Optical spectra and excited state relaxation dynamics

Appl Phys B DOI 10.1007/s00340-011-4731-9

Optical spectra and excited state relaxation dynamics of Sm3+ in Gd2 SiO5 single crystal A. Strz˛ep · R. Lisiecki · P. Solarz · G. Dominiak-Dzik · W. Ryba-Romanowski · M. Berkowski

Received: 28 March 2011 / Revised version: 9 July 2011 © Springer-Verlag 2011

Abstract Single crystals of gadolinium orthosilicate Gd2 SiO5 containing 0.5 at% and 5 at% of Sm3+ were grown by the Czochralski method. Optical absorption spectra, luminescence spectra and luminescence decay curves were recorded for these systems at 10 K and at room temperature. Comparison of optical spectra recorded in polarized light revealed that the anisotropy of this optically biaxial host affects the intensity distribution within absorption and emission bands related to transitions between multiplets rather than the overall band intensity. It has been found that among four bands of luminescence related to the 4 G5/2 →6 HJ (J = 5/2–11/2) transitions of Sm3+ in the visible and near infrared region the 4 G5/2 →6 H7/2 one has the highest intensity with a peak emission cross section of 3.54 × 10−21 cm2 at 601 nm for light polarized parallel to the crystallographic axis c of the crystal. The luminescence decay curve recorded for Gd2 SiO5 :0.5 at% Sm3+ follows a single exponential time dependence with a lifetime 1.74 ms, in good agreement with the 4 G5/2 radiative lifetime τrad = 1.78 ms calculated in the framework of JuddOfelt theory. Considerably faster and non-exponential luminescence decay recorded for Gd2 SiO5 :5 at% Sm3+ sample was fitted to that predicted by the Inokuti-Hirayama theory yielding the microparameter of Sm3+ –Sm3+ energy transfer Cda = 1.264 × 10−52 cm6 × s−1 . A. Strz˛ep () · R. Lisiecki · P. Solarz · G. Dominiak-Dzik · W. Ryba-Romanowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okólna 2, 50-950 Wrocław, Poland e-mail: A.Strzep@int.pan.wroc.pl Fax: +48-71-3441029 M. Berkowski Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668, Warsaw, Poland

It is concluded that the system under study may be of interest as a VUV-UV excited visible phosphor or laser material operating in the yellow region of the spectrum.

1 Introduction Interest in luminescent properties of trivalent samarium was marginal in the past mainly because Sm3+ absorption bands available for optical pumping could not provide practically useful pumping efficiency when employing classical lamps. Recently, the search for phosphor materials emitting in the visible has brought attention to Sm3+ -doped crystalline materials, glasses and thin films which show an ability to emit radiation in the yellow and red regions of the visible spectrum. In fact, owing to a large energy gap between the 4 G5/2 luminescent level and the next lower-energy 6 F11/2 level the multiphonon relaxation rate is small and a highly efficient visible emission of Sm3+ in virtually all inorganic matrices can be observed. Different ways have been explored to overcome low absorption efficiency of Sm3+ ions. In samarium-doped anatase TiO2 and rutile TiO2 singlephase thin films fabricated on silicon substrates by laser ablation an intense visible emission has been excited by energy transfer to Sm3+ ions from electron-hole pairs generated in TiO2 host [1]. As a consequence of energy transfer between Eu3+ and Sm3+ , the Ca3 (VO4 )2 co-doped with Eu3+ and Sm3+ showed higher red emission intensity upon the UV excitation than those observed for single-doped host [2]. Similar results have been obtained during examination of the co-activated Gd0.2 RE1.8 (WO4 )3 (RE = Eu3+ , Sm3+ ) red phosphor for light-emitting diodes [3]. Sensitization of Sm3+ emission by cerium ions has been also observed in barium fluoroborate glass [4]. Spectacular progress in the

A. Strz˛ep et al.

development of blue-emitting semiconductor lasers is expected to provide a remedy for a poor absorption efficiency of Sm3+ -doped materials. A signal enhancement of about 7.4 dB at 645 nm in a 15 mm long channel waveguide fabricated in Sm3+ -doped SU8 polymer upon the UV-laser-light excitation has been demonstrated pointing at suitability of this system for the design of planar optical waveguide amplifier [5]. Among different kinds of matrix promising for application purposes the Gd2 SiO5 (GSO) crystal deserve an attention. It has been shown that GSO:Ce3+ is a promising scintillator especially for medical and nuclear physics application [6]. Observation of efficient energy transfer from Gd3+ to Eu3+ in GSO:Eu3+ points at the potential of this system as a VUV excited phosphor emitting in the visible [7]. Also, it follows from spectroscopic assessment that the GSO:Yb3+ crystal is a promising laser material [8]. It has been ascertained that GSO compound forms optically biaxial crystals belonging to monoclinic system with a P21 /c space group [9]. In the GSO structure, Gd3+ ions occupy two non-equivalent crystallographic sites; Gd1 (coordination number CN = 9, C3v point symmetry) and Gd2 (CN = 7, Cs point symmetry). Gd2 ions create nearly linear chains along the crystallographic b axis. It has been shown that Ce3+ , Eu3+ , Er3+ , Yb3+ and Dy3+ substitute both Gd1 and Gd2 sites in the GSO crystal lattice [10]. Recent work devoted to optical properties of dysprosiumdoped GSO crystals has revealed that Dy3+ ions accommodated in Gd1 and Gd2 sites form two distinct subsystems differing in excited state relaxation dynamics and a ion-host coupling [11]. The intention of this work is to get fundamental knowledge of processes of excitation and relaxation of excited states and to assess the potential of the GSO:Sm3+ system for application as a phosphor or laser active material operating in the visible.

2 Experimental Single crystals of (Gd1−x Smx )2 SiO5 with x = 0.005 and 0.05 were grown by the Czochralski method in inductively heated iridium crucibles under nitrogen atmosphere. The starting materials of 99.99% purity were used. Detailed procedure of material preparation and conditions of a crystal growth are described elsewhere [11]. Oriented samples, in the form of plates with several millimeters of thickness, were cut out of the crystal boule and polished for optical measurement. Actual concentration of Sm3+ ions in the host crystal was checked by an inductively coupled plasma emission (ICP-ES) measurement. It has been found that the concentration ratio Sm/Gd in 0.5% sample was equal to 5.517 × 10−3 . Polarized absorption spectra at room temperature and unpolarized absorption spectra at 10 K were

recorded with a Varian Model 5E UV-VIS-NIR spectrophotometer employing a spectral bandwidth of 0.05 and 0.1 nm in the VIS and NIR region, respectively. Unpolarized survey emission spectra excited in the VUV region and VUV excitation spectra of visible emission (with SBW parameter set to 0.3 nm) were recorded using technical set-up available at SUPERLUMI station at HASYLAB, Desy in Hamburg. Technical details of this measuring system could be find elsewhere [12]. High-resolution polarized luminescence spectra were recorded with a set-up consisting of a Dongwoo Optron DM 158i excitation monochromator and DM711 emission monochromator having 750 mm focal length. The ozone-free Xenon lamp DL 80-Xe and an Argon ion laser were used as excitation sources. Luminescence decay curves were excited with a Continuum Surelite I optical parametric oscillator (OPO) pumped by a third harmonic of Nd:YAG laser and recorded with a Tektronix Model TDS 3052 digital oscilloscope. For low-temperature measurements samples were placed in a continuous flow liquid helium cryostat equipped with a temperature controller.

3 Results and discussion 3.1 Optical absorption spectra and transition intensities Optical spectra of Sm3+ are related to transitions between numerous and closely spaced multiplets of the 4f5 ground configuration. The complexity of the system can be seen in Fig. 1 showing the energy level scheme that has been constructed based on detailed analysis of Sm3+ optical spectra in LaF3 [13]. Albeit so that it does not refer rigorously to GSO:Sm3+ , it will help to get insight into the peculiarities of optical spectra considered below. Energy levels of Sm3+ can be divided into two groups. One of them consists of well separated levels related to multiplets of the 6 H and 6 F terms, with energies below to about 12 000 cm−1 . The second group, which contains levels with energies above about 17 500 cm−1 , corresponds to tens of multiplets having the crystal-field splitting comparable to their energy separation, so there is a difficulty to assign them unambiguously. Figure 2 shows room-temperature polarized absorption spectra of GSO:5 at% Sm3+ recorded with electric vector E of light parallel to crystallographic axes of the crystal. Quite intense absorption bands in the near infrared region have been assigned to transitions from the ground 6 H5/2 multiplet to multiplets of the 6 H and 6 F terms. A number of absorption transitions to quartet states, including the metastable 4 G5/2 state, expected in the 600–450 nm spectral region, are difficult to discern due to their vanishing intensity. There are, however, quite intense absorption bands in the region between 450 nm and the UV absorption edge of the host crystal. In particular, a prominent band centered at 405 nm offers a possibility of optical pumping with blue-emitting laser diodes. It

Optical spectra and excited state relaxation dynamics of Sm3+ in Gd2 SiO5 single crystal

Fig. 1 Energy level scheme of Sm3+ . Arrows indicate observed emission transitions

can be seen in Fig. 2 that the anisotropy of the Sm3+ absorption transition intensities is rather small, except for the transition around 405 nm which is markedly the lowest in the E||b spectrum. This visual impression is corroborated by the quantitative assessment expressed in terms of oscillator strengths f that were determined by numerical integration of absorption bands in polarized spectra gathered in Fig. 2. Values of oscillator strengths, obtained in this way and denoted as fE||a , fE||b and fE||c , are included in Table 1. To perform the analysis of absorption spectra in the framework of the Judd-Ofelt theory average values of oscillator strengths f average were calculated and used as input experimental data for the fitting procedure. Respective theoretical oscillator strengths were constructed as a function of matrix elements of unit tensor operators as given in Ref. [14] for each transitions contributing to absorption bands and three intensity parameters Ωt (t = 2, 4, 6) were determined by a least square fit between the experimental and theoretical oscillator strengths. It can be seen in Table 1 that the quality of the fit is quite reasonable. Three intensity parameters found (Ω2 = 1.12 (×10−20 cm2 ), Ω4 = 5.57(×10−20 cm2 ), and Ω6 = 2.78 (×10−20 cm2 ) were next used to evaluate radiative transition rates Ar and luminescence branching ratios β for the metastable 4 G5/2 level. Results of calculation are

Fig. 2 Polarized absorption spectra of the GSO:Sm (5 %at.) crystal. T = 300 K

given in Table 2. It should be noticed here that the transition from the 4 G5/2 luminescent level to the 6 H5/2 ground state fulfils the selection rules for magnetic dipole transitions J = 0, 1. To account for this, the contribution of magnetic dipole transition rate was calculated using a standard formula [15] and added to the electric dipole transition rate determined by the Judd-Ofelt approach. It may be interesting to notice that this transition appears to be mainly magnetic dipole in character. It follows from the examination of calculated luminescence branching ratios that 93% of the overall intensity of the 4 G5/2 emission is expected to occur as a consequence of four transitions, three of them in the visible (82%) and one in the far red region (11%) at around 720 nm. Sum of radiative transition rates from the 4G −1 and its 5/2 level to all terminal levels amounts to 561 s inverse, the value denoted as the radiative lifetime τrad , is 1.78 ms. 3.2 Luminescence spectra and the excited state relaxation dynamics Figure 3 shows an unpolarized survey emission spectrum for Gd2 SiO5 :0.5 at% Sm3+ excited into the host absorption

A. Strz˛ep et al. Table 1 Measured and calculated oscillator strengths for selected multiplets ν (cm−1 )

Transition from 6H

5/2

to

Oscillator strength (×10−6 ) Measured fE||a

6H 6H

Calculated fE||b

fE||c

faverage

11/2

3808

2.41

2.76

2.65

2.61

1.12

1.04

1.25

1.14

2.68 0.71

13/2

5110

6F

6 1/2, 3/2, 5/2 , H15/2

6939

12.16

11.22

13.45

12.30

12.30

6F

7/2

8077

10.46

7.59

10.70

9.58

9.61

6F

9/2

9229

6.62

5.08

6.69

6.13

6.88

6F

11/2

10 556

1.12

1.04

1.16

1.11

1.14

4 G(4)

21 259

5.87

5.25

6.44

5.85

3.74

24 610

22.39

14.65

23.09

20.04

17.71

26 514

4.89

4.06

4.92

4.61

8.16

27 590

5.43

2.97

5.68

4.69

5.25

28 839

3.35

2.26

2.94

2.85

2.79

4 7/2 , I(3)9/2 , 4M 4 15/2 , I(3)11/2 , 4 I(3) 4 13/2 , F(3)5/2 , 4M 4 17/2 , G(4)9/2 , 4 I(3) 15/2 6P , 4M 5/2 19/2 , 4L 4 13/2 , F(3)7/2 , 6 P , 4 K(1) 3/2 11/2 , 4M 4L , 21/2 15/2 , 4 G(4) 11/2 4 D(3) , 6 P , 1/2 7/2 4L 4 17/2 , K(1)13/2 4 F(3) , 4 D(2) , 6 P 9/2 3/2 5/2 4 H(1) , 4 K(1) 7/2 15/2 , 4 H(1) , 4 K(1) 9/2 17/2 , 6 P , 4 H(1) 7/2 11/2

Table 2 Calculated luminescence branching ratios and Ar parameters 4G

5/2

6H

5/2

→

ν(cm−1 )

Ar (s−1 )

β(%)

17 705

ED = 15

11.2

MD = 48 6H

7/2

16 631

247

44.0

6H

9/2

15 377

149

26.5

6H

11/2

13 594

66

11.7

6H

13/2

12 782

9

1.7

1/2

11 291

1

0.2

1

0.1

6F

6H

15/2

11 133

6F

3/2

11 037

2

0.3

6F

5/2

10 462

15

2.7

6F

7/2

9654

7

1.2

9/2

8434

2

0.4

11/2

6889

0

0.0

6F 6F

band at 201 nm and recorded at 7 K. As predicted by the assessment of radiative transition rates the spectrum recorded in the region between 550 nm and 800 nm consists of four distinct bands that can be assigned unambiguously to transi-

tions from the 4 G5/2 level to the 6 HJ (J = 5/2, 7/2, 9/2, 11/2) terminal levels. The reliability of the calculated branching ratio values can be now checked roughly by integrating numerically the emission bands in Fig. 3, equating the overall intensity to 100% and evaluating relative percentage for each band. The experimental branching ratios obtained are 10%, 52%, 31% and 7% for transitions to the terminal 6 H5/2 , 6H , 6H 6 7/2 9/2 and H11/2 levels, respectively. Thus, the agreement between the calculated and experimental branching ratios is moderately good. The inset in Fig. 3 shows an excitation spectrum recorded when monitoring the emission at 609 nm. Occurrence of a broad and intense band located between 150–225 nm could be assigned to the host absorption and charge transfer process O2− → Sm3+ . As a result of spectroscopic investigation of GSO: Eu [16] was found that the absorption band at 160-200 nm corresponds to interband transition of GSO host. The lower energetic wing of this band could be assigned to the CT process. According to [17] CT band of Sm3+ in the same matrix should occur about 1–1.1 eV above similar CT band for Eu3+ what is in good agreement with experimental data (CT in GSO: Sm3+ ∼ 5.91 eV, Eu3+ ∼ 4.96 eV). Narrow lines assigned to the 8 S7/2 →6 DJ , 8 S7/2 →6 IJ , and 8 S7/2 →6 PJ transitions

Optical spectra and excited state relaxation dynamics of Sm3+ in Gd2 SiO5 single crystal Fig. 3 Unpolarized survey luminescence spectrum of GSO:Sm (0.5 %at.) recorded at 7 K with λexc = 201 nm. Inset: excitation spectrum of luminescence monitored at 609 nm

of Gd3+ at about 250 nm, 275 nm and 310 nm, respectively provides the evidence for the energy transfer from the host and Gd3+ ions to Sm3+ ions in this system. In the following, we will restrict our consideration to peculiarities of luminescence in the visible region consisting of bands in the green, yellow and red. Figure 4 compares room-temperature luminescence spectra excited at 405 nm and recorded with light polarized parallel to crystallographic axes of the crystal. A narrow and intense line at 601 nm dominates invariably three spectra but the intensity of remaining band components is affected markedly by the crystal anisotropy. All data presented above point at a potential feasibility of laser operation in the visible related to the 4 G5/2 –4 H7/2 transition around 601 nm. The terminal level for this transition is situated well above the ground state thus fulfilling the condition of a four-level laser scheme. The inset in Fig. 4 presented the 4 G5/2 –4 H7/2 luminescence spectra that are calibrated in the cross section units σ em by inserting appropriate data into the Füchtbauer-Ladenburg relation [18]: σem (λ) =

βλ5 Ip (λ) 8πn2 cτrad p λIp (λ) dλ

(1)

where n = 1.85 is the average index of refraction, β = 0.44 is the branching ratio value for the transition, τrad = 1.78 ms is the 4 G5/2 radiative lifetime, Ip (λ) is the emission intensity versus wavelength for a given polarization p and p λIp (λ) dλ denotes the sum of integrated emission bands over three polarizations. The highest peak value of

Table 3 Values of emission cross sections found in other materials Compound

σ em (cm2 )

References

Sm:Gd2 SiO5

3.54 × 10−21

this paper

Sm:LaOCl

9.7 × 10−23

[19]

Sm:GdOCl

6.5 × 10−23

Sm:YOCl

9.7 × 10−23

Sm:BLNS glass

1.45 × 10−21

Sm:BNNS glass

9.15 × 10−22

Sm:BKNS glass

1.35 × 10−21

Sm:K2 SO4 –P2 O5 –B2 O3 glass

4.63 × 10−20

[20]

[21]

the emission cross section σem = 3.54 × 10−21 cm2 , was found for the prominent line at 601 nm. The comparison of σ em values found in other materials are listed in Table 3. It may be interesting also to compare results given above to this characterizing the visible emission of Dy3+ in GSO at around 570 nm related to the 4 F9/2 →6 H13/2 transition. The value of peak emission cross section σem = 6.8 × 10−21 cm2 , reported in [11], has been determined from unpolarized emission spectrum recorded with the GSO:1 at%Dy3+ sample. This value is higher than that for GSO:0.5 at% Sm3+ system due to relatively short radiative lifetime (τrad = 0.54 ms) of the 4 F9/2 level of Dy3+ in GSO [11]. The energy gap E of about 6 890 cm−1 between the 4G 6 5/2 luminescent level and the next lower-laying F11/2 3+ state of Sm in GSO corresponds roughly to six highest energy phonons available in the host lattice. According to the E||c

A. Strz˛ep et al.

Fig. 5 Experimental emission decays of the 4 G5/2 level in Gd2 SiO5 :xSm3+ (x = 0.5 and 5 at%) measured at room temperature (λexc = 450 nm, λem = 601 nm). The solid line represents fitted decay derived for GSO:Sm (5%) assuming dipole-dipole interactions Fig. 4 Polarized emission spectra of the GSO:Sm (0.5 %at.) crystal measured at room temperature with the excitation wavelength λexc of 405 nm. Polarized cross section spectra of 4 G5/2 →6 H7/2 emission are presented in the inset

energy-gap law, the rate of non-radiative relaxation, involving simultaneous emission of phonons consistent with a sixorder process, is very small and therefore, in the limit of low Sm3+ concentration the 4 G5/2 level decay would be governed by radiative transitions. Figure 5 compares semilog plots of 4 G5/2 luminescence decay curves recorded at room temperature for GSO crystal containing 0.5 at% and 5 at% of Sm3+ . It can be seen that the decay follows a single exponential time dependence with a lifetime value of 1.74 ms for the smaller Sm3+ concentration. The luminescence lifetime value given above agrees quite well with the calculated radiative lifetime τrad = 1.78 ms pointing at the reliability of the Judd-Ofelt analysis reported in the previous section. The 4G 5/2 luminescence decay for the GSO crystal containing 5 at% of Sm3+ is considerably faster and the respective decay curve is not exponential apparently as a consequence of an activator-activator interaction. In principle, the relaxation dynamics of the 4 G5/2 metastable level can be affected by two different kinds of non-radiative energy transfer process

between interacting Sm3+ ions, namely an energy migration and a cross-relaxation. The former process, in which excited Sm3+ ion makes a downward transition from the 4 G5/2 level to the ground state and its unexcited neighbor makes an upward transition to the 4 G5/2 level, does not involve a loss of the excitation energy but makes the distribution of the excitation over samarium ions more uniform. In the crossrelaxation process an excited Sm3+ ion undergoes a downward transition from the 4 G5/2 level to an intermediate excited level and a coupled unexcited neighbor undergoes an energy conserving upward transition to an intermediate level and finally the two interacting ions decay non-radiatively. This process competes adversely with radiative relaxation and gives rise to the phenomenon of self-quenching of luminescence. The luminescence decay recorded with GSO crystal containing 5 at% of Sm3+ consists of an initial fast and non-exponential stage followed by a nearly exponential and considerably slower decay implying that the crossrelaxation rate is considerably higher than the rate of energy migration. Accordingly, the experimental luminescence de-


tance R0 = 0.79 nm has been obtained. Resulting donoracceptor interaction parameter CDA = R06 × τ0−1 amounting to 1.264 × 10−52 cm6 × s−1 is higher than that reported for GeO2 –PbO-PbF2 glass (CDA = 4.69 × 10−53 cm6 ×s−1 ) [23] and for PbO-PbF2 glass (5.63 × 10−53 cm6 × s−1 ) [24]. 3.3 Nature of Sm3+ sites in the GSO lattice

Fig. 6 Low-temperature optical bands depicting two-site structure of Sm3+ ions in the Gd2 SiO5 matrix

cay curve is expected to be consistent with theoretical time dependence predicted by the Inokuti-Hirayama model [22]: 3 t t s Φ(t) = A exp − − α (2) τ0 τ0 where A is a constant, Φ(t) is an emission intensity after a short pulse excitation, s = 6 for dipole-dipole interaction, τ0 is an intrinsic lifetime of donor ions in the absence of acceptors and α is the parameter described by (1): 4π 3 α= 1− NA R03 (3) 3 s where denotes the Euler function, NA is the concentration of acceptor ions and R0 is the critical transfer distance defined as that separation at which the rate of energy transfer between a donor-acceptor pair is equal to the intrinsic decay rate τ0−1 . When the energy transfer process is the cross-relaxation within a system of identical ions, the acceptor concentration equals the total concentration of activators. The solid line in Fig. 5 represents the best fit of (2) treating α as a adjustable parameter with τ0 = 1.8 ms being a lifetime value for GSO crystal containing 0.5 at% of Sm3+ . The fitting procedure provides α = 3.38. Inserting NA = 9.39 × 1020 ions×cm−3 to (3), the critical dis-

As was mentioned in the Introduction, Gd3+ ions occupy two non-equivalent crystallographic positions in the GSO lattice that differ in point symmetry and number of coordination oxygens. Therefore, low-temperature absorption and emission spectra were recorded to get a more close insight into the site-location of Sm3+ ions in the system. Figure 6 shows details of unpolarized absorption and emission spectra related to the 6 H5/2 →4 G5/2 and 4 G5/2 →6 H5/2 , 6 H7/2 transitions recorded at 10 K for GSO crystals containing 0.5 and 5 at% of Sm3+ . Both initial and terminal multiplets of the 6 H5/2 ↔4 G5/2 transitions are split into three crystalfield levels each. Unless the accidental coincidence of energies corresponding to crystal-field levels of the two sites occurs, the low-temperature absorption spectrum should consist of six narrow lines. However, only three lines centered at 17 575 cm−1 , 17 710 cm−1 and 18 025 cm−1 appear in the absorption spectrum shown in Fig. 6. A careful examination of low-temperature emission spectra reveals that the absorption lines correspond to transitions of Sm3+ ions located in two non-equivalent sites. In particular, the line at 17 710 cm−1 , denoted as A5, occurs in both absorption and emission spectra. Therefore, it can be assigned to the transition between the lowest crystal-field components of ground and excited states (0-0 line) of Sm3+ ions in one of two available sites (denoted as Sm1). Similarly, the absorption line at 17 575 cm−1 , denoted as A3, which coincides with a well-defined maximum in emission spectrum recorded with GSO:0.5 at% Sm3+ and with a shoulder in emission spectrum recorded with GSO:5 at% Sm3+ , can be assigned as the 0-0 line for the second site denoted as Sm2. The origin of absorption line centered at 18 025 cm−1 cannot be determined, however. The remaining three lines in emission spectra, denoted by A1, A2 and A4, are thus related to transitions to higher crystal-field components of the ground state. It may be interesting to notice that Sm3+ concentration affects the intensity distribution of emission lines. In particular, intensities of lines A5, A3, A1 and to slight extent A2 are lower in spectrum recorded with the sample containing higher concentration of Sm3+ . Several different phenomena can be considered to account for the above dependence. The phenomenon of self-absorption is likely to be responsible for smaller intensity of A5 and A3 lines (assigned as 0-0 lines) in heavily doped sample but it cannot affect the intensity of remaining lines. Significantly different importance of luminescence self-quenching for Sm1 and

A. Strz˛ep et al. Table 4 Stark sublevels of selected multiplets SLJ

Energy (cm−1 )

Stark sublevels exp.

E

theor.a

6H

5/2

5

6

6H

13/2

9

14

1/2

1

2

6413

3/2 6H 15/2 6F 5/2 6F 7/2 6F 9/2 6F 11/2 4G 5/2

3

4

6574, 6605, 6667

3

16

6828, 6852, 6897

69

6

6

7183, 7207, 7241, 7324, 7396, 7490

307

8

8

8019, 8051, 8085, 8101, 8174, 8208, 8234, 8322

303

9

10

9170, 9195, 9230, 9279, 9294, 9343, 9405, 9448, 9472

302

9

12

10 532, 10 545, 10 575, 10 612, 10 661, 10 691, 10 741, 10 872, 10 952

320

3

6

17 575, 17 710, 18 025

450

6F 6F

a for

0, 60, 130, 191, 310

310

4924, 4934, 5029, 5061, 5112, 5131, 5180, 5227, 5320

396 93

Sm3+ in both C3v and Cs symmetry sites

Sm2 sites may be a plausible reason providing proof that the two sites are not linked by fast energy transfer. Indeed, in the GSO:5 at%Dy3+ system the Dy1 and Dy2 sites showed luminescence decay curves with mean lifetime values differing by a factor of two, roughly, indicating that the two sites behave as isolated centers and decay with different rates [11]. Such experimental evidence cannot be demonstrated for the system under study because of vanishing intensity of lines A1 and A2 in the emission spectrum recorded for GSO:5 at% Sm3+ sample. As an alternative explanation one may suppose that Sm3+ ions are located preferentially in one of two available sites when the samarium concentration increases. Such a hypothesis cannot be excluded since in contrast to Dy3+ ions the ionic radius of Sm3+ is bigger than that of Gd3+ and larger coordination sphere would be more favorable to accommodate higher concentration of samarium ions. Examination of gathered experimental data does not provide decisive arguments. Figure 6 shows also emission spectra at 10 K consisting of narrow lines denoted by B1–B8 and related to transitions from the lowest crystal-field components of the 4 G5/2 multiplet to crystal-field components of the first excited multiplet 6H 3+ 7/2 of Sm . For low point symmetry, the multiplet with J = 7/2 is split into four crystal-field levels. Accordingly, for two non-equivalent Sm3+ sites eight narrow lines should appear in the 6 H7/2 spectrum, and actually eight lines can be easily discerned in the emission spectrum recorded for GSO:0.5 at% Sm3+ sample. It can be seen that lines denoted as B1, B2, B3 and B6 are significantly weaker in the spectrum recorded for GSO:5 at% Sm3+ sample implying that these lines correspond to Sm3+ ions in one site whereas the remaining four lines (B4, B5, B7, B8) correspond to Sm3+ ions in the second site. Unfortunately, this finding does not elucidate the problem which one of two latter phenomena considered above are involved. The phenomenon of

self-absorption can be neglected in this case since the terminal level is well above the ground state. Table 4 contains energies of crystal-field levels for GSO:Sm3+ system determined from absorption and emission spectra recorded at 10 K. They are not assigned to individual Sm1 and S2 sites, however.

4 Conclusions Samarium-doped orthosilicate Gd2 SiO5 crystals show an ability to emit intense visible emission distributed in three bands located in the green, yellow and red and a slightly weaker purple-red band around 720 nm. All these emission bands correspond to transitions from the 4 G5/2 level of Sm3+ which can be efficiently excited in the VUV region by a host-sensitized energy transfer, in the UV region by the Gd3+ –Sm3+ energy transfer and in the blue region upon optical pumping into the advantageously intense 6 H5/2 → 6P 3+ ions. These features imply 3/2 absorption band of Sm the potential of the system under study as a visible phosphor. Essentially radiative decay of the 4 G5/2 level of Sm3+ in the Gd2 SiO5 :0.5 at% Sm3+ sample, combined with a high luminescence branching ratio for the 4 G5/2 →6 H7/2 transition around 610 nm and availability of strong 6 H5/2 → 6P 3/2 absorption band near 405 nm point at the potential of this system as a laser material operating in the yellow region upon optical pumping with blue laser diodes. Relatively high value of microparameter characterizing a non-radiative energy transfer between samarium ions, derived from an analysis of luminescence decay in the Gd2 SiO5 :5 at% Sm3+ sample, indicate that the self-quenching of the Sm3+ emission in the Gd2 SiO5 host is significant and should be accounted for when optimizing the emission performance of these materials. Spectroscopic measurement performed at low temperature provided evidence that in the Gd2 SiO5 :0.5 at% Sm3+


sample the Sm3+ ions in two dissimilar sites available in the crystal host contribute to absorption and emission spectra. Two different phenomena have been suggested to account for missing lines in the emission spectrum recorded for Gd2 SiO5 :5 at% Sm3+ sample but the experimental data gathered thus far are not sufficient to unambiguously indicate the phenomenon involved.

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