Upconversion Luminescence Emission Research Articles

Effect of vacuum annealing on the upconversion luminescence of β-Ba2ScAlO5: Yb3+, Ho3+ phosphor

Upconversion luminescence (UCL) materials with narrow emission peaks, long fluorescence lifetime, and large anti-Stokes shift have attracted considerable attention in biomedical, display, anti-counterfeiting, and solid-state lasers. This study presents a new synthesis method, combining air sintering and vacuum annealing, to obtain intense green UCL emission β-Ba2ScAlO5: 0.15 Yb3+, 0.005Ho3+ phosphor. After vacuum annealing, the green, red, and near-infrared emissions of Ho3+ are enhanced by factors of 40, 133, and 37, respectively. The diffuse reflectance spectra demonstrate that there is a defect energy band ranging from 330 to 860 nm in the air-treated samples, and it is reduced dramatically after vacuum annealing. This defect band covers the three wavelengths of Ho3+ emissions in the visible range and results in the reabsorption of UCL by the host lattice. X-ray Rietveld refinements and EPR spectra reveal the difference in the oxygen defects of the air-synthesized and vacuum-annealed phosphors and reflect that the absorption band is related to the oxygen defects. This research provides a new approach to obtain intense UCL emission of β-Ba2ScAlO5 hosted phosphors and offers an excellent green phosphor for application in anti-counterfeiting or displays.

Enhancing NIR-II Upconversion Monochromatic Emission for Temperature Sensing.

The upconversion luminescence (UCL) in the second near-infrared window (NIR-II) is highly attractive due to its excellent performance in high-resolution bioimaging, anticounterfeiting, and temperature sensing. However, upconvertion nanoparticles (UCNPs) are normally emitted in visible light, potentially impacting the imaging quality. Here, a monochromatic Er3+ -rich (NaErF4 :x%Yb@NaYF4 ) nanoparticles with excitation at 1532nm and emission at 978nm is proposed, both situated in the NIR-II region. The proper proportion of Yb3+ ions doping has a positive effect on the NIR-II emission, by enhancing the cross relaxation efficiency and accelerating the energy transfer rate. Owing to the interaction between the Er3+ and Yb3+ is inhibited at low temperatures, the UCL emission intensities at visible and NIR-II regions show opposite trend with temperature changing, which establishes a fitting formula to derive temperature from the luminous intensity ratio, promoting the potential application of UCL in NIR-II regions for the temperature sensing.

Strong green upconversion emission from submicron spindle-shaped SrMoO4:Yb3+,Er3.

Upconversion luminescence (UCL) is a fluorescence process where two or more lower-energy photons convert into a higher-energy photon. Lanthanide (Ln3+)-doped UCL materials often suffer from weak luminescence, especially when directly synthesized by a hydrothermal (HT) process due to the existing hydroxyl group and undesirable arrangement of dopants within host lattices which quench luminescence and limit energy transfer. Therefore, additional heat treatment processes are required to enhance their UCL emission, even though direct hydrothermal synthesis without further heat treatment has the advantages of low energy consumption, fast synthesis, and wide applicability to generate UCL materials. In this study, via a HT process without annealing, we have produced Yb3+ and Er3+ co-doped SrMoO4 submicron spindles with a strong green UCL emission which can be seen with the naked eye, which HT produced oxide-based UCL materials often fail to demonstrate. We have investigated different HT synthesis conditions, such as temperature, time, pH and dopant composition, which control the nucleation, growth, lattice structure arrangement, and ultimately their UCL properties through XRD, SEM, EDS and UCL measurements. The bright green UCL from the SrMoO4:Yb,Er submicron spindles is further enhanced by post-synthesis annealing within a molten NaNO3/KNO3 system to prevent particle size growth. The green UCL intensity from the annealed SrMoO4:Yb,Er submicron spindles surpasses samples produced by the solid-state method and is comparable to that from the commercial NaYF4:Yb,Er sample. We have further studied the temperature-dependent luminescence of both the HT-prepared and molten-salt annealed SrMoO4:Yb,Er submicron spindle samples. The strong UCL from our SrMoO4:Yb,Er submicron spindles could warrant their candidacy for bioimaging and anticounterfeiting applications.

Rationally-designed core–shell structure with double-plasmon effect for efficient and tunable upconversion luminescence emission

Rare earth-doped upconversion luminescence has great potential toward modern anti-counterfeiting applications. The coupling effect between low-cost plasmonic semiconductor nanostructures could enhance the local electric field of the surrounding material, thereby improving the light absorption of the system. Considering both light absorption and emission aspects, we propose a technological approach to combine core–shell structures and plasmon resonance for synergistically enhancing the luminescence intensity with tunable upconversion luminescence emission. As a result, the luminescent intensity of rationally-designed core–shell structure with double-plasmon effect has been elevated by two orders of magnitude than that of only core–shell structure, and its selectivity of the tricolor luminescence has been improved significantly, representing the importance to be serviced as highly-efficient color-tunable anti-counterfeiting materials.

Luminescence Thermometry Based on the Upconversion Luminescence from the Stark Sublevels of BaY2F8:Yb3+, Tm3+ Phosphor.

Visible and near-infrared (NIR) upconversion luminescence (UCL) emissions originating from the BaY2F8: Yb3+, Tm3+ systems were investigated under a laser excitation at 980nm. The BaY2F8:20mol% Yb3+, x mol% Tm3+ and BaY2F8: y mol% Yb3+, 0.5mol% Tm3+ phosphors showed prominent UCL at 800 and 810nm. The optimized doping concentrations of Yb3+ and Tm3+ in the BaY2F8 host matrix were evaluated, their spectroscopic properties were determined, and studies on their temperature-dependent behaviour were carried out. The temperature-sensing properties were studied by generating the fluorescence intensity ratio (FIR) of the UCL peaks originating from the thermally-coupled energy levels of the Tm3+ ions. The Stark sublevels of 1G4 level of Tm3+ ions were utilized to estimate the temperature-sensing abilities of the phosphor.

Enhanced upconversion luminescence and single-band red emission in β-Ba2ScAlO5: Yb3+/Er3+ phosphor through doping Cu2+

This study reports on a single-band red upconversion emission of Er3+ in a pure-phase β-Ba2ScAlO5: Yb3+/Er3+ fluorescent powder by doping Cu2+. The optimum concentration of Cu2+ ions in β-Ba2ScAlO5: Yb3+/Er3+ powder was 0.20. The red and green emission intensity of β-Ba2ScAlO5: Yb3+/Er3+ sample with optimum concentration of Cu2+ ions increased by 19.1 and 3.6 times, respectively, relative to the β-Ba2ScAlO5:Yb3+/Er3+ material lacking Cu2+ ions. The red-green integral intensity ratio of the β-Ba2ScAlO5:Yb3+/Er3+/Cu2+ phosphor reached 141.9. The potential upconversion luminescence (UCL) mechanism was explored by analyzing the down-shift luminescence (DS) spectra, upconversion luminescence spectra, and upconversion fluorescence lifetime decay curves of Yb3+/Er3+/Cu2+ triple-doped β-Ba2ScAlO5. Enhanced red emission and green upconversion emission are achieved by energy transfer between defect band by the oxygen vacancy and Er3+ energy levels. This study's findings establish the foundation for developing novel phosphors with single UC red light emission, which has potential applications in illumination displays, optical thermometry, and biomedicine.

Impacts of Ce3+ doping on temperature sensing characteristics of LuAG: Yb3+/Ho3+ up-conversion fluorescent materials

A series of LuAG: 5 mol%Yb3+/0.5 mol%Ho3+/x mol%Ce3+ (x = 0, 0.2, 0.5, 1.0, 2.0) up-conversion (UC) fluorescent materials were prepared using CO2 laser heating zone melting method. The X-ray diffraction (XRD) patterns confirm that the prepared samples have a pure LuAG crystal phase. Under excitation of a 980 nm laser, the up-conversion luminescence (UCL) spectra of the prepared samples were obtained. Two UCL emission bands of Ho3+ ions were observed, including green emission band of the 5F4/5S2→5I8 transition, and red emission band of the 5F5→5I8 transition. Due to the cross-relaxation (CR) processes between Ce3+ ions and Ho3+ ions, the doping of Ce3+ ions can effectively improve the UCL efficiency of Ho3+ ions, and the emission intensity reaches maximum when doped with 1 mol% Ce3+. The temperature sensing characteristics of LuAG: Yb3+/Ho3+ and LuAG: Yb3+/Ho3+/Ce3+ from 303 K to 723 K were studied by fluorescence intensity ratio (FIR) method. By doping 1 mol% Ce3+ ions, the absolute sensitivity of the sample has been significantly improved, with the maximal absolute sensitivity of 0.0951 K−1 (303 K), and the maximal relative sensitivity of 0.0103 K−1 (343 K). The high sensitivity and wide measurement range make LuAG: Yb3+/Ho3+/Ce3+ UCL material ideal for temperature sensing.

Squaraine Dye-Sensitized Upconversion with Enhanced Stability and Minimized Aggregation-Caused Quenching.

Upconversion nanoparticles (UCNPs) doped with lanthanides have limited brightness due to their small absorption cross section to light. However, using organic sensitizers can significantly enhance their light absorption ability. Unfortunately, the practical application of organic sensitizers has been hindered by poor stability and aggregation-caused quenching (ACQ). To address these issues, we developed a novel squaraine-based dye, SQ-739, for sensitizing upconversion luminescence (UCL). This dye has a maximum absorption at 739 nm, and shows 1 order of magnitude and 2-fold improved chemical- and photostability, compared to the commonly used cyanine-based dye IR-806, respectively. When SQ-739 is used to sensitize UCNPs, the resulting SQ-739-UCNPs exhibit excellent photostability and reduced ACQ in the presence of polar solvents. Moreover, at the single particle level, the SQ-739-UCNPs exhibit a 97-fold increase in UCL emission compared to bare UCNPs. This squaraine dye-based system represents a new design strategy for developing highly stable and efficient NIR upconversion probes.

Enhanced up- and down-conversion luminescence from Dy3+-Sm3+ co-doped B2O3-SrCO3-TeO2-Al2O3-MgO glass hosts: effects of CuO nanoparticles embedment

In the up-conversion nonlinear optical process, the absorption of lower energy photons causes the emission of photons at higher energy due to various mechanisms like sequential optical excitation, energy transfer, and nonlinear light-matter interaction processes. Improving the up- and down-conversion luminescence of different lanthanide-doped glasses via metallic nanoparticle embedment remains challenging. There has been no clear understanding of how copper oxide nanoparticles (CuONps) affect the spectroscopic properties of Dysprosium-Samarium ions (Dy3+-Sm3+)-co-doped glasses. Thus, new CuONps-infused strontium-telluro-aluminum-magnesium-borate glass samples were prepared using the standard melt-quenching technique with Dy3+-Sm3+ co-doping. X-ray diffraction (XRD) analysis confirmed the samples' glassy nature. High-resolution transmission electron microscopy (HRTEM) provided the orientation and size distribution of the embedded nanoparticles in the glass hosts. The downconversion luminescence emission (at 350 nm excitation) corresponds to 4F9/2 → 6H15/2, 4F9/2 → 6H13/2, and 4F9/2 → 6H11/2 transitions in Dy3+. Judd-Ofelt calculations for the CuONps embedded sample showed significant radiative properties. A branching ratio of 79.6% was attained. The upconversion luminescence emission (at 896 nm excitation) bands corresponded to 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions for the Dy3+-doped glasses. In addition, an intense peak due to the 4G5/2 → 6H7/2 transition in Sm3+ was observed in the co-doped glasses. It was determined that the increased up-conversion luminescence was related to localized surface plasmon resonance, energy transfer, and local field enhancements caused by CuONps inclusion. It was confirmed that excited state absorption and energy transfer mechanisms were involved in the up-conversion luminescence. Synthesized glasses may be helpful for upconversion displays, optical data storage, and solid-state lasers.

Ratiometric upconversion nanoprobes for turn-on fluorescent detection of hypochlorous acid

Ratiometric fluorescent nanoprobes have been developed for the detection of hypochlorous acid (HOCl) by simply mixing upconversion nanocrystals (UCNCs) with plasmocorinth B (PB) dye molecules, where UCNCs are adopted as the energy donor and dye molecules are introduced as the energy acceptor. Specifically, green upconversion luminescence (UCL) emission of UCNCs can be effectively quenched by PB, resulting from the spectral overlap between the absorption of PB and UCL emission of UCNCs in the green region. Furthermore, owing to the electrostatic repulsion between negatively charged UCNCs and PB dye molecules, the fluorescence quenching mechanism is based on the IFE process. Upon interaction with HOCl, PB dye molecules degrade, resulting in the recovery of green UCL emission through inhibition of the IFE process. The red UCL emission, however, remains unchanged. Therefore, the red UCL emission is employed as an internal reference signal for the ratiometric detection of HOCl. The detection limit of the nanoprobe for HOCl is quantified to be 1.12 µM, which is much lower than that of colorimetric detection. Moreover, the developed sensor has been successfully applied for quantitative monitoring of HOCl in tap water.

Emission colour tuning in KLnF4:Yb3+,Er3+/Ho3+ phosphor

KLnF4:Yb3+,Er3+/Ho3+ (Ln = Y, Yb, Gd) phosphors were prepared by solid-state reaction technique. For each of the Yb3+/Er3+ and Yb3+/Ho3+ co-doped phosphors independently, phase confirmation and morphological examination were accomplished using the X-ray diffraction technique and scanning electron microscope analysis. X-ray photoelectron spectroscopy analysis shows the charge state and existence of lanthanides and dopants. Upconversion luminescence emissions of green and red colour were detected for the synthesized phosphors. The changes in the upconverted emission intensities and colour output pertaining to the change of the lanthanide host matrix along with dopants were investigated for the prepared phosphors. In the upconversion luminescence, erbium-doped samples exhibit a predominance of red emission while holmium-doped samples display a prevalence of green emission. The rare-earth Er/Ho doped KYbF4 shows strong red emission compared to other hosts. KYF4:Yb,Er/Ho exhibits relatively significant green upconversion emissions, while Yb,Er-doped KGdF4 host displays a 10-fold increase in green/red emission ratio with an increase in the input power of 20–780 mW. Downshifting photoluminescence studies and decay curve analysis were carried out using UV light excitation for the developed phosphors. A higher lifetime of 2.2 ms was measured for the downshifting red emission in the Ho doped KYbF4 sample. This study suggests a potential for colour tunability in the upconversion and downshifting emissions through host modifications to the fluoride phosphor.

NIR-excited imaging and in vivo visualization of β-galactosidase activity using a pyranonitrile-modified upconversion nanoprobe

β-galactosidase (β-gal) is a diagnostic biomarker of primary ovarian cancers. The development of effective fluorescent probes for investigating the activity of β-gal will be beneficial to cancer diagnosis. Herein, a near-infrared (NIR) excited ratiometric nanoprobe (DCM-β-gal-UCNPs) by assembling pyranonitrile dye (DCM-β-gal) on the surface of upconversion nanophosphors (UCNPs) was designed for the evaluation of β-gal activity in vivo. Upon the interaction with β-gal, a marked decrease of upconversion luminescence (UCL) signal in the green channel was observed owing to the luminescence resonance energy transfer from the UCNPs to pyranonitrile chromophore, whereas the NIR UCL emission at 800 nm was almost no influence. Thus, the β-gal activity could be quantitatively detected by the UCL intensity ratio of UCL543 nm/UCL800 nm with the limit of detection of 3.1 × 10−4 U/mL. Moreover, DCM-β-gal-UCNPs was effectively applied for monitoring β-gal fluctuation in living cells and zebrafish by a ratiometric UCL signal excited by 980 nm laser. We envision that nanoprobe DCM-β-gal-UCNPs might be used as a potential bioimaging tool to disclose more biological information of β-gal in β-gal-associated diseases in the future.

The effect of Er3+ concentration on the kinetics of multiband upconversion in NaYF4:Yb/Er microcrystals.

In Yb-Er co-doped upconversion (UC) nanomaterials, upconversion luminescence (UCL) can be modulated to generate multiband UCL emissions by changing the concentration of activator Er3+. Nonetheless, the effect of the Er3+ concentrations on the kinetics of these emissions is still unknown. We here study the single β-NaYF4:Yb3+/Er3+ microcrystal (MC) doped with different Er3+ concentrations by nanosecond time-resolved spectroscopy. Interestingly, different Er3+ doping concentrations exhibit different UCL emission bands and UCL response rates. At low Er3+ doping concentrations (1mol%), multiband emission in β-NaYF4:Yb3+/Er3+ (20/1mol%) MCs could not be observed and the response rate of UCL was slow (5-10μs) in β-NaYF4:Yb3+/Er3+. Increasing the Er3+ doping concentration to 10mol% can shorten the distance between Yb3+ ions and Er3+ ions, which promotes the energy transfer between them. β-NaYF4:Yb3+/Er3+ (20/10mol%) can achieve obvious multiband UCL and a quick response rate (0.3µs). However, a further increase in the Er doping concentration (80mol%) makes MCs limited by the CR process and cannot achieve the four-photon UC process (4F5/2 → 2K13/2 and 2H9/2 → 2D5/2). Therefore, the result shows that changing the Er3+ doping concentration could control the energy flow between the different energy levels in Er3+, which could affect the response time and UCL emission of the Yb/Er doped rare earth materials. Our work can facilitate the development of fast-response optoelectronics, optical-sensing, and display industries.

Up-conversion luminescence and broadband white-light emission in La1-xErxGa0.5Sb1·5O6, Bi1-xErxGeSbO6

In this paper, we first synthesized La1-xErxGa0.5Sb1·5O6 and Bi1-xErxGeSbO6 phosphors with rosiaite structure and studied their spectral luminescent properties. We demonstrated the influence of the matrix composition on the intensity ratio of the Er3+ luminescence bands, due to the transitions from the 2H11/2, 4S3/2, 4F9/2, 4I9/2, and 4 I11/2 excited states to the 4I15/2 ground state upon excitation to the 4I13/2 level. It is shown that the increase in the intensity ratio of the bands corresponding to the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions is more pronounced for La1-xErxGa0.5Sb1·5O6 compounds compared to Bi1-xErxGeSbO6. This indicates that the processes of interaction in the studied matrices are different.Along with the luminescence of erbium, broadband thermal white-light emission was detected in highly doped samples after increasing the power density of the exciting radiation with λexc. = 1550 nm. For La1-xErxGa0.5Sb1·5O6 compounds, it occurs at a lower power density than for Bi1-xErxGeSbO6, which may be due to the features of interactions of RE ions with the host, as well as with the particle morphology.

Control and enhancement of upconversion luminescence of NaYF4:Yb,Er nanoparticles with multiplet independent resonance modes in multiplexed metal gratings

Resonant control of light–matter interactions is implemented usually by applying photonic resonance modes at either excitation or emission wavelengths, instead of both simultaneously, due to intrinsic limitations of conventional resonators. While, multiplet resonance modes in multiplexed gratings can be independently adjusted to control multi–frequency optical interaction processes in matters. In this work, we use multiplexed metal gratings to resonantly control and enhance upconversion luminescence (UCL) of NaYF4:Yb,Er nanoparticles by designing their doublet plasmon resonance modes locating at both the green– and red–UCL emission wavelengths, or both the excitation and one of the emission wavelengths, of the nanomaterial to improve the excitation/emission efficiencies. Experimental results show that, when the doublet plasmon resonance modes match both the excitation and green– or red–UCL emission wavelengths, the UCL enhancements can be further improved, accompanied with a large modification of the green– to red–UCL intensity ratios. The mechanisms are investigated by analyzing the internal transition processes based on measurements of dependences of the UCL emissions on excitation power and their lifetimes. The principles and resonance structures can be applied in a broad range of optical phenomena on light–matter interactions in which photons of multiple frequencies are involved.

Effects of Yb3+ concentration on up-conversion luminescence and temperature sensing characteristics of Tm3+/Yb3+:Y2O3 phosphors

Y2O3:0.5 mol%Tm3+/x mol%Yb3+(x = 3, 5, 7, 10) luminescent materials have been synthesized by the CO2 laser zone melting method. With 980 nm excitation, the up-conversion luminescence (UCL) emission bands corresponding to 1G4→3H6, 1G4→3F4, 3F2,3→3H6, and 3H4→3H6 transitions of Tm3+ ions were obtained in visible and near-infrared spectral regions, among which the luminescence emitted by 1G4→3H6, 3H4→3H6 transitions shows obvious Stark splitting. Besides, the optimal doping concentration of Yb3+ is 5 mol%. The up-conversion luminescence spectra of Y2O3:0.5 mol%Tm3+/5 mol%Yb3+ were recorded from 224 K to 873 K. The temperature sensing properties of 3F2,3 and 3H4, 1G4(a) and 1G4(b) energy levels of Tm3+ ions were investigated by the luminescence intensity ratio (LIR) method in a wide temperature range, and the possibility of using 3H4(a), 3H4(b) thermally coupled levels (TCLs) of Tm3+ ions for temperature sensing was developed. The results show that 1G4(a) and 1G4(b), 3F2,3 and 3H4, 3H4(a) and 3H4(b) of Tm3+ ions are TCLs, and the Stark sub-levels own relatively high absolute sensitivity at low temperature, whereas the 3F2,3 and 3H4 TCLs of Tm3+ ions are more suitable for high temperature measurement. The results show that the Tm3+/Yb3+:Y2O3 luminescent material can be a promising candidate for temperature sensing.

Blue and red upconversion color tuning of Tm3+ via pulse excitation based on the thermal effect.

Color-tunable upconversion luminescence has wide prospects for anti-counterfeiting and disease diagnosis/treatment. To date, achieving high-quality tunable red and blue emissions using a single excitation wavelength remains a formidable challenge, due to the large energy difference between the red and blue photons. In this Letter, based on Tm3+ upconversion luminescence, blue dominant and red dominant emissions are generated upon 980-nm excitations using a short and long pulse, respectively. The corresponding color tuning mechanisms are investigated based on the spectral observations. The proposed color tuning strategy is particularly useful for in vivo applications as the red and blue lights play important roles in biological imaging and drug release, respectively.

Upconversion luminescence thermal enhancement and emission color modulation of LiYGeO4:Er3+/Yb3+ phosphors

A series of LiYGeO4:Er3+/Yb3+ phosphors were synthesized by solid-phase sintering method, and the upconversion luminescence (UCL) thermal enhancement and emission color modulation were investigated under 980 and 1550 nm laser excitations. When Er3+ doping concentration is kept at 0.02, the UCL intensity reaches the maximum at the Yb3+ doping concentration with 0.20. With temperature increasing from 303 to 723 K, the green UCL intensities of samples reach maximum enhancement as high as 4.56 and 11.54 times under 980 and 1550 nm laser excitations, respectively. The optical temperature sensing performance of these samples was evaluated by luminescence intensity ratio (LIR) technology, and the maximum absolute sensitivities in LiYGeO4:0.02Er3+/0.40Yb3+ are 0.01020 K−1 at 625 K and 0.01026 K−1 at 655 K under 980 and 1550 nm laser excitations, respectively. In addition, the modulation of UCL color can be realized by changing laser excitation power, temperature and excitation wavelength. These results show that LiYGeO4:Er3+/Yb3+ phosphors can be used as high-temperature optical thermometer and anti-counterfeiting.

Thermometric sensing performance in Erbium modified SrBi2-xNb2ErxO9 ferroelectric ceramic for optoelectronic devices

Er3+ doped ferroelectric ceramic compositions SrBi2-xNb2ErxO9 (x = 0–5 mol %) are prepared by solid-state reaction method and sintered at 1000 °C. The ceramic's structural, ferroelectric, optical, and thermometric sensor properties have been studied. The XRD analysis confirmed the orthorhombic geometry and the A21am phase group of the ceramic. The FTIR studies revealed the typical modes (602 cm−1 and 802 cm−1) of the Aurivillius phases of bismuth layered structure ferroelectrics (BLSFs). The improved hysteresis loops are traced under the electric field < ± 50 kV/cm, and P-E parameters seem to be affected by the insertion of the Er3+ dopant. In upconversion luminescence (UCL) spectra, two intense green bands at 529 nm and 549 nm and a diminished red peak at 670 nm have been identified, corresponding to a 980 nm exciting wavelength. The increase in the lifetime with changing erbium content indicates the efficient energy transfer in Er3+ ions, which results in non-radiative transition at a higher doping level. The optimum dopant concentration is x = 0.03 and is used to measure the pump power dependence on the UCL emission intensity. This study suggests that the two photons are involved in green and red UC emissions. The maximum sensitivity achieved in SrBi2-xNb2ErxO9 ceramic at x = 0.03 is 0.0086 K−1 at 443 K, ranging between temperatures 303 K–443 K, making it a promising non-contact sensor material.

Regulative control and enhancement of multi-color upconversion luminescence with DBR cavities

Upconversion luminescence (UCL) of lanthanide-doped materials (e.g. NaYF4:Yb3+/Er3+) involves multi-step, multi-channel transitions (corresponding to multi-color emissions) in a multi-level system, and is a low-efficiency nonlinear process. Usually, the UCL is enhanced by local enhancement of the excited field or one of its multi-color emissions is promoted by matching with a resonance cavity mode based on Purcell effect. Here, we propose to regulatively control and enhance the UCL by fostering one color of the UCL emissions with a resonance mode and inhibiting the other with an anti-resonance or nonresonance mode in forbidden band in an optical cavity, so that excited-state ions (e.g. Er3+) transit to the ground state more via the fostered UCL emission channel, rather than the inhibited one. As such, high-contrast single-color UCL emission can be achieved with an enhancement beyond that of Purcell effect. For the purpose, superior mode properties of distributed Bragg reflector (DBR) cavities can be applied, whose forbidden band of the DBR layers and resonance cavity mode in the forbidden band can be independently tuned in positions to match with the UCL emissions for them to be inhibited and fostered respectively. In experimental implementation, multi-color UCL (e.g. red and green) of NaYF4:Yb3+/Er3+ nanoparticles embedded in such DBR cavities are studied. And high-contrast single-color UCL emissions are demonstrated with enhancements factors beyond Purcell factors calculated in numerical simulations. A hypothesis on modifications of intermediate transitions in the UCL processes, as feedbacks to influences of mode characteristics in the DBR cavities for regulative control, is also proposed to explain the phenomena. The work suggests a way to regulatively control multi-channel photon emissions in multi-level systems for enhanced single-channel photon emission.