**2.1 SRS in microcavities**

In nonlinear optics devices, the use of microcavities allows to take advantage of both the micrometers dimension and the increasing of the local field, combining small modal volume with high optical quality-factors (Q ). One of the most important consequences for nonlinear optics applications is that strong resonant increase of energy in microscale volumes significantly reduces the power threshold at which nonlinear optical effects occur [10]. In the case of SRS, in agreement with the observed SRS for high-*Q* cavities experimental results [10], the explicit

**131**

*Stimulated Raman Scattering in Micro- and Nanophotonics*

expression of the cavity-enhanced Raman gain shows that the improvement is inversely proportional both to the square of the radius of the spherical cavity and to

In Ref. [11], SRS from spherical droplets and microspheres, with diameters of the order of tens of micrometers and optically coupled by the use of a tapered optical fiber, has been observed. The threshold was measured whereas the coupling air gap between the taper and microsphere was changed, allowing to obtain a micrometer-scale, nonlinear Raman source with a pump threshold approximately 1000 times lower than reported before and a pump-signal conversion

Diamond as a possible material for compact, on-chip Raman lasers over a wide spectrum was introduced in Ref. [12]. A CW, low-threshold, tunable Raman laser operating at ∼2 μm wavelengths based on waveguide-integrated diamond racetrack

A high-quality-factor nanocavity using a photonic crystal with a triangular lattice structure realized by circular air holes in a suspended silicon membrane and without any p-i-n diodes, yielding a device with a cavity size of less than 10 μm, has been demonstrated in Ref. [13]. The heterostructure nanocavity is obtained by introducing a line defect waveguide with two kinds of propagation modes inside the photonic bandgap, an odd-waveguide mode and an even-waveguide mode, which were used to confine pump light and Stokes-Raman-scattered light, respectively. A continuous-wave Raman silicon laser with an extraordinary low lasing threshold of 1 μW was demonstrated. In fact, an optimized nanocavity design allows to produce a net Raman gain in the low-excitation range before TPA-induced free carrier absorption (FCA) becomes dominant, permitting a low

Multiple scattering is a well-known phenomenon, occurring in nearly all optical opaque materials. Random walk of light waves in disordered materials could carry out to a multiple scattering with a consequent strong localization of electric field. Wave character of multiply scattered light is not lost and the wave can interfere both during and after the scattering process. Considering that the scattering is elastic, optical information does not change. Furthermore, due to reciprocity, multiple scattering is, in theory, fully reversible [2]. Reciprocity means that waves following the same path in opposite directions can interfere. Interference between such counter-propagating waves is always constructive, which gives rise to the incredibly robust interference phenomenon of coherent backscattering (also called weak localization). The combination of weak localization together with reciprocity, leads to a series of interesting physical effects and to an enormous potential for new

The first experimental evidence of lasing via a Raman interaction in a bulk three-dimensional random medium was demonstrated taking advantage of barium sulfate (BaSO4) powder with particle diameters of 1–5 μm. The pump energy threshold was 1.05 mJ; at higher values, gain is stronger than losses and SRS dominates the conversion process, allowing to obtain random Raman lasing. A Raman signal of 2.0 mJ was measured at a maximum of 11.5 mJ of pump energy [2, 15]. The complicated dynamics of nonlinear pulse propagation in a turbid medium make a

theoretical approach to describing this problem very challenging.

microresonators embedded in silica on a silicon chip was demonstrated.

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

the linewidth of the Raman process.

higher than 35% [2, 11].

**2.2 SRS in photonics crystals**

lasing threshold [2, 13].

**2.3 SRS in random laser**

disorder-based optical applications [14].

expression of the cavity-enhanced Raman gain shows that the improvement is inversely proportional both to the square of the radius of the spherical cavity and to the linewidth of the Raman process.

In Ref. [11], SRS from spherical droplets and microspheres, with diameters of the order of tens of micrometers and optically coupled by the use of a tapered optical fiber, has been observed. The threshold was measured whereas the coupling air gap between the taper and microsphere was changed, allowing to obtain a micrometer-scale, nonlinear Raman source with a pump threshold approximately 1000 times lower than reported before and a pump-signal conversion higher than 35% [2, 11].

Diamond as a possible material for compact, on-chip Raman lasers over a wide spectrum was introduced in Ref. [12]. A CW, low-threshold, tunable Raman laser operating at ∼2 μm wavelengths based on waveguide-integrated diamond racetrack microresonators embedded in silica on a silicon chip was demonstrated.
