Room-Temperature Fiber-Coupled Single-Photon Sources based on Colloidal Quantum Dots and SiV Centers in Back-Excited Nanoantennas.

We demonstrate an important step toward on-chip integration of single-photon sources at room temperature. Excellent photon directionality is achieved with a hybrid metal-dielectric bullseye antenna, while back-excitation is permitted by placement of the emitter in a subwavelength hole positioned at its center. The unique design enables a direct back-excitation and very efficient front coupling of emission either to a low numerical aperture (NA) optics or directly to an optical fiber. To show the versatility of the concept, we fabricate devices containing either a colloidal quantum dot or a nanodiamond containing silicon-vacancy centers, which are accurately positioned using two different nanopositioning methods. Both of these back-excited devices display front collection efficiencies of ∼70% at NAs as low as 0.5. The combination of back-excitation with forward directionality enables direct coupling of the emitted photons into a proximal optical fiber without any coupling optics, thereby facilitating and simplifying future integration.

Silicon-Vacancy Nanodiamonds as High Performance Near-Infrared Emitters for Live-Cell Dual-Color Imaging and Thermometry.

Nanodiamonds (NDs) with color centers are excellent emitters for various bioimaging and quantum biosensing applications. In our work, we explore new applications of NDs with silicon-vacancy centers (SiV) obtained by high-pressure high-temperature (HPHT) synthesis based on metal-catalyst-free growth. They are coated with a polypeptide biopolymer, which is essential for efficient cellular uptake. The unique optical properties of NDs with SiV are their high photostability and narrow emission in the near-infrared region. Our results demonstrate for the first time that NDs with SiV allow live-cell dual-color imaging and intracellular tracking. Also, intracellular thermometry and challenges associated with SiV atomic defects in NDs are investigated and discussed for the first time. NDs with SiV nanoemitters provide new avenues for live-cell bioimaging, diagnostic (SiV as a nanosized thermometer), and theranostic (nanodiamonds as drug carrier) applications.

Fluorescence enhancement of a single germanium vacancy center in a nanodiamond by a plasmonic Bragg cavity.

Germanium vacancy (GeV) centers in diamonds constitute a promising platform for single-photon sources to be used in quantum information technologies. Emission from these color centers can be enhanced by utilizing a cavity that is resonant at the peak emission wavelength. We investigate circular plasmonic Bragg cavities for enhancing the emission from single GeV centers in nanodiamonds (NDs) at the zero phonon line. Following simulations of the enhancement for different configuration parameters, the appropriately designed Bragg cavities together with out-coupling gratings composed of hydrogen silsesquioxane ridges are fabricated around the NDs containing nitrogen vacancy centers deposited on a silica-coated silver surface. We characterize the fabricated configurations and finely tune the cavity parameters to match the GeV emission. Finally, we fabricate the cavity containing a single GeV-ND and compare the total decay-rate before and after cavity fabrication, finding a decay-rate enhancement of ∼5.5 and thereby experimentally confirming the feasibility of emission enhancement with circular plasmonic cavities.

Formation of interstitial silicon defects in Si- and Si,P-doped nanodiamonds and thermal susceptibilities of SiV- photoluminescence band.

We have produced two types of synthetic nanodiamonds Si- and Si,P-doped and have characterized the thermal susceptibilities of the spectral band of silicon-vacancy (SiV-) centers at approximately 740 nm in each case. The covered temperature range from 295 to 350 K is of interest for thermometry in biological systems. Comparison of the relative brightness of the Si- and Si,P-doped crystals shows that phosphorous significantly increases average concentration and homogeneity of distribution of SiV- centers in nanodiamonds. Moreover, linear dependence on temperature of the zero-phonon line width in Si-doped crystals is 0.061(2) nm K-1 but is 0.047(3) nm K-1, about 35% smaller in Si,P-doped nanodiamonds. This proves control of SiV- properties with additional chemical doping and close proximity of Si and P atoms.

Long-term live cells observation of internalized fluorescent Fe@C nanoparticles in constant magnetic field.

BACKGROUND: Theranostics application of superparamagnetic nanoparticles based on magnetite and maghemite is impeded by their toxicity. The use of additional protective shells significantly reduced the magnetic properties of the nanoparticles. Therefore, iron carbides and pure iron nanoparticles coated with multiple layers of onion-like carbon sheath seem to be optimal for biomedicine. Fluorescent markers associated with magnetic nanoparticles provide reliable means for their multimodal visualization. Here, biocompatibility of iron nanoparticles coated with graphite-like shell and labeled with Alexa 647 fluorescent marker has been investigated. METHODS: Iron core nanoparticles with intact carbon shells were purified by magnetoseparation after hydrochloric acid treatment. The structure of the NPs (nanoparticles) was examined with a high resolution electron microscopy. The surface of the NPs was alkylcarboxylated and further aminated for covalent linking with Alexa Fluor 647 fluorochrome to produce modified fluorescent magnetic nanoparticles (MFMNPs). Live fluorescent imaging and correlative light-electron microscopy were used to study the NPs intracellular distribution and the effects of constant magnetic field on internalized NPs in the cell culture were analyzed. Cell viability was assayed by measuring a proliferative pool with Click-IT labeling. RESULTS: The microstructure and magnetic properties of superparamagnetic Fe@C core-shell NPs as well as their endocytosis by living tumor cells, and behavior inside the cells in constant magnetic field (150 mT) were studied. Correlative light-electron microscopy demonstrated that NPs retained their microstructure after internalization by the living cells. Application of constant magnetic field caused orientation of internalized NPs along power lines thus demonstrating their magnetocontrollability. Carbon onion-like shells make these NPs biocompatible and enable long-term observation with confocal microscope. It was found that iron core of NPs shows no toxic effect on the cell physiology, does not inhibit the cell proliferation and also does not induce apoptosis. CONCLUSIONS: Non-toxic, biologically compatible superparamagnetic fluorescent MFMNPs can be further used for biological application such as delivery of biologically active compounds both inside the cell and inside the whole organism, magnetic separation, and magnetic resonance imaging (MRI) diagnostics.

Magnetocontrollability of Fe7C3@C superparamagnetic nanoparticles in living cells.

BACKGROUND: A new type of superparamagnetic nanoparticles with chemical formula Fe7C3@C (MNPs) showed higher value of magnetization compared to traditionally used iron oxide-based nanoparticles as was shown in our previous studies. The in vitro biocompatibility tests demonstrated that the MNPs display high efficiency of cellular uptake and do not affect cyto-physiological parameters of cultured cells. These MNPs display effective magnetocontrollability in homogeneous liquids but their behavior in cytoplasm of living cells under the effect of magnetic field was not carefully analyzed yet. RESULTS: In this work we investigated the magnetocontrollability of MNPs interacting with living cells in permanent magnetic field. It has been shown that cells were capable of capturing MNPs by upper part of the cell membrane, and from the surface of the cultivation substrate during motion process. Immunofluorescence studies using intracellular endosomal membrane marker showed that MNP agglomerates can be either located in endosomes or lying free in the cytoplasm. When attached cells were exposed to a magnetic field up to 0.15 T, the MNPs acquired magnetic moment and the displacement of incorporated MNP agglomerates in the direction of the magnet was observed. Weakly attached or non-attached cells, such as cells in mitosis or after cytoskeleton damaging treatments moved towards the magnet. During long time cultivation of cells with MNPs in a magnetic field gradual clearing of cells from MNPs was observed. It was the result of removing MNPs from the surface of the cell agglomerates discarded in the process of exocytosis. CONCLUSIONS: Our data allow us to conclude for the first time that the magnetic properties of the MNPs are sufficient for successful manipulation with MNP agglomerates both at the intracellular level, and within the whole cell. The structure of the outer shells of the MNPs allows firmly associate different types of biological molecules with them. This creates prospects for the use of such complexes for targeted delivery and selective removal of selected biological molecules from living cells.

Ultracompact Single-Photon Sources of Linearly Polarized Vortex Beams.

Ultracompact chip-integrated single-photon sources of collimated beams with polarization-encoded states are crucial for integrated quantum technologies. However, most of currently available single-photon sources rely on external bulky optical components to shape the polarization and phase front of emitted photon beams. Efficient integration of quantum emitters with beam shaping and polarization encoding functionalities remains so far elusive. Here, ultracompact single-photon sources of linearly polarized vortex beams based on chip-integrated quantum emitter-coupled metasurfaces are presented, which are meticulously designed by fully exploiting the potential of nanobrick-arrayed metasurfaces. The authors first demonstrate on-chip single-photon generation of high-purity linearly polarized vortex beams with prescribed topological charges of 0, - 1, and +1. The multiplexing of single-photon emission channels with orthogonal linear polarizations carrying different topological charges are further realized and their entanglement is demonstarated. The work illustrates the potential and feasibility of ultracompact quantum emitter-coupled metasurfaces as a new quantum optics platform for realizing chip-integrated high-dimensional single-photon sources.

Giant Quantum Electrodynamic Effects on Single SiV Color Centers in Nanosized Diamonds.

Understanding and mastering quantum electrodynamics phenomena is essential to the development of quantum nanophotonics applications. While tailoring of the local vacuum field has been widely used to tune the luminescence rate and directionality of a quantum emitter, its impact on their transition energies is barely investigated and exploited. Fluorescent defects in nanosized diamonds constitute an attractive nanophotonic platform to investigate the Lamb shift of an emitter embedded in a dielectric nanostructure with high refractive index. Using spectral and time-resolved optical spectroscopy of single SiV defects, we unveil blue shifts (up to 80 meV) of their emission lines, which are interpreted from model calculations as giant Lamb shifts. Moreover, evidence for a positive correlation between their fluorescence decay rates and emission line widths is observed, as a signature of modifications not only of the photonic local density of states but also of the phononic one, as the nanodiamond size is decreased. Correlative light-electron microscopy of single SiVs and their host nanodiamonds further supports these findings. These results make nanodiamond-SiVs promising as optically driven spin qubits and quantum light sources tunable through nanoscale tailoring of vacuum-field fluctuations.

Multiple channelling single-photon emission with scattering holography designed metasurfaces.

Channelling single-photon emission in multiple well-defined directions and simultaneously controlling its polarization characteristics is highly desirable for numerous quantum technology applications. We show that this can be achieved by using quantum emitters (QEs) nonradiatively coupled to surface plasmon polaritons (SPPs), which are scattered into outgoing free-propagating waves by appropriately designed metasurfaces. The QE-coupled metasurface design is based on the scattering holography approach with radially diverging SPPs as reference waves. Using holographic metasurfaces fabricated around nanodiamonds with single Ge vacancy centres, we experimentally demonstrate on-chip integrated efficient generation of two well-collimated single-photon beams propagating along different 15° off-normal directions with orthogonal linear polarizations.

Femtosecond Er-Doped All-Fiber Laser with High-Density Well-Aligned Carbon-Nanotube-Based Thin-Film Saturable Absorber.

We have studied the ultrafast saturation behavior of a high-density well-aligned single-walled carbon nanotubes saturable absorber (HDWA-SWCNT SA), obtained by a high-pressure and high-temperature treatment of commercially available single-wall carbon nanotubes (SWCNTs) and related it to femtosecond erbium-doped fiber laser performance. We have observed the polarization dependence of a nonlinear optical saturation, along with a low saturation energy level of <1 fJ, limited to the detector threshold used, and the ultrafast response time of <250 fs, while the modulation depth was approximately 12%. We have obtained the generation of ultrashort stretched pulses with a low mode-locking launching threshold of ~100 mW and an average output power of 12.5 mW in an erbium-doped ring laser with the hybrid mode-locking of a VDVA-SWNT SA in combination with the effects of nonlinear polarization evolution. Dechirped pulses with a duration of 180 fs were generated, with a repetition rate of about 42.22 MHz. The average output power standard deviation was about 0.06% RMS during 3 h of measurement.

On-chip excitation of single germanium vacancies in nanodiamonds embedded in plasmonic waveguides.

Monolithic integration of quantum emitters in nanoscale plasmonic circuitry requires low-loss plasmonic configurations capable of confining light well below the diffraction limit. We demonstrated on-chip remote excitation of nanodiamond-embedded single quantum emitters by plasmonic modes of dielectric ridges atop colloidal silver crystals. The nanodiamonds were produced to incorporate single germanium-vacancy (GeV) centres, providing bright, spectrally narrow and stable single-photon sources suitable for highly integrated circuits. Using electron-beam lithography with hydrogen silsesquioxane (HSQ) resist, dielectric-loaded surface plasmon polariton waveguides (DLSPPWs) were fabricated on single crystalline silver plates to contain those of deposited nanodiamonds that are found to feature appropriate single GeV centres. The low-loss plasmonic configuration enabled the 532-nm pump laser light to propagate on-chip in the DLSPPW and reach to an embedded nanodiamond where a single GeV centre was incorporated. The remote GeV emitter was thereby excited and coupled to spatially confined DLSPPW modes with an outstanding figure-of-merit of 180 due to a ~six-fold Purcell enhancement, ~56% coupling efficiency and ~33 µm transmission length, thereby opening new avenues for the implementation of nanoscale functional quantum devices.

Varying temperature and silicon content in nanodiamond growth: effects on silicon-vacancy centres.

Nanodidamonds containing colour centres open up many applications in quantum information processing, metrology, and quantum sensing. However, controlling the synthesis of nanodiamonds containing silicon vacancy (SiV) centres is still not well understood. Here we study nanodiamonds produced by a high-pressure high-temperature method without catalyst metals, focusing on two samples with clear SiV signatures. Different growth temperatures and relative content of silicon in the initial compound between the samples altered their nanodiamond size distributions and abundance of SiV centres. Our results show that nanodiamond growth can be controlled and optimised for different applications.

Parametrization of semiempirical models against ab initio crystal data: evaluation of lattice energies of nitrate salts.

A method to estimate the lattice energies E(latt) of nitrate salts is put forward. First, E(latt) is approximated by its electrostatic component E(elec). Then, E(elec) is correlated with Mulliken atomic charges calculated on the species that make up the crystal, using a simple equation involving two empirical parameters. The latter are fitted against point charge estimates of E(elec) computed on available X-ray structures of nitrate crystals. The correlation thus obtained yields lattice energies within 0.5 kJ/g from point charge values. A further assessment of the method against experimental data suggests that the main source of error arises from the point charge approximation.

Solid state synthesis of carbon-encapsulated iron carbide nanoparticles and their interaction with living cells.

Superparamagnetic carbon-encapsulated iron carbide nanoparticles (NPs), Fe7C3@C, with unique properties, were produced from pure ferrocene by high pressure-high temperature synthesis. These NPs combine the merits of nanodiamonds and SPIONs but lack their shortcomings which limit their use for biomedical applications. Investigation of these NPs by X-ray diffraction, electron microscopy techniques, X-ray spectroscopic and magnetic measurement methods has demonstrated that this method of synthesis yields NPs with perfectly controllable physical properties. Using magnetic and subsequent fractional separation of magnetic NPs from residual carbon, the aqueous suspensions of Fe7C3@C NPs with an average particle size of ∼25 nm were prepared. The suspensions were used for in vitro studies of the interaction of Fe7C3@C NPs with cultured mammalian cells. The dynamics of interaction of the living cells with Fe7C3@C was studied by optical microscopy using time-lapse video recording and also by transmission electron microscopy. Using novel highly sensitive cytotoxicity tests based on the cell proliferation assay and long-term live cell observations it was shown that the internalization of Fe7C3@C NPs has no cytotoxic effect on cultured cells and does not interfere with the process of their mitotic division, a fundamental property that ensures the existence of living organisms. The influence of NPs on the proliferative activity of cultured cells was not detected as well. These results indicate that the carbon capsules of Fe7C3@C NPs are air-tight which could offer great opportunities for future use of these superparamagnetic NPs in biology and medicine.

Optimal partitioning of molecular properties into additive contributions: the case of crystal volumes.

A systematic scheme to split the volume of molecular crystals into additive increments is discussed. In contrast to earlier procedures, it relies on the definition of atom types on the basis of their geometrical rather than chemical environment. In addition, the role of the relevant structural features of the compounds is explicitly taken into account. This approach provides insight into the relative influence of chemical bonds, hydrogen bonds and rings on the volume of organic crystals. Compared with group-contribution techniques, it yields very similar results with many fewer empirical parameters. Applied to estimate the densities of 42 880 crystals containing elements up to chlorine and measured at different temperatures, an average absolute deviation from experiment close to 2% is obtained.

X-ray powder diffraction structure determination of gamma-butyrolactone at 180 K: phase-problem solution from the lattice energy minimization with two independent molecules.

The crystal structure of the solid phase of the dipolar aprotic solvent gamma-butyrolactone (BL1), C(4)H(6)O(2), has been solved using the atom-atom potential method and Rietveld-refined against powder diffraction data collected at T = 180 K with a curved position-sensitive detector (INEL CPS120) using Debye-Scherrer diffraction geometry with monochromatic X-rays. It was first deduced from the X-ray experiment that the lattice parameters are a = 10.1282 (4), b = 10.2303 (5), c = 8.3133 (4) A, beta = 93.291 (2) degrees and that the space group is P2(1)/a, with Z = 8 and two independent molecules in the asymmetric unit. The structure was then solved by global energy minimization of the crystal-lattice atom-atom potentials. The subsequent GSAS-based Rietveld refinement converged to the final crystal-structure model indicator R(F(2)) = 0.0684, profile factors R(p) = 0.0517 and R(wp) = 0.0694, and a reduced chi(2) = 1.671. After further cycles of heating and cooling, a powder diffraction pattern markedly different from the first pattern was obtained, again at T = 180 K, which we tentatively assign to a second polymorph (BL2). All the observed diffraction peaks are well indexed by a triclinic unit cell essentially featuring a doubling of the a axis. An excellent Le Bail fit is obtained, for which R(p) = 0.0312 and R(wp) = 0.0511.

Calculation of the crystal densities of molecular salts and hydrates using additive volumes for charged groups.

Standard group volumes which can be used to estimate the crystal densities of molecular salts and hydrates are reported, as a complement to values derived recently for the functional groups of neutral organic compounds. These new parameters were derived from a least-squares fit of cell volumes for a set of 1132 ionic molecular crystals from the Cambridge Structural Database. Their values point to the unusual overlap between monovalent O atoms and neighbouring H atoms. Using the new group volumes presently obtained, the crystal densities of the salts are predicted with an average error of <2.5%, while previous atom-based schemes yield average errors of >3%. To illustrate the possible application of the present database, the problem of designing environmentally friendly propellants is addressed.