Energy Transfer Upconversion Research Articles

Competing effects of sensitization and energy dissipation by Ni2+ incorporation in La(Ga0.5Sc0.5)O3:Er,Ni,Nb upconverters

We investigated three pathways of energy transfer between Ni2+ sensitizers and Er3+ emitters in broadband-sensitive La(Ga0.5Sc0.5)O3:Er,Ni,Nb upconverters emitting at 0.98µm for crystalline silicon solar cells. The results were interpreted based on simplified models of the emission mechanisms. A high efficiency close to unity was successfully achieved for the Ni2+→Er3+ (4I13/2) energy transfer, meaning efficient sensitization. Energy dissipation caused by the Er3+ (4I13/2)→Ni2+ back transfer was minor. However, detrimental impact of another back transfer from the initial state of the upconversion emission Er3+ (4I11/2) competed against improvements in the absorbance and energy transfer upconversion with increasing Ni2+ and Er3+ concentrations, which limited the upconversion efficiency. Precise tuning of the Ni2+ absorption band to suppress the second back transfer can improve the upconversion performance.

(Invited) Up-Conversion Luminescence of Lanthanide Activated Phosphors

The major problem that limits solar cells’ efficiency is their insensitivity to the whole solar spectrum which is the so-called spectral mismatch. Therefore, several mechanisms have been explored based on photoluminescence (PL) to convert the solar cell spectrum where the spectral response of the solar cell is low to regions where the spectral response of the solar cell is high. Downconversion, up-conversion (UC) and downshifting are some of the mechanisms that may be applied to improve the spectral response. Upconversion nanoparticles (UCNPs) have shown some promising possibilities to be considered in this respect, however, low UC efficiency of UCNPs is still the most severe limitation of their applications. This study reports on the PL and cathodoluminescence (CL) behaviour of different phosphors. The vastly studied lanthanide pairs in UC are Er3+, Ho3+ and Tm3+ with Yb3+ as a sensitizer. Whereas, very few UC studies have been carried out for Yb3+/Tb3+ and Yb3+/Eu3+ pairs. The reason behind these two categories of lanthanides is the way they transfer their energies. Er3+, Ho3+ and Tm3+ co-doped with Yb3+ can be excited by ground state absorption (GSA), excited-state absorption (ESA) and energy transfer up-conversion (ETU) mechanisms whereas the energy transfer from Yb3+ to Tb3+/Eu3+ is due to co-operative energy transfer (CET). Since, Tb3+ and Eu3+ do not have energy levels which can absorb the near infra-red (NIR) light directly, Yb3+ is the best choice in order to obtain the UC luminescence in Tb3+/ Eu3+ doped system. Yb3+ has a single electronic transition within the 4f subshell. The transition from the lower level through ground state absorption can be easily achieved by using NIR radiation, and resulting energy from two or more excited Yb3+ ions can be utilized in the excitation of a single Tb3+ or Eu3+ ion, for gaining improved UC emission. Doping different host materials with these lanthanide pairs were investigated and tested for possible increase in solar cell efficiency. Power tuneable visible UC and infrared emissions were detected upon excitation with a 980 nm diode laser.

Intense Upconverted White Light Emission from Tm3+ - Er3+ - Yb3+ Doped Zinc Tungsten Tellurite Glasses

The different amount of rare earth ions doped zinc tungsten tellurite glasses with the compositions (70-x-y-z)TeO 2 – 20ZnO – 10WO 3 – xEr 2 O 3 – yTm 2 O 3 – zYb 2 O 3 (x = 0.3, 0.5; y = 0.3, 0.5; z = 3) have been synthesized using melt quenching technique. Blue, green, red and infrared luminescence via energy transfer and frequency upconversion mechanisms in Tm 3+ /Er 3+ /Yb 3+ triply-doped zinc tungsten tellurite glasses were investigated under single 975 nm diode laser excitation. Intense blue (Tm 3+ : 1 D 2 → 3 F 4 , 1 G 4 → 3 H 6 ; 477 nm), green ( 2 H 11/2 , 4 S 3/2 → 4 I 15/2 ; 525 nm and 549 nm), red (Er 3+ : 4 F 9/2 → 4 I 15/2 , Tm 3+ : 1 G 4 → 3 F 4 ; 659 nm) and infrared (Tm 3+ : 1 G 4 → 3 H 5 , 3 H 4 → 3 H 6 ; 807 nm) emissions were observed simultaneously at room temperature. Intense white light emission from all samples was observed with CCT values higher than 6500 K and CRI values lower than 80. The CIE coordinates of the emitted white light were found to shift to yellowish-greenish region with increasing pumping power. The possible energy transfer and upconversion mechanisms are discussed and plausible explanations are made.

Energy transfer upconversion in Er3+-Tm3+ codoped sodium silicate glass

Er3+/Tm3+ doped and codoped Na2O-SiO2-ZnO (NSZO) glasses were prepared by the conventional melt-quenching method. The amorphous nature of the prepared glasses was confirmed by the X-ray diffraction analysis. The optical absorption spectrum displayed several peaks, which correspond to Er3+ and Tm3+ dopant ions embedded into the NSZO glass. Both dopants experienced upconversion emission under 980nm excitation. Efficient energy transfer from Er3+ to Tm3+ was observed in the co-doped samples to enhance the near infrared emission of the Tm3+ ions.

Modelling the competition between photo-darkening and photo-bleaching effects in high-power ytterbium-doped fibre amplifiers

We propose an innovative, fully space–time model to take into account the seed-dependent nature of ageing penalties in high-power ytterbium-doped fibre amplifiers. Ageing is shown to be based on the on-going competition between photo-darkening and photo-bleaching phenomena. Our approach is based on the natural interplay between the excited states of co-existing ytterbium pairs and colour centres in highly doped fibres, in the presence of thermal coupling between the closely spaced excited states. As initiated from IR photons, the excitation of colour centres up to the UV band is supposed to be governed by multi-photon absorption. The interactions of interest in the kinetics of photo-bleaching then take the form of highly efficient charge transfers, which imply the reduction of some fraction of the basically trivalent ions to their divalent state. Due to the activation of ytterbium pairs by means of energy transfer up-conversion, these interactions get more and more effective at elevated operating powers. Computational results using these principles actually help to fit our experimental data regarding seeding effects, as well as fully generic trends already evidenced in the literature. This gives a fine demonstration for the need to discriminate co-active pump and signal contributions. Our self-consistent, still simplified model then consists of a valuable tool to help for a deeper understanding of the ageing issues. Furthermore, considering higher-order ytterbium aggregates, this should open new routes towards more comprehensive models.

White light generation via sequential stepwise absorption and energy-transfer frequency upconversion in Tm3+/Er3+-codoped glass

White light emission with CIE-1931 coordinates close to the standard equal-energy white light illuminant, and color temperature in the 4000–6500K range was produced in Tm3+/Er3+-codoped glasses through frequency upconversion. Multi-Stokes Raman emissions generated in an optical fiber pumped at 1.064µm were used, seeking out the 3H5 thulium absorption band around 1.2µm as the departure point. The new and efficient excitation approach allowed the generation of near infrared (800nm), red (650nm), and blue (480nm) emission due to frequency upconversion excited via ground-state absorption and excited-state absorption in Tm3+ ions. Energy-transfer followed by excited-state absorption in Er3+ ions, provided the generation of the green (530, 550nm) and red (660nm) components to compose white light. The excitation mechanism for the Tm3+ ions is assigned to stepwise multi-wavelength excitation using Stokes Raman emission in an optical fiber, followed by stepwise excited-state absorption of the pump photons. Energy-transfer Tm3+[3F4] + Er3+[4I15/2] →Tm3+[3H6] + Er3+[4I9/2] followed by excited-state absorption of multi-Stokes Raman emissions populate erbium visible emitting level. Excitation power color tuneability of the overall light emission was also achieved.

Phase-shifted response of plasmonic nanostructures: Implications to luminescence upconversion

We analyze the dynamics of a quantum dipole emitter (QDE) illuminated by a resonant electromagnetic field and placed close to a metal nanostructure, whose response to the incident field is phase shifted by 3π/2 (-π/2). It is found that, due to the phase shift in a field scattered by the nanostructure (and acting on the QDE along with the incident field), QDE dynamics is characterized by a fast QDE transition to the excited state followed by relaxation to a stable superposition (of the excited and ground states) with a close to 1 probability of the QDE to be found in the excited state. We further argue that this effect can advantageously be used for luminescence upconversion enhancement when being realized for a lower excited state in the energy transfer upconversion process since, by largely eliminating the radiative relaxation channel, the probability of excitation transfer will be increased.

Effect of Er and Dy on IR-visible up-conversion luminescence properties of (Er0.01Dy0.01La0.01Zr0.02Y0.95)2O3 transparent ceramic

Abstract In this study, we present first ever fabrication of Er/Dy co-doped Y 2 O 3 transparent ceramic and demonstration of energy transfer upconversion mechanism between Dy 3+ and Er 3+ . The spherical nanoparticles of Er/Dy co-doped Y 2 O 3 were synthesized by coprecipitation technique and transparent ceramics were fabricated by vacuum sintering of nanoparticles at 1750 °C for 5 h. A transmittance of ~ 80% (in ~ 600–2000 nm range) without Fresnel loss correction was achieved by substituting sintering aids like Zirconium and Lanthanum during preparation of nano particles without affecting the cubic phase of Er/Dy co-doped Y 2 O 3 . The upconversion luminescence spectra in visible region have been studied. The measured excitation spectrum gave evidence of the energy transfer from Dy 3+ to Er 3+ energy levels. The Judd–Ofelt intensity parameters, spontaneous emission rate, branching ratios and radiative emission lifetimes corresponding to transition wavelengths of Er 3+ were determined. The observed strong emission band at 564 nm (Er 3+ : 4 S 3/2 → 4 I 15/2 ) under 1077 nm (Dy 3+ : 6 H 15/2 → 6 F 9/2 ) excitation prove its potential as a promising gain medium for compact solid-state upconversion laser.

Upconversion Nanoparticles Y2O3 and Gd2O3 Co-Doped with Er3+ and Yb3+ with Aminosilane-Folic Acid Functionalization for Breast and Cervix Cancer Cells Detection

ABSTRACTThe upconversion nanoparticles (UCNPs) possess the ability to absorb near infrared energy (980 nm) and upconvert it to emit in the visible spectra. In this research, the UNCPs emit in red (660 nm) due to the electronic transitions between two rare earth ions: Er3+ and Yb3+, this process is called energy transfer upconversion (ETU). The UCNPs were functionalized with aminosilanes and folic acid receptors (UCNP-FR) and analyzed by transmission electron microscopy, Fourier transform infrared spectroscopy and luminescence measurements. UCNPs-FR of Y2O3 have a particle size of 70 ± 10 nm and the Gd2O3 have a 50 ± 10 nm particle size. Both showed a good luminescence spectrum in comparison with the bare ones. Cytotoxicity of different amounts between 0.001 µg/ml to 1 µg/ml of bare and functionalized UCNPs was measured with the colorimetric assay MTT in three cancer cell lines: human cervical adenocarcinoma (HeLa), human breast cancer cells MB-MDA-231. Some concentrations of bare UCNPs were cytotoxic for cancer cells; however after their functionalization they resulted to be non-cytotoxic. The functionalized UCNPs were able to bind to folate receptors which are overexpressed in cervical and breast cancers cells. It was demonstrated by confocal microscopy, that the functionalized UCNPs were internalized into the cancer cells, confirming that they can be used as biolabels for breast and cervical cancer cells.

Optical spectroscopy, energy upconversion, and white-light emission characteristics of erbium-doped calcium fluoride crystal

Absorption spectrum of Er3+-dopedCaF2 revealed absorption peaks at 255, 365, 379, 407, 441, 449, 487, 522, 539, 652, and 798 nm. When the sample was excited with an 802-nm near-infrared laser, it revealed emissions at 390, 415, 462, 555, 665, and 790 nm due to stepwise excitation and energy transfer upconversion processes. The absorption and emission peaks are identified with Er3+ spectral transitions. The sample color appears to be either white or green under near-infrared laser excitation. Emission color was found to be dependent on the pump laser wavelength used and laser power. Excitation spectral recordings were made by tuning the pump laser wavelength. Excited state lifetimes are measured to analyze the data. Color coordinates and color temperatures are measured for 802- and 405-nm laser excitations. Our studies indicate that this sample is useful for solid-state lighting applications.

Upconversion luminescence of Er3+/Yb3+ doped Sr5(PO4)3OH phosphor powders

Sr5(PO4)3OH co-doped with Er3+and Yb3+ powder phosphors were synthesized by urea combustion method. The crystal structure was analyzed using X-ray diffraction (XRD). Particle morphology was analyzed using a Jeol JSM 7800F thermal field emission scanning electron microscope (FE-SEM) and the chemical composition analysis was carried out using an Oxford Instruments AzTEC energy dispersive spectrometer (EDS) attached to the FE-SEM. Upconversion emission was measured by using a FLS980 Spectrometer equipped with a 980nm NIR laser as the excitation source, and a photomultiplier (PMT) detector. The XRD data of the Sr5(PO4)3OH powder exhibited characteristic diffraction patterns of the hexagonal structure referenced in the standard JCPDS card number 00-033-1348. The sharp peaks revealed the formation of crystalline Sr5(PO4)3OH. The powders were made up of hexagonal nanospheres. The enhanced red emission due to the 4F9/2 → 4I15/2 transitions of Er3+ was observed and was attributed to up conversion (UC) energy transfer from Yb3+. The upconversion energy transfer mechanism from Yb3+ to Er3+ is discussed.

Er3+-doped tellurite glasses for enhancing a solar cell photocurrent through photon upconversion upon 1500 nm excitation

Trivalent erbium-doped tellurite glasses have been prepared and investigated with an aim to improve the efficiency of a Silicon (Si) solar cell through photon upconversion processes. The upconversion emissions at 548 nm (4S3/2, 2H11/2 → 4I15/2), 671 nm (4F9/2 → 4I15/2), 800 nm (4I9/2 → 4I15/2) and 975 nm (4I11/2 → 4I15/2) of Er3+ ions have been observed under 1500 nm laser excitation. The 3.0 mol% Er2O3-doped glass exhibits high upconversion emission at 975 nm compared to 0.01 and 0.5 mol% Er2O3–doped glasses. Temporal evolution studies confirm that energy transfer upconversion is responsible for observed upconversion emissions, which is in good agreement with a simple rate-equations model. The upconversion intensity and photocurrent of solar cell are found to increase with the increase of pump power of the laser. A significant enhancement in photocurrent of a solar cell has been observed, when the 3.0 mol% Er2O3-doped glass is placed over a solar cell under 1500 nm laser excitation. All the results indicate that 3.0 mol% Er2O3-doped glass could be suitable for enhancing the solar cell efficiency.

Frequency noise of distributed Bragg reflector single-frequency fiber laser.

We investigated the frequency noise in the distributed Bragg reflector single-frequency fiber laser (DBR-SFFL) theoretically and experimentally. A complete theoretical analysis is demonstrated by considering the energy-transfer upconversion (ETU) process and establishing linkages between the frequency noise and the relative intensity noise (RIN) of the DBR-SFFL. The experimental results of the diverse DBR-SFFLs in different working conditions are in good agreement with the theoretical analyses. These investigations are beneficial to optimizing frequency noise property to promote the wide application of the DBR-SFFLs. The proposed results can be generally applicable to the short-linear-cavity SFFL with centimeters order of the cavity length.

Four-Dimensional Thermal Analysis of 888 nm Pumped Nd:YVO4 Dual-Rod Acousto-Optic Q-Switched Laser

A theoretical analysis upon the four-dimensional (4D) spatio-temporal temperature dependent dynamics of 888 nm pumped Nd:YVO 4 dual-rod laser is established, which is valid in both continuous-wave (CW) and acousto-optic (AO) Q-switched pulse lasers conditions. Our model can accurately solve the 4D thermal generation and temperature evolution not only in the steady Q-switched state, but also in the first few unstable giant or dwarf pulses region. Factors including ground state depletion (GSD), energy transfer upconversion (ETU), fluorescence branching ratios, temperature-dependent cross sections and nonradiative relaxations processes are comprehensively considered for precisely estimating thermal effects, valid in both the steady pulse region and the unstable region at the beginning. Moreover, temporal and spatial temperature profiles and their coupling effect on output properties at different repetition-rates are discussed. Experiments of high-power high-repetition-rate 888 nm end-pumped Nd:YVO 4 dual-rod CW and AO Q-switched lasers are also firstly presented and the experimental results enjoy good consistency with our theory.

Nanophotosensitive drugs for light-based cancer therapy: what does the future hold?

Nanophotosensitive drugs for light-based cancer therapy: what does the future hold?

The influence of Nb2O5 crystallization on the infrared-to-visible upconversion in Er3+/Yb3+ co-doped SiO2-Nb2O5 nanocomposites

This study reports on how the niobium content influences the infrared-to-visible upconversion (UC) in Er3+/Yb3+ co-doped SiO2-Nb2O5 nanocomposites produced by an alternative sol-gel route. Raman spectroscopy, Transmission electron microscopy (TEM), and Photoluminescence spectroscopy (PL) helped to evaluate the structural and luminescent properties. The Nb2O5 content impacted the nanocrystallization process—different Nb2O5 polymorphs emerged, which affected the UC luminescence. An outstanding color tunability effect arose due to the unique structural features of the nanocomposites. Different green and red emissions appeared depending on the excitation power and on the Nb content. The Nb content of 30mol% afforded luminescence of similar intensity in the green and red range. For higher than 40mol% of Nb concentration, the luminescence was more intense in the red range. The green and red emissions indicated that an Energy Transfer Upconversion (ETU) process with Yb3+→ Er3+ energy transfer occurred efficiently. In addition, cross-relaxation process between Er3+ ions is also accounted for the red emission.

The luminescence parameters of Yb3+:Er3+-doped LiLa(WO4)2 single crystal grown in the form of fiber for up-conversion green emission

This report details the first study of the luminescence properties of a single crystal grown in the form of fiber for prospective application as the gain medium for fiber laser emission at 552nm. The excited state decay processes related with the 4S3/2→4I15/2 transition in double Yb3+:Er3+-doped LiLa(WO4)2 crystal have been investigated using time-resolved fluorescence spectroscopy with a Er3+ concentration of 0.5mol% and Yb3+ with 2, 5, 7, 10 and 15mol%. Selective laser excitation of the 2F5/2 energy level of Yb3+(972nm) and selective laser excitations of the 4I11/2 and 4I13/2 energy levels of Er3+(972 and 1550nm), respectively has established that in a similar way to other optical materials, a strong energy-transfer up-conversion by way of a dipole-dipole interactions between an Yb3+ excited and Er3+ ions, the 4F5/2 level (Yb3+) populates the 4S3/2 upper laser level of the 550nm transition. The 4S3/2 energy level emits luminescence with peaks having the wavelength center at 550nm with luminescence efficiency increasing from 7% for Er3+ singly doped to 36% for Yb3+(15mol%) co-doped crystals. The 4S3/2 lifetime of Er3+ is observed to increase due to the saturation of the multiphonon relaxation rate at high excited-state density of Yb3+ ions. At high excited-state density, Yb3+ ions saturates the accepting modes inside of a critical volume of RC=39.4Å centered at an excited Er3+(4S3/2) ion, by the high-energy phonons generated from emission sideband of Yb3+ ions in Yb(x%):Er(0.5%) crystals. It is established that the green (552nm) up-conversion luminescence of Er3+ is optimized using an Yb3+ concentration of 11.5mol% for Er(0.5%):LiLa(WO4)2 crystal.

Spectra- and temperature-dependent dynamics of directly end-pumped holmium lasers

We develop a theoretical model with high accuracy for directly end-pumped Ho3+ laser system considering the influences of ground-state depletion, energy transfer up-conversion, temperature-dependent cross sections, and pump spectra shift. The heat generation in our model is precisely evaluated by calculating the transition rates of non-radiation relaxation processes among manifolds and in-band relaxation processes based on a detailed analysis of energy levels structure of holmium ions. A spatial dynamic thermal iteration method, just developed by our group, is applied to describe the coupled influences between spatial thermal effects and pump spectra. This model is verified to both adapt to the narrow-band good beam-quality pumped case and the broad-band bad beam-quality pumped case, which is in accordance with our previous reported experimental results.

Insights into Er3+↔Yb3+ energy transfer dynamics upon infrared ~1550 nm excitation in a low phonon fluoro-tellurite glass system

Highly upconverting, monolithic transparent inorganic glass co-doped with Er3+/Yb3+ ions have been explored in view of their easy integration with Si-PV cell. A ~4 fold enhancement in the photo-current of mc-Si-PV cell has been observed using a Er3+/Yb3+ co-doped sample compared to Er3+ ions singly doped glass. The role of Yb3+ ions on the enhancement of photo-current has been discussed in light of the Er3+↔Yb3+ energy transfer mechanism involved from IR to NIR and VIS upconversion process upon IR ~1550nm excitation. The influence of excitation pump power and donor Er3+ ion concentration on the energy transfer upconversion (ETU) as well as excited state absorption (ESA) energy transfer mechanisms and its effect on the upconversion emission properties have been described in detail. The prominence of ETU or ESA process were elaborated considering the decay dynamics of NIR upconversion emission upon ~1550nm excitation.

RETRACTED: Near-infrared-to-visible upconverting luminescence of Er3+-doped CdSe nanocrystals grown by chemical bath

Abstract CdSe thin films were prepared by chemical bath and doped in situ with Er3+ ions. Three impurity levels were prepared by changing the relative volume of the salt solution containing Er3+ ions in the CdSe growing solution. Changes in the grain size (∼5.5–3.5 nm) and band gap energy (∼1.80–2.25 eV) were observed in the undoped and doped CdSe films, respectively. Photoluminescence studies displayed room temperature emission exhibiting NIR-to visible upconversion. The transition bands 4I13/2 → 4I15/2, 2I9/2 → 4I15/2 and 2F9/2 → 4F15/2 in the ∼700–850 nm region were investigated. Upconversion emissions were observed from the CdSeEr sample under light excitation (325 nm). The upconversion emission intensity ratio of these transitions is attributed to the variation of the local structure around Er3+ ions. These results confirm that visible upconversion emissions of Er3+ in the CdSeEr nanocrystals are mainly produced via two-photon excited-state absorption and energy transfer upconversion processes.