Near-Ultraviolet Dielectric Metasurfaces: from Surface-Enhanced

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Near-Ultraviolet Dielectric Metasurfaces: from Surface-Enhanced Circular Dichroism Spectroscopy to Polarization-Preserving Mirrors Kan Yao, and Yuebing Zheng J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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The Journal of Physical Chemistry

Near-Ultraviolet Dielectric Metasurfaces: from Surface-Enhanced Circular Dichroism Spectroscopy to Polarization-Preserving Mirrors Kan Yao1,2 and Yuebing Zheng1,2,* 1 Department

of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, USA 2 Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, USA * Corresponding Author: [email protected]

Abstract: Circular dichroism (CD) spectroscopy is an important technique to investigate the structural information about chiral molecules. The intrinsically weak chirality of molecules often requires long optical paths and high-concentrated molecules. Metasurfaces can generate localized strong chiral fields to enhance chiral light-molecule interactions near the surface. However, losses and structural chirality of metasurfaces can diminish the inherent CD of the chiral molecules and induce interfering spectral features that disturb the chiral analysis of the molecules. Herein, we realize achiral titanium dioxide metasurfaces to enhance the near-ultraviolet CD of chiral molecules. The device is completely lossless for wavelengths greater than 360 nm and produces intense chiral hotspots in the near field. An 80-fold enhancement of optical chirality is numerically demonstrated, giving rise to a 50-fold enhancement of CD. In addition, we show that the metasurface can function as a polarization-preserving mirror and enhance optical chirality in the far field. Our results provide a scheme to ultra-compact CD spectrometers and chiral cavities.

1. INTRODUCTION Chirality is a basic property of objects about symmetry. An object is chiral if it cannot be superimposed on its mirror image. In nature, many biomolecules are chiral. Their functionalities in life processes, either favorable or harmful, can be distinctly different depending on the handedness of the molecular structures. Therefore, discriminating between chiral molecules of opposite handedness and analyzing their structures are fundamentally important for life sciences. Circular dichroism (CD) spectroscopy that measures the differential absorption of left- and right-

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circularly polarized (LCP/RCP) light is an important technique to study the structural information about biomolecules.1 For example, in the ultraviolet (UV) region where the CD bands of most chiral molecules are located,2 the far-UV CD spectrum reflects proteins’ secondary structures such as α-helix and β-sheet, and the near-UV CD feature contains information about their tertiary structures.3 Because the chiral response of molecules is intrinsically weak, in standard CD, long optical paths are required for the molecular absorption to accumulate to a detectable level. This limits the detection sensitivity and miniaturization of the devices. The recent development of plasmonics and nanophotonics allows generating localized chiral fields to facilitate the lightmolecule interactions.4-14 In particular, metasurfaces with engineered subwavelength-thick resonant nanostructures are promising platforms for surface-enhanced CD spectroscopy.10-14 Nevertheless, most devices operate in the visible and infrared region where most chiral molecules are not optically active; and even the CD of molecules can be enhanced at certain locations, the global enhancement is usually very limited due to the non-uniform distribution of the chiral fields.4-5,9 The absorption by the metasurface will also cause induced CD that disturbs analysis of the information about chiral molecules,7,12 especially when the metasurface itself is chiral.15,16 All these challenges need to be resolved for advanced CD spectroscopy and other practical applications. In this work, we report a design of achiral dielectric metasurfaces for enhancing the CD of chiral molecules at near-UV wavelengths. The metasurface comprises an array of amorphous titanium dioxide (TiO2) nanocube dimers arranged in a square lattice. Promised by the high refractive index and lossless properties of TiO2, intense and uniform chiral hotspots are generated via the interaction between nanocubes in the individual dimers and the interactions between neighboring unit cells, giving rise to strongly enhanced near-UV CD when chiral molecules are adsorbed to the nanocubes. To the best of our knowledge, this is the first design that can enhance optical chirality for two orders of magnitude in the UV region and on a lossless platform. Furthermore, we show that the metasurface can function as a polarization-preserving mirror. Unlike for conventional electric and magnetic mirrors, upon the reflection of circularly polarized light (CPL), the resulting standing wave from the metasurface is still chiral. Providing that enhanced optical chirality is achieved for both the near-field and far-field, our design could lead to ultra-compact CD spectrometers and other novel applications such as chiral lasers.


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2. METHODS To begin with, we briefly introduce several formulae to explain the physics of the problem. The chirality of an optical field can be characterized by a pseudoscalar termed optical chirality C:17,18 𝜀0

1

𝐶 ≡ 2 𝐸 ∙ ∇ × 𝐸 + 2𝜇0𝐵 ∙ ∇ × 𝐵 = ―

𝜔𝜀0 2

Im(𝐄 ∗ ∙ 𝐁).

(1)

Here, E (E) and B (B) denote the real (complex) electric and magnetic vector fields, and ε0 and μ0 are the permittivity and permeability of free space, respectively. CPL is the simplest chiral field. In a vacuum, its chirality C0 = ±ωε0|E|2/(2c) with c the speed of light and the plus/minus sign corresponding to the right/left handedness. CD occurs when a chiral object, e.g., an ensemble of chiral molecules interacts with a pair of chiral fields. By dropping the negligibly weak magnetic response of the molecules, the absorption rate A can be expressed by 𝜔

2

2

± = 2 𝛼′′|𝐄 ± | ∓ 𝜀0𝐺′′|𝐶 ± |, 𝐴𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒

(2)

where the superscripts +/– indicate the corresponding quantities subject to an LCP/RCP excitation and '' denotes the imaginary part of the molecular electric polarizability α and chiral (or mixed) polarizability G. In standard CD spectroscopy, both the electric field intensity |E|2 and the magnitude of optical chirality |C| are identical for LCP and RCP, and thus CD is given by 4

∆𝐴𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 = 𝐴 + ― 𝐴 ― = ― 𝜀0𝐺′′|𝐶|,

(3)

which is proportional to and limited by the chirality of CPL in the surrounding medium. Meanwhile, the dissymmetry factor g, given its definition as the ratio of CD to the conventional absorption1 g =

2(𝐴 + ― 𝐴 ― ) 𝐴+ + 𝐴―

,

(4) reduces to an expression that is proportional to |C|/|E|2.18 The g factor characterizes whether the CD in an absorption band is detectable and physically determines the efficiency of photoionization reaction, from which an excess of a certain enantiomer can be produced for chiral separation.5 Equations (3) and (4) together pose a fundamental requirement that to enhance CD while keeping it distinguishable from conventional absorption of the molecules, it is essential to have an intense chiral field with stronger magnetic components rather than electric components. By using nanostructures, one can achieve largely enhanced chirality. However, the enhancement arises



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predominantly from the strong electric fields that diminish the g factor. Moreover, when chiral molecules are coupled to nanostructures, the local field intensity differs for LCP and RCP excitations. Although this difference between |E+| and |E–| is small, it causes changes to the CD of the molecules12 𝜔

2

4

2

∆𝐴′𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 = 2 𝛼′′(|𝐄 + | ― |𝐄 ― | ) ― 𝜀0𝐺′′|𝐶|

(5)

and more importantly, induces additional CD from the nanostructure12,19 𝜔

2

2

∆𝐴𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒 = 2 ∫𝜀′′(|𝐄 + | ― |𝐄 ― | ) d𝑉,

(6)

where V and ε" are the volume and the imaginary part of the permittivity for the structure. The contribution from nanostructures could be negative and it is comparable in magnitude to the CD of chiral molecules. Therefore, enhancing CD via engineered nanostructures is a very challenging task, especially in the UV region where the CD bands of many biomolecules are located but the material loss becomes significant.

3. RESULTS AND DISCUSSION A. Single Dimer In order to resolve the aforementioned challenges, we first consider an isolated TiO2 nanocube dimer immersed in water. This scenario is chosen based on three considerations. First, TiO2 is a dielectric with a moderately high refractive index at optical wavelengths. The lossless interval of amorphous TiO2 covers the entire visible spectrum and extends to near-UV, terminated at about 360 nm. It has been demonstrated that high quality TiO2 prepared by atomic layer deposition (ALD) is compatible with the state-of-the-art fabrication techniques to produce high-aspect-ratio nanostructures with sub-10 nm gaps.20-22 Second, high-refractive-index dielectric nanoparticles support both electric and magnetic Mie-type resonances. When two nanoparticles are positioned closely forming a dimer, the complex interplay between the resonance modes in each particle may induce strong electric and magnetic fields in the gap, leading to accessible hotspots that facilitate light-molecule interactions.23-26 Third, biosensing is mostly performed in an aqueous environment. By properly selecting the material for the substrate, e.g. magnesium fluoride (MgF2), the impedance mismatch between solution and substrate is minimal and can be reasonably ignored. Therefore, as we will show in the following, the system investigated here takes full advantage of


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the achievable material and structure while keeping the model simple without loss of practical considerations. Figure 1(a) illustrates the schematic of the dimer. Two amorphous TiO2 nanocubes are aligned along the y-axis and separated by a gap of 10 nm. The edge lengths of the nanocubes in each dimension are Lx = 90 nm, Ly = 70 nm and Lz = 110 nm. We study the optical properties of the dimer by using the commercial finite-element electromagnetic solver COMSOL Multiphysics 5.2. In our simulations, a local radius of 15 nm is adopted at all the corners and edges to account for the roundedness from fabrication, the background medium is water with a refractive index n0 = 1.33, and the dielectric property of TiO2 is taken from Ref. 21. The incident light is an LCP plane wave propagating in the –z direction. Figure 1(b) displays the scattering (red), absorption (blue) and extinction (green) cross-section spectra, respectively. Noticeably, owing to the large band gap of TiO2, the absorption is strictly zero for wavelengths greater than 360 nm, which promises complete elimination of undesired background absorption in Eq. (6) for CD spectroscopy. The scattering spectrum exhibits a peak at 355 nm originated from the magnetic dipole resonance of individual nanocubes, as evidenced in Figure 1(c) by the strong magnetic field inside the particles. These magnetic dipole modes couple with each other, giving rise to an accessible magnetic hotspot in the dimer gap.23,27 Together with the contribution from the broadband electric dipole resonance centered at a shorter wavelength of ~330 nm, the magnetic resonance helps to form a chiral hotspot in the gap.25 As also shown in Figure 1(b), when averaged over the volume of the dimer gap (60 nm × 10 nm × 80 nm, after removal of the rounded areas), the local optical chirality is maximized at ~370 nm wavelength (black curve), slightly red-shifted from the magnetic dipole resonance, and the peak value exceeds 5.5 times the chirality of the incident LCP light. Figure 1(d) shows the distribution of the normalized optical chirality on the symmetry plane (y = 0) cutting through the gap and perpendicular to the dimer axis. The profile features a maximum enhancement factor of 9 located about 20 nm above the bottom edge, from which the chirality decreases gradually following elliptical contours. In Figure 1(e), the optical chirality is further examined on the surface of the nanocubes. As can be clearly seen, strong chirality only occurs on the two facets forming the dimer gap, and the pattern is almost the same as in Figure 1(d), meaning that the chirality density is fairly uniform within the gap. On all the other facets, the chirality is relatively weak but remains in the same sign as that for LCP. Because the dimer is achiral, for an incidence of reversed handedness, i.e. an RCP plane wave, the scattering, absorption and extinction cross-sections will



be identical to those for LCP, while chirality keeps the same profile with only the sign flipped. In other words, unlike metallic nanostructures and dielectric single particles that generate complex local chiral fields with spatially alternating handedness, dielectric dimers “squeeze” the incident CPL to a chiral hotspot in the gap and the local field exhibits uniform handedness. Compared with an earlier design made of silicon,25 the dimer studied here provides a less intense chiral field because of the lower refractive index of TiO2 and insufficient spatial overlap between the magnetic and electric hotspots (see Figures S1 and S2 in the Supporting Information). Nevertheless, it enables an unparalleled, completely lossless and achiral platform allowing the operational wavelength to extend over the near-UV region critical for CD spectroscopy. B. Dimer Array Metasurface The near-field of isolated resonant nanoparticles can be strongly modified by engineering the structures and by coupling to adjacent resonators.28-31 In the next, we arrange the TiO2 nanocube dimers into a two-dimensional (2D) array as sketched in Figure 2(a). The geometry of the dimers is not further engineered, because compared to this local optimization the collective coupling between the neighboring unit cells is more critical in determining the change of optical responses, especially the linewidth of the resonance and the near-field enhancement. In analogy to Figure 1(b) for a single unit cell, Figure 2(b) shows the reflectance (red), transmittance (green), absorbance (blue) as well as chirality enhancement (black) spectra of a dimer array with periodicity px = py = 260 nm. The periodicity is chosen by simulations from a handful of candidate values below the diffraction edge.30 The most evident feature is that a distinct peak hits total reflection at 373 nm, accompanied by the maximum of optical chirality almost at the same wavelength. Both peaks are much narrower than those for a single unit cell thanks to the collective interactions between neighboring dimers. The field distribution at the reflection peak is presented in Figure 2(c), where the strong magnetic field confined inside the cubes indicates that the mode is the magnetic dipole resonance. Other features in the spectra will be discussed in a later section. Compared with the results for an isolated dimer in Figure 1(c), here the magnetic field is further strengthened by a factor of 4. In fact, a comparable enhancement factor is obtained for electric field as well (see Figure 3 below and Figure S1 in the Supporting Information). This effect results from the short-range collective interactions of each dimer with a few of its nearest neighboring unit cells,28 which contrast with the long-range diffractive coupling in plasmonic lattices but can


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also boost the quality factor of the resonance.30 Because of the largely enhanced constituent fields, the optical chirality in the dimer gap becomes dramatically stronger. With the volume-averaged value approaching 80 read from Figure 2(b), the spatial maximum of normalized chirality exceeds 125 near the dimer gap center, as shown in Figure 2(d). This, to the best of our knowledge, is the first design that can achieve two orders of magnitude enhancement of chirality in the UV region and on a lossless platform. Similar to the isolated dimer, the dimers in the array squeeze CPL to the hotspot in the gap, while on the other facets the chirality is relatively weak but holds the same sign as for the incident CPL. However, as can be seen in Figure 2(e), due to the global field enhancement from the collective interactions, a large portion of the surfaces still displays chirality greater than 20. Hence in practice, although the chiral hotspot in the dimer gap is an ideal location for chiral molecules to attach to, the other facets also have a beneficial effect on enhancing the CD signal. In our simulations, we mainly focus on the square lattice with a lattice constant of 260 nm. Larger periodicity will introduce diffractive coupling in the spectral interval of our interest, which nevertheless is weak for dielectric resonators and does not help to enhance the local fields.31 On the contrary, reducing the lattice constant allows blue-shifts of the operational wavelengths. It is worth mentioning that the square lattice seems important for achieving the superior optical properties. When fixing py = 260 nm and gradually decreasing px, the chirality enhancement peak drops in magnitude and is blue-shifted accordingly, while the total reflection disappears (See Figure S3 in the Supporting Information for a comparison of different designs). Both effects can be attributed to the disturbed relative phase between the electric and magnetic resonances. Complete understanding of the relation between the resonance modes, phase conditions, and design variables requires thorough parameter sweeping, which is a task beyond the scope of this work and more suitable for advanced optimization algorithms.32 Although for chirality enhancement the simple scaling law may not hold as in Ref. 29, achieving pixelated devices to cover a continuous wavelength interval is still possible when the design space is sufficiently searched. The formation of the chiral hotspot is further studied by inspecting the constituent magnetic and electric fields in the gap. Interestingly, different from the case of the isolated dimer (see Figure S2), the magnetic and electric hotspots show nearly perfect overlap in the array. Figure 3(a) and (b) compare the distributions of normalized magnetic and electric fields at 374 nm wavelength on the cutting plane through the dimer gap center. Two hotspots co-occur near the center while the



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contours follow similar patterns despite slight differences near the corners. It is worth mentioning that around these hotspots the local magnetic field is enhanced more than the electric field, which according to Equation (4) leads to a much larger dissymmetry factor than that from plasmonic nanostructures. As another critical factor determining the optical chirality, the relative phase between magnetic and electric fields is also examined. Figure 3(c) presents the distribution of the phase difference between Hy and Ey in the dimer gap, which are the dominant ones among all the six components of the vectorial fields. As can be seen, although not perfectly uniform, the phase spans only a narrow range around the ideal value of π/2 inherited from the incident LCP. Therefore, it is the strong and simultaneous enhancement of magnetic and electric fields, their spatial overlap and well-preserved relative phase together giving rise to the intense chiral field in the gaps in the dimer array. The performance of the proposed metasurface for CD enhancement is then demonstrated by incorporating chiral molecules in the system. To maximize the beneficial effect of the chiral hotspots, here we consider an idealized situation that chiral molecules are located only in the gap of each dimer. Realistic conditions will be discussed in a later section. Following the previous formalism derived by Govorov and coworkers,19,25 arbitrarily oriented chiral molecules can be modeled as an isotropic and homogeneous medium described by the frequency-dependent and complex-valued relative permittivity εr 𝜀𝑟 = 𝜀𝑟0 ― 𝛾

(

1 ℏ𝜔 ― ℏ𝜔0 + iΓ

1

― ℏ𝜔 + ℏ𝜔0 + iΓ

)

(7)

and chiral Pasteur parameter κ

(

𝜅=𝛽

1 ℏ𝜔 ― ℏ𝜔0 + iΓ

1

)

+ ℏ𝜔 + ℏ𝜔0 + iΓ .

(8)

Here, εr0 is taken to be 1.5, ħ is the reduced Planck constant, ω0 = 2πc/λ0 with λ0 the molecular resonance wavelength, Γ is the resonance broadening, and γ and β are coefficients related to the dipole moments and density of the chiral molecules. The magnetic response is negligibly small and thus μr = 1. Because the CD of chiral molecules will be enhanced most when the molecular resonance spectrally overlaps the chirality maximum, without loss of generality, in simulations we choose λ0 = 375 nm coincident with the chirality peak in Figure 2(b) and Γ = 0.4 eV. In nature, there are many chiral molecules exhibiting CD near this wavelength.2 For molecules that have CD bands away from the chirality peak, an induced CD will still emerge at the peak position, whereas the inherent CD of the chiral molecules becomes complicated depending on the optical chirality at


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the molecular resonance. Figure 4(a) and (b) show the permittivity and Pasteur parameters of the chiral molecules, with γ = 0.23 eV and β = 4.36 × 10-5 eV determined from a peak molecular extinction of 10000 M-1cm-1, a peak molecular CD of 10 M-1cm-1 and a molecular density of (0.7 nm)-3, respectively. All these values are in the typical order for chiral molecules. The optical properties of the composite system, namely the metasurface with all the dimer gaps filled by the chiral molecule patches are reported in Figure 4(c) for CPL excitation. Noticeable absorption (blue curve) can be observed at two wavelength intervals. The very broad one at shorter wavelengths corresponds to the characteristic electric and magnetic dipole resonances of the individual nanocubes centered at ~366 and 374 nm, respectively; the other at 390 nm originates from the magnetic resonance of the dimers in the lattice (see Figure S4 in the Supporting Information for details), which features a magnetic hotspot in the gap rather than in the cubes. Because the metasurface is lossless, all the absorption is from the chiral molecules. Moreover, as the Pasteur parameter is four orders of magnitude smaller than the permittivity, the chiral response of the molecules is intrinsically weak and cannot be recognized when data are presented in the unit of absorbance. We thus express the CD signal separately in the unit of ellipticity in Figure 4(c) (black curve). As can be seen, the CD spectrum follows the lineshape of the chirality enhancement in Figure 2(b) and reaches a detectable maximum value of 7.5 mdeg at 374 nm,6 confirming that strong optical chirality is essential for enhancing CD. Figure 4(d) compares the CD signal from the composite and those from continuous films consisting of the same chiral molecules without the metasurface. In contrast to the chiral patches loaded in the dimer gap, chiral films exhibit a broad peak centered at the molecular resonance. The magnitude of CD from chiral films increases monochromatically as the film becomes thicker, and the peak value equals 7.5 mdeg when the thickness is about 35 nm. Compared with chiral molecule patches loaded in the dimer gap, a film of this thickness is 50 times larger in volume. In other words, the metasurface provides a 50-fold enhancement of CD. This factor is lower than the volume-averaged chirality enhancement of 80, which can be explained by Equation (5) (also see Figure S5 in the Supporting Information for decomposed CD), and the dissymmetry factor is not affected much at the molecular resonance (see Figure S6 in the Supporting Information). C. Chiral Mirrors



In practical circumstances, chiral molecules will not only be adsorbed to the dimer gap or other facets, where as shown in Figure 2(e) the chirality is considerably strong. To account for the contribution from chiral molecules that are dispersed in solution, we examine the optical field in different regions. Figure 5(a) shows the polarization-resolved reflectance and transmittance of the dimer array for CPL illumination. Interestingly, at the wavelength of the magnetic dipole resonance, over 90% of the incident power is carried by the co-polarized reflected CPL. This is opposed to conventional surfaces, from which CPL will flip handedness to the cross-polarization state upon reflection.18 Because of the structural anisotropy of the unit cell, the reflected fields polarized along and perpendicular to the dimer axis have a π/2 phase difference opposite to that for the incident fields.33 Therefore, the metasurface could function as a polarization-preserving mirror.34 Preserving the handedness of the reflected light has a beneficial effect for enhancing CD. For conventional mirrors, e.g. a metallic substrate, the chirality of the incident CPL and of the reflected CPL are in opposite signs. The total field or the standing wave, as the chirality cancels out, is achiral. In contrast, for the polarization-preserving mirror, the chirality of the incidence and reflection are in the same sign and they add up in the total field, giving rise to doubled chirality. Figure 5(b) presents the field profile along a vertical (in the z direction) probing line through the center of the dimer gap, where z = 55 nm corresponds to the top surface of the nanocubes. From the inset, one can clearly see that in the far-field, the magnetic and electric field intensities are modulated following the typical pattern for interference between two counterpropagating waves. In addition, the normalized chirality tends to be a constant of ~1.87, which unveils the reflected light is co-polarized to the incident CPL with a slight decrease in magnitude. The enhancement of chirality in the far-field by polarization-preserved reflection is nontrivial in applications. Although the metasurface alone can be used as a platform for surface-enhanced CD spectroscopy, the enhanced chirality in the far-field allows dispersed molecules away from the surface to interact with a chiral field. Moreover, when two such metasurfaces form a cavity,34 launching CPL at a small angle of incidence will induce multiple reflections between the two boundaries. Because polarization is preserved at every reflection, this process is equivalent to having the CPL propagating through a very long optical path, leading directly to applications such as ultra-compact CD spectrometers and chiral cavities for circularly polarized lasers. In fact, without the need to match molecular CD bands, this functionality is not limit to the UV region. The scheme can be readily extended to any wavelengths by choosing different materials and engineering the particle


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sizes. Because an oblique incidence will introduce a small in-plane wave vector that changes the relative phase, additional optimization of the structure or the incidence polarization states may be needed to maximize the performance. The metasurface also possesses favorable properties in the near-field. Conventional metallic surfaces reflect light in a manner similar to that of a perfect electric conductor (PEC). Because phase reversal occurs upon reflection, the resulting standing wave has a node (antinode) of the total electric (magnetic) field near the surface, which impedes effective light-matter interactions. Hence, electric mirrors are not suitable for making devices that have subwavelength active layers atop. On the other hand, magnetic mirrors, the dual of electric mirrors, can produce an antinode (node) of electric (magnetic) field near the surface like a perfect magnetic conductor (PMC), thus enabling largely enhanced interaction with electric dipoles of the molecules.35-38 However, as only one of the two constituent fields, either magnetic or electric, can be enhanced by the electric and magnetic mirrors with the other component suppressed, the optical chirality and associated interaction with chiral molecules for CD are very weak at conventional surfaces. Our design offers a solution to this challenge. Figure 5(c) and (d) compare the interference patterns from a conventional mirror (left columns) and from the dimer array metasurface (right columns). Contrary to the complementary patterns from flat mirrors, strong electric and magnetic fields are clearly seen to appear simultaneously near the nanocubes, giving rise to surface-enhanced optical chirality as shown in Figure 5(e). From this perspective, the proposed device can be considered a chiral mirror. To avoid ambiguity with the recent work on handedness-selective reflectors,34,39-41 we term our metasurface as a “near-field chiral mirror”; while for the far-field, it is a polarizationpreserving mirror. The result in Figure 5(e) ensures that dispersed chiral molecules near the surface will also contribute to the CD signal, in addition to those in the dimer gap and in the far-field.

4. CONCLUSIONS In summary, we propose and numerically study an achiral dielectric metasurface for near-UV CD of chiral molecules. Based on the fact that isolated amorphous TiO2 nanocube dimers can generate a chiral hotspot at the wavelength of magnetic dipole resonance, an appropriate design of dimer array metasurfaces is achieved, where through the collective interactions between neighboring unit cells the local chirality in the gaps is enhanced by two orders of magnitude in the near-UV region. Surface-enhanced CD is verified by filling the dimer gaps with chiral molecule



patches. The CD signal is enhanced for ~50 times compared with that from free-standing molecules. Because the metasurface is achiral and lossless, undesired background signal or absorption from the substrate is completely removed, making the enhanced CD spectra purer for representing the information about chiral molecules. Meanwhile, the chiral hotspots are composed of nearly equally enhanced magnetic and electric fields, which helps to maintain the dissymmetry factor unaffected at the resonance. We further show that although intense chiral hotspots only occur in the dimer gap, the enhancement of optical chirality is not limited to these locations but rather global. In the near-field, unlike for conventional electric or magnetic mirrors, the magnetic and electric fields are enhanced simultaneously by the metasurface to form an enhanced chiral field. More interestingly, in the far-field, as polarization is preserved upon the reflection of CPL, the resulting standing wave is still chiral. These properties result from the short-range collective interaction between neighboring dimers, which has been shown not sensitive to modest disorder of the lattice.28 With all the advantages above, the proposed metasurface functions as a unique platform for enhancing near-UV CD of chiral molecules, and its performance could be further improved by optimizing the design and the material.42 We foresee that our work will lead to practical surface-enhanced CD spectrometers and chiral cavities for numerous applications such as ultra-compact spectrometers, enantioselective sorting, and chiral lasers.

Supporting Information Additional simulations on isolated dimers of different sizes; field distributions in the isolated dimer when chirality is maximized; optical properties of metasurfaces with different lattice constants; field distributions for the magnetic dipole resonance of the dimer in the array; decomposition of CD into different components based on their origin; influence on the dissymmetry factor.

Acknowledgements K.Y. and Y.Z. acknowledge the financial support from the National Science Foundation (NSFCMMI-1761743), the National Aeronautics and Space Administration (NASA) Early Career Faculty Award (80NSSC17K0520), and the Army Research Office (W911NF-17-1-0561).

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Figure 1. (a) Schematic of a TiO2 nanocube dimer aligned along the y-axis. The dimensions of each nanocube is Lx = 90 nm, Ly = 70 nm, Lz = 110 nm and the gap size t = 10 nm. The edges and corners are all modeled with a roundedness radius of 15 nm. (b) Spectrum of volume-averaged chirality in the dimer gap (black curve) and spectra of the scattering (red curve), absorption (blue curve), and extinction (green curve) cross sections of the dimer under top illumination of an LCP plane wave. The dimer is immersed in water with a refractive index n0 = 1.33. For a normal



incidence of RCP, the chirality flips the sign while cross-sections are unchanged. (c) Normalized magnetic field distribution at 360 nm wavelength on the cutting plane through the symmetry plane of the dimer, outlined by the white contours. (d) Profile of optical chirality at 370 nm wavelength on the cutting plane through the dimer gap center. (e) Surface distribution of the optical chirality at 370 nm wavelength, observed from the gap (left) and from the end (right). The slice in the middle shows a map of chirality in the gap center as in (d). Two nanocubes are offset laterally for visualization.


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Figure 2. (a) Schematic of the metasurface based on a TiO2 nanocube dimer array. The unit cell is identical to that in Figure 1(a), and the periodicity px = py = 2a = 260 nm. (b) Reflectance (red curve), transmittance (green curve), and absorbance (blue curve) spectra of the dimer array immersed in water. The excitation is a normally incident LCP/RCP plane wave propagating in the –z direction. Only one set of the spectra is shown because the metasurface is achiral. The spectrum of volume-average chirality enhancement in the dimer gap (black curve) is plotted for LCP only,



showing a maximum value of almost 80 at 374 nm wavelength. For RCP the chirality flips the sign. (c) Normalized magnetic field distribution at 374 nm wavelength on the cutting plane through the symmetry plane of a unit cell in the dimer array. (d) Profile of optical chirality at 374 nm wavelength on the cutting plane through the dimer gap center. (e) Surface distribution of the optical chirality at 374 nm wavelength, observed from the gap (left) and from the end (right). The slice in the middle shows a map of chirality in the gap center as in (d). The nanocubes are offset laterally for visualization.


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Figure 3. Field distributions on the cutting plane through the gap center of a dimer in the array. The dimer is aligned along the y-axis and an LCP plane wave at 374 nm wavelength is launched from the top, propagating in the –z direction. (a) Magnetic field. (b) Electric field. Both are normalized to the respective intensity of the incident field. (c) The relative phase between Hy and Ey. The excellent spatial overlap between the magnetic and electric hotspots and the narrow distribution of their phase difference around π/2 ensure intense and uniform optical chirality in the dimer gap. For an incidence of RCP, the field intensity profiles in (a) and (b) do not change but the relative phase in (c) will flip the sign, leading to opposite handedness of optical chirality.



Figure 4. Dispersion of permittivity εc (a) and the Pasteur parameter κ (b) of the chiral molecules exhibiting a CD band centered at 375 nm wavelength. (c) Reflectance (red curve), transmittance (green curve), and absorbance (blue curve) spectra of the dimer array when all the gaps are filled with chiral molecules with properties given in (a) and (b). The excitation is a normally incident LCP/RCP plane wave. Only one set of the spectra is shown because the difference between the responses to LCP and RCP is very small, which cannot be recognized with the present scale on the left. The CD spectrum (black curve) is plotted with a separate scale in the standard unit on the right, which follows nicely the lineshape of the optical chirality enhancement in Figure 2(b). (d) A comparison of CD spectra for the dimer array with all the gaps filled with chiral molecule patches (black curve) and for suspended chiral films of different thickness consisting of the same chiral molecules (colored curves).


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Figure 5. (a) Polarization-resolved reflectance and transmittance spectra of the dimer array for CPL at normal incidence. The spectra hold for both LCP and RCP as the structure is achiral. (b) Spatial distribution of normalized electric (blue curves) and magnetic (red curves) field intensities and chirality enhancement (black curves) near the metasurface. The fields are probed along a vertical line through the gap center of a dimer in the array. The dimer’s top and bottom surfaces correspond to z = 55 and –55 nm, respectively. Inset: Zoom-in of the field distributions one wavelength away from the metasurface. The modulation of electric and magnetic field intensities is a typical feature for standing waves as a result of interference, while the chirality enhancement is a fingerprint revealing that the two counter-propagating waves are co-polarized CPL. (c-e) A comparison of interference patterns from conventional mirrors and from the metasurface upon LCP illumination at 373 nm wavelength. Conventional mirrors are denoted by the horizontal white line in the left panels. a = 130 nm is half of the lattice constant. (c) Normalized electric field pattern



from a perfect electric conductor (PEC) mirror (left) and from the metasurface (right). The left panel holds true for the magnetic field pattern from a perfect magnetic conductor (PMC) mirror. (d) Normalized magnetic field pattern from a PEC (left) and from the metasurface (right). The left panel holds true for the electric field pattern from a PMC mirror. Color saturation near the metasurface indicates strong local field enhancement. (e) Chirality enhancement from the metasurface. For RCP, the pattern simply flips the sign. Patterns from conventional mirrors are not shown because the standing wave is achiral and the chirality is zero everywhere.


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TOC Graphic


Near-Ultraviolet Dielectric Metasurfaces: from Surface-Enhanced

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