Low Phonon Energy Research Articles

Up-Conversion Photoluminescence of Sol–Gel Derived CaY2O4 Powders Under 980 nm Excitation

Up-conversion is an anti-stokes process that can convert near-infrared light into visible light. Besides the requirement of high optical conversion efficiency, the thermochemical stability of the host materials is critical for in practical applications, and a stable oxide host material is optimal. CaY₂O₄ follows the same ordered structure of CaFe₂O₄, which is composed of an (R₂O₄)2- (R = rare earth metal) framework of double octahedral moieties with rare earth ions residing within the framework. CaY₂O₄ is a promising host material due to its favorable characteristics such as high chemical and thermal stability, low-phonon energy, and environment friendliness. Rare earth ion (Er3+ and Yb3+)-doped nano-crystalline phosphors were prepared by a sol-gel process. The synthesized sample was characterized by X-ray diffraction analysis and the crystallite size was confirmed by Scherrer's formula and transmission electron microscopy. The surface morphology of the powders was determined by scanning electron microscopy. X-ray diffraction patterns confirmed their pure orthorhombic structure after annealing at 1,200 °C and the morphology of particles was found to be a nearly spherical shape with a diameter of the order of ~100 nm. Photoluminescence properties of the powders were measured by exciting the samples with a 980 nm diode laser at room temperature. Under the 980-nm laser excitation, the green and red up-conversion emissions were observed at around 520- 540 nm, 540-570 nm and 640-680 nm, which, are attributed to the transitions of ²H11/2 →⁴I15/2, ⁴S3/2 →⁴I15/2 and ⁴F9/2 →⁴I15/2 of Er3+ ions, respectively. The up-conversion intensity as a function of laser power shows that the up-conversion mechanism corresponding to green and red emissions occurs via a two-photon process.

Atmospheric plasma-assisted modification of nanosized LiYF4:Eu3+ with gold nanoparticles

Both gold and rare-earth doped fluoride nanoparticles have been drawing interest for their possible biological applications, for therapy or for bioimaging. Gold nanoparticles can be used as a heat source for photothermal therapy, absorbing energy through its surface plasmon resonance (SPR) band and converting it to heat, whereas the low phonon energy, long emission lifetimes and sharp emission lines of the fluoride nanoparticles can circumvent several drawbacks of other used materials for bioimaging. In this context, this article proposes a modification of nanosized europium doped-LiYF4 with gold nanoparticles using a helium atmospheric plasma to reduce Au3+ to form the gold nanoparticles. In this article, we successfully synthesized citrate stabilized gold nanoparticles assisted by a helium atmospheric plasma, which showed a broad size distribution between 30 and 109 nm and different shapes, and a SPR band at 544 nm. We also successfully modified LiYF4:Eu3+ with gold nanoparticles with the atmospheric plasma jet, which presented a SPR band at 539 nm and significantly suppressed europium luminescence possibly due to the absorption of the emitted photons of the Eu3+ by the gold SPR band.

Tm3+/Ho3+co-doped germanate glass and double-clad optical fiber for broadband emission and lasing above 2 µm

In this paper, a 2 µm broadband emission under 796 nm laser diode excitation in low phonon energy GeO2-Ga2O3-BaO glass system is co-doped with 0.7Tm2O3/(0.07-0.7)Ho2O3 (mol%). The widest emission band (where the Tm3+ → Ho3+ energy transfer efficiency is 63%) was obtained for 0.7Tm2O3/0.15Ho2O3 co-doped glass from which a double-clad optical fiber was realized and investigated. Optimization of Tm3+/Ho3+ concentration enabled the acquisition of broadband amplified spontaneous emission (ASE) in double-clad optical fiber with a full width at half maximum (FWHM): 377 nm and 662 nm for 3 dB and 10 dB bandwidth, respectively. ASE spectrum is a result of the superposition of (Tm3+: 3H4 →Η3F4) 1.45 µm, (Tm3+: 3F4 → 3H6) 1.8 µm and (Ho3+:5I7 → 5I8) 2 µm emission bands. Hence, highly rare-earth co-doped germanate glass is characterized by a remarkably broader ASE spectrum than silica and tellurite fibers showed promising lasing properties for their further application in tunable and dual wavelength lasers.

Sensitization effect between Ln3+ ions in zinc fluoride glasses for MIR applications

A system of zinc fluoride glasses were synthesized to analyse the sensitization mechanism between doped Ln3+ ions under 808 or 980 nm excitation. Differential scanning calorimetry (DSC) curve and Raman spectra indicate the favourable thermal stability and a low maximum phonon energy of the host. Judd-Ofelt (J-O) theory together with the absorption spectra were utilized to compute the Ωt for predicting radiative features of each sample which coincide with the later measured emission spectra. Broadband emission ranging from 2400 to 3300 nm with a half-width of 361 nm in Er3+/Dy3+ codoped zinc fluoride glasses under 980 nm pumping has been investigated, which may be applicated in mid-infrared fiber amplifier and broadband tunable lasers field. On the other hand, the efficient sensitization effect mechanism between Er3+ and Dy3+ around 3 μm band with an extremely high energy transfer efficiency Ƞ (96.1%) of Er3+:4I13/2→Dy:6H11/2 has been researched under 808 nm excitation. Hence, Er3+ and Dy3+ doped zinc fluoride glasses may be potential optical candidates of great development foreground in the fields of mid-infrared applications.

Sub-10 nm lanthanide-doped SrFCl nanoprobes: Controlled synthesis, optical properties and bioimaging

Alkaline-earth dihalide nanocrystals (NCs) such as SrFCl, owing to their high chemical stability and low phonon energy, are excellent host materials for lanthanide (Ln3+) doping to achieve desirable optical properties for various bioapplications. Herein, we report a novel strategy for the synthesis of sub-10 nm Ln3+-doped SrFCl NCs with efficient upconverting and downshifting luminescence through a facile one-step hot-injection method. Utilizing the temperature-dependent upconverting luminescence (UCL) from the thermally coupled 2H11/2 and 4S3/2 levels of Er3+, we showed the potential of SrFCl:Yb,Er NCs as sensitive UCL nanoprobes for non-contact thermal sensing with a maximum detection sensitivity of 0.0066 K–1, which is among the highest values for thermal sensing based on Er3+-activated UCL nanoprobes. Furthermore, by employing the intense downshifting luminescence from Tb3+ and Eu3+, we demonstrated the successful use of biotinylated SrFCl:Ce,Tb and SrFCl:Eu3+ nanoprobes for biotin receptor-targeted cancer cell imaging, thus revealing the great promise of SrFCl:Ln3+ nanoprobes for versatile bioapplications.

Exploration of the new tellurite glass system for efficient 2 μm luminescence

A new TeO2-Ga2O3-BaO (TGB) glass system has been explored to achieve high-efficient luminescence at 2 μm region. The glasses are characterized by wide infrared transmittance range, excellent thermal stability, high density and refractive index, and low maximum phonon energy. Judd-Ofelt analysis reveals high spontaneous emission probabilities of Tm3+ and Ho3+ in this glass host. As a result, intense emissions near 1.8 and 2 μm were realized in the Tm3+-doped and Nd3+/Ho3+ co-doped TGB glasses, respectively. The emission cross-sections of Tm3+ and Ho3+ are 1.024 × 10−20 cm2 and 0.829 × 10−20 cm2 with maximum gain coefficients of 4.51 and 1.55 cm−1, respectively. The combined results indicate that the TGB glass system is a potential host for obtaining high-efficient 2 μm luminescence.

BiVO4 as highly efficient host for near-infrared to visible upconversion

BiVO4 is a well-known visible-light-active semiconductor photocatalyst. Here, we report BiVO4 as a highly efficient host material for near-infrared (NIR) to visible frequency upconversion. Various lanthanide ions (Yb, Er, and Tm) doped BiVO4 upconversion phosphors were successfully synthesized through a facile microwave hydrothermal method. As-synthesized samples exhibit intense green, red, and NIR upconversion emissions under 980 nm excitation at power density as low as 0.02 W cm−2. The quantum efficiency for Yb, Er, and Tm triply doped BiVO4 under 2.0 W cm−2 was as high as 2.19% in the 500–850 nm range. Moderately low phonon energy (824 cm−1) of BiVO4 was found to be responsible for the existence of long-lived metastable energy levels leading to efficient upconversion emissions. Potential application of the material for producing latent images and security printing has been demonstrated. The high quantum efficiency of BiVO4 offers an opportunity to develop alternative upconversion materials based on the BiVO4 host for various optical applications.

Origin of Low Carrier Mobilities in Halide Perovskites

Halide perovskites constitute a new class of semiconductors that hold promise for low-cost solar cells and optoelectronics. One key property of these materials is the electron mobility, which determines the average electron speed due to a driving electric field. Here we elucidate the atomic-scale mechanisms and theoretical limits of carrier mobilities in halide perovskites by performing a comparative analysis of the archetypal compound CH3NH3PbI3, its inorganic counterpart CsPbI3, and a classic semiconductor for light-emitting diodes, wurtzite GaN, using cutting-edge many-body ab initio calculations. We demonstrate that low-energy longitudinal-optical phonons associated with fluctuations of the Pb–I bonds ultimately limit the mobility to 80 cm2/(V s) at room temperature. By extending our analysis to a broad class of compounds, we identify a universal scaling law for the carrier mobility in halide perovskites, and we establish the design principles to realize high-mobility materials.

Люминофор ближнего и коротковолнового ИК-диапазона на основе ниобата литий-лантана со структурой кубического граната

AbstractCubic garnets activated by neodymium and holmium, Li_6CaLa_2Nb_2O_12:Nd^3+,Ho^3+, were obtained by solid-state method. The main regularities of changes in the luminescence properties of Li_6C-aLa_2Nb_2O_12:Nd^3+,Ho^3+ solid solutions in the visible, near-IR, and short-wave IR ranges under 808 nm laser diode radiation are determined. The energy transfer mechanism between active centers, involving the participation of Nd^3+ ions as sensitizers for the luminescence of Ho^3+ ions, is proposed. Low phonon energy, high luminescence intensity in the range of 2.0–3.0 μm, and weak up-conversion luminescence in the region of 450–780 nm make it possible to consider the cubic lithium–lanthanum niobates Li_6CaLa_2Nb_2O_12:Nd^3+,Ho^3+ as promising phosphors of the short-wave IR range.

Unveiling lasing mechanism in CsPbBr3 microsphere cavities.

Recently, the light-matter interaction of perovskite microcavities has been widely explored for its great potential in low-threshold lasing devices. However, the mechanism of perovskite lasing remains unclear to date. In this study, we demonstrated high-quality single-mode excitonic lasing in CsPbBr3 microspheres, providing an ideal platform to study the underlying physics of lasing behavior. We show that the lasing mechanism shifts from the exciton-exciton scattering to the exciton-phonon scattering with the increase in temperature from 77 to 300 K, which was verified by temperature-dependent photoluminescence (PL), time-resolved photoluminescence (TRPL) as well as temperature-dependent Raman spectroscopy. Furthermore, by analyzing PL line width broadening with varied temperatures, we found that two different phonon modes were involved in the exciton-phonon scattering process. The scattering from the low-energy phonon (∼8.6 meV) is the dominant source of exciton-phonon coupling in the intermediate temperature range (77 to 230 K), while the high-energy phonon (∼15.3 meV) dominates from 230 K to room temperature. These results confirm the lasing mechanism in such perovskite-based micro/nano-cavities and significantly influence the development of future low-threshold lasers.

Third Order Anharmonic Effects in Many-Body Nuclear Quantum Theory

The quantum theory of third order anharmonic effects in the photon production amplitude developed by V.A. Khodel has been generalized for nuclei with pairing and for a more accurate description of low-energy collective phonons. An expression for the transition amplitude between two- and one-phonon states has been obtained. This expression has been compared to the solution of a similar problem considered within the quasiparticle–phonon model. It has been shown that this approach describes a number of new effects, including three- and four-quasiparticle correlations in the ground state (backward-going graphs).

Fluorinated graphdiyne as a significantly enhanced fluorescence material.

The chemical modification of graphdiyne (GDY) using light elements is a possible route to regulate its unique structure and optoelectronic properties. In this paper it is shown that directly heating a mixture of xenon difluoride and GDY produces partially fluorinated GDY with covalent C–F bonding and localized sp2-carbon hybridization because of the breaking of the acetylenic bond. It is seen that the fluorescence of GDY is significantly enhanced because of the fluorine doping. All the fluorinated GDYs with different doping ratios of fluorine exhibit photoluminescence from bright blue to green when the excitation wavelength varies from 260 nm to 480 nm. In addition, the doped GDY with 15.2% fluorine doping shows a strong photoluminescence and the quantum efficiency is 3.7%. The enhanced fluorescence is considered to be induced by defect states because of the doping of fluorine, suggesting its potential applications in luminescence devices, such as biological sensing and flexible light-emitting diodes.

Using nanotubes to study the phonon spectrum of two-dimensional materials.

We propose a convenient method to characterize the acoustic phonon branches of 2D monolayer materials using measurements of the infrared- and Raman-active vibrational modes of nanotubes. The relations we employ are derived from a symmetry analysis based directly on the line groups of nanotubes. We perform extensive ab initio calculations for the MoS2 monolayer and nanotubes to evaluate the method and illustrate all our results. Specifically, we show how the low-energy phonon transmission, a determining factor in thermal transport, can be easily and successfully reconstructed by this procedure.

Phonon confinement and spin-phonon coupling in tensile-strained ultrathin EuO films.

Reducing the material sizes to the nanometer length scale leads to drastic modifications of the propagating lattice excitations (phonons) and their interactions with electrons and magnons. In EuO, a promising material for spintronic applications in which a giant spin-phonon interaction is present, this might imply a reduction of the degree of spin polarization in thin films. Therefore, a comprehensive investigation of the lattice dynamics and spin-phonon interaction in EuO films is necessary for practical applications. We report a systematic lattice dynamics study of ultrathin EuO(001) films using nuclear inelastic scattering on the Mössbauer-active isotope 151Eu and first-principles theory. The films were epitaxially grown on YAlO3(110), which induces a tensile strain of ca. 2%. By reducing the EuO layer thickness from 8 nm to a sub-monolayer coverage, the Eu-partial phonon density of states (PDOS) reveals a gradual enhancement of the number of low-energy phonon states and simultaneous broadening and suppression of the peaks. These deviations from bulk features lead to significant anomalies in the vibrational thermodynamic and elastic properties calculated from the PDOS. The experimental results, supported by first-principles theory, unveil a reduction of the strength of the spin-phonon interaction in the tensile-strained EuO by a factor of four compared to a strain-free lattice.

Influence of alkaline earth oxides on Eu3+ doped lithium borate glasses for photonic, laser and radiation detection material applications

50Li2O - 20MO - (30-x)B2O3 - xEu3O3 where (M = Mg, Ca, Sr, Ba) and (x = 0, 0.1, 0.3, 0.5, 1.0, 3.0, 5.0, 7.0 glasses were prepared and characterized to study their physical & optical properties using spectroscopic measurements such as UV-Visible spectra, Luminescence spectra, Radio-Luminescence, X-Ray Luminescence etc. The electronic transitions in the UV-VIS and NIR regions are assigned to the 7F0 → 5L6, 7F0 → 5D2, 7F0 → 5D1, 7F0 → 7F6 and 7F1 → 7F6 levels centered at 394, 465, 532, 2092 and 2202 nm due to their transitions from 7F0 ground state to various excited states of Eu3+ ions, respectively. Structural studies were analyzed using Raman Spectra. Five transition bands of luminescence spectra have been observed by using an excited wavelength of 394 nm. With five emission transitions of 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3 and 5D0 → 7F4 as was reported previously in literature at 578, 591, 591, 614, 653 and 702 nm, respectively. The lower phonon energy (∼971 cm−1) and higher phonon side band intensity is observed at 5.0 mol% Eu2O3 content doped LBaB glasses while higher phonon energy (∼1176 cm−1) and lower phonon intensity observed for 5.0 mol% Eu2O3 content doped LMgB glasses. The chromaticity coordinates of Eu3+ ions in all LiMOB glasses falls in the reddish orange region. The radioluminescence (RL), emission spectra were corresponding to those from PL measurements. From RL measurement, the integral scintillation efficiency of developed glass was determined at 11.2% when compared with BGO crystal.

Effect of hydration and ammonization on the thermal expansion behavior of ZrW2O8 : Ab initio lattice dynamical perspective

The hydration and ammonization of ZrW2O8 is known to lead to positive and negative thermal expansion behaviour respectively. We report ab-initio calculations to understand this anomalous behaviour. We identify the crucial low energy phonon modes involving translations, rotations and distortions of WO4 and ZrO6 polyhedra, which lead to NTE in ZrW2O8 in pure and ammoniated forms; however, the rotation and distortion motions get inhibited on hydration and lead to positive thermal expansion. We demonstrate that the thermal expansion coefficient could be tailored by engineering the phonon dynamics of a material.

Ultrapure NIR-to-NIR single band emission of β-PbF2: Yb3+/Tm3+ in glass ceramics

An ultrapure near infrared (NIR) emission entered at 800 nm upon 976 nm laser excitation (NIR-to-NIR) was observed in β-PbF2: Yb3+/Tm3+ glass ceramics (GCs). The ultrapure NIR-to-NIR single band emission originated from the 3H4 → 3H6 transition of Tm3+ with the ratio of NIR emission, and a blue band up to 9633 was obtained. Through detailed analysis of the crystal structure and the energy transfer between Tm3+ and Yb3+, it is elucidated that the host lattice with low phonon energy cooperative with the energy mismatch results in a dramatic population of the 3H4 state as well as ultrapure NIR-to-NIR single band emission. Moreover, up-conversion (UC) emission properties and the decay lifetimes of β-PbF2: Yb3+/Tm3+ in GCs were explored thoroughly. The results illustrate that efficient cross relaxation (CR) processes between Tm3+ generate the energy redistribution in UC emission spectra, further concentrating energy to NIR emission. These two issues can be treated as crucial factors on ultrapure NIR-to-NIR single band emission in β-PbF2: Yb3+/Tm3+ in GCs. The ultrapure NIR-to-NIR single band emission through 976 nm laser excitation is advantageous for enhancing resolution and has potential application in bio-imaging fields.

Comparative study of praseodymium additives in active selenide chalcogenide optical fibers

The choice of rare earth additive when doping chalcogenide glasses can affect their mid-infrared fiber performance. Three praseodymium additives, Pr-foil, PrCl3 and PrI3 are investigated in Ge-As-Ga-Se fibers. All the fibers are X-ray amorphous and the Pr(foil)-doped fiber has the lowest overall optical loss. Pumping at 1550 nm wavelength, the Pr3+-doped fibers exhibit photoluminescence across a 3.5 to 6 μm span; photoluminescence lifetimes are 10 ms for 3H5→3H4 and 2-3 ms for (3H6, 3F2)→3H5 transitions. A fast 0.21 ms decay for (3F3, 3F4)→3H6 is observed only in the PrCl3-doped fiber due to a lower phonon energy local environment of Pr3+ ions.

Efficient improvement of 2.7 μm luminescence of Er3+:oxyfluoride glass containing gallium by Yb3+ ions codoping

Mid-infrared laser materials with low phonon energy have significant applications. However, the development of available glass systems for high-power laser gain medium have posed a great challenge. Therefore, we investigated the 2.7 μm spectroscopic properties of Er3+/Yb3+-codoped oxyfluoride glass containing gallium, which were prepared by typically melting and quenching methods. The 2.7 μm luminescence properties of the Er3+/Yb3+-codoped oxyfluoride glass containing gallium were recorded by a 980 nm laser diode. The Judd–Ofelt parameters, decay curves, emission cross section, energy transfer efficiency and quantum efficiency were calculated. The maximum emission cross section of YbFGa-0.5 is 1.63 × 10−20 cm2 by 980 nm excitation. The energy transfer efficiency is calculated to be 77.8% for the YbFGa-0.5 glass around 2700 nm. The quantum efficiency at 1530 nm is 65.6%. The result reveals that the best doping concentration ratio of Er3+:Yb3+ ions is 1:0.5, and suggests an effective energy transfer from Yb3+ to Er3+ ions.

Acoustic phonon lifetimes limit thermal transport in methylammonium lead iodide

Hybrid organic-inorganic perovskites (HOIPs) have become an important class of semiconductors for solar cells and other optoelectronic applications. Electron-phonon coupling plays a critical role in all optoelectronic devices, and although the lattice dynamics and phonon frequencies of HOIPs have been well studied, little attention has been given to phonon lifetimes. We report high-precision momentum-resolved measurements of acoustic phonon lifetimes in the hybrid perovskite methylammonium lead iodide (MAPI), using inelastic neutron spectroscopy to provide high-energy resolution and fully deuterated single crystals to reduce incoherent scattering from hydrogen. Our measurements reveal extremely short lifetimes on the order of picoseconds, corresponding to nanometer mean free paths and demonstrating that acoustic phonons are unable to dissipate heat efficiently. Lattice-dynamics calculations using ab initio third-order perturbation theory indicate that the short lifetimes stem from strong three-phonon interactions and a high density of low-energy optical phonon modes related to the degrees of freedom of the organic cation. Such short lifetimes have significant implications for electron-phonon coupling in MAPI and other HOIPs, with direct impacts on optoelectronic devices both in the cooling of hot carriers and in the transport and recombination of band edge carriers. These findings illustrate a fundamental difference between HOIPs and conventional photovoltaic semiconductors and demonstrate the importance of understanding lattice dynamics in the effort to develop metal halide perovskite optoelectronic devices.