**4. Peculiarities of nonlinear optical phenomena in nano-photonics**

Nonlinear Optics is the analysis of the mechanisms that occur as light manipulates a material's optical properties. Franken et al. [45] discovered second-harmonic generation soon after revealing the first working laser by Maiman in 1960 [46], and it is widely regarded as the start of the field of nonlinear optics. The discovery of saturation results in the luminescence of dye molecules identified by G. N. Lewis et al. [47] is the earliest known example to the author's. This section will explore the peculiarities of nonlinear optical phenomena in nanophotonics by including an overview of active nanophotonic devices, gain materials, plasmonic nanostructures, metamaterials, quantum dot lasers, and optical amplifiers.

### **4.1 Active nano-photonic devices**

Active materials have recently appeared as a potential loss compensation technique, first in nanoparticles, metamaterials, and plasmonic waveguides, and then in novel functionalities such as signal amplification and lasing in the field of nanophotonics [48]. It's important to note that in nanophotonics, the word "active" refers to the manipulation of material properties like refractive index in phasechange materials to regulate or reorganize plasmon propagation. Nanolasers and surface plasmon amplifiers have piqued the attention of researchers since they enable the theory of coherent stimulated emission to be applied to the diffraction limit and beyond. The notion of a SPASER [49], acronym of "surface plasmon amplification by stimulated emission of radiation", was originally designed to amplify oscillating localized surface plasmons (LSP's), inside metal nanoparticles and it was eventually expanded to add traveling surface plasmon-polaritons (SPP's). Aside from their significance in optoelectronic and all-optical data processing, they are also used in sensing, biological and super resolution imaging [50].

### **4.2 New gain materials**

Gain materials and parametric amplification through nonlinear effects are the key approaches for achieving optical gain in active systems. The standard active nanophotonic device scenario is depicted in **Figure 4**, in which a lossy resonator is surrounded by an active material and a bulk gain material may be used to explore this arrangement such as halide perovskite, chromophore, QD's scattered with metallic and/or dielectric nanoparticles or a gain material layer surrounding a nuclei core [50]. Material nonlinearity has been successfully used in metamaterials [51]

#### **Figure 4.**

*Multiple approaches used for representation of active nanophotonics [50].*

#### **Figure 5.** *Material gain parameter achieved for rare earth doped and other materials [50].*

and nanostructures [52] for loss correction using optical parametric amplification (OPA), while coherent amplification, which is based on pulse amplification within a cavity through positive interference, may provide gain and has recently been suggested for loss compensation in plasmonics [50]. The achievable material gain parameter of prospective materials for nanolasers including transition metal dichalcogenides (TMDCs), quantum dots, quantum wells and perovskites is shown in **Figure 5** while **Figure 6** depicts an overview of new materials for nanolasers.
