Accessing dual toroidal modes in terahertz plasmonic metasurfaces through polarization-sensitive resonance hybridization

Plasmonic metasurfaces have been quite a fascinating framework to invoke transformation of incident electromagnetic waves for a while now. Oftentimes, the building block of these metasurfaces or the unit cells consists of two or more meta-resonators. As a consequence, near-field coupling amongst these constituents may occur depending upon the spatial and spectral separation of the individual elements (meta-resonators). In such coupled structures resonance mode-hybridization can help in explaining the formation and energy re-distribution among the resonance modes. However, the coupling of these plasmonic modes is extremely sensitive to the polarization of the incident probe beam and offers ample amount of scope to harness newer physics. A qualitative understanding of the same can be attained through mode-hybridization phenomena. In this context, here, we have proposed a multi-element metastructure unit cell consisting of split ring and dipole resonators aiming to explore the intricate effects of the polarization dependency of these hybridized modes. Multi-resonator systems with varied inter-resonator spacing (sp = 3.0, 5.0, and 7.0 μm) are fabricated and characterized in the terahertz domain, showing a decrement in the frequency detuning (δ) by 30% (approx.) for a particular polarization orientation of THz probe beam. However, no such detuning is observed for the other orthogonal polarization configuration. Therefore, modulation of the resonance-hybridization is strongly dependent on the terahertz beam polarization. Further, as an outcome of the strong near-field coupling, the emergence of dual toroidal modes is observed. Excitation of toroidal modes demands thoughtful mode engineering to amplify the response of these otherwise feeble modes. Such modes are capable of strongly confining electromagnetic fields due to higher Quality (Q-) factor. Our experimental studies have shown significant signature of the presence of these modes in the Terahertz (THz) domain, backed up by rigorous numerical investigations along with multipole analysis. The calculated multipole decomposition demonstrates stronger scattering amplitude enhancements (∼7 times) at both the toroidal modes compared to off-resonant values. Such dual toroidal resonances are capable of superior field confinements as compared to single toroidal mode, and therefore, can potentially serve as an ideal testbed in developing next-generation multi-mode bio-sensors as well as realization of high Q-factor lasing cavities, electromagnetically induced transparency, non-radiating anapole modes, novel ultrafast switching, and several other applications.


Introduction
Over the past several decades, Linear Combination of Atomic Orbitals or popularly known as molecular orbital theory has been at the helm of myriad of theoretical formulations, spanning from the formations of band structures to the allocations of energy eigenstates in numerous atomic, and molecular as well as quantum systems [1,2]. The molecular orbital theory formalism is an essential tool in analysing optically interacting systems, particularly in the field of plasmonics, that deals with the collective oscillation of electrons and the associated electric fields at a metal-dielectric interface. Molecular orbital theory or its plasmonic analogue mode-hybridization has garnered tremendous attention in the recent past owing to its capability to explain plasmonic response of complex geometries in terms of elementary plasmonic eigenmodes (or resonance modes) [3,4]. The approach was first proposed by Prodan et.al, their work illustrated how the complex plasmonic resonances can be comprehended in terms of the interaction (hybridization) of elementary plasmonic eigenmodes, therefore, opening up new avenues in resonance mode engineering in optics and photonics. In this context, plasmonic metamaterials and their two-dimensional counterpart metasurfaces that are strategically designed artificial periodic structures operating in the subwavelength regime have shown impeccable capability to confine, manipulate and shape electromagnetic radiations [5,6]. Such an unparalleled control of electromagnetic fields can be achieved by simply tuning the geometry of the unit cells or meta-atoms that constructs the periodic array of the meta-device [7,8], The emergence and evolution of metamaterials have gained momentum after the seminal works on realizing negative index media came into picture about two decades ago [9,10]. Since then, these artificial photonic structures have seen a wide range of applications in realizing Electromagnetically induced transparency (EIT) [11,12], near-unity absorbers [13,14], sensors [15][16][17][18], switches [19][20][21][22], as well as extensive implementations in the studies pertaining to nonlinear-optics [23,24], quantum computation [25], observation of exceptional points [26], extraordinary transmissions [27,28] and many more. In this regard, we would like to mention that EIT-like effects are analogous to quantum-interference effects that lead to the observation of high transmission window in an otherwise absorptive background. Our outcomes as reported in this work, may showcase EIT-like traits at the transmission peak, however, as discussed later, we have chosen to highlight the exclusive emergence of dual-toroidal modes at the resonance dips owing to the intriguing physics they offer. Note that, metastructures manifest their response in terms of resonance modes that are influenced by the cumulative response of the elementary units involved, therefore, resulting in an array of meta-unit cells behaving similar to an artificially formed effective medium, with modified permittivity and permeability values that are determined by the geometry of the system [29]. Hence, with respect to tuning the resonant response as per the different application requirements, resonance mode-engineering is required in meta-geometries that often involve the coexistence of two or more resonating elements within the metamaterial unit cell. The modulation in the resonance amplitude as well as the frequency of a complex meta-molecule can be interpreted in terms of simple plasmonic 'bright' and 'dark' resonators. A plasmonic 'bright' resonator is capable of directly coupling to incident electromagnetic radiation, therefore, prone to radiative losses and manifests lower quality (Q)-factor, on the contrary, a 'dark' resonator cannot be directly excited upon the incidence of the radiation field, these resonators can only be excited through a suitable coupling mechanism with the 'bright' mode [5,30]. The complex interaction of the electromagnetic exchange among these constituent metastructures can be understood in terms of the plasmonic mode-hybridization [3]. Depending on the geometrical complexity and the resonance frequency of the constituent meta-elements involved that comprises the unit cell, the near-field effects can alter the spectral response of the system through 'conductive' [31] 'magnetic' [32] or 'electric' [33,34] interactions, resulting in a split in the spectrum. In this context, the mode-hybridization formalism simplifies the coupling mechanism in terms of the fundamental resonant eigenstates of the constituent meta-atoms, therefore, breaking the problem down in terms of symmetric (bonding) and antisymmetric (anti-bonding) elementary states [3]. A visualization of the energy distribution thus can be attained. It is worthwhile to mention that, although near-field coupling in metasurfaces has been exploited earlier across the electromagnetic spectrum, the role of electric field polarization on mode-hybridization mechanism still presents ample amount of newer physics to be explored. Generally, the elementary resonant modes ('bright' and 'dark' eigenmodes) are extremely sensitive to the polarization orientation of the incident electromagnetic radiation. In addition, certain geometries can drive near-field coupling and act as 'bright' resonators irrespective of the polarization orientation, hence, switching on the 'dark' mode, that is largely sensitive to polarization. Further, the interplay of these bright-dark modes in turn can result in the emergence of 'toroidal-dipolar modes' [35,36]. Contrary to the conventional electric and magnetic multipoles obtained through the Taylor series expansion of the electromagnetic potentials, toroidal multipoles are derived from the temporal derivative of these potentials and fields. These electromagnetic modes are visualized in terms of currents being distributed on the surface of a torus, namely toroidal currents flowing along the median of the toroid, where an array of magnetic dipoles originating from these poloidal surface currents are arranged in head-to toe configuration [37]. Owing to the low radiative cross-section, these modes are capable of attaining superior quality (Q-factor) resonant response, hence can pave the way for loss-less 'anapole' moments too. The extreme light-matter interaction achievable through toroidal/anapole configurations can lead to the realization of low-loss high Q-factor laser cavities, EIT, polarization conversion, and applications in quantum computing as well as in nonlinear optics [38][39][40][41][42]. However, toroidal modes have relatively low scattering cross-section compared to other conventional multipoles. Due to the weak coupling to the free space, the excitation and detection of these charge-current distributions are extremely challenging. Over the years only a handful of naturally occurring systems has showcased the existence of these modes [43][44][45]. However, this trade-off can be taken care of by implementing strategically designed metasurfaces, that have shown capability to suppress the conventional multipoles while strengthening the toroidal contribution multi-folds across the electromagnetic regimes [46,47]. Interestingly, the excitation of a dual-toroidal mode has also been observed in recent years, that enables tight-confinement of the time-varying magnetic fields over a larger stretch of area, resulting in superior sensing capabilities, particularly, in the sensing of biological structures that are of lower molecular weight (viruses and protein molecules) [48,49]. While a major share of the dual toroidal metamaterials is studied in stacked multilayer configuration, the design and fabrication of the same invite complexity and enhanced precision [50,51]. Therefore, a planar metasurface that demonstrates dual toroidal nature of the resonances simplifies the design algorithm and reduces the fabrication complexities. In this context, plasmonic mode-hybridization orchestrates the interactions between the constituent resonators and modulates the toroidal response. Further, the spatial asymmetry of the meta-unit cells can allow manipulation of the hybridization schemes by means of controlling the polarization orientation of the incident radiation. This in turn enables enhanced control over the polarization-dependent excitation of the toroidal modes. Therefore, with the intention to harness the toroidal modes, we propose a strategically designed planar metastructure layout in the terahertz (THz) regime that showcases polarization-dependent near-field coupling with the incident probe beam. The low-energy THz fields lie within the spectral range of energy associated with intermolecular vibrations in several complex molecules. Additionally, THz radiation widely serves as the ideal testbed for next-generation 6G communication [52], imaging [53] as well as other non-invasive biomedical applications [54]. Therefore, taking into consideration the wide range of applications the THz radiation caters to, we choose to design our structures in this regime. We observe emergence of strong mode-hybridization in the spectral response of the proposed system that is sensitive to the polarization of the incident THz probe beam. Thus, polarization-induced modulation of the spectra is expected in a mode-hybridized metastructure geometry. The proposed design further supports excitation of dual toroidal dipolar resonances courtesy of the strong mode-hybridization among the constituent meta elements. The outcomes of this study are capable of finding potential applications in realizing multiband applications requiring extreme field confinement, as well as in switching and sensing measurements.

Design optimization
We begin with the fabrication of a set of five samples as illustrated in figures 1(a)-(e). The samples M1 and M2 as depicted in figures 1(a) and (b) serve as the intrinsic (uncoupled) meta-atoms that eventually construct the coupled metasurfaces as we will discuss later on. Structure M1 is the conventional single split-ring resonator structure well known for the excitation of fundamental inductive (L)-capacitive (C) LC resonance modes [55], while M2 resembles the metallic cut-wire meta-geometry known to excite dipolar modes as their fundamental response [56]. With the intention to avoid any inter-unit cell coupling, the periodicities of the structures are taken as 152 µm and 122 µm along the X-and the Y-directions, respectively. Taking the optimum response of the THz characterization system into consideration, the intrinsic meta-atom designs are optimized to have their fundamental resonance frequency at around ω x M1 = ω y M2 = 0.5 THz (the letter in the superscript x or y denotes the polarization state, and the numeral indicates the resonance mode order as discussed later, while the subscript denotes the sample under observation). The detailed dimensions of structures M1 and M2 post-optimizations are mentioned in the caption beneath figure 1. Once the dimensions of the intrinsic structures are finalized, the design for the coupled structures is initiated that consists of two split ring resonators on both sides of the central cut-wire structure with varied inter-resonator spacing of sp = 3, 5, and 7 µm, respectively for samples M3, M4, and M5, as illustrated in figures 1(c)-(e), respectively. Figure 1(f) shows the layout of the periodic nature of the metasurface with the individual meta-unit cells serving as building blocks. The samples are fabricated atop a 0.4 µm (approximately) highly resistive intrinsic silicon wafer by means of optical photolithography technique in a clean-room environment. A thin layer of positive photoresist is spin-coated on top of the silicon wafer. For generating the desired patterns on the silicon substrate, the well-established maskless lithography approach [57] is adopted. In this regard, the desired design file is fed to the system followed by the direct laser-assisted writing process that transfers the patterns on the photoresist by means of polymerization. The radiation-exposed wafers are then developed followed by electron beam deposition method that coats the patterned substrates with a 200 nm thick layer of aluminium. A lift-off process is performed to remove the remainder of the photoresist and to pattern the metallic features atop the substrate. The fabricated samples are inspected under an optical microscope as depicted in figure 1. A table-top THz-Time Domain Spectroscopy (THz-TDS) setup, based on the 8f -confocal geometry is deployed to characterize the electromagnetic response of the samples as depicted in figure 1(g) [58]. Normal incidence of the THz beam was maintained throughout the measurement, while spectra is collected for both the orthogonal polarizations (X-and Y-polarizations). The confocal geometry of the TDS study consists of two pairs of parabolic mirrors for beam steering, that are optically aligned to excite the sample placed at the focus and collect the associated transmission spectra. THz generation and detection schemes involve an ultrafast femtosecond laser operating at 1560 nm with a repetition rate of 100 MHz and having a delay of 60 fs. The setup is arranged in a pump-probe configuration where InGaAs/InP based photoconductive antennas (PCAs) [59] are used for generation and detection purposes. The fs laser beam line is split into two parts, one part is incident on a dc biased PCA where the laser induced transient current generates THz wave packets. The generated THz beam is then steered using parabolic mirrors and falls normally on the sample under study. The transmitted THz signal through the sample is then detected using the other half of the fs laser beam that is incident on the receiver antenna. The spot size of the THz beam is optimized at around 3 mm, hence, edge diffraction effects could be minimized. A metasurface array of 1 cm × 1 cm is fabricated that maintains homogeneous excitation of the device. The obtained time-domain data is converted to frequency domain by deploying the Fast Fourier Transform (FFT) algorithm [60]. The observed transmission amplitude is normalized with respect to the silicon substrate, identical to the one used as substrate in metasurface fabrication. Thus, the normalized transmissions are represented as T(ω) = |t sample (ω)/t ref (ω)|, wheret sample (ω) andt ref (ω) are the transmission amplitudes through the metasurface samples and reference respectively.

Experimental and numerical outcomes
With the intention to observe the excitation of toroidal modes in the coupled metastructure geometry, we propose a metasurface layout based on the outcomes discussed in figures 1(h) and (i). The proposed unit cell geometry consists of a cut-wire metastructure placed at the centre of the unit cell, alongside two SRRs, one on each side of the central cut-wire, maintained at specific inter-resonator separations of sp = 3, 5, and 7 µm (M3, M4, M5) shown in figures 1(c)-(e), respectively. Conventional THz-TDS studies are performed on these tri-resonator metasurfaces to comprehend the nature of electromagnetic field modulations these metasurfaces are capable of. The outcomes of the same are illustrated in figure 2, here, column I depicts the case where the polarization of the incident THz field is taken along the length of the split gap of the SRRs (X-direction), whereas, column II depicts the transmission response when the THz field polarization is considered along the length of the central cut-wire (Y-direction). For the case of sample M3 having an inter-resonator spacing sp = 5 µm, a single resonance dip at ω x M3 = 0.5 THz as the polarization of the normal excitation beam was aligned along the X-direction. A similar trend is observed in the case of samples M4 and M5 under identical excitation conditions, both having their resonance modes at ω x M4 = ω x M5 = 0.50 THz as depicted in figure 2 column I (b) and (c), respectively. It is worthwhile to note that, the experimentally observed and the simulated responses tend to show a mismatch to a certain degree, in particular, the simulated linewidths appear sharper as compared to the experimentally observed spectra. This discrepancy can be attributed to the governing factors as follows. Firstly, the fabrication process involves a finite degree of uncertainty that stems from resolution of the lithography setup and/or minute variations in the ambient conditions during the sample fabrication. This results in minimal alterations in the geometrical parameters of the fabricated samples as compared to the numerically modelled geometry. Secondly, these samples are fabricated atop a 0.4 µm thick silicon wafer. The finite thickness of the substrate contributes to the noise as the incident THz pulse suffers a reflection off the surface (etalon). Hence, the presence of these etalon pulses results in spurious oscillations in the transmission spectra and somewhat suppresses the true response of the meta-resonators. Therefore, in order to overcome this ambiguity, the conventional practices allow one to truncate or trim the transmitted THz pulses and discard the etalon effects followed by the conventional FFT process to visualize the response in the frequency domain. However, there is trade-off associated with this process, as the transmitted pulse is truncated right before the emergence of the etalon features, a finite amount of information is lost in the due process, that results in the variation in linewidths of the experimentally observed outcomes as compared to the simulated response, that predominantly depends on the boundary conditions and mesh resolution chosen, and therefore, churning out the electromagnetic response under ideal circumstances. It is to be noted here that, for the polarization of the incident beam oriented along the X-direction, both the side SRRs serve as plasmonic 'bright' resonators, whereas, the central cut-wire acts identical to a 'dark' resonator, hence, the SRRs are able to directly couple to the incident field and oscillate at their characteristic resonance frequency of 0.5 THz. Whereas, due to the topology of the  2(f)). It is understood that, as the polarization orientation of the excitation beam is switched to the Y-direction (orthogonal polarization), the central cut-wire acts as a plasmonic 'bright' mode, whereas, the side SRRs emulate the behaviour of a 'dark' mode, and therefore, are unable to directly couple to the incident THz field and completely rely on the central cut-wire that drives the LC-resonance on the side SRRs as we shall discuss later. Hence, the splits in the spectral pattern appear courtesy of the near-field coupling between the cut-wire and SRR systems.
Further, as the inter-resonator separation is increased, the frequency detuning defined as δ = ω hf − ω lf , where, ω hf is the higher-order mode while ω lf is the lower-order mode, tends to decrease which suggests relative weakening of the near-field coupling as the value of sp is increased. The observed values for frequency detuning (δ) are 0.10, 0.09, and 0.07 THz for M3, M4, and M5, respectively. Hence, a cumulative shift of 30% in the detuning has been noted for the fabricated devices. To validate the near-field coupling induced mode-hybridization for the Y-polarized case, we have performed extended numerical studies that show the simulated response of the tri-resonator system as the inter-resonator separation is varied through 0.1-8 µm. As illustrated in figure 3, the colour-mapping depicts the transmission intensity contrast, while the x(y) axes denote the frequency (inter-resonator separation). It is observed that, as the inter-resonator separation is increased, the frequency detuning decreases as illustrated by the dashed white lines in figure 3, validating the emergence of cut-wire driven near-field coupling in the metastructure. The numerically simulated transmission spectra agree well with the recorded experimental outcomes. That boosts our confidence in order to further probe the nature of coupling as well as to construct a robust understanding of mode-hybridization in this metasurface layout. Therefore, we perform further numerical studies on the tri-resonator system discussed below.
Taking the redundant nature of the spectral response of the tri-resonator systems, we restrict our intriguing studies to metasurface M3. Further numerical analysis involves explicit calculation of the surface currents and corresponding induced electric fields (figures 4(a) and (b)) at the single resonance dip observed at around ω x M3 = 0.50 THz depicted in figure 2(a). As the X-polarized radiation is incident on the tri-resonator metasurface, the constituent side SRRs emerge as plasmonic 'bright' resonators, hence, are able to excite strong fundamental LC mode resonance at ω x M3 = 0.50 THz, whereas the central cut-wire imitates the behaviour of a plasmonic 'dark' resonator. Hence, the side SRRs attempt to set up resonant modes on the cut-wire. However, it is noted that both the SRRs resonate in phase because of the nature of incident electric field polarization ( figure 4(a)). Therefore, the inductive coupling induced by the left SRR nullifies the same imposed by the right SRR at the central dipole resonator, a direct consequence of the elementary Maxwell's equations. Therefore, the dipole resonator cannot be excited while the side SRRs solely contribute to the resonance response, and hence, we observe no split in the transmission spectrum but only the fundamental resonance of the side SRRs. The induced electric field distributions illustrated as a color plot in figure 4(b) reaffirm our assertions as there is little to no effective coupling between the constituent metastructures at the resonance frequency.
As the polarization orientation of the excitation probe is taken along the Y-direction, a clear split is recorded in the transmission response as illustrated in figure 5. Columns I and II illustrate the surface current and induced electric field distribution for the lower and higher-order mode corresponding to the spectrum illustrated in figure 2(d) at the two distinct resonance splits, namely ω y1 M3 = 0.45 THz and ω y2 M3 = 0.56 THz, respectively. For the low-frequency mode, the central cut-wire plays the role of a plasmonic 'bright' resonator, and hence, is able to couple to incident THz field, while the side SRRs are 'dark' under the current probe beam polarization configuration. As the fundamental eigenfrequencies of the cut-wire and the SRR are designed to coincide at 0.50 THz (figure 1), the central cut-wire essentially drives the circular surface currents in the side SRRs through the magneto-inductive coupling between the individual resonators. The induced magnetic field on the central cut-wire is strongest at the middle region of the structure, therefore, initiates the inductive near-field coupling between the SRRs and cut-wires. For the low-frequency mode, the induced current on the left SRR is anti-clock-wise, while that on the right SRR is clockwise, the current on the central cut-wire runs from bottom to top as depicted in figure 5(a). In the case of higher-frequency mode, the current directions on the side SRRs are simply reversed ( figure 5(c)). The corresponding electric field distributions are shown in figures 5(b) and (d), respectively. It is noteworthy to mention that at the lower-frequency mode, the circular currents that are being set up by the central cut-wire give rise to oppositely oriented magnetic moments. These moments in turn contribute to the emergence of a toroidal mode oriented along the direction of the electric dipole on the central cut-wire. Therefore, the toroidal mode and the electric dipolar mode are in phase at the lower-frequency mode. However, the modes are inversely oriented in the case of higher-order resonance mode. The former scenario can be termed as symmetric mode, while the latter can be considered as anti-symmetric in nature. Hence, the emergence of the resonance split could be explained in terms of magneto-inductive mode hybridization as illustrated in figure 5(e). The fundamental eigenstates of both the SRR and the cut-wire are tuned to 0.5 THz. As the unit cell of the tri-resonator system is formed, owing to the proximity of the resonators, both spatially as well as spectrally, the emergence of the split in the spectra can be explained in terms of interaction between these eigenstates, leading to an energy redistribution by means of plasmonic near-field coupling. The lower-order mode in this regard can be regarded as the 'bonding' mode due to the symmetric current distributions, while the higher-order 'anti-symmetric' mode signifies anti-symmetric current distributions, resulting in finite frequency detuning δ = 0.10 THz.
A detailed introspection of the induced current density reveals the excitation of oppositely oriented magnetic dipoles on the side SRRs as the polarization of the incident probe beam is considered along the Y-direction ( figure 6). At the lower-frequency mode ω y1 M3 = 0.46 THz, the induced magnetic moment on the left SRR points upwards (positive Z direction), while the magnetic moment induced on the right SRR points downwards (negative Z direction), therefore, from the definition of toroidal moments, we observe that the pair of counter-rotating magnetic moments contribute to the emergence of a toroidal dipole moment that is aligned along the negative Y-direction as illustrated in figure 6(a). For the higher-order mode ω y2 M3 = 0.56 THz, the mode hybridization aids in the formation of another toroidal dipolar mode with the current distributions-oriented opposite to that of the lower-order toroidal mode i.e. along the positive Y-direction as shown in figure 6(b). To establish our claims of the existence of the dual toroidal modes on firm grounds, we have further performed multipole analysis on the system as shown in figure 5(e). The electric dipole is calculated using the expression P α = 1 (iω) ∫ d 3 rJ α , while the toroidal modes are evaluated where, J is the current density associated with the induced electric field and the index α = x, y, z. In the case of CST studio, Maxwell's equations cannot be solved inside the metallic domain. Therefore, the Al resonators are modelled as dielectrics with permittivity value of ε = −1800 . Here, the scattering contributions of the individual multipoles involved are calculated over the stretch of the frequency range of interest (0.35 THz-0.65 THz). The corresponding multipolar contributions to the spectra are calculated as per the methodology proposed by Terekhov et al [61]. The calculated scattering cross sections show pronounced responses at frequencies that coincide with the mode-hybridized doublet resonance modes of the meta-system at ω y1 M3 = 0.46 THz and ω y2 M3 = 0.56 THz, respectively arising due to the mode-hybridized doublet formation. Therefore, the meta-atom showcases the emergence of dual toroidal dipole moment (involving SRRs) alongside the electric dipolar (due to central electric dipole resonator) contributions arising from the near-field coupled system.

Conclusions
To conclude, we have demonstrated a metasurface layout that probes the polarization dependence of the near-field coupling among the constituent resonating elements, and has formulated a qualitative understanding for the same based on plasmonic mode-hybridization schemes. We consider a tri-resonator system consisting of a cut-wire at the centre of the unit cell accompanied by two SRRs on both sides, maintaining symmetric and specified inter-resonator separation. These constituent meta-elements are optimized to have their eigenfrequencies at around 0.50 THz. The metasurfaces are further experimentally characterized, while extensive numerical studies are performed to reaffirm the experimental outcomes. In the case of X-polarized THz excitation beam, a solitary resonance dip at around 0.50 THz is observed, this can be attributed to the mutual cancellation of the inductive coupling induced by the left and right SRRs (bright mode) on the central cut-wire (dark mode), a direct consequence of elementary Maxwell's equations. Eventually, the nature of the spectral response remains unaltered as the inter-resonator separation is varied symmetrically. However, a prominent resonance frequency split resulting in the formulation of a resonance duplet is observed for the Y-polarized probe beam, with a gradual decrement in detuning by 30% (approx.) as the inter-resonator separation is increased from 3 µm to 7 µm. Effectively, the near-field coupling is initiated by the cut-wire meta element that acts as a 'bright' resonator, while the SRRs serve as 'dark' modes. Such a modulation of the spectra and the formation of the duplet could be qualitatively explained in terms of plasmonic mode-hybridizations between the constituent eigenmodes of the cut-wire and the SRRs. The formalism discusses the emergence of symmetric and anti-symmetric modes that manifest as the duplet based on the charge and surface current distributions. Therefore, it is evident that polarization plays the crucial role in manipulating near-field coupling in a strategically designed multi-resonator metasystem which is mainly driven by the near-field magnetic interactions. We further observe that, the metasurface geometry favours the simultaneous excitation of dual-toroidal modes. A pair of counter-oriented toroidal moments appear along the Y-direction as depicted by the extensive numerical analysis and current density distributions. Further, we have analytically performed multipolar decomposition of the spectrum that supports the coexistence of the dual modes, toroidal and dipolar modes. The emergence of dual-toroidal mode distribution holds tremendous potential in realizing enhanced field confinements over multiple frequency bands as compared to typical single-toroidal resonances. Particularly in the field of bio-sensing, dual toroidal modes may emerge as a promising candidate in the detection and characterization of proteins and viruses simultaneously at different frequency bands. Further, the extreme field confinement can pave the way towards the realization of non-radiating anapoles alongside applications in lasing cavities, EIT as well as ultrafast sensing and switching.