Multiphonon Relaxation Processes Research Articles

Enhanced the quantum efficiency of Tm3+ ∼2 μm laser in crystal through an energy transfer process of 3H5 level

The concentration dependence of the fluorescence and lifetime evolutions of states relevant to three near-infrared emission bands were measured and analyzed for Tm:SrF2 crystals. The fast quenching of ∼1.2 μm fluorescence (3H5) was observed as the Tm concentration increased. The cross-relaxation and multi-phonon relaxation processes between Tm3+ ions cannot explain this phenomenon. Through spectral and rate-equation analysis, two possible energy transfer mechanisms, cross-relaxation from the 3H5 level and cooperative energy transfer within Tm3+ clusters, are proposed. And based on the new modified model, the quantum efficiency of the Tm ∼2 μm lasers could surpass the traditional 200% limit. This new energy transfer has the potential for exceptionally high efficiency excitation of Tm-doped laser materials.

The impact of high pressure on the number of photons involved in upconversion luminescence of NaYF4:Er3+@NaYF4 and NaYF4:Yb3+,Er3+@NaYF4 core@shell nanoparticles and application as contactless pressure sensor

Inorganic nanomaterials doped with lanthanide ions and exhibiting upconversion (UC) luminescence are leading in research of nonlinear optical materials. Extreme conditions, such as high-pressure compression (in the GPa range), may influence the physicochemical properties of various compounds. In this work, we investigated the optical properties of upconverting NaYF4:Er3+@NaYF4 and NaYF4:Yb3+,Er3+@NaYF4 core@shell nanoparticles (NPs), focusing on the impact of high pressure on the number of photons involved in the UC mechanism. We measured the UC emission spectra of the synthesized NPs under high-pressure conditions, which allowed us to observe the pressure-induced intensity changes, band centroids spectral shifts, and band intensity ratios alterations. Importantly, by measuring the power-dependent UC emission spectra as a function of pressure, we determined and analyzed the number of photons (n) involved in the UC processes for the first time. It turned out that the n value changes with pressure as a result of diverse multi-phonon relaxation processes enhanced under high-pressure conditions. The results show potential adjustment of UC mechanisms with the use of high-pressure. The calculated band intensity ratios were also used for pressure sensing applications, resulting in the relative sensitivity reaching almost ≈20 % GPa−1 for the Er3+-doped core@shell NPs.

Dy3+,Mn4+ co-doped phosphors for synergistic luminescent dual-mode thermometer and high-resolution imaging

Fluorescent temperature measurement has become a research hotspot due to its characteristics of non-contact, miniaturization, and fast response. However, it remains a key challenge to realize excellent sensitivity and high resolution. Herein, a series of La2MgTiO6:Dy3+,Mn4+ phosphors are developed and the related energy transfer between Dy3+→Mn4+ is discovered. The coupling distance between Mn4+ and adjacent ligands is greatly affected by temperature. However, the energy differences between the emission energy levels and lower energy levels of Dy3+ hinder the multi-phonon relaxation process, which moderates the thermal quenching rate of Dy3+. Therefore, highly sensitive FIR thermometers are built according to the significant discrimination temperature dependence of Dy3+ and Mn4+. The maximum Sa and Sr reach 0.022 K−1(@563 K) and 2.622 %K−1(@544 K). In contrast, a thermometer based on the emission lifetime of Mn4+ is fabricated, which is extremely sensitive to temperature. Interestingly, Dy3+ can effectively promote the quenching of Mn4+, and then improve the sensitivity. The maximum Sr enhances from 1.621 %K−1(@503 K) to 2.305 %K−1 (@480 K). The high sensitivity and resolution of the dual-mode fluorescence thermometry have exceeded most of the current optical thermal measurement materials. Besides, the thermochromic properties presented by the designed phosphors can be combined with PDMS films to qualitatively evaluate the ambient temperature.

Enthalpy-entropy compensation in the slow Arrhenius process.

The Meyer-Neldel compensation law, observed in a wide variety of chemical reactions and other thermally activated processes, provides a proportionality between the entropic and the enthalpic components of an energy barrier. By analyzing 31 different polymer systems, we show that such an intriguing behavior is encountered also in the slow Arrhenius process, a recently discovered microscopic relaxation mode, responsible for several equilibration mechanisms both in the liquid and the glassy state. We interpret this behavior in terms of the multiexcitation entropy model, indicating that overcoming large energy barriers can require a high number of low-energy local excitations, providing a multiphonon relaxation process.

Revisiting the luminescence properties of Er3+, Tm3+ and Ho3+ doped Bi4Ge3O12 rod-type crystal fibers for the laser application

BGO single crystals doped with Er3+, Tm3+ and Ho3+ rare-earth ions were prepared in the form of rod-type crystal fibers via the micro-pulling down technique to perform an extensive investigation of their visible, near- and mid-infrared luminescence properties and to really decide on their interest as solid-state laser media in the various spectral domains. Revisiting and completing the absorption spectra already available in the literature led to the derivation of more reliable “effective” radiative lifetimes and branching ratios. Registration of the fluorescence decays of the most important emitting levels versus dopant concentrations led for the first time to an unambiguous determination of “intrinsic” emission lifetimes – corresponding to negligible contributions from inter-ionic energy-transfers - and non-radiative multiphonon relaxation rates. Such data also allowed for the determination of the dominant phonon energy which comes into play in the electron-phonon coupling responsible for these multiphonon relaxation processes and for its lattice vibration origin, i.e. a high phonon energy of the order of 820 cm−1 corresponding to the coupling between bending vibrations of (GeO4)4- tetrahedra and vibrations corresponding to motions of these (GeO4)4- tetrahedra against the Bi3+ ions for which the rare-earth dopant ions substitute. Using the above data and different methods for the calibration of the registered emission spectra in cross section unit and for the derivation of gain cross section curves finally show that mid-infrared emissions of Er:BGO, Tm:BGO and Ho:BGO around 1.55 μm, 1.9 μm and 2 μm, respectively, occur with comparable gain cross sections as in the most important reference mid-infrared laser materials, although over slightly shifted thus complementary spectral domains. It is shown that potentialities also exist with Tm:BGO in the blue, deep-red and near-infrared spectral domains around 470 nm, 800 nm and 1.45 μm, by pumping the crystals with conventional pump sources and using some suitable co-dopants to avoid detrimental self-terminating processes.

Ligand Fluorination to Mitigate the Raman Relaxation of DyIII Single-Molecule Magnets: A Combined Terahertz, Far-IR and Vibronic Barrier Model Study.

The fast Raman relaxation process via a virtual energy level has become a puzzle for how to chemically engineer single-molecule magnets (SMMs) with better performance. Here, we use the trifluoromethyl group to systematically substitute the methyl groups in the axial position of the parent bis-butoxide pentapyridyl dysprosium(III) SMM. The resulting complexes-[Dy(OLA )2 py5 ][BPh4 ] (LA =CH(CF3 )2 - 1, CH2 CF3 - 2, CMe2 CF3 - 3)-show progressively enhanced TB hys (@100 Oe s-1 ) from 17 K (for 3), 20 K (for 2) to 23 K (for 1). By experimentally identifying the varied under barrier relaxation energy in the 5-500 cm-1 regime, we are able to identify that the C-F bond related vibration energy of the axial ligand ranging from 200 to 350 cm-1 is the key variant for this improvement. Thus, this finding not only reveals a correlation between the structure and the Raman process but also provides a paradigm for how to apply the vibronic barrier model to analyze multi-phonon relaxation processes in lanthanide SMMs.

Manipulating the singular red emission in GdOF:Yb/Er nanosheets via defect engineering

Efficient tailoring of red emission in a selectively emitting Yb 3+ /Er 3+ up-conversion (UC) system is of great significance for optical applications. In this study, we adopted a facile and effective method to obtain a pure red emission by defect engineering in GdOF:Yb 3+ /Er 3+ (6/x mol%) nanosheets. It was observed that the defect engineering was realized by NaF etching of the precursors in a low-temperature water bath reaction. As a result, the probability of multi-phonon relaxation processes was increased, resulting in a red to green (R/G) ratio higher than 19 for all samples. The products perfectly inherited the sheet-like morphology of the precursors. Furthermore, the effect of calcination temperature on the UC emission selectivity was demonstrated in GdOF:Yb 3+ /Er 3+ nanocrystals. The R/G ratio of the samples obtained at 500 °C was increased by about 3–4 times compared with the samples obtained at different calcination temperatures. By combining the results with the outcomes of spectroscopic analyses, we simulated the role of defects in improving the R/G ratio of GdOF:Yb 3+ /Er 3+ samples using a model. The exciting discovery of the ability of defect engineering to modulate UC emission selectivity not only provides a general method to tailor red emission in a Yb 3+ /Er 3+ UC system, but also facilitates multicolor displays, anti-counterfeiting, and tissue bioimaging. Schematic diagram of Er 3+ up-conversion red emission enhanced via defect engineering. • The anion-exchange reaction of the layered structure materials was accomplished by low temperature water bath method. • During the anion-exchange reaction, the corrosion effect of NaF on the surface of the nanosheets produced defects. • The R/G ratio of the unmodified GdOF:Yb 3+ /Er 3+ nanosheets prepared by defect engineering were above 19. • The R/G ratio of the GdOF:Yb 3+ /Er 3+ nanosheets increased rapidly to 19.36 by controlling calcination temperature.

Predicting the impact of temperature dependent multi-phonon relaxation processes on the photon avalanche behavior in Tm3+: NaYF4 nanoparticles

Photon avalanche (PA) is an anti-Stokes process in which lanthanide-doped materials can emit upconverted luminescence in a highly non-linear manner. PA was recently demonstrated in thulium (III)-doped NaYF4 colloidal nanoparticles at room temperature, with reported nonlinearities (power factors) exceeding 30. Importantly, good rate-equations model agreement with experimental data was achieved, which enabled the derivation of the rates of energy transfer processes, providing a foundation for further developments and optimization of similar materials and their new applications. In order to apply PA to, for example, luminescent (nano)thermometry, we need to have a better understanding of the susceptibility of photon avalanche phenomenon to temperature variation. For this purpose, we study how the temperature and energy-gap-dependent multiphonon relaxation and inter-ionic energy transfer processes affect the photon avalanche behavior. As a result of these simulations, high susceptibility to temperature changes with relative sensitivity over 40 %⋅K−1 above 400 K (up to 15 %⋅K−1at room temperature) was noted. Moreover, by knocking-out the temperature dependences of the individual processes involved in the PA, we found the temperature dependence of the multiphonon relaxation of the intermediate F34 state to the ground state to be the primary one responsible for the simulated temperature dependence of PA emission. This process is observed to be more significant than temperature dependencies of energy looping and phonon assisted ground state non-resonant absorption. Although there are challenges with a single band technique, these results still show the great potential, especially when used ratiometrically with different excitation wavelengths.

Temperature dependence of red emission in YPO4:Pr3+ nanopowders

In order to compare spectroscopic properties between nanosized and that in single crystal form of YPO4:Pr3+, temperature-dependence of the red emission from 3P0 → 3H4 and 1D2 → 3H4 transitions of YPO4:0.1 at. % Pr3+ nanopowders has been investigated in details. The emission spectra of the sample in size of 20 nm, under selective excitation at 3P2, measured at different temperatures, T = 10, 100, 200 and 300 K have been recorded. The variation of emission intensity of each 3P0 → 3H4 and 1D2 → 3H4 against temperature has been investigated. Several spectral lines constitute 1D2 → 3H4 emission, assigned to transitions between 1D2 and 3H4 Stark levels are observed with no noticeable shifts between them and those observed at the single crystal. It is found that 1D2 → 3H4 emission intensity dominates that of 3P0 → 3H4 transition, pointing out the high efficiency of the multiphonon relaxation (MPR) process from 3P0 to 1D2. Fitting the temperature dependence of 3P0 → 3H4 emission intensity permits to estimate the phonons and their energies involved in non-radiatively transition between 3P0 and 1D2 levels, which are found different from single crystal. Also, fitting the multiphonon relaxation (MPR) rates against temperature, extracted from the measured 1D2 lifetime gives information that seven phonons can be involved to bridge the gap between 1D2 and 1G4. Furthermore, the possibility to use the YPO4: 0.1 at. % Pr3+ in the nano-thermometry application in temperature range T = 10–300 K was investigated.

Yb3+ influence on NIR emission from Pr3+-doped spherical yttria nanoparticles for advances in NIR I and NIR II biological windows

Highly intense NIR emissions in the first and second biological windows and a remarkable UV–Vis and NIR to NIR energy conversion were observed for Pr3+, Yb3+ co-doped yttria nanoparticles, synthesized via the homogeneous precipitation method. A single-phase body center cubic Y2O3 structure was formed for all the samples after annealing 900 °C for 2 h. Increasing the dopants content, from 0.5 to 5.0 mol %, the replacement of Y3+ by Pr3+ and Yb3+ ions lead to a lattice expansion, which could be calculated. Spherical monodispersed nanoparticles, constituted of several crystallites, were observed by TEM and SEM images. The particles size ranged from 143 to 272 nm for the as-prepared samples and from 88 to 202 nm for the samples annealed at 900 °C. The crystallite size was estimated by Debye-Scherrer and Williamson-Hall methods and ranged from 6 to 27 nm with an average of 13 nm. Photoluminescent studies evidenced pronounced Pr3+ and Yb3+ near infrared (NIR) emission under ultraviolet, visible, and NIR excitation. How the Yb3+ ions co-doping, as well as, the total rare earth content, affected the luminescence properties, were evaluated, along with the energy transfer process involving UV–Vis and NIR to NIR energy conversion between the doping ions and the host. The energy was transferred between the Y2O3 host and Pr3+, Yb3+ upon CT band excitation at 290 nm. NIR emission at 948, 1080, 1115 nm, and 1.5 μm attributed to the Pr3+ transitions 3P0,1→1G4, 1D2→3F3, 1D2→3F4, and 1D2→1G4, denoting the 1D2 population through a multiphonon relaxation process (3P0+3H4→3H6+1D2). A concentration quenching process was ascertained in the highest concentration of dopants (5.0 mol % in total), which indicated that cross-relaxation took place. Both the Yb3+-doped and Pr3+, Yb3+ co-doped samples presented the Yb3+ emission 2F5/2→2F7/2 at 976 nm. All the structural, morphological, and spectroscopic properties of the materials make these spherical yttria nanoparticles potential NIR emitting probes candidates for advances in biophotonics, especially for deep-tissue imaging using NIR I and NIR II biological windows.

A novel high-sensitive optical thermometer based on the multi-color emission in Pr3+ doped LiLaMgWO6 phosphors

Nowadays, optical thermometric materials with high sensing sensitivity in a wide temperature interval are urgently required due to the appealing prospect in the field of sensing technology. Herein, the double perovskite Pr3+: LiLaMgWO6 phosphors are successfully prepared via a high-temperature solid-state reaction method, and their concentration/temperature-dependent luminescence properties are discussed systematically. Upon UV excitation, the samples emit blue (3P0→3H4), green (3P0,1 → 3H5) and red emissions (1D2→3H4, 3P0→3F2) from Pr3+ ions. The different changing tendencies in relation to temperature are observed for the emissions from 3P0 and 1D2 multiplets, which are attributed to the effects of cross-over process to Pr3+-W6+ intervalence charge transfer (IVCT) state, cross relaxation process and multiphonon relaxation process. When the fluorescence intensity ratio (FIR) between 3P0→3F2 (654 nm) and 1D2→3H4 (616 nm) transitions is applied for temperature sensing, 0.5%Pr3+: LiLaMgWO6 displays a maximum absolute sensing sensitivity of 2421 × 10−4 K−1 at 573 K. Moreover, it can keep a comparatively high relative sensitivity (1%~3.25% K−1) and low temperature uncertainty (0.15–0.5 K) over a wide temperature range (~280 K), which are superior to those of many luminescence materials reported previously. The present study may provide useful design information to develop high-sensitive luminescent thermometers with a wide applicable temperature range.

Highly efficient ~3.4 μm emission of Er3+-doped TeO2 based glasses via resonant energy transfer and multi-phonon relaxation processes

To clarify the relation between the concentration and ~3.4 μm emission of Er3+, a series of TeO2 based glasses with Er3+ concentration from 2.0 to 10.0 mol% were prepared by conventional melting-quenching method. The physical and spectroscopic characteristics of Er3+-doped tellurite glasses were systematically investigated. The Judd-Ofelt theory was used to analyze the absorption spectra. The emission intensity of ~2.7 μm and upconversion at 550 and 670 nm were discussed. The intensity increased when the Er3+ concentration increased to 6 mol%, and then drastically decreased with further increasing Er3+ concentration. The trend of the emission properties in NIR with the Er3+ concentration was also discussed. With increasing Er3+ concentration, the emission cross section at ~3.4 μm increased from 1.37 × 10−21 cm2 to 2.53 × 10−21 cm2. The results indicate that the highly doped Er3+ concentration may be an effective route to achieve efficient ~3.4 μm emission in Er3+ doped tellurite glasses.

Luminescence and optical thermometry strategy based on emission and excitation spectra of Pr3+ doped SrMoO4 phosphors

Recent developments of luminescence ratiometric thermometry have attracted much attention owing to its merits of fast response, non-invasiveness and high spatial resolution. In this work, the synthesis, crystal structure and luminescence properties has been carried out for Pr3+-activated SrMoO4 phosphors as optical thermometry. The XRD results show that all the phosphors possess the scheelite type tetragonal structure with space group I41/a. The efficient luminescence of Pr3+ can be observed under intra-configurational (4f-4f) and charge transfer band (Mo–O) excitations, respectively. Upon different excitations, the quenching concentration of Pr3+ is diverse due to the multi-phonon relaxation and cross-relaxation processes occurring in different excited states of Pr3+ ions. The fluorescence intensity ratio (FIR) techniques based on emissions of 3P0 and 1D2 excited states of Pr3+, and the FIR in the excitations of the charge transfer band (Mo–O) and 4f-4f transitions of Pr3+ were employed for the thermometric characterizations in the 298–498 K range. Both results show remarkable performance in temperature sensing with the maximum relative sensitivity of 0.45%K−1@489 K and 0.98 %K−1@298 K, respectively. Our study demonstrates that Pr3+-activated SrMoO4 phosphors have a promising potential application in non-contact optical thermometry.

Structural characterization and photoluminescence properties of B2O3–Bi2O3–SiO2 glass containing Dy3+ ions

Bismuth borosilicate (BBS) glasses have been widely applied in laser, LED device and nonlinear optics. This work investigates the structural evolution of BBS glasses using the spectral transformation of Dy3+ ions as probe. Glass systems of composition xB2O3-(60-x) Bi2O3–40SiO2-1Dy2O3(x = 10, 20, 30, 40) (mol%) were prepared by the conventional melt quenching technique. The diffraction band of XRD pattern peaked at 2θ = 28.50° slightly shifts to smaller angle with the amount of B2O3 resulting from the more compact network. The absorption cut-off edge in ultraviolet region shifts to the shorter wavelength and band gap energy is enhanced, indicating that the increasing B–O- bonds with the stronger covalence than Bi–O bond. According to Fourier transform infrared (FTIR) and Raman spectra, the conversion of [BiO3] tetrahedral into [BiO6] octahedral takes place. Interestingly, [BO4] tetrahedral began to be transformed into [BO3] triangle when the content of B2O3 exceeds 30mol%, suggesting that the phenomenon of "boron abnormality" exists in BBS glass. The increase in Ω2 values and Y/B ratios reveals the intensifying disorder of glass network. The fluorescence intensity, decay lifetime (τexp) and quantum efficiency (η%) increase due to the enhancement of multi-phonon relaxation process of 4I13/2, 4G11/2 and 4I15/2 level populating 4F9/2 level.

An Ultrafast Shakedown Reveals the Energy Landscape, Relaxation Dynamics, and Concentration of the N3VH0 Defect in Diamond.

Atomic-scale defects can control the exploitable optoelectronic performance of crystalline materials, and several point defects in diamond are emerging functional components for a range of quantum technologies. Nitrogen and hydrogen are common impurities incorporated into diamond, and there is a family of defects that includes both. The N3VH0 defect is a lattice vacancy where three nearest neighbor carbon atoms are replaced with nitrogen atoms and a hydrogen is bonded to the remaining carbon. It is regularly observed in natural and high-temperature annealed synthetic diamond and gives rise to prominent absorption features in the mid-infrared. Here, we combine time- and spectrally resolved infrared absorption spectroscopy to yield unprecedented insight into the N3VH0 defect's vibrational dynamics following infrared excitation of the C-H stretch. In doing so, we gain fundamental information about the energies of quantized vibrational states and corroborate our results with theory. We map out, for the first time, energy relaxation pathways, which include multiphonon relaxation processes and anharmonic coupling to the C-H bend mode. These advances provide new routes to quantify and probe atomic-scale defects.

Terbium-fluorido cluster: an energy cage for photoluminescence.

We report here that energy migration during luminescence can be extremely minimized by caging the fluorescent centers in a molecular cluster of [Tb6(μ3-F)8(piv)10(Hpiv)4DMF]·xDMF·yH2O 1. Experimental and theoretical simulations reveal that bonding terbium with fluoride is the key to reducing the non-radiative multi-phonon relaxation processes, which is disparate to the common hydroxy-based lanthanide clusters.

Photoluminescence and thermal stability of Tb3+-doped K4SrSi3O9 phosphor with electron transition mechanisms

Herein, we present a series of novel Tb3+-activated K4SrSi3O9 phosphors prepared by a simple solid phase reaction process. Owing to the characteristic 5D3/5D4-7FJ transitions of Tb3+, the Tb3+-activated K4SrSi3O9 phosphors possess a green emission under near-UV excitation and high thermal quenching temperature (>500 K). The temperature/concentration-dependent luminescence and decay properties of K4SrSi3O9:Tb3+ further elucidate the effects of cross relaxation, multiphonon relaxation and energy transfer processes on the luminescence of Tb3+ 5D3 and 5D4 levels. The electron population mechanism in the system is proposed in detail, which is expected to be generally applicable to all Tb3+-doped phosphors. Moreover, a near-UV white light-emitting diode (WLED) device with excellent CIE chromaticity coordinates (0.329, 0.320) is fabricated by using K4SrSi3O9:0.07Tb3+, demonstrating the potential applicability of K4SrSi3O9:Tb3+ for near-UV WLED.

Spectroscopic investigations on Dy3+ ions doped zinc lead alumino borate glasses for photonic device applications

This paper presents the structural, optical absorption, photoluminescence (PL) and decay spectral properties of Dy3+ ions doped zinc lead alumino borate (ZPAB) glasses to elucidate their possible usage in photonic devices such as w-LEDs and lasers. A broad hump shown by the XRD spectrum recorded for an un-doped ZPAB glass confirms its non-crystalline nature. The Judd-Ofelt (J-O) intensity parameter evaluated from the measured oscillator strengths of the absorption spectral features were used to estimate various radiative parameters and also to understand the nature of bonding between Dy3+ ions and oxygen ligands. Under 350 nm excitation, the as-prepared glasses are exhibiting two emission bands 4F9/2 → 6H15/2 (blue), and 4F9/2 → 6H13/2 (yellow) at 483 and 575 nm, respectively. From the PL spectra, the Y/B ratio values, CIE chromaticity color coordinates and color correlated temperature (CCT) were evaluated. The experimental lifetimes measured from the decay profiles are decreasing with increase in Dy3+ ions concentration in these glasses which may be attributed to the cross-relaxation and non-radiative multiphonon relaxation process. Decay profiles observed for higher concentration were well fitted to Inokuti-Hirayama (I-H) model to understand the energy transfer process and subsequent decrease in experimental lifetimes. The higher values of radiative parameters, emission cross-sections, quantum efficiency, optical gain and gain band width suggest the suitability of 0.5 mol% of Dy3+ ions in these ZPAB glasses for the photonic device application.

Luminescence and upconversion processes in [formula omitted]-doped tellurite glasses

This work presents a spectroscopic investigation of tellurite glasses with the composition 65TeO2-15Li2O-20ZnO doped with different concentrations of Er2O3. Optical absorption and emission spectroscopy were performed at room temperature to determine emission properties of Er3+ in the visible and near-infrared regions. The samples presented intense green emission due to the H11/22→I15/24 and S3/24→I15/24 transitions. Emission intensities at 992 and 1550 nm exhibit a relative increase with increasing doping concentration. Luminescence decay curves for the S3/24→I15/24 transition follows a non-exponential behavior, while the I11/24→I15/24 and I13/24→I15/24 transitions presented simple exponential behavior with high lifetime values. Under 975 nm excitation, upconversion luminescences in the green and red regions are observed, with a relative increase for red emission as function of doping concentration. The possible mechanisms considering multi-phonon relaxation, energy transfer and cross-relaxation processes were discussed for green and red emissions under excitations at 488 and 975 nm. The square dependence of green emission on the excitation power indicates two photons contribution to the upconversion emission. The full width at half-maximum values of emission at 1550 nm increased with increasing Er3+ concentration, showing the potential of the studied material as infrared amplifier.

Nd3+,Ho3+-Codoped apatite-related NaLa9(GeO4)6O2 phosphors for the near- and middle-infrared region.

The apatite-like NaLa9(GeO4)6O2:Nd3+,Ho3+ phosphor is prepared using the solid-state method. Rietveld refinement of high-resolution time-of-flight neutron powder diffraction measurements indicate that this compound crystallizes in the hexagonal system with space group P63/m, Z = 1 and unit cell parameters a = 9.88903(6) Å, c = 7.25602(5) Å, V = 614.521(7) Å3 at room temperature. The 4f sites are statistically occupied by La, Nd and Na, while 6h sites are occupied by La and Nd. Luminescence in the near- and middle-IR range caused by the transitions in neodymium and holmium ions is excited under 808 nm laser diode radiation. The highest emission intensity in NaLa9-x-yNdxHoy(GeO4)6O2 is attained at trace amounts of holmium, and it decreases sharply when y increases to 0.01. The IR phosphors have a good thermal stability and exhibit a very weak upconversion emission in the red spectral range upon 808 nm excitation. A scheme of excitation and emission pathways involving ground/excited state absorption, energy transfer, cross-relaxation, nonradiative multiphonon relaxation processes in Nd3+ and Ho3+ ions has been proposed. The data analysis indicates that Nd3+ ions serve as sensitizers for Ho3+ ions in these compounds, stimulating intense 2.1 μm and 2.7 μm emissions. These apatite-related germanate phosphors are promising materials for near- and middle-infrared solid-state lighting applications.