Pr 3 + doped tellurite glasses incorporated with silver nanoparticles for laser illumination

Enhanced red fluorescence emissions of Pr3+ were observed in heavy metal germanium tellurite (HGT) glasses containing silver nanoparticles (NPs). The well-dispersed Ag NPs with a diameter ∼7 nm were evidenced by transmission electron microscope (TEM). With the introduction of Ag NPs, multichannel transition emission intensity of Pr3+ increased by ∼25% in comparison with that in the case without silver doping, which is attributed to the existence of the localized surface plasmon resonance (LSPR) referring to the characteristic absorption peaks. The larger the net emission power, the higher was the net emission photon number and the higher quantum yield in Pr3+ doped HGT glasses containing Ag NPs, presenting the effectiveness of utilizing laser. An efficient fluorescence emission and macroscopical sensitization illustrate that the Pr3+-doped HGT glasses with Ag NPs are potential materials, which improve the color-rendering index for laser illumination.


Introduction
In the past decades, nanometer-sized particles of metals have attracted increasing attention due to several tempting properties such as optical nonlinearity, thermal characteristics, and magnetic properties, [1][2][3][4] which are different from their bulk states.7][18][19][20][21][22][23][24] With the introduction of germanium ions, the distribution of phonon energy changes and thus the density of the maximum phonons is reduced.][33][34] In this study, the Pr 3+ doped heavy metal germanium tellurite glasses containing Ag NPs were fabricated by a melt-quenching method with a post-annealing treatment and the existence of Ag NPs was further observed by transmission electron microscopy.The multichannel transition emission of Pr 3+ increased by $25% in the Ag NPs embedded glasses compared with that in the silver-free glasses and the intense red uorescence emissions were captured in the glass samples under the excitation of the short-wavelength visible lights, which are generated from the emitting 3 P 0 and 1 D 2 levels of Pr 3+ , and can be used as the excitation lights for minimally invasive PDT treatment.The absolute characterization of the glass samples containing Ag NPs under the excitation of a 453 nm blue laser was carried out and the high quantum yields were derived from the absolute spectral parameters, thus proving a macroscopical sensitization with a high-power laser.An efficient uorescence emission and megascopic sensibilization in the Pr 3+ doped HGT glasses with Ag NPs under the laser excitation provide a promising orientation to develop the efficient optical devices and a tunable red uorescence is an important support for laser illumination in improving the color-rendering index.

Experiments
Pr 3+ -doped heavy metal germanium tellurite glasses (HGT) were prepared from high purity Na 2 CO 3 , Bi 2 O 3 , PbO, GeO 2 , TeO 2 , Pr(NO 3 ) 3 $6H 2 O, and AgCl powders according to the molar host composition 2Na 2 O-17Bi 2 O 3 -2PbO-19GeO 2 -60TeO 2 with an additional xwt% Pr(NO 3 ) 3 and ywt% AgCl, where x ¼ 0.22 and y ¼ 0 was labeled as HGT-Pr and x ¼ 0.22 and y ¼ 0.5 was marked HGT-PrAH.The raw materials were well-ground in an agate mortar, heated in a pure alumina crucible to 880 C and maintained at this temperature for 20 min and then quenched onto an aluminum plate.Furthermore, the glasses were subsequently annealed at 320 C for 2 h to eliminate an internal stress and then cooled down to room temperature slowly.The HGT-PrAH glass containing silver was then heat-treated at 350 C for 2 h and cooled down to room temperature slowly.For the optical measurements, the glass samples were sliced and polished into pieces with two parallel sides.
The densities of the HGT-Pr and HGT-PrAH glass samples were measured to be 6.106 and 6.110 g cm À3 , respectively, and the number densities of Pr 3+ were calculated to be 2.469 Â 10 19 and 2.461 Â 10 19 cm À3 , respectively.Using a Metricon 2010 prism coupler, the refractive indices of the HGT-Pr and HGT-PrAH glass samples were measured to be 2.1053 and 2.1062 at 635.96 nm and 2.0431 and 2.0456 at 1546.9 nm, respectively.In addition, the refractive indices at other wavelengths can be calculated by Cauchy's equation n ¼ A + B/l 2 , giving A ¼ 2.0304 and B ¼ 30 273 nm 2 for HGT-Pr glass and A 0 ¼ 2.0333 and B 0 ¼ 29 495 nm 2 for HGT-PrAH glass.
The absorption spectra were recorded using a Perkin Elmer UV-VIS-NIR Lambda 750 spectrophotometer.The visible uorescence spectra were measured by a Hitachi F-7000 uorescence spectrophotometer.Differential scanning calorimetry (DSC) was conducted using an American TA company SDT 600 at the rate of 10 C min À1 from room temperature to 1000 C under a N 2 atmosphere.A 200 kV JEM-2100 transmission electron microscope (TEM), which functioned with selected area electron diffraction (SAED), was used to investigate the nucleation of Ag NPs formed in the glasses.The specimens for TEM images were prepared by dispersing the ground glass powder in ethanol using an ultrasonic bath.The spectral power distributions of the glass samples were measured using an integrating sphere (Labsphere) with a 3.3 inch diameter, which was connected to a CCD detector (Ocean Optics, QE65000) with 600 mmcore optical ber and a 453 nm laser pigtailed with 400 mm-core ber was used as the pump source.All of the above measurements were carried out at room temperature.

Formation of silver nanoparticles
As shown in Fig. 1, the glass transition temperature (T g ) and crystallization onset temperature (T x ) of the Pr 3+ doped HGT-PrAH glass with Ag nanoparticles, which were measured by DSC, are 320 C and 402 C, respectively.
The Pr 3+ doped heavy metal germanium tellurite glasses containing Ag NPs are visually transparent and homogeneous as shown in Fig. 2(a).Fig. 2(b), which present the transmission electron microscopy (TEM) images of HGT-PrAH glass sample with the addition of Ag element under different magnications.As observed from the images, the Ag particles with various sizes and shapes are obtained in the HGT-PrAH glass, which was heat-treated at 350 C for 2 h.The rened details of the HGT-PrAH glass are investigated by adopting a larger magnication (50 nm, as shown in Fig. 2(c)), which illustrates the generation of Ag NPs with diameters about $7 nm in the HGT-PrAH glass.To conrm the nucleation of Ag NPs, the selected area electron diffraction (SAED) pattern was recorded (Fig. 2(d)), in which the white spots in the rst, second, and third rings originated from the (111), (200), and (220) crystal plane reections of metallic Ag NPs, respectively, directly proving the presence of metallic Ag in the HGT-PrAH glass. 35,36t is worth noting that the formation of Ag 0 NPs from Ag + particles occurs through two representative reactions during the high-temperature melting process 37 Ag + + e À / Ag 0 , Ag + + Ag + / Ag 2+ + Ag 0 .
(1)  Furthermore, a probable mechanism of a selective thermochemical reduction from Ag + ions to Ag 0 atoms by Te 4+ ions in the tellurite glasses is considered due to the electromotive force values or reduction potentials of the respective redox system elements, that is 38 Ag + /Ag 0 ¼ 0.799 V, Te 6+ /Te 4+ ¼ 1.02 V. (2) According to the above reduction processes, following process is likely to occur where DE 0 is the total potential of the reduction process.The viable reaction is eqn (3) (with DE 0 > 0), which illustrates the presence of Ag NPs in the glass system.In addition, it is noteworthy that there is a fraction of Ag particles that can still be present as Ag + ions, atoms, and multimers in the glass samples. 39-42

Luminescence properties
Pr 3+ possesses a variety of radiative transitions from 3 P 0 and 1 D 2 levels when excited by blue light.With the introduction of Ag NPs, it is promising to enhance the emissions of Pr 3+ .4][45][46] Compared to the HGT-Pr glass, in the HGT-PrAH glass, there is a visible increment up to $25% in the emission intensities.The Pr 3+ doped heavy metal germanium tellurite glasses exhibit a red uorescence under the excitation of 448 nm and the uorescent difference for the HGT-Pr and HGT-PrAH glass samples are shown in the inset photos (I) and (II) of Fig. 3, respectively.It is evident that the uorescence of HGT-PrAH is brighter than that of the HGT-Pr glass sample, forcefully demonstrating that the characteristic emission from Pr 3+ is intensied by introducing metallic Ag NPs in the HGT-PrAH glass.
The excitation spectra of the HGT-Pr and HGT-PrAH glass samples monitored at 595 and 646 nm are presented in Fig. 4(a) and (b), respectively.Three excitation bands peaking at 448, 474, and 487 nm are assigned to the absorption transitions of 3 H 4 / 3 P 2 , 3 H 4 / ( 1 I 6 , 3 P 1 ), and 3 H 4 / 3 P 0 , respectively, indicating that the emissions originating from the emitting 3 P 0 and 1 D 2 states can be achieved under the excitation of a commercial blue laser diode, blue and blue-greenish LEDs, and an Ar + optical laser.In addition, under the blue light excitation, the emissions from two levels of Pr 3+ are clearly enhanced in the HGT-PrAH glass compared to those in the HGT-Pr glass.The excitation band peaking at 595 nm is recorded for a 599 nm emission, which is due to the contribution of 1 D 2 / 3 H 4 emission when the 1 D 2 level undergoes excitation.
The stimulated emission cross-section s em is an important parameter to evaluate the energy extraction efficiency of the optical material.From the experimental uorescence spectra, the s em for the transition emissions of Pr 3+ can be evaluated by the Fuchtbauer-Ladenburg (FL) formula: 47 where n, A ij , I(l), and c represent a refractive index, spontaneous emission probability, uorescence intensity, and vacuum light velocity, respectively.The obtained s em proles of the Pr 3+ doped heavy metal germanium tellurite glasses in the visible region are shown in Fig. 4  perceived as the local eld enhancement around Pr 3+ induced by Ag NPs in the glass matrix. 48Thus, when the optical frequency of the incident light beam is approximately near the localized surface plasmon resonance (LSPR) of nanoparticles, a signicant luminescence enhancement can be obtained.

Localized surface plasmon resonance investigation
The measured UV/Vis/NIR absorption spectra of the HGT-Pr and HGT-PrAH glass samples are presented in Fig. 5  410 nm range for the silicate glasses with a refractive index of $1.5.][52] According to the Judd-Ofelt theory, the radiative transitions belonging to the 4f 2 conguration of Pr 3+ can be analyzed based on the absorption of Pr 3+ .][55] The intensity parameter U 2 has been identied to be sensitive to the asymmetry and the covalency of the rare-earth ions and U 4 and U 6 are related to the bulk property and rigidity of the samples, respectively.In the HGT-PrAH glass system, U 2 is larger than the value of 2.05 Â 10 À20 cm 2 in the HGT-Pr glass system without Ag NPs, which shows a strong asymmetrical and higher covalent environment owing to the introduction of Ag NPs changing the ligand eld of Pr 3+ , thus achieving the intense uorescence emission.Using these intensity parameters, some important radiative properties including spontaneous emission probabilities (A rad ), luminescence branching ratios (b), and radiative lifetime (s rad ) for the optical transitions of Pr 3+ in the HGT-Pr and HGT-PrAH glasses are calculated and listed in Table 1.The predicated spontaneous emission probabilities A rad for the transitions3 P 0 / 3 F 2 , 3 P 0 / 3 H 6 , and 1 D 2 / 3 H 4 are derived to be 15 570, 7355, and 1503 s À1 , respectively, and the relevant branching ratios b account for 23.3%, 11.0%, and 30.5%, respectively, presenting that the emissions of Pr 3+ in the tellurite glasses are effective.
The luminescence decay curves of the 3 P 0 and 1 D 2 levels for the HGT-Pr and HGT-PrAH glasses are shown in Fig. 6(a) and (d).The uorescent lifetimes (s exp ) of the 3 P 0 and 1 D 2 levels can be derived from the uorescence decays by the following formula: 56 where I(t) is the emission intensity at time t.The experimental lifetimes (s exp ) of the 3 P 0 and 1 D 2 levels for the HGT-Pr and HGT-PrAH glasses are calculated to be 8.65, 8.71, 66.14, and 67.29 ms, respectively.The energy transfer from the Ag species to the RE ions is represented by an apparent increase in lifetime of the emitting RE level and the energy transfer from Ag NPs to the RE ions is not operative since the lifetime of plasma oscillation of Ag particle (10 À14 s) is much smaller than that of the RE ions (10 À6 to 10 À3 s).Thus, the result of the single exponential tting for the decay curves leads to an increase in the lifetime when silver nanoparticles are added.The quantum efficiency (h q ) of the HGT-Pr and HGT-PrAH glasses are calculated to be 57.71% and 67.89% at the 3 P 0 level and 32.64% and 34.44% at the 1 D 2 level, respectively, by using the following equation: where s exp is the experimental lifetime, which is derived by tting the uorescence decay curves for the 3 P 0 / 1 D 2 and 1 D 2 / 1 G 4 transition emissions, and the s rad is the radiative lifetime, which is obtained from Judd-Ofelt analysis.With the introduction of Ag NPs, the quantum efficiency of the 3 P 0 and 1 D 2 levels are increased by $10.18% and $1.80%, respectively, and the increments of the quantum efficiencies from HGT-Pr to HGT-PrAH glasses occur due to the existence of the LSPR.

Absolute spectral parameters of the Pr 3+ doped HGT glasses
An integrating sphere coupled with a pumping laser was applied to measure an absolute spectral parameter, which provides an external quantum yield to evaluate the luminescence materials.The net spectral power distributions of uorescence for the HGT-Pr and HGT-PrAH glasses were captured under the excitation of the 453 nm blue laser with various pump powers and presented in Fig. 7.The intense red emission is observed in the Pr 3+ doped HGT glasses under the excitation of the blue laser as shown in the photographs in Fig. 7.Each spectral power distribution curve comprises seven emission bands in the red and near-infrared region located at 646, 688, 709, 732, 815, 869, and 1038 nm, which are assigned to 3 P 0 / 3 F 2 , 1 D 2 / 3 H 5 , 3 P 0 / 3 F 3 , 3 P 0 / 3 F 4 , 1 D 2 / 3 H 6 , 1 D 2 / 3 F 2 , and 1 D 2 / 3 F 4 transitions, respectively.Under the excitation of the 453 nm laser with a power (P laser ) of 5.13 and 15.02 mW, the net emission spectral powers for the HGT-Pr glass are obtained to 106.08 and 535.17 mW, respectively, and as high as 180.80 and 615.03 mW for the HGT-PrAH glass, respectively, conrming that the metallic Ag NPs doping HGT glasses are conducive to more efficiently utilize laser due to the LSPR of Ag nanoparticles.
According to the net spectral power distribution, the 3 P 0 and 1 D 2 levels of Pr 3+ were sensitized by introducing metallic Ag NPs  in the Pr 3+ doped HGT glasses.Table 2 demonstrates the tangible enhancement in the 3 P 0 and 1 D 2 levels under the 453 nm blue laser excitation for diverse powers, implying the sensitized difference of the disparate primary state level.With the increment of the laser power, the enhanced percentage shows an upward trend, indicating that the macroscopical sensitization can be realized under the excitation of the 453 nm blue laser.
Based on the net spectral power distributions of the Pr 3+ doped HGT glass samples, the photon distributions can be derived from where l is the wavelength, n is the wavenumber, h is the Planck constant, c is the vacuum velocity of light, and P(l) is the spectral power distribution.Under the 453 nm blue laser excitation for diverse powers, the net absorption and emission photon distribution curves of the Pr 3+ doped HGT glass samples with metallic Ag NPs are derived from eqn (8) with P(l) as shown in Fig. 8.With the increment of the laser pump power, the emission photon numbers exhibit an antrorse trend for the Pr 3+ and Ag NPs doped HGT glass samples.The quantum yield (QY) of the luminescence material is dened as the ratio of the number of photons emitted to those absorbed, which is used as a selection criterion of the luminescence materials for potential applications in solid-state lighting devices. 57The equation is expressed as follows: Through this formula, the total QYs of the Pr 3+ doped HGT glasses with and without Ag NPs under the excitation of the 453 nm blue laser with diverse powers are derived and listed in Table 3.The total QYs of the HGT-Pr and HGT-PrAH glasses are calculated to be 10.95% and 11.84% in the case of 5.13 mW pumping and 11.63% and 11.94% in the case of 15.02 mW pumping, respectively.Under the excitation of the 453 nm laser with the 5.13 mW power, the quantum yield of the HGT-PrAH glass with Ag NPs reaches 11.84%, which is higher than that in the HGT-Pr glass without Ag NPs, suggesting that the macroscopical sensitization is due to the LSPR.Higher photon release efficiency further implies the potential of the Pr 3+ doped HGT glasses containing Ag NPs for laser illumination.
In order to comprehend the mechanism of the observed luminescence enhancement, the partial energy band diagram for Pr 3+ in the vicinity of Ag NP is illustrated in Fig. 9.Under the excitation of 448 nm, initially, the 3 P 2 level is populated by the single-step ground state absorption, accompanying a sequence of non-radiative relaxation that populates the 3 P 0 state. 58ollowing this, the transitions occur from 3 P 0 to 3 H 4 , 3 H 5 , and 3 F 2 levels, emitting a series of uorescence.Moreover, the 3 P 0 level depopulates non-radiatively through multiphononassisted decays to the 1 D 2 state and then jumps to the 3 H 4 and 3 H 5 levels.The emission rates of the Pr 3+ ions located in the vicinity of Ag NPs increase and the major reason of increment in the emission rates is the mismatch of dielectric constants between the metal and the surroundings due to LSPR.Another approach to further enhancements can be described by an energy transfer (ET) from the surface of Ag NP to Pr 3+ ion, which plays a secondary role.

Color anticipation of the Pr 3+ doped germanium tellurite glass phosphor
For the practicality aspect, the intense red light with an adequate intensity and good directivity in the Pr 3+ doped HGT glasses provide advantageous surroundings in developing a laser illumination device.Fig. 10 displays the uorescence photographs corresponding to the HGT-Pr and HGT-PrAH glasses with a long-path laser under the laser excitation.In addition, the uorescence color diversication can be observed under the excitation of the 453 nm blue laser with diverse powers, demonstrating the notable tunability of color uorescence of the Pr 3+ emissions when the laser power increases from 5.13 to 15.02 mW.Moreover, there is a visible aggrandizement with regard to the brightness for the Pr 3+ doped HGT glasses with Ag NPs under the same excitation condition, suggesting the macroscopical sensitization and providing a potential development in laser illumination devices.The color coordinates of the colorful uorescence in HGT-PrAH glasses under the diverse excitation conditions are derived and marked on the CIE-1931 standard chromaticity diagram.A multicolor integral uorescence derived from the combination of the residual laser and the Pr 3+ spontaneous emission can be realized by adjusting the intensity ratio between the laser and the Pr 3+ emissions as shown in Fig. 11.
The CIE 1931 color coordinates for the white uorescence of the Pr 3+ doped germanium tellurite glasses under different excitation conditions are calculated using the following formula where l is the wavelength of the equivalent monochromatic light and x(l), y(l), and z(l) are three color-matching functions.
On varying the intensity ratios between the residual laser and the Pr 3+ emissions, the color coordinates move along the top le direction to the le boundary of the red region, passing through beneath the pure white region; the nearest-white region is point 3, whose color coordinate is derived to be (0.395, 0.225).White light would be achieved in HGT-PrAH glasses with the assistance of Tb 3+ , which emits a green uorescence, which is   benecial to the white illumination.Moreover, the Pr 3+ doped tellurite glasses have also been conrmed as promising materials for light emitting diodes with an additional Er 3+ ion doping. 59,60On the whole, the Pr 3+ doped tellurite glass is an upand-coming material for the region of the illumination.

Conclusions
The Pr 3+ doped heavy metal germanium tellurite glasses (HGT) containing AgCl was prepared to produce Ag nanoparticles with diameters $7 nm, which were evidenced by TEM images.The localized surface plasmon resonance (LSPR) band of around 500-530 nm in the prepared glass samples was demonstrated by the absorption spectra.The multichannel transition emission intensity of the Pr 3+ with Ag NPs embedded HGT-PrAH glasses increased by $25% in comparison with that in a silver-free glass sample, which illustrates the existence of the LSPR combined the absorption spectra, thus emitting a noticeable red uorescence.Net emission power and net emission photon number are calculated to be 615.03mW and 20.37 Â 10 14 cps, respectively, and the quantum yield is as high as 11.94% in the Pr 3+ doped HGT glasses with Ag NPs under the excitation of a 453 nm blue laser with a 15.02 mW power.An intense and tunable red uorescence was observed, demonstrating macroscopical sensitization with the addition of Ag NPs.Furthermore, white light would be achieved by the addition of a green component when the residual laser and the Pr 3+ emission reach an appropriate range.Moreover, the Pr 3+ doped tellurite glasses can be applied to light emitting diodes on co-doping with Er 3+ .The results indicate that the Pr 3+ -doped HGT glasses with Ag NPs are an exploitable material, which provides an efficient red uorescence in the improvement of the color-rendering index for laser illumination.

Fig. 2
Fig. 2 (a) Photograph of the HGT-PrAH glass with Ag nanoparticles under natural light.(b and c) TEM images of the HGT-PrAH glass with the scale bars of 100 nm and 50 nm.(d) The selected area electron diffraction pattern of HGT-PrAH glass.

Fig. 3
Fig. 3 Emission spectra of the HGT-Pr and HGT-PrAH glasses under a 448 nm excitation.Inset: fluorescence photos (I) and (II) of the HGT-Pr and HGT-PrAH glasses, respectively, under the excitation of 448 nm.
Fig. 4 (a and b) Excitation spectra of the HGT-Pr and HGT-PrAH glasses monitored at 595 and 646 nm.(c and d) Emission cross-section profiles of the HGT-Pr and HGT-PrAH glasses for 1 D 2 / 3 H 4 , 3 P 0 / 3 H 6 , and 3 P 0 / 3 F 2 transition emissions.

Fig. 5 (
Fig. 5 (a and b) Absorption spectra of HGT-Pr and HGT-PrAH glass samples.Inset: LSPR band of Ag NPs was observed in the HGT-PrAH glass.

Fig. 6
Fig. 6 (a and b) Fluorescence decay curves of the 3 P 0 level for the HGT-Pr and HGT-PrAH glasses.(c and d) Fluorescence decay curves of the 1 D 2 level for the HGT-Pr and HGT-PrAH glasses.

Fig. 7
Fig. 7 Net spectral power distribution in HGT-Pr (a and b) and HGT-PrAH (c and d) glasses under the 453 nm laser excitation.Inset: fluorescence photographs of HGT-Pr and HGT-PrAH glasses under the excitation of the 453 nm laser in an integrating sphere.
) where X, Y, Z are three tristimulus values.The tristimulus values for a color with a spectral power distribution P(l) are acquired by the following:

Fig. 8
Fig. 8 Net emission photon distributions of the HGT-Pr (a and b) and HGT-PrAH (c and d) glasses under the 453 nm laser excitation.Inset: details of the related net absorption photon distribution.

Fig. 9
Fig. 9 Energy level diagram of the Pr 3+ doped HGT glasses in the vicinity of Ag NP.

Fig. 10
Fig. 10 Fluorescence photographs of (a) HGT-Pr and (b) HGT-PrAH glasses under the 453 nm laser excitation with a power of 5.13 mW.Fluorescence photographs of HGT-Pr (c) and HGT-PrAH (d) glasses under the 453 nm laser excitation with a power of 15.02 mW.

Fig. 11
Fig. 11 Color coordinates in the CIE 1931 chromaticity diagrams for HGT-PrAH glasses under the 453 nm laser excitation.
(c) and (d) and the peak values (s em ) for 1 D 2 / 3 H 4 , 3 P 0 / 3 H 6 , and 3 P 0 / 3 F 2 are calculated to be 3.35 Â 10 À21 , 13.20 Â 10 À21 , and 89.88 Â 10 À21 cm 2 , respectively, for the HGT-Pr glass sample.Furthermore, the corresponding peak values (s em ) of the HGT-PrAH glass are calculated to be 3.57 Â 10 À21 , 13.99 Â 10 À21 , and 90.21 Â 10 À21 cm 2 , respectively.Although the approximations used in Judd-Ofelt theory gives the predictions that can be off by 20% or more, the efficiency evaluation still can provide the relatively comparable results.The large emission cross-sections for the emission transitions indicate that the intense uorescence in the 580-660 nm region can be realized in the Pr 3+ -doped heavy metal germanium tellurite glasses under appropriate excitation conditions.By contrast with the HGT-Pr glass sample, the HGT-PrAH glass with metallic silver NPs possesses larger values on the emission cross-section, indicating that the great red lights derived from the Pr 3+ ions are promising to improve the colorrendering index in laser illumination.To illustrate the luminescence intensication of the HGT-PrAH glass with Ag NPs, two possible mechanisms of uorescence enhancement are believed to exist in the HGT-PrAH glass with Ag NPs.The rst mechanism is based on the local eld enhancement around rare-earth ions and the second is based on the transmission of energy from Ag particles to rare-earth ions.Owing to the fact that spectral shape of the excitation curve has not changed dramatically, the process of energy transmission between Ag NPs and Pr 3+ is non-dominant in the HGT-PrAH glass.The multichannel luminescence increment of Pr 3+ is primarily

Table 1
Predicted spontaneous emission probabilities A rad , branching ratios b, and radiative lifetime s rad of the Pr 3+ doped HGT glasses with and without metallic Ag nanoparticles Transition HGT-Pr HGT-PrAH Energy (cm À1 ) A rad (s À1 ) b (%) s rad (ms) Energy (cm À1 ) A rad (s À1 ) b (%) s rad (ms)

Table 2
Multiple sensitizations of the 3 P 0 and 1 D 2 levels in the Pr 3+ doped HGT glasses with and without metallic Ag NPs under the 453 nm laser excitation

Table 3
Absorption and emission photon numbers and quantum yield in the Pr 3+ doped HGT glasses with and without metallic Ag NPs under the 453 nm laser excitation