**Figure 4.**

*Raman gain coefficients and their bandwidth are reported for different material: silicon and silica (as 'bulk material'), nanocomposite glasses (KNS) in different phases of the thermal treatment for the initial glass composition 20-25-55, silicon micro- and nano-particles (amorphous and crystalline). Features for 'ideal materials' for Raman amplification are reported, too.*

**135**

*Stimulated Raman Scattering in Micro- and Nanophotonics*

**3.2 SRS in nanostructured silicon-based materials**

size of 2 nm an important broadening of about 65 cm<sup>−</sup><sup>1</sup>

In our previous papers [2, 33–35] experimental results of spontaneous Raman scattering measurements in silicon nanostructures at the wavelength of interest for telecommunications (1.54 μm), were showed. Due to the phonon confinement model, two significant enhancements of the Raman spectra in silicon quantum dots respect to silicon were obtained: the broadening of spontaneous Raman emission and the tuning of the Stokes shift. In detail, in silicon quantum dots with a crystal

were demonstrated. Taking into account that the width of C-band telecom-

ered using silicon quantum dots, without implementing the multi-pump scheme. In this paragraph, comparison among experimental investigations of SRS in amorphous silicon nanoparticles and in silicon micro- and nano-crystals, at the wavelengths of interest for telecommunications, are reported. We considered three

1.Silicon nanocomposites dispersed in SiO2 matrix. The mean radius of the

2.Amorphous silicon nanoclusters embedded in Si-rich nitride/silicon superlattice structures (SRN/Si-SLs). The structure of the sample consists of 10 SRN layers and 9 amorphous Si (a-Si) layers for a total thickness of 450 nm.

3.Silicon nanocrystals (Si-nc) with a size of about 4 nm embedded in a silica matrix layer about 7 cm long. The sample was realized with an increasing concentration of Si-nc varying along the longer dimension of the sample, allowing

I.In silicon nanocomposites, an amplification of Stokes signal up to 1.4 dB/cm at 1542.2 nm using a 1427 nm continuous-wavelength (CW) pump laser was reported. This result demonstrates a five-fold improvement of the Raman gain respect to bulk silicon. Furthermore, a threshold power reduction of

II.In SRN/Si-SLs, a magnification of Stokes signal up to 0.87 dB/cm at 1540.6 nm by means of a 1427 nm CW pump laser was reported. This result demonstrates a four-fold enhancement of the Raman gain with respect to bulk silicon. Additionally, a threshold power reduction of about 40% is also reported

III.In Si-nc an enhancement of the Raman gain by increasing their concentration was measured, and, a remarkable improvement of the Raman gain in Si-nc respect to bulk silicon, by three to four orders of magnitude depending on the Si concentration, was proven. The amplification was carried out by using a

probe signal at 1541.3 nm and a pump signal at 1427 nm [2, 42, 43].

The obtained results are summarized in **Figure 4** where the Raman gain is plotted as a function of the Raman bandwidth for the considered nanostructured

silicon dots and the dot density were of 49 nm and 1.62 × 108

Amorphous silicon nanoclusters size was 2 nm [38–40].

Results obtained can be summarized as follows:

about 60% is also reported [36, 37, 40, 41].

, we have that more than the half of C-band could be cov-

and a peak shift of about

dots/cm2

,

*DOI: http://dx.doi.org/10.5772/intechopen.80814*

19 cm<sup>−</sup><sup>1</sup>

munication is 146 cm<sup>−</sup><sup>1</sup>

different samples:

respectively [36, 37].

to distinguish seven areas.

[38–41].

*Nonlinear Optics - Novel Results in Theory and Applications*

nanostructured 30K2O·30Nb2O5·40SiO2 (KNS 30-30-40) glass was found [31]. It is worth noting that an appropriate choice of the annealing parameters, therefore of the degree of crystallization, can allow to obtain the best compromise between the highest Raman gain and the highest nonlinear coefficients for third order effects. Moreover, Raman spectroscopy characterization of nanostructured 20K2O·25Nb2O5·55SiO2 (KNS 20-25-55) glasses are also reported. The optical and structural characteristics of the samples have been measured by the Raman set up reported in Ref. [31]. Due to dependence of the intensity of the Raman active modes on both the temperature and the frequency of the vibrational modes, the measured Stokes Raman intensity was reduced according to the procedure also described in our previous papers [28–32]. Then, in order to properly compare the Raman spectra of KNS glasses with the silica glass standard, the measured Raman spectra were modified also for the differences in reflection and angle of collection by using the procedure reported in Refs. [28–32]. Usually, due to the more extended electronic clouds, elements with high atomic number yields highest intensity Raman bands, making the polarizability more sensitive to bonds stretching. Adding niobium oxide to silica-based glasses induces an enhancement of Raman cross section respect to silica glass, being the polarizability of Nb-O bonds higher than Si-O bonds. In the studied glasses, niobium enters in the glass network creating NbO6 octahedra more or less distorted, namely with different NbO bond length, and produces several Raman bands over a wide wavenumbers range. Due to the typical glass disorder, a broadening of all Raman bands occurs and, therefore, Raman spectra of KNS

glasses outcome from the strong overlapping of several broad bands.

inhomogeneities distributed in the glass matrix [30, 32].

In **Figure 4** the Raman gain respect to the Raman gain of silica is reported as a function of the Raman bandwidth for different materials. Inter alia, in **Figure 4** is evident that glasses at initial and at different times of heat-treatment show the same bandwidth, but different gain. Hence, the nanostructuring process is nearly complete in glasses at a time between 2 and 10 h and produces nanocrystalline

*Raman gain coefficients and their bandwidth are reported for different material: silicon and silica (as 'bulk material'), nanocomposite glasses (KNS) in different phases of the thermal treatment for the initial glass composition 20-25-55, silicon micro- and nano-particles (amorphous and crystalline). Features for 'ideal* 

**134**

**Figure 4.**

*materials' for Raman amplification are reported, too.*
