Luminescence properties and energy transfer from Gd3+ to Tb3+ ions in gadolinium calcium silicoborate glasses for green laser application

Accepted Manuscript 3+ 3+ Luminescence properties and energy transfer from Gd to Tb ions in gadolinium calcium silicoborate glasses for green laser application C.R. Kesavulu, H.J. Kim, S.W. Lee, J. Kaewkhao, E. Kaewnuam, N. Wantana PII:

S0925-8388(17)30472-3

DOI:

10.1016/j.jallcom.2017.02.056

Reference:

JALCOM 40784

To appear in:

Journal of Alloys and Compounds

Received Date: 2 January 2017 Revised Date:

31 January 2017

Accepted Date: 7 February 2017

Please cite this article as: C.R. Kesavulu, H.J. Kim, S.W. Lee, J. Kaewkhao, E. Kaewnuam, N. 3+ 3+ Wantana, Luminescence properties and energy transfer from Gd to Tb ions in gadolinium calcium silicoborate glasses for green laser application, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2017.02.056. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract:-

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Revised Manuscript Luminescence properties and energy transfer from Gd3+ to Tb3+ ions in gadolinium calcium silicoborate glasses for green laser application

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C.R. Kesavulu1, H.J. Kim1,*, S.W. Lee1, J. Kaewkhao2,3, E. Kaewnuam4 and N. Wantana2,3 Department of Physics, Kyungpook National University, Daegu 702-701, Republic of Korea

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Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom Rajabhat University, Nakhon Pathom, 73000, Thailand

Physics Program, Faculty of Science and Technology, Nakhon Pathom Rajabhat University,

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Nakhon Pathom, 73000, Thailand

Physics Program, Faculty of Science and Technology, Muban Chombueng Rajabhat University,

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70150, Thailand

Abstract

The Tb3+-doped gadolinium calcium silicoborate (BSGdCaTb) glasses of composition

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(55-x) B2O3 - 10 SiO2 - 25 Gd2O3 -10 CaO -x Tb2O3, where x = 0.01, 0.05, 0.1, 0.5, 1.0, and 2.0 mol %, have been prepared by conventional melt quenching technique and are characterized through thermal, absorption, excitation, emission and decay rate measurements. The optical band

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gap (Eopt) and Urbach’s energies (∆E) have been evaluated taking into account the ultraviolet (UV) edge of absorption spectra. The Tb3+ -doped BSGdCaTb glasses exhibit strong emission in

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the green (5D4 → 7FJ, J = 3,4,5 and 6) and very weak emission in the ultraviolet blue (5D3 → 7FJ, J = 3,4,5 and 6) transitions under excitation at 275 nm, respectively. The intensity of the 5

D4 → 7F5 (green at 543 nm) and 5D3 → 7F4 (blue at 436 nm) emissions and their integrated

intensity ratios (IG/IB) and (IB/IG) under 275 nm excitation have been obtained as a functions of Tb3+ concentration, respectively. The emission intensity of Gd3+ at 312 nm decreases with increasing of Tb2O3 concentration, while emission intensity of Tb3+ increases upto 1.0 mol % concentration of Tb2O3. The Tb3+ ion decay time for 5D4 level has been determined and is found 1

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to be slightly decreases with increasing of Tb2O3 concentration. The CIE color coordinates and color purity have been calculated for Tb3+-doped glasses and it have found to be shift towards the

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bright green region with increasing of Tb3+ ion concentration in BSGdCaTb glasses. Keywords: Gadolinium calcium silicoborate glasses; Tb3+ ion; Gd3+ ion; Luminescence; Energy transfer; Decay time.

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--------------------------------------------------------------------------------------------------------------------*Corresponding author e-mail: [email protected] (H.J. Kim).

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1. INTRODUCTION

Glasses are very attractive materials as they can be doped with trivalent rare-earth ions (RE3+), and they have highly contributed to the development of lasers, optical fibers, optical waveguides, optical amplifiers and light-emitting devices, since of 4f-4f transitions which are little sensitive to the ion’s surroundings due to the shielding effect of outer 5s and 5p shell

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electrons of RE ions facilitate to obtain laser action and optical amplification at infrared region [l-4]. In oxide based glasses activated by Tb3+ ion have demonstrated to be delightful candidate for gain medium in the green region around at 543 nm, since the 5D4 → 7F5 transition of Tb3+-

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doped materials provides a four-level laser system with a lower threshold pump power to obtain strong green pulsed laser operation at 543 nm. Bjorklund et al. [5] were demonstrated for

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the first time for green laser action at 547 nm and corresponds to 5D4 → 7F5 transition from Tb3+-doped chelate in solution at room temperature. Moreover, the experimental branching ratios of the 5D4 → 7F5 (543 nm) transition are usually more than 50%, which makes the Tb3+-doped materials a promising ion for green laser applications. In order to design new, efficient and improved optical devices for specific application with enhanced performance, active work is being carried out by selected appropriate new hosts 2

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for RE3+ ions. Particularly, silicoborate glasses paved the significant advantages and improvement over the silicate and borate glass systems. These silicoborate glasses are considered to be suitable host for optically active ions because of their moderate melting point, high

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transparency, high thermal stability, low non-linear refractive index and good RE ion solubility besides physical and chemical stability [6, 7]. However there is less important in these glasses due to their high phonon energy [8]. It is well known that the glassy systems with low phonon

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energies are highly suitable for high efficiency lasers and fibers [9].

The heavy metal host glasses like gadolinium calcium silicoborate (Gd2O3-CaO-SiO3-

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B2O3) glasses have several important characteristics like high transparency, high thermal stability, high gain density, high refractive index and low energy gap. Apart from the doping materials, the glass matrix also plays an important role in the development of new optical devices. For RE oxides, intensive Gd2O3 are popular material of much interest within the glass

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matrix because of highly efficient energy transfer from the Gd3+ ions to the incorporated activators, high thermal neutron detection cross-section and enhance the luminescence light yield [10, 11]. Hence, the selection of suitable host matrix and RE ion plays a vital role in developing

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new RE-doped optical devices. Recently, authors studied B2O3-SiO2 - Gd2O3 -CaO host glasses doped with Er3+ and Nd3+ ions for optical and spectroscopic properties for visible and NIR laser

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applications [12, 13] and also Wantana et al. [14], studied the energy transfer from Gd3+ to Sm3+ and luminescence characteristics of Cao-Gd2O3-SiO2-B2O3 glasses for scintillation applications. In this paper, similar host of gadolinium calcium silicoborate glasses doped with different Tb3+ ion concentration were prepared and investigated their luminescence properties and energy transfer from Gd3+ to Tb3+ ions as a function of the Tb3+ ion concentration for strong green laser applications in the visible region.

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2. EXPERIMENTAL STUDIES Tb3+-doped gadolinium calcium silicoborate glasses with a chemical composition of (55-x)B2O3 - 10SiO2 - 25Gd2O3 -10CaO - xTb2O3, glasses (here after referred as BSGdCaTb0.05,

BSGdCaTb0.1,

BSGdCaTb0.5,

BSGdCaTb1.0,

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BSGdCaTb0.01,

and

BSGdCaTb2.0, for x = 0.01, 0.05, 0.1, 0.5, 1.0, and 2.0 mol %, respectively) have been prepared by conventional melt quenching technique. About 20g of the batch compositions was thoroughly

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mixed in an agate mortar and the homogeneous mixture was taken in an alumina crucible and kept in an electric furnace at a temperature of 1400 oC for about 90 min. The melt was poured

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onto a preheated brass mold and annealed at 460 oC for 5 h to remove thermal stress and strains and then cooled to room temperature. Afterwards, these glass samples were polished to attain good transparency and flat surfaces for optical measurements. Those glass samples photograph images are shown in Fig. 1.

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The physical properties such as optical path length was measured by using screw gauge, density was determined by Archimedes’s method using distilled water as an immersion liquid and refractive index was measured using an Abbe refractometer at sodium wavelength

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(589.3 nm) with 1-bromonapthalene (C10H7Br) as a contact liquid. The physical and optical

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properties of the Tb3+-doped gadolinium calcium silicoborate glasses are presented in Table 1. The differential thermal analyzer (DTA) was measured by using SDT Q600 V20.9 Build 20 differential thermal analyzer with a heating rate of 5 oC/min. Absorption spectra were measured with a UV-vis-NIR spectrophotometer (Shimadzu UV-3600) in the wavelength range of 3502500 nm with a spectral resolution of 1 nm. The excitation, emission and decay curves were recorded by using a fluorescence spectrophotometer (Cary-Eclipse) with xenon lamp as a light source. All measurements were carried out at room temperature. 4

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3. Results and discussion 3.1. Physical properties All the physical parameters for BSGdCaTb glasses have been calculated from reference

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[15] and were depicted in Table 1. Density is a useful physical parameter used to evaluate the degree of structure compactness, modifications of the geometrical configurations of the glass network, changes in coordination and variation of the dimensions of the interstitial holes [16].

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The variation of density (g/cm3) and molar volume (cm3/mol) as a function of Tb3+ ion concentration in BSGdCaTb glasses are shown in Fig. 2. It is observed from the Fig. 2 that the

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both density and molar volume increases with increase of Tb3+ ion concentration. It is due to more rigidity/denser nature of the present BSGdCaTb glasses. This is further confirmed by the formation of non-bridging oxygen (NBO) and expands the structure. The high values of density are due to the presence of heavy elements such as Gd3+ and Tb3+ ions in the glass network [16].

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From Table 1 can be concluded that the distance between Tb and Tb is higher than the polaron radios (rp) for all samples and also the values of both rp and inter nuclear distance (ri) are decreases with the increase of Tb3+ ion concentration. Therefore both ri and rp are linked with

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concentration of Tb3+ ion, as the glass network is bounded by Tb3+ interstices, Tb-O bond strength increases. The increase in Tb-O bond strength generates stronger field strength (F)

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around the Tb3+ ions and hence compactness of glass structure increases with the addition of Tb2O3 content [17] as can be seen in Table 1. 3.2. Thermal analysis

Fig. 3 shows the DTA curve of BSGdCaTb10 glass. From DTA curve, the glass

transition temperature (Tg), the onset crystallization temperature (Tx) and the peak crystallization temperature (Tp) are found to be 505, 720 and 790 oC, respectively. To estimate thermal stability of glasses, the parameter ∆T, which means the difference between Tx and Tg (∆T = Tx− Tg) is 5

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usually chosen as a rough measure of glass formation ability or glass stability against crystallization [18]. The larger value of ∆T gives a larger working range during operations for fiber drawing. If ∆T > 100 oC, the glass can be considered as a glass with relatively good thermal

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stability [19]. The ∆T for BSGdCaTb10 glass is found to be 215 oC. Therefore, the present BSGdCaTb10 glass which exhibits a high thermal stability is the best candidates for rod/fiber fabrication due to the correspondingly small chance of crystallization problems and thermal

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damages. However, the evaluated ∆T is not accurate value due to the absence of Tp. Therefore, the Saad and Poulain [20] obtained another criterion parameter S is utilized to evaluate more

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accurately the thermal stability of the prepared glass, which reveals the resistance to devitrification after the formation of the glass and can be defined by


=

  ×(  ) 

(1)

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where (Tp-Tx) is related to the rate of devitrification transformation of the glassy phase. In addition, the high value of ∆T delays the nucleation process. The obtained ∆T and S values of the present BSGdCaTb10 glass is found to be 215 oC and 29.80 oC, respectively, which are

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higher than the other RE doped bismuth silicate (∆T= 87oC; S= 9.41 oC) [21], barium gallo-

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germanate (∆T = 163 oC; S = 8.41 oC) [22] and tellurite (∆T = 102 oC; S = 4.40 oC) [22] glasses. The larger values of ∆T and S indicate better thermal stability and ability of devitrification transformation of the glassy phases. Hence, the present gadolinium calcium silicoborate glasses possess good thermal stability against crystallization.

3.3. Optical absorption spectra The room temperature optical absorption spectra of BSGdCaTb1.0 glass in UV-visible and near infrared spectral regions are shown in Fig. 4. As shown in Fig. 4, In the UV-visible 6

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region, the only one absorption peak observed at 485 nm is assigned to the transition from ground state 7F6 to the higher excited state 5D4 of Tb3+ ions. In the NIR region the detected absorption bands at wavelengths 1876 and 2191 nm arise due to transitions from the ground state F6 to the excited states, 7F0,1,2 and 7F3, respectively. The inset figure shows the intensity of the

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F6 → 5D4 (485 nm) transition with respect to different Tb3+ ion concentrations. As can be seen

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from the inset Fig. 4, the absorption intensity increases linearly with the Tb3+ ion concentrations,

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it indicates that the successful incorporation of Tb3+ ions in the present BSGdCaTb glass matrices. From the absorption spectra of Tb3+-doped BSGdCaTb glasses, we could not obtain the

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Judd-Ofelt intensity parameters due to the very strong overlapping of the UV absorption as can be seen in Fig. 4. This is a common problem in the Tb3+-doped glasses, where very few reliable Judd-Ofelt calculations are found in the literatures [23-26]. 3.4. Optical band gap and Urbach’s energy analysis

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The optical band gap energy is an important parameter for describing solid-state materials. The study of the fundamental absorption edge in the UV-region is a useful method for the investigation of optical band gap energy for non-crystalline/amorphous materials. The

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equations:

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absorption coefficient α(ν) was calculated from the absorbance (A) using the following







(ν) =     = 2.303 ,

(2)

where A is the absorbance at frequency ν and d is the thickness of the sample. For an absorption by an indirect transition, the equation takes the form:

 = ℎν

%

#ν & −   , $

(3)

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where B is a constant called band tailing parameter, hν is the incident photon energy and Eopt is the optical band gap energy. Using the above equations, the optical band gap energy (Eopt) values were determined by plotting (αhν)1/2 as a function of photon energy hν for different

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concentration of Tb2O3 doped samples, respectively. One can find the Eopt for indirect transitions by extrapolating the linear region of the curve to the hν axis [27] as shown in Fig. 5. The optical band gap energies of the present work lie in the range of 3.63 - 3.77 eV for Tb3+-doped

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BSGdCaTb glasses, respectively. This is the same order as those reported in literature for Tb3+-

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doped low silica calcium aluminate glasses [28]. The Eopt values as a function of varying Tb3+ ion concentration are shown graphically in the inset Fig. 5. Eopt values are found to decrease slightly with the increasing of Tb3+ ion content in the present glasses as can be seen in the inset Fig. 5 and it is due to the structural changes in the glass matrix that is increase of non-bridging oxygen’s (NBO’s). The increase in NBO’s to shift the valence band towards the conduction band

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of the host matrix which is leads to have decrease in the optical band gap energy [29]. The Urbach’s energy is well known that the shape of the fundamental absorption edge in

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the exponential (Urbach) region can yield information on the disorder effects. The optical transition between localized tail states (as adjacent to the valance band) and the extended states

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in the conduction band (as positioned above the mobility edge) is known as the Urbach’s energy. The lack of crystalline long-range order in amorphous/glassy materials is associated with a tailing of density of states. At lower values of the absorption coefficient (α), the extent of the exponential tail of the absorption edge characterized by the Urbach’s energy is given by [30], #ν

 (ν) = ' ()*  ,

(4)

∆,

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where α0 is a constant, ∆E is the Urbach’s energy and ν is the frequency of radiation. The Urbach’s energy indicates the width of the band tails of the localized states. The optical absorption coefficient just below the absorption edge shows exponential variation with photon

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energy, indicating the presence of Urbach’s tail. From the curves of ln (α), against photon energy hν for BSGdCaTb glasses are shown in Fig. 6. The Urbach’s energy is calculated taking the reciprocals of the slopes of the linear portion (in the lower photon energy) of these curves. The

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Urbach’s energies obtained in the present work are found to be in the range of 0.29 -0.40 eV for the BSGdCaTb glasses and were listed in Table 1. The Urbach’s energy (∆E) values as a

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function of varying Tb3+ ion concentration are graphically depicted in the inset Fig. 5. ∆E values are found to be increases with the increasing of Tb3+ ion content in the present BSGdCaTb glasses as can be seen in the inset Fig. 6 and it is due to the defects produced within the localized states [31]. The lower values of Urbach’s energy suggest that the presence of less number of

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defects which assist the long range order in the studied BSGdCaTb glasses. From Table 1, it is observed that the band gap and the Urbach’s energy values are follow the opposite trend that is, when the optical band gap values decreases the Urbach’s energy values are found to be increase

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due to the structural changes produced by the Tb3+ ion concentration in the studied glasses which

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may delocalize some of the localized states present in the energy states [31]. 3.5. Excitation and emission spectra analysis Fig. 7 shows the excitation spectra of BSGdCaTb glasses measured by monitoring the

green emission of Tb3+ ion at 543 nm. As can be seen from Fig. 7, first two excitation transitions at 275 (8S3/2 → 6I15/2) and 312 (8S3/2 → 6P7/2) nm are associated to host bands of Gd3+ ion and other five transitions at 340 (7F6 → 5L6), 352 (7F6 → 5L9), 369 (7F6 → 5L10), 378 (7F6 → 5G6 + 5

D3) and 485 (7F6 → 5L6) nm are attributed to Tb3+ ion excitations. Among the all excitations, the 9

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intense and strong excitation peak at 275 nm is used to investigate the emission spectra characteristics for the BSGdCaTb glasses.

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The emission spectra of BSGdCaTb glasses were measured by exciting at 275 nm in the wavelength range from 280 to 650 nm and were shown in Fig. 8. The emission spectra exhibits two group of transitions, which can be assigned to transition originating from 5D3 and 5D4 states to the ground state multiplet 7Fj of Tb3+ ions. And also additionally one more emission peak was 8

S7/2) due to Gd3+ ion in the host matrix as shown in the inset

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observed at 312 nm (6P7/2 →

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Fig. 8. The exhibited first group of intense green emission transitions from 5D4 → 7F3-6 (centered at 623, 587, 543 and 488 nm, respectively) for Tb3+ ions as shown in Fig. 8 and the second group of very weak blue emission transitions from 5D3 → 7F3-6 (centered at 458, 436, 414 and 379 nm, respectively) for Tb3+ ions as shown in the inset Fig. 8.

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From the emission spectra of BSGdCaTb glasses, we have noticed that these emission causes by the energy transfer from the host (Gd3+) to dopant (Tb3+) ions. The emission intensities of Gd3+ at 312 nm decreases with increasing of Tb3+ ion concentration, consequently the

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emission intensities of Tb3+ at 543 nm increases up to 1.0 mol % of Tb3+ ion concentration and then decreases at the higher concentration of Tb3+ ion as shown in Fig. 9. The quenching of Gd3+

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emission intensity consequently with increasing of Tb3+ emission intensity with various Tb3+ ion concentrations are quit evidence of energy transfer mechanism involved from host (Gd3+) to activator (Tb3+) ions. The Gd3+-Tb3+ energy transfer still occur even in the higher than 1.0 mol % of Tb ion concentration for BSGdCaTb glasses, it was influenced by the concentration quenching effect, which makes Tb3+ emission intensity quenches. These energy transfer mechanism from

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Gd3+ to Tb3+ ion could be as follows: 6P7/2 [Gd3+] + 7F6 [Tb3+] → 8S7/2 [Gd3+] + 5D4 [Tb3+] and their emission channels are shown in the partial energy level diagram of Fig. 10.

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In addition, as can be seen from Fig. 8, the 5D4 level emission intensity enhances with increasing of Tb3+ ion concentration until 1.0 mol % Tb3+ ion content consequently the 5D3 level emission intensity decrease with increasing of Tb3+ ion concentration. The variation of the

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integrated emission intensities of blue (5D3 → 7F4) to the green (5D4 → 7F5) transitions referred as the blue-to-green emission intensity ratio (IB/IG) as a function of Tb3+ ion concentration is

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shown in Fig. 11(a). It can be noticed that the IB/IG decreases with increasing of Tb3+ ion concentration. This is attributed to the energy transfer through cross-relaxation (CR) process from 5D3 to 5D4 levels as follows: 5D3 [Tb3+] + 7F6 [Tb3+] → 5D4 [Tb3+] + 7F0 [Tb3+]. The energy difference between 5D3 and 5D4 levels matches well with the energy gap between 7F6 and 7F0 energy levels [26], as can be seen in Fig. 10. This process becomes more prominent at higher

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Tb3+ ion concentrations. Therefore, the IB/IG parameter is useful to measure the degree of fluorescence quenching of the blue emission in the most of Tb3+-doped materials [24, 26, 28, 3234]. Moreover, on the other hand, the variation of the integrated emission intensities of green

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(5D4 → 7F5) to the blue (5D3 → 7F4) transitions referred as the green-to-blue emission intensity

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ratio (IG/IB) as a function of Tb3+ ion concentration is shown in Fig. 11(b). It can be observed that the IG/IB increases with increasing of Tb3+ ion concentration. The IG/IB parameter gives information on the asymmetry of the local environment around Tb3+ ions. Therefore, the present Tb3+-doped BSGdCaTb glasses suggesting that the higher asymmetry around Tb3+ ions as well as higher covalency in Tb3+-O2- bonds [34]. From the emission spectra, the experimental branching ratios (βexp) are obtained for 5

D4→ 7FJ emission levels in the present BSGdCaTb glasses and are presented in Table 2. The 11

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βexp can be determined from the relative areas of the emission transitions. In general, βexp is a critical parameter to the laser designer, since it characterizes the possibility of attaining stimulated emission from any specific transition. In the present BSGdCaTb glasses, the value of

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βexp for the 5D4 → 7F5 (543 nm) lasing transition was found to be higher compared to other transitions. The value of βexp are found to be increases from 57.4 to 59.7 % with increasing of Tb3+ ion concentration from 0.01 to 2.0 mol %, respectively for the 5D4 → 7F5 transition as

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shown in Table 2. Moreover, the βexp for the 5D4 → 7F5 transition is greater than 50 % suggesting

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that present BSGdCaTb glasses can be used as efficient solid state green lasers. 3.6. Decay curve analysis

Luminescence decay time analysis is very useful for understanding the energy transfer mechanism and luminescence quenching behavior of Tb3+ ions. Fig. 12 shows the decay curves

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for the 5D4 level for varying Tb3+ ion concentrations obtained by monitoring the green emission around at 543 nm attributed to 5D4 → 7F5 transition with exciting at 275 nm. It can be seen that decay curves exhibit single exponential nature for all the concentrations in the studied

and

2.16

ms

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BSGdCaTb glasses. The experimental lifetimes are obtained to be 2.31, 2.27, 2.24, 2.20, 2.18 for

BSGdCaTb0.01,

BSGdCaTb0.05,

BSGdCaTb0.1,

BSGdCaTb0.5,

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BSGdCaTb1.0 and BSGdCaTb2.0 glasses, respectively as can be seen in Fig. 12 and Table 3. It is noticed that the lifetime of the 5D4 level decreases very slightly as the Tb3+ concentration increases. This indicates that energy migration does not affect the 5D4 lifetime as observed in most Tb3+-doped glasses [26, 35, 36]. The multiphonon relaxation rate is also expected to be negligible due to the large energy gap (~15,285 cm-1) between the 5D4 emitting level and the next lower lying 7F0 level and as well as the absence of CR channels for 5D4 level. The long

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fluorescence decay time of Tb3+ ions in these glasses can reduce the pump threshold to get the strong green laser emission from the 5D4 level. 3.7. CIE Chromaticity diagram and color purity analysis

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In general, the emission color of two glasses is compared by means of color coordinates and is good certification for photoluminescence applications. In 1931, the Commission International de I’Eclairage (CIE) established a universal quantitative model of color spaces [37].

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The chromaticity coordinates of Tb3+ doped BSGdCaTb glasses are calculated and listed in Table 3 from their corresponding emission spectra excited by 275 nm as shown in Fig. 13, the

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obtained CIE color coordinates of all the Tb3+ doped BSGdCaTb glasses are lie in the green region, which are very close to those (0.290, 0.600) of European Broadcasting Union (EBU) illuminant green [34]. The CIE color coordinates of the BSGdCaTb glasses slightly varies from bluish green to bright green with increasing of Tb3+ ion concentration as can be seen in Fig. 13.

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This is good agreement with green-to-blue intensity ratio (IG/IB), while increasing the IG/IB ratio with increase of Tb3+ ion concentration. Therefore, all the color coordinates slightly shift towards

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bright green region with increasing of Tb3+ ion concentration. In addition to that, the quality of this green light was inspected with the correlated color

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temperature (CCT) values are calculated according to McCamy’s approximation, the CCT values of the present glasses can be calculated through the following expression [38]. CCT = − 449n3 + 3525n2 − 6823n + 5520.33

(5)

where n = (x-xe)/(y-ye) is the inverse slope and (xe = 0.332, ye = 0.186) is the epicenter of the isotemperature lines. The obtained CCT values of the studied BSGdCaTb glasses and compared with other reported systems are presented in Table 3 and the values are found to be in the range of 5878 to 6041 K. Therefore, the CCT values of the present BSGdCaTb glasses lie in between 13

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daylight CIE D55 (5500 K) [39] and commercially available white light LED (6400 K) [40] (see Table 3). The color purity (CP) of the specific dominant color is the weighted average of the

wavelength CP is given by the expression [41]:

/(001 )& 2 (331 )&

/(04 01 )& 2 (34 31 )&

× 100%

(6)

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-. =

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(x, y) coordinate relative to the coordinate of the illuminant and the coordinate of the dominant

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where (x, y), (xi, yi) and (xd, yd) are the color coordinates of the light source, the 1931 CIE standard illuminant source C coordinates (illuminant C = (0.310, 0.316)) and the dominant color coordinates, respectively. The dominant color coordinates (xd, yd) are determined by drawing a straight line from the standard illuminant source C (xi, yi) through the color coordinates (x, y) of the sample until the line intersects to the outer locus of points along the spectral edge of the

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CIE1931 chromaticity diagram. Therefore, all the BSGdCaTb glasses green emission CP are determined from eqn. (6) and were found to be 73 - 78% with increasing of Tb3+ ion concentrations and are listed in Table 3. Which are higher than those of Tb3+-doped zinc

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phosphate glass [34] and Tb3+-doped zinc aluminate film phosphor [41] as can be seen in

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Table 3. Overall, the present BSGdCaTb2.0 glass may be emit bright green light emission with high CP of 78% and an experimental branching ratio is higher than 50% of the green (5D4 → 7F5) transition, so that it could be useful for solid state lighting technology as a green laser source. 4. Conclusions

The gadolinium-calcium silica borate (BSGdCaTb) glasses doped with Tb3+ ions have been prepared and characterized their physical, thermal, optical and luminescence properties. From the physical parameter results, the both density and molar volume increases with increase of Tb3+ 14

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ion concentration. The high values of density are due to the presence of heavy elements such as Gd3+ and Tb3+ ions in the glass network and hence compactness of glass structure increases with the addition of Tb2O3 content. From the excitation results, we observed the strongest excitation at

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275 nm for Gd3+ ions and it generates the emissions of Tb3+ ions through energy transfer from host matrix to activator. The emission spectra of BSGdCaTb glasses exhibit the very weak blue (5D3 → 7F3-6) and strong green emissions (5D4 → 7F3-6) under 275 nm excitation. From the

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emission spectra of the present BSGdCaTb2.0 glass showed strong green emission at 543 nm corresponding to 5D4 → 7F5 transition with high experimental branching ratio of 59.7%,

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correlated color temperature of 6036 K and high color purity of 78% compared to other Tb3+doped BSGdCaTb glasses. The decay time of the 5D4 level exhibits single exponential nature for all concentrations of Tb3+ ion with a small decay time variation from 2.31 to 2.16 ms. The long decay time of the 5D4 level could reduce the pump threshold to get good laser emission at 543

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nm. Hence, the present results suggest that the BSGdCaTb glasses could be a potential candidate for green color display devices and solid state green laser applications.

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Acknowledgments

These investigations have been funded by the Ministry of Science and Technology, Korea

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(MEST) (No.2015R1A2A1A13001843). J. Kaewkhao would like to thanks National Research Council of Thailand (NRCT) and National Research Foundation of Korea (NRF) under NRCTNRF project. E. Kaewnuam would like to thanks Muban Chombueng Rajabhat University for support.

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[37] E. Fred Schubert, Light Emitting Diodes, 2nd edition, Cambridge University Press (2006) pp. 292 (Chapter 17). [38] C.S. McCamy, Color. Res. Appl. 17 (1992) 142-144.

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Caption for figures: Fig. 1. Photograph of BSGdCaTb glasses doped with various mol % of Tb3+ concentrations. Fig. 2. The variation of density (g/cm3) and molar volume (cm3/mol) as a function of Tb3+ ion

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concentration (mol %) in BSGdCaTb glasses. Fig. 3. DTA curve of the BSGdCaTb10 glass.

Fig. 4. Optical absorption spectra of BSGdCaTb1.0 glass in UV-visible and NIR regions. Inset

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shows the linear dependence of intensity of the 7F6 → 5D4 (485 nm) transition with Tb3+ concentrations.

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Fig. 5. hν versus (αhν)1/2 plot for BSGdCaTb0.01 and BSGdCaTb2.0 glasses. Inset shows variation of optical band gap energy values (Eopt) with Tb3+ ion concentrations for BSGdCaTb glasses. Fig. 6.

hν versus Ln(α) plot for BSGdCaTb0.01 and BSGdCaTb2.0 glasses. Inset shows

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variation of Urbach’s energy values (∆E) with Tb3+ ion concentrations for BSGdCaTb glasses. Fig. 7. Excitation spectra of BSGdCaTb glasses for different Tb3+ concentrations under λem = 543 nm.

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Fig. 8. Emission spectra of BSGdCaTb glasses for different Tb3+ concentrations under λex = 275 nm.

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Fig. 9. Variation of Gd3+ intensity at 312 nm (6P7/2 → 8S7/2) and Tb3+ intensity at 543 nm (5D4 → 7F5) as a function of Tb3+ ion concentration in BSGdCaTb glasses. Fig. 10. Partial energy level diagram showing the possible energy transfer from Gd3+ to Tb3+ ions and emission transitions with cross relaxation (CR) channels in BSGdCaTb glasses under 275 nm excitation.

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Fig. 11. Integrated emission intensity ratios (a) IB/IG and (b) IG/IB as a function of Tb3+ ion concentration in BSGdCaTb glasses. Fig. 12. Decay curves for the 5D4 level of BSGdCaTb glasses for different concentrations under

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excitation at 275 nm.

Fig. 13. 1931 CIE chromaticity diagram for Tb3+ doped BSGdCaTb glasses under at 275 nm

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excitation with photographs of bright green emission under UV lamp excitation.

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Tables: Table 1. The physical and optical properties of BSGdCaTb glasses.

BSGdCa Tb0.05

BSGdCa Tb0.1

BSGdCa Tb0.5

BSGdCa Tb1.0

BSGdCa Tb2.0

Density (g/cm3)

4.053

4.054

4.055

4.064

4.101

4.157

Average molecular weight (g)

140.562

140.681

140.829

142.014

143.495

146.457

Molar volume (cm3/mol)

34.680

34.702

34.730

34.944

34.990

35.231

Concentration (×1020 ions/cm3)

0.0173

0.0868

0.1734

0.8617

1.7214

3.4189

Refractive index

1.6530

1.6533

1.6534

1.6535

1.6528

1.6532

Polaron radius (rP) (Å)

34.000

19.898

12.545

7.355

5.841

4.648

Inter-nuclear distance (ri) (Å)

84.366

49.372

39.209

22.989

18.258

14.528

0.4014

0.6365

1.8514

2.9354

4.6358

3.77

3.76

3.72

3.69

3.65

3.63

0.29

0.31

0.32

0.33

0.35

0.40

Urbach’s energy, ∆E (eV)

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0.1374

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Indirect band gap, Eopt (eV)

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Field strength F(× 1016 cm-2)

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BSGdCa Tb0.01

Properties

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5

Glass

7

F6

7

F5

D4 7

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Table 2. Experimental branching ratios (β exp, %) of the 5D4 → 7F6,5,4, 3 emissions.

F4

7

F3

24.1

57.4

11.5

7.0

BSGdCaTb0.05

24.6

59.0

10.9

5.5

BSGdCaTb0.1

24.8

59.3

10.7

5.2

BSGdCaTb0.5

24.6

BSGdCaTb1.0

24.6

BSGdCaTb2.0

24.7

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BSGdCaTb0.01

10.6

5.2

59.6

10.5

5.2

59.7

10.5

5.1

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59.6

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Table 3. Experimental decay lifetime (τexp, ms) for 5D4→7F5 (543 nm) transition, CIE color coordinates (x, y), correlated color temperature (CCT, K) and color purity (CP, %) for Tb3+-

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doped BSGdCaTb glasses and compared with other reported systems.

Glass

τexp

(x, y)

BSGdCaTb0.01

2.31

(0.311, 0.597)

BSGdCaTb0.05

2.27

(0.303, 0.605)

6010

75

BSGdCaTb0.1

2.24

(0.301, 0.608)

6041

75

BSGdCaTb0.5

2.20

(0.301, 0.610)

6038

76

BSGdCaTb1.0

2.18

(0.301, 0.611)

6037

76

BSGdCaTb2.0

2.16

(0.301, 0.612)

6036

78

2.76

(0.290, 0.581)

--

67

--

(0.290, 0.600)

--

--

--

(0.332, 0.347)

5500

--

--

(0.315, 0.336)

6400

--

--

(0.340, 0.560)

--

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ZnAl2O4:Tb [41]

CP

5878

73

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Zinc phosphate (ZP5Tb) [34] EBU illuminant green [34] Daylight CIE D55 [39] Commercial white light LED [40]

CCT

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BSGdCaTb0.01

BSGdCaTb0.05

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Figures:-

BSGdCaTb0.1

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Fig. 1

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BSGdCaTb0.5 BSGdCaTb1.0

BSGdCaTb2.0

Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7

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Fig. 8

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Fig. 9

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Fig. 10

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Fig. 11

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Fig. 12

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(0.311, 0.597) (0.303, 0.605) (0.301, 0.608) (0.301, 0.610) (0.301, 0.611) (0301, 0.612)

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(f)

(a) (b) (c) (d) (e) (f)

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(a)

Fig. 13

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Research highlights
Tb3+-doped gadolinium calcium silicoborate glasses were prepared and characterized.
Luminescence and energy transfer from Gd3+ to Tb3+ ions have been studied.

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Intensity ratios (IB/IG) and (IG/IB) under 275 nm excitation were studied as a functions of

Tb3+ concentration.


BSGdCaTb2.0 glass exhibits a strong green emission at 543 nm with βexp of 60% and

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color purity of 78%.

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BSGdCaTb glasses could be considered as a good candidate for solid state green laser

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applications.