arXiv subjects

Elke Neu

Publications and source records attributed to Elke Neu.

At least 19 recordsLinked to original sources

arXiv2026

Methods for traceable scanning magnetometry using single nitrogen vacancy centers in diamond: determining orientation, distance and localization

Individual, scannable nitrogen vacancy (NV) centers in single crystal diamond nanostructures enable nanoscale, quantitative imaging of magnetic stray fields. Nevertheless, important parameters like distance between the NV center and the sample and the orientation of the NV high symmetry axis are often not known precisely and enter data evaluation as free fitting parameters. We here use scanning NV imaging on micro-patterned, perpendicularly magnetized stripes and discs. From these measurements, we directly infer NV - sample distance d_NV and the NV's azimuthal orientation without the need for an external vector magnet control. We determine d_NV = 31.5 nm, while we infer the azimuthal orientation with a precision of 3{\deg}. We additionally employ commercially available silicon needles to image the apex topography of our diamond nanostructures to detect surface contamination. Simultaneously, monitoring NV fluorescence as a function of the needle's position allows us to estimate the lateral placement of the NV inside the diamond nanostructure.

cond-mat.mes-hall↗

arXiv2026

Towards Application of Nanodiamonds for in-situ Monitoring of Radicals in Liquid Phase Chemical Reactions

In many chemical reactions, short-lived radical intermediates play a crucial role, while detecting such short-lived species in-situ remains challenging. The optically readable electronic spin of nitrogen-vacancy (NV) centers in diamond is a nanoscale sensor for such radical species: its longitudinal spin relaxation time (T$_{1}$) reacts to magnetic fluctuations from the unpaired electrons of radical species in its local environment. In this setting, we demonstrate the successful in-situ detection of the nitroxide radical 2,2,6,6-Tetramethylpiperidinyloxyl (TEMPO) using NV center-based T$_1$ relaxometry after depositing nanodiamonds onto the inner wall of a glass cuvette. A significant concentration-dependent shortening of the relaxation time was observed, from 197\:\textmu s $\pm$ 21\:\textmu s without radical to 66\:\textmu s $\pm$ 30\:\textmu s at a concentration of 1\:M TEMPO. At the same time, the nanodiamonds remain firmly attached to the cuvette even after the solution under investigation has been exchanged. The detection is sensitive in the nanomolar (nM) range and the determined signal-to-noise ratio is between 1.6 and 3.

cond-mat.mes-hall↗

arXiv2026

Direct Laser Writing of Ferromagnetic Nickel Utilizing the Principle of Sensitized Triplet-Triplet Annihilation Upconversion

Direct laser writing of ferromagnetic microstructures is of great interest for sensing and data storage in compact three-dimensional architectures. However, reliable direct laser writing of metallic and even more so ferromagnetic materials remains a major challenge. Here, we present a novel photoresist suitable to direct laser write ferromagnetic nickel based on sensitized triplet-triplet annihilation upconversion. By combining an in-situ photochemical deoxygenation process with a sensitized triplet-triplet annihilation upconversion process as well as a photoreduction of Ni2+ ions, the deposition of metallic nickel is enabled under ambient conditions. Using this approach, nickel structures are fabricated as a proof of concept. Scanning electron microscopy and EDX analysis confirm the spatially confined deposition of nickel, while magnetic characterization by vibrating sample magnetometry and scanning NV magnetometry demonstrate the ferromagnetic nature of the printed structures. This work presents a major step forward in extending the possibilities of direct laser writing to metallic and ferromagnetic materials.

cond-mat.mtrl-sci↗

arXiv2026

FRET between NV centers in diamond and chlorophyll molecules: a novel resource for multimodal sensing and imaging in plant cells

This work demonstrates efficient Forster resonance energy transfer (FRET) between ensembles of shallow nitrogen-vacancy (NV) centers located 7 nm and 9 nm below a single-crystal diamond surface and a naturally occurring fluorophore, namely a mixture of chlorophyll a and b molecules extracted from Arabidopsis thaliana. The broad fluorescence band of NV centers spectrally overlaps with the absorption of chlorophyll molecules, enabling FRET. As a result, depositing a chlorophyll layer on the diamond surface reduces the NV fluorescence lifetime from approximately 14 ns to below 4 ns, indicating efficient energy transfer. Laser-induced photobleaching of chlorophyll restores the unquenched NV lifetime. In contrast, NV centers located deeper within the diamond at depths of 40 nm and 72 nm remain unaffected, confirming that the observed quenching originates from a short-range FRET mechanism. The NV ensembles retain their optically detected magnetic resonance contrast while FRET is observed, demonstrating preservation of their spin properties. These proof-of-principle experiments establish the feasibility of combining FRET-based distance measurements with magnetic sensing using optically readable spins.

physics.bio-ph↗

arXiv2025

Influence of Platinum Thin Films on the Photophysical and Quantum Properties of Near-Surface NV Centers

Nitrogen-vacancy (NV) centers in diamond are optically addressable spin defects with great potential for nanoscale quantum sensing. A key application of NV centers is the detection of external spins at the diamond surface. Among metals, platinum thin films - widely used in spintronics, catalysis and electrochemistry - provide a particularly interesting system for such studies. However, the interaction between NV centers and metals is known to affect their quantum sensing capabilities. In this work, we study five platinum-covered diamond samples containing shallow NVs created via nitrogen implantation with different energies (2.5-60 keV) and investigate the optical and quantum properties of NV ensembles beneath the metal films. We find a substantial reduction of the photoluminescence lifetime and a pronounced decrease of the NV$^{-}$ population for NV ensembles located near the platinum layer. As a result, optically detected magnetic resonance experiments could only be efficiently performed on diamonds implanted with at least 20 keV, where we observed a strong increase in the T$_{2}$ coherence time beneath the platinum thin films. Our study describes the various processes affecting NV centers near platinum films and provides guidance for the integration of thin metal films with near-surface NV centers.

cond-mat.mes-hall↗

arXiv2025

Nanoscale Dipolar Fields in Artificial Spin Ice Probed by Scanning NV Magnetometry

We investigate dipolar coupling fields in two square-lattice artificial spin ice (ASI) systems with different lattice constants using scanning probe microscopy based on a single nitrogen-vacancy (NV) center in diamond. This technique offers unprecedented spatial resolution, operates under ambient condition, and provides quantitative stray field measurements, making it uniquely suited for studying nanoscale magnetic textures. Our approach combines fluorescence quenching imaging and continuous-wave optically detected magnetic resonance (cODMR). A comparison of the two ASI samples, which differ in their lattice constants of 1000 nm and 910 nm respectively, reveals differences in the appearance of ice-rule violations - deviations from the lowest energy configuration in ASI vertices. We attribute these variations to varying coupling strengths dictated by the lattice constant. From the cODMR data, we extract both axial and transverse components of the local magnetic field relative to the NV axis. Micromagnetic modeling of these measurements allows for an iterative determination of the external magnetic field orientation, the detection of subtle magnetization tilts induced by weak external fields (well below the nanomagnets' switching threshold), and an estimation of the effective saturation magnetization, thereby accounting for deviations in nanomagnet dimensions. These findings provide crucial insights into the tunable magnetic interactions in ASI, paving the way for the design of advanced magnonic and spintronic devices.

cond-mat.mes-hall↗

arXiv2025

Medusa 84 SiH -- A novel high Selectivity Electron Beam Resist for Diamond Quantum Technologies

We investigate the novel electron beam resist Medusa 84 SiH by Allresist GmbH (Germany) for nanostructuring of single crystal diamond and its effects on the spin properties of nitrogen vacancy (NV) centers in nanopillars as prototypes for photonic structures. We find contrast curves comparable to those of resists previously used for this task (Hydrogensilsequioxane FOx). We present a minimum selectivity for diamond etching of 6$\pm$1. Using an adhesion-promoting silicon interlayer enables fabrication yields of up to 98\%. We measure $T_2$ times of up to $\sim$25 $\mu$s before and after processing, demonstrating that the manufactured structures are usable for diamond-based quantum sensing.

physics.ins-det↗

arXiv2025

Nitrogen-Vacancy Centers in Epitaxial Laterally Overgrown Diamond: Towards Up-scaling of Color Center-based Quantum Technologies

Providing high-quality, single-crystal diamond (SCD) with a large area is desirable for up-scaling quantum technology applications that rely on color centers in diamond. Growth methods aiming to increase the area of SCD are an active research area. Native color centers offer a sensitive probe for local crystal quality in such novel materials e.g., via their reaction to stress. In this work, we investigate individual native nitrogen-vacancy (NV) centers in SCD layers manufactured via laterally overgrowing hole arrays in a heteroepitaxially grown large-scale substrate. Heteroepitaxy has become a common tool for growing large SCDs; however, achieving the high crystal quality needed for quantum applications remains a challenge. In the overgrown layer, we identify NV centers with spin-decoherence times in the order of hundreds of microseconds, comparable to high-purity homoepitaxial SCD. We quantify the effective crystal strain in different regions of the overgrown layer, indicating a low stress overall and a stress reduction in the diamond layer above the holes.

cond-mat.mtrl-sci↗

arXiv2024

On the experimental properties of the TS defect in 4H-SiC

When annealing a 4H silicon carbide (SiC) crystal, a sequence of optically active defect centers occurs among which the TS center is a prominent example. Here, we present low-temperature photoluminescence analyses on the single defect level. They reveal that the three occurring spectral signatures TS1, TS2 and TS3 originate from one single defect. Their polarization dependences expose three different crystallographic orientations in the basal plane, which relate to the projections of the nearest neighbor directions. Accordingly, we find a three-fold level-splitting in ensemble studies, when applying mechanical strain. This dependency is quantitatively calibrated. A complementary electrical measurement, deep level transient spectroscopy, reveals a charge transition level of the TS defect at 0.6 eV above the valence band. For a future identification, this accurate characterization of its optical and electronic properties along with their response to mechanical strain is a milestone.

physics.app-ph↗

arXiv2021

Robust Magnetometry with Single NV Centers via Two-step Optimization

Shallow Nitrogen-Vacancy (NV) centers are promising candidates for high-precision sensing applications; these defects, when positioned a few nanometers below the surface, provide an atomic-scale resolution along with substantial sensitivity. However, the dangling bonds and impurities on the diamond surface result in a complex environment which reduces the sensitivity and is unique to each shallow NV center. To avoid the environment's detrimental effect, we apply feedback-based quantum optimal control. We first show how a direct search can improve the initialization/readout process. In a second step, we optimize microwave pulses for pulsed Optically Detected Magnetic Resonance (ODMR) and Ramsey measurements. Throughout the sensitivity optimizations, we focus on robustness against errors in the control field amplitude. This feature not only protects the protocols' sensitivity from drifts but also enlarges the sensing volume. The resulting ODMR measurements produce sensitivities below 1$\mu$T\,Hz$^{-\frac{1}{2}}$ for an 83\% decrease in control power, increasing the robustness by approximately one third. The optimized Ramsey measurements produce sensitivities below 100\,nT\,Hz$^{-\frac{1}{2}}$ giving a two-fold sensitivity improvement. Being on par with typical sensitivities obtained via single NV magnetometry, the complementing robustness of the presented optimization strategy may provide an advantage for other NV-based applications.

quant-ph↗

arXiv2021

Optimized Planar Microwave Antenna for Nitrogen Vacancy Center based Sensing Applications

Individual nitrogen vacancy (NV) color centers in diamond are versatile, spin-based quantum sensors. Coherently controlling the spin of NV centers using microwaves in a typical frequency range between 2.5 and 3.5 GHz is necessary for sensing applications. In this work, we present a stripline-based, planar, {\Omega}-shaped microwave antenna that enables to reliably manipulate NV spins. We find an optimal antenna design using finite integral simulations. We fabricate our antennas on low-cost, transparent glass substrate. We demonstrate highly uniform microwave fields in areas of roughly 400 x 400 {\mu}m^2 while realizing high Rabi frequencies of up to 10 MHz in an ensemble of NV centers.

cond-mat.mtrl-sci↗

arXiv2020

Introduction to Quantum Optimal Control for Quantum Sensing with Nitrogen-Vacancy Centers in Diamond

Diamond based quantum technology is a fast emerging field with both scientific and technological importance. With the growing knowledge and experience concerning diamond based quantum systems, comes an increased demand for performance. Quantum optimal control (QOC) provides a direct solution to a number of existing challenges as well as a basis for proposed future applications. Together with a swift review of QOC strategies, quantum sensing and other relevant quantum technology applications of nitrogen-vacancy (NV) centers in diamond, we give the necessary background to summarize recent advancements in the field of QOC assisted quantum applications with NV centers in diamond.

quant-ph↗

arXiv2019

Single crystal diamond pyramids for applications in nanoscale quantum sensing

We present a new approach combining top down fabrication and bottom up overgrowth to create diamond photonic nanostructures in form of single-crystalline diamond nanopyramids. Our approach relies on diamond nanopillars, that are overgrown with single-crystalline diamond to form pyramidal structures oriented along crystal facets. To characterize the photonic properties of the pyramids, color centers are created in a controlled way using ion implantation and annealing. We find very high collection efficiency from color centers close to the pyramid apex. We further show excellent smoothness and sharpness of our diamond pyramids with measured tip radii on the order of 10 nm. Our results offer interesting prospects for nanoscale quantum sensing using diamond color centers, where our diamond pyramids could be used as scanning probes for nanoscale imaging. There, our approach would offer signifikant advantages compared to the cone-shaped scanning probes which define the current state of the art.

cond-mat.mes-hall↗

arXiv2019

Plasma treatments and photonic nanostructures for shallow nitrogen vacancy centers in diamond

We investigate the influence of plasma treatments, especially a 0V-bias, potentially low damage O$_2$ plasma as well as a biased Ar/SF$_6$/O$_2$ plasma on shallow, negative nitrogen vacancy (NV$^-$) centers. We ignite and sustain using our 0V-bias plasma using purely inductive coupling. To this end, we pre-treat surfaces of high purity chemical vapor deposited single-crystal diamond (SCD). Subsequently, we create $\sim$10 nm deep NV$^-$ centers via implantation and annealing. Onto the annealed SCD surface, we fabricate nanopillar structures that efficiently waveguide the photoluminescence (PL) of shallow NV$^-$. Characterizing single NV$^-$ inside these nanopillars, we find that the Ar/SF$_6$/O$_2$ plasma treatment quenches NV$^-$ PL even considering that the annealing and cleaning steps following ion implantation remove any surface termination. In contrast, for our 0V-bias as well as biased O$_2$ plasma, we observe stable NV$^-$ PL and low background fluorescence from the photonic nanostructures.

cond-mat.mtrl-sci↗

arXiv2019

Reliable Nanofabrication of Single-Crystal Diamond Photonic Nanostructures for Nanoscale Sensing

In this manuscript, we outline a reliable procedure to manufacture photonic nanostructures from single-crystal diamond (SCD). Photonic nanostructures, in our case SCD nanopillars on thin (< 1$\mu$m) platforms, are highly relevant for nanoscale sensing. The presented top-down procedure includes electron beam lithography (EBL) as well as reactive ion etching (RIE). Our method introduces a novel type of inter-layer, namely silicon, that significantly enhances the adhesion of hydrogen silsesquioxane (HSQ) electron beam resist to SCD and avoids sample charging during EBL. In contrast to previously used adhesion layers, our silicon layer can be removed using a highly-selective RIE step which is not damaging HSQ mask structures. We thus refine published nanofabrication processes to ease a higher process reliability especially in the light of the advancing commercialization of SCD sensor devices.

cond-mat.mtrl-sci↗

arXiv2019

Nanoscale sensing based on nitrogen vacancy centersin single crystal diamond and nanodiamonds:achievements and challenges

Powered by the mutual developments in instrumentation, materials andtheoretical descriptions, sensing and imaging capabilities of quantum emitters insolids have significantly increased in the past two decades. Quantum emitters insolids, whose properties resemble those of atoms and ions, provide alternative waysto probing natural and artificial nanoscopic systems with minimum disturbance andultimate spatial resolution. Among those emerging quantum emitters, the nitrogen-vacancy (NV) color center in diamond is an outstanding example due to its intrinsicproperties at room temperature (highly-luminescent, photo-stable, biocompatible,highly-coherent spin states). This review article summarizes recent advances andachievements in using NV centers within nano- and single crystal diamonds in sensingand imaging. We also highlight prevalent challenges and material aspects for differenttypes of diamond and outline the main parameters to consider when using color centersas sensors. As a novel sensing resource, we highlight the properties of NV centersas light emitting electrical dipoles and their coupling to other nanoscale dipoles e.g.graphene.

physics.app-ph↗

arXiv2019

Near-field energy transfer between a luminescent 2D material and color centers in diamond

Energy transfer between fluorescent probes lies at the heart of many applications ranging from bio-sensing and -imaging to enhanced photo-detection and light harvesting. In this work, we study F\"orster resonance energy transfer (FRET) between shallow defects in diamond --- nitrogen-vacancy (NV) centers --- and atomically-thin, two-dimensional materials --- tungsten diselenide (WSe$_2$). By means of fluorescence lifetime imaging, we demonstrate the occurrence of FRET in the WSe$_2$/NV system. Further, we show that in the coupled system, NV centers provide an additional excitation pathway for WSe$_2$ photoluminescence. Our results constitute the first step towards the realization of hybrid quantum systems involving single-crystal diamond and two-dimensional materials that may lead to new strategies for studying and controlling spin transfer phenomena and spin valley physics.

quant-ph↗

arXiv2019

Spin Measurements of NV Centers Coupled to a Photonic Crystal Cavity

Nitrogen-vacancy (NV) centers feature outstanding properties like a spin coherence time of up to one second as well as a level structure offering the possibility to initialize, coherently manipulate and optically read-out the spin degree of freedom of the ground state. However, only about three percent of their photon emission are channeled into the zero phonon line (ZPL), limiting both the rate of indistinguishable single photons and the signal-to-noise ratio (SNR) of coherent spin-photon interfaces. We here report on the enhancement of the SNR of the optical spin read-out achieved by tuning the mode of a two-dimensional photonic crystal (PhC)cavity into resonance with the NV-ZPL. PhC cavities are fabricated by focused ion beam (FIB) milling in thin reactive ion (RIE) etched ultrapure single crystal diamond membranes featuring modes with Q-factors of up to 8250 at mode volumes below one cubic wavelength. NV centers are produced in the cavities in a controlled fashion by a high resolution atomic force microscope (AFM) implantation technique. On cavity resonance we observe a lifetime shortening from 9.0ns to 8.0ns as well as an enhancement of the ZPL emission by almost one order of magnitude. Although on resonance the collection efficiency of ZPL photons and the spin-dependent fluorescence contrast are reduced, the SNR of the optical spin read-out is almost tripled for the cavity-coupled NV centers.

quant-ph↗