Color Centers Research Articles

Radiophotoluminescence response of LiF:Mg,Ti pellets irradiated with clinical proton beams in the 70–200 MeV energy range

Lithium fluoride doped with Mg and Ti (LiF:Mg,Ti) has been used for several decades as thermoluminescent dosimeter material, but recently also its radiophotoluminescence has been investigated for applications in dosimetry. In this work, LiF:Mg,Ti pellets (TLD-100) were irradiated in a water phantom at CNAO (Pavia, Italy) with therapeutic proton beams at five energies from 70 to 200 MeV in the dose range from 2 to 20 Gy. After irradiation, their visible radiophotoluminescence spectra were measured in controlled conditions and the spectrally-integrated red emission response of radiation-induced color centers has been investigated. The radiophotoluminescence signal, excited by a 445 nm continuous wave laser, was integrated within a 50 nm-wide band around the emission peak of the F2 color centers, located around 670 nm in LiF. The spectrally-integrated signal of the samples irradiated at the energy of 147.7 MeV exhibited a linear dependence with dose. Moreover, this radiophotoluminescence response appears independent from Linear Energy Transfer in the range from 0.8 to 1.6 keV/μm in all the samples irradiated at the dose of 5 Gy. Such independence was found up to 10.3 keV/μm in samples irradiated within two spread out Bragg peaks made of 36 energy components in the entire investigated proton energy range. The radiophotoluminescence response of the TLD-100 pellets was compared to that of nominally-pure LiF crystals irradiated in the same conditions, which show a similar behavior. The results are encouraging for the exploitation of TLD-100 pellets as passive solid-state radiophotoluminescent dosimeters for proton therapy.

Vacancy-related color centers in two-dimensional silicon carbide monolayers

Vacancy-related color centers in two-dimensional silicon carbide monolayers

Single-Shot Readout of a Nuclear Spin in Silicon Carbide.

Solid-state qubits with a photonic interface is very promising for quantum networks. Color centers in silicon carbide have shown excellent optical and spin coherence, even when integrated with membranes and nanostructures. Additionally, nuclear spins coupled with electron spins can serve as long-lived quantum memories. Pioneering work previously has realized the initialization of a single nuclear spin and demonstrated its entanglement with an electron spin. In this Letter, we report the first realization of single-shot readout for a nuclear spin in SiC. We obtain a deterministic nuclear spin initialization and readout fidelity of 94.95% with a measurement duration of 1ms. With a dual-step readout scheme, we obtain a readout fidelity as high as 99.03% within 0.28ms by sacrificing the success efficiency. Our Letter complements the experimental toolbox of harnessing both electron and nuclear spins in SiC for future quantum networks.

Small multimodal thermometry with detonation-created multi-color centers in detonation nanodiamond

Detonation nanodiamond (DND) is the smallest class of diamond nanocrystal capable of hosting various color centers with a size akin to molecular pores. Their negatively charged nitrogen-vacancy center (NV−) is a versatile tool for sensing a wide range of physical and even chemical parameters at the nanoscale. The NV− is, therefore, attracting interest as the smallest quantum sensor in biological research. Nonetheless, recent NV− enhancement in DND has yet to yield sufficient fluorescence per particle, leading to efforts to incorporate other group-IV color centers into DND. An example is adding a silicon dopant to the explosive mixture to create negatively charged silicon-vacancy centers (SiV−). In this paper, we report on efficient observation (∼50% of randomly selected spots) of the characteristic optically detected magnetic resonance (ODMR) NV− signal in silicon-doped DND (Si-DND) subjected to boiling acid surface cleaning. The NV− concentration is estimated by continuous-wave electron spin resonance spectroscopy to be 0.35 ppm without the NV− enrichment process. A temperature sensitivity of 0.36K/Hz in an NV− ensemble inside an aggregate of Si-DND is achieved via the ODMR-based technique. Transmission electron microscopy survey reveals that the Si-DNDs core sizes are ∼11.2 nm, the smallest among the nanodiamond’s temperature sensitivity studies. Furthermore, temperature sensing using both SiV− (all-optical technique) and NV− (ODMR-based technique) in the same confocal volume is demonstrated, showing Si-DND’s multimodal temperature sensing capability. The results of the study thereby pave a path for multi-color and multimodal biosensors and for decoupling the detected electrical field and temperature effects on the NV− center.

Exploration of a high-precision measurement scheme for current sensors based on diamond NV color centers

A high-precision measurement scheme for current sensors based on diamond NV color centers was verified. In the research process, diamond crystals containing a large number of NV color centers are applied as the probe of the sensor, and the magnetic field generated by the current is measured to explore the relationship between the current intensity and the fluorescence intensity of the NV color centers and the polarization resonance, to carry out simulation experiments, and finally to obtain the results of the effect of the concentration of the NV color centers on the performance of the sensor. The results of simulation experiments show that the accuracy of the current measurement is within 100-500 A, and the result is 0.3%. By applying NV color centers to current measurement, good results have been obtained in terms of accuracy and frequency response range, enabling high-precision current measurement as well as providing monitoring and analysis of high-frequency current inrush.

Two-hole “desorption” mechanism of interstitial-vacancy pair generation visualized by avalanche-like color-center yield in synthetic diamond under ultrashort-pulse laser exposure

A model sectorial synthetic Ib-diamond plate with a minor concentration of substitutional atomic nitrogen C-centers was exposed in its bulk by tightly focused 0.2-ps, 525-nm laser pulses, coming at 80-MHz repetition rate and nJ-level variable pulse energies. Structural conversion into NV- and other carbon vacancy/interstitial-related centers was observed by infrared, optical and 3D confocal photoluminescence (PL) microspectroscopy, exhibiting in the optical range the ultra-broad (UV-near-IR) and smooth (structureless) absorption/photoluminescence spectra. PL intensity measured at the selected 480 and 530-nm wavelengths demonstrates its third power of the corresponding pulse energy dependences (∝E3) and avalanche-like dynamics as a function of laser exposure. Our analysis of electron-hole plasma photoexcitation and related generation of non-thermalized Auger carriers as a function of laser pulse energy (∝E1/2 and ∝E3/2, respectively) indicates that the known two-hole Auger-induced surface desorption mechanism could be anticipated in bulk diamond for interstitial-vacancy Frenkel pair generation, which rate could be evaluated via structural conversion rate for C-centers. At higher pulse energies tremendous interstitial-vacancy Frenkel pair generation rates could result not only in structural conversion of impurity-based color centers, but also in ultrafast “non-thermal” structural disordering in diamond.

Polariton creation in coupled cavity arrays with spectrally disordered emitters

Integrated photonics has been a promising platform for analog quantum simulation of condensed matter phenomena in strongly correlated systems. To that end, we explore the implementation of all-photonic quantum simulators in coupled cavity arrays with integrated ensembles of spectrally disordered emitters. Our model is reflective of color center ensembles integrated into photonic crystal cavity arrays. Using the Quantum Master equation and the Effective Hamiltonian approaches, we study energy band formation and wavefunction properties in the open quantum Tavis–Cummings–Hubbard framework. We find conditions for polariton creation and (de)localization under experimentally relevant values of disorder in emitter frequencies, cavity resonance frequencies, and emitter-cavity coupling rates. To quantify these properties, we introduce two metrics, the polaritonic and nodal participation ratios, that characterize the light-matter hybridization and the node delocalization of the wavefunction, respectively. These new metrics combined with the Effective Hamiltonian approach prove to be a powerful toolbox for cavity quantum electrodynamical engineering of solid-state systems.

Engineering Quantum Light Sources with Flat Optics.

Quantum light sources are essential building blocks for many quantum technologies, enabling secure communication, powerful computing, and precise sensing and imaging. Recent advancements have witnessed a significant shift toward the utilization of "flat" optics with thickness at subwavelength scales for the development of quantum light sources. This approach offers notable advantages over conventional bulky counterparts, including compactness, scalability, and improved efficiency, along with added functionalities. This review focuses on the recent advances in leveraging flat optics to generate quantum light sources. Specifically, the generation of entangled photon pairs through spontaneous parametric down-conversion in nonlinear metasurfaces, and single photon emission from quantum emitters including quantum dots and color centers in 3D and 2D materials are explored. The review covers theoretical principles, fabrication techniques, and properties of these sources, with particular emphasis on the enhanced generation and engineering of quantum light sources using optical resonances supported by nanostructures. The diverse application range of these sources is discussed and the current challenges and perspectives in the field arehighlighted.

A scalable cavity-based spin–photon interface in a photonic integrated circuit

A central challenge in quantum networking is transferring quantum states between different physical modalities, such as between flying photonic qubits and stationary quantum memories. One implementation entails using spin–photon interfaces that combine solid-state spin qubits, such as color centers in diamond, with photonic nanostructures. However, while high-fidelity spin–photon interactions have been demonstrated on isolated devices, building practical quantum repeaters requires scaling to large numbers of interfaces yet to be realized. Here, we demonstrate integration of nanophotonic cavities containing tin-vacancy (SnV) centers in a photonic integrated circuit (PIC). Out of a six-channel quantum microchiplet (QMC), we find four coupled SnV-cavity devices with an average Purcell factor of ∼7. Based on system analyses and numerical simulations, we find with near-term improvements this multiplexed architecture can enable high-fidelity quantum state transfer, paving the way toward building large-scale quantum repeaters.

Transitions of nitrogen optical centers induced by femtosecond laser pulses in treated natural diamond

Diamond is a unique optical material with ultra-hard lattice, which properties can be altered by optically active doping defects combined with carbon interstitials and vacancies. Direct laser writing by focused ultrashort laser pulses enables local modification of color centers in diamonds with excellent three-dimensional positioning accuracy, providing unique capabilities for quantum technologies, magnetometry, laser generation and photoluminescence (PL) micromarking. To shed light on currently unknown underlying atomistic transformations, we exposed electron-irradiated and annealed natural IaAB1-type diamond with an initially high content of various nitrogen-vacancy centers (NV, H3, N3 and H4) to millions of 0.2-ps, 525-nm laser pulses focused inside the sample. The modified areas were characterized by Fourier-transform infrared (FT-IR) and ultraviolet–visible transmission spectroscopy and confocal Raman/PL microspectroscopy with excitation wavelengths of 405 nm and 532 nm. We report on a local decrease in originally strong PL yield of NV, H3 and H4 centers with a monotonic dependence on both pulse energy and exposure time, as well as a rise of H1b and H1c lines in the FT-IR spectra accompanied by a drop in the concentration of H1a center. The results indicate that laser-induced processes make possible both creation and destruction of NV, H3 and H4 centers influenced by their initial concentration and the parameters of laser modification, which can be useful for adjusting of laser writing regimes for PL micromark inscription and other local modification of diamond for photonics applications.

Development of a large volume line scanning, high spectral range and resolution 3D hyperspectral photoluminescence imaging microscope for diamond and other high refractive index materials

Hyperspectral photoluminescence (PL) imaging is a powerful technique that can be used to understand the spatial distribution of emitting species in many materials. Volumetric hyperspectral imaging of weakly emitting color centers often necessitates considerable data collection times when using commercial systems. We report the development of a line-scanning hyperspectral imaging microscope capable of measuring the luminescence emission spectra for diamond volumes up to 2.20 × 30.00 × 6.30 mm with a high lateral spatial resolution of 1–3 µm. In an single X-λ measurement, spectra covering a 711 nm range, in a band from 400–1100 nm, with a spectral resolution up to 0.25 nm can be acquired. Data sets can be acquired with 723 (X) × 643 (Y) × 1172 (λ) pixels at a rate of 6 minutes/planar image slice, allowing for volumetric hyperspectral imaging with high sampling. This instrument demonstrates the ability to detect emission from several different color centers in diamond both at the surface and internally, providing a non-destructive method to probe their 3D spatial distribution, and is currently not achievable with any other commonly used system or technique.

Optically stimulated luminescence of Durolon polycarbonate

This paper reports for the first time the study of optically stimulated luminescence (OSL) of amorphous Brazilian bisphenol-A polycarbonate (Durolon) exposed to beta radiation. The OSL was studied from two perspectives: characterizing the luminescent signal as a function of radiation dose and examining repeatability. Computational analyses are applied to OSL curves via the deconvolution method. Fluorescence measurements employing the emission - excitation matrix (EEM) technique and optical absorption (OA) are incorporated to characterize the polymer defects and color centers. OSL measurements were carried out in a commercial reader Risø (model DA-20) coupled with a90Sr/90Y beta source. The sample was illuminated using 470 nm blue LEDs. The OSL response exhibited an exponential decay signal after 1 Gy, with linear dose-response and consistent repeatability. The continuous-wave-OSL curve was well adjusted with two distinct components. As result, Durolon promising potential as a luminescent dosimeter, supported by its desirable OSL signal characteristics.

Femtosecond-laser-induced creation of G and W color centers in silicon-on-insulator substrates

Femtosecond-laser-induced creation of G and W color centers in silicon-on-insulator substrates

The characterization of diamond single crystal growing along (100) after annealing treatment under high pressure and high temperature

Diamond has many desirable properties. In this study, temperature gradient method was used to prepare a series of type Ib diamond single crystals, which were processed again to obtain and characterize Type IaA diamonds with even more desirable properties. Before annealing, Fourier-transformed infra-red(FTIR) spectra of diamond showed peaks at 1130 cm−1 and 1344 cm−1; After annealing, the peak at 1282 cm−1 was appeared in FTIR spectra of diamonds This data indicated that the isolated nitrogen in diamond aggregated after high-pressure and high-temperature (HPHT) method annealing experiment. Moreover, photoluminescence (PL) spectra suggested that no nitrogen-related color centers existed in the diamond before annealing. In contrast, after annealing, a peak at 637 nm appeared in the PL spectra, indicating the appearance of NV−colors centers type Ib diamond post-annealing. The experimental results show that type Ib diamond crystals can be decolorized after 60 min of annealing under 7 GPa and 1850 °C, which makes part of the area transparent. Under 7 GPa and 2100 °C, type Ib diamond crystals can produce NV color center when annealed for 3 min. With increasing annealing temperature and prolonged annealing duration, the density and size of etch pits on the crystal surface escalated. Therefore, this study provides a guideline for the decolorization of diamonds by HPHT annealing. Moreover, this study also reports a method to change the crystal type and obtain NV color centers under ultra-high-temperature annealing conditions of 7 GPa pressure and 2100 °C temperature, which can be applied to develop quantum materials.

Spectral stability of V2 centres in sub-micron 4H-SiC membranes

Colour centres in silicon carbide emerge as a promising semiconductor quantum technology platform with excellent spin-optical coherences. However, recent efforts towards maximising the photonic efficiency via integration into nanophotonic structures proved to be challenging due to reduced spectral stabilities. Here, we provide a large-scale systematic investigation on silicon vacancy centres in thin silicon carbide membranes with thicknesses down to 0.25 μm. Our membrane fabrication process involves a combination of chemical mechanical polishing, reactive ion etching, and subsequent annealing. This leads to highly reproducible membranes with roughness values of 3–4 Å, as well as negligible surface fluorescence. We find that silicon vacancy centres show close-to lifetime limited optical linewidths with almost no signs of spectral wandering down to membrane thicknesses of ~0.7 μm. For silicon vacancy centres in thinner membranes down to 0.25 μm, we observe spectral wandering, however, optical linewidths remain below 200 MHz, which is compatible with spin-selective excitation schemes. Our work clearly shows that silicon vacancy centres can be integrated into sub-micron silicon carbide membranes, which opens the avenue towards obtaining the necessary improvements in photon extraction efficiency based on nanophotonic structuring.

Growth and characterization of 6LiI:Ag crystal scintillators for thermal and epithermal neutron detection on the lunar surface

This paper discusses crystal growth and characterization of 1.0-inch 6LiI:Ag scintillators for thermal and epithermal neutron detection on the lunar surface as part of the Lunar Vehicle Radiation Dosimeter project. Zone purification method was employed to purify the raw material of 6LiI, and the crystals were grown using the Bridgman method. X-ray luminescence spectrum was measured to confirm the presence of Ag+ in the crystal lattice of 6LiI. The feasibility of neutron detection under 252Cf fast neutron irradiation was investigated. A hydrogen-rich paraffin wax moderator was used to slow down fast neutrons to obtain thermal and epithermal neutrons. The charge comparison method was used to perform pulse shape discrimination. The maximum value of the Figure of Merit was obtained to be 1.3, which demonstrates the capability of using 6LiI:Ag crystal scintillators in high neutron and gamma mixed fields. The energy resolution of the thermal neutron peak varied with the thickness of scintillators, achieving 12.0 % for the 0.5 cm thick samples. The potential for further improvement of the scintillation performance of 6LiI:Ag crystal relies on reducing impurities and color centers.

Cryogenic Thermal Shock Effects on Optical Properties of Quantum Emitters in Hexagonal Boron Nitride.

Solid-state quantum emitters are vital building blocks for quantum information science and quantum technology. Among various types of solid-state emitters discovered to date, color centers in hexagonal boron nitride have garnered tremendous traction in recent years, thanks to their environmental robustness, high brightness, and room-temperature operation. Most recently, these quantum emitters have been employed for satellite-based quantum key distribution. One of the most important requirements to qualify these emitters for space-based applications is their optical stability against cryogenic thermal shock. Such an understanding has, however, remained elusive to date. Here, we report on the effects caused by such thermal shock that induces random, irreversible changes in the spectral characteristics of the quantum emitters. By employing a combination of structural characterizations and density functional calculations, we attribute the observed changes to lattice strain caused by cryogenic temperature shock. Our study sheds light on the stability of the quantum emitters under extreme conditions─similar to those countered in outer space.

NV color center fluorescence signal measurement based on FPGA architecture

Nitrogen-vacancy (NV) quantum defects in diamond are sensitive detectors of magnetic fields. Then, the existing sensing technologies based on diamond NV centers rely on bulky and expensive instruments, which must be replaced by a more compact design. Here we combine NV quantum sensing technology with integrated circuit technology to create a compact and easy-to-operate platform. Using the existing integrated circuit technology, we integrate the lock-in amplifier, microwave generator and TTL modulator together. with this platform, we can obtain a magnetic sensitivity of 5.72μT/Hz 1/2.

Unraveling sources of emission heterogeneity in Silicon Vacancy color centers with cryo-cathodoluminescence microscopy

Diamond color centers have proven to be versatile quantum emitters and exquisite sensors of stress, temperature, electric and magnetic fields, and biochemical processes. Among color centers, the silicon-vacancy (SiV[Formula: see text]) defect exhibits high brightness, minimal phonon coupling, narrow optical linewidths, and high degrees of photon indistinguishability. Yet the creation of reliable and scalable SiV[Formula: see text]-based color centers has been hampered by heterogeneous emission, theorized to originate from surface imperfections, crystal lattice strain, defect symmetry, or other lattice impurities. Here, we advance high-resolution cryo-electron microscopy combined with cathodoluminescence spectroscopy and 4D scanning transmission electron microscopy (STEM) to elucidate the structural sources of heterogeneity in SiV[Formula: see text] emission from nanodiamond with sub-nanometer-scale resolution. Our diamond nanoparticles are grown directly on TEM membranes from molecular-level seedings, representing the natural formation conditions of color centers in diamond. We show that individual subcrystallites within a single nanodiamond exhibit distinct zero-phonon line (ZPL) energies and differences in brightness that can vary by 0.1 meV in energy and over 70% in brightness. These changes are correlated with the atomic-scale lattice structure. We find that ZPL blue-shifts result from tensile strain, while ZPL red shifts are due to compressive strain. We also find that distinct crystallites host distinct densities of SiV[Formula: see text] emitters and that grain boundaries impact SiV[Formula: see text] emission significantly. Finally, we interrogate nanodiamonds as small as 40 nm in diameter and show that these diamonds exhibit no spatial change to their ZPL energy. Our work provides a foundation for atomic-scale structure-emission correlation, e.g., of single atomic defects in a range of quantum and two-dimensional materials.

Generation of genuine all-way entanglement in defect-nuclear spin systems through dynamical decoupling sequences

Multipartite entangled states are an essential resource for sensing, quantum error correction, and cryptography. Color centers in solids are one of the leading platforms for quantum networking due to the availability of a nuclear spin memory that can be entangled with the optically active electronic spin through dynamical decoupling sequences. Creating electron-nuclear entangled states in these systems is a difficult task as the always-on hyperfine interactions prohibit complete isolation of the target dynamics from the unwanted spin bath. While this emergent cross-talk can be alleviated by prolonging the entanglement generation, the gate durations quickly exceed coherence times. Here we show how to prepare high-quality GHZM-like states with minimal cross-talk. We introduce the M-tangling power of an evolution operator, which allows us to verify genuine all-way correlations. Using experimentally measured hyperfine parameters of an NV center spin in diamond coupled to carbon-13 lattice spins, we show how to use sequential or single-shot entangling operations to prepare GHZM-like states of up to M=10 qubits within time constraints that saturate bounds on M-way correlations. We study the entanglement of mixed electron-nuclear states and develop a non-unitary M-tangling power which additionally captures correlations arising from all unwanted nuclear spins. We further derive a non-unitary M-tangling power which incorporates the impact of electronic dephasing errors on the M-way correlations. Finally, we inspect the performance of our protocols in the presence of experimentally reported pulse errors, finding that XY decoupling sequences can lead to high-fidelity GHZ state preparation.