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We demonstrate an adaptive wave-front shaping of optical beams transmitted through fiber bundles as a powerful resource for multisite, high-resolution bioimaging. With the phases of all the beamlets delivered through up to 6000 different fibers within the fiber bundle controlled individually, by means of a high-definition spatial light modulator, the overall beam transmitted through the fiber bundle can be focused into a beam waist with a diameter less than 1 μm within a targeted area in a biotissue, providing a diffraction-limited spatial resolution adequate for single-cell or even subcellular bioimaging. The field intensity in the adaptively-focused continuous-wave laser beam in our fiber-bundle-imaging setting is more than two orders of magnitude higher than the intensity of the speckle background. Once robust beam focusing was achieved with a suitable phase profile across the input face of the fiber bundle, the beam focus can be scanned over a targeted area with no need for a further adaptive search, by applying a physically intuitive, wave-front-tilting phase mask on the field of input beamlets. This method of beam-focus scanning promises imaging speeds compatible with the requirements of in vivo calcium imaging.

We demonstrate a versatile framework for cellular brain imaging in awake mice based on suitably tailored segments of graded-index (GRIN) fiber. Closed-form solutions to ray-path equations for graded-index waveguides are shown to offer important insights into image-transmission properties of GRIN fibers, suggesting useful recipes for optimized GRIN-fiber-based deep-brain imaging. We show that the lengths of GRIN imaging components intended for deep-brain studies in freely moving rodents need to be chosen as a tradeoff among the spatial resolution, the targeted imaging depth and the degree of fiber-probe invasiveness. In the experimental setting that we present in this paper, the head of an awake mouse with a GRIN-fiber implant is fixed under a microscope objective, but the mouse is free to move around an in-house-built flat-floored air-lifted platform, exploring a predesigned environment, configured as an arena for one of standard cognitive tests. We show that cellular-resolution deep-brain imaging can be integrated in this setting with robust cell-specific optical neural recording to enable in vivo studies with minimal physical restraints on animal models. The enhancement of the information capacity of the fluorescence signal, achieved via a suitable filtering of the GRIN-fiber readout, is shown to open routes toward practical imaging modalities whereby the deep-brain neuronal dynamics and axonal connections underpinning the integrative functions of essential brain structures can be studied in awake rodent models.

The effect of laser radiation with a wavelength near 1.55 µm on the photoluminescence properties of “nitrogen-vacancy” (NV) color centers in diamond has been studied experimentally. The effect of radiation of an infrared laser on the charge state of color centers redistributes the spectral intensity of photoluminescence in the ensemble of NV centers. In the case of neutral NV centers, the quenching of photoluminescence reaching about 60% has been observed. A feature of the laser excitation of NV centers by this method is the absence of competition with stimulated emission and two-photon excitation, which is observed with an increase in the power of infrared radiation at other wavelengths.

We present all-optical high-resolution temperature measurements in the brain of freely behaving mice using an implantable thermometer that integrates a reconnectable two-section fiber-optic probe a...

We present an experimental framework and methodology for in vivo studies on rat stroke models that enable a real-time fiber-optic recording of stroke-induced hydrogen peroxide and pH transients in ischemia-affected brain areas. Arrays of reconnectable implantable fiber probes combined with advanced optogenetic fluorescent protein sensors are shown to enable a quantitative multisite time-resolved study of oxidative-stress and acidosis buildup dynamics as the key markers, correlates and possible drivers of ischemic stroke. The fiber probes designed for this work provide a wavelength-multiplex forward-propagation channel for a spatially localized, dual-pathway excitation of genetically encoded fluorescence-protein sensors along with a back-propagation channel for the fluorescence return from optically driven fluorescence sensors. We show that the spectral analysis of the fiber-probe-collected fluorescence return provides means for a high-fidelity autofluorescence background subtraction, thus enhancing the sensitivity of real-time detection of stroke-induced transients and significantly reducing measurement uncertainties in in vivo acute-stroke studies as inherently statistical experiments operating with outcomes of multiply repeated measurements on large populations of individually variable animal stroke models.

The retrosplenial cortex (RSC) plays a key role in spatial navigation and contextual encoding. It was also shown that RSC is activated in presence of landmark-like objects and during object recognition task. However, the essence of RSC activity during place and object exploration remains unclear. Here we utilized the optic-fiber photometry calcium imaging to record the RSC activity during exploratory behavior in novel object and novel place recognition tasks in mice. We showed that object exploration led to decrease in RSC calcium activity independently of behavioral task. Moreover, another form of exploratory behavior - rearing, was now accompanied by such decrease in RSC activity. Altogether, we showed the specific change in RSC activation during object exploration in mice.

Accurate, high spatial resolution thermal measurements have provided fundamental insights into many fields of study; however, existing thermometer technology often suffers from one or more limitati...

Ischemic cerebral stroke is one of the leading causes of death and disability in humans. However, molecular processes underlying the development of this pathology remain poorly understood. There are major gaps in our understanding of metabolic changes that occur in the brain tissue during the early stages of ischemia and reperfusion. In particular, it is generally accepted that both ischemia (I) and reperfusion (R) generate reactive oxygen species (ROS) that cause oxidative stress which is one of the main drivers of the pathology, although ROS generation during I/R was never demonstrated in vivo due to the lack of suitable methods. In the present study, we record for the first time the dynamics of intracellular pH and H2O2 during I/R in cultured neurons and during experimental stroke in rats using the latest generation of genetically encoded biosensors SypHer3s and HyPer7. We detect a buildup of powerful acidosis in the brain tissue that overlaps with the ischemic core from the first seconds of pathogenesis. At the same time, no significant H2O2 generation was found in the acute phase of ischemia/reperfusion. HyPer7 oxidation in the brain was detected only 24 h later. Comparison of in vivo experiments with studies on cultured neurons under I/R demonstrates that the dynamics of metabolic processes in these models significantly differ, suggesting that a cell culture is a poor predictor of metabolic events in vivo., Highlights • The dynamics of stroke-induced changes in intracellular pH and H2O2 was recorded. • Ischemia leads to immediate acidification of the affected part of the brain. • Only slight increase in H2O2 in the brain tissue occurs during ischemia/reperfusion. • The pronounced increase in H2O2 in the brain occurs next day after I/R. • Neurons show different I/R-induced pH and H2O2 dynamics in vitro and in vivo.

High-resolution thermal and magnetic vector gradient imaging is demonstrated using diamond quantum sensors in a microstructured fiber probe platform. These sensors are capable of in situ magnetic field and temperature measurements with 160 pT / Hz and 25 mK / Hz sensitivities.

We demonstrate a reconnectable implantable ultraslim fiber-optic microendoscope that integrates a branching fiber bundle (BFB) with gradient-index fiber lenses, enabling a simultaneous fluorescence imaging of individual cells in distinctly separate brain regions, including brain structures as distant as the neocortex and hippocampus. We show that fluorescence images of individual calcium-indicator-expressing neurons in the brain of freely moving transgenic mice can be recorded, via the implanted BFB probe, in parallel with time- and cell-resolved traces of calcium signaling, thus enabling correlated circuit-dynamics studies at -multiple sites within the brain of freely moving animals.

All-optical thermometry presents a promising avenue of investigation for sensitive biological and electronic systems, with methods free of radio-frequency interference of particular importance. In this work, an all-optical scanning fiber thermometer is tested and shown to possess $5\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ spatial and $20\phantom{\rule{0.2em}{0ex}}\mathrm{mK}/\sqrt{\mathrm{Hz}}$ temperature resolution. This probe is then used to generate noncontact scanning thermal images of active circuit elements with microscale thermal features.

We present high-resolution, all-optical thermometry based on ensembles of germanium-vacancy (GeV) color center in diamond and implement this method of thermometry in the fiber-optic format. Due to the unique properties of diamond, an all-optical approach using this method opens a way to produce back-action-free temperature measurements with resolution below 0.1 K in a wide range of temperatures.

Thermogenetics is a promising innovative neurostimulation technique, which enables robust activation of neurons using thermosensitive transient receptor potential (TRP) cation channels. Broader application of this approach in neuroscience is, however, hindered by a limited variety of suitable ion channels, and by low spatial and temporal resolution of neuronal activation when TRP channels are activated by ambient temperature variations or chemical agonists. Here, we demonstrate rapid, robust and reproducible repeated activation of snake TRPA1 channels heterologously expressed in non-neuronal cells, mouse neurons and zebrafish neurons in vivo by infrared (IR) laser radiation. A fibre-optic probe that integrates a nitrogen−vacancy (NV) diamond quantum sensor with optical and microwave waveguide delivery enables thermometry with single-cell resolution, allowing neurons to be activated by exceptionally mild heating, thus preventing the damaging effects of excessive heat. The neuronal responses to the activation by IR laser radiation are fully characterized using Ca2+ imaging and electrophysiology, providing, for the first time, a complete framework for a thermogenetic manipulation of individual neurons using IR light., Current approaches to thermogenetic manipulation of neuronal activity lack sufficient spatiotemporal resolution. Here the authors show that neurons expressing snake TRPA1 channels are activated at high temporal resolution with IR light and this technique can be used to elicit behaviour in zebrafish larvae.

The latest breakthroughs in quantum technologies, such as satellite quantum communications, present new challenges, imposing stringent restrictions on weight, size, and power consumption of quantum information systems. Here, we show that nonlinear and quantum optics provides powerful resources to confront these challenges by offering attractive solutions for photon-pair counting and quantum-entanglement detection. We demonstrate a low-cost, readily miniaturizable photon-pair counting module, which consumes less than 100 μAh during a sub-10 ms power-on/off measurement cycle, thus providing a meaningful performance as a promising component for satellite quantum technologies.

A bundle of individually addressable optical fibers is shown to enable three-dimensional optical readout from single neurons in live brain, as well as in intact brain extracted from transgenic mice. With individual fibers in the bundle being only a few microns in diameter, single neurons are readily resolved in brain images transmitted by the fiber bundle. The third dimension is added by scanning the fiber in the longitudinal direction, with the fluorescence return read out from only one of the fibers in the bundle.

We have fabricated the upconversion nanoparticles (NPs), which are nanocrystallites of 10 nm, doped with Yb3+ and Er3+ ions, prepared through hydrothermal method. Monitoring of their photophysical properties had been realized at different fabrication stages by using electron microscopy and luminescence spectroscopy under photoexcitation of the NPs with the 100 mW laser at 980 nm wavelength. The performed monitoring allows us to observe dramatic changes in the upconversion properties of the NPs during the sequence of the fabrication stages and revealed very strong correlation between the NPs’ upconversion efficiency and their structural specificities.

We show that waveguide sensors can enable a quantitative characterization of coronavirus spike glycoprotein–host-receptor binding—the process whereby coronaviruses enter human cells, causing disease. We demonstrate that such sensors can help quantify and eventually understand kinetic and thermodynamic properties of viruses that control their affinity to targeted cells, which is known to significantly vary in the course of virus evolution, e.g., from SARS-CoV to SARS-CoV-2, making the development of virus-specific drugs and vaccine difficult. With the binding rate constants and thermodynamic parameters as suggested by the latest SARS-CoV-2 research, optical sensors of SARS-CoV-2 spike protein–receptor binding may be within sight.

The methods of controlling the charge state of nitrogen–vacancy centers using pulsed infrared laser field are studied. The effect of radiation intensity at a wavelength of about 1 μm on the ionization of these color centers was shown. The obtained results can be useful for multiphoton microscopy of biological objects, high-resolution microscopy and sensory applications.

A monolithic, ultra-compact fiber probe integrating a nitrogen-vacancy diamond microcrystal, a dual-core photonic-crystal fiber (PCF), and a microwave transmission line is shown to enable a highly sensitive dual-channel vectorial magnetic field measurements by means of simultaneous optical detection of magnetic resonances through the two fiber cores. A reliable detection of microscale magnetic-field variations as low as ∼10 µT/(Hz)1/2 is demonstrated using a dual-core PCF with a core-to-core separation of 6 µm. Such a fiber probe is shown to provide a unique tool for a vectorial three-dimensional mapping of weak magnetic fields induced by microscale objects.

Reconnectable bundles consisting of thousands of optical fibers are shown to enable high-quality image transmission, offering a platform for the creation of implantable fiberscopes for minimally invasive in vivo brain imaging. Experiments on various lines of transgenic mice verify the performance of this fiberscope as a powerful tool for chronic in vivo neuroimaging using genetically encoded calcium indicators, neuronal activity markers as well as axon growth regulators and brain-specific protein drivers in deep regions of live brain.

Optically detected electron spin resonance in fiber-coupled nitrogen–vacancy (NV) centers of diamond is used to demonstrate a fiber-optic quantum thermometry of individual thermogenetically activated neurons. Laser-induced temperature variations read out from single neurons with the NV-diamond fiber sensor are shown to strongly correlate with the fluorescence of calcium-ion sensors, serving as online indicators of the inward Ca2+ current across the cell membrane of neurons expressing transient receptor potential (TRP) cation channels. Local laser heating above the TRP-channel activation threshold is shown to reproducibly evoke robust action potentials, visualized by calcium-ion-sensor-aided fluorescence imaging and detected as prominent characteristic waveforms in the time-resolved response of fluorescence Ca2+ sensors.

We demonstrate a fiber-optic source of random numbers in which random-number generation (RNG) is implemented via an inherently quantum process—detection of correlated photon pairs produced by spontaneous four-wave mixing (FWM) inside an optical fiber. The detection times of individual photon pairs, generated in our RNG scheme via phase-matched FWM in a highly nonlinear photonic-crystal fiber with a suitable dispersion profile, are converted into megabit-size binary sequences, serving as strings of random numbers. The randomness of these bit strings is verified by the National Institute of Standards and Technology Statistical Test Suite. We identify FWM regimes in which mode-locked short-pulse laser sources can provide a high peak power needed to drive photon-pair generation in optical fibers without an excessive loss of randomness due to unwanted patterns in the RNG photon-pair output.

We demonstrate that a background-free readout of two-photon fluorescence from nitrogen-vacancy (NV) centers in a strongly fluorescing environment can be accomplished by all-optical means via a multiphoton charge-state modulation of NV centers in a mixture of negatively charged and neutral NV centers. A 100 fs, 1060 nm output of an ytterbium fiber laser is ideally suited for this modality of multiphoton microscopy, providing, as our experiments show, an efficient two-photon excitation of both NV− and NV0 charge states, but keeping the nonlinearity of n-photon ionization needed for NV−/NV0 charge-state modulation to a minimum, n=3.

Four-wave mixing (FWM) of ultrashort laser pulses in a highly nonlinear photonic-crystal fiber is shown to provide a multimodal source of broadband quantum states of light. In the regime of low pump powers, generation of two-photon states and heralded single photons with a bandwidth up to ≈6 THz is demonstrated within an FWM-sideband tunability range of ≈55 THz. In the modality of heralded single-photon generation, three-detector measurements of the conditional second-order correlation function reveal a strong antibunching in the signal-photon channel, with more than 99.2% of the signal counts identified as truly single-photon states. At high pump powers, two-detector signal-idler correlation function measurements indicate broadband bright squeezed vacuum generation.

We demonstrate an ultraslim all-fiber microendoscope for depth-resolved imaging that integrates a gradient-index (GRIN) fiber and an imaging fiber bundle. The desired imaging depth is achieved through a high-precision adjustment of the length of the GRIN fiber. A fusion splicing of this fiber to an imaging fiber bundle, with provisions for a carefully tailored adiabatic conical transition region, keeps these fiber components robustly aligned, enabling a high-quality image transmission with no compromise on fiber-probe flexibility. With the maximum outer diameter of this microendoscope scalable down to ≈50 μm, the range of envisaged applications spans from depth-resolved brain imaging, deep optical biopsy, microendoscopy, and optical coherence tomography to single-neuron imaging and single-cell neuromodulation studies in awake animal models.

Cognitive tests on representative groups of freely behaving transgenic mice are shown to enable a quantitative characterization of reconnectable implantable fiber-optic neurointerfaces for optogenetic neurostimulation. A systematic analysis of such tests provides a robust quantitative measure for the cognitive effects induced by fiber-optic neurostimulation, validating the performance of fiber-optic neurointerfaces for long-term optogenetic brain stimulations and showing no statistically significant artifacts in the behavior of transgenic mice due to interface implantation.

Optical coupling between a single, individually addressable neuron and a properly designed optical fiber is demonstrated. Two-photon imaging is shown to enable a quantitative in situ analysis of such fiber-single-neuron coupling in the live brain of transgenic mice. Fiber-optic interrogation of single pyramidal neurons in mouse brain cortex is performed with the positioning of the fiber probe relative to the neuron accurately mapped by means of two-photon imaging. These results pave the way for fiber-optic interfaces to single neurons for a stimulation and interrogation of individually addressable brain cells in chronic in vivo studies on freely behaving transgenic animal models, as well as the integration of fiber-optic single-neuron stimulation into the optical imaging framework.

We demonstrate high-resolution magnetic field imaging with a scanning fiber-optic probe which couples nitrogen-vacancy (NV) centers in diamond to a high-numerical-aperture photonic-crystal fiber integrated with a two-wire microwave transmission line. Magnetic resonance excitation of NV centers driven by the microwave field is read out through optical interrogation through the photonic-crystal fiber to enable high-speed, high-sensitivity magnetic field imaging with sub 30 μm spatial resolution.

Thermal activation of transient receptor potential (TRP) cation channels is one of the most striking examples of temperature-controlled processes in cell biology. As the evidence indicating the fundamental role of such processes in thermosensation builds at a fast pace, adequately accurate tools that would allow heat receptor logic behind thermosensation to be examined on a single-cell level are in great demand. Here, we demonstrate a specifically designed fiber-optic probe that enables thermal activation with simultaneous online thermometry of individual cells expressing genetically encoded TRP channels. This probe integrates a fiber-optic tract for the delivery of laser light with a two-wire microwave transmission line. A diamond microcrystal fixed on the fiber tip is heated by laser radiation transmitted through the fiber, providing a local heating of a cell culture, enabling a well-controlled TRP-assisted thermal activation of cells. Online local temperature measurements are performed by using the temperature-dependent frequency shift of optically detected magnetic resonance, induced by coupling the microwave field, delivered by the microwave transmission line, to nitrogen—vacancy centers in the diamond microcrystal. Activation of TRP channels is verified by using genetically encoded fluorescence indicators, visualizing an increase in the calcium flow through activated TRP channels.

A photonic-crystal fiber is used to demonstrate a fully fiber-integrated optical interrogation of nitrogen vacancies in diamond nanoparticles, where both pump radiation and the photoluminescent response of nitrogen vacancies are coupled to the fiber modes. A properly designed fiber is shown to provide a high-contrast guided-mode-coupled zero-phonon-line photoluminescent response from nitrogen vacancies, which is at least an order of magnitude higher than the Raman background signal from the fiber.

An accurate fiber dispersion measurement is demonstrated for different types of solid- and hollow-core photonic-crystal fibers (PCFs) using cross-correlation frequency-resolved optical gating (XFROG) with ultrashort light pulses. In the multimode regime, the XFROG technique is shown to enable a simultaneous dispersion measurement for several modes in parallel, offering a powerful fiber characterization tool for ultrafast science and metrology.

A silicon nanowaveguide ring resonator is combined with a photonic-crystal fiber (PCF) frequency shifter to demonstrate an all-optically tunable synthesis of ultrashort pulse trains, modulated by ultrafast photoinduced free-carrier generation in the silicon resonator. Pump–probe measurements performed with a 50-fs, 625-nm second-harmonic output of a Cr:forsterite laser, used as a carrier-injecting pump, and a 1.50–1.56-μm frequency-tunable 100-fs soliton output of a photonic-crystal fiber, serving as a probe, resolve tunable ultrafast oscillatory features in the silicon nanowaveguide resonator response.

Spectral broadening in the regime of normal dispersion in a large-mode-area all-silica photonic band-gap fiber (PBGF) is shown to enable compression of an optical parametric amplifier output with a peak power of 3 MW, a central wavelength of 1140 nm, and an initial pulse width of about 200 fs to a minimum pulse width of 20 fs, corresponding to a maximum peak power of 20 MW. The spectral bandwidth of the PBGF output generated in our experiments is sufficient to support compression to a two-cycle pulse width.

A monolithic dual-core photonic crystal fiber (PCF) probe employing an integrated nitrogen-vacancy diamond microcrystal and microwave transmission line is shown to enable highly sensitive dual-channel vectorial magnetic field measurements by means of optical detection of magnetic resonances with both fiber cores. Reliable detection of microscale spatial magnetic-field variations on the order of 10 μT is demonstrated with a PCF probe possessing a core-to-core separation of 6 μm. Such a fiber probe is shown to provide a powerful tool for three-dimensional vectorial mapping of weak magnetic fields emanating from spatially localized objects.

Frequency-resolved optical gating measurements with a sub-10-fs time resolution demonstrate that optical nonlinearities of fluorinated polymers (FPs) enable an ultrafast switching of ultrabroadband wavelength- and time-division multiplexed optical signals in the near-infrared (NIR) and short-wavelength infrared (SWIR) ranges. In the NIR region, the operation bandwidth of fluorinated polymer thin-film switches is mainly limited by material dispersion. In the SWIR range, material dispersion of a fluorinated polymer is much weaker. Low group delays (less than 40 attoseconds per 1 mm of polymer thickness per 1 THz of data-packet bandwidth) allow ultrafast optical switching of subpetahertz data packets.

Highly efficient collection of the fluorescent response of silicon nanoparticles loaded in micro- and nanowaveguide fiber structures is experimentally demonstrated. Micro- and nanowaveguide channels of a fiber structure are used to optically excite nanoparticles, capture their fluorescent response, and deliver the fluorescence signal to the detector. Enhancement of the evanescent field in the modes of nanowaveguide channels improves the efficiency of optical excitation of an analyte.

We demonstrate a new method of generation of frequency-tunable megawatt femtosecond pulses in the infrared based on multisoliton supercontinuum generation in a large-mode-area photonic-crystal fibre (PCF) followed by the temporal compression of the PCF output. Photonic-crystal fibres with a core area of about 710 μm2 are employed to convert microjoule femtosecond laser pulses with a spectrum centreed at 1.39 μm into pulses with a central wavelength of 1520 nm, a pulse width of 210 fs, and a peak power of about 1 MW.

We report direct spectral and temporal characterization of ultrashort ultraviolet (UV) pulses resulting from third-harmonic generation by high-intensity Ti: sapphire laser radiation in the atmosphere. The experimental technique implemented in this work enables the measurement of the pulse width and the chirp of the UV field, as well as the electron density of the plasma produced by laser pulses. The bandwidth of the third harmonic generated in our experiments by laser pulses with an initial pulse width of 45 fs supports transform-limited UV pulses as short as 30 fs.

The complex study of the polarization-controlled supercontinuum generation in a dual-core square lattice photonic crystal fiber made of multicomponent glass was accomplished. The fiber was excited by 100-fs pulses at a 1250-nm wavelength in the anomalous dispersion region and the registered spectra exhibited soliton fission and Raman-induced self-frequency shift processes. The study also involved a detailed analysis of the infrared-to-visible light conversion exhibiting a dispersive wave origin. The special dual-core properties were investigated by separate spectral analysis of each of the two cores and the near-field profile registration. The emphasis was on the visible part of the spectrum where the input energy and polarization direction dependences were studied. The increase in the input energy allowed for the tuning of the wavelength of the visible spectral features, a further rotation of the polarization direction had an effect on the spectral dependence of the light distribution between the cores. The dual-core fiber exhibited a significant coupling performance and the spectral dependence of the visible light distribution is in good agreement with the simulated coupling length spectral characteristics. Single-core excitation in the linear regime revealed the possibility of coupling 50% of the energy to the other core and the same polarization-controlled redirection possibilities as that at the nonlinear experiments. Dual-core excitation of the fiber enhances the light redirection effect with the application potential for the polarization-controlled directional coupler accompanied by nonlinear frequency conversion.

Photonic-crystal fibers are shown to allow efficient spectral transformation of nanosecond laser pulses through parametric four-wave mixing and stimulated Raman scattering. Regimes providing highly efficient transformation of nanosecond laser pulses into white-light broadband radiation (supercontinuum) are identified. A strong parametric coupling between Stokes and anti-Stokes Raman sidebands around the wavelength of zero group-velocity dispersion is shown to increase the bandwidth and to improve the spectral quality of supercontinuum radiation.

We report the delivery of high-energy nanosecond pulses (10 ns pulse width) from a Q-switched Nd:YAG laser through the fundamental mode of a hollow-core photonic crystal fiber at 1064 nm. At a pressure of 450 mbar, energies greater than 1 mJ were delivered in a single spatial mode through the hollow-core fiber, coupling efficiencies over 80% were demonstrated.

Differential optical detection of a magnetic resonance induced in nitrogen-vacancy (NV) centers in diamond is shown to enable a high-spatial-resolution room-temperature magnetic-field gradiometry on a fiber platform. An ultracompact design of this fiber-based solid-state magnetic gradiometer is achieved by integrating an NV-diamond magnetic sensor with a two-fiber opto-microwave interface, which couples NV centers to microwave and optical fields, used to resonantly drive and interrogate the spin of NV centers.

Stimulated emission depletion (STED) is the key optical technology enabling super-resolution microscopy below the diffraction limit. Here, we demonstrate that modulation of STED in the time domain, combined with properly designed lock-in detection, can radically enhance the contrast of fluorescent images of strongly autofluorescent biotissues. In our experiments, the temporally modulated STED technique, implemented with low-intensity continuous-wave laser sources, is shown to provide an efficient all-optical suppression of a broadband fluorescent background, allowing the contrast of fluorescent images of mammal brain tissues tagged with nitrogen-vacancy diamond to be increased by five orders of magnitude.

We demonstrate a scanning fiber-optic probe for magnetic-field imaging where nitrogen-vacancy (NV) centers are coupled to an optical fiber integrated with a two-wire microwave transmission line. The electron spin of NV centers in a diamond microcrystal attached to the tip of the fiber probe is manipulated by a frequency-modulated microwave field and is initialized by laser radiation transmitted through the optical tract of the fiber probe. The two-dimensional profile of the magnetic field is imaged with a high speed and high sensitivity using the photoluminescence spin-readout return from NV centers, captured and delivered by the same optical fiber.