All-Dielectric Chiral Metasurfaces Based on Crossed-Bowtie Nanoantennas.

Circular dichroism spectroscopy is a technique used to discriminate molecular chirality, which is essential in fields like biology, chemistry, or pharmacology where different chiral agents often show different biological activities. Nevertheless, due to the inherently weak molecular-chiroptical activity, this technique is limited to high concentrations or large analyte volumes. Finding novel ways to enhance the circular dichroism would boost the performance of these techniques. So far, the enhancement of light-matter interaction mediated by plasmons is the most common way to develop chiral plasmonic structures with extraordinarily strong chiroptical responses. However, absorptive losses of metals at optical frequencies has hindered its practical use in many scenarios. In this work, we propose an all-dielectric low-loss chiral metasurface with unit cells built by high-refractive-index crossed-bowtie nanoantennas. These unit cells, built of silicon, strongly increase the chiroptical effect through the simultaneous interaction of their electric and magnetic modes, which in contrast to other recent proposals shows at the same time a high concentration of the electric field in its gap that leads to the presence of hotspots. The proposed structure exhibits a circular dichroism spectra up to 3-fold higher than that of previous proposals that use complex plasmonic or hybrid nanostructures, making it a clear alternative to develop low-loss metasurfaces with potential applications in chiral target sensing/biosensing. For completeness, single triangular shaped and symmetric (achiral) bowtie nanostructures were also studied as possible candidates for a detection up to the single-molecule level due the lack of a circular dichroism background of the nanostructures themselves.

Introduction

Chirality refers to a geometrical or structural handedness of an object, meaning that its mirror image cannot be superimposed on itself. This phenomenon is ubiquitous in nature and plays a key role in life sciences, analytical chemistry, biochemistry, and medical sciences.[1,2] An example of chiral structures in chemistry are the enantiomers, also known as optical isomers. In pharmacology, for example, different enantiomers have the same chemical structure, but their biological activities can make a significant difference between an effective treatment or an ineffective one (with serious side-effects).[3,4] From the optical point of view, chiral materials present different refractive indexes when interacting with left circularly polarized (LCP) light or right circularly polarized (RCP) light, giving rise to optical activity (OA) and circular dichroism (CD).[5,6] The first phenomenon consists of the rotation induced in the polarization plane of linearly polarized light (which is just a superposition of LCP and RCP lights) and the second is the difference in absorption when excited by left and right circularly polarized lights. Both phenomena are extremely connected by the Kramers–Kronig relations.

CD spectroscopy, that is, the differential absorption of left- and right-hand circularly polarized (LCP and RCP) light, is traditionally used to discriminate molecular chirality.[7] Nevertheless, due to the inherently weak molecular-chiroptical activity, this technique is limited to high concentrations or large analyte volumes. On the other hand, recent advances in metamaterials,[8,9] optoelectronics,[10−12] biosensing,[13−15] enantioselective forces,[16−18] nanometrology,[19] and ultrafast data processing[20,21] stem from the combined advances in the tailoring, patterning, and manipulation of light through confinement and enhancement at nanometric scales. The ability of plasmonic structures to localize and enhance light at dimensions much smaller than the incident wavelength,[22] through the resonant coupling of electromagnetic waves to collective oscillations of free electrons in metals, has been extensively used for these purposes. In particular, the last few years have witnessed an explosive development of chiral plasmonic structures with extraordinarily strong chiroptical responses.[23−35] However, absorptive losses associated with the inter- and intraband transitions in metals at optical frequencies constitute a major hurdle for practical use in many scenarios.

The use of high-refractive-index (HRI) dielectric nanostructures, where Ohmic losses inherent to metals can be significantly diminished, has emerged as a promising solution to control the near- and far-field optical response at the nanoscale.[36,37] In contrast to their plasmonic counterpart, where the electromagnetic enhancement is governed only by electric resonances, HRI dielectric nanoparticles can exhibit both electric and magnetic resonances.[38−40] Important applications are envisaged, like optical cloaking,[41] harmonic generation,[42,43] nanoscale lasers,[44] or directionality of light.[45−48] Despite these advances, their use in some applications are limited by the relatively mild intensity of the electric-field enhancement and the occurrence of the magnetic hotspot inside the nanoparticles, thus being hard to access. To surpass these limitations, some recent works[49,50] demonstrated that in analogy to plasmonic aggregates, ensembles of two or more dielectric particles exhibit large enhancements of the electric and magnetic fields in the gap (hotspots). Inspired by these works, here we propose an all-dielectric low-loss chiral metasurface with unit cells built by high-refractive-index crossed-bowtie nanoantennas. These unit cells, built of silicon, strongly increase the chiroptical effect through the simultaneous interaction of their electric and magnetic modes. Recent proposals[51] use dielectric nanospheres of different sizes to produce CD signals, where the excitation of strong hotspots can be hindered by the curvilinear surface of the spheres. Our platform shows a high concentration of the electric field in its gap that leads to the presence of hotspots being directly accessible to nearby emitters, thus opening the route for new low-loss surface-enhanced spectroscopies, sensing, and other applications in the field of nanophotonics. An additional advantage of the crossed-bowtie unit cell is that it can be excited at normal incidence, whereas in the case of spherical oligomers of different unit sizes, the electric field needs to be obliquely placed along the line joining two diameters to generate a hotspot. As a result, the proposed structure exhibits a circular dichroism spectra up to 3-fold higher than those of previous proposals that use complex plasmonic or hybrid nanostructures,[52] making it a clear alternative to develop low-loss metasurfaces with potential applications in chiral target sensing/biosensing. We also highlight that this chiral structure can be spectrally tuned to exhibit its maximum circular dichroism at the desired wavelength by just engineering its size, shape, and periodicity.

Results and Discussion

It is known that high-refractive-index dielectric nanoparticles, when excited with light, can retain the electromagnetic field inside due to the generation of strong displacement currents. This effect, for resonant frequencies, can lead to electric and/or magnetic resonant modes produced by the inner field distributions. Analogous to metallic nanoparticles, under surface plasmon resonance conditions, HRI dielectric particles can also exhibit enhanced electric field distributions at their boundaries. When resonant nanoparticles, metallic or dielectric, are coupled, forming dimers, trimers, or more complex aggregates, their electric modes in the case of plasmonics, and electric and magnetic modes in the case of dielectrics, overlap and lead to an enhancement and spectral shift of the extinction cross sections. Figures a–c show the extinction spectra of a single triangular scatterer, a bowtie, and two crossed bowtie scatterers, respectively. All of them made of amorphous silicon[53] and surrounded by air. They are illuminated by left and right circularly polarized plane waves with the wavevector perpendicular to them and along the z-axis. These systems were considered as deposited on a glass (ng = 1.5) substrate and surrounded by air. The corresponding extinction spectra, σext = σscat + σabs, were numerically calculated with the finite-difference time-domain (FDTD) method. σscat and σabs correspond to the scattering and absorption cross sections associated with each platform. The length of the side and the height of each triangle is constant here, l = 190 nm and h = 100 nm, to keep their resonances in the optical range and a tip-to-tip length of 20 nm. The optical constants for silicon were obtained from Palik.[53]

Figure 1

Extinction spectra, σext, for (a) a single triangular scatterer, (b) a dielectric bowtie nanoantenna, and (c) two crossed-bowtie nanoantennas. Triangular scatterers are considered as made of Si, with sides l = 190 nm and height h = 100 nm. The tip-to-tip gap was taken as g = 20 nm. Results for right/left circularly polarized (RCP/LCP) light, incident along the z-axis, are presented by solid/dashed (red/blue) lines. The corresponding electric and magnetic near-field intensities, at the highest resonance wavelengths, are presented in (d)–(f) and (g)–(i), respectively.

Three overlapping peaks with a small difference in width, amplitude, and spectral position can be clearly identified in all cases for incident wavelengths of λ ≈ 570, 520, and 430 nm, corresponding to magnetic dipolar, electric dipolar, and magnetic quadrupolar resonances, respectively.[49] We observe that both the magnetic and the electric resonances are slightly shifted (the magnetic resonance to the blue and the electric resonance to the red), and their intensities vary consequently. In the same way that metallic nanoparticles can concentrate electric fields in small volumes, when dielectric nanoparticles are coupled to form dielectric antennas, they also provide a reasonably strong electric field enhancement and localization of electromagnetic energy in the antenna gap. Calculations demonstrate that intense and localized electric and magnetic near-fields are present in the gap of dielectric bowties and crossed-bowties. Figures d–f,g–i show that the normalized electric and magnetic near-fields for LCP incident light at the magnetic resonance wavelength exhibit a rotational symmetry on the xy plane in all cases, leading to a lack of optical chirality, that is, σRCP = σLCP. This can also be understood in terms of the mirror symmetry along the z-axis that all of the systems present. However, we must remark here that all of these nonchiral structures have an associated near-field optical chirality, C, at each resonance, as can be observed in Figure S1 of the Supporting Information. The optical chirality, , quantifies the interaction strength of the electromagnetic field with the chiral photonic structure, thus being responsible for the CD spectroscopic signal. This term reaches its maximum value, , for circularly polarized light (CPL) in free-space.[54] Hence, the enhancement of the electric and magnetic field amplitudes, for which Im(E*·B) ≠ 0, is a key element to improve the CD signal.[54]

The first approach to introduce chiral optical activity in the system consists of breaking the mirror symmetry of the structure along the z-axis. This can be done in three different ways: changing the side lengths of each arm, the heights, and both together, as depicted in Figures a–c, respectively. The corresponding CD signal for each case, in the extinction spectra, is calculated as follows[29]with σ being the extinction cross section for RCP and LCP illumination, which was calculated via the commercial software Lumerical FDTD, for wavelengths in the range from λ = 300 to 900 nm, and considering all of the media as isotropic.

Figure 2

Crossed-bowtie nanoantennas with a broken mirror symmetry along the z-axis by changing the (a) side lengths, (b) heights, and (c) simultaneously the heights and side lengths of the building dielectric scatterers. (d–f) Extinction spectra, σext, of the systems shown in (a)–(c), respectively, for RCP/LCP incident light are presented by solid/dashed (red/blue) lines. Results were calculated for systems with symmetry breaking by changing the (a) side lengths, (b) heights, and (c) side lengths and heights, simultaneously, for the building dielectric scatterers. (g–i) CD spectra calculated for the systems (a)–(c), respectively.

In the first case, shown in Figure a, side-lengths of the dielectric triangles were increased from 170 to 230 nm, in steps of 20 nm, following the anticlockwise direction. A small differences in the extinction spectra of the system is observed (Figure d), thus producing a negligible effect in the CD spectral response, as can be seen in Figure g. A second symmetry-breaking procedure consisted of varying the scatterer heights from 80 to 140 nm, in steps of 20 nm, following the anticlockwise sense, as depicted in Figure b. Although differences in the extinction spectra remain small (see Figure e), the CD amplitude was slightly increased when compared with the previous case, as noted from Figures g and 2h. In the last case, we simultaneously changed the side lengths and heights, in steps of 20 nm, again following the anticlockwise direction. The latter case is depicted in Figure c. The corresponding extinction spectra, for RCP and LCP incident lights, are plotted in Figure f. In this case, we find the largest CD signal, as noted from Figure i, showing this configuration as a clear candidate to be used later as the periodic unit cell for the design of a two-dimensional chiral all-dielectric metasurface. We must remark that the studied chiral platforms present enhanced and robust CD signals, in contrast to previous plasmonic proposals[52] requiring precise control of the cooling and the seed structure to exhibit an about three times smaller chiroptical activity.

As mentioned before, enhanced CD signals require the enhancement of the electric and magnetic near-fields to produce higher Im(E*·B) terms. A deep insight can be obtained from the electric and magnetic field maps in Figure . The near-field maps are only shown for the wavelengths corresponding to the maximum amplitudes of the CD signal extracted from Figures h,i. Thus, Figures a,b [e,f] and Figures c,d [g,h] are for RCP [LCP] polarization at λ = 597.12 and 511.29 nm, respectively. In the case of Figures a,b and Figures e,f, we can observe that there is only a small difference for the interaction of RCP and LCP incident lights with the structure, where the electric hotspots are mainly located at the external boundaries of the scatterers and the magnetic fields only exhibit a small difference in their amplitudes. On the other hand, Figures c,d and Figures g,h exhibit strong changes in their amplitudes and symmetries, with the additional advantage of having the hotspots located at the gap region. The latter hotspots can be exploited to improve light–molecule interactions in chiral-sensing applications, for example, by placing a chiral molecule at the gap region. The optical chiralities for these structures, associated with each resonance, are shown in Figure S2 of the Supporting Information.

Figure 3

Near-field profiles for RCP and LCP incident lights, (a–d) corresponding to the wavelength (λ = 597.122 nm) of the maximum amplitude in Figure h and (e)–(h) to the wavelength (λ = 511.294 nm) of the maximum amplitude in Figure i, are presented in the upper and lower panels, respectively.

To show the robustness of our proposal, the effects induced by small shifts and asymmetries, in the tip-to-tip gaps of the system described in Figure c, were also studied. In Figures a,b, we considered two different possibilities, as illustrated in the corresponding insets. In the first case, we considered a small misalignment of 10 nm around the center of both crossed bowties (see Figure a). In the second, we explored two different tip-to-tip gap distances, 20 and 30 nm, for the longitudinal and transversal bowties, respectively, as depicted in Figure b. The extinction spectra, for RCP and LCP polarizations, do not suffer any considerable change when compared with the ones in Figure c, thus indicating a robust behavior. The same can be concluded for the corresponding CD spectra in Figures d,e when compared with results in Figure i. Another feature we explored was to double the gap sizes in Figure f (20 nm), that is, using gaps of 40 nm. The latter results are presented in Figures c,f, from where a small decrease in the CD amplitude can be seen. This result is consistent with the literature since longer distances between scatterers lead to smaller mode interactions.[49]

Figure 4

RCP (solid) and LCP (dashed) scattering spectra for the system in Figure f under misalignment and asymmetries. A misalignment of 10 nm around the center was considered in (a) (see inset). In (b), different tip-to-tip gap sizes, 20 and 30 nm, were used (see inset). Symmetric gaps of 40 nm were considered in (c) (see inset). The corresponding CD spectra are presented in (d)–(f).

In Figure , we compare the results for the structure in Figure c with the ones for the same system but made of gold. The preferential absorption of CPL by a chiral molecule is described by the dissymmetry factor, g, defined as follows[60]with Ue = (ε0/4)|Ẽ|2 being the time-average electric energy density. G″ and α″ are the molecule’s chirality and electric polarizability, that is, the chiral asymmetry (g) in the absorption of light by a molecule is proportional to both the chirality of matter and the chirality of the electromagnetic field. This equation can be rewritten as follows[60]from where it can be noted that g can be enhanced above gCPL by increasing the local density of chirality C or decreasing the local field intensity. In Figures a–d, we present the scattering-to-absorption cross-sectional ratios, the CD spectrum, and the dissymmetry factor ĝ for LCP and RCP polarizations, respectively. The same results are presented in the case of the structure made of gold in Figure e–h. We clearly observe large scattering-to-absorption ratios for the dielectric system, indicating a reduction in the losses’ contribution when compared with their plasmonic counterparts. Moreover, a larger CD signal is also noted for the dielectric system. In Figures c,d and Figures g,h, we present the results for the dissymmetry factor, for both systems, for each polarization of the incident light. Results indicate that these systems exhibit a high chiroptical activity, large CD, and C fields with chiral hotspots. However, the main disadvantage of using chiral platforms, dielectric or plasmonic, is associated with the strong chiroptical background from the structure, as noted from the latter results. In this respect, symmetric structures from Figure , where C values are enhanced for each polarization, can be competitive with some recent proposals[59−61] to improve CD measurements from dielectric nanoparticle systems.

Figure 5

Ratio of the scattering to absorption, CD spectrum, and dissymmetry factors for LCP and RCP incident lights are presented for the systems made of (a–d) Si and (e–h) Au, respectively.

The final objective of this work is to propose an all-dielectric chiral metasurface, as illustrated in the insets of Figure a,b, for potential applications in highly efficient chiral biosensors. For this purpose and based on its high chiral optical activity, we considered the crossed-bowtie structure shown in Figure c as the unit cell to build the metasurface. It was arranged in a squared lattice with a size period length Δ concentric with the central gap. The corresponding CD signal was calculated from the transmittance spectra as followswith TRCP and TLCP being the transmittances for RCP and LCP incident lights, respectively. The transmittance and CD spectra are shown in Figures a,b and Figures c,d, respectively, for Λ = 800 and 600 nm. The increase in chiral optical activity, while decreasing the length of the period, is explained by the increase in the overlap of the electric near-field tails between the neighboring cells. This can be noted from Figure , where we present the corresponding chiral near-fields, normalized by the CCPL of the free-space-propagating CPL light, within a unit cell for periods Λ = 800 nm in Figure a,b and Λ = 600 nm in Figure c,d. Results are presented for RCP and LCP incident lights. It has been previously observed and explained[55,56] that in the case of plasmonic nanoparticles, the near-field overlap among neighboring scatterers resemble the electronic bands comprising well-localized atomic orbitals. Thus, we show here how to design the corresponding chiral “meta-atoms”, made as a crossed-bowtie systems, to build all-dielectric chiral metasurfaces with enhanced chiroptical responses. We also emphasize that the low-loss structure, observed from the scattering/absorption ratio for the unit cells, proposed in this work exhibits a CD spectral response higher than those in recent proposals that use complex plasmonic or hybrid nanostructures.[52,57]

Figure 6

Transmission spectra for all-dielectric chiral metasurfaces, considered as built by the periodic repetition of the system studied in Figure f, with period lengths of (a) 800 nm and (b) 600 nm. Results are presented for RCP (solid) and LCP (dashed) incident lights. The corresponding CD spectra are presented in (c) and (d).

Figure 7

Normalized optical chirality near-fields for (a, b) Λ = 800 nm and (c, d) Λ = 600 nm. Results are presented for a unit cell. Results for nine unit cells can be seen in Figure S3 in the Supporting Information.

Conclusions

We have demonstrated that all-dielectric metasurfaces, consisting of a two-dimensional periodic arrangement of conveniently designed Si crossed-bowtie nanoantennas, can generate chiral hotspots and robust CD signals. Based on the separation distance of the crossed-bowtie systems, we can control the overlap between the corresponding electric and magnetic modes responsible for maximizing the optical chirality of the proposed metasurfaces. The corresponding electric hotspots are located at the central gap region, which opens up the possibility of using this metasurface for circular dichroism spectroscopy. Finally, it is worth mentioning that the proposed metasurface can be relevant in chiral biosensing applications, such as optical enantiomer discrimination after proper functionalization of the metasurface.[58] However, for a detection up to the single-molecule level, an array of symmetric systems can be better suited to avoid the strong background of CD from chiral bowties.