs spatially resolved magnetic resonance to provide a robust mapping of magnetic fields. By considering the monolithic OLED as an array of individual virtual sensors, we achieve sub-micron magnetic field mapping with field sensitivity of ~160 µT Hz^-1/2 µm^-2. Our work demonstrates a chip-scale OLED-based laser free magnetic field sensor and an approach to magnetic field mapping built on a commercially relevant and manufacturable technology.

Orientation-Dependent Interaction between the Magnetic Plasmons in Gold Nanocups and the Excitons in WS2 Monolayer and Multilayer

The integration of two-dimensional transition metal dichalcogenides with plasmonic nanostructures is extremely attractive for the investigation of the resonance coupling between plasmons and excitons, which offers a framework for the study of cavity quantum electrodynamics and is of great potential for exploring diverse quantum technologies. Herein we report on the coupling between the magnetic plasmons supported by individual asymmetric Au nanocups and the excitons in WS₂ monolayer and multilayer. Resonance coupling with the strength varying from weak to strong regimes is realized by adjusting the orientation of the individual Au nanocups on WS₂ monolayer. Different energy detunings between the magnetic plasmons and the excitons are achieved by varying the size of the Au nanocup. The Rabi splitting energies extracted at zero detuning are up to 106 meV. The anticrossing feature is observed in the measured scattering spectra and simulated absorption spectra, which indicates that the resonance coupling between the magnetic plasmons in the Au nanocup and the excitons in WS₂ monolayer enters the strongly coupled regime. A dependence of the coupling strength on the layer number is further observed when the Au nanocups are coupled with WS₂ multilayer. Our study suggests a promising approach toward the realization of different coupling regimes in a simple hybrid system made of individual Au nanocups and two-dimensional materials.

Resolving the Spatial Variation and Correlation of Hyperfine Spin Properties in Organic Light-Emitting Diodes

Devices that exploit the quantum properties of materials are widespread, with quantum information processors and quantum sensors showing significant progress. Organic materials offer interesting opportunities for quantum technologies owing to their engineerable spin properties, with spintronic operation and spin resonance magnetic-field sensing demonstrated in research grade devices, as well as proven compatibility with large-scale fabrication techniques. Yet several important challenges remain as moving toward scaling these proof-of-principle quantum devices to larger integrated logic systems or spatially smaller sensing elements, particularly those associated with the variation of quantum properties both within and between devices. Here, spatially resolved magnetoluminescence is used to provide the first 2D map of a hyperfine spin property-the Overhauser field-in traditional organic light-emitting diodes (OLEDs). Intra-device variabilities are found to exceed ≈30% while spatially correlated behavior is exhibited on lengths beyond 7 µm, similar in size to pixels in state-of-the-art active-matrix OLED arrays, which has implications for the reproducibility and integration of organic quantum devices.

Floquet spin states in OLEDs

Electron and hole spins in organic light-emitting diodes constitute prototypical two-level systems for the exploration of the ultrastrong-drive regime of light-matter interactions. Floquet solutions to the time-dependent Hamiltonian of pairs of electron and hole spins reveal that, under non-perturbative resonant drive, when spin-Rabi frequencies become comparable to the Larmor frequencies, hybrid light-matter states emerge that enable dipole-forbidden multi-quantum transitions at integer and fractional g-factors. To probe these phenomena experimentally, we develop an electrically detected magnetic-resonance experiment supporting oscillating driving fields comparable in amplitude to the static field defining the Zeeman splitting; and an organic semiconductor characterized by minimal local hyperfine fields allowing the non-perturbative light-matter interactions to be resolved. The experimental confirmation of the predicted Floquet states under strong-drive conditions demonstrates the presence of hybrid light-matter spin excitations at room temperature. These dressed states are insensitive to power broadening, display Bloch-Siegert-like shifts, and are suggestive of long spin coherence times, implying potential applicability for quantum sensing.

Perdeuterated Conjugated Polymers for Ultralow-Frequency Magnetic Resonance of OLEDs

The formation of excitons in OLEDs is spin dependent and can be controlled by electron-paramagnetic resonance, affecting device resistance and electroluminescence yield. We explore electrically detected magnetic resonance in the regime of very low magnetic fields (<1 mT). A pronounced feature emerges at zero field in addition to the conventional spin- 1 / 2 Zeeman resonance for which the Larmor frequency matches that of the incident radiation. By comparing a conventional π-conjugated polymer as the active material to a perdeuterated analogue, we demonstrate the interplay between the zero-field feature and local hyperfine fields. The zero-field peak results from a quasistatic magnetic-field effect of the RF radiation for periods comparable to the carrier-pair lifetime. Zeeman resonances are resolved down to 3.2 MHz, approximately twice the Larmor frequency of an electron in Earth's field. However, since reducing hyperfine fields sharpens the Zeeman peak at the cost of an increased zero-field peak, we suggest that this result may constitute a fundamental low-field limit of magnetic resonance in carrier-pair-based systems. OLEDs offer an alternative solid-state platform to investigate the radical-pair mechanism of magnetic-field effects in photochemical reactions, allowing models of biological magnetoreception to be tested by measuring spin decoherence directly in the time domain by pulsed experiments.

OLEDs as models for bird magnetoception: detecting electron spin resonance in geomagnetic fields

Certain species of living creatures are known to orientate themselves in the geomagnetic field. Given the small magnitude of approximately 48 μT, the underlying quantum mechanical phenomena are expected to exhibit coherence times in the microsecond regime. In this contribution, we show the sensitivity of organic light-emitting diodes (OLEDs) to magnetic fields far below Earth's magnetic field, suggesting that coherence times of the spins of charge-carrier pairs in these devices can be similarly long. By electron paramagnetic resonance (EPR) experiments, a lower bound for the coherence time can be assessed directly. Moreover, this technique offers the possibility to determine the distribution of hyperfine fields within the organic semiconductor layer. We extend this technique to a material system exhibiting both fluorescence and phosphorescence, demonstrating stable anticorrelation between optically detected magnetic resonance (ODMR) spectra in the singlet (fluorescence) and triplet (phosphorescence) channels. The experiments demonstrate the extreme sensitivity of OLEDs to both static as well as dynamic magnetic fields and suggest that coherent spin precession processes of coulombically bound electron-spin pairs may play a crucial role in the magnetoreceptive ability of living creatures.

Nonresonant Transmission Line Probe for Sensitive Interferometric Electron Spin Resonance Detection

Electron spin resonance (ESR) spectroscopy measures paramagnetic free radicals, or electron spins, in a variety of biological, chemical, and physical systems. Detection of diverse paramagnetic species is important in applications ranging from quantum computation to biomedical research. Countless efforts have been made to improve the sensitivity of ESR detection. However, the improvement comes at the cost of experimental accessibility. Thus, most ESR spectrometers are limited to specific sample geometries and compositions. Here, we present a nonresonant transmission line ESR probe (microstrip geometry) that effectively couples high frequency microwave magnetic field into a wide range of sample geometries and compositions. The nonresonant transmission line probe maintains detection sensitivity while increasing availability to a wider range of applications. The high frequency magnetic field homogeneity is greatly increased by positioning the sample between the microstrip signal line and the ground plane. Sample interfacing occurs via a universal sample holder which is compatible with both solid and liquid samples. The unavoidable loss in sensitivity due to the nonresonant nature of the transmission line probe (low Q) is recuperated by using a highly sensitive microwave interferometer-based detection circuit. The combination of our sensitive interferometer and nonresonant transmission line provides similar sensitivity to a commercially available ESR spectrometer equipped with a high-Q resonator. The nonresonant probe allows for transmission, reflection, or dual-mode detection (transmission and reflection), where the dual-mode results in a √2 signal enhancement.