Energy Transfer between Er³⁺ and Pr³⁺ for 2.7 μm Fiber Laser Material

Abstract

Energy transfer mechanisms between Er³⁺ and Pr³⁺ in Er³⁺/Pr³⁺ codoped germinate glass are investigated in detail. Under 980 nm LD pumping, 2.7 μm fluorescence intensity enhanced greatly. Meanwhile, 1.5 μm lifetime and fluorescence were suppressed deeply due to the efficient energy transfer from Er³⁺:⁴I_13/2 to Pr³⁺:³F_3,4, which depopulates the ⁴I_13/2 level and promotes the 2.7 μm transition effectively. The obvious change in J-O parameters indicates that Pr³⁺ influences the local environment of Er³⁺ significantly. The increased spontaneous radiative probability in Er³⁺/Pr³⁺ glass is further evidence for enhanced ⁴I_11/2 → ⁴I_13/2 transition. The Er³⁺:⁴I_11/2→Pr³⁺:¹G₄ process is harmful to the population accumulation on ⁴I_11/2 level, which inhibits the 2.7 μm emission. The microscopic energy transfer coefficient of Er³⁺:⁴I_13/2→Pr³⁺:³F_3,4 is 42.25 × 10⁻⁴⁰ cm⁶/s, which is 11.5 times larger than that of Er³⁺:⁴I_11/2→Pr³⁺:¹G₄. Both processes prefer to be non-phonon assisted, which is the main reason why Pr³⁺ is so efficient in Er³⁺:2.7 μm emission.

1. Introduction

Mid-infrared laser, especially ~3 μm laser, has extensive potential application, such as remote sensing and laser microsurgery. The abundant level systems of Er³⁺, Dy³⁺ and Ho³⁺ ions make the production of ~3 μm laser possible [1,2,3]. Therefore, a series of methods were used to enhance the intensity of photoluminescence around 2.7 μm, including codoping with other rare-earth ions, such as Tm³⁺, Nd³⁺, Yb³⁺ and Pr³⁺ [1,4,5,6,7,8]. Among them, Pr³⁺ is very effective at sensitizing Er³⁺:2.7 μm emission. Furthermore, Er³⁺/Pr³⁺ codoped tunable CW laser has been obtained in the ZBLAN fiber [9,10]. However, represented by ZBLAN glass, fluoride glass performs with small ΔT (=Tx_p − Tg, Tx_p is the crystallization peak temperature) value, poor chemical and thermal stability, and difficulties in glass preparation and fiber drawing. Researchers have done a lot of work in developing new mid-IR glass to overcome the shortcomings of fluoride glasses. Many results has been reported on chalcogenide, fluorophosphate, tellurite, germanate and PbO-Bi₂O₃-Ga₂O₃ glass [11,12,13,14,15]. Nonetheless, the chalcogenide glass requires a complex fabrication route, especially in refining raw materials and moisture removal process. Besides, the character of bismuth trioxide glass depends on the melting condition. Among the remains of choices, germinate glass does have proper phonon energy and higher glass transition temperature for resisting laser damage. Thus, the Er³⁺/Pr³⁺ codoped germanate glass can have preferable spectroscopic properties as a candidate for 2.7 μm fiber laser material.

Except for Pr³⁺, no laser output has been reported in other RE³⁺ sensitized Er³⁺ glasses. In order to understand the sensitizing mechanisms of Pr³⁺ to Er³⁺ further, in this work, we studied the energy transfer (ET) dynamics as well as the macroscopic ET parameters in a Pr³⁺/Er³⁺ codoped germanate glass. The energy transfer mechanism in Er³⁺/Pr³⁺ codoped glass is focused to demonstrate the sensitizing effect of Pr³⁺.

2. Experimental

Glass was prepared following the molar composition 56GeO₂-15PbO-14Na₂O-12Ga₂O₃-3PbF₂-Er₂O₃-0.5Pr₂O₃, named GPNG glass. 3 mol% PbF₂ was introduced to reduce the hydroxyl groups. All the high-purity powders were well-mixed and melted at 1150 °C for 30 min in an electrical furnace. The melting glass was bubbled with high-purity oxygen gas, and then the melts were poured into preheated stainless-steel mold and annealed for 10 h. The homogeneous samples were cut into 20 mm × 20 mm × 1 mm and well polished.

Refractive index was measured by a Specro-Ellipsometer (Woollam W-VASE, error limit ±0.05%). With 1 nm steps, the absorption spectra were recorded in the range of 400–1700 nm with a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer. Under 980 nm, laser diode (LD)’s pumping, fluorescence spectra were tested by a computer-controlled TRIAX 320 type spectrometer and the FLSP920 fluorescence spectrophotometer (Edinburgh Analytical Instruments Ltd., Livingston, UK). The emission spectra ranged in 2550–2800 nm, 1400–1700 nm, 500–700 nm and the lifetime of Er³⁺:⁴I_13/2 level was also obtained accordingly. The power of 980 nm LD, the width of the slit to collect signals and the position of the samples were fixed to the same condition in the experiment setup in order to accurately compare the intensity of 2.7 μm emission. All the measurements were carried out at room temperature.

3. Results and Discussions

3.1. Absorption Spectra and Infrared Transmittance Spectrum

Figure 1 displays the absorption spectra of GPNG glass singly doped with 1 mol% Er₂O₃, 0.5 mol% Pr³⁺ and coped with both Er₂O₃ and Pr₂O₃ at room temperature. The labeled transitions for both RE³⁺ ions correspond to the ground state to the specific higher levels. The absorption bands at 980 nm illustrate that 980 nm LD can excite this glass.

The mid-infrared transmittance spectrum of Er³⁺/Pr³⁺ codoped sample is shown in the inset of Figure 1. The transmittance reaches 86% and extends to 6.0 μm in the present glass. A typical absorption band of OH⁻ groups appears at around 3 μm. Due to the overlap with the emission wavelength, the OH⁻ groups are regarded as a catastrophe in mid-infrared laser materials. The content of OH⁻ groups can be represented by absorption coefficient, which is defined by:

α_OH =-In(T_b/T)/L

(1)

Where l is the sample thickness (cm), T_b and T are the lowest transmittance (%) in OH⁻ group absorption band and the transmittance of baseline, respectively. The calculated OH⁻ absorption coefficient is 0.67 cm⁻¹. This peak is small but more work can be done to decrease the OH⁻ absorption further.

Figure 1. Absorption spectra of Er³⁺, Pr³⁺ and Er³⁺/Pr³⁺ codoped samples.

Figure 1. Absorption spectra of Er³⁺, Pr³⁺ and Er³⁺/Pr³⁺ codoped samples.

3.2. Fluorescence Spectra and Judd-Ofelt Analysis

Figure 2 shows the measured photoluminescence spectra of Er³⁺ singly doped and Er³⁺/Pr³⁺ codoped GPNG glass that correspond to the Er³⁺:⁴I_13/2→⁴I_15/2 and Er³⁺:⁴I_11/2→⁴I_13/2 transition, respectively. It is clearly observed that 2.7 μm emission is significantly enhanced. Meanwhile, the 1.5 μm emission is deeply quenched by Pr³⁺ codoping. In addition, the lifetime of 1.5 μm decrease sharply by Pr³⁺ codoping. The lifetime of Er³⁺:1.5 μm in singly doped sample is 3.23 ms, while that of codoped sample too weak to be detected. According to the previous study [16], this phenomenon suggests that energy transfer occurs between Er³⁺ and Pr³⁺ owing to the equally energetic spacing of multiplet-to-multiplet transitions. Pr³⁺ ions obviously depopulate the Er³⁺:⁴I_13/2 level in this ET process.

Figure 2. 1.5 μm (a) and 2.7 μm (b) emission in Er³⁺ singly doped and Er³⁺/Pr³⁺ codoped GPNG glasses.

Figure 2. 1.5 μm (a) and 2.7 μm (b) emission in Er³⁺ singly doped and Er³⁺/Pr³⁺ codoped GPNG glasses.

Judd-Ofelt [17] analysis is widely employed to determine the spontaneous emission transition probabilities, radiative lifetime and branching ratios. The Judd-Ofelt intensity parameters of Er³⁺ and Er³⁺/Pr³⁺ glass were calculated in Table 1. Previous studies indicated that Ω₆ is inversely proportional to the covalence of Er-O band. Ω₂ represents symmetry around Er³⁺ ions. In addition, Ω₆ is also related to the optical basic of host materials. Table 1 shows that Ω₂ of GPNG glass is higher than other systems, while Ω₆ is smaller than fluoride and fluorotellurite glass.

Table 1. Judd-Ofelt intensity parameters in various glasses (unit: 10⁻²⁰ cm²).

Parameters	Germanate		Fluorophosphate	Fluoride	Silicate	Fluorotellurite
	Er3+	Er3+/Pr3+
Ω2	6.60	13.23	5.14 ± 0.10	2.98	4.23	4.38
Ω4	1.75	2.81	1.02 ± 0.08	1.40	1.04	3.05
Ω6	0.99	2.26	0.91 ± 0.06	1.04	0.61	1.04
Reference	this work		[18]	[19]	[20]	[4]

Table 1. Judd-Ofelt intensity parameters in various glasses (unit: 10⁻²⁰ cm²).

Parameters	Germanate		Fluorophosphate	Fluoride	Silicate	Fluorotellurite
	Er3+	Er3+/Pr3+
Ω2	6.60	13.23	5.14 ± 0.10	2.98	4.23	4.38
Ω4	1.75	2.81	1.02 ± 0.08	1.40	1.04	3.05
Ω6	0.99	2.26	0.91 ± 0.06	1.04	0.61	1.04
Reference	this work		[18]	[19]	[20]	[4]

Theoretically, J-O parameters are influenced by the properties of the host. However, results in this work show that J-O parameters change obviously when Pr³⁺ added in, and this is because the local environment of Er³⁺ has been interfered largely by Pr³⁺, which I nduces the variations on Ω_2,4,6. As a result, the spontaneous radiative probability (A_rad) increased from 36.69 s⁻¹ to 60.93 s⁻¹. Moreover, the calculated radiative lifetime of ⁴I_11/2 level decreased from 4.55 ms to 2.16 ms (Table 2). Obviously, increasing A_rad promotes the ⁴I_11/2→⁴I_13/2 transition as well as the 2.7 μm photoluminescence.

Table 2. Calculations of A_rad, branching ratio (β) and radiative lifetime in Er³⁺ and Er³⁺/Pr³⁺ samples.

InitialLevel	EndLevel	Er3+			Er3+/Pr3+
		Arad	β	τ (ms)	Arad	β	τ (ms)
4I13/2	4I15/2	183.17	100.00%	5.46	334.21	100.00%	2.99
4I11/2	4I15/2	182.92	83.29%	4.55	402.18	86.84%	2.16
	4I13/2	36.69	16.71%		60.93	13.16%
4I9/2	4I15/2	183.28	77.57%	4.23	297.32	71.91%	2.42
	4I13/2	49.82	21.08%		112.95	27.32%
	4I11/2	3.19	1.35%		3.19	0.77%
4F9/2	4I15/2	1882.77	90.80%	0.48	3428.00	90.09%	0.26
	4I13/2	104.40	5.03%		188.15	4.94%
	4I11/2	80.91	3.90%		178.36	4.69%
	4I9/2	5.36	0.26%		10.75	0.28%
4S3/2	4I15/2	1249.87	62.83%	0.50	2853.25	63.07%	0.22
	4I13/2	632.44	31.79%		1443.76	31.91%
	4I11/2	39.90	2.01%		89.01	1.97%
	4I9/2	67.14	3.37%		138.05	3.05%
2H11/2	4I15/2	11581.49			22658.36
4F7/2	4I15/2	3390.03			7067.49
4F5/2	4I15/2	779.07			1778.47
2H9/2	4I15/2	1631.01	33.35%	0.20	3228.73	33.05%	0.10
	4I13/2	2325.23	47.54%		4722.89	48.35%
	4I11/2	844.79	17.27%		1640.00	16.79%
	4I9/2	39.18	0.80%		76.81	0.79%
	4F9/2	50.44	1.03%		99.76	1.02%

Table 2. Calculations of A_rad, branching ratio (β) and radiative lifetime in Er³⁺ and Er³⁺/Pr³⁺ samples.

InitialLevel	EndLevel	Er3+			Er3+/Pr3+
		Arad	β	τ (ms)	Arad	β	τ (ms)
4I13/2	4I15/2	183.17	100.00%	5.46	334.21	100.00%	2.99
4I11/2	4I15/2	182.92	83.29%	4.55	402.18	86.84%	2.16
	4I13/2	36.69	16.71%		60.93	13.16%
4I9/2	4I15/2	183.28	77.57%	4.23	297.32	71.91%	2.42
	4I13/2	49.82	21.08%		112.95	27.32%
	4I11/2	3.19	1.35%		3.19	0.77%
4F9/2	4I15/2	1882.77	90.80%	0.48	3428.00	90.09%	0.26
	4I13/2	104.40	5.03%		188.15	4.94%
	4I11/2	80.91	3.90%		178.36	4.69%
	4I9/2	5.36	0.26%		10.75	0.28%
4S3/2	4I15/2	1249.87	62.83%	0.50	2853.25	63.07%	0.22
	4I13/2	632.44	31.79%		1443.76	31.91%
	4I11/2	39.90	2.01%		89.01	1.97%
	4I9/2	67.14	3.37%		138.05	3.05%
2H11/2	4I15/2	11581.49			22658.36
4F7/2	4I15/2	3390.03			7067.49
4F5/2	4I15/2	779.07			1778.47
2H9/2	4I15/2	1631.01	33.35%	0.20	3228.73	33.05%	0.10
	4I13/2	2325.23	47.54%		4722.89	48.35%
	4I11/2	844.79	17.27%		1640.00	16.79%
	4I9/2	39.18	0.80%		76.81	0.79%
	4F9/2	50.44	1.03%		99.76	1.02%

3.3. Energy Transfer Mechanisms between Er³⁺and Pr³⁺

Previous study [21] has demonstrated possible mechanisms based on multiplets of Er³⁺ and Pr³⁺ ions in Figure 3. Pumped by 980 nm LD, ground state ions are excited to ⁴I_11/2 level (ground state absorption, GSA). The involved energy transfer processes based on the ⁴I_11/2 level are as follows: Er³⁺: ⁴I_11/2 + a photon→Er³⁺: ⁴F_7/2 (ESA1), Er³⁺: ⁴I_11/2 + ⁴I_11/2→Er³⁺:⁴I_15/2 + ⁴F_7/2 (ETU1), ⁴I_11/2→⁴I_13/2 transition with 2.7 μm emission, and energy transfer from Er³⁺: ⁴I_11/2 level to Pr³⁺: ¹G₄ level (ET1). After ETU1 and some nonradiative transitions among ⁴F_7/2, ²H_11/2, ⁴S_3/2 and ⁴F_9/2, the above-mentioned levels are populated. These excited ions contribute to upconversion spectra for green and red emission. Based on lower level ⁴I_13/2, ⁴I_13/2 + ⁴I_13/2→⁴I_15/2 + ⁴I_9/2 (ETU2) and nonradiative relaxation from ⁴I_9/2 level to ⁴I_11/2 level populate the ⁴I_11/2. Therefore, the ETU2 process is beneficial to 2.7 μm emission. Moreover, ET2 from Er³⁺:⁴I_13/2 to Pr³⁺:³F_3,4 depletes the Er³⁺: ⁴I_13/2 level. It is observed that the intensity of red emission is higher than that of green emission in Figure 4. The upper level of red emission (⁴F_9/2) is mainly populated by ESA2: ⁴I_13/2 + a photon→⁴F_9/2. ⁴I_13/2 level is significantly depleted, which is valuable to 2.7 μm emission.

Figure 3. Energy transfer mechanisms of Er³⁺/Pr³⁺ codoped glass.

Figure 3. Energy transfer mechanisms of Er³⁺/Pr³⁺ codoped glass.

Figure 4. Upconversion spectra of GPNG glasses.

Figure 4. Upconversion spectra of GPNG glasses.

As it mentioned above, codoping with Pr³⁺ can enhance the intensity of 2.7 μm emission through energy transfer processes. To profoundly investigate this system, Forster and Dexter’s [22,23] method is used to quantitatively analyze the energy transfer microscopic parameters between Er³⁺ and Pr³⁺. The energy transfer probability rate between donor (D) Er³⁺ and acceptor (A) Pr³⁺ can be expressed as [24]:

(2)

Where |H_DA| is the Hamiltonian between the donor and acceptor. S_DA^N is the integral overlap between m-phonon emission sideband of donor ions and k-phonon absorption line shapes of acceptor ions. N is the total phonons in the transfer process (m + k = N). Since m and k have several compound modes, accumulation of different m, k combination is necessary to determinate the integral overlap under N phonon assisted:

(3)

S_DA(0,0,E) is the overlap between donor emission and acceptor absorption with zero phonon participation. S₀^D and S₀^A are the Huang-Rhys factors in germanate glass. In the resonance and quasi resonance energy transfer process, the overlap S_DA is calculated by Miyakawa and Dexter [25]:

(4)

(5)

In the case of m-phonon emission by the donor and no phonon absorption by the acceptor, the integral overlap is:

(6)

Where ∆E = mħω₀, the max phonon energy peak is taken as ħω₀ in glass material. Considering the fact that the measurements are carried out at finite temperature T, the phonon population at T should be counted:

(7)

(8)

Where E₁ = mħω₀, E₂ = kħω₀ and E = E₁ + E₂. λ_m⁺=1/(1/λ - mħω₀) and λ_k^-=1/(1/λ + kħω₀) represent the m-phonon translated wavelength when the donator emits or the acceptor absorbs k-phonon, respectively.

If we just focus on m-phonon creation process, the probability rate of energy transfer can be obtained by:

(9)

where R is the distance between donor and acceptor. C_DA is the energy transfer coefficient, which can represent the efficiency of energy transfer. Thus, C_DA can be expressed by:

(10)

The calculation results by Equations (2)–(10) are listed in Table 3. It also shows the contribution percentage of different phonon numbers to the probability rate involved in the energy transfer. As a result, the transfer coefficient of ET1 Er³⁺: ⁴I_11/2→Pr³⁺: ¹G₄ is 3.67 × 10⁻⁴⁰ cm⁶/s while that of ET2: Er³⁺: ⁴I_13/2 → Pr³⁺: ³F_{3, 4} is 42.25 × 10⁻⁴⁰ cm⁶/s. The coefficient of ET2 is 11.5 times larger than that of ET1; therefore, codoping with Pr³⁺ ions efficiently depletes the lower level of 2.7 μm.

Results also show that non-phonon assisted energy transfer is predominant in the ET process, indicating that energy match is the main reason why Pr³⁺ is very effective in sensitizing Er³⁺:2.7 μm emission.

Table 3. Energy transfer microscopic parameter of Er³⁺ and Pr³⁺ in germanate glass.

Energy Transfer	N (% Phonon Assisted)				Transfer Coefficient (10−40 cm6/s)
Er3+: 4I11/2→Pr3+: 1G4	0	1	2	3	3.67
	55	32	12	1%
Er3+: 4I13/2→Pr3+: 3F3, 4	0	1	2		42.25
	82	15	3%

Table 3. Energy transfer microscopic parameter of Er³⁺ and Pr³⁺ in germanate glass.

Energy Transfer	N (% Phonon Assisted)				Transfer Coefficient (10−40 cm6/s)
Er3+: 4I11/2→Pr3+: 1G4	0	1	2	3	3.67
	55	32	12	1%
Er3+: 4I13/2→Pr3+: 3F3, 4	0	1	2		42.25
	82	15	3%

4. Conclusions

In summary, Er³⁺/Pr³⁺ codoped GPNG glass has much enhanced 2.7 μm emission and greatly decreased 1.5 μm fluorescence due to the efficient energy transfer from Er³⁺: ⁴I_13/2 to Pr³⁺: ³F_3,4, which depopulates the ⁴I_13/2 level and promotes the ⁴I_11/2→⁴I_13/2 transition effectively. Fluorescence lifetime decreases with Pr³⁺ introduction, proving the population evacuation on ⁴I_13/2. Interestingly, J-O parameters change obviously, indicating that Pr³⁺ influences the local environment of Er³⁺ significantly. The increased spontaneous radiative probability in Er³⁺/Pr³⁺ glass is further evidence for enhanced ⁴I_11/2→⁴I_13/2 transition. Pr³⁺ can also suppress the upconversion luminescence, and this will increase the quantum efficiency of the system. ET process of Er³⁺: ⁴I_11/2→Pr³⁺: ¹G₄ is harmful to the population accumulation on ⁴I_11/2 level, which inhibits the 2.7 μm emission. The calculation of energy transfer microscopic indicates that the transfer coefficient of Er³⁺: ⁴I_13/2 to Pr³⁺: ³F_3,4 is 42.25 × 10⁻⁴⁰ cm⁶/s, which is 11.5 times larger than that of Er³⁺: ⁴I_11/2→Pr³⁺: ¹G₄, and both processes are preferred to be non-phonon assisted, which is the main reason why Pr³⁺ is so efficient in Er³⁺:2.7 μm emission. This result provides another proof to illustrate the energy transfer mechanisms between Pr³⁺ and Er³⁺.