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Abstract
Circular dichroism (CD), as one of the most representative chiroptical effects, provides a simple strategy for the detection and characterization of the molecular chirality. The enhancement and sign reversal of CD are of great importance for its practical applications in chiral bio-sensing, chirality switching and optical filtering, etc. Here, we realize considerable adjustments and the sign reversal of CD in quasi-three-dimensional (quasi-3D) combined Archimedean spiral nanostructures. With special local and lattice configurations, the nanostructures have both right-handed and left-handed geometric chirality, which are designed based on the proximity effect of stencil lithography. We find that the CD response of the nanostructures becomes obvious once its height exceeds 200 nm and can be adjusted by the further increase of the height or the change of the blade spacing of the nanostructures. The CD reversal is achieved by utilizing the competition of two chiral centers when the height or blade spacing exceeds a critical value. Further analysis of the scattering power of multipole moments reveals that the CD modulation is determined by both magnetic dipole moment and electric quadrupole moment. Benefiting from the highly sensitive CD response to the height, the extreme sign reversal of CD is achieved when a sub-10-nm ultrathin medium layer is anchored on the surface of the nanostructures, which provides a promising strategy for ultra-sensitive chiral bio-sensing.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
If an object cannot coincide with its mirror image through any translation and rotation, the object is considered to possess chirality. Such an object and its mirror image are called enantiomers [1,2]. In optics, right- and left-handed circular polarization (RCP and LCP) lights are a pair of enantiomers. RCP and LCP lights, interacting differently with chiral objects, result in chiroptical effects, such as circular dichroism (CD) [3], which is the absorption difference between RCP and LCP lights [4,5]. However, natural chiral materials often show weak CD due to the mismatching between the wavelength of light and the molecular size. Extensive researches have shown that plasmonic nanostructures can be used to effectively enhance the CD amplitude by several orders of magnitude [6–9]. This is of great significance in both fundamental studies and practical applications and can be applied in optical filtering [10], selective thermal radiation [11] and biosensors [12], etc. In addition, the structure can be converted to its enantiomer by using external stimuli, such as photo-induced excitation [13,14], temperature [15] and electron tunneling effect [16], with which the CD reversal can be realized. However, the traditional CD reversal schemes require drastic changes in structural geometries, leading to a dramatic increase in manufacturing complexity and difficulty.
Here, we demonstrate an ultra-sensitive CD reversal in quasi-three-dimensional (quasi-3D) combined Archimedean spiral nanostructures based on the designs of stencil lithography. The reason to choose stencil lithography is that compared with other three-dimensional (3D) fabrication method [17–19], stencil lithography [20] is not only desirable for large-scale fabrication, but also adaptable for a variety of substrates and deposition materials. Meanwhile, the fabrication accuracy of such technology can reach as high as 20 nm [21,22]. More importantly, based on the proximity effect of stencil lithography [23], the combined Archimedean spiral pattern on the stencil shrinks laterally as the height of the nanostructures increases, which breaks the mirror symmetry of the nanostructures and consequently transforms the spiral nanostructures from achiral to chiral. The proximity effect of the stencil lithography relies on aperture clogging, which naturally occurs on the stencil membrane during material deposition, and leads to lateral pattern shrinkage, projecting 2D patterns as quasi-3D nanostructures on the target substrate [24,25]. By controlling the height of the quasi-3D nanostructures or changing the geometric parameters of the stencil, the amplitude and the sign of CD are successfully tuned. Further analysis of the scattering power of various multipole moments reveals that the amplitude and the sign of the CD are determined by both magnetic dipole moments and electric quadrupole moments. By utilizing this design principle, the sign of CD is readily reversed when a sub-10-nm layer of medium is anchored on the structural surface, providing a potential way for ultra-sensitive chiral bio-sensing.
2. Simulated methods and models
The proposed quasi-3D chiral nanostructure is a kind of three-blade spiral wedge structure arranged in a hexagonal lattice, which is designed based on the proximity effect of stencil lithography. Due to the fascinating performance of Archimedean spiral in various chiral models [26–30], the combined Archimedean spirals are employed to construct the nano-pattern of initial 2D stencil unit cell as shown in Figs. 1(a)-(c).
Fig. 1. Schematic diagram of the structure design. (a) Plot of an Archimedean spiral curve, where g represents the distance between two adjacent circles. (b) A geometric pattern made of combined Archimedean spirals. ${w_{\max }}$ is the maximum width of the blade. (c) Top-view of the initial 2D stencil unit cell arranged in a hexagonal lattice with a separation of 1.2 µm, which consists of three milled holes with different rotation angles. (d-e) Side-view (d) and top-view (e) of the simulated quasi-3D combined Archimedean spiral nanostructure, respectively.
In Cartesian coordinate, the Archimedean spiral can be expressed by
3. Results
3.1 CD engineering by the height of the nanostructures
Since the maximum height of the quasi-3D chiral nanostructures in this design is determined by ${\textrm{h}_{\max }} = {\textbf 1}{\textbf .69}{\textrm{w}_{\max }}$ (where ${\textrm{w}_{\max }}$ is the maximum width of the holes on the stencil) [23], one can change the height of the nanostructure by controlling the scale of gold atoms deposited on the stencil, which can be readily realized by engineering the deposition time and rate during the stencil lithography. To investigate the CD responses of the chiral nanostructures with different heights, the numerical simulations are performed with finite element method (FEM) by using a commercial software CMOSOL. Periodic boundary conditions within the x-y plane are applied to simplify the simulations. To ensure the accuracy of calculation, the minimum value of the spatial grid is set to 2 nm to construct the gold nanostructures. The optical transmission is simulated under the normal incidence of the circularly polarized light propagating along the z-axis direction. The refractive index and extinction coefficient of gold are described by the Lorentz-Drude model [32]. The refractive index of the SiO2 substrate is set to 1.45. The thickness of the substrate is set to semi-infinite. The optical transmission spectra were obtained under the normal incidence of RCP and LCP lights, respectively. When the height increases from 200 to 450 nm, it can be seen from Figs. 2(a)-(c) that two resonance modes are excited as noted by the green and blue arrows. The resonance mode in the short wavelength region changes obviously, whereas the mode in the long wavelength region almost does not change. Here, we focus on the resonance mode in the short wavelength region.
Fig. 2. Engineering of CD by controlling the height of the nanostructures. (a)-(c) Calculated transmission spectra of the chiral nanostructures under normal incidence of RCP and LCP lights with h=200 nm (a), 300 nm (b) and 400 nm (c), respectively. The green arrows point to the resonance in the short wavelength region, and the blue arrow points to the resonance in long wavelength region. (d) CD spectra of the chiral nanostructures with different heights. As the LCP and RCP reflections are identical under C3 rotational symmetry, in this work the CD in absorption is defined as $CD = {A_{LCP}} - {A_{RCP}} = {T_{RCP}} - {T_{LCP}}$, where A and T are absorption and transmission, respectively. (e) Top-view of the simulated combined Archimedean spiral nanostructure array at h=350 nm and s=240 nm. In the purple hexagon, the three blades are rotated in RH with respect to the chiral center R (denoted by the purple dot). In comparison, in the red hexagon, the three blades are rotated in LH with respect to the chiral center L (denoted by the red dot). Two chiral centers R and L are intertwined with each other.
When the height of the chiral nanostructure is less than 200 nm, the transmission spectra of RCP and LCP light are almost the same. However, when the height is larger than 200 nm, sharp resonances appear in both RCP and LCP excitations, with the amplitudes and the positions of the resonances changing accordingly, which is caused by the breaking of mirror symmetry due to increased shrank top part. Interestingly, as the height reaches 300 nm, a sharp resonance dip appears in the transmission spectrum of RCP light, as noted by the green arrow in Fig. 2(b), where the LCP transmission spectrum shows no dip. However, when the height increases to 400 nm, the LCP transmission spectrum possesses a more pronounced resonance dip while the transmission dip of the RCP light becomes relative shallower (Fig. 2(c)). Therefore, when the height increases from 200 to 450 nm, the CD response changes dramatically around λ=1600 nm and the sign of CD has undergone a reversal as shown in Fig. 2(d). Specifically, the CD dip and peak values change from -0.17 at λ=1538 nm (h=300 nm) to +0.42 at λ=1646 nm (h=400 nm). Furthermore, the dip of CD is almost zero when h=330 nm, which means there is a critical height where the transmissions of RCP and LCP lights are the same. Besides the CD amplitude modulation and sign reversal, the positions of the CD dip and peak are also tuned with a maximum red shift of 188 nm (λd=1504 nm at h=250 nm and λp = 1692 nm at h=450 nm).
In order to study the reason for the amplitude engineering and sign reversal of CD, we analyze the properties of periodically arranged quasi-3D chiral nanostructure. One feature of our design is that the right handed (RH) nanostructure has three twisted blades connected by a chiral center, as enclosed by the purple dashed hexagon in Fig. 2(e). When the RH nanostructure is arranged into a hexagonal lattice, another type of subunit consisting of three intercellular blades is formed in the red dashed hexagon, which is a left-handed (LH) nanostructure. As a result, the periodic array has two chiral units (named as R and L subunits) which share the same twisted blades. The intracellular and intercellular chiral units can interact strongly with each other through the surface lattice resonances (SLRs) that are formed by the coupling between local plasmonic resonances and diffractive lattice modes [33,34], the feature of which can be clearly identified in the sharp dip of the transmission spectra (see Figs. 2(b)–2(c) and following Fig. 3). Generally speaking, two chiral centers with opposite handedness can have two CD responses with opposite signs at different wavelengths. However, due to the threefold rotational symmetry of both the twisted unit cells and the hexagonal lattice, the two CD responses in our special geometries may occur in almost the same wavelength band and compete with each other when the height of the nanostructures increases. As a result, the final CD response is formed as a positive peak or negative dip. This competition of two chiral centers determines CD response in the spectrum and is very sensitive to the height of the nanostructures.
Fig. 3. Engineering of CD through controlling the unit displacements. (a) Schematic diagram of changing the spacing among adjacent blades on the stencil. The spacing of the red and blue patterns are s=240 nm and 60 nm, respectively. (b-c) Top-view of the simulated combined Archimedean spiral nanostructure array at s=160 nm and 60 nm, respectively, when h=350 nm. (d-e) Calculated RCP and LCP transmission spectra under different spacing as noted. The dashed arrows denote the region where the resonances change in different strength. (f) CD spectra of the chiral nanostructures with different spiral blade spacings as noted.
3.2 CD engineering through unit displacements
To study the competition induced chiral effects, the pattern on the stencil is kept unchanged but the spacing (s) between two adjacent spiral blades are changed as shown in Fig. 3(a). The definition of s is the end of the outer spiral of the blade to θ=π of the outer spiral of the adjacent blade on the stencil. In this way, the height of the chiral nanostructures is maintained at h=350 nm and the s between two adjacent blades is decreased from 240 nm to 60 nm as shown in Fig. 2(e) and Figs. 3(b)-(c). In such a case, the spacing among the blades of the RH unit (enclosed by the purple hexagons) becomes smaller, while that of the LH unit (enclosed by the red hexagons) get larger when the positions of the R and L remain unchanged. In this case, the RH unit influences the properties of the nanostructures more intensively than the LH unit, and the coupling between RCP light and the nanostructures is gradually increased, while decreases for the case of LCP light. This can be verified by the results in Figs. 3(d)-(e), where the transmission spectra of RCP light change more dramatically than those of LCP light, as noted by dashed arrows. This results in a dramatic change of the CD spectra as shown in Fig. 3(f). Interestingly, the CD peak value decreases gradually with the decrease of s, accompanied with a small blue-shift of the resonance. Importantly, when the s is smaller than 160 nm, the signs of CD in the short wavelength region are reversed, i.e. the previous CD peak changes into CD dip. Meanwhile, the amplitude at the CD dip increases with the further decrease of the spacing. Specifically, the CD peak value is +0.16 at λ=1592 nm when s=240 nm, while the CD dip value is -0.23 at λ=1556 nm when s=60 nm. In such a way, the CD modulation is successfully achieved by merely tuning the spacing of the adjacent spiral blades of the nanostructures, which further reveals that the competition between the two chiral centers determines the CD response.
It should be mentioned that the chiral SLRs play important roles in such CD reversal. First, the transmission spectra show similar asymmetric Fano-type lineshapes as those in Figs. 2(b)–2(c). Second, it is found that near the CD reversal wavelengths, the contributions from local chiral plasmonic resonances are greatly enhanced by the diffractive lattice modes (see below multipolar analysis). Third, in the long wavelength region which is far away from the chiral SLRs wavelengths (determined by product of the period and the effective refractive index of the modes), the symmetric CD peaks are not reversed, as shown in Fig. 3(f).
3.3 Electromagnetic multipolar analysis
It can be seen from Fig. 2(d) and Fig. 3(f) that changing the height or spacing of spiral blades can control the competition between chiral centers, which consequently engineer the CD amplitude and sign. To understand such kinds of CD modulation, the electromagnetic multipole moments excited by RCP and LCP lights are analyzed. Specifically, the scattering power of multipole excitation decomposition, including the electric dipole (ED) moment, magnetic dipole (MD) moment, electric quadruple (EQ) moment and magnetic quadruple (MQ) moment, are calculated using the Cartesian multipole decomposition method as follows [35]:
It can be seen from Fig. 4 that various multipolar moment modes are excited in the short wavelength region. Interestingly, when h changes from 300 nm to 350 nm as shown in Figs. 4(a)-(c) and Figs. 4(d)-(f) (under the same s=240 nm), the scattering powers contributed by MD and EQ change significantly and the differences of the scattering power between RCP and LCP excitations are reversed. Similarly, when s is switched between 240 nm and 60 nm as shown in Figs. 4(d)-(f) and Figs. 4(g)-(i) (under the same h=350 nm), the relative scattering power contributed by MD and EQ is also reversed. This implies that MD and EQ together determine the CD amplitude and sign.
Fig. 4. Electromagnetic multipolar analysis. Calculated scattering power from various multipole moments under RCP and LCP incidence when (a-b) h=300 nm s=240 nm; (d-e) h=350 nm s=240 nm; (g-h) h=350 nm s=60 nm. ED, electric dipole moment; MD, magnetic dipole moment; EQ, electric quadruple moment; MQ, magnetic quadruple moment. (c, f, i) Differential scattering power between RCP and LCP excitations ($\Delta S = {S_{RCP}} - {S_{LCP}}$) for three different parameters of the chiral nanostructures.
3.4 Potential application in bio-sensing
The facile CD engineering effects, especially the drastic CD sign reversal scheme, provides an interesting strategy for ultra-sensitive chiral bio-sensing. To test this idea, a thin layer of medium with a refractive index of 1.43 used to mimic the protein is covered on the surface of the quasi-3D chiral nanostructures as shown in Fig. 5(a), where the height of the initial nanostructures is 325 nm and the spacing among adjacent spiral blades is 240 nm. When adding the thin layer, the values of a and θ are slightly increased and the thickness of upper surface are added, while the g parameters of the two Archimedean spirals are kept unchanged. The minimum value of the mesh grid in within the added layer is set to 1 nm. Benefiting from the extremely sensitive CD response of the chiral nanostructures to its height and spacing, the well-defined CD response is red shifted by 18 nm and the CD dip becomes a CD peak (λd=1564 nm at t=0 nm and λp=1582 nm at t=8 nm), as shown in Fig. 5(b). Strikingly, the sign of the CD is reversed by such a simple coating, i.e. the CD sign can be reversed by coating a thin layer of protein with thickness of less than 10 nm. This indicates that the proposed chiral nanostructures can be used as an effective tool for potential applications in ultra-sensitive bio-sensing. It should be mentioned that once the medium is deposited onto the nanostructure, the tuning of refractive index in the realistic range can not reverse the CD sign due to the special geometric design (results not shown).
Fig. 5. CD reversal by sub-10-nm thin layer. (a) Schematic diagram of the nanostructures covered by a thin-layer medium with a refractive index of 1.43 and a thickness of t nm (as marked in red color) with h=325 nm and s=240 nm. (b) Calculated CD spectra of the nanostructures with an initial height of 325 nm and a layer of nanofilm with thickness of 8 nm.
4. Conclusion
In this paper, a new type of quasi-3D chiral nanostructures has been constructed by considering the proximity effect of stencil lithography. Both amplitude and sign of the CD can be readily tailored by changing the height or blade spacing of the chiral unit cells. It has been found that the nanostructures have two chiral centers, with which the competition between them determines the amplitude and sign of the CD response. Such special bi-chiral geometry provides a new way to control the CD reversal by changing the geometric chirality of the nanostructures. Further analysis of the scattering power of multipole moments revealed that the CD enhancement and sign reversal are mainly determined by the chiral excitation of the magnetic dipole moments and the electric quadrupole moments. Finally, it is exciting to find that the sign of CD can be readily reversed by covering protein with thickness of less than 10 nm. This work provides useful knowledge for the exploration of chiroptics and may find potential applications in ultra-sensitive bio-sensing.
Funding
National Natural Science Foundation of China (61975016,12074446, 61775244); Beijing Municipal Natural Science Foundation (1212013, Z190006).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented herein are not publicly available currently but can be obtained from the authors upon reasonable request.
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