Super-resolution Fluorescence Quenching Microscopy of Graphene

Lately, fluorescence quenching microscopy (FQM) has been introduced as a new tool to visualize graphene-based sheets. Even though quenching of the emission from a dye molecule by fluorescence resonance energy transfer (FRET) to graphene happens on the nanometer scale, the resolution of FQM so far is still limited to several hundreds of nanometers due to the Abbe limit restricting the resolution of conventional light microscopy. In this work, we demonstrate an advancement of FQM by using a super-resolution imaging technique for detecting fluorescence of color centers used in FQM. The technique is similar to stimulated emission depletion microscopy (STED). The combined “FRET+STED” technique introduced here for the first time represents a substantial improvement to FQM since it exhibits in principle unlimited resolution while still using light in the visible spectral range. In the present case we demonstrate all-optical imaging of graphene with resolution below 30 nm. The performance of the technique in terms of imaging resolution and contrast is well described by a theoretical model taking into account the general distance dependence of the FRET process and the distance distribution of donor centers with respect to the flake. In addition, the change in lifetime for partially quenched emitters allows extracting the quenching distance from experimental data for the first time

Relaxometry and Dephasing Imaging of Superparamagnetic Magnetite Nanoparticles Using a Single Qubit

To study the magnetic dynamics of superparamagnetic nanoparticles, we use scanning probe relaxometry and dephasing of the nitrogen vacancy (NV) center in diamond, characterizing the spin noise of a single 10 nm magnetite particle. Additionally, we show the anisotropy of the NV sensitivity’s dependence on the applied decoherence measurement method. By comparing the change in relaxation (<i>T</i>₁) and dephasing (<i>T</i>₂) time in the NV center when scanning a nanoparticle over it, we are able to extract the nanoparticle’s diameter and distance from the NV center using an Ornstein–Uhlenbeck model for the nanoparticle’s fluctuations. This scanning probe technique can be used in the future to characterize different spin label substitutes for both medical applications and basic magnetic nanoparticle behavior

Nanoengineered Diamond Waveguide as a Robust Bright Platform for Nanomagnetometry Using Shallow Nitrogen Vacancy Centers

Photonic structures in diamond are key to most of its application in quantum technology. Here, we demonstrate tapered nanowaveguides structured directly onto the diamond substrate hosting shallow-implanted nitrogen vacancy (NV) centers. By optimization based on simulations and precise experimental control of the geometry of these pillar-shaped nanowaveguides, we achieve a net photon flux up to ∼1.7 × 10⁶ s^–1. This presents the brightest monolithic bulk diamond structure based on single NV centers so far. We observe no impact on excited state lifetime and electronic spin dephasing time (<i>T</i>₂) due to the nanofabrication process. Possessing such high brightness with low background in addition to preserved spin quality, this geometry can improve the current nanomagnetometry sensitivity ∼5 times. In addition, it facilitates a wide range of diamond defects-based magnetometry applications. As a demonstration, we measure the temperature dependency of <i>T</i>₁ relaxation time of a single shallow NV center electronic spin. We observe the two-phonon Raman process to be negligible in comparison to the dominant two-phonon Orbach process

Toward Optimized Surface δ‑Profiles of Nitrogen-Vacancy Centers Activated by Helium Irradiation in Diamond

The negatively charged nitrogen-vacancy (NV) center in diamond has been shown recently as an excellent sensor for external spins. Nevertheless, their optimum engineering in the near-surface region still requires quantitative knowledge in regard to their activation by vacancy capture during thermal annealing. To this aim, we report on the depth profiles of near-surface helium-induced NV centers (and related helium defects) by step-etching with nanometer resolution. This provides insights into the efficiency of vacancy diffusion and recombination paths concurrent to the formation of NV centers. It was found that the range of efficient formation of NV centers is limited only to approximately 10 to 15 nm (radius) around the initial ion track of irradiating helium atoms. Using this information we demonstrate the fabrication of nanometric-thin (δ) profiles of NV centers for sensing external spins at the diamond surface based on a three-step approach, which comprises (i) nitrogen-doped epitaxial CVD diamond overgrowth, (ii) activation of NV centers by low-energy helium irradiation and thermal annealing, and (iii) controlled layer thinning by low-damage plasma etching. Spin coherence times (Hahn echo) ranging up to 50 μs are demonstrated at depths of less than 5 nm in material with 1.1% of ¹³C (depth estimated by spin relaxation (T₁) measurements). At the end, the limits of the helium irradiation technique at high ion fluences are also experimentally investigated

Photoinduced Modification of Single-Photon Emitters in Hexagonal Boron Nitride

Fluorescent defects recently observed under ambient conditions in hexagonal boron nitride (h-BN) promise to open novel opportunities for the implementation of on-chip photonic devices that rely on identical photons from single emitters. Here we report on the room-temperature photoluminescence dynamics of individual emitters in multilayer h-BN flakes exposed to blue laser light. Comparison of optical spectra recorded at successive times reveals considerable spectral diffusion, possibly the result of slowly fluctuating, trapped-carrier-induced Stark shifts. Large spectral jumpsreaching up to 100 nmfollowed by bleaching are observed in most cases upon prolonged exposure to blue light, an indication of one-directional photochemical changes possibly taking place on the flake surface. Remarkably, only a fraction of the observed emitters also fluoresce on green illumination, suggesting a more complex optical excitation dynamics than previously anticipated and raising questions on the physical nature of the crystal defect at play

Addressing Single Nitrogen-Vacancy Centers in Diamond with Transparent in-Plane Gate Structures

For many applications of the nitrogen-vacancy (NV) center in diamond, the understanding and active control of its charge state is highly desired. In this work, we demonstrate the reversible manipulation of the charge state of a single NV center from NV^– across NV⁰ to a nonfluorescent, dark state by using an all-diamond in-plane gate nanostructure. Applying a voltage to the in-plane gate structure can influence the energy band bending sufficiently for charge state conversion of NV centers. These diamond in-plane structures can function as transparent top gates, enabling the distant control of the charge state of NV centers tens of micrometers away from the nanostructure

Protecting a Diamond Quantum Memory by Charge State Control

In recent years, solid-state spin systems have emerged as promising candidates for quantum information processing. Prominent examples are the nitrogen-vacancy (NV) center in diamond, phosphorus dopants in silicon (Si:P), rare-earth ions in solids, and V_Si-centers in silicon-carbide. The Si:P system has demonstrated that its nuclear spins can yield exceedingly long spin coherence times by eliminating the electron spin of the dopant. For NV centers, however, a proper charge state for storage of nuclear spin qubit coherence has not been identified yet. Here, we identify and characterize the positively charged NV center as an electron-spin-less and optically inactive state by utilizing the nuclear spin qubit as a probe. We control the electronic charge and spin utilizing nanometer scale gate electrodes. We achieve a lengthening of the nuclear spin coherence times by a factor of 4. Surprisingly, the new charge state allows switching of the optical response of single nodes facilitating full individual addressability