**2. Plasmonics nano-antenna**

The optical field could be transformed into localized energy via a structure called optical nano-antennas (ONAs). Their structures have an ability to control and manipulate the optical field at subwavelength scales [2].

ONAs require engineering accuracies of the characteristic dimensions down to a few nano-meters while about to the wavelength scale in other antennas [3–6]. However, this downscaling holds the technological challenges of nano-scale antenna engineering [6, 7]. The antenna performance can be strongly enhanced by plasmon resonances that lead to high and confined fields. The optical excitation of ONAs with a suitable wavelength can produce very high near-field because of their LSPR [4].

The interaction of an intense electromagnetic field with electrons ejected freely at the interface between dielectric/metal results in a quantum electromagnetic phenomenon called surface plasmon resonance (SPR). plasmonic is a field that deals with SPR. The interesting potential for engineering many devices and patterns involving nano-photonic devices are based on plasmonic nano-structures.

Plasmonic nano-antennas (PNAs), are able to controlling and confining EM field at the nano-scale. The performance evaluation of PNAs is depending on two important parameters, the absorption of the light and field improvement locally. A wide research areas invest the high light absorption, such as thermal emitters, solar thermal applications, thermal photoluminescences, and sensors. The improved electric fields at resonance wavelength can modulate the optical properties in the vicinity of molecules, so that, enhancing their light-matter interactions [8–10].

The tuning of the plasmon resonance for both absorption and emission to the excitation or the emission of species is the interesting research recently. The exciting EM field is enhanced several order of magnitude due to the production of what socalled hot spots when perfect nano-structures are designed. The structures working at plasmonic resonances open the ability to implement antennas working in the visible. The hot areas could be used to excite the effects at nonlinear regime so to match the EM field effectively. SERS and tip-enhanced Raman spectroscopy are the practical techniques that show the influence of such hot areas to observe the emitters with its sensitivity down to a single molecule [11].

The construction structure of PNAs is depending mainly on putting a gap at the sub-wavelength scale between two metallic areas, are gained distinguishable importance. This is mainly because of the hot spots in PNAs produce intensive EM field in nano-size overcoming the restriction of the diffraction. The confinement of the light field by BNAs is observed to be several order of magnitudes in the nano-scale smaller than the incident wavelength, as improved by the dimensions of the gap [12].

The resonance wavelength decisively depends on the shape, dimensions, and material of the antenna, a numerous variation of plasmonic antenna structures published proposed, such as bowties, nano-rings, nano-rod, and Yagi-Uda antennae. The sharp resonance wavelengths with narrow-band spectra with sharp are a major challenge for applications that require devices operating over a wide range of frequencies. For example, antennas used to improve energy harvesting efficiency of photovoltaic devices. Broadband PNAs are also highly wanted for SERs, fluorescence enhancement, and higher harmonic generation, which are multi-wavelength and broadband in nature [13].

## **2.1 Metal nano-antennas**

The intensity of the light, the dimensions of the components, and the material used are the essential parameters that produce plasmons excited by optical frequency. The suitable matters for this type of excitation are noble metals (i.e., gold (Au), silver (Ag), copper (Cu), and aluminum (Al)) so result in an important enhancement of the design and devices.

The nano-structures of metals with around area included in any design of optimized parameters need mainly the optical properties of PNAs especially in the medical field. Among many metals, gold is a perfect metal against high-temperature oxidation with the best plasmonic characteristics and especially suitable for biocompatible applications.

Distinguished spectroscopic features (spatial, spectral) are observed in noble metal nano-antennas (MNAs) such as Au NPs and Ag NPs. MNAs have those features resulting from the oscillations of electrons collectively in the conduction band, which is LSPR [10].

MNAs can confine and improve near IR and visible field in superficially by the excitation of LSPR. The 'EM hot region (spot)' that could be created on the nanoantennas has excessively utilized the absorption of light locally leading to an increase the weak intensity in the nonlinear optical process [14].

Au nano-particles (Au NPs) could effectively absorb IR and visible field energy in quite concentrated sizes, getting them properly controllable heat source in subwavelength size. In addition to the great importance the mechanism of those phenomena to be investigated, the capability to generate point like heat nano-heat encourage a broad area of research in physics, chemistry, and biology. The aforementioned properties of Au NPs especially as a nano-sources are promising researchers in the catalysis at nano-size, photonics, and in the field of medicine for cancer cells destroying photothermally [15].
