The absorption of the samples (A) and (C) was measured, as is shown in Fig.1. From Fig.1, we can find that the absorption of Er³⁺ ion in sample (A) is basically the same as that in sample (C). They are very similar in wave shape, peak intensity, and peak wavelength. The baseline of the absorption of sample (A) is enhanced greatly because of the absorption and scatter of the Ag NPs. It can be found that the absorption line of sample (A) in the range of 1 600~2 500 nm is parallel to that of sample (C), which mainly results from the scatter of the Ag NPs. Meanwhile in the range of 400~1 500 nm, the absorption line of sample (A) emerges a small broad spectral peak position at about 520 nm, which results from the absorption of surface plasmon polariton of the Ag NPs. The phenomenon of that the absorption peak of surface plasmon polariton of the Ag NPs in Telluride glass is small is coincided with the report of references^{[9, 12, 13]}. Finally, Figure 1 shows that the absorption peaks of Er³⁺ ion in sample (A) are positioned at (1 497.0 nm, 1 531.5 nm), 976.5, 799.0, 652.5, 544.0, 522.0, 489.0, 451.5, 443.5, 406.8, 379.0 and 365.5 nm, respectively. It can be recognized that these absorption peaks are the absorption transitions of ⁴I_15/2→ ⁴I_13/2, ⁴I_15/2→ ⁴I_11/2, ⁴I_15/2→ ⁴I_9/2, ⁴I_15/2→ ⁴F_9/2, ⁴I_15/2→ ⁴S_3/2, ⁴I_15/2→ ²H_11/2, ⁴I_15/2→ ⁴F_7/2, ⁴I_15/2→ ⁴F_5/2, ⁴I_15/2→ ⁴F_3/2, ⁴I_15/2→ ²H_9/2, ⁴I_15/2→ ⁴G_11/2, and ⁴I_15/2→ ⁴G_9/2 of the Er³⁺ ion^{[1, 10]}, respectively.

Fig.1

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	Fig.1 The absorption of the samples (A) Er3+(0.5%)Ag(0.2%): Telluride glass with 80 nm average diameter for Ag NPs (blue) and (C) Er3+(0.5%): Telluride glass (green)

The excitation spectra of the samples (A), (B) and (C) were measured. First, as is shown in Fig. 2, we select 550.0 nm visible luminescence wavelength as fluorescence measurement wavelength to measure the visible excitation spectra in the wave range of 250.0~508.0 nm. We found 5 excitation peaks positioned au 365.5, 379.0, 406.5, 451.0, and 488.5 nm, respectively. Moreover, we easily recognized that they are the absorption transitions of the ⁴I_15/2→ ⁴G_9/2, ⁴I_15/2→ ⁴G_11/2, ⁴I_15/2→ ²H_9/2, ⁴I_15/2→ (⁴F_3/2, ⁴F_5/2), and ⁴I_15/2→ ⁴F_7/2 of the Er³⁺ ion^{[1, 10]}. Their peak intensities of the visible excitation spectra 6.83× 10², 2.42× 10³, 2.90× 10², 2.59× 10², 6.73× 10² for sample (A), and are about 3.69× 10², 1.85× 10³, 1.82× 10², 1.70× 10², 4.74× 10² for sample (C). For the above mentioned 5 peaks, the intensities of the visible excitation spectra of the sample (A) are 1.85, 1.30, 1.59, 1.52, and 1.42 times larger than that of sample (C), respectively.

Fig.2

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	Fig.2 The visible (1) and infrared (2) excitation spectra of (A) Er3+(0.5%)Ag(0.2%): Telluride glass with the average diameter of 80 nm for Ag NPs (blue), (B) Er3+(0.5%)Ag(0.2%): Telluride glass with the average diameter of 50 nm for Ag NPs (red) and (C) Er3+(0.5%): Telluride glass (green) when being monitored at 550.0 nm (1) and 1 530.0 nm (2) luminescence wavelength

Then, as is also shown in Fig.2, we select 1 530 nm near infrared luminescence wavelength as fluorescence measurement wavelength to measure the near infrared excitation spectra in the wave range of 250.0~850.0 nm. We found 9 excitation peaks positioned in 365.5, 378.0, 406.5, 451.0, 488.5, 520.5, 543.5, 652.5, and 801.0 nm, respectively. It was recognized that they are the absorption peaks of the ⁴I_15/2→ ⁴G_9/2, ⁴I_15/2→ ⁴G_11/2, ⁴I_15/2→ ²H_9/2, ⁴I_15/2→ (⁴F_3/2, ⁴F_5/2), ⁴I_15/2→ ⁴F_7/2, ⁴I_15/2→ ²H_11/2, ⁴I_15/2→ ⁴S_3/2, ⁴I_15/2→ ⁴F_9/2, and ⁴I_15/2→ ⁴I_9/2 transitions^{[1, 10]}. Their peak intensities of the near infrared excitation spectra are about 1.13× 10³, 3.22× 10³, 4.86× 10², 4.60× 10², 1.07× 10³, 2.68× 10³, 4.39× 10², 1.11× 10³, 4.21× 10² for sample (A), and 5.36× 10², 2.12× 10³, 2.37× 10², 2.25× 10², 5.79× 10², 1.81× 10³, 2.13× 10², 6.46× 10², 2.28× 10² for sample (C). For the above mentioned 9 peaks, the intensities of the near infrared excitation spectra of the sample (A) are about 2.12, 1.52, 2.05, 2.05, 1.85, 1.49, 2.06, 1.71, and 1.85 times larger than that of sample (C), respectively.

The luminescence spectra of the samples (A), (B) and (C) were measured. First, as is shown in Figs.3 and 4, we select the 365.5 and 379.0 nm excitation peaks as excitation wavelength to measure the visible luminescence spectra in the wave range of 385~780 nm. We found 4 luminescence peaks positioned at 408.0, 525.0, 546.0, and 658.5 nm, respectively. They are, respectively, the fluorescence transitions of ²H_9/2→ ⁴I_15/2, ²H_11/2→ ⁴I_15/2, ⁴S_3/2→ ⁴I_15/2, and ⁴F_9/2→ ⁴I_15/2 of Er³⁺ ions^{[1, 10]}. Their peak intensities of the visible luminescence spectra are about 2.70× 10⁰, 2.56× 10¹, 8.94× 10¹, 7.50× 10^-1 for sample (A), 1.57× 10⁰, 1.84× 10¹, 5.48× 10¹, 4.85× 10^-1 for sample (B), and 1.11× 10⁰, 1.43× 10¹, 3.54× 10¹, 3.45× 10^-1 for sample (C), when they are excited by the 365.5 nm light. Similarly, their peak intensities of the visible luminescence spectra are about 5.87× 10⁰, 1.20× 10², 3.67× 10², 3.04× 10⁰ for sample (A), 3.58× 10⁰, 8.99× 10¹, 2.43× 10², 2.15× 10⁰ for sample (B), and 2.84× 10⁰, 8.31× 10¹, 2.04× 10², 1.85× 10⁰ for sample (C), when they are excited by the 379.0 nm light. It is easy to calculate that the peak intensities of the above 4 visible luminescence of sample (A) are about 1.44~2.52 times larger than that of the sample (C). Moreover, the peak intensities of the above 4 visible luminescence spectra of sample (B) are about 1.08~1.55 times larger than that of the sample (C). In other words, the largest enhancement is about 2.52 times larger for visible luminescence.

Fig.3

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	Fig.3 The visible (1) and infrared (2) luminescence spectra of the samples (A) Er3+(0.5%)Ag(0.2%): Telluride glass with 80 nm average diameter for Ag NPs (blue), (B) Er3+(0.5%)Ag(0.2%): Telluride glass with 50 nm average diameter for Ag NPs (red), and (C) Er3+(0.5%): Telluride glass (green) when excited by 365.5 nm light

Then, as is also shown in Figs.3 and 4, we select the 365.5 and 379.0 nm excitation peaks as excitation wavelength to measure the near infrared luminescence spectra in the wave range of 928~1 680 nm. It is found that near infrared luminescence peaks are positioned at 979.0 and 1 530.0 nm. They are, respectively, the fluorescence transitions of ⁴I_11/2→ ⁴I_15/2 and ⁴I_13/2→ ⁴I_15/2 of Er³⁺ ions^{[1, 10]}. Their peak intensities of the near infrared luminescence spectra are about 2.84× 10¹, 2.36× 10² for sample (A), 2.16× 10¹, 2.01× 10² for sample (B), and 1.41× 10¹, 1.10× 10² for sample (C), when they are excited by the 365.5 nm light. Similarly, their peak intensities of the near infrared luminescence spectra are about 8.97× 10¹, 7.32× 10² for sample (A), 8.04× 10¹, 6.95× 10² for sample (B), and 6.28× 10¹, 4.81× 10² for sample (C), when they are excited by the 379.0 nm light. The peak intensities of the above 2 near infrared luminescence spectra of sample (A) are about 1.43~2.14 times larger than that of the sample (C). Similarly, the peak intensities of the above 2 near infrared luminescence spectra of sample (B) are about 1.28~1.82 times larger than that of the sample (C). That is, the largest enhancement time is about 2.14 times larger for near infrared luminescence.

Fig.4

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	Fig.4 The visible (1) and infrared (2) luminescence spectra of the samples (A) Er3+(0.5%)Ag(0.2%): Telluride glass with 80 nm average diameter for Ag NPs (blue), (B) Er3+(0.5%)Ag(0.2%): Telluride glass with 50 nm average diameter for Ag NPs (red), and (C) Er3+(0.5%): Telluride glass (green) when excited by 379.0 nm light

The fluorescence lifetime of 550 nm fluorescence of sample (A), (B) and (C) are measured when pulse xenon lamp is used as a pumping source. The measured results are shown in Fig. 5. The 550 nm fluorescence lifetime of sample (A) is about τ _A(550)=43.5 μ s, that of sample (B) is about τ _B(550)=43.2 μ s, and that of sample (C) is about τ _C(550)=48.6 μ s.

Fig.5

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	Fig.5 The fluorescence lifetime of 550 nm fluorescence of (A) Er3+(0.5%)Ag(0.2%): Telluride glass with 80 nm average diameter for Ag NPs (blue), (B) Er3+(0.5%)Ag(0.2%): Telluride glass with 50 nm average diameter for Ag NPs (red), and (C) Er3+(0.5%): Telluride glass (green) when excited by 378 nm pulsed light

The method of introducing nanosilver particles in luminescence glass is mainly through silver chloride or silver nitrate. In the process of melting glass raw materials, silver chloride or silver nitrate is melted. In the process of subsequent condensation, the silver atoms solidified into nano clusters with the formation of solid glass. Or, the Ag NPs may be obtained by the special heat-treated process. In these ways, nanosilver is generally very small within the few nanometers range, which causes a small number of luminescence enhancements of Er³⁺ ion induced by silver surface Plasmon in luminescence glass. However, we are experimenting with the direct introduction of nanosilver particles that ranges from 10 to more than 100 nm. Because the melting point of silver is 961° . And we control the raw materials of Telluride luminescent glass to be melted at less than 880° . Therefore, the size of the nanoscale particles of Ag will hardly decrease. The most important thing is that we have thus enhanced and controlled the size r of the silver nanoparticles in the Telluride luminescence glass.

According to Mie theory^[8], the extinction interface C_e of the spherical metal particles in rough metal surface is about

Ce=Ca+Cs(1)

The scattering cross section C_s is the reason of fluorescence enhancement, the absorption cross section C_a is the reason of the fluorescence decrease. In which

Cs=k46π|α|2∝r6(2)

Ca=k×Im(α)∝r3(3)

k=2πnλ0(4)

α=4πr3εm-εεm+2ε(5)

k is the wave vector of the incident light in the medium, λ ₀ is the wavelength of the incident light, n is the refractive index of the medium, α is the polarizability of the sphere when the radius is r, ε _m is the dielectric constant of the metal, ε is the dielectric constant of the medium. From the above formulas, the scattering cross-section C_s are proportional positive ratio to r⁶, and the absorption cross-section C_a are proportional positive ratio to r³. After we have greatly enhanced the size r of nanosilver particles, the increase of the scattering cross-section C_s is much larger than that of the absorption cross-section C_a. As a result, the enhancement of the fluorescence is greatly improved.

We discovered from the previous experiments of luminescence spectra that the visible luminescence enhancement was 1.44~2.52 times the of A/C for sample (A), which is obviously larger than that of 1.08~1.55 of B/C for sample (B). Moreover, it is fact that the infrared luminescence enhancement is 1.43~2.14 times of A/C for sample (A), which is also larger than that of 1.28~1.82 of B/C for sample (B).

From the previous experiments of fluorescence lifetime dynamics, we know that τ _A≈ τ _B < τ _C. This means that the luminescence enhancement of sample (B) relative to sample (C) results from the Spontaneous radiation enhancement effect. Meanwhile, it also means that the luminescence enhancement of sample (A) relative to sample (B) results from the Size r effect, which is illustrated above.