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Deep-Subwavelength Resolving and Manipulating of Hidden Chirality in Achiral Nanostructures Shuai Zu, Tianyang Han, Meiling Jiang, Feng Lin, Xing Zhu, and Zheyu Fang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01380 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Deep-Subwavelength Resolving and Manipulating of Hidden Chirality in Achiral Nanostructures Shuai Zu1‡, Tianyang Han1‡, Meiling Jiang1, Feng Lin1, Xing Zhu1,2,4, and Zheyu Fang1,2,3,* 1

School of Physics, State Key Lab for Mesoscopic Physics, Peking University, Beijing 100871, China 2

Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China 3

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Collaborative Innovation Center of Quantum Matter, Beijing 100871, China

Key Laboratory of Nanoscale Measurement and Standardization, National Center for Nanoscience and Technology, Beijing 100190, China

ABSTRACT: Chiral state of light plays a vital role in light-matter interactions and the consequent revolution of nanophotonic devices and advanced modern chiroptics. As the lightmatter interaction goes into the nano- and quantum world, numerous chiroptical technologies and quantum devices require precise knowledge of chiral electromagnetic modes and chiral radiative local density of states (LDOS) distributions in detail, which directly determine the chiral lightmatter interaction for applications like chiral light detection and emission. With classical optical techniques failing to directly measure the chiral radiative LDOS, deep-subwavelength imaging and control of circular polarization (CP) light associated phenomena are raised into the agenda.

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Here, we simultaneously reveal the hidden chiral electromagnetic mode and acquire its chiral radiative LDOS distribution of a single symmetric nanostructure at the deep-subwavelength scale by using CP-resolved cathodoluminescence (CL) microscopy. The chirality of the symmetric nanostructure under normally incident light excitation, resulting from the interference between the symmetric and antisymmetric modes of the V-shaped nanoantenna, is hidden in the near-field with a giant chiral distribution (~99%) at the arm-ends, which enables the circularly polarized CL emission from the radiative LDOS hot-spot and the following active helicity control at the deep-subwavelength scale. The proposed V-shaped nanostructure as a functional unit is further applied to the helicity-dependent binary encoding and the two dimensional display applications. The proposed physical principle and experimental configuration can promote the future chiral characterization and manipulation at the deep-subwavelength scale, and provide direct guidelines for the optimization of chiral light-matter interactions for the future quantum study.

KEYWORDS: Cathodoluminescence, aluminum plasmonics, reciprocity theorem, circular polarization, deep-subwavelength.

In the past two centuries, circular polarization (CP) spectroscopy with focus on the classic chiroptical phenomena, such as the optical activity (OA) and circular dichroism (CD), has greatly benefited the understanding of natural materials with the lack of mirror symmetry, such as crystals, molecules, proteins and DNA, promoting the development of chemistry, biology, and material science.1 As the development of modern nanofabrication techniques, various artificial chiral materials2,

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present a superior chiroptical response than their counterparts in nature.

Especially, metallic nanostructures with the collective excitation of free electrons, known as


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surface plasmons, have been demonstrated to show giant chiroptical effects.4-6 Moreover, the optical chirality7 can be increased by plasmonic nanostructures, leading to a strong CD enhancement for the ultrasensitive optical detection and characterization of chiral molecules,8 facilitating the development of the pharmaceutical and chemical industries. More intriguingly, plasmonic nanostructures exhibit an incredible talent for the manipulation of CP light in ways never found in nature, like the asymmetric transmission9 in planar chiral metamaterials with CP conversion efficiencies depending on the propagation direction of light, and the extrinsic chirality10 in achiral nanostructures arising from the mutual orientation of the structure and incident light. Despite controlling the intensity and phase of CP, more fascinating properties such as negative refraction,11 optical spin-orbit interaction (SOI)12 and Pancharatnam−Berry phase13 have been achieved by tailoring the chirality parameter, which present an talent for controlling the propagation of CP light such as optical Spin-Hall effect,14 and find various applications in super-lenses,15 optical spin routing,16,

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unidirectional

transmission18 and helicity multiplexed holography,19 etc. With the revolution of nanophotonics and quantum optics in recent years, CP light plays an even more important role in the light-matter interaction. Exceptional physical phenomena emerge including valley polarization,20, 21 propagation-direction-dependent emission, scattering and absorption of single photons,22 leading to advanced applications for the nanotechnology and information processing.23 As the light-matter interaction enters the quantum world, numerous chiroptical technologies and quantum devices require the detailed distribution of chiral electromagnetic fields and chiral radiative local density of states (LDOS) at the deepsubwavelength scale, which can be used to determine the efficiency of chiral light detection and emission. Dark-field optical microscopy can realize the measurement of CD in planar chiral


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objects at single nanostructure level,24, 25 but chiral electromagnetic modes that result in CD are still hidden in the near-field. A potential solution is scanning near-filed optical microscopy (SNOM) with resolution beyond diffraction limit, which has been successfully applied to imaging the chiral near-filed electromagnetic mode.26, 27 However, the measured distribution is inevitably perturbed by the scanning probe, and the squeezed detection volume leads to the huge decrease of signal intensity with requiring large excitation laser power and long collection time. As a non-invasive technique, cathodoluminescence (CL) microscopy and spectroscopy with nanoscale resolution have been successfully used in the electromagnetic modes investigation of plasmonic nanostructures28-31 and photonic crystals.32 The scanning electron beam functions as a virtual dipole source, enabling flexible mode excitation and control, and the radiative LDOS that governs the radiative spontaneous decay of quantum emitters can be simultaneously probed with the CL photon emission.33 In this work, we realize the imaging and control of hidden chirality of a single symmetric nanostructure at the deep-subwavelength scale with the CP-resolved CL microscopy. As demonstrated by the reciprocity between electron beam excitation and plane wave illumination, the detected CP-resolved CL image can perfectly imitate both the near-field chiral electromagnetic distribution that resulted from the mode interference in the nanoantenna, and the chiral radiative LDOS that governs the chiral photon emission. With the largest degree of CP approaching 90%, the emitted CL from the chiral radiative LDOS hot-spot can be further applied to realize the active switchable helicity-dependent binary encoding and the two-dimensional display, and facilitate the applications in information storage and processing. We believe that the basic physical principle and technique for deep-subwavelength CL helicity imaging and control are general for the chiral characterization and manipulation, which can greatly benefit the


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development of nanophotonic device design and find advanced applications in the quantum optical science and technology in the future. RESULTS AND DISCUSSION

Figure 1. Design of the Al nanoantenna with optical plane wave simulations as guided by the reciprocity theorem. (a) Schematic of the CP-resolved reciprocity between electron beam −1 excitation and CP light illumination. LCP and RCP electric field components (E+1 1 and E1 ) of CL


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emission in the far-field with electron beam excitation are reciprocally related to the electric field −1 (E+1 2 and E2 ) in the vicinity of the nanostructure under LCP and RCP light excitation,

respectively. (b) Geometry of the V-shaped nanoantenna including two rectangular arms (l = 73 nm, w = 46 nm) and a 1/3 circle (θ = 120°, r = 46 nm). The nanostructure was designed on the Si/SiO2 substrate with the thickness of 30 nm, and a ~3 nm Al2O3 layer on top of the nanoantenna was formed after exposure to the air. (c) The simulated scattering spectra of the nanoantenna under plane wave excitations with the polarization along y and x axes, corresponding to the excitation of the symmetric mode (purple line) and the antisymmetric mode (green line). The dashed line corresponds to the CP light excitation. (d, e) The corresponding electric field distributions (d) and surface charge distributions (e) at the resonant wavelength (marked by the arrow in (c)) for the symmetric mode (at 520 nm), the antisymmetric mode (at 720 nm), and CP light (at 630 nm). (f) The calculated dipole moment based on the multipole expansion method for the symmetric mode and the antisymmetric mode represented by electric dipoles ps and pas along y and x axes, respectively. The grey line shows the phase difference ∆ϕ = arg(ps) – arg(pas) of these two modes. The dashed line at −90° is drawn as a guide for the eye. CP-resolved reciprocity between electron beam excitation and CP light illumination. The underlying physical basis for the chiral electromagnetic mode imaging with CP-resolved CL microscopy is the reciprocity relation between electron beam excitation and optical plane wave illumination (Figure 1a). By employing the Lorentz reciprocity theorem (Supporting Section I), we demonstrated that the electron beam generated CP-resolved CL can be directly related to the normally incident optical plane wave induced electric field via the relation:


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E1σ (r0 , ω) =

∞ iωe −iωt e Eσ2 ( x0 , y0 , z, ω) ⋅ n z e−iω z / v dz 2 ∫ −∞ 4πε 0c R

(1)

Here, equation (1) was obtained solely based on Lorentz reciprocity theorem (Supporting Section I). e, ε0 and c are the electron charge, permittivity and light speed in vacuum. v is the velocity of electron, r = (x0, y0, z) is the impinging position of the moving electron along –z axis, nz is the unit vector along the +z direction and r0 = (0, 0, R) is the far-field CL detection position in vacuum. Here, the induced electric field of CL emission that generated by resonant plasmonic modes is resolved into left-handed circular polarization (LCP, σ = +1) and right-handed circular polarization (RCP, σ = −1) components Eσ1 (r0, ω), which are reciprocally connected with the electric field Eσ2 (r, ω) that strongly localized in the vicinity of the nanostructure under normally incident LCP and RCP plane wave excitation, respectively. With the electron beam moving along +z direction, only the z component of Eσ2 (r, ω) can be accessed. The CL emission is related to the integral of the z component of Eσ2 (r, ω) with specific phase e−iωz/v, reflecting the chirality of electric field along z direction under plane wave excitation. Therefore, CP-resolved CL microscopy can be used as an approach to reveal CP-dependent electromagnetic modes of the nanostructure. By tailoring the geometry of nanostructures, electromagnetic modes depend heavily on the CP light illumination resulting in chiral near-field distributions,34 which can benefit the CP-resolved CL measurement. Design of the chiral electromagnetic mode with optical plane wave simulations. We start the design of the chiral electromagnetic mode with convenient optical plane wave simulations rather than electron beam simulations that are time-consuming and need to excite massive positions to form mode distributions of the nanostructure. Chiral electromagnetic modes can be effectively excited by far-field CP light and analyzed by performing several simulations, which


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can facilitate the following chiral electromagnetic mode imaging with CP-resolved CL microscopy. Without loss of generality, the achiral nanostructure without far-field CD under normally incident light excitation was chosen. Figure 1b shows the designed anisotropic symmetric nanoantenna on the Si substrate with a 100 nm SiO2 layer. Comparing with the noble metals, Al with its natural abundance, low cost and superior optical property in the visible region was chosen as the constituent material.35 When exposed to the air, a Al2O3 layer (~3 nm) on top of the nanoantenna was formed by sacrificing the surface Al, and protected the inside from corrosion.36 This symmetric nanoantenna can support two fundamental modes, referred to as the symmetric mode and the antisymmetric mode37 under y- and x- polarized normally incident plane wave excitations, as shown in the finite-difference time-domain (FDTD) calculated scattering spectra of Figure 1c, respectively (See Methods). Figure 1d shows corresponding electric field distributions of these two modes at the resonant wavelength (upper two panels) with the characterized symmetric and antisymmetric surface charge distributions (Figure 1e, upper two panels) about the symmetry axis (y axis). When the excitation polarization turns to CP, both of these two modes can be simultaneously excited without scattering CD as shown in Figure 1c (dashed line). The simulated scattering spectra under normally incident light excitation are the same for LCP and RCP in the whole wavelength range, which is caused by the mirror symmetry of the V-shaped nanoantenna. However, electric field distributions (Figure 1d, lower two panels) show a giant near-field chirality (~99%, Figure S1) at the arm-ends, which is resulted from the two modes interference with the largest spectral overlap at 630 nm (Figure 1c). Besides, the electric field distributions at other wavelengths show similar behavior and the near-field chirality shows a broadband feature (Figure S2). Here, we explore a huge chirality that is hidden in the


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near-field of the symmetric nanostructure, which is unobservable by conventional far-field detection techniques and can be acquired by the electron beam with specific excitation conditions. Formation mechanisms of hidden chiral electromagnetic modes. The mode interference phenomenon can be further quantitatively analyzed by using the multipole expansion method38, 39 (Supporting Section II) to explain the formation of hidden chiral electromagnetic modes under CP light excitation. The symmetric and antisymmetric modes show dominant electric dipolar responses (Figure S3) with dipole moments represented by ps = i/ω∫jyd3r and pas = i/ω∫jxd3r along y and x axes, respectively. Here, jx and jy are the x and y components of the polarization current induced in the nanoantenna. The phase difference ∆ϕ = arg(ps) – arg(pas) of these two modes is close to −90° with equal amplitudes around 630 nm (Figure 1f). With the additional phase difference −90° (90°) arising from LCP (RCP) excitation, the total phase difference between the two modes is −180° (0°), leading to a final dipole moment along the left (right) arm of the nanoantenna with the strong electric field localized at its arm-end. In detail, the electric field and surface charge distributions for the symmetric (antisymmetric) mode show even (odd) parity about the y axis (Figure 1d,e), resulting in the constructive (destructive) interference at the left arm-end and the accompanying destructive (constructive) interference at the right arm-end for the LCP (RCP) excitation that were protected by the nanoantenna structure symmetry. The analysis method for this hidden chiral electromagnetic mode formation under optical plane wave excitation is insightful, and provides a convenient tool for the design of other complicated nanostructures to achieve strong near-field chirality. As demonstrated by the reciprocity relation between electron beam excitation and optical plane wave illumination, this giant hidden chirality


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can enable the CP CL emission from the chiral near-field hot-spots, which greatly benefits the following CP-resolved CL measurement and further manipulation.

Figure 2. Samples and optical setup for CL imaging. (a) The 45°-tilted scanning electron microscope (SEM) image of V-shaped nanoantennas. Inset: top view of an individual nanostructure. (b) The optical path of the SEM-CL detection platform with the bandpass filter for the total CL mapping and filter groups applied to collecting LCP and RCP CL images. Measurements of hidden chiral electromagnetic modes and chiral radiative LDOS distributions. As demonstrated by the reciprocity theorem between electron beam excitation and plane wave illumination, these chiral electromagnetic modes of the nanoantenna that hide in the near-field can be imaged by the CP-resolved CL microscopy. To reveal the hidden chirality in symmetric nanostructures, the V-shaped nanoantenna (Figure 2a) was fabricated on a Si/SiO2 wafer by electron-beam lithography (EBL) with the 30 nm Al deposition and following lift-off process (see Methods). As shown in Figure 2b, the energetic electron beam (30 keV) passing through the pinhole of a parabolic mirror was tightly focused on the Al nanoantenna, and the generated CL emission was collected by the parabolic mirror and finally acquired by a high sensitive photomultiplier tube (PMT). Total CL and CP-resolved CL images at specific


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wavelengths can be measured by applying different bandpass filters, quarter-wave plates and linear polarizers before the PMT (see Methods and Figure 2b). Figure 3a shows a series of bandpass CL images with the center wavelengths from 488 to 788 nm. In the total CL series, the symmetric mode is dominant for the wavelengths shorter than 543 nm with CL hot-spots localized at the tip and arm-ends of the V-shaped nanoantenna. On the contrary, the antisymmetric mode is dominant for the wavelengths longer than 700 nm with CL hot-spots mainly locating at the arm-ends. The CL images of the symmetric and antisymmetric modes well imitate the near-field distributions under optical plane wave illumination (Figure 1d). Besides, the corresponding CP-resolved CL images show a broadband chiral feature at the armends, especially for the wavelengths falling between 591 and 650 nm with the strongest interference between the symmetric and antisymmetric modes, which also shows a good agreement with the simulated plane wave excitation results (Figure 1d and Figure S2). This huge chirality is hidden in the near-field of the symmetric structure, and hardly detected by the conventional optical microscopy and even SNOM at such deep-subwavelength scale. Here, we reveal this hidden chirality and its related electromagnetic mode distribution by our CP-resolved CL microscopy. Moreover, as demonstrated previously33,

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that the total CL emission is

generally related to the projection of the radiative LDOS along the z axis, we extended the concept and resolved the radiative LDOS into LCP and RCP light. The CP-resolved CL images exactly reflect the generalized chiral radiative LDOS that are localized transversely in real space (xy plane) and in momentum space along the z axis, which governs the radiative decay of quantum emitters into either LCP or RCP channel (Supporting Section III). The chiral electromagnetic mode distributions and the chiral radiative LDOS in the nanostructure were successfully obtained simultaneously by detecting the CP-resolved CL emission.


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Figure 3. CL responses of the Al nanoantenna. (a) Total, LCP and RCP bandpass CL images with different center wavelengths. (b, c) The averaged total (points in (b)), LCP and RCP (points in (c)) CL intensities with different collection wavelengths in the left and right arm-end regions (L-arm and R-arm) marked by dashed boxes in the inset of (b). Error bars in vertical and horizontal directions correspond to the standard deviation of the CL intensity and the effective 3dB bandwidth of the bandpass filter, respectively. The dashed lines represent the simulated total, LCP and RCP spectra for the single point excitation with the electron beam impinging position marked in Figure 4a. The shadow regions are the analytical two orthogonal Lorentz


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oscillators model fitted results. (d) The degree of CP for the CP-resolved CL emission under the left and right arm-end excitations corresponding to (c) with the maximum ~90%. Evolution and analysis of the CL spectra. To further investigate the evolution of the CL signal, the total, LCP and RCP CL spectra were calculated by averaging the CL intensity around the arm-ends (marked by dashed boxes in the inset of Figure 3b). The total CL spectra (Figure 3b) have two resonant peaks at 540 and 670 nm, corresponding to the excitation of the symmetric and antisymmetric modes, respectively. Moreover, the CP-resolved CL spectra (Figure 3c) show a broadband chiral feature with the intensity of LCP (RCP) CL emission larger than RCP (LCP) CL emission at the left (right) arm-end, and the maximum chirality is located at the wavelengths between 591 and 650 nm where the mode interference is drastically enhanced. The FDTD simulated total, LCP and RCP spectra (dashed lines in Figure 3b,c) under the single point excitation (with electron beam positions marked in Figure 4a) show the deviation from measured CL spectra for the wavelengths outside the range of 591 to 650 nm, which is mainly caused by the proximity effect where finite data extracted regions inevitably introduce the response of whole arm-ends, leading to the decrease of the measured chirality compared with the simulation. Because the CL radiation comes from the dipolar excitation of the symmetric and antisymmetric modes, an analytical two orthogonal Lorentz oscillators model was proposed to take into account the proximity effect and quantitatively describe the CP-resolved CL spectra. The displacement of the symmetric mode (antisymmetric mode) is defined as xs(as) = aa(as)/(ω −ωa(as)+iγa(as)), and the phase difference between these two modes for the left arm-end excitation is ∆θ = θs−θas. Hence, the corresponding total CL intensity is |xs|2+|xas|2. The LCP (σ = +1) and RCP (σ = −1) CL intensities are |xas−σixsei∆θ|2. For the right arm-end excitation, ∆θ


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changed to ∆θ −180° enforced by the mirror symmetry of the nanoantenna with the displacement unchanged. The fitted CL spectra are plotted as shadow regions in Figure 3b,c with the parameters aa(as) = 0.21 (0.07), γa(as) = 0.36 (0.22) eV, ωa(as) = 2.36 (1.86) eV and ∆θ = 182.81°, which shows an excellent agreement with the experiment. As a consequence of the mirror symmetry, the total CL spectra are the same for the left and right arm-end excitations with equal but helicity-reversed CP-resolved CL emissions. Only the FDTD calculated and the analytically fitted results for the left arm-end excitation are plotted to clearly show the results. In order to quantitatively describe the helicity of the CP-resolved CL emission, the degree of CP is defined as (ILCP−IRCP)/(ILCP+IRCP), where ILCP (IRCP) is the intensity of LCP (RCP) CL emission. As shown in Figure 3d, the maximum of the absolute value for the degree of CP is ~90% with opposite signs for the left and right arm-ends at the wavelength of 630 nm, which means that the moving of the electron beam from the left arm-end to the right arm-end of the V-shaped nanoanntenna allows the helicity switching from LCP to RCP and vice versa.


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Figure 4. Basic principle of the real space CL helicity switching. (a) Energy level diagram of the approximate LCP and RCP dipole transitions and CL emissions at the left and right arm-ends (Larm and R-arm), and the corresponding electric field distributions at 630 nm with circles representing the impinging positions of the electron beam. (b) The intensities of LCP and RCP dipole transitions calculated by the multipole expansion method under the left arm-end excitation. (c) The corresponding degree of CP dipole transitions for the left and right arm-end excitations with the absolute value larger than 92% at 630 nm. (d) The calculated amplitudes and phase differences of the symmetric and antisymmetric dipolar modes for these two excitation positions.


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CP dipole transitions and real space CL helicity switching. To clarify the underlying physics of the CL helicity switching in the symmetric nanostructure, the plasmonic resonance under the electron beam excitation was expanded into CP dipole modes by using the multipole expansion method38, 39 (Supporting Section II). Figure 4a shows a representative energy level diagram of approximate LCP and RCP dipole transitions and CL emissions at the left and right arm-ends, and corresponding mode distributions at 630 nm with circles at the arm-ends representing the impinging positions of the electron beam. In the energy level diagram, the minor RCP and LCP components of dipole transitions at the left and right arm-ends were not shown for simplicity, respectively. Due to the mirror symmetry of the V-shaped nanoantenna and electron beam excitation positions, the approximate LCP and RCP dipole transitions at the left and right arm-ends are mirror symmetric with the same energy. As the nanoantenna resonance is dominant by electric dipole modes, the transition dipole moment can be defined as pσ = pσ (nx + σ in y ) / 2 for LCP (σ = +1) and RCP (σ = −1) CL emissions along +z axis. For the left (right) arm-end electron beam excitation, the transition is mainly contributed from the LCP (RCP) dipole, which is confirmed by the multipole expansion results as the intensity of p2σ plotted in Figure 4b. The degree of CP dipole transitions, following the definition of CP-resolved CL emission, was calculated as larger than 92% at 630 nm (Figure 4c), which demonstrates that the CP dipole transitions at the two arm-ends have opposite helicities. To further explain the formation of the CP dipole transitions in this symmetric nanostructure, the corresponding plasmonic modes (Figure 4a) were further expanded into the symmetric ps and antisymmetric pas modes as mentioned before for these two electron beam excitation positions. The calculated amplitudes of the dipole moments for the symmetric and antisymmetric modes along y and x axes show the characteristic dipole resonance line shapes with equal amplitudes for


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both excitation positions that protected by the mirror symmetry of the nanostructure. Thus, we only show one set of the results for the sake of clarity (Figure 4d). However, the phase difference ∆ϕ between the symmetric and antisymmetric modes is close to 90° (−90°) with the amplitude ratio ps/pas approaching one at 630 nm, which directly leads to the approximate LCP (RCP) dipole transition at the left (right) arm-end. The corresponding surface charge variations with the time (Figure 5) intuitively reflect CP dipoles, which shows a counterclockwise (clockwise) rotation of the dipole moment for the left (right) arm-end excitation. By tuning the symmetric and antisymmetric modes hybridization, CP dipole transitions with opposite helicity can be achieved by moving the electron beam between the left and right arm-ends, leading to the CL emission helicity switching in the real space (Figure 4a).

Figure 5. The surface charge distribution variations with the time at 630 nm under these two arm-end excitations. The left (right) arm-end excitation shows a counterclockwise (clockwise) rotation of the dipole moment. Helicity-dependent binary encoding and two dimensional display. This CL helicity switching phenomenon provides a freedom for the manipulation of the CL emission with chiral states “stored” in the arm-ends of the nanostructure like a memory cell (Figure 6a). The LCP and


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RCP photons at the left and right arm-ends can be read out with the electron beam scanning, respectively. By using filter groups for the LCP detection with the bandpass of 657 nm, the left and right arm-ends can be lightened on and off, which are defined as 1 and 0 for the binary encoding, respectively. Two binary sequences were designed according to the ASCII codes of capital letters “C” and “L” (Figure 6b) with the fabricated V-shaped nanoantenna arrays as shown in Figure 6c,d. As expected, the left and right arm-ends in all of the structures were lightened on and off with the LCP detection. Hence, the binary sequences for “C” and “L” can be realized and outputted by simply moving the electron-beam along the scan path i and ii (Figure 6e). Although the V-shaped nanoantenna is larger than the unit cell of the electronic memory, the V-shaped nanoantenna realized the direct information conversion from electrons to photons, which is useful for integrated optoelectronic applications. With the detection polarization changed to RCP, complementary CL images and binary sequences outputs can be realized for the helicity-dependent imaging and information output (Figure S4). The binary encoding can also be realized with nanorods under the linear polarization detection, which outputs the same binary sequences under LCP and RCP detections. For the V-shaped nanoantenna, the binary encoding is helicity-dependent with complementary binary sequences outputted under LCP and RCP detection. When the detection polarization changed to linear polarization, all of the left and right arm-ends in the structures were lightened on, and no effective information can be outputted. Therefore, the helicity-dependent binary encoding is suitable for secrecy applications, which cannot be realized by nanorods.


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Figure 6. Applications of the Al nanoantenna for the helicity-dependent binary encoding. (a) Left: schematic of functional units with LCP and RCP photons “stored” in the left and right armends, respectively. Right: the LCP CL image of a single nanostructure with the bandpass of 657 nm. The left and right arm-ends were lightened on and off, which are defined as 1 and 0 for the binary encoding. (b) The ASCII codes of capital letters “C” and “L” for the binary encoding with the LCP detection, and (c, d) the corresponding SEM images and LCP CL images of the designed pattern. (e) CL-position relationships with the electron-beam moving along the scan path i and ii marked in (c) and (d) that output the binary sequences of capital letters “C” and “L”. Moreover, the arm-end in the nanoantenna can also function as the luminescence center with the light and dark states switched by the detection helicity, which can be applied to the two dimensional helicity-dependent display for secrecy applications. As a prototype, nanoantenna arrays were designed (Figure 7a) to display the capital letters “CL” with the LCP detection (Figure 7b), while showing a disordered image for the RCP detection (Figure S5). Other binary


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sequences and display images can be obtained by properly arranging nanoantenna patterns, facilitating applications of information storage and display at the deep-subwavelength scale.

Figure 7. Two dimensional helicity-dependent display. The SEM image (a) and LCP CL image (b) for the helicity-dependent two dimensional display of capital letters “CL”. CONCLUSIONS Various chiroptical phenomena, such as classic CD and optical SOI effect, are inseparable from CP-dependent light-matter interactions and electromagnetic mode interferences. The physical insight of chiral effects is obscured in the near-field of the nanostructure, and hardly detected by traditional optical characterization methods and even SNOM at the deepsubwavelength scale. By using the CP-resolved CL microscopy with the deep-subwavelength resolution, the hidden chirality of a single symmetric nanostructure emerges in front of the eyes as guided by the reciprocity between electron beam excitation and far-field optical excitation. The measured CP-resolved CL image perfectly reflects both the near-field chiral electromagnetic


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mode distribution and the chiral radiative LDOS distribution of the nanostructure, which greatly benefits the design of chiral light-matter interactions. The mode interference in the symmetric Vshaped nanoantenna was revealed with a giant near-field chirality (~99%), which enables the CP CL emission (with the largest degree of CP approaching 90%) from the radiative LDOS hot-spot. Intriguingly, the accurate moving of the electron beam impinging position allows the real space tuning of the mode hybridization and the following active switching of the CL helicity at the deep-subwavelength scale. After the proposed V-shaped nanoantenna was designed as functional patterns, the helicity-dependent binary encoding and the two dimensional display were realized, and can find applications in many areas including quantum information storage and processing. Our proposed physical principle and experimental configuration for the chiral imaging and manipulation of the near-field electromagnetic mode and the radiative LDOS at the deepsubwavelength scale is attractive, and can benefit the development of modern interdisciplines in chiral nanophotonics and quantum optics. METHODS Sample fabrication. The V-shaped nanostructures were fabricated on the Si/SiO2 substrate with a standard EBL process followed by lift-off process and Ar ion irradiation. In detail, positive resist (MircoChem PMMA A2 950) was spin-coated onto the substrate with a thickness of ~60 nm. Structures were patterned by using a focused 30 keV electron beam controlled by the Nano Pattern Generation System (NPGS) module, which is equipped onto the SEM (FEI Quanta 450 FEG). 30 nm Al was deposited on the substrate by using an electron-beam evaporator (LJUHV E-400L). Ar ion irradiation was performed for one minute to clean up the residual PMMA in the final process.


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CL measurements. CL images were acquired by a CL detector system (Gatan MonoCL4 Plus), which is equipped onto the SEM (FEI Quanta 450 FEG). The emission was collected by the high sensitive PMT (HSPMT, 160-930 nm). For detecting specific wavelengths of the CL emission, different bandpass filters were placed into the optical path. For CP-resolved CL detection, two sets of quarter-wave plate (Thorlabs, AQWP10M-980 and Union Optics, WPA4225-490-690) and linear polarizer (Thorlabs, LPVIS100 and Union Optics, SHP1025) combinations were used for different detection wavelength ranges. Specifically, the LCP and RCP CL emissions can be selectively collected by locating the fast axis of the wave plate by ±45° with respect to the polarization axis of the polarizer (Figure 2b). Besides, the substrate background signal (Ibackground) was subtracted from each pixel (Iraw) in the raw CL image, and the resulting CL signal of each pixel (ICL) were corrected based on the collection efficiency of the system with the correction equation expressed as ICL = (Iraw− Ibackground)/∆λ3dB/ηsystem. ∆λ3dB is the effective 3dB bandwidth of each bandpass filter, and ηsystem is the corresponding collection efficiency including contributions of the PMT, quarter-wave plates and linear polarizers at each center wavelength of the bandpass filter. Numerical simulations. Full-field electromagnetic wave simulations were performed by using the commercial FDTD method solver (FDTD solutions, Lumerical). The simulation domain included the structure with perfect matched layers in all directions. For the calculation of scattering, total-field scattered-field sources with linear and circular polarization were used to illuminate the structure along the −z axis. In the calculation of CL emissions, the electron-beam moving along –z axis was regarded as a linear current density J(r, t) = evδ(z+vt)δ(x−x0)δ(y−y0)nz, where e is the electron charge, v is the velocity of electron, r = (x0, y0, z) is the position of the electron-beam, and nz is the unit vector along the +z direction. In the frequency domain, it


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corresponds to a current density as J(r, ω) = ee−iωz/vδ(x−x0)δ(y−y0)nz, and then the current density was modeled as a series of dipoles with a temporal phase delay (−z/ν) related to the electron velocity, ν = 0.34c, corresponding to 30 keV electron energy (c is the velocity of light in vacuum). A reference simulation (without the nanostructure and substrate) was also run to subtract any background signal created by only the electron beam that could obscure the signal from the nanostructure. The CL spectra were calculated in the far-field by integrating the Poynting vector normal to an arbitrary surface in the upper z half-plane for the wavelengths ranging from 400 to 900 nm. In the far-field region, the time-averaged magnitude of the Poynting vector in sphere coordinate can be expressed as Ptotal = ε0c(|Eθ|2+|Eϕ|2)/2, PLCP =

ε0c(|Eθ-iEϕ|2)/4 and PRCP = ε0c(|Eθ+iEϕ|2)/4 for the total, LCP and RCP CL spectra calculation. The far-field region in the upper surface is vacuum. In the simulations, we used Palik data for the Al, Al2O3 and Si complex refractive indices. The refractive index of SiO2 was taken as 1.5.

ASSOCIATED CONTENT Supporting Information. CP-resolved reciprocity theorem between electron beam excitation and plane wave illumination, multipole expansion method for the dipole moment calculation, the relation between chiral radiative LDOS and CP-resolved CL, the total and RCP CL images of designed patterns for the helicity-dependent binary encoding and the two dimensional display, the influence of the parabolic mirror on the polarization state of CL emissions, simulated CP CL spectra of the Au dimer nanostructure, CP-resolved emission spectra of electric dipoles located at the left arm-end of the V-shaped nanoantenna, the influence of the nanoantenna rotation angle on the CP-resolved CL emissions, and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.


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Competing financial interest. The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions Z.F., S.Z. and T.H. conceived the idea and designed the research. S.Z. performed numerical simulations and theoretical analyses. T.H. and M.J. fabricated the samples. T.H., S.Z. and M.J. performed the CL measurement. S.Z., T.H., M.J., F.L., X.Z. and Z.F. wrote the manuscript. All authors discussed the results and commented on the manuscript. ‡S.Z. and T.H. contributed equally to this work. ACKNOWLEDGMENT This work is supported by the National Key Research and Development Program of China (Grant No. 2017YFA0205700), National Basic Research Program of China (Grant Nos. 2015CB932403 and 2017YFA0206000), National Science Foundation of China (Grant Nos. 11674012, 61422501, 11374023, and 61521004), Beijing Natural Science Foundation (Grant No. L140007), Foundation for the Author of National Excellent Doctoral Dissertation of PR China (Grant No. 201420), and National Program for Support of Top-notch Young Professionals (Grant No.W02070003). REFERENCES (1) Valev, V. K.; Baumberg, J. J.; Sibilia, C.; Verbiest, T. Chirality and Chiroptical Effects in Plasmonic Nanostructures: Fundamentals, Recent Progress, and Outlook. Adv. Mater. 2013, 25, 2517-2534.


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