Compositional and thermal treatment effects on Raman gain and bandwidth in nanostructured silica based glasses
Introduction
Fused silica has been, for the past century, the key material used for long and short haul transmission of optical signals, because of its good optical properties and attractive figure of merit (i.e. trade-off between Raman gain and losses). A breakthrough in fiber optics communications was achieved with the reduction of the water absorption peak at 1400 nm [1], which opened up the available communication range to span from 1270 to 1650 nm, corresponding to about 50 THz bandwidth [1], [2]. This dramatic increase in bandwidth reduces the usefulness of existing Er-doped fiber amplifiers, leaving Raman gain as the main mechanism for future amplification needs. However, pure silica and germanium-doped silica fibers, currently utilized as Raman gain media in telecommunications, have quite low Raman gain coefficients and a limited usable spectral bandwidth of approximately 5 THz [3], [4]. Therefore the investigation of new materials possessing both large Raman gain coefficients and broader spectral bandwidth than fused silica is becoming mandatory in order to satisfy the increasing telecommunications demands [5], [6].
The ‘ideal’ material for Raman amplification should have wide, flat and high Raman gain over all the range of interest. Unfortunately, as a general rule, due to the physics behind the Raman effect, there is a tradeoff for Raman amplification: in nature we have materials with strong Raman efficiency and small bandwidth (for example silicon) or materials with small amplification but with a very large bandwidth (for example silica). Of course, this tradeoff is a fundamental limitation towards the realization of micro-/nano-sources with broad emission spectra.
Among the advanced materials for Raman amplification, one of the most interesting classes is that of oxide glasses. This is because the optical properties of oxide glasses can be easily modulated acting on both the chemical composition and the microstructure.
Although tellurite- [7], [8], [9], [10], [11], [12], [13], [14] and chalcogenide- [15], [16], [17] based glasses combine large scattering intensity and bandwidth giving very high Raman gains, up to 40 times larger than SiO2, their fabrication demands particular wise precautions, including controlled melting atmosphere.
Thus, silicon dioxide-based glasses, due to their compatibility with the existing optical fibers technology, are more attractive than tellurite-like materials. To improve the stimulated Raman scattering (SRS) of these glasses, suitable dopants (heavy metal oxides as Ta2O5, Bi2O3, Nb2O5) have been added to silica, but a Raman gain only twice that of pure silica was obtained [18].
In other systems, such as niobium–phosphate glasses with high niobium content, some broadening of the bandwidth and a higher peak Raman gain (but not more than ≈ 10 times) than in silica glass has been reported [19], [20]. In this work in order to improve the optical features (mostly SRS) of silica-based glasses, rather than to explore new glass compositions, we have played on the glass structure. Indeed, in a glass, structural modifications can occur as a consequence of proper heat treatments performed in the glass transition range producing glass-ceramics with nanocrystals uniformly dispersed in the glass matrix (glass-crystals nanocomposites) [21], [22], [23], [24].
Our investigation is motivated by the fact that the optical nanocomposite approach offers opportunities to produce high-performance and relatively low-cost optoelectronic media suitable for many applications. In addition, from a fundamental point of view, one of the most recent fascinating research fields is nonlinear optics at nanoscale. SRS in electrons-confined and photons-confined materials is of great importance from both fundamental and applicative points of view. Concerning the fundamental one, despite there are a number of investigations, both experimental and theoretical ones, on the relation between nanostructure and Raman gain, the question is still “open” [25], [26], [27], [28]. As to the applicative point of view, there are some important prospectives, for example to produce micro/nanosources, with improved performances.
As mentioned, a clear relationship between glass nanostructuring and Raman gain has not been established yet, even though, in our recent paper, a correlation between local structure and SRS in bulk nanostructured 30K2O⋅30Nb2O5⋅40SiO2 (KNS 30–30–40) glass was found [29]. This glass belongs to the K2O–Nb2O5–SiO2 (KNS) system. It is characterized by a large glass-forming range from which transparent and stable glasses showing interesting non-linear optical properties can be obtained. The composition of all KNS glasses studied in our previous works are reported on the ternary plot in Fig. 1 [21], [22], [23], [30], [31], [32]. Indeed, it was shown that a strong correlation exists among local structure and SHG in bulk nanostructured KNS glasses [24], [31], [32]. In particular, SHG shows a maximum in correspondence of the early stages of nanostructuring, that are characterized by the segregation within the amorphous matrix of nanosized inhomogeneities [32].
This paper is addressed to elucidate the role both of the initial glass composition and of the local structure of nanostructured KNS glasses on Raman gain. In particular, the ellipsometric and the Raman spectroscopy characterization of nanostructured KNS 23–27–50 and KNS 20–25–55 glasses (circles in the ternary plot of Fig. 1) have been performed. The phase transformations occurring upon thermal treatment in glasses with these two compositions had been investigated in a previous paper by structural characterization [24]; in spite of their small composition difference (23–27–50, 20–25–55), they exhibit quite different local structure and nanostructuring mechanisms as summarized in Fig. 1. In the KNS 23–27–50 glass nanostructuring occurs firstly by binodal phase separation. In fact for 2 h annealing at Tg, this glass is still amorphous as shown by the trace b of the XRD pattern (inset of Fig. 1.2), even if at this stage it is no more homogeneous as seen by SEM image (Fig. 1.2). Only for a prolonged annealing nanocrystals grow from the niobium-rich regions. On the contrary, the KNS 20–25–55 glass skips the step of phase separation undergoing more directly nanocrystallization. TEM images of samples of this glass annealed 2 and 24 h at Tg are shown in Fig. 1.3 and 1.4, respectively, and nanocrystals can be clearly seen just after 2 h [24]. Therefore, even though upon slow heating the nanocrystals of both glasses transform into the same crystalline phase, they reach the final state in a different way: in the KNS 23–27–50 they undergo a second-order phase transition, while in the KNS 20–25–55 glass they undergo a first-order phase transition [24]. Ultimately, the ability to manipulate the microstructure of these KNS glasses by chemical composition and/or by thermal treatment called us for research on their optical properties.
Section snippets
Experimental
Two potassium niobium silicate (KNS) glasses with composition 20K2O⋅25Nb2O5⋅55SiO2 (KNS 20–25–55) and 23K2O⋅27Nb2O5⋅50SiO2 (KNS 23–27–50) were obtained by melt-quenching technique, as reported in a previous paper [31]. Their glass transition temperatures (Tg) are 705 and 680 °C respectively. It was also reported that from KNS glasses with high Nb2O5 content, within 25 and 30 mol%, transparent nanostructured glasses can be easily produced by heat treatment at the respective Tg for time up to 24 h
Results and discussion
In a previous paper, it was ascertained that nanostructuring of as-quenched potassium niobiosilicate glasses (KNS), intended as segregation within the amorphous matrix of nanosized inhomogeneities and/or nanocrystals, is driven by phase separation and it can be induced and tailored by annealing at the glass transition temperature, Tg, and/or by fine modulation of the composition [24]. In KNS 20–25–55 and KNS 23–27–50 glasses nanostructuring evolves starting from nucleation of niobium-rich
Conclusions
Silica based amorphous and nanostructured glasses with composition 20K2O⋅25Nb2O5⋅55SiO2 and 23K2O⋅27Nb2O5⋅50SiO2 have been prepared and investigated with specific reference to Raman gain and bandwidth. A significant gain coefficient enhancement and a bandwidth broadening with respect to pure silica glasses are demonstrated. In addition, starting from K2O–Nb2O5–SiO2 (KNS) glass-forming system, by an easy and cheap process, a transparent nanocomposite material has been realized and characterized.
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