Imaging of Plasmonic Chiral Radiative Local Density of States with

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Imaging of Plasmonic Chiral Radiative Local Density of States with Cathodoluminescence Nanoscopy Shuai Zu, Tianyang Han, Meiling Jiang, Zhixin Liu, Qiao Jiang, Feng Lin, Xing Zhu, and Zheyu Fang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03850 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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Imaging of Plasmonic Chiral Radiative Local Density of States with Cathodoluminescence Nanoscopy Shuai Zu1‡, Tianyang Han1‡, Meiling Jiang1, Zhixin Liu1, Qiao 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 light-matter interactions as an emerging aspect of quantum optics enable exceptional physical phenomena and advanced applications in nanophotonics through the nanoscale exploitation of photon-emitter interactions. The chiral radiative properties of quantum emitters strongly depend on the photonic environment, which can be drastically altered by plasmonic nanostructures with a high local density of states (LDOS). Hence, precise knowledge of the chiral photonic environment is essential for manipulating the chirality of light-matter

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interactions, which requires high resolution chiral characterization techniques. In this work, chiral radiative LDOS distributions of single plasmonic nanostructures that directly govern the chiral radiative spontaneous decay of quantum emitters are imaged at the nanoscale by using cathodoluminescence nanoscopy, enabling precise and highly efficient control of chiral photon emission for chiroptical technologies. Radiative LDOS hot-spots with the chirality larger than 93% are obtained by properly designing chiral plasmonic modes of Au nanoantennas. After fabricating monolayered WSe2 nanodisks (NDs) at chiral radiative LDOS hot-spots and forming ND/Au hybrid nanostructures, the chiral radiative properties of WSe2 NDs are significantly modified, leading to chiral photoluminescence. Our experimental concept and method provide an effective way to characterize and manipulate chiral light-matter interactions at the nanoscale, facilitating future applications in chiral quantum nanophotonics such as single-photon sources and light emission devices.

KEYWORDS: Cathodoluminescence, plasmonics, chiral radiative local density of states, WSe2 monolayers, quantum emitters. With the exploitation of plasmonic resonances, metallic nanostructures exhibit unprecedented talents for the manipulation of light-matter interactions at the nanoscale.1 Due to the plasmonic light localization and field enhancement effects,2, 3 the efficiency of many photophysical processes can be drastically increased,4, 5 promoting potential applications in nanophotonics, such as light detection6-8 and emission.9, 10 Moreover, the high local density of states (LDOS) of plasmonic nanostructures that directly determines the radiative properties of quantum emitters allows the flexible control of their spontaneous decay rate and light emission efficiency, facilitating quantum optical applications like single-photon devices and light communication.11,

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Intriguingly,


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abundant electromagnetic modes of metallic nanostructures also provide a versatile platform for tailoring the photonic environment and further precisely engineering electromagnetic properties of light emission including polarization states and radiation directions, leading to various nanophotonic applications like spin-polarized photon emission13-16 and unidirectional radiation.17 Recent progress in quantum optics demonstrates the important role of chirality in light-matter interactions.18 The radiative properties of quantum emitters can be highly affected by their chiral photonic environment, leading to fascinating chiral quantum optical phenomena like nonreciprocal photon–emitter interactions with potential applications in quantum information processing.19 By utilizing the unique talent of plasmonic nanostructures for the control of electromagnetic field, chiral photonic environment or LDOS can be intentionally engineered to manipulate chiral light-matter interactions.20-22 However, the interaction between photons and quantum emitters is mainly confined at the nanoscale, and it is impractical to fabricate hybrids of quantum emitters and plasmonic nanostructures without the precise knowledge of LDOS distributions. Thus, nanoscale chiral characterization techniques of LDOS are required, which is vital for the manipulation of chiral light-matter interactions and can further provide direct guidelines for the design of light-matter interaction systems in numerous chiroptical devices. In general, the spontaneous decay of quantum emitters can be influenced by radiative and nonradiative channels, corresponding to photon emission and energy dissipation, respectively.23 In contrast to the nonradiative decay, the radiative decay through photon emission into the farfield allows the division of different light polarization states, making it possible to characterize the chirality of light-matter interactions through the chiral radiative LDOS.24,

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As a nanoscale

characterization technique, cathodoluminescence (CL) nanoscopy has been successfully used in the detection of radiative LDOS in plasmonic and photonic nanostructures. 26-28 With the electron


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beam functioning as a virtual dipole source, CL photon emissions are generally related to the projection of the radiative LDOS along the moving direction of electrons.29, 30 After resolving CL emissions into left-handed circular polarization (LCP) and right-handed circular polarization (RCP) components, the generalized chiral radiative LDOS that governs the radiative decay into either LCP or RCP channels can be further acquired.25 In this work, we realize the nanoscale imaging and utilizing of the chiral radiative LDOS in single plasmonic nanostructures by using CL nanoscopy. The fine tailoring of chiral plasmonic modes in Au nanoantennas facilitates the formation of chiral radiative LDOS hot-spots with the chirality larger than 93%, which enables the precise control of light-matter interactions for chiral photon emission. With a combination of electron beam lithography (EBL) and reactive ion etching (RIE) methods, monolayered WSe2 nanodisks were patterned at the chiral radiative LDOS hotspots of Au nanoantennas, resulting in the significant modification of chiral radiative properties and the consequent chiral photoluminescence (PL) from nanodisk/Au hybrid nanostructures. The characterization and exploitation methods of the chiral radiative LDOS at the nanoscale open a way for the manipulation of chiral light-matter interactions and may find potential applications like highly efficient chiral light emission or single-photon devices. Relation between Radiative LDOS and CL. The radiative decay of quantum emitters that corresponds to photon emission into the far-field can be straightforwardly modelled by classical electrodynamic theory (Supporting Section I). For a monochromatic point electric dipole p along a unit vector n at a position r0, the power radiated into the far-field is given by Prad = 2|p|2rad n (r0, )/40, where 0 is the permittivity of the far-field region assumed to be vacuum and rad n (r0,

) is the projected radiative LDOS defined as29-31

nrad (r0 ,  ) 

2 2 c

  n  G 4

T



*

(, r0 ,  )  G  (, r0 ,  )   nd 

(1)


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Here, Ω denotes a solid angle in a spherical coordinate, c is the light speed in vacuum, and G  (, r0 ,  )  reir / c lim G(r, r0 ,  ) is the far-field asymptote of the electric Green tensor for r / c 

Maxwell’s equations.30 Similarly, CL measures the far-field radiation originated from swift electrons interacting with a sample. For a specific electron trajectory re(t) = (x0, y0, z = vt), the intensity of CL emissions detected in a solid angle Ω0 can be expressed as29, 30

e 2   ˆ  ˆ  CL ( x0 , y0 ,  )  n z  G ind ,  )  G ind , )  n z d   (, x0 , y0 ,  (, x0 , y0 , 3  2  0c v v     T

*

(2)

0

where e is the electron charge, nz is the unit vector along the z direction and ˆ  i z / v Gind Gind  (, x0 , y0 ,  / v,  )   e  (, re (t),  ) dz is the Fourier transform of the induced far-

field asymptotic Green tensor obtained by subtracting the free-space part from the far-field asymptotic Green tensor. As shown in eqs 1 and 2, the CL intensity is generally related to the projection of the radiative LDOS along the z axis, which can be viewed as a generalized radiative density of states that is localized transversely in real space (xy plane) and local in momentum space along the remaining z axis. For plasmonic nanostructures, a measurement of total CL emissions could then probe the projected radiative LDOS along the electron trajectory. Moreover, electromagnetic properties of far-field radiations like polarization states are sensitive to localized plasmonic modes of metallic nanostructures.32 By resolving radiated power into LCP and RCP contributions, chiral CL emissions are shown to directly render the chiral radiative LDOS that governs the radiative decay into either LCP or RCP channels (Figure S1), enabling the characterization of chiral light-matter interactions like chiral photon emission. Au Nanostructure for Chiral CL Emissions. Under the excitation of moving electrons (or an approximate z-direction dipole), LCP and RCP CL emissions that well imitate the z-projected LCP


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and RCP radiative LDOS can be generated from the plasmon resonance in metallic nanostructures (Figure 1a), which are reciprocally related to the z-component of the electric field in the vicinity of nanostructures under LCP and RCP plane wave excitations, respectively.25 Therefore, the CL response (or the radiative LDOS) of plasmonic nanostructures can be easily inferred from their near-field distributions, allowing the achievement of desired CL responses through the plasmonic mode design. Figure 1b shows the designed Au rectangle nanoantenna with the length of 132 nm and the width of 108 nm, which was fabricated on a SiO2/Si wafer by EBL with the deposition of 30 nm Au and 2 nm Ti as an adhesion layer. The thickness of the SiO2 layer on Si wafer was chosen as 100 nm to avoid the Fabry-Perot like modulation due to the SiO2 layer (Figure S2). The nanoantenna can support two fundamental modes with characteristically dipolar mode distributions (Figure 1c), corresponding to x- and y-polarized, normally incident, plane wave excitations, respectively. Under circularly polarized light illuminations, both of these modes can be excited, and the subsequent hybridization of the two modes results in the formation of chiral plasmonic modes (Figure 1c). The total phase difference between the two modes approaches 0°(180°) for the LCP (RCP) light excitation, leading to the formation of dipole moments mainly along the top-right (topleft) direction (Figure S3). The absolute value for the z-component of the electric field shows prominent near-field enhancements at top-right and bottom-left (top-left and bottom-right) corners of the nanoantenna under the LCP (RCP) light excitation, which was further confirmed by the evolution of surface charge distributions with different phases (Figure S4). The prominent difference of electric field distributions under LCP and RCP light excitations indicates that chiral CL emissions can be generated from the corners of the rectangle nanoantenna under the electron beam excitation as guided by the reciprocity.


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Figure 1. Design of the Au nanostructure. (a) Schematic of LCP and RCP photon emissions under the electron beam excitation. (b) Scanning electron microscope (SEM) image of the Au rectangle nanoantenna. (c) Finite-difference time-domain (FDTD) calculated near-field distributions for the absolute value of the electric field z-component and snapshots of surface charge distributions at the resonant wavelength under plane wave excitations with x-polarization (at 699 nm), ypolarization (at 774 nm) and circular polarization (at 728 nm). The hybridization of x- and ypolarized modes results in the formation of chiral plasmonic modes under LCP and RCP excitations. By using CL nanoscopy, plasmonic modes and their hybridization for chiral CL emissions can be further revealed, providing direct guidelines for the chiral radiative LDOS imaging (Supporting Section II). Total CL spectra were individually measured for three different electron beam impinging positions of the nanoantenna, as shown in Figure 2a. With the impinging position located at the left-end and the top-end of the nanoantenna, dipolar modes along x and y axes were selectively excited, which show characteristic plasmon resonances at 680 nm and 769 nm, respectively. Corresponding bandpass CL images at the resonance wavelengths were acquired with


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characteristically dipolar mode distributions (Figure 2b), which well imitate the near-field distributions shown in Figure 1c. When the impinging position changed to the top-left corner, two resonant peaks appeared in the CL spectrum, resulting from the simultaneous excitation of the two dipolar modes along x and y axes (Figure S5). With the largest spectral overlap between the two modes at 732 nm, the bandpass CL image exactly reflected a superposed distribution of the two modes. FDTD simulations were also performed to calculate the CL spectra for three different impinging positions (See Methods), which have a good agreement with experimental results, as shown in Figure 2c. As indicated by chiral near-field distributions shown before (Figure 1c), chiral CL emissions were generated for the top-left corner excitation (Figure 2c, shadow regions), caused by the simultaneous excitation of two dipolar modes along x and y axes. The RCP CL emission is dominant over LCP in a broad spectral range with the largest chirality locating at 736 nm, facilitating the following chiral radiative LDOS imaging.


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Figure 2. CL responses of the Au nanoantenna. (a) Measured total CL spectra of three different excitation positions marked by red squares in the inset. (b) Total bandpass CL images with the center wavelengths of 680 nm, 769 nm and 732nm. (c) FDTD simulated total CL spectra corresponding to the three excitation positions in (a). The shadow regions show simulated LCP and RCP spectra with the excitation of the top-left corner in the nanoantenna. Nanoscale CL Imaging of Chiral Radiative LDOS. By resolving CL emissions into LCP and RCP contributions, LCP and RCP CL bandpass images were acquired with the center wavelength of 732 nm, which directly reflect LCP and RCP radiative LDOS distributions of the nanoantenna projected along the z axis, as shown in Figure 3a,b. The helicity of CL emissions can be distorted by the parabolic mirror.33 To minimize the influence of the parabolic mirror on the CL helicity, the long axis of Au nanoantennas was put along the symmetry axis of the parabolic mirror (Figure S6). Similarly to the near-field distributions of chiral plasmonic modes (Figure 1c), LCP radiative LDOS hot-spots located at top-right and bottom-left corners of the nanoantenna, while RCP radiative LDOS hot-spots located at top-left and bottom-right corners. LCP and RCP radiative LDOS intensities were further extracted along the outline of the Au nanoantenna to better analyze the evolution of chiral radiative LDOS (Figure 3c). The intensity of LCP and RCP radiative LDOS changed alternately with LCP dominant around top-right and bottom-left corners and RCP dominant around top-left and bottom-right corners. In order to quantitatively describe the difference between LCP and RCP radiative LDOS, the radiative LDOS chirality is defined as (LCP−RCP)/(LCP+RCP), where LCP and RCP are LCP and RCP radiative LDOS intensities respectively. In Figure 3d, the chirality of radiative LDOS larger than 93% was obtained with a positive (negative) sign at top-right and bottom-left (top-left and bottom-right) corners. As the chiral radiative LDOS directly determined the radiative decay into LCP and RCP channels, the


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chirality of photon emission from quantum emitters positioned at the corners of the nanoantenna can be highly modified, enabling LCP and RCP light emissions.

Figure 3. CL measurements of the chiral radiative LDOS. (a, b) LCP (a) and RCP (b) radiative LDOS distributions measured by the bandpass CL imaging with the center wavelength of 732 nm. (c, d) LCP and RCP radiative LDOS intensities (c), and the corresponding radiative LDOS chirality (d) along the outline of the Au nanoantenna marked by arrow lines in (a) and (b). Chiral PL from WSe2 ND/Au Hybrids. With a combination of the two-step EBL process and the RIE method, commercially available WSe2 monolayers with the PL central wavelength of 748 nm can be etched into nanodisks (NDs) and then precisely positioned at chiral radiative LDOS hot-spots of the Au nanoantenna (See Methods). As shown in Figure 4a, WSe2 monolayers on a SiO2/Si substrate were first spin-coated with a 60 nm poly(methyl methacrylate) (PMMA) layer and patterned with PMMA disks by using a standard EBL process. With PMMA disks acting as


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the mask, WSe2 NDs can be fabricated by using the RIE method. A second step of EBL process was finally performed to fabricate Au nanoantennas on the top of WSe2 NDs with the center of NDs locating at the top-left corner of the nanoantennas. The fabricated WSe2 ND/Au hybrid nanostructures were shown in Figure 4b with a high success rate of positioning WSe2 NDs at chiral radiative LDOS hot-spots of the Au nanoantenna. Figure 4c,d show chiral PL spectra of WSe2 NDs and ND/Au hybrids measured by a home-built optical microscope (Supporting Section II). LCP and RCP PL spectra of bare WSe2 NDs both have a resonant peak at 748 nm, showing equal intensities in the whole spectral range (Figure 4c). However, PL spectra of ND/Au hybrids present dominant RCP photon emission with the radiative property of WSe2 NDs highly modified by the chiral radiative LDOS (Figure 4d). Comparing with bare WSe2 NDs, the PL chirality of ND/Au hybrids, following the definition of CL chirality, changes from 0% to negative 23%, providing a unique way to make achiral luminescent materials to chiral for light emission devices.


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Figure 4. Fabrication and chiral PL measurements of WSe2 ND/Au hybrids. (a) Schematic of the two-step EBL process and the RIE method for fabricating WSe2 NDs at the corner of the nanoantenna. (b) The SEM image of WSe2 ND/Au hybrids. (c, d) LCP and RCP PL spectra, and the corresponding PL chirality for WSe2 NDs (c) and ND/Au hybrids (d). The PL spectra in (c) and (d) are normalized by the maximum value of LCP or RCP PL intensities for each figure. Conclusion. The chirality of the photonic environment provides a unique freedom for the manipulation of light-matter interactions. Various quantum optical and nanophotonic applications require precise knowledge of the chiral photonic environment, which can provide a direct guideline for the engineering of chiroptical phenomena and the subsequent design of nanophotonic devices. With the CL nanoscopy, chiral radiative LDOS distributions in single plasmonic nanostructures can be imaged at the nanoscale, which directly reflect the chiral radiative spontaneous decay of quantum emitters. By finely tailoring chiral plasmonic modes in Au nanoantennas, the chirality of radiative LDOS hot-spots is larger than 93%, enabling the highly efficient control of chiral photon emission from quantum emitters. The PL with the chirality approaching 23% can be further acquired by positioning WSe2 NDs at chiral radiative LDOS hot-spots in Au nanoantennas, promoting the exploitation of light emission devices. We believe that the basic concept and technique for the characterization and utilization of the chiral radiative LDOS are unique for the manipulation of chiral light-matter interactions at the nanoscale, which can greatly benefit the design of nanophotonic devices and inspire the development of chiral quantum optical science and technology. Methods. Sample fabrication. SiO2/Si substrates were sequentially rinsed with acetone, ethanol and deionized water in an ultrasonic bath for 5 min and dried with the nitrogen flow. Then, a positive resist (MircoChem PMMA A2 950) layer was spin-coated onto the substrate with the


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thickness of ~60 nm. Au nanostructures were fabricated on a SiO2/Si substrate by using a standard EBL process with the exposure dose of ~313 C/cm2 and a subsequent lift-off process. Under the control of the Nano Pattern Generation System module, structures were patterned by using a focused 30 keV electron beam of the SEM (FEI Quanta 450 FEG). By using an electron beam evaporator (DE400DHL, DE Technology), a 2 nm Ti adhesion layer and a 30 nm Au layer were deposited on the substrate with the chamber pressure of ~1e-7 Torr and the deposition rate of 0.18 Å/s and 0.5 Å/s, respectively. UV ozone cleaning (Samco UV-1) was performed for 10 min to remove the residual PMMA. To fabricate ND/Au hybrid nanostructures, chemical vapor deposition grown WSe2 monolayers on a sapphire substrate were first transferred onto the same SiO2/Si substrate by using the PMMA nanotransfer method.34 In detail, a PMMA layer (950K, 4% in anisole, Alfa Aesar) was spin-coated onto the sapphire substrate with the rotation speed of 1500 rpm and baked under 80 C for 30 s and 10 min, respectively. After that, the sapphire substrate was immersed and corroded by the 2 mol/L KOH solution to make the PMMA/WSe2 film peel from the substrate. A prepared SiO2/Si substrate was used to fish out the PMMA/WSe2 film and rinsed several times with deionized water. The PMMA layer was removed with acetone and isopropanol. Then, WSe2 monolayers on the SiO2/Si substrate were spin-coated with a 60 nm positive resist (MircoChem PMMA A2 950) layer. By using a standard EBL process with the exposure dose of ~150 C/cm2, PMMA disks with the designed radius of 50 nm were fabricated on WSe2 monolayers. With PMMA disks acting as the mask, WSe2 NDs were fabricated by using a 10 s Ar (3 sccm)/CF4 (30 sccm) RIE (ME-3A) process with the etching power of 30 W. The residual PMMA was then removed by acetone. A second step of EBL process was performed to fabricate


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Au nanoantennas on WSe2 NDs with the center of NDs locating at the top-left corner of the nanoantennas. Numerical Simulations. Electromagnetic field simulations were performed with the commercial FDTD method solver (FDTD solutions, Lumerical). In the simulation, perfect matched layers were used in all directions. Total-field scattered-field sources with linear and circular polarizations were used to excite electromagnetic modes in Au nanostructures. For the calculation of CL emissions, the linear current density generated by the electron-beam was regarded as j(r, )  eeiz/v(xx0)(yy0)nz,35 which can be modeled as a series of dipoles with a temporal phase delay (z/ν). Here, e is the electron charge, v is the velocity of electrons, (x0, y0) is the excitation position of the electron-beam, and nz is the unit vector along the z axis. CL spectra were calculated in the far-field of the upper z half-space by integrating the Poynting vector normal to an arbitrary surface. In a spherical coordinate system, the time-averaged magnitude of the Poynting vector is Ptotal =

0c(|E|2+|E|2)/2, PLCP = 0c(|EiE|2)/4 and PRCP = 0c(|E+iE|2)/4 for the calculation of total, LCP and RCP CL spectra, where  and c are the permittivity and light speed in a vacuum. In the simulation, we used Johnson and Christy data for the Au refractive index and Palik data for the Si refractive index. The refractive index of SiO2 was taken as 1.45. ASSOCIATED CONTENT Supporting Information. Relation between chiral radiative LDOS and CL, the simulated reflection spectrum of the SiO2/Si substrate, simulated scattering spectra and dipole moments under plane wave excitations, the evolution of surface charge distributions with time, simulated near-field distributions under the electron beam excitation, the CL response of Au nanoantennas with different rotation angles at the bandpass of 732 nm, LCP and RCP PL spectra of WSe2


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monolayers and WSe2 ND/Au hybrids, CL and PL measurements, and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions Z.F. and S.Z. 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. T.H., M.J. and Z.L. performed the PL measurement. S.Z., T.H., M.J., Z.L., Q.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. Notes The authors declare no competing ﬁnancial interest. 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


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The graphic for the Table of Contents


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Imaging of Plasmonic Chiral Radiative Local Density of States with

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