Line Shape Research Articles

Formation and Application of Polymer Spherulite-like Patterns of Halloysite Nanotubes by Evaporation-Induced Self-Assembly.

Halloysite nanotubes (HNTs) are one-dimensional clay nanomaterials featuring distinct tubular structures and unique surface charges. HNTs can readily form ordered assembly structures under specific conditions, which shows significant potential applications in optical and biological fields. In this study, sodium hexametaphosphate (SHMP) was employed as a stabilizer to prepare polymer spherulite-like patterns via the evaporation-induced self-assembly (EISA) technique. The incorporation of SHMP enhanced the repulsion force among the nanotubes and the surface potential, which facilitated the orderly deposition of HNTs. The influence of HNT concentration, SHMP concentration, drying temperature, and substrate on the polymer spherulites-like pattern has been investigated in detail. The optimal conditions were 10 wt % HNT dispersion, 0.6 wt % SHMP concentration, 30 °C as drying temperature, and glass substrates. In addition, by changing the droplet volume and shape of the three-phase contact line, patterns of different sizes and shapes can be achieved. Bovine serum albumin or metal salt compounds were incorporated into the dispersion of SHMP-modified HNTs, which altered the charge and the self-assembled patterns with different area ratios. Thus, this technology can be utilized for the analysis and comparison of protein and metal ion concentration accurately. This study creates the correlation between the structural parameters and the preparation process involved in creating polymer spherulite-like patterns of modified HNTs and offers fresh insights into potential applications for the self-assembly of HNT droplets in the realms of anticounterfeiting and solution concentration analysis.

Engineering 2D Material Exciton Line Shape with Graphene/h-BN Encapsulation.

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton line shape and charge state. Fano-like asymmetric spectral features are produced in WS2, MoSe2, and WSe2 van der Waals heterostructures combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe2/graphene with a neutral exciton red shift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

Nonlocal Metasurfaces‐Enabled Analog Light Localization for Imaging and Lithography

AbstractLocalization of light spots in a given image is a common task in digital processing but is challenging in analog. Herein, a periodic nonlocal metasurface that has an optical transfer function defined in reciprocal space is proposed to resolve this issue. Assuming an ideal optical transfer function that encodes that of a local lens, the selective spot size reduction of incident Gaussian beams and sharpening of a patterned incidence with a Gaussian line shape are demonstrated. A realistic nonlocal metasurface comprising a trilayer structure with air grating on top that operates as a 2D analog light localizer under unpolarized incidence is presented. Nonlocal metasurfaces, combined with conventional optics, are expected to improve the resolution by sharpening the spatial features and find applications in diverse scientific fields such as medical, materials, and life science.

Fano Resonance Thermo-Optic Modulator Based on Double T-Bus Waveguides-Coupled Micro-Ring Resonator

For the silicon optical computing chip, the optical convolution unit based on the micro-ring modulator has been demonstrated to have high integration and large computing density. To further reduce power consumption, a novel, simple Fano resonant thermo-optic modulator is presented with numerical simulation and experimental demonstration. This designed Fano resonator comprises double T-shaped waveguides and a micro-ring with a radius of 10 μm. Compared with the free use of bus waveguides, our double T-shaped waveguides generate a phase shift, along with a Fano-like line shape. The experimental results show that the resonant wavelength shift of the designed modulator is 2.4 nm with a driven power of 20 mW. In addition, the maximum spectral resolution and the extinction ratio are 70.30 dB/nm and 12.69 dB, respectively. For our thermo-optic modulator, the optical intensity power consumption sensitivity of 7.60 dB/mW is three times as large as that of the micro-ring modulator. This work has broad potential to provide a low-power-consumption essential component for large-scale on-chip modulation for optical computing with compatible metal oxygen semiconductor processes.

Transient stimulated Raman scattering spectroscopy and imaging

Stimulated Raman scattering (SRS) has been developed as an essential quantitative contrast for chemical imaging in recent years. However, while spectral lines near the natural linewidth limit can be routinely achieved by state-of-the-art spontaneous Raman microscopes, spectral broadening is inevitable for current mainstream SRS imaging methods. This is because those SRS signals are all measured in the frequency domain. There is a compromise between sensitivity and spectral resolution: as the nonlinear process benefits from pulsed excitations, the fundamental time-energy uncertainty limits the spectral resolution. Besides, the spectral range and acquisition speed are mutually restricted. Here we report transient stimulated Raman scattering (T-SRS), an alternative time-domain strategy that bypasses all these fundamental conjugations. T-SRS is achieved by quantum coherence manipulation: we encode the vibrational oscillations in the stimulated Raman loss (SRL) signal by femtosecond pulse-pair sequence excited vibrational wave packet interference. The Raman spectrum was then achieved by Fourier transform of the time-domain SRL signal. Since all Raman modes are impulsively and simultaneously excited, T-SRS features the natural-linewidth-limit spectral line shapes, laser-bandwidth-determined spectral range, and improved sensitivity. With ~150-fs laser pulses, we boost the sensitivity of typical Raman modes to the sub-mM level. With all-plane-mirror high-speed time-delay scanning, we further demonstrated hyperspectral SRS imaging of live-cell metabolism and high-density multiplexed imaging with the natural-linewidth-limit spectral resolution. T-SRS shall find valuable applications for advanced Raman imaging.

Efficient Access to Elusive 1D 13C NMR Spectra through Highly Resolved 1H, 13C-Long-Range Correlation Spectroscopy.

A method for obtaining 1D 13C NMR spectra from natural products or metabolites using proton detection is described. The approach delivers singlets for every 13C signal without conducting any broadband 1H decoupling (CPD) and is based on calculating 13C projections from constant-time HMBC and conventional HSQC experiments, recorded at high digital resolution and processed to pure phases. Paramount to the proposed method is the implication of nonuniform sampling and echo processing. The echo processing produces phase-sensitive 2D CT-HMBC spectra with narrow 13C signal line shapes. Two simple HMBC pulse sequences are utilized with the suppression of homo- and heteronuclear couplings. Due to the removal of the 1H multiplet structure in F1 (no tilt at higher digital resolution), 13C singlets arise. An overall increase in 13C signal-to-noise (SINO) for all types of carbon multiplicities is observed, making the proposed technique superior compared to direct 13C excitation. For otherwise difficult-to-measure quaternary carbon atoms, a SINO enhancement of up to 6 and 12 depending on F1 resolution (3 and 6 Hz/point) is reported. Echo/anti-Echo signal detection cleans up the spectrum. Nonuniform sampling (NUS) lays the groundwork to significantly reduce the total acquisition time. Final 1D 13C projections are obtained by combining the 13C projection from CT HMBC and conventional HSQC. This orthogonal concept of combining the 13C projections from different spectra inherently minimizes the risk of missing 13C cross-peaks by inappropriate setting of long-range nJHC coupling delays and the shortcoming of T2 relaxations. The advantages and some limitations of the concept are discussed.

Signatures as an object of autoforgery (self-forgery)

The study presents the results of research aimed at isolating the graphic features most frequently and least frequently modified by people committing autoforgery (self-forgery) of signatures in situations where the appearance of their natural signatures is not known to the recipient. The research covered a total of over 12,000 signatures from 200 individuals. The most successful attempts at autoforgery of legible and illegible signatures of each test subject were selected for the final evaluation. It was found that autoforgery changes are most often focused on the most striking features of the signatures, such as the structure of letters in the initial part of the signature, size, readability, impulse, and slope. Secondary features, more difficult to notice or those whose existence the writers are not aware of (such as the presence or absence of additions, the arrangement of letters in relation to each other, the shape and direction of signature lines, the format of legible signatures) are usually omitted in autoforgery activities. Detecting autoforgery can be a big challenge for experts, because in practice, any significant differences between the questioned signature and comparative signatures are often mistakenly considered to be the result of forgery. Therefore, in order to detect autoforgery, it is necessary to analyze the structure of easily noticeable features that most influence the so-called pictorial effect of the signature in combination with the unattractive features that remain unchanged in most cases of autoforgery. The more characteristic the latter are, the more their consistency in the questioned and comparative material proves self-forgery, regardless of the differences in the primary features. In the case of a forged signature, the opposite is true: the most easily noticeable features of the signature are imitated by the forger, and the differences occur mainly in secondary features.

Changes in Moisture Desorption Rate of Shelled Corn Layers in a Low-Temperature Deep-Bed Drying Column

To optimize grain drying processes, high-moisture shelled corn was dried in a device using a combination of three temperatures (45, 55 and 65 °C) and hot-air velocities (0.7, 1.1 and 1.3 m·s-1). The grain moisture and temperature, and the relative humidity (RH) of interstitial airflow were determined intelligently in grain layers of 7.5 to 32.5 cm with 5.0 cm intervals. The change in RH of interstitial airflow in grain layers of 7.5-27.5 cm with drying time all revealed the shape of a hyperbolic line and took turns to lag. These RH lines shortened with an increased drying temperature from 45 to 65 °C and hot-air velocity from 0.7 to 1.3 m·s-1. The temperature lines were raised and shortened with an increase in drying temperature and hot-air velocity. A modified moisture diffusion exponential equation was developed to analyze the moisture desorption rate of corn kernels. For the average sorption rate curve from grain layers of 2.5-32.5 cm at 1.3 m·s-1 of hot air, the transition points from adsorption to desorption occurred at 2.4 h, 2.7 h, and 3.0 h with drying temperatures of 65 and 45 °C, respectively, and represented the earliest and largest initial desorption rates among the three hot-air velocities. The moisture desorption rate of samples increased with increasing drying temperature. Compared with the average value of grain layers, at the same hot-air velocity, the kernel effective diffusivity (Deff) values dried at 55 °C were correspondingly higher than those dried at 45 and 65 °C. At the same drying temperature, the kernel Deff values tended to increase with an increase in hot-air velocity, whereas the activation energy decreased. These results suggested that an increase in drying temperature and hot-air velocity may considerably shorten adsorption time and increase desorption rates of shelled corn in deep bed drying.

A dual-route optical emission spectroscopy diagnostic with wide spectral range and high wavelength resolution on HL-2A tokamak

A dual-route optical emission spectroscopy (D-OES) diagnostic is newly developed to monitor the optical emission from the X-point plasma region on the HL-2A tokamak. This diagnostic is composed of an imaging system, a beam-splitting system for dual-route measurements, fiber bundles, a spectrometer system, and a control and acquisition system. One route is used to obtain wide-spectral-range spectra, and the other route is used to acquire high-wavelength-resolution line shapes. The spectral resolution of the wide-range spectrometers is 0.8 nm with a coverage of 800 nm (@200–1000 nm). The spectral resolution of the high-resolution spectrometer is 0.01 nm with a coverage of 6 nm (@200–660 nm). The spatial resolution of each route of D-OES is about 4 cm with 11 channels. The temporal resolution is 16 ms at maximum in the single-channel mode. Wide-range spectra (containing Balmer series and a Fulcher band) and highly resolved line shapes are obtained by D-OES in the hydrogen glow discharge in the lab. D-OES measurements are carried out in the high-density deuterium experiments of HL-2A. The electron density and deuterium temperature in the X-point multifaceted asymmetric radiation from the edge (MARFE) region are derived simultaneously by fitting the measured shape. The density is observed to increase from ~ to ~ , and the temperature drops from ~14.4 eV to ~2.3 eV after the onset of MARFE in the discharge #38260.

New theoretical study of potassium perturbed by He and a comparison to laboratory spectra

The visible and near-infrared spectra of late L- and T-type dwarf stars are dominated in large part by the resonance lines of neutral Na and K. It is the collision broadening of these atomic lines by H2 and He in the stellar atmosphere that determines the continuum from below 0.5 µm to above 0.9 µm in the spectrum. Their line profiles can be detected as far as 3000 cm−1 from the line center in T dwarfs and consequently an accurate and detailed determination of the complete profile, including the extreme far wing, is required to model the contribution of these strong alkali resonance lines to brown dwarf spectra. We report on our new calculations of unified line profiles of K perturbed by He using ab initio potential data for the conditions prevailing in cool substellar brown dwarfs and hot dense planetary atmospheres with temperatures from Teff = 500 to 3000 K. For such objects with atmospheres of H2 and He, conventional laboratory absorption spectroscopy can be used to examine the line wings and test the line shape theories and molecular potentials. We find that an analytical Lorentzian profile is useful for a few cm−1 from the line center, but not in the line wings, where the radiative transfer is a consequence of the K–He radiative collisions that are sensitive to the interaction potentials. Tables of the K–He absorption coefficients of the resonance lines allow accurate model atmospheres and synthetic spectra. For this purpose, we present new opacities from comprehensive line shape theory incorporating accurate ab initio potentials. Use of these new tables for the modeling of emergent spectra will be an improvement over previous line shape approximations based on incomplete or inaccurate potentials. We also present Lorentzian impact parameters obtained in the semi-classical and quantum theory for the K 4s − 4p resonance line centered at 0.77 µm specifically for the line core regime.

Temperature and density dependence of Kr L-shell spectrum in hot dense plasmas

Kr L-shell spectroscopy modeling results are discussed in this paper, focusing on the n = 4 to n = 2 line transitions of Be- and Li-like Kr ions. Collisional radiative atomic kinetic and Stark-broadened spectral line shape calculations show electron temperature Te and density ne sensitivity in the spectrum. The combination of the Te dependence due to the relative intensity of Be-like to Li-like line emissions in the range from 1.5 to 3 keV and the ne sensitivity from the Stark broadening effect on the line shapes in the range from 5×1023 to 2×1024/ cc results in a spectrum that can be employed to diagnose Te and ne. Two different collisional radiative atomic kinetic models i.e., Prismspect [J. J. MacFarlane, et al., Int. Fusion Sci. Appl. Conf. Proc. 457 (2003)] and ABAKO [Florido, et al., PRE, 80, 056402 (2009)] produce similar results in level populations and spectra. In x-ray spectroscopy of implosion cores, this Kr L-shell spectrum may prove useful in an intermediate Te range in which Ar is too ionized for its K-shell to be of diagnostic value and Kr is not ionized enough for its K-shell emission to be useful.

International Journal of Scientific Research in Science and Technology

The coastal zone is an interface between land and sea. The interaction between land and sea involves physical and chemical processes, which produce characteristic shapes of coast line. Thus, in a coastal zone the terrestrial ecosystem has its influence on the sea and vice versa. Veraval is one of the important port cities located along the western coast of Gujarat in India. The Veraval coast is situated in Gujarat state and its water is polluted by the discharge of Fish processing industry effluent. The intertidal area of Fish processing industry at Veraval coast was selected for the study and the investigation was carried out at Veraval at Lat 210 35’N; Long 690 36E’. Survey was conducted during pre-monsoon, monsoon and post monsoon seasons during the year of 2018. In order to achieve the aims of the present study, following parameters were studied to assess species diversity of seaweeds along increasing distance from the discharge point. Number of seaweed species showed a trend similar to that of species diversity was more related to species richness. Ratio of species number for Green, Brown and Red algae indicated that red algae were in greater proportion than green and brown algae. Seaweed species diversity was minimum near Chemical industry effluent discharge point during post-monsoon, pre-monsoon and monsoon seasons, whereas the diversity increased as the discharge point from the outfall increased.

Correlating Intrinsic Stellar Parameters with Mg ii Self-reversal Depths

The Mg ii h&k emission lines (2803, 2796 Å) are a useful tool for understanding stellar chromospheres and transition regions due to their intrinsic brightness, relatively low interstellar medium (ISM) absorption interference, and abundance of archival spectra available. Similar to other optically thick chromospheric emission lines such as H i Lyα, Mg ii emissions commonly present with a self-reversed line core, the depth and shape of which vary from star to star. We explore the relationship between self-reversal and the stellar atmosphere by investigating the extent to which fundamental stellar parameters affect self-reversal. We present a search for correlations between photospheric parameters such as effective temperature, surface gravity, and metallicity with the Mg ii k self-reversal depth for a group of 135 FGKM main-sequence stars with high-resolution near-ultraviolet spectra from the Hubble Space Telescope. We modeled the observed Mg ii k line profiles to correct for ISM attenuation and recover the depth of the emission line’s self-reversal in relation to the intensity of the line. We used the PHOENIX atmosphere code to homogeneously determine the stellar parameters by computing a suite of stellar atmosphere models that include a chromosphere and transition region, and using archival photometry to guide the models of each star. We quantify the sensitivity of the visible and near-infrared photometry to chromospheric and photospheric parameters. We find weak trends between Mg ii k self-reversal depth and age, rotation period, Mg ii luminosity, temperature, and mass. All stars in our sample older than ∼2 Gyr or rotating slower than ∼10 days exhibit self-reversal.

High-energy-resolution off-resonant spectroscopy with self-seeded x-ray free-electron laser pulses.

This paper presents the implementation of high-energy-resolution off-resonant spectroscopy (HEROS) measurements using self-seeded x-ray free-electron laser (XFEL) pulses. This study systematically investigated XFEL conditions, including photon energy and accumulated shot numbers, to optimize the measurement efficiency for copper foil samples near the K-edge. The x-ray absorption spectra reconstructed using HEROS were compared with those derived from fluorescence-yield measurements. The HEROS-based spectra exhibited consistent line shapes independent of the sample thickness. The potential application of HEROS to high-temperature copper was also explored. HEROS offers distinct advantages including scan-free measurement of x-ray absorption spectra with reduced core-hole lifetime broadening and self-absorption effects. Using self-seeded XFEL pulses, HEROS facilitates single-shot-based pump-probe measurements to investigate the ultrafast dynamics in various materials and diverse conditions.

FEFF analysis of americium oxides

The Am N4,5 (4d3/2 and 4d5/2) and Am O4,5 (5d3/2 and 5d5/2) x-ray absorption spectroscopy (XAS) of americium sesquioxide (Am2O3) and americium dioxide (AmO2) has been evaluated with FEFF, a Green's function–based, multiple scattering code. Taking guidance from the intermediate coupling model (ICM), applicable to local and nonmagnetized samples, it is possible to completely reconstruct the experimental results for the N4,5 spectra, including the observed differences between the Am2O3 and the AmO2 cases. Although complicated by a more asymmetric line shape and difficult background variations, the FEFF analysis confirms the absence of core hole angular momentum coupling in Am O4,5 spectroscopy.

GaN/AlN bilayers for integrated photonics

III-Nitride semiconductors offer a versatile platform for integrated photonic circuits operating from the ultra-violet to the near-infrared spectral range. Either pure AlN or pure GaN waveguiding layers have usually been investigated so far. In this work, we report on the study of GaN/AlN bilayers epitaxially-grown on a sapphire substrate for photonic circuits. Quality factors up to 410,000 are demonstrated with microring resonators in the near-infrared spectral range. We emphasize the peculiar advantages of these bilayers for nonlinear photonics: GaN offers a larger nonlinear susceptibility as compared to AlN. More importantly, both materials exhibit nonlinear susceptibilities with opposite signs that can be advantageous for nonlinear conversion. Thick epitaxial III-nitride bilayers are associated with the occurrence of cracks in the epi-layers and multimode waveguide propagation. We show that the multimode character can lead to peculiar resonance line shapes with the capacity to control full transmission and reflection by phase engineering.

Deep-Ultraviolet and Helicity-Dependent Raman Spectroscopy for Carbon Nanotubes and 2D Materials.

Recent progress of Raman spectroscopy on carbon nanotubes and 2D materials is reviewed as a topical review. The Raman tensor with complex values is related to the chiral 1D/2D materials without mirror symmetry for the mirror in the propagating direction of light, such as chiral carbon nanotube and black phosphorus. The phenomenon of complex Raman tensor is observed by the asymmetric polar plot of helicity-dependent Raman spectroscopy using incident circularly-polarized lights. First-principles calculations of resonant Raman spectra directly give the complex Raman tensor that explains helicity-dependent Raman spectra and laser-energy-dependent relative intensities of Raman spectra. In deep-ultraviolet (DUV) Raman spectroscopy with 266 nm laser, since the energy of the photon is large compared with the energy gap, the first-order and double resonant Raman processes occur in general k points in the Brillouin zone. First-principles calculation is necessary to understand the DUV Raman spectra and the origin of double-resonance Raman spectra. Asymmetric line shapes appear for the G band of graphene for 266 nm laser and in-plane Raman mode of WS2 for 532 nm laser, while these spectra show symmetric line shapes for other laser excitation. The interference effect on the asymmetric line shape is discussed by fitting the spectra to the Breit-Wigner-Fano lineshapes.

Design of the New Production Line in the Automotive Company

Innovation is one of the key competitiveness strategies for small, medium, and large companies. Production line innovations using Lean methods are currently one of the most important ways to change the shape of the production line in a positive direction. Emphasis must be placed on the resulting innovation and the form of the production line, which should be as efficient and functional as possible and meet the required parameters of productivity and quality assurance. The primary goal of production line innovations using Lean methods is the elimination of bottlenecks in the production process that do not add value and the balancing of individual assembly stations. The aim of the contribution was to design a semi-automatic line for the completion and testing of seat locks under the name D3P2. The result of the solution is the creation of a robotic workplace that will ensure a higher production capacity and the related packaging of finished products. On the assembly line, an analysis of line balancing was carried out using the Yamazumi graph to eliminate unnecessary assembly process activities, Cycle time and Tact time. An analysis of the measurements of individual production activities and places was carried out on the line, and the proposal for a new production line was subsequently shared. Its main goal was to meet all customer requirements through high flexibility. The result of the analysis and applied measures was a reduction in the cycle time of the new line, balancing of individual posts and economic benefits.

Blue-Emitting Cs(Pb,Cd)Br3 Nanocrystals Resistant to Electric Field-Induced Ion Segregation.

High-performance solution-processed perovskite light-emitting diodes (PeLEDs) have emerged as a good alternative to the well-established technology of epitaxially grown AIIIBV semiconductor alloys. Colloidal cesium lead halide perovskite nanocrystals (CsPbX3 NCs) exhibit room-temperature excitonic emission that can be spectrally tuned across the entire visible range by varying the content of different halogens at the X-site. Therefore, they present a promising platform for full color display manufacturing. Engineering of highly efficient PeLEDs based on bromide and iodide perovskite NCs emitting green and red light, respectively, does not face major challenges except low operational stability of the devices. Meanwhile, mixed-halide counterparts demonstrating blue luminescence suffer from the electric field-induced phase separation (ion segregation) phenomenon described by the rearrangement (demixing) of mobile halide ions in the crystal lattice. This phenomenon results in an undesirable temporal redshift of the electroluminescence spectrum. However, to realize spectral tuning and, at the same time, address the issue of ion segregation less mobile Cd2+ ion could be introduced in the lattice at Pb2+-site that leads to the band gap opening. Herein, we report an original synthesis of CsPb0.88Cd0.12Br3 perovskite NCs and study their structural and optical properties, in particular electroluminescence. Multilayer PeLEDs based on the obtained NCs exhibit single-peak emission centered at 485 nm along with no noticeable change in the spectral line shape for 30 min which is a significant improvement of the device performance.

Monodispersed and Monofunctionalized DNA-Caged Au Nano-Clusters with Enhanced Optical Properties for STED Imaging.

The performance of Stimulated Emission Depletion (STED) microscopy depends critically on the fluorescent probe. Ultrasmall Au nanoclusters (Au NCs) exhibit large Stokes shift, and good stimulated emission response, which are potentially useful for STED imaging. However, Au NCs are polydispersed in size, sensitive to the surrounding environment, and difficult to control surface functional group stoichiometry, which results in reduced density and high heterogeneity in the labeling of biological structures. Here, this limitation is overcome by developing a method to encapsulate ultrasmall Au NCs with DNA cages, which yielded monodispersed, and monofunctionalized Au NCs that are long-term stable. Moreover, the DNA-caging also greatly improved the fluorescence quantum yield and photostability of Au NCs. In STED imaging, the DNA-caged Au NCs yielded ≈40nm spatial resolution and are able to resolve microtubule line shapes with good labeling density and homogeneity. In contrast, without caging, the Au NCs-DNA conjugatesonly achieved ≈55nm resolution and yielded spotted,poorly resolved microtubule structures, due to the presence of aggregates. Overall, a method is developed to achieve precise surface functionalization and greatly improve the monodispersity, stability, as well as optical properties of Au NCs, providing a promising class of fluorescent probes for STED imaging.