Slow light and photon echoes in the 7F0→5D0 transition of Sm2+ in single-crystal BaFCl.

Slow light effects induced by transient spectral hole-burning in the 7F0→5D0 transition of Sm2+ in BaFCl at 688 nm are reported and a probe pulse delay of 1.25 µs was observed through a 5 mm thick crystal. This delay corresponds to a reduction of the group velocity vG of the transmitted light to ∼4000 m/s. An analysis of the dependence of the slow light effect on the probe pulse timing indicates some broadening of the spectral hole caused by relatively fast excitation energy transfer. We also demonstrate two-pulse (2PE) and (three-pulse) stimulated photon echoes (SPE) for the first time for Sm2+ in the solid state and a homogenous linewidth of 16 kHz (∼2.5·10-8 nm) was obtained at 1.8 K. The echoes in the optically dense medium were very efficient and revealed spectral diffusion on the 100-µs time scale possibly due to flipping of the fluorine and chlorine nuclear spins in the environment of the Sm2+ ions. Furthermore, the SPE also indicates relatively fast energy transfer, commensurate with the hole decay.

Ultra-high-resolution greyscale fluorescence images via UV-exposure of thin flexible phosphor films.

Thin films of BaFCl:Sm nanocrystals prepared using a polymer binder were used to create fluorescence images. The phosphor films were exposed to a UV-C mercury lamp light source via chromium-coated quartz greyscale masks to create 4 µm resolution greyscale fluorescence images. The mechanism relies on the highly efficient conversion of Sm3+ to Sm2+ ions upon exposure to UV-C light which displays a large linear dynamic range. The red fluorescence around 688 nm of the Sm2+ is then read-out using blue-violet illumination under a laser scanning confocal microscope. The greyscale images with 16 greyscale levels had a resolution equivalent to ∼125 line pairs per mm or ∼6400 dpi. Improvements in the resolution would be possible using collimated UV-C laser exposure of the film or the use of higher resolution photomasks. Ultra-high resolution binary fluorescence images were also created with resolutions down to 2 µm (∼250 line pairs per mm, ∼12 700 dpi). Downstream applications of the technology could include tailored covert or overt anti-counterfeiting labelling.

Reversible Mn valence state switching in submicron α-Al2O3:Mn by soft X-rays and blue light - a potential pathway towards multilevel optical data storage.

The generation of Mn4+ in α-Al2O3:Mn3+ by soft X-ray exposure is demonstrated with a large dynamic range of the X-ray generated Mn4+ luminescence signal, indicating the potential use of α-Al2O3:Mn3+ for multilevel optical data storage. Samples with a range of Mn concentrations (0.05, 0.1, 0.2, 0.4, 0.6 and 1.2 atom%) were prepared via a facile combustion method and the sample with 0.4 atom% was found to display the highest luminescence intensity. The stored information can be read out via the R-lines (2E → 4A2) under ∼470 nm (4A2 → 4T2), or ∼630 nm (4A2 → 2T1) excitation with the latter being preferred since photobleaching is minimized. Interestingly, the Mn4+ valence state can be fully switched back to Mn3+ by blue light exposure (e.g., 462 nm laser diode). The stored information could be repeatedly written and erased, showing no significant deterioration over five consecutive cycles, with less than 5% uncertainty.

Spin-forbidden near-infrared luminescence from a F3+ colour centre generated upon annealing in mechanochemically prepared nanocrystalline BaLiF3.

We report the properties of a unique colour centre in mechanochemically synthesized inverse perovskite BaLiF3 submicron crystals that are luminescent at ∼765 nm. The spin-forbidden luminescence with a lifetime of 5 ms is attributed to a F3+ (F-centre aggregate) in the fluoride octahedra, with three fluoride anion vacancies (3F+) filled with two electrons (2e-). The Zeeman splitting of the electronic origin and its temperature dependence indicate that the transition is from a singlet excited state to a triplet ground state. The F3+ emission occurred after annealing (≥500 °C) the mechanochemically prepared pure BaLiF3 nanocrystals and is characterized by a structured emission with a relatively narrow zero-phonon line. A reduction of photoluminescence intensity of the F3+ band upon increasing X-ray dose was observed. Importantly, it is observed that the F3+ luminescence is stable in the dark but bleaches upon exposure to natural sunlight. Our results point to the potential for a new colour centre-based nano-laser in the near-infrared region. Additionally, our experiments also indicate that BaLiF3 : F3+ has some potential for data storage, and X-ray imaging and dosimetry.

Ultra-slow electron spin-lattice relaxation in a low magnetic field enables massive optical spin polarization in the4A2ground state of Cr3+in ruby.

Ruby (α-Al2O3doped with Cr3+) has been an archetypal material in the development of optical spectroscopy of the solid state for the last 150 years and was the first material that was demonstrated to lase. Notwithstanding the vast literature on ruby, one effect was somehow missed: in a magnetic fieldB∥c∼ 235 mT, the spin-lattice relaxation timeT1for the |+3/2⟩ level in the4A2ground state is massively lengthened to ∼12 s at 1.4 K as demonstrated in this study. This very long relaxation time enables optical pumping of the |+3/2⟩ level via theR1(±1/2) lines and a considerable +3/2 spin polarization of ∼95% is readily achieved. The observed magnetic field dependence can be quantitatively described using the one-phonon relaxation process.

Luminescence and photoionization of X-ray generated Sm2+ in coprecipitated CaF2 nanocrystals.

We report photoluminescence and photoionization properties of Sm2+ ions generated by X-irradiation of nanocrystalline CaF2:Sm3+ prepared by coprecipitation. The nanocrystals were of 46 nm average crystallite size with a distribution of ±20 nm and they were characterised by XRD, TEM and SEM-EDS. At room temperature, the X-irradiated sample displayed broad electric dipole allowed Sm2+ 4f55d (A1u) → 4f6 7F1 (T1g) luminescence at 725 nm that narrowed to an intense peak at 708 nm on cooling to ∼30 K. The narrow f-f transitions of Sm3+ were also observed. The X-irradiation-induced reduction of Sm3+ + e- → Sm2+ as a function of X-ray dose was investigated over a very wide dynamic range from 0.01 mGy to 850 Gy by monitoring the photoluminescence intensities of both Sm2+ and Sm3+ ions. The reverse Sm2+ → Sm3+ + e- photoionization can be modelled by employing dispersive first-order kinetics and using a standard gamma distribution function, yielding an average separation of 13 Å between the Sm2+ ions and the hole traps (e.g. oxide ion impurities). The present results point towards potential applications of Sm doped CaF2 nanocrystals in the fields of dosimetry and X-ray imaging.

On the origins of the green luminescence in the "zero-dimensional perovskite" Cs4PbBr6: conclusive results from cathodoluminescence imaging.

There is great interest in the use of highly-efficient all-inorganic halide perovskites CsnPbBr2+n for optoelectronic applications. There however remains considerable debate as to the origins of the green luminescence in the zero-dimensional phase of the perovskite Cs4PbBr6, with theories suggesting it originates either from defects in the Cs4PbBr6 lattice or CsPbBr3 impurities/inclusions. The confusion has arisen due to the two phases being miscible and typically co-existing. Moreover, low impurity levels of CsPbBr3 in Cs4PbBr6 are difficult to detect by XRD measurements, yet have much stronger photoluminescence than bulk CsPbBr3 that exhibits quenching, further contributing to the confusion as to the origins of the green photoluminescence. With the rise of significant debate and misconceptions, we provide conclusive evidence that the green emission from Cs4PbBr6 is indeed due to nanocrystalline CsPbBr3 impurities. This is demonstrated by undertaking cathodoluminescence and EDX measurements on samples prepared mechanochemically by ball-milling. Cathodoluminescence imaging clearly shows the presence of small crystals embedded in/or between larger crystallites of Cs4PbBr6 and they emit around 520 nm. EDX shows that the smaller crystal inclusions have a Pb : Br ratio that is approximately 2 times higher, confirming the CsPbBr3 phase, which has the expected size-dependent shift to shorter wavelengths (about 528 to 515 nm). These studies make significant inroads into understanding these lead halide perovskites for their use in a variety of optoelectronic and photovoltaic applications.

Towards rewritable multilevel optical data storage in single nanocrystals.

Novel approaches for digital data storage are imperative, as storage capacities are drastically being outpaced by the exponential growth in data generation. Optical data storage represents the most promising alternative to traditional magnetic and solid-state data storage. In this paper, a novel and energy efficient approach to optical data storage using rare-earth ion doped inorganic insulators is demonstrated. In particular, the nanocrystalline alkaline earth halide BaFCl:Sm is shown to provide great potential for multilevel optical data storage. Proof-of-concept demonstrations reveal for the first time that these phosphors could be used for rewritable, multilevel optical data storage on the physical dimensions of a single nanocrystal. Multilevel information storage is based on the very efficient and reversible conversion of Sm3+ to Sm2+ ions upon exposure to UV-C light. The stored information is then read-out using confocal optics by employing the photoluminescence of the Sm2+ ions in the nanocrystals, with the signal strength depending on the UV-C fluence used during the write step. The latter serves as the mechanism for multilevel data storage in the individual nanocrystals, as demonstrated in this paper. This data storage platform has the potential to be extended to 2D and 3D memory for storage densities that could potentially approach petabyte/cm3 levels.

Mechanochemically prepared SrFCl nanophosphor co-doped with Yb3+ and Er3+ for detecting ionizing radiation by upconversion luminescence.

We report a novel method for detecting ionizing radiation by employing the phenomenon of upconversion luminescence. Nanocrystalline SrFCl:Yb3+/Er3+ was prepared by ball-milling and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The photoluminescence properties of nanocrystalline SrFCl:Yb3+, SrFCl:Er3+ and SrFCl:Yb3+/Er3+ before and after X-irradiation were investigated. The results demonstrate that both Yb3+ and Er3+ ions in the SrFCl host are reduced to their divalent state upon X-ray exposure. Under 980 nm infrared excitation, SrFCl:Yb3+/Er3+ nanocrystals displayed efficient upconversion luminescence. The upconversion luminescence intensity gradually decreased with increasing X-irradiation in a double exponential fashion with rate constants of k1 = 0.08 Gy-1 and k2 = 0.01 Gy-1. In comparison with other X-ray storage phosphors, the present system shows a much higher stability of stored information since it is not subject to photobleaching in the read-out process. This is the first report on detecting ionizing radiation by upconversion luminescence, with the potential for improved read-out performance over traditional storage phosphors. Possible applications of the present phosphor include bioimaging and in vivo cell-level X-ray dose monitoring.

Ultra-slow light propagation by self-induced transparency in ruby in the superhyperfine limit.

Self-induced transparency is reported for circularly polarized light in the R1(-3/2) line of a 30 ppm ruby (α-Al2O3:Cr3+) at 1.7 K in a magnetic field of B‖c=4.5 T. In such a field and temperature, a 30 ppm ruby is in the so-called superhyperfine limit resulting in a long phase memory time, TM=50 µs, and a thousand-fold slower pulse propagation velocity of ∼300 m/s was observed, compared to the ∼300 km/s measured in the first observation of self-induced transparency (SIT) ∼50 years ago, that employed a ruby with a 500 ppm chromium concentration in zero field and at 4.2 K. To date, this is the slowest pulse propagation ever observed in a SIT experiment.

Controlled Generation of Tm2+ Ions in Nanocrystalline BaFCl:Tm3+ by X-ray Irradiation.

An investigation of the photoluminescence properties of divalent thulium generated by X-ray irradiation (X-irradiation) of nanocrystalline BaFCl:Tm3+ (250 ± 50 ppm) is reported. The X-irradiated samples show typical Tm2+ f-f luminescence with an excited state lifetime τ = 0.98 ± 0.05 ms in the near-infrared (∼1140 nm), but no d-f luminescence is observed for the BaFCl host material within the visible range. The Tm2+ ions are relatively stable under dim room light but are photoionized rapidly by sunlight or by fluorescent tube lighting. To study the X-ray storage mechanism, photoluminescence intensities of both Tm3+ and Tm2+ transitions were measured, and the reverse photoionization of Tm2+ was investigated as a function of the laser power density and the initial radiation dose. The photoionization data was then modeled by employing equations based on dispersive first order kinetics using a standard Γ distribution function for the separation between the reduced thulium ions and the hole traps. In accord with previous reports, the hole traps are most likely oxide impurities such as O2-, and the average of the separation is found to be ∼6-7 Å, i.e., a few interionic spacings.

Photoreduction of Sm(3+) in Nanocrystalline BaFCl.

We demonstrate that exposure of nanocrystalline BaFCl:Sm(3+) X-ray storage phosphor to blue laser pulses with peak power densities on the order of 10 GW/cm(2) results in conversion of Sm(3+) to Sm(2+). This photoreduction is found to be strongly power-dependent with an initial fast rate, followed by a slower rate. The photoreduction appears to be orders of magnitude more efficient than that for previously reported systems, and it is estimated that up to 50% of the samarium ions can be photoreduced to the divalent state. The main mechanism is most likely based on multiphoton electron-hole creation, followed by subsequent trapping of the electrons in the conduction band at the Sm(3+) centers. Nanocrystalline BaFCl:Sm(3+) is an efficient photoluminescent X-ray storage phosphor with possible applications as dosimetry probes, and the present study shows for the first time that the power levels of the blue light have to be kept relatively low to avoid the generation of Sm(2+) in the readout process. A system comprising the BaFCl:Sm(3+) nanocrystallites embedded into a glass is also envisioned for 3D memory applications.

Room temperature hole-burning of X-ray induced Sm(2+) in nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm(3+) prepared by mechanochemistry.

Alloyed nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5 doped with Sm(3+) ions was prepared by a facile ball milling method at room temperature. Spectral hole-burning properties of Sm(2+) ions from X-irradiated sample were investigated in the (7)F0-(5)D0 transition between 2.5 K and room temperature. The alloying allows a "chemical" broadening of the inhomogeneous width of the (7)F0-(5)D0 f-f transition to 40 cm(-1); spectral holes with a homogeneous width of 5 cm(-1) can be burnt, yielding a figure-of-merit of Γinh/Γhom = 8. Mechanochemical preparation methods have a significant potential for the preparation of functional materials for applications in frequency domain optical data storage and as X-ray storage phosphors by allowing the preparation of tailored solid solutions.

Mechanochemical preparation of nanocrystalline BaFCl doped with samarium in the 2+ oxidation state.

We report a facile mechanochemical preparation method for nanocrystalline BaFCl doped with samarium in the 2+ oxidation state by ball milling BaCl2, BaF2, and SmI2 under a nitrogen atmosphere. The resulting phosphors were characterized by powder X-ray diffraction; electron microscopy, X-ray photoelectron spectroscopy; and photoluminescence, photoexcitation, cathodoluminescence, and diffuse reflectance spectroscopy. This is the first report of a direct preparation method of Sm(2+) doped alkaline earth fluorohalides at room temperature and points to a significant potential for the preparation of a wide range of related X-ray storage phosphors containing rare earth ions in divalent and trivalent cationic states by mechanochemical methods.

Optical spectroscopy of graphene quantum dots: the case of C132.

We have reinvestigated the optical spectroscopy of C132 graphene quantum dots by absorption, selective fluorescence, excitation and time-resolved spectroscopy, the external heavy atom effect, and DFT based quantum chemical calculations. In particular, wavelength-selective photobleaching provides strong evidence for the assignment of the intrinsic absorption and emission features of the quantum dots and indicates that emissions observed at ∼670 and ∼630 nm and associated relatively narrow features that display vibrational progressions in the selective excitation spectra are due to different species. The emitting state that leads to a broad emission (1700 cm(-1)) centered around 750 nm appears to be a "near-dark" singlet state with a relatively long lifetime of ∼30 ns. Simulated spectra, based on the nuclear ensemble approach, are in qualitative agreement with this finding and indicate very low oscillator strengths with some significant electron-vibrational intensity borrowing.

Controlling pulse delay by light and low magnetic fields: slow light in emerald induced by transient spectral hole-burning.

Slow light based on transient spectral hole-burning is reported for emerald, Be(3)Al(2)Si(6)O(18):Cr(3+). Experiments were conducted in π polarization on the R(1)(± 3/2) line (E2 ← A(2)4) at 2.2 K in zero field and low magnetic fields B||c. The hole width was strongly dependent on B||c, and this allowed us to smoothly tune the pulse delay from 40 to 154 ns between zero field and B||c = 15.2 mT. The latter corresponds to a group velocity of 16 km/s. Slow light in conjunction with a linear filter theory can be used as a powerful and accurate technique in time-resolved spectroscopy, e.g., to determine spectral hole-widths as a function of time.

Effects of postannealing on the photoluminescence properties of coprecipitated nanocrystalline BaFCl:Sm3+.

Nanocrystalline BaFCl:Sm(3+), as prepared by coprecipitation from aqueous solutions, is an efficient photoluminescent X-ray storage phosphor. In the present study, we report effects on its photoluminescence properties resulting from postannealing treatment in air in the temperature range between 100 to 900 °C. Interestingly, upon annealing at temperatures from 200 to 600 °C in air, a small fraction of the Sm(3+) ions in nanocrystalline BaFCl can be reduced to Sm(2+) ions. In addition to the creation of Sm(2+) ions, two different sites of Sm(3+) ions, denoted as sites A and B, are observed when the nanocrystalline BaFCl:Sm(3+) is annealed between 500 to 900 °C. The temperature dependence of photoluminescence properties of the two different sites in the 500 °C annealed sample reveals that the Sm(3+) ions at site A are possibly located at or near the crystallite surface, whereas site B is situated in a very ordered environment. To the best of our knowledge, this is the first report on the reduction of Sm(3+) ions doped in alkaline-earth fluorohalides to Sm(2+) ions by annealing in air.

Cathodoluminescence microanalysis of irradiated microcrystalline and nanocrystalline samarium doped BaFCl.

BaFCl:Sm3+ is an efficient photoluminescent storage phosphor for ionizing radiation. Cathodoluminescence (CL) microanalysis enables the Sm2+ and Sm3+ oxidation states of samarium doped BaFCl to be easily identified, provides information about electron-beam and X-ray induced modification of BaFCl:Sm, and enables the synthesis dependent spatial distribution of samarium dopants of <100 ppm concentration to be determined with sub-100 nm resolution at 295 K. CL spectroscopy of BaFCl:Sm particles reveals broad CL emissions at ≈ 360 and ≈500 nm associated with V k (Cl-) and oxygen-vacancy defects in the BaFCl host lattice and fine structure CL emissions associated with major 4GJ → 6HJ (Sm3+) and 5DJ → 7FJ (Sm2+) transitions. CL microanalysis shows samarium dopants are uniformly distributed in conventional sintered microcrystalline BaFCl:Sm. In contrast, CL investigations reveal that for BaFCl:Sm nanoparticles, which have been prepared using a co-precipitation method, with greatly improved Sm3+ → Sm2+ conversion efficiency, the samarium dopants are concentrated near the particle surface resulting in a BaFCl:Sm3+ shell surrounding the BaFCl core, which is stable to energetic irradiation.

Slowing light down by low magnetic fields: pulse delay by transient spectral hole-burning in ruby.

We report on the observation of slow light induced by transient spectral hole-burning in a solid, that is based on excited-state population storage. Experiments were conducted in the R1-line (2E←4A2 transition) of a 2.3 mm thick pink ruby (Al2O3:Cr(III) 130 ppm). Importantly, the pulse delay can be controlled by the application of a low external magnetic field B||c≤9 mT and delays of up to 11 ns with minimal pulse distortion are observed for ~55 ns Gaussian pulses. The delay corresponds to a group velocity value of ~c/1400. The experiment is very well modelled by linear spectral filter theory and the results indicate the possibility of using transient hole-burning based slow light experiments as a spectroscopic technique.

Photoinduced electron transfer and persistent spectral hole-burning in natural emerald.

Wavelength-selective excited-state lifetime measurements and absorption, luminescence, and hole-burning spectra of a natural African emerald crystal are reported. The (2)E excited-state lifetime displays an extreme wavelength dependence, varying from 190 to 37 µs within 1.8 nm of the R(1)-line. Overall, the excited state is strongly quenched, in comparison to laboratory-created emerald (τ=1.3 ms), with an average quenching rate of ∼6 × 10(3) s(-1) at 2.5 K. This quenching is attributed to photoinduced electron transfer caused by a relatively high concentration of Fe(2+) ions. The forward electron-transfer rate, k(f), from the nearest possible Fe(2+) sites at around 5 Å is estimated to be ∼20 × 10(3) s(-1) at 2.5 K. The photoreductive quenching of the excited Cr(3+) ions by Fe(2+) is followed by rapid electron back-transfer in the ground state upon deactivation. The exchange interaction based quenching can be modeled by assuming a random quencher distribution within the possible Fe(2+) sites with the forward electron-transfer rate, k(f), given as a function of acceptor-donor separation R by exp[(R(f)-R)/a(f)]; R(f) and a(f) values of 13.5 and 2.7 Å are obtained at 2.5 K. The electron transfer/back-transfer reorganizes the local crystal lattice, occasionally leading to a minor variation of the short-range structure around the Cr(3+) ions. This provides a mechanism for spectral hole-burning for which a moderately high quantum efficiency of about ∼0.005% is observed. Spectral holes are subject to spontaneous hole-filling and spectral diffusion, and both effects can be quantified within the standard two-level systems for non-photochemical hole-burning. Importantly, the absorbance increases on both sides of broad spectral holes, and isosbestic points are observed, in accord with the expected distribution of the "photoproduct" in a non-photochemical hole-burning process.