Fluorescein dye-doped boric acid glasses were prepared by the rapid cooling method using an appropriate mixture of H₃BO₃, Pb₃O₄, ZnO, and C₂₀H₁₂O₅ in the composition range 70H₃BO₃-(10-X) Pb₃O₄-20ZnO-(X) C₂₀H₁₂O₅ where X = 0.05, 0.1, 0.5 and 0.3 respectively. Table 2 shows the glass compositions for various prepared samples. Suitable amounts of each chemical were taken and weighed in a weighing machine. After weighing, all the chemicals were thoroughly mixed using a mortar and pestle. The composition of each sample was taken in an open silica crucible and kept in an electric muffle furnace for heat treatment. To decompose the boric acid, initially, the samples were maintained at room temperature and then the temperature was raised to 1050˚C - 1150˚C [5]. Base glass 70H₃BO₃-(10-X) Pb₃O₄-20ZnO was first melted at 1050˚C for 1 hour and quenched. The resulting frit was ground, mixed with fluorescein (0.05 - 0.30 mol %) and re melted at 350˚C for 15 minutes.

After that the molten liquid was poured quickly between two stainless-steel plates kept at room temperature to form a glassy composition of the required shape. The obtained glasses were light greenish colored in nature. After that, the synthesized glass samples were polished to make them transparent. The obtained glasses and the experimental procedure are shown in Figure 1.

Figure 1. Schematic diagram of synthesis glass by using the rapid colling process in muffle furnace.

The physical parameters, such as the density, number of rare earth ions, refractive index, electronic polarizability [6][7], and molar refractivity, are calculated using the following equations and tabulated in Table 1.

Table 1. Physical parameter.

The refractive index (n) of the prepared glass is calculated using,

The electronic polarizability (α_e) is determined following the expression,

The molar refractivity R_m of the prepared glasses is obtained by,

where M is the average molecular weight and ρ is the density in g∙cm⁻³.

Briefly, the glass composition is shown in Table 2. We have synthesized samples (A, B, C, and D) as below:

Table 2. C₂₀H₁₂O₅ dye doped BAG composition.

The XRD spectra of the synthesized sample have been recorded by using the SmartLab 3 kW/Rigaku model diffractometer. The broad peaks show the glassy nature of the synthesized sample [8]. The XRD plot show one broad characteristic of peak amorphous materials and there are no signs of crystalline phases. No crystalline fluorescein peaks observed indicating dissolution. As the concentration increases the peaks are broader as compared to the undoped sample and peaks shifted to some but higher angle [9]. Corresponding XRD patterns of Fluorescein dye-doped and undoped boric acid glasses are shown in Figure 2.

Figure 2. XRD spectra of undoped and fluorescein dye doped Boric Acid Glass (BAG).

Figure 3 shows the FTIR spectra of Fluorescein dye-doped BAG glass which is recorded using the Spectrum Two/Perkin Elmer model. The various [10] absorption bands and their assignments obtained from FTIR spectra are shown in Table 3. The bands are assigned in the region 425.84 - 449.95 cm⁻¹ due to the bending vibrations of B-O-B linkages [11], in the region 649.76 - 918.20 cm⁻¹ the bands are obtained due to ZnO vibrations due to ZnO units, the bands are identified in the region 918.20 - 1153.95 cm⁻¹ due to the B-O symmetric stretching vibrations of BO₄ units. At around 1504 cm⁻¹ the vibration band [12] was observed due to the vibrational modes of the hydroxyl or water group in the glass system matrix. COO⁻ of fluorescein proof of retention are present in the region around 1691 cm⁻¹ due to low concentration in the glasses [13]. In the region, 3400 - 3500 cm⁻¹ bands are assigned due to CH banding vibrations, and in the region, 3600 cm⁻¹ bands are due to OH banding vibrations. Figure 3 shows the FTIR spectrum for C₂₀H₁₂O₅ doped and undoped BAG glass for various concentrations [14]. The corresponding assignment of FTIR is shown in Table 3.

Figure 3. FT-IR spectra of undoped and C₂₀H₁₂O₅ ion doped boric acid glass.

Table 3. The FTIR assignments and their band positions (cm⁻¹) of Fluorescein dye-doped glass samples.

Continued

Laser Raman spectroscopy is also useful in the study of vibrational modes of C₂₀H₁₂O₅ doped Boric Acid Glass (BAG). Raman spectral studies using the 532 nm as the exciting sources have been done for undoped and Fluorescein dye-doped boric acid glass using the WITEK model Alpha 300R. Experimental data for the sample are given in Figure 4 exposed by 532 nm. Figure 4 shows the Raman spectra of Fluorescein dye-doped boric acid glass for excitation of wavelength 532 nm.

Figure 4. Raman spectra of undoped and fluorescein dye-doped Boric Acid Glass at 532 nm.

Raman spectra show differences in the lines but show five peaks as in Figure 4. Raman spectra for undoped glass show 2 to 3 well-shaped peaks as in Figure 4. Also, the plot of doped glass shows more peaks than undoped glass and has shifted to a lower wavelength as in Figure 4. Raman Peak positions (cm⁻¹) for Fluorescein dye-doped boric acid glass samples at different wavelengths are tabulated in Table 4.

Table 4. Raman peak positions (cm⁻¹) for Fluorescein dye-doped boric acid glass samples at different wavelengths [15] are tabulated below.

In the range of 200 nm to 800 nm, UV-VIS absorption spectra have been recorded. A typical Absorption plot for Fluorescein dye-doped boric acid glass is shown in Figure 5. As we increased the concentration of C₂₀H₁₂O₅, the higher concentrated sample showed more peaks than the low concentration or undoped sample. Furthermore, the undoped [16] sample showed nearly three peaks at 322 nm, 362 nm, and 584 nm but the doped sample with a concentration of 0.05% C₂₀H₁₂O₅ shows five to six peaks at 376, 452,489, 522, 546, and 653 nm, also the concentration with 0.1% C₂₀H₁₂O₅ doped sample shows eight peaks at nearly 317, 357, 380, 407, 450, 488, 546 and 653 nm. This can be seen in Figure 5, As we increase concentration [17], the peaks increased in strength and shifted to higher wavelengths, whereas some peaks are at the same position in the doped and undoped sample as shown in Figure 5 [18].

Technically, the undoped glass prepared was colorless and there was no band structure in their spectra in UV and visible regions, but one band was seen nearly at 362 nm in the UV region [19], The samples prepared were transparent, and the doped glass containing C₂₀H₁₂O₅ attained a light pink color and obtained some bands in the UV-VIS region. The bands around 320 - 376 nm is due to π-π transition of benzoid rings. The bands around 452 - 490 nm is due to π-π transition of xanthene core (monomer). The bands around 522 - 546 nm is due to J-aggregate/anionic form of fluorescein. The bands around 653 nm are due to n-π shoulder due to carbonyl interaction with Pb²⁺ matrix (Figure 6).

Figure 5. UV-VIS absorption spectra of undoped and C₂₀H₁₂O₅ doped Boric Acid Glass.

(A) (B)

Figure 6. Absorption Tauck’s plot of undoped and C₂₀H₁₂O₅ doped Boric Acid Glass (A) direct band gap, (B) indirect band gap.

Tauc’s Plot can be used to determine the [14] energy band gap (E_g), by using the relation,

where α is an absorption coefficient, A is any arbitrary constant and E_g is the energy band gap. For the indirect band gap, we use n = 1/2 and direct band gap n = 2. The hv value can be calculated by using the relation 1240/wavelength.

The E_g value for the undoped sample is nearly equal to 3 eV which is compared to other borate glasses as in reference [16]. The Indirect band gap value for the undoped sample is 3.07 eV, 0.05% C₂₀H₁₂O₅ doped glass is 3.01 eV, and 0.1% C₂₀H₁₂O₅ doped glass is 2.82 eV approximately which is comparable to other C₂₀H₁₂O₅ doped Glasses [20] as shown in Table 5. As we increased the concentration of C₂₀H₁₂O₅, the optical band gap decreased (Figure 7).

Table 5. Direct band gap, Indirect band gap, and refractive index of the C₂₀H₁₂O₅ doped and undoped boric acid glass.

Figure 7. Energy level diagram of fluorescein dye doped glass.

Finally, the refractive index (n) is calculated by using the relation between the refractive index (n) and optical band gap (E_g), using the following relations [7]

where n values are found to be decreased with C₂₀H₁₂O₅ content.

The nonlinear transmission behavior of fluorescein dye-doped boric acid glass (BAG) was investigated using a continuous-wave He-Ne laser (λ = 632.8 nm, 20 mW) as the excitation source. The schematic of the experimental setup is shown in Figure 8. The laser beam was focused to a spot diameter of ~1.5 mm on the sample, corresponding to a peak intensity of ~1.13 W/cm². The sample thickness was measured as 1.2 ± 0.05 mm using a digital micrometer. The incident power was varied using a calibrated neutral density (ND) filter wheel (OD 0.1 - 2.0). The transmitted power was measured using a calibrated photodiode power meter. To avoid thermal damage, a dwell time of 5 sec was maintained at each power level and measurements were performed for three successive cycles, with reproducibility within ±8% [21]-[24].

Figure 8. Schematic diagram for measuring saturation absorption using He-Ne laser 632.8 nm 20 mW, ND filter wheel, sample thickness 1.2 mm.

Figure 9 shows the saturation absorption curve (transmitted power vs incident power) for Sample C (0.10 mol% fluorescein). At low incident power (<5 mW), the transmittance increases linearly, following Beer-Lambert law. Above ∼8 mW, the absorption saturates due to ground state depletion (S₀ → S₁) of fluorescein molecules, and the transmitted power shows sub-linear increase. This saturable absorption is crucial for optical limiting applications where the material attenuates high-intensity light, protecting sensitive optical components and photodetectors [25][26].

Figure 9. Saturation absorption behavior of fluorescein dye-doped BAG (Sample C) showing transmittance vs incident intensity-sub-linear at high intensity due to ground state bleaching.

Figure 10 shows the incident power vs transmitted power when cycled in increasing and decreasing directions. An optical hysteresis loop is observed with a loop width of ΔI ≈ 0.8 mW. At a given incident power (e.g., 10 mW), two distinct transmitted values are observed depending on the history (increasing/decreasing branch).

Figure 10. Optical hysteresis loop of fluorescein dye-doped BAG (Sample C) showing thermal hysteresis (ΔI = 0.8 mW)-revised from bistability to photo-thermal hysteresis.

This behavior is attributed to photo-thermal induced refractive index change (thermal lensing) rather than pure electronic optical bistability [27]-[30]. The absorbed 632.8 nm energy by fluorescein dye is partially converted to heat via non-radiative S1 → S0 internal conversion, leading to local temperature rise, change in refractive index (dn/dT) and formation of a thermal lens. This thermal effect produces a hysteresis loop which is often misinterpreted as electronic bistability. Therefore, we have revised the earlier claim of “optical bistability” to “thermal optical hysteresis” in accordance with the observed mechanism. The hysteresis width decreases from 1.2 mW (Sample D, 0.30%) to 0.6 mW (Sample B, 0.05%) with decreasing dye concentration, confirming its dye-concentration dependence. Such thermal hysteresis can be utilized in all-optical switching, optical memory and optical limiting devices operating at low power CW lasers [31][32].