Energy transfer characteristics of Nd³⁺/Yb³⁺-codoped phospho-silicate oxyfluoride glasses for ~ 1.0 µm laser applications

Abstract

Nd³⁺/Yb³⁺-codoped phospho-silicate oxyfluoride (PNSBYNd_0.5Yb_0.5) glasses were prepared by melt-quenching technique and the structural and photoluminescence (PL) properties were investigated. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to assess glass microstructure and dispersed elements in the glass, respectively. A high-intense and broad Raman band appeared at 1179 cm⁻¹ is attributed to υ_as (Si–O–Si) stretching vibrations of Q³ units owing to high-alkali (Na⁺) silicates and O–P–O (PO)²⁻ symmetric stretching vibrations in Q² units. The g value of unpaired electrons in PNSBYNd_0.5Yb_0.5 glasses was assessed by electron spin resonance (ESR) spectra and found to be 2.0168, more significant than the standard value (2.0023). Various bismuth ionic states (Bi²⁺, Bi³⁺ and Bi⁵⁺) were related in the Bi 4f_7/2 and Bi 4f_5/2 XPS spectra; however, Bi³⁺ ions were dominated. From the PL spectrum, 874 nm band for the ⁴F_3/2 → ⁴I_9/2 transition of Nd³⁺ ions was dominated over 1053 nm emission band perceived in PNSBYNd_0.5Yb_0.5 glasses due to its quasi-three-level system. Energy transfer (ET) and cross-relaxations (CR) were unveiled in Nd/Yb-doped PNSBYNd_0.5Yb_0.5 glasses upon 808 nm diode laser excitation. The Nd³⁺ emission bands 874 and 1053 nm were merged with the Yb³⁺ band at 975 nm due to CR and ET from ⁴F_3/2 → ²F_7/2. The high intensity of the 975 nm laser may be a suitable candidate for NIR laser and amplification applications.

1 Introduction

Rare earth (RE) ions-doped materials are of growing research interest owing to many applications such as lasers [1, 2], display devices [3], lighting technology [4], solar energy conversion [5], telecommunications [6], remote sensing [7], and laser cooling of solids [8, 9]. In addition to various RE ions, ytterbium (Yb³⁺) ions are frequently used for high-power infrared fiber lasers, photovoltaic and thermal materials, phosphors, glasses, and unique nanomaterials for medical [10] and biotechnology applications [11]. The Yb³⁺ ion has an absorption band in the near-infrared at 980 nm, enabling the design of materials to be used as spectral converters inefficient solar cells. Glasses doped with Yb³⁺ materials may find technical applications in telecommunications and optical fiber light sources [12].

Yb³⁺-doped materials are essential in diode-pumped solid-state laser (DPSSL) research. These materials are more suitable for today’s new high-energy laser applications than Nd³⁺ materials due to the moderate electronic level structure of Yb³⁺ ions with only two energy states, ²F_7/2 and ²F_5/2 [13]. Nd³⁺ and Yb³⁺ materials have minor quantum defects, long lifetimes, and wide bandwidths. Nevertheless, this offer establishes the potential use of Yb³⁺-doped materials in innovative ‘ultrafast’ laser generation. However, doping with Yb³⁺ ions presents several problems. Among them a strong influence of the host matrix on spectral and laser properties [14].

In high-power glass-based lasers, phosphate glass is mainly used as the matrix for Yb³⁺ ions due to its high RE ion solubility, gain factor and excellent spectral properties. However, the main drawbacks of phosphate glasses are their chemical robustness and thermos-mechanical limitations. The literature has reported that the mechanical properties of phosphate glasses are effectively improved by doping with SiO₂. Furthermore, the Stark cleavage of Yb³⁺-doped phosphate glasses was enhanced by including SiO₂ material, allowing the glasses to reach the laser energy effectively. On the other hand, the glass composition and local coordination of RE ions are effectively controlled by SiO₂ doping in phosphate-based glasses, which adversely affects the spectral properties of glasses [15]. Created by Jinhang Wang et al. [16] fabrication process of Er³⁺/Yb³⁺ phosphor–silicate glass by 3D printing with digital photo processing (DLP) based on sol–gel technology.

Novel glass hosts are indispensable for enhancing Nd laser emission cross-section and bandwidth. In addition, Nd glass fibers based on fluoride are attributed to low signal transmission loss in the NIR region. Mainly, various glasses doped with Nd³⁺ ions exhibit three NIR emission bands at around 0.94, 1.06 and 1.34 μm corresponding to the transmissions from higher energy state ⁴F_3/2 to the lower energy states ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2, respectively. However, 1.06 μm band fluorescence is used for high-power laser applications whereas to produce a high-power source by frequency doubling of blue light, in addition, 0.92 µm radiation for pumping ytterbium-doped fiber lasers and amplifiers at 0.98 µm and using Raman fiber lasers as frequency converters [17,18,19].

In the current work, Yb₂O₃- and Nd₂O₃-doped phospho–silicate (PNSB, PNSBY, PNSBYNd_0.5, PNSBYYb_0.5, and PNSBYNd_0.5Yb_0.5) oxyfluoride glasses were synthesized and used SEM, EDS, XPS, and photoluminescence techniques to characterize the morphology, binding energy, valence state, and emission properties.

2 Experimental procedure

Nd/Yb codoped phospho-silicate oxyfluoride glasses made by melt-quench technique. The composition of the glass and the labels are given below. All the chemicals are procured from Alpha Asar with an average assay of 99–99.99%. A total composition mass of 15 g was placed in an agate mortar and then ground for 2 h to obtain a homogeneous powder. Furthermore, this homogeneous powder was placed in an aluminum crucible and then melted at 1200–1300 °C for one hour using an electric furnace. After, the melt was abruptly removed from the furnace and quenched in the air on a brass plate at room temperature (RT). Additionally, these samples were annealed at 350 °C for 10 h and cooled to room temperature to release the stresses of the glasses. These samples were polished for optical quality for photoluminescence and powders for thermal and structural characterizations.

• 1. 60 P₂O₅ + 25 NaF₂ + 10 SiO₂ + 5 Bi₂O₃ (PNSB),

• 2. 55 P₂O₅ + 25 NaF₂ + 10 SiO₂ + 5 Bi₂O₃ + 5 YF₃ (PNSBY),

• 3. 54.5 P₂O₅ + 25 NaF₂ + 10 SiO₂ + 5 Bi₂O₃ + 5 YF₃ + 0.5 Yb₂O₃ (PNSBYYb_0.5),

• 4. 54.5 P₂O₅ + 25 NaF₂ + 10 SiO₂ + 5 Bi₂O₃ + 5 YF₃ + 0.5 Nd₂O₃ (PNSBYNd_0.5),

• 5. 54 P₂O₅ + 25 NaF₂ + 10 SiO₂ + 5 Bi₂O₃ + 5 YF₃ + 0.5Nd₂O₃/0.5 Yb₂O₃ (PNSBYNd_0.5Yb_0.5).

3 Characterization techniques

A scanning electron microscope (JEOL JSM-IT500) with an embedded EDS device was utilized to explore the elemental analysis. The elemental compositions as well as electronic states of the elements of the prepared glasses, including their binding energies were examined by XPS (Model: PHI 5000 Versa Probe II, FEI Inc.) with AES Module (Ar ion as well as C60 sputter guns). Brolight portable optical spectrometer (300–1100 nm) was used to record the photoluminescence of glasses upon 808 and 915 nm laser excitation.

4 Results and discussion

4.1 XRD

X-ray diffraction (XRD) profile of the PNSB glass is displayed in Fig. 1. The profile possesses few humps only; no abrupt peaks that indicates crystallinity were found. This indicates that the glass is highly disordered structure and confirms its non-crystallinity.

Fig. 1

X-ray diffraction pattern of PNSB glass

4.2 SEM and EDS analyses

The amorphous nature of PNSB glass is shown in Fig. 2a at 20 μm scale. Rigid structure refers to the amorphous nature of the manufactured glass. All elements in the composition of the PNSB glass are shown in the EDS spectrum of Fig. 2b. A strong peak is observed in the EDS spectrum due to the high content of vitrified phosphate. A small amount of aluminum (Al) is seen in the spectrum because the crucible is made of aluminum oxide. Carbon and nitrogen, on the other hand, are produced during sample preparation. The percentage of elemental presence was recorded during the measurement of the EDS spectra. Carbon (C), oxygen (O) and phosphate (P) have high weight percentages (%) presented in the table shown in Fig. 2c.

Fig. 2

Morphological structure of Bi³⁺ ions-doped phospho–silicate (PNSB) glass. a SEM micrograph, b elemental mapping (EDS) spectra and c table of elements with their percentage

4.3 Raman spectrum

Raman spectrum of PNSB glass was measured under 514 nm laser excitation in the frequency range of 100–1500 cm⁻¹, as shown in Fig. 3. Raman spectrum of PNSB glass perceived seven bands positioned at 329, 513, 721, 997, 1179, and 1456 cm⁻¹. These bands are well resolved and the Raman band at 339 cm⁻¹ is associated with Bi–O bond of BiO₆ octahedra for PNSB glass [20,21,22,23,24]. A small band at 512 cm⁻¹ is ascribed to Si–O vibrations [25]. The band at 722 cm⁻¹ is due to υ_s(P–O–P) stretching vibration from bridging oxygen (BO) due to Q¹ units for PNSB glass and υ_s(Si–O–Si) stretching vibration from silicon. A band with asymmetrical υ_as(P–O–P) stretching of non-bridging oxygen (NBO) was observed at 997 cm⁻¹ in PNSB glass. Raman bands positioned at higher-frequency region (> 1000 cm⁻¹) are because of localized Si–O stretching vibrations of tetrahedral silicates [26]. A high-intense and broad band at 1184 cm⁻¹ is attributed to υ_as(Si–O–Si) stretching vibrations of Q³ units owing to high-alkali (Na⁺) silicates [27] and O–P–O (PO)²⁻ symmetric stretching vibrations in Q² units. A weak band was found at 1456 cm⁻¹ due to stretching of NBO in Q³ tetrahedra v(P=O) (Q³) [28].

Fig. 3

Raman spectrum of PNSB glass

4.4 ESR spectrum

Electron spin resonance (ESR) spectrum of PNSBY glass was studied for magnetic behavior and free electron availability in the Q-band region at room temperature under applied 0–0.6 T magnetic field. Adding of 5 mol% YF₃ increases the availability of free electrons, which leads to the magnetic moment. [Kr]4d¹ 5s² is the electronic configuration of yttrium (Y) with an angular momentum of 2, a magnetic momentum of − 2, and a spin of + 1/2 exhibits magnetic behavior. Since the crystal field splitting for Y³⁺ is typically smaller than that developed by the Zeeman interaction in X-band ESR experiments. Moreover, the ESR spectrum is depicted as a spin Hamiltonian including the interaction between Zeeman and the crystal field given by Eq. (1).

$$H = H_{{{\text{ZEEMAN}}}} + H_{{{\text{exch}}}} + H_{{{\text{SS}}}} ,$$

$$H = g_{0} \mu_{{\text{B}}} BS + JS_{1} S_{2} + D[S_{z}^{2} - ({1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-0pt} 3})S(S + 1)] + E\left(S_{x}^{2} - S_{y}^{2}\right ),$$

(1)

where B is the magnetic field, µ_B is the Bohr magneton, and S_x, S_y, S_z are electron spin operators, H_exch is a Hamiltonian of electron exchange interaction with an isotropic electron exchange coupling constant, J, H_SS is a Hamiltonian of dipolar electron spin interaction with D and E being the zero-field splitting constants; the value of g-factor of the Y³⁺ is isotropic and equals to g₀ as in a case of free ion [29, 30]. The d orbitals are uncompressed for iron and manganese compared to the f orbitals of RE³⁺ ions. This leads to a more covalent character of metal–ligand interactions in d-element complexes and also influences the correlation of strength, distribution width and zero-field splitting parameters.

The electron Lande splitting factor (g) or g-factor is the ratio of the electron’s magnetic moment to the electron’s total spin angular momentum. The magnitude of g is contingent on the orientation of the unpaired electron-containing molecules/ions concerning the applied magnetic field. The g values of unpaired electrons in gas atoms/molecules/ions are given as Russell–Saunders bonds (L–S coupling) and is applied in Eq. (2).

$$g = 1 + \frac{J(J + 1) + S(S + 1) - L(L + 1)}{{J(J + 1)}}.$$

(2)

For a free electron, S = 1/2, L = 0 and J = 1/2, the value of g come out to be 2.0 using Eq. (2). The actual value of free electron is 2.0023. For most free radicals, the orbital involvement to the magnetic moment is insignificant and the g-value is around equal to the free electron value of 2.0023. The slight deviation (± 0.05) frequently detected for most free radicals is described as a mixture of low excited and ground states. The properties of transition metals (TM) are evaluated mainly by the relative sizes of the crystalline field and spin–orbit coupling. These two interactions have opposite effects on orbital degeneracy [31].

Fast electron exchange interaction was identified due to its signal behavior. A deviation of the ESR line shape from the Lorentzian due to the scattering effect because of the non-diagonal component of the magnetic susceptibility at a large linewidth of 45 mT. Resonant magnetic field (H_r) occurred at 335.16 mT for PNSBY glasses displayed in Fig. 4a. The g value of PNSBY glasses are observed at 2.0168, 2.1595, 2.9748, and 3.7661. These g values for PNSBY glasses revealed that the availability of free electrons responsible for the paramagnetic behavior of Y³⁺ ions is due to its value being more significant than that of 2.0023 exposed in Fig. 4b.

Fig. 4

ESR spectra of a magnetic field vs intensity and b g-factor vs intensity of 5 mol% of Bi₂O₃ and Y₂O₃

4.5 XPS analysis

Binding energies and valency states of the elements of the PNSB glass were analyzed using X-ray photoelectron spectroscopy (XPS), which revealed binding energy (BE) in the range of 0–1200 eV. Silicon in the Si2p state, phosphorous in the P2p state, bismuth in the Bi4f state, oxygen in the O1s state and sodium in the Na1s corresponding BEs were revealed at ~ 101 eV, 132 eV, 158 eV, 530 eV, and 1069 eV, respectively for PNSB glass as displayed in Fig. 5a. The PNSB glass exhibits the P2p_3/2 peak position having BE with phosphate PO₄³⁻ units at 132 eV [32]. Spectral peaks clearly indicate that the glass specifying the prime role of PO₄³⁻ units in the glass is revealed in Fig. 5b with a broad peak. Subsequently, this causes for the existence of P2p_3/2 to be in a 5 + valance state. Owing to PO₄³⁻ units orthophosphate-type structure in these glasses [33]. The full-width at half-maximum (FWHM) of P2p_3/2 peak is 1.85 for PNSB glass. When P2p of PNSB glass is deconvoluted, it exhibits two peaks positioned at 131.8 and 132.5 eV for 2p_3/2, and 2p_1/2, respectively, in which P2p_3/2 peak was dominated.

Fig. 5

XPS spectra of a PNSB glass and binding energies of b P2p, c Si2p and d Bi4f

Si2p spectra of PNSB glass was shown in Fig. 5c. The BE of the Si2p_3/2 peak at around 101 eV is due to Q³ class of bonding units; whereas, the Si2p_1/2 peak at approximately 101.6 eV is owing to Q⁴ class of bonding units. The FWHM of the Si2p peak is 2.05 eV for PNSB glass. The BE peaks of Bi for 4f_7/2 and 4f_5/2 are centered at 157.75 eV and 163.11 eV, respectively, for PNSB exposed in Fig. 5d. In addition, FWHM of Bi 4f_7/2 and Bi 4f_5/2 peaks of PNSB glass are 1.20 and 1.31 eV, respectively. Additionally, when Bi4f peaks were deconvoluted for PNSB glass, the ionic states of Bi ions were found to be Bi²⁺ at 157.5 eV, Bi³⁺ at 158 eV and Bi⁵⁺ at 158.6 eV for Bi4f_7/2 and Bi²⁺ at 162 eV, Bi³⁺ at 163 eV and Bi⁵⁺ at 163.6 eV. However, 3 + ionic states of Bi were dominated and revealed in Bi 4f_7/2 and Bi 4f_5/2 peaks of PNSB glass.

4.6 Optical absorption

The optical absorption bands 349, 445, 523, 583, 629, 683, 629, 683, 746, 799, 874, and 973 nm were revealed in neodymium oxide (Nd₂O₃) and ytterbium oxide (Yb₂O₃) Nd/Yb-doped PNSBYNd_0.5Yb_0.5 in the UV–visible–NIR absorption spectrum measured in the range 270–1100 nm corresponding to the transmissions ⁴I_9/2 → ²I_11/2, ²K_15/2 + ⁴G_11/2 + ²D_3/2, ⁴G_9/2 + ⁴G_7/2, ²G_7/2 + ⁴G_5/2, ²H_9/2, ⁴F_9/2, ⁴S_3/2 + ⁴F_7/2, ⁴F_5/2 + ²H_9/2, and ⁴F_3/2 for Nd³⁺ ions and ²F_7/2 → ²F_5/2 for Yb³⁺ ions displayed in Fig. 6. A broad absorption band was found at 445 nm overlapping of three bands due to ²K_15/2 + ⁴G_11/2 + ²D_3/2, three intense peaks were observed at 583 (²G_7/2 + ⁴G_5/2), 746 (⁴S_3/2 + ⁴F_7/2), and 799 (⁴F_5/2 + ²H_9/2) nm. The highest intensity of 583 nm is due to the hypersensitive behavior of Nd³⁺ ions that follows the selection rules of |ΔL|≤ 2, |ΔS|≤ 0 and |ΔJ|≤ 2 [17].

Fig. 6

Optical absosption spectrum of PNSBYNd_0.5Yb_0.5 glass in the range UV–visible–NIR

4.7 Photoluminescence

The luminescence properties of 0.5 mol% (Nd₂O₃ and Yb₂O₃) embedded in PNSBY glasses were studied. As illustrated in Fig. 7, when Yb³⁺ ions were pumped with a wavelength of approx. 915 nm in the PNSBYYb_0.5 glass, a broad and intense emission band was observed at 975 nm due to the transitions ²F_5/2 → ²F_7/2. The FWHM of the broad emission band at 975 nm for PNSBYYb_0.5 glass is about 10.55 nm. The luminescence conversion process obviously depends on the energy transfer (ET) of the RE ions concerning their structure and the local environment. When the RE ion concentration was changed, the fluorescence characteristics caused by ET changed significantly. However, there are few reports on the consequences of structural deformation on the emission mechanism of Y₂O₃:Bi conversion with changes in Yb³⁺ ion concentration [34].

Fig. 7

Emission spectrum of PNSBYYb_0.5 glass under 915 nm laser excitation

In addition to the broad emission in the NIR region at the active Bi³⁺ binding site, a band at 950–1100 nm was also observed in the PNSBYYb₀₅ glass. A simple schematic diagram of the energy level of the Yb³⁺ ion and the occurrence of Yb³⁺ emission, as reported in previous reports, shows an ET process at the NIR Bi³⁺ emission center for the Yb³⁺ ion. In particular, when increasing the amount of Yb₂O₃, the emission intensity of Yb³⁺ ions is similar to that of Bi³⁺ ions, which means that the efficiency of ET is very high [35].

The ionic radii of Y³⁺ ions are around 90 pm, which is very convenient for replacing lanthanides, while the radii of Bi³⁺ and Bi⁵⁺ ions are 103 and 76 pm, respectively. Adding 5 mol% YF₃ increased the emission intensity of Yb³⁺ compared to increasing the Bi₂O₃ content by 5 mol% in the PNSB glass. This demonstrates that the positions of the Y³⁺ atoms agree well with the Bi³⁺/Yb³⁺ ions. The high transparency of Y³⁺ ions reduces the absorption of PNSBYYb₀₅ glasses, ameliorating the thermal stability, optical properties and structural rigidity of glasses [36,37,38,39]. This is may be because of the ET between Y³⁺, Bi³⁺, and Yb³⁺ ions. NaF, YF₃, and NaYF₄ nanocrystals are expected to form in PNSBYYb₀₅ glass prepared in this way [40].

0.5 mol% of Nd₂O₃-doped PNSBYNd₀₅ glasses were recorded for PL emission in the range of 750–1100 nm excited by the wavelength of 808 nm diode laser. Two emission bands were observed within the region at 876 nm for the ⁴F_3/2 → ⁴I_9/2 transition and 1053 nm for the ⁴F_3/2 → ⁴I_11/2 transition displayed in Fig. 8. Highest emission was perceived for ⁴F_3/2 → ⁴I_9/2 transition at 876 nm which is near to 0.9 µm that can be very useful for fiber laser in under water communication. Nevertheless, it’s not easy to get emission for ⁴F_3/2 → ⁴I_9/2 (0.9 µm) transition instead of the usual ⁴F_3/2 → ⁴I_11/2 (1.06 µm) transition. This is achieved if the laser functions in a quasi-three-level system and the gain medium in the four-level system for the ⁴F_3/2 → ⁴I_11/2 transition will be at all times larger than that of the ⁴F_3/2 → ⁴I_9/2 transition [41,42,43,44,45]. In a three-level system, the lower laser level matches with the ground state, in which the unexcited medium absorbs emitted photons. On the other hand, the upper pump level overlaps with the upper laser level in which the absorption at pumping photons saturates [46].

Fig. 8

Emission spectrum of PNSBYNd_0.5 glass under 808 nm laser excitation

Lasers with lower NIR wavelengths are used for the second-harmonic conversion procedure in the blue spectrum laser emission. This results in the wavelength 800–950 nm covert into the blue spectrum of 400–445 nm wavelength. Related to diode lasers, this laser shows smaller spectral width and better coherence length, which attracts in the field of interferometer experiments and the sources for the differential absorption lidar (DIAL) in water vapor detection, which has wavelengths overlap the absorption peaks of water vapor [47]. Compared to 1053 nm emission, 876 nm is dominated in the 0.5 mol% Nd₂O₃-doped PNSBYNd₀₅ glasses. The Stark effect is the reason for splitting the ⁴F_3/2 → ⁴I_9/2 band in the Nd³⁺ ions.

0.5 mol% of each PNSBYNd_0.5Yb_0.5 phospho-silicate oxyfluoride glass doped with Nd₂O₃ and Yb₂O₃ excited in the range 750–1100 nm by 808 nm diode laser is shown in Fig. 9. When Nd³⁺ ions were pumped to a wavelength of 808 nm, they were excited to the ⁴I_9/2 → ⁴F_3/2 levels, so the ions were pumped to their closest energy state ⁴F_9/2. Furthermore, Nd³⁺ ions were pumped from ⁴F_9/2 to ⁴I_11/2 by an excited state adsorption (ESA) process illustrated in partial energy level diagram in Fig. 10. Due to the short lifetime of Nd³⁺ ions at the ²I_11/2 level, they were de-excited in the ⁴G_9/2 level via a non-radiative (NR) transition and then de-excited at the ⁴F_3/2 level, as shown in Fig. 9. Additionally, the ⁴F_3/2 transition with a lifetime longer than the ⁴I_11/2 level for Nd³⁺ ions, leading to spontaneous and induced emissions. ET was predicted between the ⁴F_3/2 level and the ²F_5/2 level from Nd³⁺ ions to Yb³⁺ ions due to cross relaxation (CR) and led to emission at the wavelength of 874 nm, which is used for excitation of Yb³⁺ ions.

Fig. 9

Emission spectrum of PNSBYNd_0.5Yb_0.5 glass under 915 nm laser excitation

Fig. 10

Partial energy level diagram of Nd and Yb ions in PNSBYNd_0.5Yb_0.5 under 808 nm laser excitation

Three emission bands are detected for PNSBYNd_0.5Yb_0.5 glasses at 874, 975 and 1052 nm corresponding to transitions ⁴F_3/2 → ⁴I_9/2, ²F_5/2 → ²F_7/2 and ⁴F_3/2 → ⁴I_11/2. Among them, the strong bands at 874 and 1052 nm are due to Nd³⁺ ions, and the highest band at 975 nm is due to Yb³⁺ ions. However, similar emission trends of PNSBYNd₀₅ glasses were reported for PNSBYNd_0.5Yb_0.5 glasses. A slight band shift and decreased band intensity at 1053 nm were observed toward the lower wavelength range (1052 nm). This suggests that ET takes place from the Nd³⁺ ions ⁴F_3/2 → ⁴I_11/2 transitions to Yb³⁺ ions ²F_5/2 → ²F_7/2; apart from this, ET also observed between the transitions from Nd³⁺ ions ⁴F_3/2 → ⁴I_9/2 transitions to Yb³⁺ ions ²F_5/2 → ²F_7/2 transitions displayed in Fig. 10. As a result, the intensity of the Yb band at 975 nm increases and shows greater intensity than the others.

5 Conclusion

Phospho-silicate oxyfluoride glasses (PNSBY) doped with Nd/Yb ions were explored for elemental analysis, phosphate and bismuth ion binding energies, and photoluminescence properties. The solid, stone-like morphology confirmed the amorphous behavior of the PNSB glass. EDS spectrum was used to monitor the elemental compositions and their concentrations. A P2p_3/2 valence state is observed in the phosphorus XPS spectrum. In the XPS spectrum, the content of Bi, F and more oxygen were the reason for changes in the binding energy values from the actual values of the PNSB glass. Two spectral peaks of Bi ⁴F_5/2 and ⁴F_7/2 were visible in this glass. In a 0.5 mol% Yb₂O₃-doped PNSBYYb_0.5 glass, a broad emission band at 975 nm was observed when Yb³⁺ ions were excited by the wavelength of 915 nm diode laser. A 0.5 mol% of Nd₂O₃-doped PNSBYNd_0.5 glass emits at 876 nm owing to ⁴F_3/2 → ⁴I_9/2 band was dominated compared to 1053 nm band due to quasi-three-state system of Nd³⁺ ions in PNSBYNd_0.5 glasses. Nd/Yb-codoped PNSBYNd_0.5Yb_0.5 glasses were investigated for energy transfer studies, those have shown ET from ⁴F_3/2 to ⁴F_7/2, and the intensity of Yb band at 975 nm was enhanced due to CR from 1053 and 876 nm. As a result, the emission at 975 nm is beneficial for developing 1.0 µm laser and amplifiers for under water communication applications.