DURATA PROIECTULUI:      Ianuarie 2012 - Decembrie 2016
PROJECT DURATION:        January 2012 - December 2016

CONDUCATOR PROIECT:   Institutul National de Cercetare-Dezvoltare pentru Fizica Laserilor, Plasmei si Radiatiei, Magurele, Bucuresti, Romania
PROJECT LEADER:            National Institute for Laser, Plasma and Radiation Physics, Magurele, Bucharest, Romania

Contract value:                    1.500.000 lei

Valoare contract:                 1.500.000 lei

The research team:              GEORGESCU Vasile Serban, PhD

Echipa de cercetare:            CINTEZA Liudmila, PhD (Assoc. Prof.)

                                          TOMA Octavian, PhD

                                          VOICULESCU Ana-Maria, PhD

                                          MATEI Elena Cristina, PhD student

                                          STEFAN Angela, PhD student

                                          IONESCU Alina

Project Summary

OBJECTIVES
The main objective of this project is the finding of the optimal compositions of phosphors (host + dopants) excited in IR (940 - 980 nm) and emitting in NIR or UV for specific applications in biology, medicine and purification of the wastewaters.
2012 / Objective 1: Synthesis and characterization of phosphors doped with erbium and ytterbium. Although the emission in visible of this system was studied extensively, erbium transition ⁴S_3/2 to ⁴I_13/2 with emission in NIR was much less studied. We aim to study this transition with important applications as biomarker that can be excited and detected deeper in biological tissues.
2013 / Objective 2: Synthesis and characterization of phosphors doped with thulium and ytterbium. This system was also extensively studies for upconversion pumped visible emission; for this system, the NIR transition ³H₄ to ³H₆ is very efficient even in oxide hosts and is worthy of a systematic study; the interesting UV transition ¹D₂ to ³H₆ (~360 nm) can activate the photocatalytic activity of TiO₂ for water treatment; its initial level ¹D₂ is isolated from the next lower-energy level by a large gap and thus possesses a high quantum efficiency. Studies of the parameters of the various energy transfer processes from Yb³⁺ to Tm³⁺ are also missing from scientific literature.
2014 / Objective 3a: Synthesis and structural investigations of phosphors doped with Ho³⁺ and Yb³⁺. Solid state reaction and sol-gel method will be used to synthesize the phosphors. The X-ray diffraction and FTIR will be used for structural investigations.
2015 / Objective 3b: Objective 3b. Investigation of the emission properties of phosphors doped with Ho³⁺ and Yb³⁺. The Ho³⁺ ion has many transitions; the NIR transition of interest is ⁵I₃ to ⁵I₇; the UV transition of interest is (⁵G₄, ³K₇) to ⁵I₈ (~392 nm). The population mechanisms of the initial levels of these transitions will be investigated; a special attention will be paid to the characterization of the energy transfer processes Yb³⁺ - Ho³⁺, which have not been quantitatively described up to now.

OBIECTIVE
Obiectivul principal al proiectului este gasirea compozitiei optime a fosforilor (gazda + dopant) excitati in IR (la 940 - 980 nm) cu emisie in infrarosu apropiat sau in UV pentru aplicatii in biologie, medicina si purificarea apelor uzate.
2012:
Valoare 2012: 600.000 lei
Obiectivul 1: Sinteza si caracterizarea fosforilor dopati cu erbiu si ytterbiu. Desi sistemul a fost studiat extensiv, tranzitia erbiului ⁴S_3/2 →  ⁴I_13/2 cu emisie in infrarosu apropiat a fost mult mai putin studiata. Vom studia sistematic aceasta tranzitie cu aplicatii importante pentru biomarkeri care pot fi excitati si detectati in adancimea tesuturilor biologice.
2013:
Valoare 2013: 225.745.43 lei
Obiectivul 2: Sinteza si caracterizarea fosforilor dopati cu tuliu si ytterbiu. Sistemul a fost studiat extensiv pentru emisia in vizibil pompata prin conversie superioara; pentru acest sistem, tranzitia in IR apropiat ³H₄ la ³H₆ este foarte eficienta chiar si in oxizi si merita un studiu sistematic; tranzitia in UV ¹D₂ la ³H₆ (~360 nm) poate activa activitatea fotocatalitica a TiO₂ pentru tratarea apelor uzate, nivelul initial ¹D₂ fiind separat de urmatorul printr-o banda interziza larga are o eficienta cuantica ridicata. Lipsesc din literatura datele privind diferitele procese de transfer de energie Yb - Tm.
2014:
Valoare 2014: 200.000 lei
Obiectivul 3a: Sinteza şi investigarea structurală a fosforilor dopaţi cu Ho³⁺ şi Yb³⁺. Ca metode de sinteză, se vor utiliza reacţia în fază solidă şi metoda sol-gel. Investigarea structurală se va face prin difracţie de raze X şi FTIR.
2015:
Obiectivul 3b: Investigarea proprietăţilor de emisie ale fosforilor dopaţi cu Ho³⁺ şi Yb³⁺. Sunt de interes tranzitiile ⁵I₃ la ⁵I₇ (in IR apropiat) si (⁵G₄,³K₇) la ⁵I₈ (~392 nm). Vor fi investigate mecanismele de populare a acestor nivele pentru pompaj in IR; o atentie deosebită va fi acordată caracterizarii proceselor de transfer de energie Ho - Yb.

REZULTATE/RESULTS
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2012:
- Au fost sintetizate, prin metoda sol-gel Pechini, pulberi nanometrice de langasit (La₃Ga₅SiO₁₄) dopate cu Er³⁺ si Yb³⁺ sau dopate cu Eu³⁺ si tratate termic la temperaturi pana la 1000^oC.
- Au fost sintetizate, prin reactie in faza solida, probe ceramice de langatat (La₃Ga_5.5Ta_0.5O₁₄) si langanit (La₃Ga_5.5Nb_0.5O₁₄) dopate cu Er³⁺ si Yb³⁺. Aceste probe au un continut redus de centri de culoare.
- Probele obtinute au fost caracterizate prin difractie de raze X, FTIR si spectroscopie optica (reflectanta difuza / absorbtie, luminescenta excitata direct sau prin conversie superioara, cinetica nivelelor metastabile).
- Au fost studiate proprietatile de conversie superioara IR-vizibil si IR-UV si dependenta lor de conditiile de tratament termic in cazul pulberilor nanometrice si au fost evidentiate mecanismele de populare a nivelelor metastabile. 
- Rezultatele obtinute au fost publicate in reviste cotate ISI si prezentate la conferinte internationale.
- Using a modified Pechini sol-gel route, langasite (La₃Ga₅SiO₁₄) nano-powders doped with Er³⁺ and Yb³⁺ or with Eu³⁺ were synthesized and annealed in air at temperatures up to 1000ºC.
- Using the solid-state synthesis, ceramic samples of langatate (La₃Ga_5.5Ta_0.5O₁₄) and langanite (La₃Ga_5.5Nb_0.5O₁₄) doped with Er³⁺ si Yb³⁺, with low concentration of color centers, were obtained.
- The obtained samples were characterized by XRD and optical spectroscopy (diffuse reflectance / absorption, luminescence excited directly and by upconversion, kinetics of the metastable levels).
- The dependence on the annealing temperature of IR-IR and IR-UV upconversion properties of Er³⁺ and Yb³⁺ doped nano-powders and ceramic samples was analyzed.
- The results were published in ISI journals or presented at international conferences.

2012: PUBLICATIONS/CONFERENCES
1. A. M. Voiculescu, S. Georgescu, S. Nastase, C. Matei, D. Berger, C. Matei, A. Stefan, and O. Toma, “Upconversion luminescence of Er³⁺/Yb³⁺ co-doped nanolangasite synthesized by a modified Pechini route,” J. Sol-Gel Sci. Technol. 64, 667-672 (2012).
2. A. M. Voiculescu, S. Georgescu, S. Nastase, O. Toma, C. Matei, and A. Stefan "Upconversion processes in langasite nanopowders doepd with erbium and ytterbium," ICFE - 8th International Conference on f-elements, Udine, Italy, August 26-31, 2012, presentation OPT 32P.
3. C. Matei, S. Georgescu, D. Berger, S. Nastase, A. M. Voiculescu, and A. Stefan, "Luminescence properties in ZrO₂ nanoparticles doped with Er and Yb," ICFE - 8th International Conference on f-elements, Udina, Italy, August 26-31, 2012, presentation OPT 31P.
4. A. M. Voiculescu, S. Georgescu, L. Gheorghe, A. Achim, C. Matei, and A. Stefan, "Infrared-excited red, green, violet and UV-luminescence from langasite crystal doped with erbium and ytterbium," ICOM 2012, 3rd International Conference on the Physics of Optical Materials and Devices, Belgrad, Serbia, 3-6 September 2012, presentation P1.2-31, Book of Abstracts ISBN: 978-86-7306-116-0, pg 145.
5. A. M Voiculescu, S. Georgescu, O. Toma, and S. Nastase, "Properties of luminescence for nanopowders compounds from the langasite family doped with erbium and ytterbium," International Conference on Nanoscience and Technology (ICNT+T), July 23-27 2012, Paris, France, presentation PO3-21, Europhysics Conference Abstracts Vol. 36D, ISBN No. 2-914771-0, pg. 97.
6. A. M. Voiculescu, S. Georgescu, C. Matei, A. Stefan, and O. Toma, "Synthesis and characterization of langasite nanopowders doped with erbium and ytterbium," The 8th General Conference of Balkan Physical Union, 5-7 July 2012, Constanta, Romania, presentation SP-P11.
7. S. Georgescu, A. M. Voiculescu, C. Matei, A. Stefan, and O. Toma, Upconversion luminescence in La₃Ga_5.5Ta_0.5O₁₄ codoped with Er³⁺ and Yb³⁺," International Conference on Optics "Micro-to-Nano-Photonics, September 3-6 2012, Bucharest, Romania, presentation I.I.6.
8. A. M. Voiculescu, S. Georgescu, C. Matei, and S. Nastase, "Upconversion luminescence of nanolangasite doped with Er³⁺ and Yb³⁺," Physics Conference TIM-12, 27-30 November 2012, Timisoara, Romania; Abstract Book pg. 14, presentation API F205.
9. C. Matei, S. Georgescu, A.M. Voiculescu, and S. Nastase, "Synthesis by Pechini method of Eu³⁺: nanolangasite," Physics Conference TIM-12, 27-30 November 2012, Timisoara, Romania; Abstract book pg. 74, presentation API 74.

● Raportul stiintific pe anul 2012 este disponibil aici: | 2012 | 
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2013:
- Au fost sintetizate, prin reacţie in faza solidă, probe ceramice de langatat (La₃Ga_5.5Ta_0.5O₁₄) şi langanit (La₃Ga_5.5Nb_0.5O₁₄) dopate cu Tm³⁺ si Yb³⁺ cu concentraţii diferite de dopanţi, precum si probe de langasit şi langatat dopate numai cu Tm³⁺ (1 at.% în raport cu La³⁺). Aceste probe au un conţinut redus de centri de culoare.
- Au fost sintetizate, prin metoda Pechini sol-gel, puberi nanometrice de langasit (La₃Ga₅SiO₁₄) dopate cu Yb³⁺ şi Tm³⁺.
- Au fost sintetizate, prin metoda Pechini sol-gel, pulberi nanometrice de LaGaO₃ şi LaAlO₃ dopate cu Yb³⁺ şi Tm³⁺.
- Au fost sinterizate, prin reactie în faza solidă, probe ceramice de LaGaO₃ dopate cu Yb³⁺ şi Er³⁺ precum şi probe ceramice de LaGaO₃ dopate cu Yb³⁺ şi Tm³⁺.
- Probele obtinute au fost caracterizate prin difracţie de raze X, FTIR şi spectroscopie optica (reflectanta difuza/absorbtie, luminescenta excitata direct sau prin conversie superioara, cinetica nivelelor metastabile).
- Rezultatele obtinute au fost publicate in reviste cotate ISI si prezentate la conferinte internationale.
- Langatate (La₃Ga_5.5Ta_0.5O₁₄), langanite (La₃Ga_5.5Nb_0.5O₁₄) and langasite (La₃Ga₅SiO₁₄) ceramic samples, doped with various concentrations of Tm³⁺ and Yb³⁺, and having low concentration of color centers, were synthesized by solid-state reaction; the same synthesis method was used for LaGaO₃ doped with Er³⁺ and Yb³⁺ and for LaGaO₃ doped with Tm³⁺ and Yb³⁺.
- Langasite, LaGaO₃ and LaAlO₃ nano-powders doped with Yb³⁺ and Tm³⁺ were synthesized by a modified Pechini route.
- The obtained samples were characterized by XRD and optical spectroscopy (diffuse reflectance / absorption, directly excited and upconversion luminescence, kinetics of the metastable levels).
- The results were published in ISI journals or presented at international conferences.

2013: PUBLICATIONS/CONFERENCES
1. A. M. Voiculescu, S. Georgescu, C. Matei, A. G. Stefan, L. Gheorghe, A. Achim, and F. Voicu, “Infrared-excited, violet and UV luminescence from langasite crystal doped with erbium and ytterbium”, Rom. J. Phys. 58, 136-142 (2013).
2. S. Georgescu, A. M.Voiculescu, C. Matei, A. G. Stefan, and O. Toma, “Violet and near-ultraviolet upconversion luminescence in La₃Ga_5.5Ta_0.5O₁₄ codoped with Er³⁺ and Yb³⁺, Physica B 413, 55–58 (2013).
3. S. Georgescu, A. M. Voiculescu, C. Matei, C. E. Secu, R. F. Negrea, and M. Secu, “Ultraviolet and visible up-conversion luminescence of Er³⁺/Yb³⁺ co-doped CaF₂ nanocrystals in sol-gel derived glass-ceramics,” Journal of Luminescence 143, 150-156 (2013).
4. O. Toma and S. Georgescu, “Selectively excited Er³⁺ luminescence in calcium lithium niobium gallium garnet,” Rom. J. Phys. 58 (3-4), 365-372 (2013).
5. C. Matei, S. Georgescu, A.-M. Voiculescu, A. Stefan, and R. Birjega, “Synthesis by solid state reaction of Langanite doped with Tm³⁺ and Yb³⁺ and luminescence properties,” INDLAS 2013, 20-24 May 2013 Bran, Romania.
6. A. M. Voiculescu, S. Georgescu, C. Matei, A. Stefan, and R. Birjega, “Upconversion luminescence properties of La₃Ga_5.5Ta_0.5O₁₄ doped with Yb and Tm,” INDLAS 2013, 20-24 May 2013 Bran, Romania.
7. A. M. Voiculescu, S. Georgescu, C. Matei, A. Stefan, O. Toma, and S. Nastase, "Synthesis and characterization of Europium-doped nano Langasite," 3rd International Conference on RARE EARTH MATERIALS (REMAT) Advances in Synthesis, Studies and Applications, 26-28 April 2013, Wroclaw, Poland.
8. A. M. Voiculescu, S. Georgescu, C. Matei, A. Stefan, O. Toma, F. Voicu, and R. Birjega, “Synthesis and characterization of La₃Ga_5.5Ta_0.5O₁₄ doped with Yb and Tm for upconversion luminescence”, Materials, Methods & Technologies, 15th International Symposium, 10-14 June 2013, Sunny Beach, Bulgaria, presentation P66.
9. C. Matei, S. Georgescu, A. M. Voiculescu, F. Voicu, and R. Birjega, “Upconversion luminescence of Tm, Yb - doped langanite ceramic synthesized by solid state reaction”, Materials, Methods & Technologies, 15th International Symposium, 10-14 June 2013, Sunny Beach, Bulgaria, presentation P67.

● Raportul stiintific pe anul 2013 este disponibil aici: | 2013 | 
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2014:
In cadrul acestei etape, am invesigat compuşii oxidici CaSc₂O₄ şi SrY₂O₄ dopaţi cu ioni de pământuri rare şi am continuat investigarea langatatului (La₃Ga_5.5Ta_0.5O₁₄) dopat cu ioni de pământiri rare.
- Fosforii au fost sintetizaţi prin două metode: sinteză în fază solidă şi sol-gel.
- Pulberile obţinute prin sol-gel au fost tratate termic în aer la temperaturi între 800^oC şi 1200^oC.
- Fosforii au fost dopaţi cu Ho³⁺, Ym³⁺, Yb³⁺ şi Er³⁺.
- Fosforii obţinuţi au fost caracterizaţi prin difracţie de raze X şi spectroscopie optică (absorbţie, luminescenţă, cinetica nivelelor metastabile.
- Probele ceramice de CaSc₂O₄ dopat cu Ho³⁺ sau Tm³⁺ precum şi cele de langatat dopat cu Tm³⁺ sau Er³⁺ au fost caracterizate prin metoda Judd-Ofelt; calibrarea spectrelor de absorbţie a fost făcută fie utilizând timpul de viaţă al unui nivel care se dezexcită radiativ (⁴I_13/2 - Er³⁺, ¹G₄ - Tm³⁺ sau ⁵I₇ - Ho³⁺), fie utilizând componenta de dipol magnetic a unei tranziţii (³H₆ → ³H₅ - Tm³⁺).
- A fost investigată luminescenţa Tm³⁺ excitată prin conversie superioară în pulberea nanometrică de SrY₂O₄:Tm:Yb obţinută pein sol-gel.
- CaSc₂O₄, SrY₂O₄ and La₃Ga_5.5Ta_0.5O₁₄ (langatate) doped with rare-earth ions were obtained using the solid-state synthesis.
- The sol-gel method (followed by annealing in air at temperatures between 800^oC and 1200^oC) was used for the synthesis of SrY₂O₄ doped with Tm³⁺ and Yb³⁺.
- The obtained phosphors were characterized by XRD and optical spectroscopy. The ceramic samples of CaSc₂O₄ (doped with Ho³⁺ or Tm³⁺) as well the langatate doped with Er³⁺ were investigated using the Judd-Ofelt approach; the absorption spectra were calibrated using either the lifetime of a level known to decay mainly radiatively (⁴I_13/2 - Er³⁺, ¹G₄ - Tm³⁺ or ⁵I₇ - Ho³⁺), or using the magnetic-dipole contribution to an absorption transition (³H₆ → ³H₅ - Tm³⁺).
- The upconversion luminescence of the SrY₂O₄:Tm:Yb nano-powder (obtained using the sol-gel approach) was investigated.

1. S. Georgescu, A. Stefan, A. M. Voiculescu, O. Toma, C. Matei, R. Birjega, „Peculiarities of the Ho³⁺-Yb³⁺ energy transfer in CaSc₂O₄:Ho:Yb”, J. Luminescence 154, 142-147 (2014).
2. O. Toma, S. Georgescu, „Excited-state absorption in erbium-doped calcium lithium niobium gallium garnet,” J. Luminescence 154, 553-558 (2014).
3. S. Georgescu, O. Toma, C. Matei, A. M. Voiculescu, A. Ştefan, ”Judd–Ofelt analysis of Tm³⁺ in La₃Ga_5.5Ta_0.5O₁₄ ceramic with granular structure,” J. Luminescence 157, 35-38 (2015).
4. S. Georgescu, A. M. Voiculescu, C. Matei, A. Stefan, O. Toma, R. Birjega, ”Upconversion luminescence in langatate ceramics doped with Tm³⁺ and Yb³⁺,” J. Luminescence 154, 74-79 (2014).
5. O. Toma, S. Georgescu, ”Judd-Ofelt analysis of Er³⁺ ions in calcium lithium niobium gallium garnet,” J. Luminescence 147, 259-264 (2014).

Au fost prezentate două lucrări la conferinţa internaţională ”The 14th International Balkan Workshop on Applied Physics”, 2-4 iulie 2014, Constanţa:
6. C. Matei, S. Georgescu, A. M. Voiculescu, A. Stefan, O. Toma, „Luminescence Properties in Langatate Ceramics Doped with Tm³⁺ and Yb³⁺,” The 14th International Balkan Workshop on Applied Physics, 2-4 iulie 2014, Constanţa, oral presentation S2-OP2, Book of Abstracts p. 111.
7. A. M. Voiculescu, S. Georgescu, C. Matei, A. Stefan, O. Toma, ”Synthesis and Characterization of La₃Ga_5.5Ta_0.5O₁₄ Doped with Holmium and Ytterbium,” The 14th International Balkan Workshop on Applied Physics”, 2-4 iulie 2014, Constanţa, poster presentation S2-P20, Book of Abstracts p. 127.

● Raportul stiintific pe anul 2014 este disponibil aici: | 2014 | 
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● Short resume of the results, in English:  | here|

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2015:
In anul 2015, am continuat investigarea compuşilor oxidici La₃Ga_5.5Ta_0.5O₁₄ (langatat) şi CaSc₂O₄ dopaţi cu ioni de pământuri rare (Er³⁺, Ho³⁺, Tm³⁺, şi Yb³⁺). Astfel:
- A fost extinsă metoda Judd-Ofelt pentru probe ceramice care, datorită structurii lor granulare, împrăştie lumina transmisă prin probă;
- Pentru calibrarea spectrelor de absorbţie, s-au folosit două abordări: (i) utilizarea tăriei liniei de dipol magnetic pentru tranziţia ³H₆ → ³H₅ a Tm³⁺, care este tabelată; (ii) timpul de viaţă măsurat al unui nivel metastabil care poate fi considerat egal cu timpul de viaţă radiativ, datorită gap-ului energetic mare dintre acest nivel şi nivelul următor;
- Timpii de viaţă radiativi au fost comparaţi cu cei experimentali şi au fost astfel estimate eficienţele cuantice ale nivelurilor emiţătoare;
- Metodele de calibrare propuse pot fi aplicate şi pentru materiale transparente (sticle, cristale, ceramici transparente, fără împrăştiere a luminii), pentru care grosimea probei este cunoscută, dar concentraţia de ioni activatori nu este cunoscută precis. Se evită, astfel, erorile de măsurare a concentraţiei;
- Pentru CaSc₂O₄:Er au fost analizate cantitativ procesele de relaxare încrucişată în care sunt implicate nivelurile cu emisie în verde şi a fost determinat microparametrul de transfer de energie dipol-dipol;

Rezultatele obţinute în cadrul acestei etape au fost publicate în următoarele lucrări:
[1] S. Georgescu, A. Ştefan, A.M. Voiculescu, O. Toma, “Judd–Ofelt analysis of ceramic La₃Ga_5.5Ta_0.5O₁₄ doped with Er³⁺”, J. Lumin. 162 (2015) 168-173.
[2] S. Georgescu, A. Ştefan, O. Toma, A.M. Voiculescu, “Judd–Ofelt analysis of Ho³⁺ doped in ceramic CaSc₂O₄”, J. Lumin. 162 (2015) 174-179.
[3] S. Georgescu, A. Ştefan, A.M. Voiculescu, O. Toma, “Judd–Ofelt analysis of Tm³⁺ doped in CaSc₂O₄ ceramic samples”, J. Lumin. 166 (2015) 130-136.
[4] S. Georgescu, A. Ştefan, O. Toma, “Judd–Ofelt and energy-transfer analysis of Er³⁺ doped in CaSc₂O₄ ceramic samples”, J. Lumin. 167 (2015) 186-192.
[5] A.M. Voiculescu, S. Georgescu, C. Matei, A. Stefan, O. Toma, “Synthesis and characterization of La₃Ga_5.5Ta_0.5O₁₄ doped with holmium and ytterbium”, Romanian J. Phys. 60 (2015) 495-501.

A fost prezentată o lucrare la Conferinţa Internaţională “THE 8^th INTERNATIONAL CONFERENCE ON ADVANCED MATERIALS, ROCAM 2015”, Bucureşti, 7-10 iulie 2015
[6] A. Ştefan, S. Georgescu, O. Toma, C. Matei, “Upconversion luminescence in CaSc₂O₄ codoped with Er³⁺ and Yb³⁺” (prezentare orală).

During 2015, we continued the investigation of the oxide compounds La₃Ga_5.5Ta_0.5O₁₄ (langatate) şi CaSc₂O₄ doped with rare-earth ions (Er³⁺, Ho³⁺, Tm³⁺, and Yb³⁺). The main results are:
- The Judd-Ofelt method was extended for ceramic samples which scatter the transmitted light. Various methods were used to calibrate the absorption spectra:  
    (i) the lifetime of an energy level which, due to the large gap between this level and the lower next one, has a radiative decay (⁴I_13/2 (Er³⁺), ¹G₄ (Tm3+));   
    (ii) the strength of a magnetic-dipole transition for transitions containing both magnetic-dipole and electric-dipole (³H₆ → ³H₅, Tm³⁺).;
- The quantum efficiencies of the emitting levels of Er³⁺, Tm³⁺ and Ho³⁺ were estimated.

2016:
In cursul anului 2016 activitatea a fost îndreptată în două direcţii:
    (i) modelarea matematică a proceselor care conduc la emisia prin conversie superioară;
    (ii) măsurarea eficienţei emisiei prin conversie superioară pentru fosforul CaSc₂O₄:Er:Yb.
Pentru modelarea matematică a proceselor care au loc în fosforii dopaţi cu Er³⁺ şi Yb³⁺, am inclus următoarele procese de transfer de energie:
        (1) (⁴I_15/2 (Er³⁺), ²F_5/2 (Yb³⁺)) → (⁴I_11/2 (Er³⁺), ²F_7/2 (Yb³⁺)), urmat de tranziţie multifononică pe ⁴I_13/2 (Er³⁺);
           (2) (⁴I_11/2 (Er³⁺), ²F_5/2 (Yb³⁺)) → (⁴F_7/2 (Er³⁺), ²F_7/2 (Yb³⁺)) urmat de tranziţii multifononice rapide pe (²H_11/2, ⁴S_3/2) (Er³⁺);
           (3) (⁴I_13/2 (Er³⁺), ²F_5/2 (Yb³⁺)) → (⁴F_9/2 (Er³⁺), ²F_7/2 (Yb³⁺)) (tranziţie asistată de fononi);
           (4) procesele de back-transfer Er³⁺ → Yb³⁺ (⁴I_11/2 (Er³⁺), ²F_7/2 (Yb³⁺)) → (⁴I_15/2 (Er³⁺), ²F_5/2 (Yb³⁺)) şi, respectiv, ((²H_11/2, ⁴S_3/2) (Er³⁺), ²F_7/2 (Yb³⁺)) → (⁴I_13/2 (Er³⁺), ²F_5/2 (Yb³⁺));
           (5) procesele de relaxare încrucişată în sistemul Er³⁺, ((²H_11/2, ⁴S_3/2), ⁴I_15/2) → (⁴I_9/2, ⁴I_13/2).
- Au fost obţinute formule analitice simple pentru populaţiile nivelurilor emiţătoare în verde (²H_11/2, ⁴S_3/2) şi în roşu (⁴F_9/2) şi pentru raportul intensităţilor roşu/verde. Formulele obţinute evidenţiază rolul proceselor incluse în modelare şi descriu bine datele experimentale.
- Schimbarea raportului între emisia în roşu şi cea în verde cu creşterea concentraţiei de Yb³⁺ a fost ilustrată cu diagrame CIE.
- Eficienţa emisiei prin conversie superioară, pentru pompaj în IR la 980 nm, a fost măsurată pe probe ceramice de CaSc₂O₄:Er:Yb, folosind sfera integratoare. Cele mai bune rezultate au fost obţinute pentru CaSc₂O₄:Er(1%):Yb(5%), şi anume 1.4%.

Rezultatele obţinute au fost publicate în Journal of Luminescence:
[1] A. Stefan, O. Toma, S. Georgescu, “Upconversion luminescence in CaSc₂O₄ doped with Er³⁺ and Yb³⁺,” J. Lumin. 180, 376-383 (2016).
şi prezentate in trei comunicari la două conferinţe internaţionale:
[2] O. Toma, S. Georgescu, and A. Stefan, “Excited-state absorption in erbium-doped ceramic langatate,” 2016 International Conference on Defects in Insulating Materials (ICDIM 2016)", 10-15 July 2016, Lyon, France (poster presentation);
[3] O. Toma, S. Georgescu, “Excited-state absorption in Er-doped partially disordered calcium lithium niobium gallium garnet,” 2016 International Conference on Defects in Insulating Materials (ICDIM 2016)", 10-15 July 2016, Lyon, France (poster presentation)
[4] A. Stefan, S. Georgescu, and O. Toma, “Peculiarities of Er³⁺ - Yb³⁺ energy transfer in CaSc₂O₄:Er:Yb,” 19th International Conference on Dynamical Processes in Excited States of Solids (DPC 2016), 17-22 July, 2016, Paris, France; Book of Abstracts, p. 202 (P29, poster presentation).

During 2016, the activity was focused on two directions: (i) mathematical modeling of the processes leading to upconversion emission; (ii) measuring the upconversion efficiency of the CaSc₂O₄:Er:Yb phosphor.
- Simple analytical expressions were obtained for the populations of the emitting levels (²H_11/2, ⁴S_3/2) (green emission) and ⁴F_9/2 (red emission) as well as for the ratio of red and green emissions. The obtained expressions evidence the contribution of the various energy-transfer processes to upconversion efficiency and describe the experimental data.
- The change of the green to red ratio with increasing Yb³⁺ concentration was illustrated with CIE diagrams.
- The upconversion efficiency of CaSc₂O₄:Er:Yb phosphor was measured with the integrating sphere, for IR pumping at 980 nm. The best result (1.4%) was obtained for a CaSc₂O₄:Er(1%)Yb(5%) ceramic sample.
The results were published in Journal of Luminescence and presented at international conferences.