High Near-Field Enhancement in Plasmonic Coupled Nanostructure for Spaser Application

We theoretically demonstrated a kind of plasmon coupled elliptical/crescent nanostructure to achieve a vast range of applications based on nanolaser or spaser with high intensity. To overcome the ohmic losses, the plasmon ellipse is composed of the gold film substrate with a gain media. A simple ellipse has been chosen from which a variety of dimer configurations have been formed by the symmetry alteration technique which is then tested for different light polarizations and gap variations. The proposed model supports localized surface plasmon resonance mode (LSPR). Moreover, the localized surface plasmon resonance (LSPR) property of the proposed nanostructure is numerically analysed by the finite-element method (FEM), and the results show that the electric field intensity (EFI) can be amplified to a large value by symmetry breaking in the elliptical nanostructure. Various plasmon modes can be excited by selecting the appropriate gain media. In addition to this, a compact tuneable multi-wavelength nanolaser (spaser) can be developed by using this model.


Introduction
Surface plasmon (SP) is the photon-excited collectively coherent oscillation of electrons at the surface of metallic nanostructures or films. Once the resonant interaction of surface charge oscillation and the electromagnetic field of the light has been realized, the restricted charges on the metal surface will dramatically enhance the electromagnetic field nearby and confine the electromagnetic energy into subwavelength regions, which exhibit strong resonant properties in optical spectra, namely surface plasmon resonance (SPR) [1]. The SPR reveals unique optical properties and spurs by new synthetic chemical methods of producing small particles in a variety of shapes quickly and inexpensively, advances micro/nanofabrication techniques [2][3][4][5], and has prominent applications in surface-enhanced Raman scattering (SERS) [6,7], optical sensing [8,9], perfect light absorption [10][11][12], imaging and cloaking [13,14], light speed manipulation [15][16][17], and alternative optical devices [18][19][20][21][22][23][24]. Nano-optics is now undergoing a period of explosive growth where new ideas, developments, and impressive results appear daily. It is concerned with the science of concentrating optical energy into regions with subwavelength dimensions (typically tens of nanometres). Yet, despite all this progress, there is still the need for a coherent, intense, ultrafast (with pulse durations down to a few femtoseconds), source of optical energy concentrated to nanoscale areas, similar to the laser but on a much smaller scale. Despite all this, heating problem that is associated with nanoparticles is also an issue of great concern. Since interaction of light with nanosized particles produces huge amount of heat due to metal presence in the structures which causes ohmic losses and suffers the performance of the systems, so for this reason, [25] proposed such a source that is based on surface plasmons so-called spaser (surface plasmon amplification by stimulated emission of radiation), and researchers are now working to develop and exploit this idea. Conventional semiconductor lasers are subject to the diffraction limit, and so their size cannot be less than half a wavelength, hindering the miniaturization of lasers [26]. Unlike conventional lasers, SP-based nanolasers can reduce the mode size and physical size of lasers to less than half a wavelength simultaneously, forming a nanoscale coherent light source far beyond the diffraction limit [27]. To achieve a lower threshold and lower loss transmission, a variety of nanolaser structures have been designed, which can be generally classified into metal-dielectric-metal structure nanolasers, metal nanoparticles plasmonic nanolasers [28][29][30][31], nanowire surface plasmonic nanolasers, array nanowires plasmonic nanolasers, W-G mode plasmonic nanolasers, etc. [32,33]. Three major physical phenomena photon density of states enhancement, nonradiative decay enhancement, and enhancement of incoming field concerning the photonic process and devices associated with electroluminescence, photovoltaics, photodetectors, and Raman scattering have been briefly discussed by [34]. A vast range of examples has been mentioned including metal nanospheres and nanorods showing their maximal outcomes on basis of shape, size, and designs. Secondary radiation phenomena are also emphasized along with the decisive role of extinction spectra that overlap with the incident and emitted wavelength. Furthermore, multiple extinction peaks for Al and Ag along with an enhancement value of 1071.6 have been achieved which is a very high value for spaser-based applications. Besides all this, complex nanostructures have been deployed for investigating resonance phenomena, and those structures are much difficult to fabricate for practical realization of a model. In [35], the authors have explained mathematical equations and material lists for the practical realization of photonic crystals including mirrors, waveguides, microcavities, and much more that can be used for a vast range of applications and provided a platform for studying and developing more plasmonic streams for spasers. Finite integration technique (FIT) is used by [36] to design nanoprisms and associated dimer. Edge rounding along with resonance modifications has been investigated, first without placing a nano-sphere between the bowtie configuration and then introducing the nano-sphere. The near-field enhancement obtained was about 250 and 130 respectively. Numerical calculations showed that the curvature radius of the edges strongly affects the plasmonic peaks and blueshifts have been observed with an increase in curvature. Also, the spectral properties are strongly influenced by the presence of a nanoparticle between the gap of the prism. An aggregated metallic system with two or more nanoprisms has been reported by [37], able to produce a high local field of up to 250. The proposed system is based on a FIT with geometrical alteration to study the response of extinction peaks. Finally, a hybridization model for prisms has been proposed which is useful for the prediction of optical properties and optimizing the assembly processes. An assembly of anisotropic nanoparticles with controlling their angular position using capillary force based approach has been theoretically and experimentally reported by [38]. The model is able to eliminate undesired impure particles from the system, along with emphases on interparticle forces that have been focused. The researchers in [39] showed the fabrication of polymer-grafted metallic nanocube arrays for tunable electromagnetic properties. The system produced a local field of up to 400 and is dependent on polymer chain length or grafting density.
For observation of multiple resonances, a dimer is based on identical and mismatched gold crescents, which are highly structured and polarization-dependent nanostructures. The nanostructures are wrapped by a gain media layer like silica, and for simplicity, we have set its value to 2.25. The mode interactions with distinct angular momentums and line shapes are stronger in the asymmetric plasmonic nanodimers. Unlikely with homodimers, the symmetry-reduced nanostructures sidestep the crossings of the plasmonic modes resulting in a sharp resonance in the spectrum and possess better sensitivity than the symmetric correspondents. The resonance observed in the mismatched nano-crescent dimer (NCD) can be enhanced and tuned by varying the distance between two nano-crescents or modifying the size of one of the nano-crescent. Here, we elucidate the results for the dimer separations only down to 3 nm, 5 nm, 7 nm, 9 nm, and 11 nm because, for such distances, the quantum mechanical effect, such as tunnelling of electrons through dimer junctions, alters the classical response. Due to the high polarization sensitivity of the proposed NC dimer, we have a different response for x-y polarizations; therefore, the dimer can be used as a polarization-sensitive beam filter [34]. Furthermore, we have a spectral shape of lines with Fano resonance, which is thought to be a key element for highly dispersive metamaterials, and this is engineered through asymmetric spectral lines with steep variations.

Model
In this paper, we investigate various patterns of crescent dimers formed from a single nano-ellipse by scratching a portion and then splitting to form a NCD. This arrangement is done with special interest for the formation of spectral shapes of line. We found some arrangements that seem to be much beneficial for spectral line shapes and nearfield enhancement (NFE). Polarization tuning and strong asymmetric spectral line shapes make the proposed NCD an ideal testbed for spaser applications like switching, plasmon-induced transparency, and SERS. The optical properties of the NCDs are carried out using COMSOL with RF module, while Johnson and Christy data model has been utilized for finding the dielectric constant value of gold [35] with air as an embedding medium. Figure 1 shows the schematic diagram of a heterodimer. The dimensions of the NCD are a 1 ,b 1 /a 2 ,b 2 , where 'a 1 ' and 'b 1 ' are the semi-axis of one nano-crescent (NC) and 'a 2 ' and 'b 2 ' are the semi-axis of the second NC, with crescent thickness 't' respectively. The gap between two nano-crescents is denoted by 'S'. The illuminating electromagnetic wave is linearly polarized, i.e. first, we calculated the optical properties of NCD for an x-polarized case and then for a y-polaroid one. Furthermore, we have broken the symmetry of one of the NC to transform the dimer for the investigation of plasmonic responses and resonances.

Optical Properties of the NCD
In this work, we examine different configurations of the NCD structure, listed in Table 1.

Type I NCD/Effect of Polarization
We analyse the optical properties of the 'type I' dimer composed of two nano-crescents with dimensions; a 1 = a 2 = 100 nm, b 1 = b 2 = 60 nm, t = 35 nm with a separation S = 3 nm respectively shown in Table 1. Hot spots are formed in the regions where incident fields become concentrated and amplified. One more important parameter  Table 1 Various Nano-Crescent Dimer arrangements of a (NCD)

Type Geometry
Type I

Type II
Type III of a hot spot plane is the strong inhomogeneity of the incoming fields. We show that, in our case, the electrons are confined in two gap positions on the sharp edges of the nano-crescent dimer, and these charge electrons experience a plasmonic field created by external illumination. In this dimer case, the plasmon fields become non-uniform and greatly amplified at the edges due to the hot spot formation. Figure 2a depicts the extinction cross-section obtained for x-y polarized cases; red line represents x-polarized cases in which the light is efficiently coupled with the dimer. A small activation in the spectrum can be observed at 620 nm due to dark octupolar mode, along with a formation of Fano resonance, while a low energy peak at 688 nm appeared due to the mixing of dipolar modes with octupolar modes. The peak at 859.9 nm shows the mixing of dipole and quadrupole modes and at the tail of this peak at 970.7 nm represents low energy bonding modes. A strong peak can be observed at 1381 nm due to the hybridization of bright dipolar modes. The green line in the plot represents the y-polarized case in which a strong peak at 1155 nm can be seen and is of similar fashion as of x-polaroid version mainly due to combination bright dipolar modes that give birth to strong plasmonic effect, and we attained two hot spots along with high energy spike. Another small rise can be seen at 739.9 nm mainly occurred due to dark modes. Figure 2b-c depicts the formation of hot spots and near-field enhancement for x-y polarized cases respectively. It is visible that two hot spots were formed at the sharp edges of each NC, due to the confinement of huge electric field between these two Fig. 2 a Extinction cross-section of simple nano-crescent dimer (NCD) type I for x-y polarization, b near-field enhancement (NFE) of type I NCD for x-polarization luminance, c NFE of type I NCD for y-polarization luminance gap places. The near-field enhancement for x-polarized NCD was recorded to be 910, and for y-polaroid case, this value reaches to almost 310.

Effect of the Gap Variation 'S' on the Optical Properties of Type I NCD
Here, we have examined the effect of gap variation 'S' on Type I NCD while keeping all the other parameters fixed as described in 'Type I NCD/Effect of Polarization'. By giving variation in distance 'S', we have formed the same geometry as Type I and named the configurations as Type IS 3 for S = 3 nm, Type IS 5 for S = 5 nm, Type IS 7 for S = 7 nm, Type IS 9 for S = 9 nm, and Type IS 11 for S = 11 nm respectively. Figure 3a depicts the extinction plot which shows the dependence of the hybridized plasmonic modes on different values of the gap region 'S'. For Type IS 3 and Type IS 5 same type of mode, hybridization occurred which can be seen by red and bar-type lines. A small rise appeared at about 620 nm due to dark octupolar modes followed by three consecutive heights at 659.5 nm (Type IS 7 ), 656.1 nm (Type IS 9 ), and 662.9 nm (Type IS 11 ) represents the mixing of dipolar and quadrupolar modes; similarly, simultaneous peaks at 688 nm give the same nature. A narrow bonding dipole mode hybridization forms peaks for all separation cases at about 850 to 860 nm. At the vicinity of narrow and bright modes, a height appeared due to the mixing of quadrupolar modes at 970.7 nm, and at the same point, a Fano resonance can be noticed by a purple curve. Fig. 3 a Extinction cross-section of simple nano-crescent dimer (NCD) type I for gap variation along x-polarization; b-f near-field enhancement (NFE) of type I NCD for x-polarization luminance with separation between nano-crescents (NC) S = 3 nm, 5 nm, 7 nm, 9 nm, and 11 nm respectively A peak at 1234 nm for green and purple lines shows strong bright dipolar modes; a slight red shifting is found at 1296 nm, while the same peak is further red-shifted and peak formed at 1381 nm. Figure 3b-f depicts the formation of hot spots and NFE for S = 3-11 nm respectively. The NFE values for each case were recorded to be 910 (3 nm and 5 nm), 207, 191, and 188 (7 nm, 9 nm, and 11 nm) respectively. A drastic drop in NFE value happened by increasing the gap between NCs which shows that NFE is highly sensitive to gap variation and is inversely proportional to the distance between the nanostructures.

Effect of Symmetry Breaking on the Optical Properties of Type I NCD
Metallic nanostructures support plasmon resonances whose energies depend sensitively on the geometrical shape of the structure. This tunability has stimulated considerable experimental and theoretical research [36]. An important topic in the field of plasmonics is the effect of symmetry breaking. For nanostructures that are small compared to the wavelength of the incident light, only plasmons with finite dipole moments can be excited. For highly symmetric nanostructures such as a nanoshell, the symmetry breaking induced by a displacement Fig. 4 a Schematic of symmetry broken Type I NCD; b extinction cross-section of simple symmetry broken nano-crescent dimer (NCD) type I for x-polarization; c-g near-field enhancement (NFE) of symmetry broken type I NCD for x-polarization luminance of the dielectric core for the metallic shell renders higher-order multipolar modes dipole active and therefore visible in the optical spectrum of the particle [37]. We have now engineered the Type I NCD in such a way that size of one of the NC has been altered for various configurations. Here, we fixed the size of one NC as a 1 /b 1 /t = 100/60/35 nm, while the size of the other NC has been altered by different values to a 2 /b 2 /t = 90/50/30, 80/40/25, 70/30/20, 60/20/15, and 50/10/10 nm to form Type IA, Type IB, Type IC, Type ID, and Type IE respectively, while the separation between two NCs has been kept constant, i.e. S = 3 nm for each case (Fig. 4a). Figure 4b depicts the extinction spectra for symmetry broken cases. The peaks at 1234 nm, 1188 nm, 1155 nm (Type IA and Type IB), and 1145 nm (Type IC, Type ID, and Type IE) represent the hybridization of bright antibonding dipolar modes for each case respectively. The amplitude of the peaks drops sequentially according to the size decaying, and the smallest height is shown by the green line. The peaks at 800.2 nm, 820.8 nm, 815.6 nm, and 831.5 nm show the strong hybridization of quadrupolar modes with the same pattern of amplitude as their red-shifted versions except for Type IE since it depicts broad antibonding dipolar spectra at 805.3 nm and 853.8 nm. Mixing of octupolar modes occurs at 652.8 nm except for the blue line which occurred at 639.7 nm. Fano resonances appeared at different wavelengths on the spectrum mainly due to dark octupolar modes at 853.8 nm, 831.5 nm, 859.7 nm, 970.7 nm, 1009 nm, and 1025 nm. Figure 4c-g gives the charge distribution or NFE for Type (IA, IB, IC, ID, and IE) respectively. The NFE value for each case is 171, 116, 163, 129, and 114. Once again, close-sized NCD, i.e. Type IA, has the maximum value due to efficient coupling of light with the nano-structure which energized bright antibonding modes along with other higher-order modes. Similarly, the value decreases with decreasing the size of one of the NC except for Type IB mainly due to activation of dark modes. The NFE values at different wavelengths are listed in Table 2.

Type II NCD/Effect of Polarization
Next, we have engineered type II NCD by keeping all the parametric values the same as that of type I, instead we have rotated one crescent by an angle θ = 180° to form a type II NCD as shown in Table 1 for detailed examination of optical properties for x-y polarization cases, effects of gap variation, and symmetry breaking. The type II NCD is an ideal test bed for studying because it supports multipolar progression, light polarization, and dark plasmonic modes that can be spectrally tuned by a simple incident polarization axis. Type II offers the possibility for dark plasmon to spectrally overlap a bright mode which strengthens the plasmonic effect. Furthermore, the designed model provides both even (dark) and odd (bright) parity plasmons to be transformed to interact with each other. In Fig. 5a, we show the type II extinction plot along x-y position. The case contains contributions from all angular momentums that resulted in the generation of five peaks for x-polarized luminance and three peaks for y-polarized luminance. The peak formed at 1494 nm shown by blue line is highly redshifted and is mainly due to the negative parity of dipoles as clear from its height. However, peaks at 1199 nm and 865.4 nm are formed due to strong hybridization of positive parities luminous modes that strongly interacted with even dark modes and given birth to these high energy peaks. The peak at 680.6 nm is blue-shifted but still at a good height level compared to the peak that appeared at 1001 nm. Similarly, y-polarized case shows a red-shifted peak at 1381 nm appeared due to the mixing of dipolar modes of both nano-crescents. The other low heightened peaks raised at 921.6 nm and 699.3 nm respectively due to poor light luminance resulting in the mixing of dark quadrupolar modes. Figure 5b-c shows the near-field enhancements for x-y polarized cases respectively and it is visible that charge distribution for x-polarized cases is much better than y polarized cases. The charges are distributed in the gap positions and along the surface of each NC. Hot spots are formed at four different points, two at the tips while two in between gaps that produced huge plasmonic effect resulting in the NFE of about 238. Similarly, y-polarized case shows relatively less charge distribution due to weaker hybridization resulted in near-field enhancement of about 110.

Effect of the Gap Variation 'S' on the Optical Properties of Type II NCD
Here, we have introduced separation 'S' in the Type II NCD while keeping all the other parameters fixed as described in 'Type I NCD/Effect of Polarization'. By giving variation in distance 'S', we have formed the same geometry as Type II and named the configurations as Type IIS 3 for S = 3 nm, Type IIS 5 for S = 5 nm, Type IIS 7 for S = 7 nm, Type IIS 9 for S = 9 nm, and Type IIS 11 for S = 11 nm respectively. Figure 6a depicts the extinction spectrum for the Type II gap variation case in which Type IIS 3 and Type IIS 11 gives the identical spectral line shape; a small peak at 615 nm shows the dark octupolar modes, and at the tail, a Fano resonance appears, quadrupolar modes of both ellipses interact with each other, and a resonance originated at 680.6 nm followed by a dark octupolar height at 775.9 nm. Two high energy spikes appeared at 865.4 nm and 1199 nm due to the hybridization of dipolar bright modes. A quadrupolar interaction generates a resonance at 1001 nm, while a same nature peak red-shifted peak appears at 1494 nm. Type IIS 5

Type III NCD/Effect of Polarization
In this section, we investigate the optical properties of a type III NCD. The building parameters are kept constant as that of types I and II. Only an axial rotation of θ = 180° is given to one nano-crescent to form a Type III NCD as shown in Table 1. This configuration is the one in which the single tip side of each NC faces each other with identical dimensions forming a nanoantenna known as 'bowtie antenna'. The extinction spectra of such configuration for x-y polarization are shown in Fig. 8a. Blue line in the spectrum represents x-polaroid case for which the strong peak appears at 1059 nm due to efficient coupling of light with the bowtie antenna resulting in the activation of even parities which causes the hybridization of dipolar-dipolar bonding modes. After this, the lines go down and a small height appeared at 805.3 nm due to the antisymmetric alignment of the weak quadrupolar mode. Similarly, for y-polarization, the dark modes were not actively reacted with bright modes resulted in the low height red-shifted peak occurred at the same position as that of x-axis at the wavelength of 1059 nm but shows less energy. Another peak appears which represents dark modes at 739.9 nm. Furthermore, a little spike can be observed at 597.7 nm Fig. 7 a Schematic of symmetry broken Type II NCD; b extinction cross-section of simple symmetry broken nano-crescent dimer (NCD) type II for x-polarization; c-g near-field enhancement (NFE) of symmetry broken type II NCD for x-polarization luminance which also shows the poor interaction of light with the bowtie antenna compared to x-polaroid case. These peaks can be tuned by slight geometrical alteration or varying the gap and this is highly suitable for high sensitive biosensors [36]. Figure 8b-c corresponds to surface charge distributions, the surface charge distributions are strongly bonded with incident light polarization. A strong correlation between geometry and local field enhancement can be observed. The mapping clearly shows the fundamental dipole mode for each NC and hybridized dipolar mixing of charges. A strong coupling occurred when the light was incident from x-axis due to which a hot spot can be seen between the gaps of the two nanostructures. This point reveals the strong antibonding modes that led to growing the near-field enhancement value up to 492. Furthermore, y-polarized map shows poor coupling due to which bonding modes were formed and unable to activate dark modes. Also, the charge distributions were mainly formed on the sharp edges of the nano-structures and a clear hot spot was not formed between the gaps of the dimer. NFE for this case was found to be about 70 which is almost 7 times less than the x-polarized case. Fig. 8 a Extinction cross-section of simple nano-crescent dimer (NCD) type III for x-y polarization; b near-field enhancement (NFE) of NCD type III for x-polarization luminance; c near-field enhancement (NFE) of NCD type III for y-polarization luminance

Effect of the Gap Variation 'S' on the Optical Properties of Type III NCD
In this section, we have brought gap variation 'S' in the Type III NCD while fixing all the parameters same as of 'Type I NCD/Effect of Polarization'. By varying the separation 'S' a same Type III geometry has been formed and given names for simplicity such as Type IIIS 3 for S = 3 nm, Type IIIS 5 for S = 5 nm, Type IIIS 7 for S = 7 nm, Type IIIS 9 for S = 9 nm and Type IIIS 11 for S = 11 nm respectively. Figure 9a depicts the extinction spectrum for the Type III gap variation case in which all the cases show almost identical line shape. Since Type IIIS 3 has minimum separation so strong dipole-dipole hybridization forms a broad peak at 1059 nm and is more red-shifted compared to other peaks. Another peak appeared due to the mixture of broad dipolar and quadrupolar modes at 805.3 nm and is again a slight red-shifted compared to others and a Fano resonance can be noticed at 646.2 nm mainly due to dark octupolar modes. The same kind of hybridization occurred for other separation cases and peaks formed and almost similar fashion peaks appeared on the spectrum details given in Table 3. Figure 9b-f provides the formation of spots and near-field enhancement for gap variations. Once again NFE value shows a close relationship with the separation and the highest NFE value is obtained from Type IIIS 3 which goes up to 492 followed by a drastic drop in NFE value to 107 for Type IIIS 5 . Remaining NFE values are listed in Table 3.

Effect of Symmetry Breaking on the Optical Properties of Type III NCD
The parameters are set the same as 'Effect of Symmetry Breaking on the Optical Properties of Type II NCD' while the geometry followed is Type III, such configuration is shown in Fig. 10a. Figure 10b shows the extinction spectrum in which Type IIIA shows bright dipolar-dipolar mode mixing at 1145 nm and 956,2 nm while octupolar hybridization for the same case produced low energy spikes at 748.5 nm and 815.6 nm. Type IIIB and Type IIIC show the same nature of peaks while Type IIID shows octupolar mixing at 703.1 nm, quadrupolar hybridization at 790.3 nm, and 1059 nm with dipole-dipole mode formation at 949.1 nm. Type IIIE shows octupolar hybridization at 612 nm, 723.1 nm, 805.1 nm, and 1125 nm, while dipolar-dipolar mixing formed a peak at 935.1 nm. Similarly, Fig. 10c-g gives the formation of hot spots and NFE. The highest value of NFE is shown by Type IIIA since it has the minimum separation due to which effective plasmonic mixing occurred which resulted a high value. Further achieved NFE values are listed in Table 4.  Figure 11 gives a comparison for the best case among all the discussed cases above in terms of NFE and multiple wavelength spectrum ranges. Type I (x), i.e. when light was incident on the Type I NCD from x-direction so it produced a huge NFE value of about 910 which is the highest value recorded for this study, along with a vast range of operational wavelength with 4 peaks. Type II (x) produced a NFE value of about 238 but with the highest wavelength range with 5 peaks. Type III (x) generated a NFE value of about 492 but with only 2 peaks along the wavelength spectrum. Table 5 gives a comparison of the current study with other studies in terms of enhancement value, i.e. NFE for various applications. Fig. 9 a Extinction cross-section of simple nano-crescent dimer (NCD) type III for x-polarization; b-f near-field enhancement (NFE) of type III NCD for x-polarization luminance with separation between nano-crescents (NC) S = 3 nm, 5 nm, 7 nm, 9 nm, and 11 nm respectively  Type IIIS 3 Type IIIS 5 Type IIIS 7  Type IIIS 9 Type IIIS 11  Type IIIA  Type IIIB Type IIIC  Type IIID  Type IIIE   No. of  Peaks   2  3  2  2  2  2  2  3  3  3  3  4 Wavelength ( Fig. 10 a Schematic of symmetry broken Type III NCD b Extinction Cross-section of simple symmetry broken Nano-crescent dimer (NCD) type III for x-polarization, c-g Near Field Enhancement (NFE) of symmetry broken type III NCD for x-polarization luminance

Conclusion
We have investigated the generation of higher-order plasmonic modes in a simple crescent structure, its possible symmetry broken versions, and gap variations illuminated by a linear x-y polarized light. A simple Type I NCD uniform charge distribution along with the formation of hot spot was found with a good line shape of extinction spectra. The same parameters were applied to a Type II and III NCD structure which caused an efficient coupling of dark quadrupolar modes with bright dipolar modes creating multiple peaks in the spectrum. Then we performed symmetry breaking on Types I, II, and III respectively since the symmetry breaking technique is an ideal methodology for performance enhancement layout. This introduced numerous heights at different wavelengths due to the strong mixing of even and odd modes. We extended our findings to form symmetry breaking and gap variations with an axial rotation from θ = 0°-180° that produced variants of NCD configuration. This introduced multi-wavelengths starting from two peaks to seven peaks along with giant NFE for x-polaroid along with its variant cases. Moreover, Type I and its variants produced the highest NFE followed by Type III, while Type II and its sub-classes produced relatively small NFE values. To conclude, Types I, II, and III along with their variants are ideal candidates for producing pronounced tunable resonances and higher-order dark modes in the visible and near-infrared regions, which may be useful for several spaser applications like electromagnetic-induced transparency, slow light, switching, sensing, bio-medical, and SERS applications. Furthermore, such type of model may be realized using different synthesis methods based on atom by atom or molecule by molecule for the preparation of nanoparticles. Laser ablation, evaporation-condensation, dynamic light scattering (DLS), lithography, gamma radiation, etc. can be used to fabricate the model for several realistic and practical approaches.