Thermal, Optical, and Emission Traits of SM³⁺-Ion-Doped Fluoride/Chloride/Oxide Glass for Red/Orange Laser Fiber Applications

Abstract

This study examined spectroscopic, thermal, and other qualities, such as the lasing parameters, of Sm³⁺-doped glass with the composition 40P₂O₅–30ZnO–20LiCl–10BaF₂. The ellipsometric data were used in a Sellmeier dispersion relation to estimate the refractive index values of the glasses investigated. The measured absorption spectra of the doped glass reveal the presence of various absorption bands assigned to transitions from the ⁶H_5/2 ground state attributed to Sm³⁺-ion-excited states. We studied the decay of the ⁴G_5/2 level of the Sm³⁺ ions in the doped glass by analyzing its absorption and emission fluorescence spectra. The Judd–Ofelt hypothesis allowed us to determine that the quantum efficiency of the ⁴G_5/2–⁶H_7/2 transition is high: 96% and 97% for glass doped with 4.05 $\times$ 10¹⁹ ions/cm⁻³ and 11 $\times$ 10¹⁹ ions/cm⁻³, respectively. Furthermore, this glass exhibits efficient red/orange enhanced spontaneous emission that matches the excitation band of the photosensitizer material used in medical applications.

1. Introduction

Phosphate glasses possess high thermal stability, relatively low melting temperatures, and unique optical properties, along with a broad range of transparency in the UV-VIS spectral range; however, they have a low refractive index [1]. Therefore, phosphate glasses doped with rare-earth (RE) ions are widely studied for their applications in waveguides, optical detectors, fiber optic amplifiers, and lasers [2]. The addition of various fluorides to RE-ion-doped phosphate glasses enables the creation of host matrices that combine the significant optical properties of fluoride glasses and the high stability of phosphate glasses. Oxides increase the mechanical resistance and thermal stability of the glasses, while low-frequency fluorides reduce the nonradiative decay losses caused by multifamily relaxation [3], improve the quantum efficiency and lifetimes of rare-earth ions’ luminescence, and act as flux agents that reduce the melting temperature of the glasses. When samarium ions are doped into fluorophosphate glass, the resulting glass exhibits distinctive optical behavior due to its emissions in the UV–visible region through various emission channels from the ⁴G_5/2 level of Sm³⁺ ions [4]. Samarium ions (Sm³⁺) are regarded as excellent luminescent centers among the rare-earth (RE³⁺) ions due to their strong emissions at longer wavelengths in the red/orange region. The strong luminescence in the red/orange region, as well as the high quantum efficiency value of Sm³⁺-doped P₂O₅–Na₂O–BaO–Al₂O₃ glasses, makes them a potential candidate for laser emission [5].

This characteristic makes them highly suitable for applications in solid-state lasers, LEDs, and display devices. Additionally, the lowest emitting level of Sm³⁺ ions, ⁴G_5/2, exhibits high quantum efficiency and various quenching emission channels [6,7]. The ⁴f₅ configuration of samarium ions with strong orange-red fluorescence in the visible region is used in underwater communications, color displays, visible semiconductor lasers [8,9,10], and phosphors for white light-emitting diodes (LEDs) [11,12]. Although extensive research has been conducted on phosphate glasses doped with Sm³⁺ ions [13,14,15,16], there is still a need to explore new and advanced compositions of phosphate glass due to their potential applications.

In [17], the researchers confirmed the formation of defects and the conversion of Sm³⁺ into Sm²⁺ ions in Sm-doped fluorophosphate glasses exposed to X-ray irradiation. Additionally, Hamdy et al. [18] synthesized compositions containing 20, 30, and 50 mol% NaF, AlF₃, and PF₅ doped with Sm³⁺ to investigate the impact of Sm³⁺ on optical transmission and photoluminescence (PL) spectroscopy. The authors of [19] discussed the influence of the chemical composition of [(45 − x) P₂O₅–10AlF₃–45 NaF x Sm₂O₃](where x equal 0 to 1 mol%) glasses on their optical properties, mainly their linear and non-linear parameters. The study performed by [20] focused on examining the thermal, structural, and luminescent characteristics of sodium barium metaphosphate glasses doped with Sm³⁺. Samarium-doped fluoroaluminate and fluorophosphate glasses studied by Chicilo et al. [21] proved to be excellent candidates for high-resolution, large-dynamic-range microbeam radiation therapy (MRT) dosimetry. It is worth noting that Sm³⁺ ions are ideal for doping because their excited energy level ⁴G_5/2 has a high quantum efficiency with various quenching emission channels. It emits strong reddish-orange light and possesses sufficient energy to initiate photodynamic reactions. Recently, lasers with excellent directivity and high intensity, as well as LEDs with relatively narrow spectral bandwidths and high fluence rates, have been specifically developed for photodynamic therapy (PDT) treatments. However, scattered or misdirected light from the target area can unintentionally expose large areas of normal tissue to high power densities, potentially leading to side effects such as inflammation, pain, swelling, burns, and scarring. While LEDs can be integrated with optical fibers, their low coupling efficiency limits their use. Additionally, their narrow excitation spectrum restricts the use of multiple photosensitizers, reducing the efficiency of therapy. Amplified spontaneous emission (ASE) fluorescence produced in rare-earth (RE) ion-doped glass channel waveguides offers a broadened bandwidth that can be tuned by adjusting the RE ion concentration and pumping power. This type of light source delivers sufficient intensity, excellent directivity, and high coupling efficiency, making it a promising option for use in photodynamic therapy (PDT) treatments [22].

A fluorophosphate glass channel waveguide produces amplified spontaneous emission (ASE) fluorescence with a broad width of 600–730 nm. This light source has excellent efficiency, strong intensity, and good directivity, thus making it an attractive option for use in photodynamic therapy (PDT) [22,23]. Red light in the 600–730 nm spectral band has 50–200% greater penetrating power than light in the 400–500 nm region, and it possesses more energy to initiate a photodynamic reaction that produces ¹O₂ according to most tissue models [22]. A diverse range of light sources, both coherent and incoherent, have demonstrated their effectiveness in achieving anti-tumor effects in PDT for various superficial and interstitial treatment sites [24,25,26]. Thus, in this paper, we have focused on the lasing parameters of Sm³⁺-doped fluorophosphate glasses as a candidate light source for photodynamic therapy (PDT). In the glass compositions, we included BaF₂ ions as network modifier ions with a low field strength and ZnO to improve the mechanical strength, chemical durability, and hygroscopic nature, which, in turn alter, the optical, electrical, and magnetic properties of phosphate glasses.

2. The Experimental Section

The composition of the glasses was chosen as 40P₂O₅–30ZnO–20LiCl–10BaF₂ with different concentrations of Sm³⁺ ions: sample 1 (SM1) at 4.05$\times$10 ¹⁹ cm⁻³ and sample 2 (SM2) at 11$\times$10¹⁹ cm⁻³. The following chemicals were used for batch production: phosphorus oxide (P₂O₅), zinc oxide (ZnO), lithium chloride (LiCl), barium fluoride (BaF₂), and samarium oxide (Sm₂O₃). All of the raw materials were carefully mixed. The fluorophosphate glasses were obtained by melting 50 g batches in a gold crucible in an electric furnace at a temperature of 850°C in an air atmosphere [27]. The densities of the glasses studied were determined using Archimedes’ method (

Table 1. Volume concentration (N), glass matrix composition (mol%), density, and refractive index at 633 nm.

Sample	Composition (mol%)	N (1019 cm−3)	Density (g/cm3)	n
SM	40P2O5–30ZnO–20LiCl–10BaF2	----	3.872	1.588
SM1	40P2O5–30ZnO–20LiCl–10BaF2–XSm2O3	4.05	3.792	1.585
SM2	40P2O5–30ZnO–20LiCl–10BaF2–YSm2O3	11.00	3.821	1.579

).

Their refractive indexes were determined with use of a Woollam M-2000 ellipsometer; the spectra were recorded in the 190–1700 nm spectral range. The transmittance and reflectance spectra were recorded using Jasco V-570 spectrophotometers [27]. Luminescence spectra were measured using the Optron Dong Woo fluorometer system [28]. The luminescence decay curves were measured following short-pulse excitation provided by an optical parametric oscillator with the third harmonic of a Nd YAG laser [29,30]. Changes in the thermal behavior of the 40P₂O₅–30ZnO–20LiCl–10BaF₂ glasses were investigated with the DTA/DSC method using the PerkinElmer DTA-7 System.

Under a flowing air environment (80 mL min⁻¹), the glass samples were ground into powders with grain sizes of 0.1 to 0.3 mm and then heated in platinum crucibles at a rate of 10 °C min⁻¹.

Alumina oxide (Al₂O₃) was used as a reference material. Several thermal parameters characteristic of the glassy state have been established, i.e., the glass transformation temperature (T_g) and the glass crystallization temperature (T_c). Using the midpoint of the corresponding transformation phase as the glass conversion temperature (T_g) and the start and maximum of the glass crystallization peak as the parameters for the glass melting and solidification processes, we were able to obtain the glass heating and solidification temperatures.

While the midpoint of the corresponding transformation step was used to estimate the glass conversion temperature (T_g), the start (T_x) and maximum (T_c) of the glass crystallization peak were used to calculate the glass crystallization temperature.

The characteristic glass temperature values that appeared in DSC were determined using Proteus Thermal Analysis (Version 5.0.0.). The amorphous nature of the glasses studied was confirmed using the X-ray diffraction (XRD) method and a Philips X’Pert X-ray diffractometer with CuK_α radiation. All of the research was conducted at room temperature. The refractive index of the bulk glasses was investigated using transmission mode ellipsometric data. The ellipsometric results were registered in the range of 400–1700 nm, and n was modeled using the Sellmeier dispersion function. Therefore, the Sellmeier refractive index (n)’s normal dispersion, which was determined from our ellipsometric data collected as a function of the wavelength λ (see Equation (1)) [31,32], was fitted to the low-absorption region, which is 400–1700 nm.

n² = A + B_λ²/(λ² − C²) − D_λ²

(1)

where A, B, C, and D are the fitting parameters.

The refractive index dispersions of the glasses studied are quite similar to each other, as are the values of n measured at 633 nm (see Table 1). The addition of Sm₂O₃ to the glasses led to a decrease in the refractive index compared to the sample SM without a dopant.

It is worth mentioning that ellipsometry is not a direct method for determining the refractive index.

3. Results and Discussion

3.1. Thermal Analysis

The broad hump in the XRD profile shows the long-term structural instability (i.e., random atomic arrangements) of the glasses prepared (Figure 1). Therefore, based on XRD analysis of all the glasses, it can be found that they are amorphous. The differential scanning calorimetry (DSC) curves of all of the glasses studied showed distinct endothermic events associated with the glass transition T_g, as well as the melting point T_m. The exothermic event T_c associated with crystallization was observed. During heating, the glasses studied demonstrate several characteristic temperatures. The range of vitreous state transitions (T_g,endset − T_g,onset) decreases progressively when the Sm³⁺ ion concentration in the glass increases, although the transition temperature increases (Figure 2).

The increase in T_g refers to structural changes in the glass network with increasing Sm³⁺; furthermore, the narrowing of the transition range of the vitreous state (i.e., a reduction in the relaxation time) supports the theory of increased rigidity in the material’s internal structure (

Table 2. Thermal characteristics of Sm³⁺-doped fluorophosphate glasses.

Glass ID	Tg	Tg range	Tx	Tc	Tm
SM	357	23	426	523	720
SM1	369	14	440	526	725
SM2	372	5	445	540	733

Abbreviations: T_g—glass transition temperature, T_x—endothermic onset of the maximum crystallization peak in glass; T_c—maximum crystallization peak in glass, T_m—endo thermic peak (glass melting temperature).

). According to [33], this process occurs via the breaking of chemical bonds rather than the displacement of structural units when the time required for structural strain relaxation shortens, maintaining the network’s continuity. To determine the thermal behavior of glasses, many parameters have been established, mainly considering the dependencies between values of characteristic temperatures. To evaluate the stability of the glass, the following parameters were utilized: the Hruby (K_H) = (T_x − T_g)/(T_m − T_x) [34], the Weinberg (K_W) = (T_x − T_g)/T_m), the Lu and Liu (K_LL) = T_x/(T_g + T_m)), and the Angell K_A = (T_x − T_g) parameters [35]. The resistance of the resulting material to crystallization is expressed by the term (T_x − T_g) in the suggested formula; a narrower gap indicates an increased tendency to crystallize. As the concentration of Sm³⁺ ions in the glasses increased, their K_A was in the 69–73 °C temperature range. Greater Sm³⁺ doping results in a higher KA value, increasing the glass’s thermal stability.

The stronger thermal stability of glass against devitrification on heating was further verified by the fact that the values of other thermal stability metrics, such as K_w, K_H, and K_LL, increased with larger Sm³⁺ concentrations. Not only can one infer a material’s stability from all these variables but one can also infer its glass-forming ability, which is the ease with which a liquid vitrifies upon cooling.

Table 3. Stability parameters of the Sm³⁺-doped fluorophosphate glasses.

Glass ID	Hruby/KH	Weinberg/KW	Lu and Liu/KLL	KA
SM	0.234	0.095	0.395	69
SM1	0.249	0.097	0.402	71
SM2	0.253	0.099	0.403	73

Abbreviations: K_A—Angell parameter, K_H—Hruby parameter, K_W—Weinberg parameter, K_LL—Lu and Liu parameter.

shows that the stability and glass-forming ability of 40P₂O₅–30ZnO–20LiCl–10BaF₂ glasses are enhanced when the Sm³⁺ ion concentration in their structure increases. Both Mawlud et al. and Mnjeet et al. [36,37] found similar correlations in their studies.

3.2. Absorption and Excitation Spectra

The transmittance and reflectance spectra allowed us to calculate the dispersion of the absorption coefficient of the fluorophosphate glasses doped with the Sm³⁺ ions. The results are illustrated in Figure 3 and Figure 4.

As shown in Figure 3 and Figure 4, the absorption spectra of the Sm³⁺-doped fluorophosphate glasses registered in the UV-VIS-NIR region showed 19 peaks. As can be seen in Figure 3 and Figure 4, these peaks are caused by Sm³⁺ ions’ transition from the ⁶H_5/2 ground state to various energy levels [38,39].

The absorption spectra allowed us to observe a large number of energy transitions of the samarium ions in the glass from the system P₂O₅–ZnO–LiCl–BaF₂. Above 450–500 nm, in the SM1 and SM2 glasses, one may find the high energy manifolds built from the ⁴D, ⁴G, ⁴I, ⁴L, and ⁴M quartets and ⁶P sextet terms. Because many ^2S+1L_J splitters overlap, the UV-VIS bands that are visible are less intense and difficult to identify. Shifts in the absorption edge towards the UV region are observed with an increasing Sm³⁺ ion content. However, the bands that formed in the near-infrared spectrum are more distinct and strongly differentiated. In the spectral range of 500–250 nm, there is a comparable effect; however, in contrast, intense absorption peaks at 402 nm occurred, and this band is associated with transitions ending in ⁴M_19/2, ⁶P_3/2, ⁴L_15/2, ⁶P_7/2, ⁴D_3/2, ⁴D_7/2, ³P_3/2, and ⁴P_5/2 closely spaced multiples.

In addition, the absorption spectra of the glasses tested were supplemented by photoluminescent excitation spectra, which are shown in Figure 5.

Figure 6 shows the absorption cross-section (ACS) of the ⁶H_5/2 → ⁶F_7/2 transition, which is fundamental for samarium-doped fiber amplifiers. The ACS is defined as σ_abs = α/N and is easily obtained from the absorption spectra shown in Figure 3.

The most significant transitions of Sm³⁺ ions occur in the range of 1000–1300 nm, corresponding to the ⁶H_5/2 → ⁶ F_9/2 and ⁶H_5/2 → ⁶F_7/2 transitions of Sm³⁺ ions (Figure 6).

Figure 7 presents the absorption cross-section (σ_abs) calculated for the SM1 and SM2 glasses, and the maximum value at 402 nm was σ_abs = 17.0067 × 10⁻²¹ cm² for SM2 and σ_abs = 14.1206 × 10⁻²¹ cm² for SM1.

3.3. Judd–Ofelt Analysis

Here, we analyzed the absorption data using the conventional Judd–Ofelt (J-–O) theory (Figure 3 and Figure 4). Numerical integration of suitable absorption bands, excluding the background absorption of the glass arrays, was used to establish the experimental oscillator strengths for transitions from the Sm³⁺ ions’ ground level ⁶H_5/2 to subsequent excited levels.

Many intense transitions can be observed in the PLE spectrum (Figure 5), like those in the absorption spectra. In

Table 4. Judd–Ofelt intensity parameters (Ω_t) for fluorophosphate glasses doped with Sm³⁺ ions.

Glass ID	Ω2 (10−20 cm2)	Ω4 (10−20 cm2)	Ω6 (10−20 cm2)	σrms (10−6)
SM1	0.4327	1.1224	1.0071	0.354
SM2	0.9897	1.9765	1.4576	0.327

, the computed J–O parameters (Ω₂, Ω₄, and Ω₆) can be observed. Judd–Ofelt parameters are a useful tool for determining the radiative properties of Sm³⁺-doped glasses, which are strongly dependent on the host matrix. The values of the J–O parameters obtained [40,41] are comparable to values found in other phosphate glasses doped with Sm³⁺ and show the same trend, namely Ω₄ > Ω₆ > Ω₂ [42,43]. The values of Ω₄ and Ω₆ are associated with structural qualities like the viscosity and stiffness of the material, whereas the value of Ω₂ indicates the covalency of oxygen atoms and the asymmetry of the RE ion sites, according to J–O theory. The ion sites exhibit less asymmetry, leading to dominating covalent interactions between oxygen ligands and Sm³⁺ ions, as shown by the high value of Ω₄ compared to Ω₂ and Ω₆. We may find out more about the luminescence activators by calculating the spectroscopic quality factor using the Ω₄/Ω₆ ratio. The glasses examined show promise as luminescence activators, with computed spectroscopic quality factor values of 1.1 (SM1) and 1.5 (SM2), respectively [44,45].

Three phenomenological spectroscopic parameters were calculated. In addition, the value of fitting quality (σ_rms) esti mated from the root mean square (RMS) deviation [46] was determined and is presented in Table 4. This value implies a good match between the measured P_exp and the calculated P_cal oscillator strengths.

The radiative transition probability (W_r) and the total radiative lifetime (τ_rad = 1/W_r) of the emission level ⁴G_5/2 were estimated using the J-O parameters determined. In the conventional J-O approach, the Ω_t parameter values are derived using an averaging process that takes into account all the absorption bands, known as the least-squares method. The samples’ observed lifetimes of the excited state ⁴G_5/2 (Figure 8a,b) were considerably lower than the expected radiation equivalent of τrad (refer to

Table 5. This study compares the experimentally obtained lifetimes of the ⁴G_5/2 emission band in fluorophosphate glasses (τ_exp) with the lifetimes estimated using the J-O methods (τ_rad ^J-O) and quantum efficiencies (η).

Glass ID	4G5/2 Lifetime (ms)
	τexp	τrad J-O	η (%)
SM1	3.134	3.856	96
SM2	3.758	4.865	97

) and those predicted by the following relationship [42]:

$$\mathsf{\eta }={\displaystyle \frac{{\tau }_{exp}}{{\tau }_{rad}}}\times 100\%$$

(2)

This gives more than 96% of the quantum efficiency of the excited state for the material under study.

In the case of samples SM1 and SM2, one deals with high-concentration Sm³⁺ ions, leading to effective cross-relaxation (CR) between them [47]. The CR mechanism causes a decrease in the Sm³⁺:⁴G_5/2 lifetime for SM1 to 3.134 ms when compared to the J-O radiative lifetime of 3.856 ms; for sample SM2, the lifetime decreases to τ_exp = 3.758 ms, compared to a radiative lifetime of 4.865 ms.

3.4. Emission Spectra

The photoluminescence (PL) emission spectra of Sm³⁺-doped fluorophosphate glasses are shown in Figure 9c, with images of the glass emission presented in Figure 9a,b.

Figure 9c displays the photoluminescence spectra produced by the glasses stimulated at 402 nm. The emission bands seen at 561.5 nm, 597.5 nm, 643.5 nm, and 708 nm, respectively, are caused by the ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, ⁴G_5/2 → ⁶H_9/2, and ⁴G_5/2 → ⁶H_11/2 transitions of Sm³⁺. The strength of the green and reddish-orange emission bands is significantly affected by the kind of glass host. ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_9/2 (red/orange) are the two distinct transition peaks of the Sm³⁺ ions in the visible emission spectrum. A decrease in the higher energy levels of ⁴G_5/2 may occur when higher sub-levels of energy are near a Sm³⁺ ion in the ground state because the energy can be absorbed by the Sm³⁺ ion.

The glasses tested, like glasses in other research, show that the exceptional intensity of the ⁴G_5/2 → ⁶H_7/2 transition makes them ideal for various applications, including color displays, high-density optical storage devices, and medical diagnostics. Our data closely align with those on other glasses described in the referenced studies [48,49,50].

Figure 10 shows a simplified diagram of the emission energy of Sm³⁺. From SM1 and SM2, it can be seen that as the Sm³⁺ ions are pumped with a 402 nm excitation wavelength, they are excited to the ⁶P_3/2 level, and then some of the Sm³⁺ ions relax non-radioactively into lower levels of ⁴G_5/2 and ⁴F_3/2 and then decay into ⁶H_9/2.

Figure 11 displays a CIE 1931 chromaticity diagram, which illustrates the effect of the emission transitions at a 402 nm excitation wavelength. For the manufactured glasses SM1 and SM2, the color coordinates (x, y) are (0.551, 0.352) and (0.581, 0.398), respectively. Thus, the glasses produced in this study have the potential to serve as materials for optical gain in orange/red laser applications.

4. Conclusions

In this study, glasses with a base chemical composition of 40P₂O₅–30ZnO–20LiCl–10BaF₂ doped with Sm³⁺ were successfully prepared using a melt quenching technique. The glasses obtained fulfil the criteria to be considered superior materials for channel waveguide applications. They are compositionally and structurally homogeneous to prevent scattering and ensure consistent guiding properties and are free from significant defects and impurities that could introduce additional loss or alter the refractive index profile. From the DCS curve profiles, the glass transition temperature (T_g), the onset of the glass crystallization peak (T_x), the maximum glass crystallization peak (T_c), glass melting temperature (T_m), and thermal stability parameters were analyzed. The range of the vitreous state transition (T_g__endset − T_g__onset) was observed to decrease, while the glass transition temperature shifted towards higher temperatures with an increase in the Sm³⁺ ion concentration. The values of the thermal stability parameters calculated (K_w, K_H, K_LL) increased with a greater Sm³⁺ content, confirming the higher thermal stability of glass against devitrification on heating. Based on thermal studies, we confirmed very good glass stability. The glasses under investigation showed normal refractive index values for fluorophosphate glasses, according to ellipsometric measurements, and the Sellmeier model provided an excellent description of this index’s dependency on wavelength. The Judd–Ofelt parameters calculated for the 40P₂O₅–30ZnO–20LiCl–10BaF₂ glasses are as follows: SM1 (Ω₂ = 0.4327 × 10⁻²⁰ cm², Ω₄ = 1.1224 × 10⁻²⁰ cm², Ω₆ = 1.0071 × 10⁻²⁰ cm²) and SM2 (Ω₂ = 0.9897 × 10⁻²⁰ cm², Ω₄ = 1.9765 × 10⁻²⁰ cm², Ω₆ = 1.4576 × 10⁻²⁰ cm²). The large quantum efficiency of Sm³⁺/SM1/⁴G_5/2 (96%) and SM2 (97%) was reported for all samples. The absorption cross-section data for the ⁴G_5/2 – ⁶H_7/2 transition were determined. The CIE chromaticity coordinate (x,y) values corresponded to the orange-red region at the concentrations studied. The prominent transition peaks of the Sm³⁺ ions in the visible emission spectrum wavelength (red/orange) make our glasses useful as laser fiber sources that could be used in photodynamic therapy (PDT) treatment.