Effects of Fano Resonance on Optical Chirality of Planar Plasmonic

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Effects of Fano resonance on optical chirality of planar plasmonic nanodevices Yongsop Hwang, Seojoo Lee, Sejeong Kim, Jiao Lin, and Xiaocong Yuan ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01007 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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Effects of Fano resonance on optical chirality of planar plasmonic nanodevices Yongsop Hwang,†,‡ Seojoo Lee,¶ Sejeong Kim,§ Jiao Lin,∗,†,‡ and Xiao-Cong Yuan∗,† †Nanophotonics Research Center, Shenzhen University, Shenzhen, 518060, China ‡School of Engineering, RMIT University, Melbourne, VIC 3001, Australia ¶Department of Physics, Korea University, Seoul 02841, Republic of Korea §School of Mathematical and Physical Sciences, University of Technology Sydney, Ultimo, NSW 2007, Australia E-mail: [email protected]; [email protected]

Abstract The effects of Fano resonance on the optical chirality of planar plasmonic nanodevices in the visible wavelength range are experimentally observed and theoretically explained. The nanodevice consists of a nanodisk at the center with surrounding six gold nanorods with an orientation angle to exhibit optical chirality under dark-field illumination. The chiral response induced by the gold nanorods is affected by the presence of the nanodisk with different diameters which causes Fano resonance of different coupling strength. An intriguing change to the opposite selection preference of different handedness of the circularly polarized light has been clearly observed experimentally. This change of the preference is understood based on the coupled localized surface plasmon model. Moreover, electrostatic analysis and the time-dependent simulations provide a further understanding of the phenomenon. The observed and understood

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effects of Fano resonance on optical chirality enables effective manipulation of chiral characteristics of planar subwavelength nanodevices.

Introduction Optical chirality which exhibits different responses between right- and left-handed circularlypolarized (RCP and LCP) light has attracted attention due to its intriguing physical characteristics 1 as well as its applications, for instance, in pharmaceuticals 2 and in molecular chemistry. 3 Recent advances in nanotechnology have enabled nanostructured systems exceeding the chiral responses of conventional materials. 4–6 Moreover, planar chiral nanostructures have been developed to reduce the fabrication complexity and increase the capability to be integrated into optical circuits with other devices. 7–9 Yet, due to the inherent mirror symmetry of the 2-dimensional structures, achieving significant chiral signal in planar nanostructure at subwavelength scale is still challenging despite some successful reports. One of the promising approaches to the strong optical chirality of a planar nanodevice is using metallic nanostructures to support localized surface plasmon (LSP) resonances. 10,11 LSP resonances are the oscillations of collective electrons which can be excited by light on the surfaces of metallic nanostructures. The optical energy is confined to nanoscale volumes by the LSP resonances with potential for high-density integration of optical components. 12,13 Various nanodevices are designed and experimentally demonstrated based on LSP resonances exploiting the strong confinement and size reduction. 14–17 In particular, the LSPs couple evanescently among one another when the metal nanoparticles are placed in proximity, which alters the resonance properties enabling intriguing optical phenomena such as plasmon induced transparency, 18–20 plasmonic edge states, 21 and Fano resonances. 22,23 On the other hand, Fano resonances occur by an interaction of two resonances with a narrow and a broad bandwidth, 22 where destructive and constructive interferences are observed depending on the wavelength. 23 The characteristic asymmetric line shape of Fano resonances

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Figure 1: (a) A schematic of the dark-field illumination with two different spins onto the planar plasmonic nanostructure. (b) The scattering spectra of the hexamer of nanorods and the nanodisk from the dark-field illumination of differently rotating circularly polarized waves. The SEM images are displayed along with the spectra. (c) The scattering spectra of the combined nanodevice of the hexamer and the nanodisk along with the SEM image. The scale bar is 100 nm. has also been found in the plasmonic particle systems supporting localized surface plasmon (LSP) resonances. 24,25 For instance, plasmonic Fano resonances appear in structures such as particle oligomers, 26 disk/ring assemblies, 27 and nanoantenna assemblies. 28 Since their narrow spectral width and large field enhancement, plasmonic systems supporting Fano resonances have various applications including plasmonic rulers, 29 color routing, 30 and biosensors. 31 There have been a theoretical study on circular dichroism induced by Fano resonances in planar chiral oligomers 32 and an experimental demonstration on metallic arrays in the reflection optics configuration by Shuai Zu and the colleagues. 10 Since Zu’s experimental demonstration has been performed on an array under dark-field illumination, the analysis for the effects of Fano resonance has an inherent weakness as the focused spot of the incident light must be aligned on a single unit cell of the array. Consequently, the reflection from the misaligned neighbor cells causes a peak shift and an offset. Another weakness of their analysis was caused by the fact that they used an absolute value of the circular dichroism as the measure of plasmonic chirality. Their analysis was concentrated on the amount of the difference between the responses from RCP and LCP illuminations and did not examine which of the two circular polarizations is preferred and how the preference is affected by Fano

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resonance. For this reason, a systematic analysis of Fano resonance of a single plasmonic chiral device has to be performed with a chiral signal with both positive (RCP > LCP) and negative (LCP < RCP) values as the measure of optical chirality. Here, we present rigorous study on the effects of Fano resonance on the optical chirality in 2-dimensional plasmonic nanostructures both in experiment and theory. In our previous work, 11 we showed that the hexamers composed of gold nanorods placed on a glass substrate exhibit strong chiral response under dark-field illumination. In this article, we systematically demonstrate the effects of Fano resonance on the optical chirality of subwavelength plasmonic nanodevices by introducing a nanodisk at the center of the hexamers. Theoretical analysis using a coupled LSP model and the numerical simulations are also presented.

Experiment A combined structure of two plasmonic nanodevices with different resonance characteristics, one with and the other without optical chirality, is required to investigate the effects of Fano resonance on chiral signals so that the interference between the two resonance spectra can be controlled by varying the geometry of the achiral nanodevice. Therefore, a plasmonic nanostructure consisting of six gold nanorods and one nanodisk is designed in order to investigate the effects of Fano resonance on chirality as shown in Fig. 1(a). As the hexamer shows strong chiral signal under dark-field illumination whereas the nanodisk shows the almost identical response for the spin states of the incident waves, the significance of Fano resonance can be engineered by changing the diameter of the nanodisk. The schematic shown in Fig. 1(a) artistically describes how the positive and negative spins of the incident light induce the LSP modes in the nanostructure. The scattering spectra of the nanorods and the disk are separately measured under dark-field illumination and depicted in Fig. 1(b). The gold nanorod in the hexamer is 90 nm long, 34 nm wide, and 30 nm thick. The diameter of the hexamer ring, the center-to-center distance between the two opposite nanorods, is

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340 nm. Each nanorod is rotated for 60◦ to counter-clockwise from its radial axis. The gold nanodisk has a diameter of 170 nm and a thickness of 30 nm. The gold nanostructure adheres to a glass substrate by 2 nm germanium adhesion layer. The detailed methods of the nanofabrication and the optical measurement are described in Methods. The hexamer of gold nanorods shows a peak at 680 nm in the scattering spectra for both RCP and LCP incident light due to the induced surface plasmons. As a result of the existence of the glass substrate and the dark-field illumination, the plasmonic nanorods show the obvious difference between RCP and LCP in their scattering intensity as we reported in our previous research. 11 On the other hand, the nanodisk presents a peak at 700 nm without chiral response as expected from its geometric symmetry. The scattering spectra of the combined nanostructure of the rods and the disk are shown in Fig 1(c). A dip between the two peaks is clearly observed as a result of the coupling which is a characteristic of Fano resonance. Remarkably, the change of the preference for the circular polarization is observed in the wavelength range of the disk resonance; scattering for RCP is now greater than LCP in the range of 720 − 830 nm for the combined structure while LCP scattering was stronger than RCP in the measured wavelength range about the resonance for the rods hexamer without the disk.

Mode analysis We examined the mode characteristics of the nanodevice for a better understanding of the notable sign change of the chiral response. Electrostatic eigenmode analysis is used to understand the characteristics of the modes supported by the combined nanostructure with the same physical dimensions of the experiment shown in Fig. 1. The dielectric function reported by Johnson and Christy 33 is taken for gold. First, the surface charge density profiles are obtained using the boundary element method 34,35 for the nanorod hexamer and the nanodisk separately (see Fig. 2). Apparently, the LSP dipole mode of the individual nanorod is excited on each nanorod and their serial arrangement is a strongly supported

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disk mode

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Figure 2: Surface charge distributions of the LSP modes supported by the rods, the disk, and the combined nanodevice obtained by the boundary element method. The corresponding spectra of scattering cross-section are displayed accordingly. mode which can be excited by a circularly polarized beam. A dipole mode of LSP is also induced at the disk. The spectra obtained by the boundary element methods are narrower than the measured scattering spectra shown in Figs. 1(b) and (c). The spectral broadening observed in the experimental data could occur from the nonuniformity of the fabricated six nanorods as well as the roughness of the surface. Next, the surface charge distributions of the combined structure are obtained. Two representative coupled modes are generated by combining two structures which are rods dominant mode and a disk dominant mode shown on top and bottom, respectively. Interestingly, the rods dominant coupled mode is not substantially affected by the nanodisk whereas the disk dominant coupled mode is affected by the presence of the nanorods. It is expected that one of the two coupled modes will be more strongly excited depending on the spin of the incident light. The corresponding spectra of scattering cross-section are displayed accordingly for the linearly-polarized plane wave incident light which excites basis modes whose linear superpositions can form the depicted charge distributions. The characteristic Fano shape showing a dip between the two peaks can be clearly observed from the combined nanostructure as a result of the coupling

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of the rods mode and the disk mode.

Variation of the disk size We introduced different size of nanodisks into the nanorod hexamer to examine the effects of Fano resonance by shifting the resonance wavelength of the disk as increasing the diameter. The dimensions of the fabricated nanorods were measured to be 34 × 90 nm2 while gold nanodisks were fabricated varying the diameter in the increment of 30 nm, as shown in Fig. 3(b). The orientation angle of the nanorods is fixed to be −60◦ since similar configurations with the selected angle have been reported to have strong chiral responses. 10,11,32 The corresponding scattering and chirality spectra are provided in Figs. 3(a) and (b), respectively. For comparison, the scattering spectra of the disks without the nanorods are shown in Supporting Information. It is observed that the valley between two scattering peaks which is a characteristic feature of Fano resonance becomes more significant as the disk diameter increases. When there is no disk at the center, the scattered intensity of RCP is greater than LCP over the measured wavelength range. As Fano resonance becomes more significant by the increase of the disk size, however, the scattered intensity of LCP becomes greater than RCP in the longer wavelength. To quantitatively analyze the chiral response, we define a chiral signal 11 ∆=

SRCP − SLCP , SRCP + SLCP

(1)

where SRCP and SLCP are the scattered intensities under the dark-field illumination of RCP and LCP light, respectively. Optical chirality can be quantified using a few different measures such as circular differential scattering, 36 circular dichroism in absorption, 32 and optical chirality flux. 37 Some of the measures are introduced and compared in Supporting Information. Here, we use the chiral signal in Eq. 1 since it quantifies the polarization selectivity in the dark-field scattering by providing a difference normalized by the total scattering of both spins. The experimentally obtained chiral signal spectra for different disk diameters 7


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are shown in Fig. 3(b). The change to the opposite selection preference from RCP to LCP is clearly observed for the structures with d = 140, 170, and 200 nm. The observed negative ∆ shifts to longer wavelength as increasing the disk diameter due to the red-shift of the disk mode. The combined nanostructures with the opposite orientation angle (60◦ ) of the nanorods also show the same effects of Fano resonance with the reversed optical chirality (See Supporting Information).

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Figure 3: The measured (a) scattering and (b) chiral signal spectra for different disk diameters d. The SEM images of the fabricated gold nanostructures are displayed. The scale bar is 100 nm. Gray dashed lines are shown in (b) to indicate the baselines of zero chiral signal of the corresponding nanodevices. The change to the opposite selection preference from RCP to LCP is clearly observed for the structures with d = 140, 170, and 200 nm. The wavelength range of ∆ < 0 is shaded for visual guidance.

FDTD simulations Three-dimensional full-wave simulations using the finite-difference time-domain (FDTD) method (Lumerical Inc.) have been performed for the further understanding of the effects of Fano resonance caused by the nanodisk at the center of the nanostructure, and the obtained 8


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Figure 4: FDTD simulation results of (a) scattering cross-section and (b) chiral signal spectra for different disk diameters d. Color dashed lines are shown in (b) to indicate the baselines of zero chiral signal of the corresponding nanodevices. The electric-field intensity profiles of the nanostructures with (c) d = 200 nm and (d) d = 140 nm at the wavelength λ = 725 nm as indicated in (b).

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scattering and the chiral signal spectra are shown in Fig. 4. The diameter of the central disk is varied in the range of d = 100 − 200 nm with the increment of 10 nm. The resonance wavelength of the hexamer without the central disk obtained by the simulation is 725 nm both for RCP and LCP (see Supporting Information). The scattering cross-section of the RCP is obtained greater than the LCP in the wavelength range of 680 − 900 nm which agrees with the experimental results. On the other hand, the calculated resonance wavelength of the nanodisk with the nanorods shifts from 635 − 855 nm as increasing the disk diameter. The two resonances caused by the nanorods and the nanodisk overlap the most when the disk diameter d is 140 nm with the resonance wavelength of 715 nm. In result, the scattering spectra at d = 140 nm show two peaks of similar scattering cross-section. Interestingly, the corresponding chiral signal spectrum shows a dip at the resonance wavelength which indicates the reversed preference of rotational direction of the circularly polarized light. The electric field intensity of the given structure at λ = 725 nm is depicted in Fig. 4(d), where the obvious difference between the RCP and the LCP is shown: The disk mode is dominant in RCP while the rods mode is in LCP. It should be noted that the disk mode itself has no preference for RCP and LCP, thus the preference originates from the coupling with the rods mode. On the contrary, the resonance of the disk with d = 200 nm has little overlap with the rods mode, hence the rods mode is dominant both for RCP and LCP as displayed in Fig. 4(c). Consequently, the chiral signal of the nanodevice at the given wavelength has the same positive sign as the hexamer without the disk. Despite the qualitative agreement of the experimental and numerical scattering characteristics, it is observed that the experimental Fano dips shown in Fig. 3(a) are less pronounced than the numerical results shown in Fig. 4(a). The fabrication imperfection could cause spectral broadening of both the rods and the disk modes, thus the depth and the sharpness of the Fano dips could have been diminished in the experiments.

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Figure 5: (a) Coupling constant G and coupling strengths (b) fr G and (c) fd G between the rods and the disk extracted from the experimental scattering spectra as functions of the disk diameter d. Insets show the G, fr G, and fd G obtained from the FDTD simulation results.

Coupled localized surface plasmons We apply a theoretical model of coupled LSPs 38 for a further understanding of the effects of Fano resonance. When there is a nanostructure consisting of multiple nanoparticles, the

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general expression of the excitation amplitude of the jth mode of mth nanoparticle is

j ajm (ω) = fm (ω)

I

j τm (r0 )ˆ n0 · E0 (r0 )dS 0

j ≈ fm (ω)pjm · E0 ,

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where f (ω), τ (r0 ), p, and E0 are the polarizability per unit volume, the surface dipole, the dipole moment, and the electric field of the incident light, respectively. To apply the coupled LSPs model, we assume the hexamer of six nanorods as one system and the central disk the other, and the LSPs excited on both systems couple to each other with the coupling constant G as indicated in the inset of Fig. 5(a). Subsequently, the excitation amplitudes of the rods and the disk as independent systems are noted as ar and ad , respectively, while those of the coupled LSPs are a ˜r and a ˜d . The excitation amplitudes of the coupled nanostructure are given by 

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where D = 1 − fr fd G2 is the determinant, and fr,d are the polarizabilities per unit volume of the rods hexamer and the disk, respectively. 38 Here, the coupling constant G is determined by the physical dimensions of the structures such as the length of nanorods, the diameter of the disk, and the gap between the hexamer and the disk, hence it is independent of the wavelength and the polarization of the incident light. The polarizabilities f , however, are functions of the wavelength and the incident polarization, so are the excitation amplitudes a, in general. In our combined system of the rods and the disk, both fr and fd are expected to depend on the wavelength. Regarding the incident polarization, on the other hand, only fr for the rods is expected to vary by different 12


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circular polarizations while fd for the disk remains constant due to its circular symmetry. Then fr G and fd G represent the actual coupling strength from the disk to the rods and vice versa, respectively. Next, we extract G, fr G, and fd G from the experimental data of the dark-field scattering spectra shown in Figs. 3(a) and S1(a). The scattered intensity and the excitation amplitude are related as I = |a|2 , hence ar and ad can be directly obtained from the measured scattered intensity of the rods and the disk, respectively. The scattered intensity of the combined structure of the hexamer and the disk is defined as I˜ = |˜ ar |2 + |˜ ad |2 . By substituting the experimental data at the resonance wavelength ω0 of the rods to Eq. 3, the coupling constants G and the polarizabilities fr (ω0 ) and fd (ω0 ) are obtained for different disk diameters as shown in Fig. 5. The dots indicate experimental data and the connecting curves are interpolated for visual aid. The coupling constants and the coupling strengths from the FDTD simulation results are shown in the insets for comparison. The increase of G is observed as the diameter of the central disk increases for both the experiment and the simulation as shown in Fig. 5(a). This is understood that the coupling depending on the physical structure becomes stronger as the distance between the central disk and the ring of the six nanorods gets closer. The coupling strength fr G which represents the coupling from the disk to the rods also becomes stronger as the disk diameter increases both for RCP and LCP. It is observed that the coupling strength for RCP is greater than LCP means the coupled mode of the rods is more strongly influenced by the disk mode for RCP than LCP. Intriguingly, fd G decreases as d increases which means that the coupling from the rods to the disk becomes weaker by increasing the disk diameter. Accordingly, it can be interpreted that the coupled disk mode gets less affected by the rods mode as the resonance wavelength of the disk mode shifts to the longer wavelength away from the resonance of the rods mode. This asymmetric tendency of the coupling strength, the increase of fr G and the decrease of fd G, must be noted as a result of Fano resonance which would be closely related to the change of the selection preference of the circular polarization. In summary,

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the analysis using the coupled LSPs model supports that the transition from ∆ > 0 to ∆ < 0 can be understood as a result of Fano resonance since the stronger the coupling, the more significant the transition.

Conclusion To conclude, we have observed an intriguing transition to the opposite selection preference of different handedness of the circularly polarized light which is originated from the Fano resonance. We have systematically demonstrated the effects of Fano resonance on the chiroptical response of subwavelength plasmonic nanodevices. Theoretical analysis using a coupled LSP model has explained the effects of Fano resonance in terms of the coupling between the rods and the disk modes. In addition, FDTD simulations supported the experimental results and the theoretical model. Furthermore, the dependence of the diameter of the nanodisk has been studied both theoretically and experimentally. Since our experiment, simulations, and theoretical analysis have been performed on a single device, we could accurately investigate the chiral response under dark-field illumination avoiding the interference caused by misaligned neighboring devices in case of a periodic structure. This study enriches the scientific understanding of the effects of Fano resonance in two-dimensional plasmonic chiral systems and provides insights on optical chirality in coupled systems including destructive interferences.

Methods Nanofabrication The plasmonic nanostructures were fabricated by means of electron-beam lithography (Vistec EBPG 5000 Plus ES) on glass substrates, using poly(methyl methacrylate) (Micro-Chem, 950k A2) and Copolymer (Micro-Chem, EL6) as a bilayer resist stack for a standard lift-off

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process and a sacrificial chromium layer (30 nm) that provides for charge dissipation during the exposure. The structures were developed with a 1:3 (by volume) mixture of methyl isobutyl ketone/2-propanol for 90 s, rinsed with 2-propanol, and dried with a nitrogen gun. A 30 nm layer of Au was deposited by electron-beam evaporation, using 2 nm of germanium as the adhesion layer. A subsequent lift-off step with acetone produced the nanostructures. Normal incidence images of the resulting structures were obtained by scanning electron microscopy (FEI, NovaNanoSEM 430).

Optical measurement A transmission optics setup was installed to characterize the optical properties of the planar plasmonic nanodevices. The light source was a halogen lamp. A linear polarizer and a quarter-wave plate and were placed before the dark-field condenser with the NA range of 0.80-0.95 in order to generate the circularly polarized dark-field incident waves. The forward scattered light was collected using a 40x objective with NA of 0.60. The scattering spectra were acquired with a Nikon TiU microscope coupled to a cooled CCD camera (Princeton Instrument PIXIS).

FDTD simulations Three-dimensional numerical modeling has been conducted using the finite-domain timedifference (FDTD) method (Lumerical Inc.). The computational domain is defined as a volume of 1 µm3 divided by a 2-nm spatial grid. The complex refractive index reported by Johnson and Christy is used to determine the optical parameters of the plasmonic nanostructures. 33 The dark-field illumination with circular polarization is created using four linearly polarized Gaussian sources. The circularly polarized light is generated by interfering two linearly polarized waves with π/2 phase difference. Afterward, two beams with different numerical apertures (NA), 0.95 and 0.80, are employed to create a dark-field beam for each circular polarization. The two beams are in antiphase (π phase difference) for the beam of 15


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smaller NA to be subtracted from that of larger NA to form a ring-shaped incident beam in consequence. The total-field scattered-field method is applied to obtain the scattered intensity by the nanodevices.

Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website. The measured and simulated scattering spectra of the disks without the nanorods are shown. The simulated scattering spectra of the nanorods without the disks are shown. Quantification of chirality using different measures is discussed. The combined nanostructures with the opposite orientation angle (60◦ ) of the nanorods are discussed.

Acknowledgement This work was partially supported by the National Natural Science Foundation of China under Grant Nos. 61490712, 61427819, U1701661; National Key Basic Research Program of China (973) under grant No.2015CB352004; the leading talents of Guangdong province program No. 00201505; the Natural Science Foundation of Guangdong Province under No.2016A030312010; and the Science and Technology Innovation Commission of Shenzhen under grant Nos. KQTD2015071016560101, ZDSYS201703031605029. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). YH acknowledges the discussions on theory with B. Hopkins.

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Effects of Fano Resonance on Optical Chirality of Planar Plasmonic

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