Open Access
Add to BibSonomy
Abstract
We propose a new method for fabricating hybrid metasurfaces by combining Mie and plasmonic resonances. Our approach involves obtaining an ultrasmooth gold film and separately structuring monocrystalline silicon (c-Si) nanoantenna arrays, which are then wet-transferred and finally immobilized onto the gold film. The experimental and simulation analysis reveals the importance of the native oxide layer of Si and demonstrates fascinating dispersion curves with nanogap resonances and bound states in the continuum. The localized field enhancements in the nanogap cavities result from the coupling between multipolar Mie resonances and their mirror images in the gold film. This effective method improves our understanding of hybrid modes and offers opportunities for developing active metasurfaces, such as depositing c-Si nanoantenna arrays onto stretchable polydimethylsiloxane substrates or electro-optic and piezoelectric sensitive lithium niobate films for potential applications in MEMS, LiDAR, and beyond.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nanophotonics research focuses on manipulating light at the nanoscale to achieve strong interactions between light and matter [1]. One approach is the study of nanogap cavities, which have been extensively investigated due to their strong near-fields [2–8]. These cavities can be made using full-metal or all-dielectric materials. However, metallic structures suffer from limited quality ($Q$) factors due to ohmic loss, while dielectrics offer lower losses and support electric and magnetic Mie resonances. Hybrid nanostructures that combine plasmonic and Mie resonances have been proposed to leverage their unique advantages [9]. Monocrystalline silicon (c-Si) is the preferred dielectric material for constructing these hybrid architectures in the visible and near-infrared bands, due to its low absorption coefficients ($k$), high refractive index ($n$), and CMOS-compatible fabrication process [10–14]. Si nanospheres and nanopillars on metallic films have been shown to enhance Purcell factors, emission directionality, and strong coupling for quantum emitters [15–21]. Si nanoparticles can generate non-radiative modes such as bound states in the continuum (BICs) and anapole modes, resulting in significant near-field enhancements [22–24]. Si nanoparticle arrays (NPAs) on plasmonic mirrors have applications in various fields such as optoelectronics [25], nonlinear optics [22], white-light generation [26], and color displays [27].
The current method for fabricating Si NPAs on metallic mirrors involves a bottom-up coating process [22,26,27], but it faces challenges. The nanoscale roughness of the metal surface hinders the propagation of surface plasmon polaritons (SPPs), and achieving atomic-scale smoothness is difficult [28]. High absorption losses are common especially in the visible and near-infrared band when using amorphous or polycrystalline Si [22,27], and the complexity of depositing c-Si often leads to the use of silicon-on-insulator (SOI) substrates, which poses challenges in separating the SOI wafer substrates [26,29,30]. Additionally, a dielectric layer is required between Si and the metal during dry etching to protect the metal [26,27]. However, this layer increases the mode volume of nanogap resonances and weakens their confinement ability. Another approach involves etching Si nanoparticles on a quartz substrate and transferring them wet onto the metal, but it produces only single nanoparticles, and dispersion occurs during wet transfer [19].
Here, we propose a new method that involves obtaining an ultrasmooth gold film and structuring c-Si NPAs separately. The NPAs are then wet-transferred and self-bonded onto the gold film, precisely determining their location relative to plasmonic mirrors. The sample is analyzed using transmission electron microscopy (TEM), revealing the importance of the native oxide layer of Si in correcting the difference between measured and simulated reflection spectra. Microscope back-focal-plane (BFP) imaging shows a fascinating "emoji" pattern in the dispersion curves for transverse magnetic (TM) polarization modes, with flat bands (FBs) representing nanogap resonances separated by BICs and a parabolic-dispersive arc. Interestingly, this pattern changes to a single FB for transverse-electric (TE) polarization modes. Near-field calculations demonstrate that these FBs are localized in the nanogap cavities due to the coupling between multipolar Mie resonances and their mirror images in the gold film. These findings offer a practical approach to fabricating hybrid metasurfaces and improve our comprehension of hybrid modes, thereby opening up many possibilities for developing active optoelectronic devices [31].
2. Constructing the hybrid nanogap arrays
To overcome the challenges of current techniques, we have combined the "floating off" method with the "template stripping" method [28], to prepare our sample (see Fig. 1(a)). The c-Si NPAs were first patterned by electron beam lithography and reactive-ion etching (IRE) on a commercial SOI substrate from ShinEtsu Inc., with the device layer having a thickness of 100 nm. We then spin-coated a resist polymer of polymethyl methacrylate (PMMA) onto the c-Si NPAs. After fully etching the oxide layer of the SOI with hydrofluoric acid (HF), we transferred the sample to deionized water where the PMMA layer floated off due to aqua tension in solution. To produce the substrate for our hybrid metasurface, we template stripped a 160 nm thick gold film on a quartz substrate with an ultrasmooth surface (Ra = 0.57 nm), which enables us to fish the PMMA slab floating on the water’s surface. We left the PMMA slab on the gold film sample to air-dry and later baked it on a hotplate to fix the c-Si NPAs on the gold film using surface dangling bonds. Finally, we lifted off the PMMA with acetone to obtain the c-Si NPAs on an Au film sample.
Fig. 1. Sample fabrication and the measurement setup. (a) Schematic of the fabrication processes. (b) Schematic of the sample and incidence plane for measurements. $\theta$ is the incidence angle of in-plane waves. $k_{i}$ and $k_{r}$ define the incidence and reflection wave vectors, respectively. $E_{TE}$ and $E_{TM}$ represent different polarizations. (c) Bright-field microscopic image of two c-Si nanoparticle arrays with different periods on a gold mirror. (d) Schematic of the Fourier setup. (e) Scanning electron microscope image of the sample from the side-view. Scale bars: 16 $\mu$m (c) and 160 nm (e).
The hybrid metasurface is made up of c-Si NPAs and a 160 nm gold mirror arranged in a square lattice (Fig. 1(b)). The height of c-Si cylinders is 90 nm. When observed from the side using a scanning electron microscope (SEM), it is clear that the c-Si nanoparticles are firmly attached to the gold film through a combination of wet transfer and surface adsorption methods (see Fig. 1(e)). By adjusting the period ($P$) and diameter ($D$) of the lattice and nanoparticles respectively, the sample can display various structural colors, as seen in the bright-field microscope image shown in Fig. 1(c).
To obtain a comprehensive understanding of the color change, we utilized a Fourier-imaging setup to analyze the momentum-resolved reflection spectra of the samples (see Fig. 1(d)). The setup works by reflecting a broadband beam using a beam splitter (BS), which is then focused onto the sample via a microscope objective (OL, Nikon CFI Plan Fluor 100x, NA=0.9). The reflected light is then collected by the same OL before passing through a series of lenses including the Fourier lens (FL), delayed lens (DL), and tube lens (TL). The DL serves to switch the Fourier images to bright-field microscope images of the sample. Additionally, a linear polarizer (LP) positioned between the FL and DL is used to analyze different polarization modes. The front-focal-plane of the FL coincides with the BFP, i.e., the Fourier plane, of the OL so that the FL copies the BFP image of the OL to infinity. At the end of the setup, the TL focuses the Fourier images onto the slit of the spectrometer (Andor Shemrock 500i) coupled with the EMCCD (Andor iXon Ultra 888). During the measurement, the optical beam’s incidence plane is perpendicular to the metasurface and parallel to the lattice’s axis. To define the TM and TE modes of the reflection beam, the axis of the lattice is rotated parallel or vertical to the spectrometer’s slit. We recorded the reflectance ($R$) of the sample referenced to a calibrated silver mirror. Due to the presence of a 160 nm thick gold film, the transmittance ($T$) was ignored, and the absorption ($A$) of the sample was calculated as $1-R$.
Initially, we only consider the reflection spectra for normal incidence, where the incident angle ($\theta$) of the plane waves is 0. Experimental measurements exhibit a reflection dip in the wavelength range from 700 to 800 nm, which is marked on Fig. 2(b) by the gray-dashed curve. Using the finite-difference time-domain method (FDTD), we conduct a simulation to assign the electromagnetic properties of this dip resonance and calibrate the geometry parameters for our sample. In this simulation, we utilize in-plane wave sources and establish periodic boundary conditions in the in-plane directions ($x$- or $y$-axis), as well as introduce perfectly matched layer (PML) boundaries in the out-of-plane direction ($z$-axis). The $n$ and $k$ of c-Si and gold were determined using ellipsometry, as shown in Figures S1b and c. At the onset, we disregard the native oxide layer that contains the c-Si nanoparticles. Consequently, the simulated outcome (represented by the blue-solid curve in Fig. 2(b)) is significantly different from the experimental results, exhibiting a shallow dip with a slight blueshift at the short wavelength band, and a complete absorption resonance between 800 and 900 nm. Upon adding a 3 nm oxide layer ($n$ = 1.45) to the surface of the c-Si nanoparticles, we observe a significant blueshift of approximately 126 nm for the zero reflection dip. This outcome shows excellent agreement with the experimental results (black-solid curve in Fig. 2(b)).
Fig. 2. Hybrid nanocavity. (a) Transmission electron microscope image and element analysis for the cross-section of the sample. Green, red, and blue colors represent the elements of O, Si, and Au, respectively. (b) Normal-incidence-reflection spectrum of a metasurface. The diameter of silicon nanoparticles is $D$ = 130 nm and the period of the array is $P$ = 420 nm. The gray-dashed curve is the experimental result. The blue- and black-solid curves are the simulated spectra with a zero and 3 nm oxide spacer between Si nanoparticles and the gold film (indicated by the white arrow in panel a), respectively. (c) (d) The simulated electric and magnetic fields from the cross-section view at the $yz$-plane. The resonance corresponds to the reflection dip of the black-solid curve in panel b. The yellow arrows in panel c represent the electric field vectors projected into the $yz$-plane. Scale bars: 20 nm (a) and (d).
To confirm our hypothesis, we conducted a TEM measurement on a cross-section of the c-Si nanoparticles. The resulting images show a 3 nm oxide layer surrounding the nanoparticles, which acts as an oxide spacer between the nanoparticles and the Au mirror (Figs. 2(a) and S1a). Despite still being puzzled by the significant difference in spectra between the oxide spacer set as 0 nm and 3 nm, calculations were conducted on the near-field distribution, corresponding to the full absorption dip in the black-solid curve in Fig. 2(b). Observing the side view along the $yz$-plane, it was noted that the electric field is primarily located in the oxide spacer, with a maximum field enhancement factor of up to 60-fold (Fig. 2(c)). The nanocavity’s extremely strong field sensitivity to the oxide spacer is especially noteworthy for thicknesses from 0 to 10 nm [19,23]. Furthermore, the magnetic field vectors projected to the $yz$-plane demonstrate a curl property, implying a magnetic dipole ($MD$) oscillation in the spacer along the $x$-axis (Fig. 2(d)). In contrast to the electric field confined in the nanogap, the magnetic field penetrates the c-Si nanoparticles and the Au mirror, with a maximum enhancement factor beyond 22-fold. We also conducted simulations to assess amorphous silicon (a-Si) NPAs on a gold film, employing identical parameters to those used for c-Si. Notably, the broader linewidth associated with a-Si results in only 90% absorption of the hybrid structure. Moreover, the corresponding near-field enhancement within the nanogap is weakened by a factor of 1.3 compared to that of c-Si (Figure S2).
After clarifying the discrepancy between our simulations and measurements, we delve deeper into the physics underlying the zero reflection dip or perfect absorption peak. The reflectance of the metasurface is analytically modeled by the equation: [32]
3. Observation of angular dispersion pattern
Next, we continue our analysis by characterizing the momentum-resolved and polarization-dependent responses of a metasurface with dimensions $P$ = 420 nm and $D$ = 120 nm. We present the measured TM- and TE-polarized reflection spectra in Figs. 3(a) and (d), respectively. The dispersion curves for the TM polarization modes exhibit an interesting "emoji" pattern (Fig. 3(a)). We define the profile of the emoji pattern using a blue-dashed horizontal line and a green-dashed arc. The emoji’s eyes are mirror-symmetric and are separated by regions with nearly unit-one reflectance at the $\Gamma$ point. The linewidth of the arc band becomes sharper as the momentum increases until its reflectance reaches unit-one. Importantly, the whole emoji pattern is located below the diffraction limit defined by two linear dispersion curves of SPP bands. The black dashed curves in Fig. 3(a) represent the fitted SPP bands complying with the momentum-conservation equation: [33]
Fig. 3. Momentum-resolved spectra. (a) (d) The measured dispersion curves for the sample with $P$ = 420 nm and $D$ = 120 nm under TM and TE polarization, respectively. (c) (f) The corresponding simulations. The horizontal dashed line in panels a and d indicates two different flat bands FB$_1$ and FB$_2$, respectively. The black-dashed curves in panels a and d are the fitted dispersion curves for the propagating surface plasmon polaritons (SPPs) on the gold film. The pentacles in panels a and c indicate the position of an accidental bound state in the continuum (BIC$_1$), and a symmetric-protected BIC$_2$. (b) (e) The variation in the reflection spectra obtained by cutting through the corresponding dispersion curves in panels a, c, d, and f. The solid-line curves represent the experiments, and the dashed-lines are the simulations. The blue- or green-dashed-dotted curves are the guides to the eye for the variation of dips in the flow of spectra.
To carefully analyze the band shifts, we extract the reflection spectra from the dispersion curves shown in Fig. 3(a). We plot these spectra as a function of the normalized momentum ${k}_{\parallel }$/${k}_{0}$ from 0 to 0.7 in Fig. 3(b). The reflection dips corresponding to the eyes of the emoji (denoted by the blue dashed-dotted curve) appear when ${k}_{\parallel }/{k}_{0}$ is greater than 0.1. Interestingly, the resonances do not shift until the SPP bands are crossed, which indicates that the left band is a flat band (FB$_{2}$) with a non-dispersive region. On the other hand, the reflection dips associated with the mouth of the emoji (denoted by the green dashed-dotted curve) are exhibited by a redshift in the resonances. As the momentum increases, the dips become shallower, until they eventually disappear. This phenomenon is consistently observed via simulations (Fig. 3(c)), and the measured spectra (the solid-line curves in Fig. 3(b)) agree with the corresponding simulations (the dashed-line curves in Fig. 3(b)).
Both the transitions of the reflection spectra for the eyes and mouth of the emoji pattern exhibit behavior consistent with BICs, discrete states completely decoupled from the radiation field despite their spectra lying within a continuum of radiation modes [34]. The former is attributed to a typical symmetry-protected BIC$_{2}$ at the $\Gamma$ point, which arises from complete the forbidding of coupling between bound modes and radiation continuum resulting from the C$_{2}$ symmetry of the c-Si NPAs and the propagating plane-waves. When the incident angle of $\theta$ deviates from 0$^{\circ }$, the symmetry is broken, and the BIC is coupled with the radiation mode to form a quasi-BIC. The absence of SPP-related mode results in the disappearance of BIC$_{2}$ for TE-polarized light, as shown in Fig. 3(d). The latter is an instance of an accidental BIC$_{1}$ belonging to Friedrich-Wintgen BIC, caused by mirror-image-induced electric dipoles ($ED$s) resonance coupling with SPP at higher incidence angles [23,24]. We utilize two pentacles to signify the placement of two BICs in the dispersion maps (refer to Fig. 3(a) and (c)). In the TE polarization modes, only one flat band (FB$_{1}$) is observed within the SPP cone. The incidence plane waves can have wide angles of up to 57$^\circ$ to excite FB$_{1}$.
We have observed two FBs and BICs in an array with $P$ = 420 nm. When we decrease the period to the subdiffraction regime, we measured and simulated the reflection dispersions for arrays with $P$ = 320 nm and $D$ = 140 nm, as shown in Figs. 4(a) and (c). We find that the absorption of all bands decreased when the coupling with SPPs is limited to the subdiffraction condition. Apart from the two FB modes discussed previously, we also observe a new resonance (FB$_3$) smaller than 600 nm for the transverse electric (TE) modes, as indicated by the orange-dashed line in Fig. 4(c). The momentum-dependent variation for FB$_3$ behaves similarly to FB$_2$, with a symmetric-protected BIC$_3$ at the $\Gamma$ point. However, FB$_3$ exhibits a desirable $Q$ of 54 when ${k}_{\parallel }/{k}_{0}$ = 0.3 despite being among the short wavelengths between 500 nm and 600 nm.
Fig. 4. Tuning the flat-bands. (a) (c) The measured (left) and simulated (right) dispersion curves for the samples with $P$ = 320 nm and $D$ = 140 nm under TM and TE polarization, respectively. Another flat band appears in the wavelength range from 500 to 600 nm. The green-, blue-, and orange-dashed horizontal lines indicate the FB$_1$, FB$_2$, and FB$_3$ bands, respectively. (b) The resonance wavelengths of the FB$_1$, FB$_2$, and FB$_3$ bands change as a function of diameter when the period remains constant at $P$ = 420 nm. (d) The resonance wavelengths of the FB$_1$, FB$_2$, and FB$_3$ bands change as a function of period when the diameter ($D$) remains constant at $D$ = 140 nm. The dashed curves in panels b and d are guides to the eye. (e) The mechanism for the origin of FBs. The in-plane electric dipoles ($ED_{x}$ or $ED_{y}$) in the c-Si nanoparticles coupled with the mirrored dipoles in the gold film lead to magnetic dipoles in the nanogaps, corresponding to FB$_1$. Meanwhile, out-of-plane electric ($ED_{z}$) or magnetic dipoles ($MD_{z}$) result in FB$_2$ and FB$_3$, respectively. (f) The resonance wavelengths of the FB$_1$, FB$_2$, and FB$_3$ bands change as a function of the oxide spacer when the period $P$ = 320 nm and the diameter $D$ = 140 nm.
4. Formation mechanism of the nanogap resonances
To manipulate the FB modes, we investigate the resonances by tuning the geometries of the nanoparticle arrays, including both $P$ and $D$. First, we fix the period as a constant ($P$ = 420 nm) and vary the diameter of the nanoparticles from 100 nm to 160 nm with 10 nm intervals. We repeat the experimental and numerical investigations following the foregoing approach and plot the results in Figures S5 and S6. The resonance wavelengths of FB$_1$ - FB$_3$ as a function of $D$ are summarized in Fig. 4(b). We find that the emergence of FB$_3$ requires a $D$ larger than 120 nm, implying that lateral geometry influences the formation of a loop of replacement current in c-Si nanoparticles. Additionally, the slight changes in the FB$_2$ value, along with the pronounced shifts in the resonances of FB$_1$ and FB$_3$, indicate that FB$_2$ are insensitive to $D$.
Next, we keep the diameter constant ($D$ = 140 nm) and tune the period of the array from 320 nm to 620 nm with 100 nm intervals. We exclusively conduct simulations that fully conform to experimental data (see data in Figure S7). FB$_1$ and FB$_3$ are almost insensitive to the period, implying that the radiative coupling between the nanoparticles in the array is negligible (Fig. 4(d)). In this case, the redshift of FB$_2$ resonances is pronounced as $P$ increases due to the effective coupling with the delocalized SPPs (Figure S7). In the aforementioned discussion, we determined that the formation of FB$_1$ is facilitated by the contribution of the nanogap cavities. Therefore, we performed numerical simulations for samples with different thicknesses of oxide spacer, showing the resonance wavelength as a function of the oxide spacer in Fig. 4(f). It is noteworthy that the resonances of FB$_2$ and FB$_1$ are jointly blue-shifted with an increasing oxide spacer, but FB$_3$ did not change significantly, indicating that the formation of FB$_2$ also originates from the nano-gap cavity, which can be confirmed in the analysis of near-field distribution later on.
We have elucidated how macroscopic parameters, such as the momentum and polarization of the incidence beam, and the geometries of the metasurfaces, affect FB resonances. Additionally, we investigate the internal structures of nanoparticles and nanogap cavities to reveal the microscopic mechanisms behind the formation of relevant FBs. By simulating their near-fields on the same sample described in Fig. 4(a) and (c), we plot the field distributions excited with an incidence angle of $\theta$ = 23$^{\circ }$ to explore all three FBs in Fig. 5. The results of FB$_{1}$ shown in Fig. 5(a) and (b) correspond to the previous plot in Fig. 2(c). Hence, we attribute this resonance to the oscillation of $MD$s within the nanocavity along the $x$-axis as $MD_{x}$. FB$_2$ involves the oscillation of electric dipoles within the nanocavity along the $z$-axis as $ED_{z}$ (Figs. 5(c) and (d)), which depends on the height of the nanoparticles. Height predominantly affects the FB$_3$ resonance. Beyond a specific cylinder height threshold, the resonance wavelength of FB$_1$ becomes insensitive to height. FB$_2$ resonance remains largely unaffected by height variations (see Figure S4). For both FB$_1$ and FB$_2$, the light is confined within the 3 nm oxide spacer, resulting in an extremely strong field enhancement with a factor over 40-fold. For FB$_3$, the electric field extends within the c-Si nanoparticles (Figs. 5(e)-h). The field enhancement within the nanocavity of FB$_3$ is relatively modest, with a maximum factor of 10-fold. However, due to the low-loss of c-Si, this mode exhibits a high $Q$. The loop of the electric field vectors is parallel to the $xy$-plane, indicating an $MD$ oscillating along the $z$-axis as $MD_{z}$. $MD_{z}$ necessitates critical lateral geometries for the c-Si nanoparticles, evidencing that FB$_3$ is sensitive to the aforementioned $D$.
Fig. 5. Near-field simulations for FB modes. (a) and (b) Simulated spatial distribution of the electric field intensity $\left | E \right |$ normalized to the incident field intensity $\left | E_{0} \right |$ in the $\it {yz}$- and $\it {xy}$-plane of a unit cell of the array with $\it {P}$ = 320 nm and $\it {D}$ = 140 nm. The resonance wavelengths correspond to FB$_{1}$ calculated with the incidence angle of $\theta$ = 23$^{\circ }$. The color scale represents the electric field enhancements, and the yellow arrows are the real components of the electric vector field projected in $\it {yz}$ or $\it {xy}$-planes. The contours of the nanoparticles are indicated by white rectangles and circles. The blue-horizontal-dashed lines indicate the surface of gold film. (c) and (d), (e)-(h) are the results of FB$_{2}$ and FB$_{3}$, respectively. Different from other plots, (g) and (h) represent the magnetic field distributions. Note that the $\it {xy}$-planes are recorded in the center of the oxide spacer. Scale bar: 25 nm.
By analyzing the near-field distribution, we can gain a deeper understanding of the physical characteristics of the FB modes. In Fig. 4(e), we depict a schematic of the mechanism responsible for the mode generation. The coupling between the in-plane $ED$s ($ED_{x}$ or $ED_{y}$) present in c-Si nanoparticles and the mirror-imaged electric dipoles in the gold film gives rise to the $MD$s in the oxide spacer, contributing to FB$_1$. Similarly, polarization of the out-of-plane electric ($ED_{z}$) or magnetic dipoles ($MD_{z}$) in c-Si nanoparticles results in FB$_2$ and FB$_3$, respectively.
By applying the mirror imaging method, we can infer that knowledge of the polarized multipoles attributed to c-Si nanoparticles allows us to determine the imaged multipoles in the gold film. The oscillations of $ED$ and $ED$ in the mirror image are either in phase or anti-phase. Consequently, the electric field experienced on both sides of the oxide spacer (as seen in Fig. 5(a)) and at the center of the oxide spacer (as seen in Fig. 5(c)) is greatly enhanced by approximately 40-fold through the interaction of $ED$ and "$ED$ image". The strongly localized field in the 3 nm nanogap acts as a hotspot, indicating a significantly high optical local density of states suited for sensing, enhanced surface spectroscopy, and boosted Purcell factors [9]. For FB$_3$, the magnetic field in the oxide spacer is the outcome of $MD$ interference in the c-Si nanoparticles and the gold film due to their antiphase oscillations. Additionally, the strong fields extend to the nanoparticles. The radiation of $MD_{z}$ is fully suppressed in the direction perpendicular to the device, leading to the FB$_3$ as a symmetry-protected BIC. Such BIC’s high $Q$ and polarization singularity allow for the generation of efficient nonlinear optical processes with topological charges [26].
For accurate attribution of the optical bands seen in the momentum-resolved dispersion curves (Figs. 6, S8 and S9), we employ the multipole decomposition approach discussed in the literature and our recent papers [11,35–37], which allows us to determine the Cartesian components of the multipoles excited inside the nanoparticles. By simulating the dispersion of the NPAs on the gold film and recording the electric field amplitude inside the nanoparticle for the unit cell of the array, we calculate the scattering cross-section of each multipole, such as $ED_{x, y}$, $ED_{z}$, $MD_{x, y}$, $MD_{z}$, and $MQ$, based on the field values. In Figs. 6(a) and (e), we present the calculated scattering cross-sections for samples with $P$ = 320 nm and $D$ = 140 nm, respectively, which show marked similarity to the reflection spectra. Additionally, Panels b-d and f-i in Fig. 6 further support our conclusion that $ED_y$, $MD_z$, and $ED_z$ dominate the polarized multipoles in the nanoparticles for FB$_1$, FB$_2$, and FB$_3$, respectively, which is consistent with the near-field calculations.
Fig. 6. Multipole decomposition for the c-Si nanoparticle array with $\it {P}$ = 320 nm and $\it {D}$ = 140 nm on the gold film. (a) and (e) The total scattering cross-section simulated under TM- and TE-polarized light, respectively. (b)-(d) and (f)-(i) Contribution of different electromagnetic modes to the total scattered cross-section in panels a and e, respectively.
Our analysis of BICs and FB modes confirms the feasibility of our fabrication method, which offers several advantages compared to conventional processes. To the best of our knowledge, we present experimental results in comparison with commonly utilized methods in Supplement 1. Specifically, our method produces an ultra-flat gold film with atomic roughness, improving the propagation capability of SPPs on the gold film. Nanopatterning the NPAs on the SOI substrate ensures perfect conductivity for electric beam lithography and eliminates the need for expensive conductive adhesive. The structure of the transferred c-Si NPAs remains intact. By quantitatively measuring the variation of mode resonances with structural parameters, we find that the parameters of the c-Si NPAs align with the established values. Additionally, the presence of a native oxide layer for the construction of a less than 10 nm nanogap cavity enhances extreme near-field confinement and allows for strong light-matter interactions in two-dimensional materials with atomic thickness [33,38]. This process leverages the template stripped gold film’s exceptional smoothness, the minimal loss properties of c-Si in the visible band, and the synergistic collective resonances inherent in the array structure. This amalgamation allows us to discern hybrid nanogap resonances characterized by $Q$-factors ranging from 10 to 100. These resonances encompass both enduring modes and nano-scale optical field confinement. Importantly, our approach exhibits compatibility with a broad target substrate that are insoluble in water. It opens up many possibilities for the development of novel hybrid metasurfaces. For example, the transfer of c-Si NPAs onto a flexible substrate like polydimethylsiloxane enables the creation of mechanically tunable metasurfaces for phase control of light [39–41]. Furthermore, the transfer onto circuits of lithium niobate or conductive polymers allows for applications such as beam steering of LiDAR, electro-optic modulation, and acousto-optic modulation [42–46].
5. Conclusions
In summary, we present a new method for creating hybrid metasurfaces by combining ultrasmooth gold films with structured c-Si NPAs. The approach involves wet-transfer and self-bonding of NPAs onto the gold film, enabling precise positioning relative to plasmonic mirrors. Advanced microscopy techniques reveal the importance of the native oxide layer of Si in correcting reflection spectra. Fascinating dispersion patterns are observed for different polarization modes, with nanogap resonances separated by BICs in TM modes and a single FB in TE modes. Near-field calculations and multipolar decomposition confirm that these FBs are localized in the nanogap cavities, facilitated by coupling between multipolar Mie resonances and their mirror images in the gold film. The method provides numerous advantages, including the production of ultra-flat gold films, preservation of intact c-Si NPA structures, and enhanced near-field confinement. These advancements pave the way for the development of innovative optoelectronic devices. Additionally, the physical processes involved in the adsorption of Si NPAs onto the surface of heterogeneous materials without any distortion raise fundamental questions within the nanoscale community.
Funding
National Natural Science Foundation of China (62005188, 62120106001, 62275184); Natural Science Foundation of Jiangsu Province (BK20190825, BK20200859); Priority Academic Program Development of Jiangsu Higher Education Institutions (22KJA480003); Hui-Chun Chin and Tsung-Dao Lee Chinese Undergraduate Research Endowment.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. L. Novotny and B. Hecht, Principles of Nano-optics (Cambridge University, 2012).
2. R. Zhang, Y. Zhang, Z. Dong, S. Jiang, C. Zhang, L. Chen, L. Zhang, Y. Liao, J. Aizpurua, Y. e. Luo, J. L. Yang, and J. G. Hou, “Chemical mapping of a single molecule by plasmon-enhanced raman scattering,” Nature 498(7452), 82–86 (2013). [CrossRef]
3. G. M. Akselrod, C. Argyropoulos, T. B. Hoang, C. Ciracì, C. Fang, J. Huang, D. R. Smith, and M. H. Mikkelsen, “Probing the mechanisms of large purcell enhancement in plasmonic nanoantennas,” Nat. Photonics 8(11), 835–840 (2014). [CrossRef]
4. J. J. Baumberg, J. Aizpurua, M. H. Mikkelsen, and D. R. Smith, “Extreme nanophotonics from ultrathin metallic gaps,” Nat. Mater. 18(7), 668–678 (2019). [CrossRef]
5. R. Regmi, J. Berthelot, P. M. Winkler, M. Mivelle, J. Proust, F. Bedu, I. Ozerov, T. Begou, J. Lumeau, H. Rigneault, M. F. García-Parajó, S. Bidault, J. Wenger, and N. Bonod, “All-dielectric silicon nanogap antennas to enhance the fluorescence of single molecules,” Nano Lett. 16(8), 5143–5151 (2016). [CrossRef]
6. J. Cambiasso, G. Grinblat, Y. Li, A. Rakovich, E. Cortés, and S. A. Maier, “Bridging the gap between dielectric nanophotonics and the visible regime with effectively lossless gallium phosphide antennas,” Nano Lett. 17(2), 1219–1225 (2017). [CrossRef]
7. S. Hu, M. Khater, R. Salas-Montiel, E. Kratschmer, S. Engelmann, W. M. Green, and S. M. Weiss, “Experimental realization of deep-subwavelength confinement in dielectric optical resonators,” Sci. Adv. 4(8), eaat2355 (2018). [CrossRef]
8. L. Sortino, P. Zotev, S. Mignuzzi, J. Cambiasso, D. Schmidt, A. Genco, M. Aßmann, M. Bayer, S. Maier, R. Sapienza, and A. I. Tartakovskii, “Enhanced light-matter interaction in an atomically thin semiconductor coupled with dielectric nano-antennas,” Nat. Commun. 10(1), 5119 (2019). [CrossRef]
9. Y. Yang, O. D. Miller, T. Christensen, J. D. Joannopoulos, and M. Soljacic, “Low-loss plasmonic dielectric nanoresonators,” Nano Lett. 17(5), 3238–3245 (2017). [CrossRef]
10. I. Staude and J. Schilling, “Metamaterial-inspired silicon nanophotonics,” Nat. Photonics 11(5), 274–284 (2017). [CrossRef]
11. Z. Zhang, P. Liu, W. Lu, P. Bai, B. Zhang, Z. Chen, S. A. Maier, J. G. Rivas, S. Wang, and X. Li, “High-q collective mie resonances in monocrystalline silicon nanoantenna arrays for the visible light,” Fundam. Res. 3(5), 822–830 (2023). [CrossRef]
12. P. Liu, Z. Zhang, M. Lang, W. Lu, P. Bai, Z. Chen, S. Wang, and X. Li, “Manipulating the directional emission of monolayer semiconductors by dielectric nanoantenna arrays,” J. Opt. 24(2), 024005 (2022). [CrossRef]
13. F. Shen, Z. Zhang, Y. Zhou, J. Ma, K. Chen, H. Chen, S. Wang, J. Xu, and Z. Chen, “Transition metal dichalcogenide metaphotonic and self-coupled polaritonic platform grown by chemical vapor deposition,” Nat. Commun. 13(1), 5597 (2022). [CrossRef]
14. Z. Zhang, C. Xu, C. Liu, M. Lang, Y. Zhang, M. Li, W. Lu, Z. Chen, C. Wang, S. Wang, and X. Li, “Dual control of enhanced quasi-bound states in the continuum emission from resonant c-si metasurfaces,” Nano Lett. 23(16), 7584–7592 (2023). [CrossRef]
15. A. B. Vasista, S. Tiwari, D. K. Sharma, S. K. Chaubey, and G. P. Kumar, “Vectorial fluorescence emission from microsphere coupled to gold mirror,” Adv. Opt. Mater. 6(22), 1801025 (2018). [CrossRef]
16. H. Sugimoto and M. Fujii, “Broadband dielectric–metal hybrid nanoantenna: Silicon nanoparticle on a mirror,” ACS Photonics 5(5), 1986–1993 (2018). [CrossRef]
17. V. A. Milichko, D. A. Zuev, D. G. Baranov, G. P. Zograf, K. Volodina, A. A. Krasilin, I. S. Mukhin, P. A. Dmitriev, V. V. Vinogradov, S. V. Makarov, and P. A. Belov, “Metal-dielectric nanocavity for real-time tracing molecular events with temperature feedback,” Laser Photonics Rev. 12(1), 1700227 (2018). [CrossRef]
18. G. Yang, Y. Niu, H. Wei, B. Bai, and H.-B. Sun, “Greatly amplified spontaneous emission of colloidal quantum dots mediated by a dielectric-plasmonic hybrid nanoantenna,” Nanophotonics 8(12), 2313–2319 (2019). [CrossRef]
19. A. Maimaiti, P. P. Patra, S. Jones, T. J. Antosiewicz, and R. Verre, “Low-loss hybrid high-index dielectric particles on a mirror for extreme light confinement,” Adv. Opt. Mater. 8(6), 1901820 (2020). [CrossRef]
20. F. Deng, H. Huang, J.-D. Chen, S. Liu, H. Pang, X. He, and S. Lan, “Greatly enhanced plasmon–exciton coupling in si/ws2/au nanocavities,” Nano Lett. 22(1), 220–228 (2022). [CrossRef]
21. S. K. Chaubey, S. Tiwari, G. M. A. D. Paul, A. Rahman, and G. Pavan Kumar, “Mirror-coupled microsphere can narrow the angular distribution of photoluminescence from ws2 monolayers,” Appl. Phys. Lett. 120(26), 261109 (2022). [CrossRef]
22. L. Xu, M. Rahmani, K. Zangeneh Kamali, A. Lamprianidis, L. Ghirardini, J. Sautter, R. Camacho-Morales, H. Chen, M. Parry, I. Staude, G. Zhang, D. Neshev, and A. E. Miroshnichenko, “Boosting third-harmonic generation by a mirror-enhanced anapole resonator,” Light: Sci. Appl. 7(1), 44 (2018). [CrossRef]
23. J. Xiang, Y. Xu, J.-D. Chen, and S. Lan, “Tailoring the spatial localization of bound state in the continuum in plasmonic-dielectric hybrid system,” Nanophotonics 9(1), 133–142 (2020). [CrossRef]
24. X. Xiao, Y. Lu, J. Jiang, and Y. Chen, “Manipulation of optical bound states in the continuum in a metal-dielectric hybrid nanostructure,” Photonics Res. 10(11), 2526–2531 (2022). [CrossRef]
25. A. J. Traverso, J. Huang, T. Peyronel, G. Yang, T. G. Tiecke, and M. H. Mikkelsen, “Low-loss, centimeter-scale plasmonic metasurface for ultrafast optoelectronics,” Optica 8(2), 202–207 (2021). [CrossRef]
26. L. Zhou, M. Panmai, S. Li, Y. Mao, W. He, H. Huang, and S. Lan, “Lighting up si nanoparticle arrays by exploiting the bound states in the continuum formed in a si/au hybrid nanostructure,” ACS Photonics 9(9), 2991–2999 (2022). [CrossRef]
27. M. Zheng, Y. Yang, D. Zhu, Y. Chen, Z. Shu, K. K. Berggren, M. Soljacic, and H. Duan, “Enhancing plasmonic spectral tunability with anomalous material dispersion,” Nano Lett. 21(1), 91–98 (2021). [CrossRef]
28. P. Nagpal, N. C. Lindquist, S.-H. Oh, and D. J. Norris, “Ultrasmooth patterned metals for plasmonics and metamaterials,” Science 325(5940), 594–597 (2009). [CrossRef]
29. D. Sell, J. Yang, S. Doshay, K. Zhang, and J. A. Fan, “Visible light metasurfaces based on single-crystal silicon,” ACS Photonics 3(10), 1919–1925 (2016). [CrossRef]
30. Z. Zhou, J. Li, R. Su, B. Yao, H. Fang, K. Li, L. Zhou, J. Liu, D. Stellinga, C. P. Reardon, T. F. Krauss, and X. Wang, “Efficient silicon metasurfaces for visible light,” ACS Photonics 4(3), 544–551 (2017). [CrossRef]
31. A. M. Shaltout, V. M. Shalaev, and M. L. Brongersma, “Spatiotemporal light control with active metasurfaces,” Science 364(6441), eaat3100 (2019). [CrossRef]
32. C. Qu, S. Ma, J. Hao, M. Qiu, X. Li, S. Xiao, Z. Miao, N. Dai, Q. He, S. Sun, and L. Zhou, “Tailor the functionalities of metasurfaces based on a complete phase diagram,” Phys. Rev. Lett. 115(23), 235503 (2015). [CrossRef]
33. S. Wang, S. Li, T. Chervy, A. Shalabney, S. Azzini, E. Orgiu, J. A. Hutchison, C. Genet, P. Samorì, and T. W. Ebbesen, “Coherent coupling of ws2 monolayers with metallic photonic nanostructures at room temperature,” Nano Lett. 16(7), 4368–4374 (2016). [CrossRef]
34. C. W. Hsu, B. Zhen, A. D. Stone, J. D. Joannopoulos, and M. Soljačić, “Bound states in the continuum,” Nat. Rev. Mater. 1(9), 16048 (2016). [CrossRef]
35. E. Radescu and G. Vaman, “Exact calculation of the angular momentum loss, recoil force, and radiation intensity for an arbitrary source in terms of electric, magnetic, and toroid multipoles,” Phys. Rev. E 65(4), 046609 (2002). [CrossRef]
36. R. Paniagua-Domínguez, Y. F. Yu, A. E. Miroshnichenko, L. A. Krivitsky, Y. H. Fu, V. Valuckas, L. Gonzaga, Y. T. Toh, A. Y. S. Kay, B. Luk’yanchuk, and A. I. Kuznetsov, “Generalized brewster effect in dielectric metasurfaces,” Nat. Commun. 7(1), 10362 (2016). [CrossRef]
37. V. R. Tuz and A. B. Evlyukhin, “Polarization-independent anapole response of a trimer-based dielectric metasurface,” Nanophotonics 10(17), 4373–4383 (2021). [CrossRef]
38. S. Wang, Q. Le-Van, F. Vaianella, B. Maes, S. Eizagirre Barker, R. H. Godiksen, A. G. Curto, and J. Gomez Rivas, “Limits to strong coupling of excitons in multilayer ws2 with collective plasmonic resonances,” ACS Photonics 6(2), 286–293 (2019). [CrossRef]
39. S. M. Kamali, A. Arbabi, E. Arbabi, Y. Horie, and A. Faraon, “Decoupling optical function and geometrical form using conformal flexible dielectric metasurfaces,” Nat. Commun. 7(1), 11618 (2016). [CrossRef]
40. E. Arbabi, A. Arbabi, S. M. Kamali, Y. Horie, M. Faraji-Dana, and A. Faraon, “Mems-tunable dielectric metasurface lens,” Nat. Commun. 9(1), 812 (2018). [CrossRef]
41. T. C. Tan, Y. K. Srivastava, R. T. Ako, W. Wang, M. Bhaskaran, S. Sriram, I. Al-Naib, E. Plum, and R. Singh, “Active control of nanodielectric-induced thz quasi-bic in flexible metasurfaces: a platform for modulation and sensing,” Adv. Mater. 33(27), 2100836 (2021). [CrossRef]
42. J. Ji, Z. Wang, J. Sun, C. Chen, X. Li, B. Fang, S.-N. Zhu, and T. Li, “Metasurface-enabled on-chip manipulation of higher-order poincaré sphere beams,” Nano Lett. 23(7), 2750–2757 (2023). [CrossRef]
43. F. Ye, Y. Yu, X. Xi, and X. Sun, “Second-harmonic generation in etchless lithium niobate nanophotonic waveguides with bound states in the continuum,” Laser Photonics Rev. 16(3), 2100429 (2022). [CrossRef]
44. A. Fedotova, L. Carletti, A. Zilli, F. Setzpfandt, I. Staude, A. Toma, M. Finazzi, C. De Angelis, T. Pertsch, D. N. Neshev, and M. Celebrano, “Lithium niobate meta-optics,” ACS Photonics 9(12), 3745–3763 (2022). [CrossRef]
45. A. Boes, L. Chang, C. Langrock, M. Yu, M. Zhang, Q. Lin, M. Lončar, M. Fejer, J. Bowers, and A. Mitchell, “Lithium niobate photonics: Unlocking the electromagnetic spectrum,” Science 379(6627), eabj4396 (2023). [CrossRef]
46. J. Karst, M. Floess, M. Ubl, C. Dingler, C. Malacrida, T. Steinle, S. Ludwigs, M. Hentschel, and H. Giessen, “Electrically switchable metallic polymer nanoantennas,” Science 374(6567), 612–616 (2021). [CrossRef]
