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Down- And Up-Conversion Phenomena in Gd(Al,Ga)O Crystals With Pr And Yb Ions 3

5

12

3+

3+

Jaros#aw Komar, Radoslaw Lisiecki, Robert Marek Kowalski, Boguslaw Macalik, Piotr Solarz, Michal Marcin Glowacki, Marek Berkowski, and Witold Ryba-Romanowski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03441 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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The Journal of Physical Chemistry

Down- And Up-conversion Phenomena In Gd3(Al,Ga)5O12 Crystals Doped With Pr3+ And Yb3+ Ions J. Komar*1, R. Lisiecki1, R. Kowalski1, B. Macalik1, P. Solarz1, M. Głowacki2, M. Berkowski2, W. Ryba-Romanowski1 1

Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okólna 2, 50-422 Wrocław

2

Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warszawa

*

[email protected] 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

Single crystals of Gd3Al2.5Ga2.5O12, single-doped with Pr3+ ions and double-doped with Pr3+ and Yb3+ ions were fabricated by the Czochralski technique. Transition intensities and relaxation dynamics of Pr3+ ions were determined employing the Judd-Ofelt treatment. Crystal field splitting of excited multiplets of incorporated luminescent ions were determined based on optical spectra recorded at liquid helium temperature. The Pr3+ → Yb3+ energy transfer phenomena were determined analyzing the effect of Yb3+ concentration on luminescence spectra and decay curves of Pr3+ ions. It was concluded that observed downconversion phenomenon involves a quantum cutting mechanism consisting of a two-step energy transfer from Pr3+ to Yb3+. We observed a phenomenon of non-resonant conversion of femtosecond pulses of infrared light into visible Pr3+ emission that was weakly affected by wavelength of incident light at least in the 1100 – 1600 nm region. It was concluded that excitation mechanisms consist of multiphoton absorption of incident infrared radiation involving inter-configurational 4f2 – 4f15d transitions of Pr3+ and/or indirect excitation of Pr3+ ions by energy transfer from electrons created in the conduction band of the host.

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1. Introduction Trivalent praseodymium ions in wide band-gap media show a specific energy level structure with several metastable levels within the 4f2 configuration and low energy levels within the excited 5d configuration offering thereby a potential for application in the field of lasers, phosphors or scintillators. Numerous published papers were devoted to visible laser operation in various Pr3+-doped crystals, e.g.: a simultaneous laser action in YAG:Pr3+ at blue and orange wavelength 1, random-lasing at 604 nm YAlO3 crystal at wavelength 746.9 nm

3

2

and a laser performance of Pr3+-doped

have been reported in the past. More recently,

Pr:YLF orange laser operation at cryogenic temperature 4 and nonlinear mirror mode-locked Pr:YAlO3 laser performance 5 have been studied. Considerable attention has been paid to phenomena of up-conversion of infrared radiation around 1000 nm to visible luminescence in hosts single-doped with Pr3+ and double-doped with Pr3+ and Yb3+. For example, in Ref.

6

Authors reported efficient conversion of infrared

light delivered by Ti-Sapphire ring laser in 920 – 1060 nm range into visible luminescence in the low phonon-energy KPb2Cl5:Pr3+ system. It has been then concluded that a two-photon excitation gives rise to the 1D2 luminescence whereas a three-photon excitation mechanism is responsible for the 3P0 luminescence. Much interest has been directed to systems co-doped with praseodymium and ytterbium because a strong Yb3+ absorption followed by Yb3+ → Pr3+ energy transfer enhances markedly the up-conversion efficiency. In fact, an efficient upconversion has been observed in sol-gel derived Yb3+ – Pr3+ co-doped SiO2-LaF3 nano-glassceramics, whereas no up-conversion was observed in single-doped Pr3+ samples 7. It has been then ascertained that the up-conversion in Yb3+ - Pr3+ co-doped samples occurred for both the CW and pulsed excitation, a finding supposed to indicate that the second step excitation involves the Yb3+ - Pr3+ energy transfer rather than an excited state absorption (ESA) of Pr3+ ions. On the other hand, rather weak up-converted luminescence has been observed in Yb3+ – 3 ACS Paragon Plus Environment


Pr3+ co-doped fluoride glasses and glass ceramics 8. This discouraging finding has been attributed to the contribution of adverse cross relaxation processes and/or of a back energy transfer Pr3+ → Yb3+ . Interest in down-conversion phenomena in crystals and glasses containing Pr3+ and Yb3+ ions is stimulated by attempts to enhance efficiencies of solar cells employing a phenomenon of near-infrared quantum cutting

9,10

. In this phenomenon one high energy photon of light in

the UV-blue region incident on a luminescent material is converted to two infrared photons emitted near 1000 nm. Numerous published papers have been devoted to investigation of down-conversion efficiency and mechanism involved in various hosts double doped with Yb3+–RE3+ ions (RE = Pr, Er, Tm, Ho)

11–16

. In particular, various possible mechanisms of

quantum-cutting in Pr3+ – Yb3+ codoped oxyfluoride glass ceramics have been considered and observed down-conversion phenomenon has been attributed to one-step energy transfer (Pr3+:3P0→1G4) →(Yb3+:2F5/2→2F7/2) which results in generation of two infrared photons via the Yb3+:2F5/2→2F7/2 and Pr3+:1G4→3H5 transitions

17

. The enhanced 1G4 emission has been

observed in NaLaF4:Pr3+, Yb3+. The 3P0→1G4 (Pr3+) / 2F7/2 → 2F5/2 (Yb3+) energy transfer step was considered but no experimental proof for the second energy step in the down-conversion process between Pr3+ and Yb3+ was provided 18. On the other hand, investigation of quantum cutting in Yb/Pr – codoped NaY(WO4)2 has revealed that the conversion of a blue photon at 456 nm into two infrared photons near 1000 nm proceeds with quantum efficiency of 158% and the conversion of an UV photon at 310 is able to provide three infrared photons near 1000 nm with quantum efficiency of 258%

19

. Results mentioned above indicate that the up-

conversion phenomena and efficiency of quantum cutting in Pr3+ – Yb3+ system, as well as mechanisms involved depend strongly on the host material. In the present paper we consider down- and up-conversion phenomena in Gd3Al2.5Ga2.5O12 (GAGG) crystals containing Pr3+ and Yb3+ ions. Partial substitution of aluminum ions by 4 ACS Paragon Plus Environment

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gallium ions in this host crystal creates a certain structural disorder resulting in a significant inhomogeneous broadening of spectral lines of incorporated luminescent rare earth ions 20. It has been demonstrated that rare earth-doped GAGG is able to provide multi-purpose materials. When doped with Pr3+ it shows scintillating properties with the light yield around 4500 photon/MeV 21. Doping with cerium leads to much higher yield of about 46000 photons/ MeV

22

. When doped with Yb3+ it shows efficient laser operation near 1000 nm in pulsed

regime 23 and in CW regime 24. A Gd3Ga3Al2O12:Eu3+ (GGAG) nanophosphor is able to show efficient red emission implying that it can be used as a red phosphor to warm the colour in white LED systems 25. Intention of the investigation reported in the present paper is to determine optical features of materials under study that may be relevant for practice and to elucidate mechanism involved in processes of excitation, energy transfer and emission characteristics observed.

2. Experimental Gd3Al2.5Ga2.5O12 crystals nominally doped with 0.7 at. % Pr3+, and co-doped with 0.7at.% Pr3+ + 1.5 at.% Yb3+, 0.7at.% Pr3+ + 3.0 at.% Yb3+ and 0.7at.% Pr3+ + 10 at.% Yb3+ with nominal Al:Ga ratio 1:1 were grown by the Czochralski method under a nitrogen atmosphere on oriented seed with a pulling rate of 2.5 mm/h and a speed of rotation 20 rpm. The temperature adopted in the growth procedure was ca. 1860°C ± 20°C. We used a Malvern MSR4 puller with automatic diameter control based on weighing the crucible. Starting materials - Gd2O3 (4N or 99.99% purity), Al2O3 (4N5 or 99.995% purity) and Ga2O3 (5N or 99.999% purity) - were dried at 1000 °C for 4 hours before weighing. Suitable admixtures of Pr2O3 and Yb2O3 were introduced to GAGG in place of Gd2O3. Powders were pressed into cylindrical pellets under pressure of 200 kPa and calcinated at 1350 °C for 6 hours before melting in a crucible. Transparent crystals with 20 mm of the diameter and 60 5 ACS Paragon Plus Environment


mm in length were grown with a convex crystal melt interface from inductively heated iridium crucible of 40 mm in diameter. Samples in the form of parallelepipeds 10x6x4 mm3 were cut out from as grown boules and polished for spectroscopic measurement. Inductively coupled plasma (ICP-AES) measurement was performed to determine actual concentration of Pr3+ ions in fabricated crystals which may be lower than intentional because praseodymium ions substitute markedly smaller gadolinium ions in this host. Application of the ICP-AES method posed some problems due to heavy interference between the lines related to gallium and praseodymium ions. The line corresponding to praseodymium emission at 417 nm showed strong interference with the 418 nm line of gallium, while the line at 422 nm line pointed at unrealistically high praseodymium concentration consistent with a segregation coefficient around 1.4. Solely a line at 406 nm provided a reasonable Pr3+ concentration value amounting to 0.45 at.%. This value agrees well with the segregation coefficient of 0.64 reported for Pr3 in Gd3Ga3Al2O12 samples obtained by a pulling-down method 21. Absorption spectra were measured with an Agilent Cary 5000 spectrophotometer employing a SBW (spectral band-width) of 0.1 nm in the visible and ultra-violet range and 0.7 nm in the infrared. In addition, the 3P0 and 1D2 bands were measured at low temperature using SBW of 0.03 nm. Survey luminescence spectra in the visible were recorded employing StellarNet SILVER-Nova 25 TEC BW16 CCD spectrometer. High resolution luminescence spectra were recorded employing a Dongwoo Optron DM711 monochromator having a focal length of 750 mm, coupled with Hamamatsu R3896 photomultiplier or InGaAs detector depending on spectral region. An ozone free DL180-Xe Lamp coupled to an excitation monochromator Dongwoo Optron DM152i, a laser diode emitting at 445 nm, or an Apollo Instruments F4980-6 Laser diode emitting infrared radiation at 980 nm were used for excitation purposes. To record time resolved spectra and luminescence transients the samples were excited employing 6 ACS Paragon Plus Environment

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an optical parametric oscillator Opotek Opolette 355 LD emitting 5 ns pulses. Luminescence decay curves were acquired with a photomultiplier connected to a digital Tektronix MDO 4054B oscilloscope. A femtosecond laser (Coherent Model “Libra”) coupled to optical parametric amplifier (Light Conversion Model “OPerA”) delivering 100 fs pulses at wavelength tuned between 1000 nm and 1500 nm was used to excite up-converted visible luminescence. Spectra of this luminescence were recorded with a grating monochromator (Princeton Instr. Model Acton 2500i) coupled to a streak camera (Hamamatsu Model C5680) An Oxford Instr. Model CF 1204 continuous flow liquid helium cryostat equipped with a temperature controller was used to record absorption spectra at 4.2 K.

3. Results and discussion 3.1. Analysis of absorption spectra of GAGG:Pr3+ Figure 1 shows a room temperature survey absorption spectrum for GAGG: 0.45 at% Pr, 1.5 at% Yb. It consists of bands related to transitions between the ground 3H4 multiplet and excited multiplets of Pr3+ ions and to the 2F7/2 → 2F5/2 transition of Yb3+ ions around 980 nm. The transition to the first excited level 3H5 is beyond the range of our measurement system, so the first band visible in Fig. 1 at long wavelength side at around 2300 nm corresponds to transition to 3H6 multiplet.



α [cm-1]

4

Absorption coefficient α [cm-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3

P2,1I6,3P1

3

0,06 1

0,04

G4

0,02 0,00 950

2

1000

1050

1100

Wavelength [nm]

1

Yb3+ 2 F7/2 →2F5/2

3

P0

3

F3,4 3

F2

1

D2

3

H6

0 500

1000

1500

2000

2500

Wavelength [nm] Fig. 1. Room temperature absorption of GAGG crystal containing 0.45 at% Pr3+ and 1.5 at% Yb3+ ions. Inset shows details of the 3H4 → 1G4 absorption spectrum recorded for GAGG crystal single doped with 0.45 at% Pr3+. We assign a group of bands between 1750 nm and 2000 nm to transitions ending on the 3F2 multiplet and that between 1250 nm and 1750 nm to transitions ending on closely spaced 3F3 and 3F4 multiplets. Near 1000 nm there is an intense band related to ytterbium ion transition 2

F7/2 → 2F5/2, which hides a much weaker praseodymium ion transition to 1G4 state. Detailed

spectrum of the latter transition recorded for a GAGG: 0.45 at% Pr3+ is shown in the inset. In the visible there are two groups of bands. One of them, centered around 600 nm is related to a transition to the 1D2 state and the other one located between 440 nm and 500 nm to transitions ending on 3P0, 3P1, 1I6 and 3P2 states. Assignment of the lowest energy band in this group to a transition to the 3P0 , which is well separated from the next higher energy bands is rather easy. Assignment of bands located at higher energy is not trivial, however. To perform calculation 8 ACS Paragon Plus Environment

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of transition intensities within the Judd-Ofelt theory we assume that the dissimilarity of transition wavelengths of a rare earth ion in different garnet crystals is not large and we rely on experimental data and CF calculation reported by Antic-Fidancev et al.

26

for GGG:Pr3+

and Pr3Ga5O12 crystals. Analysis of low temperature optical spectra presented later on justifies this assumption. Standard Judd-Ofelt theory predicts that the values of oscillator strengths are equal to  n2 + 2  8π 2 mc  f = n 3hλ (2 J + 1)  3n 

2

∑

Ωt φa U ( t ) φb

2

,

t = 2,4,6

(1)

where m is electron mass, c is speed of light in vacuum, h is Planck constant, J is total angular momentum of initial state, n is refractive index, Ωt are Judd-Ofelt intensity parameters and φa U ( t ) φb are matrix elements of doubly reduced unit tensor operators. On the other hand oscillator strengths can be derived from absorption spectrum using the formula:

f exp =

4.32 ⋅ 10 −9 A(ν )dν c⋅d ∫

(2)

As the reduced matrix elements U (t ) can be thought to be independent of the host, and their values are given in appropriate tables 27, this gives a possibility to obtain the values for the Ωt parameters using a least squares fitting method, commonly employed for this purpose. Having the Ωt parameters one can calculate the transition speeds between any given states using the equation:

AJ ′J

 n2 + 2    = n 3  3h(2 J ′ + 1)λ  3n  64π 4e 2

2

∑

Ωt φa U (t ) φb

2

t = 2, 4, 6

(3)

Where e is electron charge, λ is mean wavelength of the transition considered (in centimeters), and all other variables have the same meaning as in previous equations. Having calculated AJJ ′ the values, one can obtain the branching ratios and lifetime using equation:



β=

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AJ ′J , ∑ AJ ′J

(4)

1 ∑ AJ ′J

(5)

J

τ=

J

Attempts to determine Ωt parameters for GAGG:Pr3+ with all oscillator strengths obtained from experiment were not successful since the fitting procedure gave negative value of Ω2. When removing the 3P2 multiplet from calculations the Ωt parameters were positive but they predicted unreasonable values of radiative transition rates (573 µs for 3P0 and 42 µs for 1D2). This problem, frequently encountered when dealing with praseodymium doped crystals, is due to the fact that Equation (1) has been achieved based on several assumptions. The most relevant one, namely the assumption that excited 5d configuration is sufficiently high to be considered as degenerate, is not fulfilled since the lowest 5d state of praseodymium ions has quite low energy (in GAGG crystal it has energy of only about 35350cm-1). This causes serious problems with standard Judd-Ofelt theory. To overcome this shortcoming the modified approaches have been proposed, for example by including branching ratios in the fit and/or eliminating hypersensitive transitions 28 or taking into account the varying influence of 5d states

29

. Determination of branching ratios experimentally is not easy due to overlap

between the emissions from the 3P0 and 1D2 states, we adopt in the following the latter approach. Further referred as the Kornienko method

29

, this approach consists in modifying

the equations for oscillator strengths and transition rates by a multiplicative factor that takes into account the low lying 5d levels. The modified oscillator strengths are

(

(

f ′ = f ⋅ 1 + 2α EJ + EJ ′ − 2 E 0f

))

(6)

where

α=

1 1 = 2∆E5d 2 E4 f 5d − E f 0

(

), 10 ACS Paragon Plus Environment

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and f is oscillator strength calculated from eq.(2), EJ and E J ′ are energies of initial and final state respectively, E 0f is the center energy of the 4f states (taken to be 9940 cm-1 after Kornienko

29,30

), ∆E5 d is energy difference between center of all the 4f-4f states and the first

5d state, E4 f 5d is energy of the lowest lying 5d state, in case of GAGG crystal equal to 35350cm-1. The values of intensity parameters Ωt can be obtained as with traditional method employing a least squares fitting procedure. This correction causes oscillator strengths of lower lying multiplets to be weakened in relation to traditionally calculated ones, while for the multiplets above ~20000 cm-1 oscillator strengths appear higher. Analogously one can obtain the modified expression for transition rates:

AJ′J ′ = AJJ ′ (1 + 2α (EJ + EJ ′ − 2 E 0f )) with all variables having same meaning as for equation (6). Branching ratios and radiative lifetimes can be calculated using equations (4) and (5). Results of fitting procedure performed employing this modified approach are given in

Table 1. RMS deviation for the fit obtained was 0.65·10-6. Three Ωt parameters obtained are as follows: Ω2=0.66·10-20, Ω4=2.199·10-20, Ω6=2,491·10-20

Transition Oscillator strength [·10-6] energy [cm-1]

Experimental Theoretical

3

H6

4421

0.45

0.33

3

F2

5348

1.61

1.70

3

F3+3F4 6732

5.47

5.55

1

G4

9898

0.22

0.29

1

D2

16697

2.07

1.24

3

P0

20525

2.19

1.39

3

P1+1I6

21512

2.08

2.66 11 ACS Paragon Plus Environment


rms

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0.65

Table 1. Comparison of experimental and theoretical oscillator strength obtained using the Kornienko method. Table 2 presents calculated radiative transition rates for the 3P0, 1D2 and 1G4 multiplets. In the last column experimental lifetimes for the 3P0 and 1D2 multiplets are shown. It can be seen that there is a reasonable agreement between theoretical and experimental values

Transition 3

1

1

P0→

D2→

G4→

λ[µm]

A [s−1]

ΣiAij [s-1]

β[%]

Tr [µs]

Texp [µs]

47640

0.0

21

19

206

168

631

-

1

D2

2.61

22.3

1

G4

0.941

572.6

1.2

3

F4

0.725

3258.7

6.8

3

F3

0.707

0.0

0.0

3

F2

0.659

22423.0

47.1

3

H6

0.621

9312.7

19.5

3

H5

0.538

0.0

0.0

3

H4

0.487

12050.6

25.3

1

G4

1.47

718.5

3

F4

1.00

2006.6

41.4

3

F3

0.957

118.2

2.4

3

F2

0.881

271.3

5.6

3

H6

0.815

258.5

5.3

3

H5

0.678

15.7

0.3

3

H4

0.599

1463.3

30.2

3

F4

3.51

64.2

3

F3

2.93

14.2

0.9

3

F2

2.23

4.8

0.3

3

H6

1.85

402.5

25.4

4852.2

1585.3

14.8

4.0


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3

H5

1.35

1020.5

64.4

3

H4

1.04

79.1

5.0

Table 2. Radiative transition rates A, branching ratios β, and radiative lifetimes Tr obtained from the Kornienko modification of the Judd-Ofelt method. Experimental lifetime values Texp are given in the last column.

Low temperature absorption spectra were measured at 4.2 K. to determine energies of crystal field components of excited multiplets involved in luminescence phenomena. At such a low temperature the transitions between multiplets of rare earth ions in crystals originate in the lowest crystal field component of the initial multiplet, unless the crystal field splitting of an initial multiplet is unusually small. Accordingly, a single line for the 3H4(1) → 3P0 transition is predicted. In fact, a single line located at 20591 cm-1 has been observed for GGG:Pr3+ 26. Corresponding absorption spectrum recorded for GAGG: 0.45 at% Pr3+ is shown in Fig. 2.



14

Absorption coefficient [cm-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

12

Experiment Fit Peak 1 Fit Peak 2 Fit Peak 3 Cumulative Fit Peak

10 8 6 4 2 0 20540

20560

20580

Wavenumber [cm -1] F ig 2. Absorption band related to the 3H4→3P0 transition measured at 4.2 K. Lines represent Gaussian curves fitted to the data represented by dots. In contrast to GGG:Pr3+ system it consists of a double line and a shoulder on the lower energy side. The overall spectral width of this transition is ca. 19 cm-1 (25 cm-1 for GAGG containing 0.45 at% Pr3+ and 10 at% Yb3+). Numerical decomposition of the band reveals that it is consistent with three components, a dominant peak at 20564 cm-1, a weaker one at 20574 cm-1 and a shoulder at 20555 cm-1. Fig. 3 shows the spectrum of 3H4(1) → 1D2 transition recorded at 4.2 K.


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Page 15 of 39

4

Absorption coefficient [cm-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

H4→1D2

3

2

1

0 16,4

16,6

16,8

17,0

17,2 3

17,4

17,6

-1

Wavenumber [10 cm ]

Fig 3. Absorption band related to the transition measured at 4.2K The energy difference between the highest energy component at 17226 cm-1 and the lowest energy component 16398 cm-1 implies that the overall splitting of the 1D2 multiplet amounts to 800 cm-1. Corresponding components have been located at 16430 cm-1 and 17220 cm-1 for GGG:Pr3+ implying the 1D2 splitting of 790 cm-1 26. A marked dissimilarity resides in that the 3

H4 → 1D2 spectrum recorded for GGG:Pr3+ contains five components in agreement with

theoretical prediction whereas eight components contribute to spectrum for GAGG:Pr3+. Results presented above indicate that praseodymium ions can enter non-equivalent sites in GAGG host that can be assigned to defects, ligand vacancies, antisites. It is worth mentioning here that low temperature absorption spectra of GAGG:Er3+ point at occurrence of at least two different sites for erbium ions

20

. Results of low temperature absorption measurement for

GAGG:Pr3+ are gathered in Table 3.



Page 16 of 39

Number components

Multiplet Experimental band components

of Overall splitting

Exp.

Theor.

[cm-1]

9

9

676

3

H4

0, 26, 49, 300, 348, 408, 527, 556, 676

3

H5

2280, 2298, 2324, 2347, 2393, 2569, 2600, 2624, 10 2671, 2806

11

526

3

H6

4302, 4338, 4373, 4407, 5118

5

13

816

3

F2

5350, 5411, 5492

3

5

142

3

F3

6469, 6483, 6494, 6551

4

7

82

3

F4

6753, 6780, 6807, 6969, 7098, 7124, 7270, 7281, 9 7404

9

651

1

G4

9726, 9806, 9838, 10117, 10251

5

9

549

1

D2

16398, 16432, 16472, 16506, 16905, 16924, 8 16957, 17226

5

800

3

P0

20555, 20564, 20574

1

n/a

3

P1,3P2, I6

20835, 20898, 20975, 21055, 21084, 21118, 20 21169, 21190, 21552, 21655, 21709, 21745, 21891, 22011, 22181, 22207, 22313, 22398, 22455, 22617

13+3+5=21

n/a

1

3

Table 3. Gathered energies of band components. It can be seen that except for the 3H4 and 3F4 multiplets the number of observed lines is lower than theoretically predicted. This concerns in particular the 3H6 multiplet for which five lines were observed (eight lines have been observed for GGG:Pr3+) and 1G4 multiplet for which five lines were observed (none observed for GGG:Pr3+). The spectral positions of components for the ground 3H4 state and first excited multiplet 3H5 that appear in the first two rows in Table 3 were determined based on luminescence spectra recorded at liquid nitrogen temperature. It is worth noticing that widths of lines related to transitions between individual crystal field levels for GAGG:Pr3+ are not smaller than ca. 15 cm-1, a value roughly five times


Page 17 of 39

larger than those reported for ordered YAG

31

32

and LuAG

crystals doped with

praseodymium.

3.2. Luminescence spectra and excited state relaxation of GAGG:Pr3+

Room temperature luminescence spectra of GAGG: 0.45 at% Pr3+ recorded in the visible region and in near infrared are shown in Fig. 4 and Fig. 5, respectively. The spectrum in Fig. 4

1,0

P0→3H6

3

P0→3H4

3

0,8

D2→3H4

1

P0→3H5

3

0,6

Lum. int. [a.u.]

was acquired using a 445 nm laser diode as a source of excitation.

Luminescence Intensity [a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 µs 70 µs

600

620

640

660

Wavelength [nm] 0,4

P0→3F2

3

3 3 P0→3F3 P0→ F4

3

0,2

0,0 500

550

600

650

700

750

800

Wavelength [nm] Fig 4. The room temperature emission spectrum measured upon excitation with laser diode at wavelength of 445 nm. Inset shows time resolved spectra It suffers from a certain instrumental broadening owing to rather poor resolution of StellarNet SILVER-Nova 25 TEC BW16 CCD spectrometer employed in this measurement. Nevertheless, spectral characteristics of praseodymium luminescence can be revealed, 17 ACS Paragon Plus Environment


Page 18 of 39

especially high intensities of bands related to the 3P0-3H5 and 3P0-3F3 transitions which contribute to the spectrum despite the fact that their transitions moments calculated using the Judd-Ofelt theory are vanishing. This behavior is rather typical for garnets

33

but it is also

encountered in other hosts, for example LaF3 34. This is due the fact that the low lying d levels influence higher lying f-states. This can also be seen as a change of parity of the 4f state’s wave functions. L and S cease to be good quantum numbers. As a result selection rules are relaxed. While the sample is excited around 445 nm the 3P0 bands dominate the emission spectrum, although the transitions from the 1D2 state may contribute too. To determine this contribution the time resolved luminescence spectra were recorded around 620 nm, where the 3

P0 → 3H6 and 1D2 → 3H4 transitions occur. In this experiment a train of 5 ns pulses at 445 nm

delivered by an optical parametric oscillator pumped by a third harmonic of YAG:Nd laser excited a sample and resulting luminescence was observed employing a set-up consisting of a grating monochromator and a photomultiplier coupled with a boxcar integrator. Results of measurement are shown in the inset to Fig. 4. A spectrum recorded for a short delay after an excitation pulse is related to transition from the short-lived 3P0 level, and that recorded for a delay of 70 µs is related to transition from the long-lived 1D2 level. Observation of the 1D2 luminescence indicates clearly that the 3P0 and 1D2 levels are bridged by a multiphonon relaxation. Slightly smaller value of experimental 3P0 lifetime than that calculated (given in Table 2) may be due to the contribution of this nonradiative process. Room temperature luminescence spectra of GAGG: 0.45 at% Pr3+ in near infrared, shown in Fig. 5, were recorded employing a grating monochromator coupled with an InGaAs detector.


Page 19 of 39

3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

P0→ G4

1

3

1

3

GAGG:0.45%Pr

G 4→ H 4 D2→ F4 Exc. 445 nm 1 1

3

G 4 → H5

1

D2 → G 4

Exc. 612 nm

800

1000

1200

1400

1600

Wavelength [nm]

Fig 5. Emission spectra of GAGG:0.45%Pr3+ in the NIR region The upper graph was obtained upon a CW excitation with a diode laser emitting at 445 nm whereas the lower graph was obtained upon excitation at 612 nm provided by a lamp coupled to an excitation monochromator. Assignment of bands in the spectra relies on low temperature spectra considered above. It is easy to assign the band that appears in upper graph between 800- 980 nm to the 3P0 → 1G4 transition since it is not present in the lower graph. Remaining bands appear in the two graphs, however. Therefore, relative contributions of the 1D2 → 3F4 and 1G4 → 3H4 transitions to the band peaking around 1000 nm cannot be assessed. At this point mechanisms involved in excitation of the 1G4 luminescence deserve some attention. Feeding of the 1G4 level via multiphonon relaxation from the 1D2 level is expected to be very weak because energy separation between these levels (ca. 6000 cm-1) is large as compared to the phonon cut-off energy of the host (ca. 800 cm-1). Two other processes are likely to contribute. One of them is the 1D2 → 1G4 radiative transition, predicted by the Judd19 ACS Paragon Plus Environment


Ofelt analysis. Another one consists of a cross-relaxation process in which an excited praseodymium ion makes a downward 1D2 → 3F3 transition and a coupled unexcited ion makes the 3H4 → 1G4 transition. The contribution of cross-relaxation processes may be responsible for markedly smaller value of experimental 1D2 lifetime than that calculated (given in Table 2).

3.3. Down-conversion in GAGG:Pr,Yb The effect of incorporation of ytterbium ions on Pr3+ luminescence in GAGG crystal can be inferred from Fig. 6 and Fig.7 which compare semilog plots of decay curves for the 3P0 and 1

D2 luminescence.

1

Luminescence intensity [a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 39

τavg=168µs

0.45% Pr 0.45% Pr 1.5% Yb 0.45% Pr, 3%Yb 0.45% Pr 10%Yb

0,1

τavg=86µs 0,01

τavg=59µs τavg=11µs 1E-3 0

100

200

300

400

Time [µs]

Fig 6. Luminescence decay curves for emission originating from 1D2 level


500

Page 21 of 39

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


τavg=19.0µs

0.45% Pr 0.45% Pr 1.5% Yb 0.45% Pr, 3%Yb 0.45% Pr 10%Yb

0,1

τavg=10.1µs τavg=9.3µs 0,01

τavg=2.8µs 1E-3 0

20

40

60

Time [µs]

Fig 7. Luminescence decay curves for emission originating from 3P0 level The decay curves were acquired at room temperature when exciting directly the metastable levels in question. It follows from these plots that Yb3+ ions are able to quench markedly both the 3P0 and 1D2 luminescence implying the contribution of the Pr3+ → Yb3+ energy transfer. As a consequence of the energy transfer involved the decay curves of Pr3+donor ions are not consistent with a single exponential time dependence. Therefore, we follow a commonly used approximation

35

and evaluate so called mean lifetime values τavg of donor ions in the

presence of Yb3+ acceptor ions, defined as

τ avg =

∫ t ⋅ I (t )dt ∫ I (t )dt



Next, a rough assessment of the energy transfer efficiency quenching efficiency values

η = 1−

τ , where τ0 denotes a lifetime of praseodymium donor ions in the absence of τ0

acceptors, was made. Evaluated τavg values are indicated in Figures 5 and 6 and η values are given in third column of Table 4. Yb [ at. %] τ [µs] η [%] Pda[103s-1] 3

P0

0

19.0

n/a

n/a

1.5

10.1

45,8

44,5

3

9.3

52,1

57,3

10

2.8

85,3

304,5

1

D2

0

168

n/a

n/a

1.5

86

48,8

5,7

3

59

64,9

11,0

10

11

93,5

85,0

Table 4 Lifetimes of 3P0 and 1D2 multiplets and derived energy transfer efficiencies.

The η values determined for the 3P0 and 1D2 luminescence do not differ markedly. However, a strong dissimilarity of corresponding τ0 values results in dissimilar energy transfer rates. In the last column of table 4 we present calculated values of energy transfer rates Pda =

 1 η0  − 1 , where η0 and η denote quantum efficiencies of luminescent level of  τ0  η 

donor ions in absence and presence of acceptor ions, respectively. Results presented above point at efficient Pr3+ – Yb3+ energy transfer in GAGG host. 22 ACS Paragon Plus Environment

Page 22 of 39

Page 23 of 39

In the following we will restrict our attention to a down conversion process which removes the excitation of the 3P0 level of Pr3+ because it is able to show very high energy transfer rates and may be potentially useful to improve sensitivity of silicon solar cells. Possible mechanisms responsible for this down conversion have been discussed in

36

. Those likely to

be involved in the system under study will be examined referring to energy level schemes shown in Fig. 8.

a)

b) 3

3

P2

1

20

Energy [103cm-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

I6

P2 3 I6 3P1 P0

3

P1 3 P0

1

20

1

D2

D2

15

15

1 10

1

G4

3

F3,4

2

F5/2

3

5

F 3 2 H6 3 H5 3

0

Yb

3+

Pr

3+

1

2

3

5

10

2

G4

F5/2

F3,4

3

F2 H6 3 H5 3

2

H4

Yb

2

F7/2 0

3+

3

Pr

3+

H4

F7/2

3+

Yb

Fig 8. Down-conversion mechanism considered in this paper Mechanism depicted in Fig. 8 (a) consists of a two-step energy transfer from Pr3+ to Yb3+, namely: (Pr3+: 3P0 → 1G4) and (Yb3+: 2F5/2 ← 2F7/2) (first step) and (Pr3+: 1G4 → 3H4) and (Yb3+: 2F5/2 ← 2F7/2) (second step). It provides two photons emitted by ytterbium ions around 1000 nm. Mechanism depicted in Fig. 8 (b) is a one-step energy transfer which provides one photon related to transition (Yb3+: 2F5/2 → 2F7/2) around 1000 nm and one photon related to transition (Pr3+: 1G4 → 3H5) around 1300 nm. The mechanisms mentioned above are considered as quantum cutting (QC) since one absorbed photon in the blue region results in emission of two photons in near IR. 23 ACS Paragon Plus Environment


Figure 9 compares luminescence spectra recorded in the spectral region 850 nm – 1550 nm for GAGG0.45%Pr3+ containing ytterbium ions with concentrations 1.5at% and 10at%.

0.45%Pr,10%Yb 0.45%Pr,1.5%Yb


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Exc. 445 nm

×50

1000

1200

1400

Wavelength [nm]

Fig 9. Emission spectra for GAGG co-doped with ytterbium and praseodymium. Intensities of bands shown in the 1250 – 1550 nm region are magnified by a factor of 50. Excitation wavelength was 445 nm. In a spectrum recorded for a smaller Yb3+ concentration a prominent band related to the 2F5/2 → 2F7/2 transition of Yb3+ stretches from ca. 960 nm to ca. 1150 nm with a maximum at 1028 nm is. A relatively narrow 0–0 line of Yb3+ transition appears at 971.6 nm. In addition, there is a small feature between 906 – 950 nm and a small feature at ca. 1094 nm. Their spectral positions are consistent with those of Pr3+ bands that contribute to spectra for GAGG:0.45at%Pr3+ shown in Fig. 5. Accordingly, the former feature can be assigned to the 3P0 → 1G4 transition and the latter one to the 1D2 → 3F4 and 1G4 → 3H4 transitions. Similarly, we assign a weak band located between 1300 nm and 1500 nm to the 1

G4 → 3H5 and 1D2 → 1G4 transitions. Luminescence spectrum for a sample containing 10at%

of ytterbium consists of a huge band related to the 2F5/2 → 2F7/2 transition, the contributions of 24 ACS Paragon Plus Environment

Page 24 of 39

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the 3P0 → 1G4, the 1D2 → 3F4 and 1G4 → 3H4 transitions are too weak to be discerned and the 0-0 line disappears due to a self-absorption effect. A band component peaking at 1500 nm, related to the 1D2 → 1G4 transition is also negligibly weak, but a small contribution of the 1G4 → 3H5 transition is discernible. Based upon these findings we conclude that the mechanism consisting of two-step energy transfer (Scheme (a) in Fig. 8) governs the quantum cutting in GAGG:Pr,Yb system. Commonly, the efficiency ηQE of this QC process is determined employing the relation 11,34 : η QE = η Pr (1 - η ET ) + 2η ETη Yb , where ηET denotes the Pr3+ → Yb3+ transfer efficiency, ηPr is efficiency of praseodymium emission and ηYb denotes the efficiency of ytterbium emission, both frequently assumed to be unity 37,38. Following this approach and inserting values of ηET determined above we obtain ηQE values ranging from 145% to 185% When ηYb = 1. However, this optimistic assumption is not usually valid because the 2F5/2 emission of ytterbium may be markedly quenched by praseodymium ions. Figure 10 compares decay curves of Yb3+ luminescence recorded at room temperature for samples under study. The radiative lifetime of ytterbium ions in GAGG was evaluated in the past to be 1.00 ms

39

. Luminescence lifetime of GAGG: 2 at% Yb3+ was determined based on measurement

performed with powdered sample and alternatively employing the pinhole method 40 to avoid effects of self-absorption 24. From this measurements the ηYb in the absence of Pr3+ ions was found to be 0.91. Values of ηYb for remaining samples were determined based on τavg values indicated in Fig. 10 as equal to 0.63, 061 and 0.17 for samples with concentration 1.5%, 3% and 10% respectively.



1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 39

Yb 2% 0.45%Pr 1.5%Yb 0.45% Pr 3%Yb 0.45%Pr 10%Yb

τavg=955µs

τavg=603µs

0,1

τavg=587µs τavg=165µs

0,01

1E-3 0

500

1000

1500

2000

2500

Time [µs]

Fig

10.

Luminescence

decay

curves

for

emission

related

to

ytterbium

ions

transition 2F5/2→2F7/2. Taking into account that ηPr is equal to 0.90, and based on these data ηQE values amounting. to 0.989, 0.9998 and 0.405 for samples with concentration 1.5%, 3% and 10% respectively were obtained.

3.4. Up-conversion of infrared radiation to visible luminescence Attempts to obtain up-converted visible luminescence upon CW excitation at wavelength 975 nm for GAGG crystal single-doped with 0.45 at% Pr3+ ions were unsuccessful likely because of very weak intensity of the 3H4 → 1G4 transition. Indeed, it follows from absorption spectra shown in Fig. 1 (inset) that the absorption coefficient at 975 nm amounts to ca. 0.02 cm-1, a value that is not high enough to provide efficient feeding of the 1G4 level. This 26 ACS Paragon Plus Environment

Page 27 of 39

conclusion is supported by the occurrence of up-converted visible emission that was observed for GAGG crystal double-doped with Pr3+ and Yb3+.

GGAG:0.7%Pr,1.5%Yb Luminescence Intensity [a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Exc. 975 nm Upconversion

Slope 2.06

1

2

3

Power [W]

350

400

450

500

550

600

650

700

750

800

Wavelength [nm]

Fig 11. Up-conversion spectrum for GAGG crystal doped with 0.45% Pr and 1.5% Yb. Inset shows log-log plot of luminescence intensity vs. pumping power Figure 11 shows a room temperature spectrum recorded for GAGG: 0.45 at% Pr3+, 1.5 at% Yb3+ upon CW excitation at 975 nm. Inset presents a log-log plot of integrated upconverted luminescence versus power of incident infrared light. Points indicate experimental data and a straight line results from a fitting procedure. Obtained slope value of 2.06 indicates that the up-conversion is a sequential two-photon process, commonly observed in various crystals and glasses double-doped with Pr3+ and Yb3+ 41. A high absorption of ytterbium ions is a key factor that ensures efficient feeding of the 2F5/2 level followed by energy transfer to the 1G4 level of Pr3+ and subsequent feeding of the 3P0 level by an intra-ion 1G4 - 3P0 excited state absorption and/or by a second-step Yb3+ → Pr3+ energy transfer.



Unlike the CW excitation the intensity of up-converted luminescence excited by 6ns pulses at 975 nm was marginal in GAGG:Pr,Yb system. This finding indicates that the population of the 1G4 intermediate level grows with a certain delay to meet steady-state conditions. In contrast to this, the conversion of ultrashort infrared pulses into visible Pr3+ luminescence in GAGG was found to be very efficient. Singularity of this up-conversion process resides in that the intensity of up-converted luminescence depends weakly on wavelength of incident infrared pulses, at least in the 1000 nm- 1600 nm region. Figure 12 compares luminescence

Intensity[a.u.]

spectra of GAGG:Pr3+ excited with 100fs light pulses at several different wavelengths.

1200

ex c

1400

[n m ]

1300

1500

λ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

1600 475

500

525

550

575

600

625

650

675

λem[nm]

Figure 12. Up-conversion spectra of GAGG:Pr3+ obtained for several excitation wavelengths in near infrared It follows from this figure that a mechanism involved does not fulfil actually or seemingly conditions of resonance which determine feasibility of up-conversion with CW or long-pulse excitation. Extremely high peak powers of incident infrared pulses (hundreds of GW) justify a consideration of following possible excitation mechanisms: (i) feeding of the 3P0 level resulting from a single photon or multiphoton absorption of a broad-band visible 28 ACS Paragon Plus Environment

Page 29 of 39

supercontinuum generated in the host, (ii) multiphoton absorption of incident infrared radiation involving inter-configurational 4f2 – 4f15d transitions of Pr3+ and (iii) indirect excitation of Pr3+ ions by energy transfer from electrons created in the conduction band of the host.

1

exc 1100 nm

0,1

I[a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,01

1E-3 500

600

700

800

900

1000

1100

λ[nm] Fig.13. Spectrum of visible part of supercontinuum generated in GAGG by incident 100 fs pulses at wavelength 1100 nm Figure 13 shows a spectrum of the anti-Stokes, visible part of supercontinuum generated in GAGG crystal by incident 100fs pulses at wavelength 1100 nm. The spectrum stretches to ca. 550 nm thereby excluding a mechanism of a single-photon excitation involving the 3H4 → 3PJ transitions of Pr3+ ions in the region 450 – 500 nm. A two-photon absorption of supercontinuum radiation between 900 – 1000 nm may feed the 3PJ levels but the efficiency of this process is not likely to account for a high intensity of up-converted emission observed. We consider a mechanism (ii) mentioned above because the lowest energy 5d levels of 29 ACS Paragon Plus Environment


excited configuration of praseodymium ions are located below the bottom of the conduction band of GAGG. In absorption spectrum shown in Fig. 14 there are two broad and smooth bands peaking at ca. 282.5 nm and 238.4 nm that we assign to inter-configurational 4f2 – 4f15d transitions of Pr3+ and numerous narrow lines related to intra-configurational transitions of Gd3+ ions.

300

Absorption coefficient α [cm-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 39

250

200

150

100

50

0 200

250

300

350

Wavelength [nm]

Fig 14. Room temperature absorption spectrum of GAGG:Pr3+ in UV region.. Widths of the broad bands are quite large, e.g. the lower energy band peaking at ca. 282.5 nm stretches from about 300 to 265 nm (33333 – 37735 cm-1) thereby offering a possibility of simultaneous absorption of 4-5 photons of incident infrared femtosecond pulses at wavelengths ranging from 1100 to 1500nm. A mechanism (iii) mentioned above, consisting of indirect excitation of Pr3+ions implies the occurrence of multiphoton absorption process able to create electrons in the conduction band of the host. The order of this multiphoton excitation (MPE) process is higher as compared to 30 ACS Paragon Plus Environment

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that considered above since the band-gap value of GAGG host of 5.93 eV is rather large

20

.

Thus, a simultaneous absorption of 5 photons is needed for incident pulses at 1100 nm and the order of the process increases to 7 for incident pulses at 1500 nm. Nevertheless, the efficiency of high-order MPE processes that produce free electrons is large enough to counterbalance the self-focusing of femtosecond infrared pulses providing thereby a stable supercontinuum generation

in

GAGG

host.

Intense

radioluminescence spectra reported in

21

3

P0

luminescence

lines

that

contribute

to

for GAGG:Pr3+ corroborate the feasibility of this

excitation mechanism. It follows from these considerations that the mechanisms (ii) and (iii) mentioned above may be involved in phenomena observed.

4. Conclusions Experimental lifetime of the 3P0 and 1D2 levels of Pr3+ were found to be shorter than those determined by a modified Judd-Ofelt treatment. This dissimilarity was attributed to the contribution of non-radiative cross relaxation processes. Furthermore, the significant uncertainty of Judd-Ofelt theory should be considered as well. Optical spectra of Pr3+ and Yb3+ ions recorded at liquid helium temperature point at a strong inhomogeneous line broadening effect and at the presence of non-equivalent sites of luminescent ions in the host lattice. Examination of the effect of Yb3+ concentration on luminescence decay curves of Pr3+ ions revealed that transfer of the praseodymium 3P0 excitation to the 2F5/2 level of Yb3+ occurs in all samples studied and attains the highest efficiency of 93% in GAGG:0.45%Pr,10%Yb sample. Effect of Yb3+ concentration on infrared luminescence spectra related to transitions from the 3P0, 1D2 and 1G4 levels of Pr3+ in 850 – 1500nm region was examined to identify mechanism responsible. Based on gathered results it was concluded that the quantum cutting 31 ACS Paragon Plus Environment


mechanism involved in observed down-conversion phenomenon consists of a two-step energy transfer from Pr3+ to Yb3+, namely: (Pr3+: 3P0 → 1G4) and (Yb3+: 2F5/2 ← 2F7/2) (first step) and (Pr3+: 1G4 → 3H4) and (Yb3+: 2F5/2 ← 2F7/2) (second step). Efficiency of down-conversion mentioned above is affected adversely by the contribution of multiphonon relaxation to the decay of the 1G4 level, however. Up-conversion of incident radiation at 975 nm into visible luminescence was observed for GAGG:Pr,Yb samples under CW excitation and attributed to sequential two-photon process involving a two-step Yb3+ – Pr3+ energy transfer. An intense up-converted visible luminescence was observed upon excitation by femtosecond pulses of infrared light at different wavelengths between 1100 nm and 1600 nm. Based on gathered experimental data it was concluded that the excitation mechanisms consist of multiphoton absorption of incident infrared radiation involving inter-configurational 4f2 → 4f15d transitions of Pr3+ and/or indirect excitation of Pr3+ ions by energy transfer from electrons created in the conduction band of the host.

Abbreviations 4N - 99.99%, 4N5 - 99.995%, 5N - 99.999%, ca. - circa CW - continuous wave, ESA - excited state absorption, GAGG - Gd3Al2.5Ga2.5O12, GGAG - Gd3Ga3Al2O12, GGG - Gd3Ga5Al2O12, ICP-AES - inductively coupled plasma atomic emission spectroscopy, IR – infrared, LED - light emitting diode LuAG - Lu3Al5O12, QC - quantum cutting, RE - rare-earth, SBW - spectral band-width, UV – ultraviolet, YAG - Y3Al5O12, YLF - LiYF4,

Acknowledgments This work was supported by the National Science Centre, Poland under grant number DEC 2016/21/B/ST5/00890. 32 ACS Paragon Plus Environment

Page 32 of 39

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