1 Introduction

The propagation of an intense femtosecond laser pulse through a nonlinear medium leads to the broadening of the pulse spectrum in different regions of the optical spectrum, known as Supercontinuum (SC) generation. The generation of SC can be used in other domains of ultrafast science, including time-resolved absorption and emission spectroscopy, optical parametric amplification, photonics, and nonlinear optics [1,2,3,4]. The first observation of the supercontinuum generation from condensed media was reported by Alfano and Shapiro using laser pulses on a borosilicate glass sample [3]. Advancements in the creation of novel photonic crystal fibers and glasses, like tellurite and chalcogenide glasses, have resulted in the emergence of ultra-broadband supercontinuum sources[5,6,7]. The process of spectral broadening is complicated and can be initiated through different mechanisms, including self-phase modulation (SPM), the generation of free electron plasma, and self-steepening [8,9,10,11]. The broadening of the supercontinuum can be accomplished by modifying either the input pulse parameters (such as pulse shape, polarization, phase, etc.) or by tuning the structural and optical properties of the medium[10].

Due to the ease of fabrication, shaping, and unique properties such as optical transparency, large opportunity for tuning the material properties of glasses is available which can further help in understanding the laser-matter interactions which lead to the large spectral broadening of the pulses. Among the glass formers, borate glasses have the least mass and possess superior properties such as low melting temperature, and exhibit high nonlinear optical properties[12]. Furthermore, the ability to modify the coordination with oxygen between three and four creates diverse anionic environments that help in the induction of various metal cations in the glass network [13]. Besides, the addition of sodium ions into the borate glass network improves optical transparency and enhances mechanical and chemical stability [14].

Glasses incorporating Sb2O3 have garnered significant attention due to their high electronic polarizability, refractive index, and nonlinear optical susceptibility, making them promising candidates for application in ultrafast nonlinear optical devices [15]. The nonlinear optical properties of the glass matrix can be enhanced with the inclusion of antimony due to the existence of a lone pair of electrons in the Sb3+ cation[16].

This manuscript is an attempt to gain an in depth understanding of the formation of different structural units within the glass network and their evolution with the progressive introduction of antimony, a strongly nonlinear material, and to examine its implications on the supercontinuum generation.

2 Experimental methods

2.1 Glass preparation

Glasses of composition 40B2O3-10Na2O-(50-x)ZnO-xSb2O3 (x in mol%) were synthesized through the conventional melt quenching method, where x = 0,10,20,30, & 40 mol% and were labeled as NBZS0, NBZS1, NBZS2, NBZS3, NBZS4 respectively. Precise amounts of analytical grade materials H3BO3, Na2CO3, ZnO, and Sb2O3 were accurately weighed for each sample and thoroughly mixed. The obtained mixtures were finely powdered using an agate mortar and then transferred to a silica crucible. Subsequently, the mixtures were heated at 450 °C for 40 min in an electrically heated muffle furnace to facilitate decarbonization. The temperature was then raised gradually to approximately 900—925 °C depending on the antimony concentration to achieve a homogeneous melt. The melts were rapidly quenched between two preheated stainless-steel plates. The obtained glasses were annealed in an air atmosphere near the glass transition temperature of 3200C for four hours to alleviate internal stresses incurred during quenching. Finally, all the glasses were ground by using silica carbide powders of different grit sizes (600 to 1200) to obtain a uniform thickness of 1 mm and then fine polished using a diamond paste to obtain high optical quality glasses.

2.2 Characterisation techniques

The density of the glass samples was determined by employing the Archimedes principle, using water as an immersion liquid. The UV–visible absorbance spectra of the glass samples were acquired using a UV–Vis-NIR (Varian model 5000) spectrophotometer, having a spectral resolution of 0.1 nm. X-ray Photoelectron Spectroscopy (XPS) data were collected using a Thermo-scientific K-alpha surface analysis system where the Al-Kα will act as the X-ray source. Renishaw laser confocal Raman microscope, having an excitation wavelength of 785 nm, has been used to acquire the Raman spectra of the glass samples. FTIR measurements were carried out using a Thermo-Nicolet (model 6700) Fourier Transform Infrared Spectrometer.

Femtosecond laser pulses from an amplified Ti: Sapphire laser system (Libra, 4W, manufactured by Coherent Co) was utilized for supercontinuum(SC) generation from the glasses. The laser pulses possess a pulse width of 70 fs, a repetition rate of 1 kHz, and an excitation wavelength of 800 nm. The fs laser beam, with an 8 mm diameter, was focused onto the sample using a lens with a focal length of 150 mm. The samples were positioned 135 mm away from the lens, resulting in a 0.8 mm spot size on the sample surface. Proper care was taken to ensure that no material damage occurred inside the glass. The generated SC spectra were recorded using an optical fiber-coupled spectrometer (Maya 2000, Ocean Optics). The stability and reproducibility of the generated SC were confirmed through repeated measurements at various positions on the glass sample. While recording the supercontinuum spectrum, a proper care is taken to avoid damage to the glass sample by the focussed fs laser beam. The position of the sample along the focus is chosen such that no optical damage occurs within the glass sample for a given pulse energy. This is important as the localised damage or modification which happens at the point of laser incidence may change its refractive index or absorption coefficient and thereby affect supercontinuum generation. Localised damage may further have a cascading effect and lead to more permanent damage to the sample in the form burn centers and make the sample unusable or the resultant measurement unreliable.

3 Results and discussions

The prepared glasses are optically transparent and with the increase in antimony concentration, initially colourless glass samples change to a pale yellow colour. This is attributed to the absorption transition taking place between the highest occupied molecular orbital, HOMO, (Sb, 5 s + O 2pπ) and the least unoccupied molecular orbital, LUMO, (Sb, 5p) pairs [16]. The density (ρ), and molar volume (Vm) of the glass samples were evaluated to understand the structural compactness and the cross-linking in the glass network, and the respective values are depicted in Table 1. The observed increase in the density is due to the substitution of ZnO with Sb2O3, which possesses a higher molecular mass of 291.52 amu. The molar volume of the glass also increases with the increased addition of the antimony. This is because the antimony forms bonds with extended bond lengths, inducing a free volume expansion within the glass network, consequently increasing the molar volume [17].

Table 1 Density (ρ), Molar Volume (Vm), Oxygen Packing Density (OPD), Optical Band Gap (Eg), Refractive index (n), Molar Refraction (Rm), and Oxide ion polarizability (α0) of the NZBS glass samples
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Figure 1 shows the UV–visible absorption spectra of the glass samples. It is observed that there is a red shift in the absorption edge wavelength with the enhanced addition of antimony concentration. This shift indicates a reduction in bond ionicity, signifying that electrons become more loosely bound to their respective bonds[18]. The optical bandgap values were evaluated using the Tauc relation

$$\alpha hv=B({hv-{E}_{g})}^{n}$$
(1)

where α is the absorption coefficient, h is the Planck constant, v is the frequency, B is the band tailing parameter, n depends on the nature of the transition involved, and Eg is the optical band gap[19]. The evaluated optical band gap values are tabulated in Table 1. A noticeable reduction in the optical band gap is evident with the introduction of antimony into the glass composition, specifically from NBZS0 to NBZS1. A further reduction in the optical band is observed with the rise in antimony concentration. The observed colour change from colourless to pale yellow at higher concentrations of antimony as mentioned earlier is due to the decrease in the optical bandgap between the HOMO (Sb5s + O2pπ) and LUMO (Sb5p) pairs. The Sb3+ ions have a localized lone pair of electrons in their 5 s orbital, creating a density of states at the top of the valence band which leads to a reduction in the optical bandgap [16].

Fig. 1
figure 1

UV–Vis absorption spectra of the NBZS glasses

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The refractive index of the glass influences the interaction of incident light with the electrons in the glass system, and it is calculated from the optical band gap (Eg) using;

$$\frac{({n}^{2}-1)}{({n}^{2}+2)}=1-\sqrt{\frac{{E}_{g}}{20}}$$
(2)

where n is the refractive index [20]. From Table 1, it is observed that the refractive index of the glass increases with the increase in the antimony concentration. Generally, glasses with a higher refractive index (n > 2) exhibit higher polarizability, optical basicity, and large third-order nonlinear susceptibility. The Molar refraction (Rm) quantifies the overall polarizability of one mole of a material and can be calculated using the Lorentz-Lorentz relationship [21]:

$${R}_{m}=\left[\frac{({n}^{2}-1)}{({n}^{2}+2)}\right]{V}_{m}$$
(3)

where n is the refractive index, and Vm is the molar volume. As observed from Table 1, the Rm values show a gradual increase with the improved addition of antimony concentration. This indicates that there is an enhancement in the formation of highly polarizable Non-Bridging Oxygens (NBOs) within the glass network as the concentration of antimony increases.

Dimitrov and Komatsu proposed an equation to estimate the oxide ion polarizability(α0) based on the energy band gap;

$${\alpha }_{0}\left({E}_{g}\right)=\left[\left(\frac{{V}_{m}}{2.52}\right)\left(1-\sqrt{\frac{{E}_{g}}{20}}\right)-\sum {\alpha }_{i}\right]*{\left({N}_{{O}^{2-}}\right)}^{-1}$$
(4)

where Σαi is the sum of molar cation polarizability and NO2− is the number of oxide ions in the glass chemical composition [22]. It is observed that there is an increase in the oxide ion polarizability (α0) with antimony addition. The rise in oxide ion polarizability indicates that there is an enhancement in the ability of the oxide ions to donate electrons [21, 22]. Dimitrov et al. proposed that oxide ion polarizability is directly correlated with nonlinear optical susceptibility (χ3) [21]. Hence, the third-order nonlinear susceptibility values are higher for the glass matrix having the largest antimony content.

The Supercontinuum (SC) generation from the glasses was recorded at different incident pump powers. The normalized spectra are presented in a contour plot, as depicted in Fig. 2. The generated SC exhibits asymmetric broadening around the pump wavelength, with a more pronounced broadening observed towards the lower wavelength side (blue-shifted) as opposed to the higher wavelength side (red-shifted). This asymmetric spectral broadening is a universal phenomenon in a normal dispersive medium, as previously reported in earlier studies [11, 23]. This happens due to the pulse-splitting scenario in which a steep edge is formed at the trailing front of the trailing sub-pulse, resulting in a sharp drop in the nonlinear phase at the middle of the pulse. This, in turn, translates into a broad blue shifted pedestal in the SC spectrum [1, 4].

Fig. 2
figure 2

Supercontinuum generation from the different NBZS glasses

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In order to assess the extent of SC broadening, the generated spectra of the glasses using femtosecond laser pump powers of 0.25W. 0.5W, 0.75W, and 1W are depicted in Fig. 3. It is observed that at a lower pump power of 0.25W, greater spectral broadening is observed for glass having a higher concentration of antimony (i.e., NBZS4 glass). This broadening diminishes with the decrease in the antimony content. However, the observed trend changes for a pump power of 0.5W, showing more pronounced spectral broadening in the following order: NBZS0 > NBZS2 > NBZS3 > NBZS4 > NBZS1. For pump powers of 0.75W and 1W, the order of SC broadening changed to NBZS0 > NBZS1 > NBZS2 > NBZS3 > NBZS4. Particularly, at 1W, the blue edge cut-off power of the NBZS0 glass sample extends to around 505 nm. Hence, at higher pump powers of 0.75 and 1W, glasses having a lower concentration of antimony exhibit a better spectral broadening. The energy ratio of the optical bandgap to the incident photon, Eg/hv, was estimated for all the glasses, and it was greater than 2, where the maximum value of 3.09 was obtained for NBZS0 glass. This was in tune with the understanding that the extent of spectral broadening increases with the increase in the Eg/hv ratio [10].

Fig. 3
figure 3

SC spectra of NZBS glasses at various pump powers a 0.25W, b 0.5W, c 0.75W, and d 1W

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However, for the NBZS4 glass, with the increase in the input pump powers (i.e., 0.5W, 0.75W, and 1W), even though there is an increase in the spectral content under the curve for the NBZS4 glass, there is no noticeable SC spectral broadening (Fig. 3), i.e., the SC spectral broadening saturates around 610 nm, and there is no further broadening with improved laser pump powers. To comprehend the enhanced spectral broadening in glasses with lower bandgaps (i.e. NBZS4) at reduced pump powers, as well as the saturation observed in spectral broadening at higher pump powers, a thorough investigation of the structural parameters of the glasses needed to be done.

3.1 Structural characteristics

Generally, symmetric vibrational modes are strong scatters in Raman spectroscopy but weak in IR spectroscopy. Conversely, asymmetric vibrational modes tend to exhibit strong scattering intensity in IR spectra and weak intensity in Raman spectra. Therefore, a comprehensive analysis involves examining both types of vibrational modes. Here, structural investigation was conducted through Raman, FTIR, and XPS analyses.

3.1.1 Raman spectroscopy

The Raman spectra of a pure borate glass consists of an intense peak around 806 cm−1 that arises from a symmetric ring breathing vibrations of the boroxol rings [24]. The introduction of alkali oxide and other modifiers disrupts the characteristic boroxol ring, resulting in the creation of various structural groups, that can be characterized well through Raman spectroscopy. The introduction of Na2O and ZnO resulted in the destruction of boroxol ring structures and the appearance of several newer bands in the 500–1600 cm−1 region (Fig. 4). Deconvolution of the Raman spectra was carried out using the Gaussian peak fitting to identify the exact position of the peaks and their intensity variations, as shown in Fig. 4. The optimal fit was determined through visual assessment and quantified using R2 values. The R2 values obtained for NBZS0, NBZS1, NBZS2, NBZS3, and NBZS4 are 0.997, 0.998, 0.999, 0.999, and 0.999, respectively. From Fig. 4a for NBZS0 glass, the appearance of the band peak at 777 cm−1 arises from the symmetric ring breathing vibrations of BO4− containing six-membered pentaborate groups [25, 26]. The intense peak at 845 cm−1 is attributed to the presence of pyroborate groups [24, 27]. The peak at 690 cm−1 arises from the symmetric breathing vibrations of metaborate chains. Similarly, the band at 1410 cm−1 appears from the B-O− stretching from metaborate rings and chains [28]. Furthermore, the band at 940 cm−1 arises from the stretching vibrations of BO33− orthoborate units [24, 27]. In addition, the band at 1290 cm−1 is from the asymmetric stretching vibrations of the same [26]. The small peak around 1030 cm−1 is due to the presence of some CO33−, carbonate species [26, 27]. The band at 280 cm−1 corresponds to the bending modes of ZnO4 units [28]. The broad peak with band peaks from 1350–1400 cm−1 is associated with the stretching of B-O− bonds from metaborate units [29].

Fig. 4
figure 4

Deconvoluted Raman bands of the glass matrices a NBZS0, b NBZS1, c NBZS2, d NBZS3 and e NBZS4

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The introduction of antimony into the glasses leads to the disappearance of bands corresponding to the borate structural groups, and the bands corresponding to the SbO3 pyramids become predominant. This is due to the high Raman scattering intensity of Sb-O vibrational modes [16]. From Fig. 4b-e, it is observed that the peaks around 440 cm−1 and 500 cm−1 arise from the asymmetric and symmetric bending vibrational modes of SbO3 units [16]. Furthermore, the peak at 620 cm−1 occurs from the asymmetric stretching vibrations of Sb-O-Sb bridges [25]. The peak around 700 cm−1 can be attributed to the common vibration of Sb-O-B bridges, whose intensity is higher at lower concentrations of antimony and decreases with further improved antimony addition. Antimony acts as a network former at lower concentrations through the formation of Sb-O-B due to the covalent nature of the Sb-O bonds [16]. The smaller peaks at around 180 cm−1 arise from the SbO3 group modes [16, 18]. The assigned bands are tabulated in Table 2.

Table 2 Assignment of Raman bands observed in NBZS glasses
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3.1.2 FTIR spectroscopy

To have a precise understanding of the transition in boron coordination from + 3 to + 4 and the formation of non-bridging oxygens (NBO) within the glass network, IR spectroscopy stands out as the optimal method. The incorporation of modifiers into the glass matrix results in the depolymerization of boroxol rings, giving rise to the formation of different BO4 and BO3 groups, as evidenced in the spectra. In the borate network, as shown in Fig. 5, vibrational modes in the region from 780 to 1110 cm−1 are attributed to the stretching vibrations of different BO4 structural groups, and 1110–1350 cm−1 constitute the stretching vibrations from the trigonal BO3 groups [13], respective values are listed in Table 3. The band at 500–780 cm−1 comprises bending vibrational modes of borate groups and also from the vibrations of other metal cations in the glass network. However, due to the formation of multiple bands across different regions of the spectra, deconvolution was necessary to assess all the infrared features. The experimentally obtained spectrum was fitted using Gauss peak curves. An optimal peak fit was achieved for NBZS0, NBZS1, NBZS2, NBZS3, and NBZS4 spectra with R2 values of 0.995, 0.995, 0.996, 0.996 and 0.996, respectively.

Fig. 5
figure 5

Deconvoluted FTIR spectra of the a NBZS0, b NBZS1, c NBZS2, d NBZS3 and e NBZS4 glasses

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Table 3 Assignment of FTIR bands observed in NBZS glasses
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For, the NBZS0 glass, the small intense peak around 570 cm−1 arises from the asymmetric bending vibrations from the BO3 units. The peak at 650 cm−1 is assigned to the in-plane bending vibrations of orthoborate units [26]. The high intense peak at 705 cm−1 arises from the bending of B-O-B linkages from the borate network [30]. The pronounced peak at 920 cm−1 originates from the stretching vibrations of pentaborate groups [30, 31]. Additionally, the minor intense peak at around 1020 cm−1 is attributed to the B-O stretching vibration of other BO4− tetrahedral units, i.e., triborate and diborate groups [31, 32]. The broad peak at 1280 cm−1 arises from the asymmetric B-O stretching vibrations of BO3 groups [30, 31].

The inclusion of antimony in the glass matrix modifies the spectra and generates a prominently intense band around 590 cm−1. This peak is attributed to the asymmetric stretching vibrations of trigonal SbO3 units. Similarly, the small peak at 700 cm−1 corresponds to the symmetric stretching vibrational modes of the same [16]. The peak at 1300 cm−1 arises from the asymmetric stretching vibration of BO33− units [31]. It is observed that there is an increase in the concentration of NBO containing BO33− units with the improved addition of antimony in the glass network.

The ratio of area under the curve of regions spanning from 780 to 1100 cm−1 and 1100 to 1350 cm−1 of the IR spectrum provides an insight into the concentration of BO4 and BO3 structural units formed within the glass network with antimony concentrations. The obtained BO4/BO3 ratio values for NBZS0, NBZS1, NBZS2, NBZS3, and NBZS4 are 1.87, 1.64, 1.52, 1.39 and 1.22 respectively. This reveals that there is a progressive enhancement in the formation of BO3 units having Non-Bridging Oxygens (NBO) with the improved addition of antimony.

3.1.3 XPS analysis

To comprehend the electronic structure of the glasses, the XPS spectra of O1s and Sb3d binding energies were measured and are illustrated in Fig. 6. Due to the spin–orbit coupling, the Sb3d spectra split into, Sb3d5/2 and Sb3d3/2 with a separation of about 9.3 eV. However, a complicated behaviour arises due to the overlapping of the binding energies of Sb3d5/2 and O1s. To assess the contribution of each element, deconvolution is necessary. This can be achieved by using the constraint by fixing the peak area ratio of Sb3d5/2 to Sb3d3/2 at 3:2 and Sb3d splitting around 9.3 eV [33]. An optimal curve fitting is achieved using Gaussian-Lorentz curves, wherein the O1s peak is resolved into two constituents, O1s(1) and O1s(2). The shifts in binding energy peaks and their full width at half maximum (FWHM) of Sb3d and O1s peaks for different glasses are summarised in Table 4.

Fig. 6
figure 6

Experimental and deconvoluted XPS spectra of O1s and Sb3d peaks of a NBZS0, b NBZS1, c NBZS2, d NBZS3 and e NBZS4 glasses

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From Table 4, it is observed that there is a shift in the Sb3d binding energies towards smaller values as the antimony content increases. Moreover, the O1s(1) peaks exhibit greater intensity than the O1s(2) peaks. The O1s(1) peak, exhibiting higher binding energy, is attributed to atomic bonds formed with Boron, particularly bonds connecting BO3 [B(3)] and BO4 [B(4)] groups, such as B(3)-O-B(3), B(3)-O-B(4), and B(4)-O-B(4) as well as, the O1s(2) peak, displaying lower binding energy, is associated with Sb-O-B and Sb-O-Sb bonds [22, 34, 35]. A decrease in the O1s(1)/O1s(2) ratio is observed in Table 4. indicates a decrease in the concentration of bridging B-O-B bonds with improved addition of antimony. Furthermore, the shift in the O1s(2) binding energies to lower values indicates an increase in the electron density of oxide ions associated with these bonds.

Table 4 Binding energy, FWHM of Sb3d and O1s XPS peaks, and the ratio of O1s(1)/O1s(2) peak areas of NZBS glasses
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Due to the high unit field strength of boron, the B3+ strongly attracts the electron charge density of the neighbouring oxide ions [21]. However, the introduction of Sb3+, characterized by its higher cation polarizability (1.111 Å) and having a lone pair of electrons in the valence shell, exerts a comparatively lesser influence on the surrounding oxide ions [21, 22]. This is due to the repulsive interaction between the outermost lone pair electron of Sb5s and the valence electron of oxygen [22]. Consequently, this creates a highly polarizable glass network in the glass matrices having a higher concentration of antimony, resulting in an easily distortable electron cloud density when exposed to a high-intensity electromagnetic field.

The supercontinuum generation primarily depends on the extent of self-focussing occurring in the medium, further enhancing the self-phase modulation [1, 10]. When a femtosecond laser pulse interacts with the medium, multiphoton excitations occur, causing electrons to excite from the valence band to the conduction band, where they become free [10, 23]. These generated free electrons have a negative change in the index of refraction, which retards the self-focusing process [23]. Generally, the self-focusing phenomenon stops when the free electron density reaches a value of Nstop is approximately 1018 cm−3[23].

For glass systems with lower antimony and consequent larger band gap, accumulate enough nonlinear phase before the free electron generated in the matrix reaches the saturated value of approximately 1018 cm−3, resulting in the generation of a spectrally broad supercontinuum. However, for the glasses having a higher concentration of antimony due to their high polarizability and lower optical bandgap, the threshold value of free electron density reaches at lower pump power itself, retarding the self-focusing, i.e., the laser pulse does not have enough propagation distance to accumulate the nonlinear phase, leading to lower spectral broadening of SC. The observed trend in spectral broadening at pump powers of 0.5W, 0.75W, and 1W is attributed to this phenomenon. However, at 0.25W, the NBZS4 glass sample exhibits more pronounced spectral broadening. This can be attributed to the initiation of multiphoton excitation at lower pump powers, facilitated by its high polarizability and lower optical bandgap. In contrast, this pump power (0.25W) remains below the threshold pump power required to induce multiphoton excitation in the remaining glass samples.

In summary, the minimum critical power required for initiating supercontinuum generation is media-dependent. For the NBZS4 glass sample, due to its high polarizability and lower optical bandgap, the critical power needed to generate supercontinuum is minimal. Therefore, the multiphoton excitation happens at lower pump powers generating an enhanced supercontinuum. The generated SC saturates at a particular pump power (0.25W) and does not lead to any enhancement with further higher pump powers. Nevertheless, for NBZS0 with a larger optical bandgap, the critical power for that saturated supercontinuum is higher. Hence for lower pump powers the generated SC is weaker and it enhances with the progressive increase in the laser pump powers.

4 Conclusions

Transparent Antimony zinc sodium borate glasses were prepared through the melt quenching method, and supercontinuum generation from these glasses were studied in detail. From the Raman, FTIR, and XPS analysis confirms the formation of BO33− structural groups with the progressive addition of antimony indicating the formation of NBO in the glass matrix. The reduction in the bandgap serves as confirmation of an increase in the formation of NBO. The calculated physical characteristics, such as refractive index, molar refraction, and oxide ion polarizability, suggest that an increased concentration of antimony creates a glass network that is highly polarisable. Among the glasses, the NBZS0 glass having a high optical bandgap displays a greater extent of spectral broadening, where it broadens around lower wavelengths to 505 nm. The lower optical band gap and highly polarisable nature of antimony containing glasses (NBZS4) facilitate a faster excitation of electrons from the valence band to the conduction band through multiphoton excitation. Hence, the threshold saturation of free electron density in the conduction bands occurs rapidly leading to insufficient accumulation of nonlinear phase for supercontinuum broadening in glasses with higher antimony content.