anti-Stokes Photoluminescence Research Articles

Resonant Multiple-Phonon Absorption Causes Efficient Anti-Stokes Photoluminescence in CsPbBr3 Nanocrystals.

Lead halide perovskite nanocrystals, such as CsPbBr3, exhibit efficient photoluminescence (PL) up-conversion, also referred to as anti-Stokes photoluminescence (ASPL). This is a phenomenon where irradiating nanocrystals up to 100 meV below gap results in higher energy band edge emission. Most surprising is that ASPL efficiencies approach unity and involve single-photon interactions with multiple phonons. This is unexpected given the statistically disfavored nature of multiple-phonon absorption. Here, we report and rationalize near-unity anti-Stokes photoluminescence efficiencies in CsPbBr3 nanocrystals and attribute them to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals. Moreover, the developed microscopic mechanism has immediate and important implications for applications of ASPL toward condensed phase optical refrigeration.

The Use of Diatoms in the Synthesis of New 3D Micro-Nanostructured Composites (SiO2/CaCO3/Corg/NdVO4NPs and SiO2/CaO/Corg/NdVO4NPs) Exhibiting an Intense Anti-Stokes Photoluminescence.

New 3D micro-nanostructured composite materials have been synthesised. These materials comprise SiO2/CaCO3/Corg/NdVO4NPs and SiO2/CaO/Corg/NdVO4NPs, exhibiting strong upconversion luminescence. The synthesis was accomplished by metabolically doping diatom cells with neodymium and vanadium. Subsequently, the biomass of these doped diatoms was subjected to pyrolysis at 800 °C. The morphology, structure, and physicochemical properties of the doped diatom biomass as well as dried (SiO2/CaCO3/Corg/NdVO4NPs) and pyrolysed (SiO2/CaO/Corg/NdVO4NPs) samples were characterised using scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD), thermal analysis (TG), and fluorescence spectroscopy (FS). Studies have shown that the surface of diatom shells is covered with trigonal prismatic nanocrystallites (nanoparticles) of NdVO4 with dimensions of 30-40 nm, forming the crystallite clusters in the form of single-layer irregular flakes. The synthesised composites produced intense anti-Stokes fluorescent emission in the visible region under xenon lamp excitation in the near-infrared (λex = 800 nm) at room temperature in an ambient atmosphere. Such materials could be attractive for applications in solar spectrum conversion, optical sensing, biosensors, or photocatalysts.

Anti‐Stokes Luminescence in Multi‐Resonance‐Type Thermally‐Activated Delayed Fluorescence Molecules

AbstractPhoton‐upconversion in organic molecular systems is one of the promising technologies for future energy harvesting systems because these systems can generate excitons that possess higher energy than excitation energy. The photon‐upconversion caused by absorbing ambient heat as additional energy is particularly interesting because it could ideally provide a light‐driving cooling system. However, only a few organic molecular systems have been reported. Here, we report the anti‐Stokes photoluminescence (ASPL) derived from hot‐band absorption in a series of multi‐resonance‐type thermally‐activated delayed fluorescence (MR‐TADF) molecules. The MR‐TADF molecules exhibited an anti‐Stokes shift of approximately 0.1 eV with a high PL quantum yield in the solution state. The anti‐Stokes shift corresponded well to the 1–0 vibration transition from the ground state to the excited singlet state, and we further evaluated a correlation between the activation energy for the ASPL intensity and the TADF process. Our demonstration underlines that MR‐TADF molecules have become a novel class of ASPL materials for various future applications, such as light‐driving cooling systems.

Anti-Stokes Luminescence in Multi-Resonance-Type Thermally-Activated Delayed Fluorescence Molecules.

Photon-upconversion in organic molecular systems is one of the promising technologies for future energy harvesting systems because these systems can generate excitons that possess higher energy than excitation energy. The photon-upconversion caused by absorbing ambient heat as additional energy is particularly interesting because it could ideally provide a light-driving cooling system. However, only a few organic molecular systems have been reported. Here, we report the anti-Stokes photoluminescence (ASPL) derived from hot-band absorption in a series of multi-resonance-type thermally-activated delayed fluorescence (MR-TADF) molecules. The MR-TADF molecules exhibited an anti-Stokes shift of approximately 0.1 eV with a high PL quantum yield in the solution state. The anti-Stokes shift corresponded well to the 1-0 vibration transition from the ground state to the excited singlet state, and we further evaluated a correlation between the activation energy for the ASPL intensity and the TADF process. Our demonstration underlines that MR-TADF molecules have become a novel class of ASPL materials for various future applications, such as light-driving cooling systems.

Photoluminescence upconversion in monolayer WSe2 activated by plasmonic cavities through resonant excitation of dark excitons

Anti-Stokes photoluminescence (PL) is light emission at a higher photon energy than the excitation, with applications in optical cooling, bioimaging, lasing, and quantum optics. Here, we show how plasmonic nano-cavities activate anti-Stokes PL in WSe2 monolayers through resonant excitation of a dark exciton at room temperature. The optical near-fields of the plasmonic cavities excite the out-of-plane transition dipole of the dark exciton, leading to light emission from the bright exciton at higher energy. Through statistical measurements on hundreds of plasmonic cavities, we show that coupling to the dark exciton leads to a near hundred-fold enhancement of the upconverted PL intensity. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. We show that a precise nanoparticle geometry is key for a consistent enhancement, with decahedral nanoparticle shapes providing an efficient PL upconversion. Finally, we demonstrate a selective and reversible switching of the upconverted PL via electrochemical gating. Our work introduces the dark exciton as an excitation channel for anti-Stokes PL in WSe2 and paves the way for large-area substrates providing nanoscale optical cooling, anti-Stokes lasing, and radiative engineering of excitons.

Visible upconverted and NIR luminescence properties of Er3+ and Er3+/Yb3+-doped Lu3Sc2Ga3O12 nanocrystals

Trivalent erbium and ytterbium embedded in Lu3Sc2Ga3O12 nanocrystals in the garnet phase were synthesised employing the citrate sol-gel method. The crystalline phase and size, grain size, and effect of ion concentration and laser pump power on the photoluminescence properties of lanthanide ions were systematically characterised. XRD results showed that the present phosphor was in the cubic garnet phase with an average crystallite size of 27 nm and a lattice constant of 12.325 Å. Morphology images revealed the particles to be nearly spherical and agglomerated. Partial energy level diagrams have been constructed using diffused reflectance and excitation spectra. Stokes and anti-Stokes photoluminescence spectra were observed under 980 nm laser excitation. The near-infrared (NIR) photons (980 nm) were successfully converted into visible radiation. Upconverted luminescence spectra consisted of two strong peaks located in the green and red regions, corresponding to the 2H11/2,4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of the Er3+ ion. The intensity ratio of red to green emission bands increased with increasing Er3+ or Yb3+ concentrations as well as with the pump power of laser. Power-dependent photoluminescence spectra revealed that there were two photons involved in the upconversion mechanism. The decay profiles of the thermally coupled 2H11/2,4S3/2 levels of Er3+ showed a non-exponential nature accompanied by a shortening of lifetime as the concentration increased. CIE colour co-ordinates were located in the green region and shifted to the yellow region with increasing Yb3+ ion concentration. NIR emission spectra exhibited an intense peak at around 1535 nm, attributed to the 4I13/2 → 4I15/2 transition of Er3+ ion, the intensity of which was observed to increase with the incorporation of Yb3+ ions up to 5 mol%, confirming the existence of Yb3+-to-Er3+ energy transfer processes. Therefore, the present phosphors can be used for photovoltaic cells, solid-state light emitting diodes, and display devices.

Improved sensing of metal ions via abnormal enhancement of the anti-stokes emission of the near-infrared fluorescent probe

Sensing of toxic and heavy metals is of great importance for protecting environment and health monitoring. Here we demonstrate an improved fluorescent probe operating in the near-infrared (NIR). The probe represents a NIR tricarbocyanine dye with 2,2′-dipicolylamine meso-group which is able to chelate zinc, cobalt and palladium ions. We show that the probe operation via monitoring of the anti-Stokes photoluminescence provides much better sensitivity of the probe compared to its Stokes photoluminescence. Particularly, it is shown that the anti-Stokes photoluminescence increases by more than an order of magnitude and can reveal additional features of the dye-metal complex not observed via the Stokes emission.

Anti-Stokes Photoluminescence in Halide Perovskite Nanocrystals: From Understanding the Mechanism towards Application in Fully Solid-State Optical Cooling.

Anti-Stokes photoluminescence (ASPL) is an up-conversion phonon-assisted process of radiative recombination of photoexcited charge carriers when the ASPL photon energy is above the excitation one. This process can be very efficient in nanocrystals (NCs) of metalorganic and inorganic semiconductors with perovskite (Pe) crystal structure. In this review, we present an analysis of the basic mechanisms of ASPL and discuss its efficiency depending on the size distribution and surface passivation of Pe-NCs as well as the optical excitation energy and temperature. When the ASPL process is sufficiently efficient, it can result in an escape of most of the optical excitation together with the phonon energy from the Pe-NCs. It can be used in optical fully solid-state cooling or optical refrigeration.

Optical cooling at the optimal SPR angle of a glass–ITO–CdSe/ZnS(QDs) interface

In practice, cooling effects must dominate over heating associated with nonradiative decay and multiphonon emission to attain optical cooling in semiconductors. Although quantum confinement nanostructures (e.g., quantum dots [QDs]) have been used to reduce the self-absorption of anti-Stokes luminescence, their low quantum efficiency precludes high light extraction, thus reducing net cooling generation. Superficial plasmon resonance (SPR) generated in some metal–dielectric interfaces provides an efficient way to tune and enhance the optical properties and photoluminescence (PL) of QDs. In this work, the optimal SPR angle (θSPR) was used to enhance the PL of CdSe/ZnS core–shell QDs, and hence their cooling properties, in a glass–indium tin oxide–QD interface. Three excitation wavelengths (λexc = 405, 532, and 640 nm) were used to evaluate the cooling properties. The best results were obtained when λexc = 640 nm. A PL increment and temperature decrement (Δt) of 2-fold and 4 °C, respectively, were obtained at θSPR in 50 min due to the generation of anti-Stokes PL by the QDs and the enlargement of such generated PL because of the presence of the SPR effect at the interface.

Organic All-Photonic Artificial Synapses Enabled by Anti-Stokes Photoluminescence.

All-photonic synaptic devices with the merits of visible signals and high spatiotemporal resolution are promising to break the Von Neumann bottleneck. Although organic synapses outperform their inorganic counterpart for easy molecular modulation and lower energy consumption, the organic all-photonic artificial synapse has never been reported. Here, all-photonic synaptic characteristics were unprecedentedly observed in an organic semiconductor, (3,6-dimethyl-9H-carbazol-9-yl)(thiophen-2-yl) methanone (S2OC), with anti-Stokes photoluminescence. Impressively, the intensity of fluorescence from the higher excited state (S3) exhibited synaptic performance, which constantly increased with irradiation time through a channel composed of intersystem crossing, triplet-triplet annihilation, and energy transfer. More importantly, the relationship between the molecular structure and synaptic performance was established. Based on the synaptic photoplasticity property, noncontacted multilevel anticounterfeiting and imaging recognition were realized in all-photonic synapse arrays. This work provides a universal strategy for tuning the performances of organic synapses upon regulating the molecular structures, which paves the way for the application of organic semiconductors in artificial intelligence.

Dual-Modal Nanoplasmonic Light Upconversion through Anti-Stokes Photoluminescence and Second-Harmonic Generation from Broadband Multiresonant Metal Nanocavities.

Metal nanocavities can generate plasmon-enhanced light upconversion signals under ultrashort pulse excitations through anti-Stokes photoluminescence (ASPL) or nonlinear harmonic generation processes, offering various applications in bioimaging, sensing, interfacial science, nanothermometry, and integrated photonics. However, achieving broadband multiresonant enhancement of both ASPL and harmonic generation processes within the same metal nanocavities remains challenging, impeding applications based on dual-modal or wavelength-multiplexed operations. Here, we report a combined experimental and theoretical study on dual-modal plasmon-enhanced light upconversion through both ASPL and second-harmonic generation (SHG) from broadband multiresonant metal nanocavities in two-tier Ag/SiO2/Ag nanolaminate plasmonic crystals (NLPCs) that can support multiple hybridized plasmons with high spatial mode overlaps. Our measurements reveal the distinctions and correlations between the plasmon-enhanced ASPL and SHG processes under different modal and ultrashort pulsed laser excitation conditions, including incident fluence, wavelength, and polarization. To analyze the observed effects of the excitation and modal conditions on the ASPL and SHG emissions, we developed a time-domain modeling framework that simultaneously captures the mode coupling-enhancement characteristics, quantum excitation-emission transitions, and hot carrier population statistical mechanics. Notably, ASPL and SHG from the same metal nanocavities exhibit distinct plasmon-enhanced emission behaviors due to the intrinsic differences between the incoherent hot carrier-mediated ASPL sources with temporally evolving energy and spatial distributions and instantaneous SHG emitters. Mechanistic understanding of ASPL and SHG emissions from broadband multiresonant plasmonic nanocavities marks a milestone toward creating multimodal or wavelength-multiplexed upconversion nanoplasmonic devices for bioimaging, sensing, interfacial monitoring, and integrated photonics applications.

Size-Dependent Phonon-Assisted Anti-Stokes Photoluminescence in Nanocrystals of Organometal Perovskites.

Anti-Stokes photoluminescence (ASPL), which is an up-conversion phonon-assisted process of the radiative recombination of photoexcited charge carriers, was investigated in methylammonium lead bromide (MALB) perovskite nanocrystals (NCs) with mean sizes that varied from about 6 to 120 nm. The structure properties of the MALB NCs were investigated by means of the scanning and transmission electron microscopy, X-ray diffraction and Raman spectroscopy. ASPL spectra of MALB NCs were measured under near-resonant laser excitation with a photon energy of 2.33 eV and they were compared with the results of the photoluminescence (PL) measurements under non-resonant excitation at 3.06 eV to reveal a contribution of phonon-assisted processes in ASPL. MALB NCs with a mean size of about 6 nm were found to demonstrate the most efficient ASPL, which is explained by an enhanced contribution of the phonon absorption process during the photoexcitation of small NCs. The obtained results can be useful for the application of nanocrystalline organometal perovskites in optoelectronic and all-optical solid-state cooling devices.

Reduced photothermal heating in diamonds enriched with H3 point defects

Solid-state laser refrigeration of semiconductors remains an outstanding experimental challenge. In this work, we show that, following excitation with a laser wavelength of 532 nm, bulk diamond crystals doped with H3 centers both emit efficient up-conversion (anti-Stokes) photoluminescence and also show significantly reduced photothermal heating relative to crystals doped with nitrogen–vacancy (NV) centers. The H3 center in diamond is a highly photostable defect that avoids bleaching at high laser irradiances of 10–70 MW/cm2 and has been shown to exhibit laser action, tunable over the visible band of 500–600 nm. The observed reduction of photothermal heating arises due to a decrease in the concentration of absorbing point defects, including NV-centers. These results encourage future exploration of techniques for H3 enrichment in diamonds under high-pressure, high-temperature conditions for the simultaneous anti-Stokes fluorescence cooling and radiation balanced lasing in semiconductor materials. Reducing photothermal heating in diamond through the formation of H3 centers also opens up new possibilities in quantum sensing via optically detected magnetic resonance spectroscopy at ambient conditions.

Anti-Stokes photoluminescence from CsPbBr3 nanostructures embedded in a Cs4PbBr6 crystal

Lead halide perovskites possess high photoluminescence (PL) efficiency and strong electron-phonon interactions, and therefore the optical cooling using up-conversion PL has been expected. We investigate anti-Stokes PL from green-luminescent Cs$_4$PbBr$_6$, whose origin is attributable to CsPbBr3 nanostructures embedded in a Cs$_4$PbBr$_6$ crystal. Because of the high transparency, low refractive index, and high stability of Cs$_4$PbBr$_6$, the green PL displays high external quantum efficiency without photo-degradation. Time-resolved PL spectroscopy reveals the excitonic behaviors in recombination process. The shape of the PL spectrum is almost independent of excitation photon energy, which means that the spectral width is determined by homogeneous broadening. We demonstrate that the phonon-assisted process dominates the Urbach tail of optical absorption and anti-Stokes PL at room temperature. Anti-Stokes PL is observed down to 70 K. We determine the temperature dependence of the Urbach energy and estimate the strength of the electron-phonon coupling. Our spectroscopic data show that CsPbBr3 nanostructures have potentially useful features for optical cooling.

Anti-Stokes Photoluminescence Regulated by Lattice Distortion of CsPbX3 Nanocrystals in Glasses

Anti-Stokes Photoluminescence Regulated by Lattice Distortion of CsPbX₃ Nanocrystals in Glasses

Yb-doped Y–Al–O thin films with a self-organized columnar structure and their anti-Stokes photoluminescence properties

We fabricated ytterbium-doped yttrium aluminum oxide (Yb:Y–Al–O) thin films by radio-frequency magnetron sputtering and evaluated their crystallinity and anti-Stokes photoluminescence (PL) properties for optical refrigeration. The Yb:Y–Al–O films that were grown on c-sapphire substrates had better transparency than the films deposited on fused-quartz substrates. The better transparency is considered to be a result of the smaller mismatch between the thermal expansion coefficients of Yb:Y–Al–O and c-sapphire. We found that the thin films on the c-sapphire substrates consist of densely packed sub-micron columnar crystals that are aligned perpendicular to the substrate. In these films, we confirmed the existence of perovskite, garnet, and monoclinic phases despite using a single-phase sputtering target. The excitation wavelength dependence of anti-Stokes PL is used to investigate the energy transfer process between trivalent Yb ions in neighboring columnar crystals. The data indicate that the resonant energy transfer from Yb3+ ions at a specific seven-coordinated site of the monoclinic phase to Yb3+ ions in neighboring columnar crystals is faster than the radiative relaxation at the energy-donor site.

Anti-Stokes photoluminescence of perovskite CsPbBr-=SUB=-3-=/SUB=- nanocrystals in a fluorophosphate glass matrix

The anti-Stokes photoluminescence (PL) of perovskite CsPbBr3 nanocrystals in a fluorophosphate glass matrix has been found and experimentally studied upon optical excitation to the low-energy edge of the photoluminescence band. The intensity of anti-Stokes PL depends linearly on the pumping power and increases rapidly with increasing temperature. A simple three-level model is proposed that describes well the main regularities of the observed phenomenon. Keywords: perovskites, nanocrystals, anti-Stokes photoluminescence, CsPbBr3, fluorophosphate glass.

Optical modulation of anti-stokes photoluminescence of the Y-=SUB=-2-=/SUB=-O-=SUB=-2-=/SUB=-S : Er

The luminescence and excitation spectra of visible photoluminescence of Y2O2S : Er ceramics (1 at.%) at different optical pumping have been studied. It is shown that the anti-Stokes luminescence of single crystals doped with erbium has a high intensity when pumping the metastable state 4I13/2 of impurities. The resonant wavelengths of IR-photons for two-photon excitation of the visible glow Y2O2S : Er. are determined. A method of photooptical modulation of anti-Stokes luminescence has been developed, which makes it possible to determine the nature of the corresponding electronic transitions inside the excited glow centers. Keywords: Luminescence, excitation, photooptical modulation.

Impact of the thermal properties of the environment on the hot-band absorption-assisted single-photon upconversion.

Single-photon hot-band absorption-assisted anti-Stokes photoluminescence (ASPL) is a non-equilibrium process which causes a local cooling effect and which therefore is accompanied by a reverse heat flux from the warmer environment. Here, we demonstrate that the thermal properties of the medium, i.e., its thermal conductivity and specific heat capacity, play a significant role in driving the ASPL of an emitter placed in it. Exploiting seven different solvents and the near-infrared tricarbocyanine dye as a single-photon upconverter, we show an obvious correlation of activation energy and the quantum yield (QY) of ASPL with the solvent thermal conductivity and specific heat capacity, respectively. A linear fit of the above correlations leads to the predictions that the maximum QY of ASPL should be observed in a vacuum where it should reach a value of ∼10% for the exploited dye, which is close to the QY of its Stokes emission and that the high thermal conductivity of the solvent assists in a stepwise population of the hot band, which thus facilitates the hot-band absorption-assisted ASPL. These findings lead to the conclusion that the selection of a solvent with appropriate thermal properties may help one to control the efficiency of the one-photon energy upconversion.

Effect of Nanocrystal Size on Anti-Stokes Photoluminescence in Lead Halide Perovskites

Effect of Nanocrystal Size on Anti-Stokes Photoluminescence in Lead Halide Perovskites