Midrefractive Dielectric Modulator for Broadband Unidirectional

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Mid-refractive dielectric modulator for broadband unidirectional scattering and effective radiative tailoring in the visible region Pu Liu, Jiahao Yan, Curong Ma, Zhaoyong Lin, and Guowei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05123 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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ACS Applied Materials & Interfaces

Mid-refractive dielectric modulator for broadband unidirectional scattering and effective radiative tailoring in the visible region

Pu Liu†*a, Jiahao Yan†, Curong Ma, Zhaoyong Lin and Guowei Yang*b State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China †

These authors contributed equally to this work.

*a Corresponding author: [email protected] *b Author to whom correspondence should be addressed: [email protected]

KEYWORDS: mid-refractive dielectric, broadband, unidirectional scattering, radiative tailoring, visible region

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ABSTRACT Nanoantennas have found many applications in ultrasmall sensors, single-molecule detection and all-optical communication. Conventional nanoantennas are based on noble-metal plasmonic structures, but suffer from large ohmic loss and only possess dipolar plasmon modes. This has driven an intense search for all-dielectric materials beyond noble metals. Here, we propose mid-refractive nanospheres as a novel all-dielectric material to realize broadband unidirectional radiation and effective radiative tailoring in the visible region. Mid-refractive all-dielectric materials such as boron nanospheres possess broad and overlapping electric and magnetic dipole modes. The internal interaction between these two modes can route the radiation almost on the one side covering the whole visible range. Unlike the elaborate design in plasmonic nanostructures to obtain strong coupled broad and narrow modes, the bright mode in boron nanospheres is intrinsic, independent and easily coupled with adjacent narrow modes. So the strong interaction in boron-based heterodimer is able to realize an independent and precise tailoring of the radiant and subradiant states by simply changing the particle sizes respectively. Our findings imply mid-refractivity materials like boron are excellent building blocks to support electromagnetic coupling operation in nanoscale devices, which will lead to variety of emerging applications such as nanoantennas with directing exciton emission, ultrasensitive nanosensors, or even potential new construction of photonic metamaterials.

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INTRODUCTION In nanophotonics, metallic plasmonic structures have been widely used to manipulate the optical field at the nanoscale through localized surface plasmon resonance. These approach are very attractive for applications such as chemical and biological sensing,1-3 directing or enhancing single-molecule emission,4-6 and metamaterials with engineered electric and magnetic responses.7-9 However, conventional plasmonic materials have several disadvantages that restrict their applications. First, plasmonic materials like gold (Au) and silver (Ag) suffer from high optical loss at optical frequencies. Second, noble metals are rare and too expensive for large-scale fabrication. Third, plasmonic nanoparticles usually exhibit only electric dipole resonance, which cannot tailor and direct optical fields as desired. To control fields and radiation, one of major challenges facing current plasmonics is the manufacture of nanostructures whose optical properties can be operated artificially.10,11 Because of the absence of a natural broadband dipole mode, so far optical Fano resonances have typically been based on hybrid plasmon modes generated in complicated nanostructures.12,13 To fabricate effective nanoscale transmitters, receivers and sensors, some complex plasmonic structures that hold multiple plasmon modes to direct scattered radiation have also been proposed.14-16 In recent years, alternative approaches using efficient dielectric materials to construct optical antennas and metamaterials have been developed.17-25 Dielectric nanoparticles with moderately high refractive index (n) and very low absorption (for example, silicon (Si) and germanium in the visible–near-infrared range) have been

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shown to exhibit intriguing anisotropic light-scattering properties originating from their intrinsic electromagnetic interaction.26,27 However, the electric dipole (ED) and magnetic dipole (MD) modes in a Si sphere are narrow and detached in spectra, so they cannot perform well in internal or external electromagnetic (EM) couplings. It thus remains a challenge to design a new generation of all-dielectric materials to realize effective radiative tailoring covering the whole visible region. Here, we propose mid-refractive materials (MRMs) as novel all-dielectric materials that operate in the visible region. We experimentally demonstrate that a single MRM nanosphere behaves as a directional nanoantenna, exhibiting broadband unidirectional scattering at visible wavelengths. The appropriate n of MRMs broadens their ED and MD modes. In addition, the resonant modes in MRMs covering the visible range are stable and intrinsic, and possess low nonradiative damping compared with those of conventional plasmonic materials. A series of MRM nanospheres composed of materials such as boron (B), silicon carbide (SiC), aluminum nitride (AlN) and zinc oxide (ZnO) are fabricated through a convenient bottom-up technique based on laser-induced ablation in a liquid,28-31 and demonstrated to possess intrinsic broad modes.32,33 Taking B nanospheres as a typical example of MRMs, we demonstrate that a single B nanosphere can behave as a new broadband unidirectional nanoantenna, radiating a light signal on almost only one side in the visible region because of an internal broadband electromagnetic interaction.34 In addition, we experimentally demonstrate strong, tunable Fano-type resonances from two kinds of B-based heterodimers originating from the interference of the

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intrinsic broad mode from the MRM and the narrow modes from Si and Au nanospheres. The strong interaction in the B-based heterodimers is able to realize independent, precise tailoring of the radiant and sub-radiant states by simply changing the particle sizes. Fine control of the optical properties can be accomplished through tuning the size of the B nanosphere to adjust its broad modes, while coarse tuning can be accomplished by tuning the size of Au or Si spheres possessing narrow modes. The B-based heterodimers serve as an example for the design of structures with highly complex optical line shapes, and reveal a new pathway for the implementation of tunable optical nanodevices and sensing.

EXPERIMENTAL SECTION Dielectric spheres of various sizes were synthesized using the fs-LAL process. The preparation of B nanospheres is described as an example. Crystalline B particles (50 mg, purity of 99.9999%, Alfa Aesar, China, Tianjin) were added to absolute alcohol (10 mL, Guangzhou Chemical Reagent) to form a suspension. Then, the third harmonic produced by a Q-switched Nd:YAG laser with a wavelength of 355 nm, pulse width of 3 ns, repetition frequency of 50 Hz, pulse energy of 40 mJ and beam size of 0.5 mm was carefully focused on the middle of the suspension. During laser irradiation, the suspension was stirred with a magnetic stirrer to uniformly disperse the raw B particles in the liquid environment. After laser ablation for 2 h, the suspension was left to stand for 30 min to allow the raw B particles to sink to the bottom of the vessel and separate from the preliminary product. The suspension was

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decanted into a new bottle and then ablated by a carefully focused femtosecond laser (Legend Elite Series ultrafast laser, Coherent Inc.) with a wavelength of 800 nm, pulse width of 35 fs, repetition frequency of 1 kHz and pulse energy of 4 mJ. After 2 h of laser irradiation, the products dispersed in the liquid were collected. Other MRM nanospheres were synthesized following a similar procedure, although some details were different. For example, to fabricate Si nanospheres, the ultrafast laser was focused on a monocrystalline Si target immersed in deionized water in a quartz chamber for 10 min to form a Si colloid. Heterogeneous nanospheres dispersed in liquid were prepared by mixing starting materials to form a suspension. The suspension was stirred with a magnetic stirrer in a clean quartz chamber while being irradiated by the second harmonic produced by a Q-switched Nd:YAG laser with a wavelength of 532 nm, pulse width of 10 ns, repetition frequency of 10 Hz and pulse energy of 100 mJ. After laser irradiation for 5 min, the products were collected and dialyzed carefully with deionized water. Finally, one drop of each suspension was transferred onto an indium tin oxide (ITO)-coated glass substrate and dried at 40 °C in a vacuum oven for further measurements. The back- and forward-scattering spectra of specific nanostructures were collected by a dark-field optical microscope (Olympus BX51) integrated with a quartz–tungsten–halogen lamp (100 W), a monochromator (Acton SpectraPro, 2300i), and CCD camera (Princeton Instruments, Pixis 400BR_eXcelon). A dark-field objective (100×, numerical aperture 0.80) was used to illuminate the B nanospheres with the white excitation light in backward scattering measurements. Meanwhile, an

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oil-immersed dark-field condenser with a numerical aperture of 1.4 was used to illuminate the B spheres from the bottom in forward-scattering measurements. Scattering light was collected by a dark-field objective (100×, numerical aperture 0.80). The scattered spectra from individual nanostructures were corrected by first subtracting the background spectra taken from the adjacent regions without spheres and then dividing them by the calibrated response curve of the entire optical system. These processes ensure that the scattering intensities from back and forward measurements are able to be compared. Finally back and forward spectra, near-field distributions, and far-field scattering patterns were calculated using the finite-difference time-domain (FDTD) method (FDTD Solutions 8.6.0, Lumerical Solutions, Inc.). The normal incident total-field scattered-field (TFSF) plane wave at visible wavelengths (300–900 nm) combined with two planar detectors in forward and backward directions were used to simulate the dark-field scattering in experiments.

RESULTS AND DISCUSSION Unidirectional scattering from individual boron nanospheres. The visualized scattering properties among B, Si and Au nanospheres are shown in Figure 1a. The arrows with different colors illustrate the directional radiation at different wavelengths. For a B nanosphere, ED and MD modes overlap and couple effectively (see Figure 1c), so the forward scattering always dominates over a broad spectral range. Conversely, for a Si nanosphere, depending on the relative strength between the

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narrow, detached ED and MD (Figure 1b), the scattering direction varies with wavelength. For an Au nanosphere, only the plasmon dipole-like mode interacts with the optical field and it has no directivity (Figure 1d). The simulated forward and backward spectra of 130-nm spheres of Si, B and Au clearly reveal the directivity (Figure S1, Supporting Information). Lower n of MRM weakens the confinement of light inside the sphere and makes the resonant modes broader.23 According to Kerker's theory,34 two broad, comparable dipole modes will lead to obvious unidirectional scattering. This property of the MRMs benefits the directional light transmission. When combined with other materials, MRMs can serve as a stable, broad, bright background and provide effective EM coupling. Remarkably, in a B-Si dimer (Figure 1e), the intrinsic broadband ED and MD act as a super-radiant mode. Noted that, just this super-radiant mode couples with the sub-radiant mode of Si couple, and induce the transfer energy to each other unit, which giving rise to a characteristic Fano line shape. Similarly, in a B-Au dimer (Figure 1f), because of the broadband MD and ED in B, the hybridized plasmon mode exhibits a distinct red shift with enhanced intensity compared with that of the B nanosphere. One unique property of MRMs is the low absorption loss, so the B-based heterodimers can avoid the nonradiative damping that always occurs in plasmonic structures. Furthermore, comparison of the imaginary part of permittivity between B and Au (Table S1, Supporting Information) also confirms the low-loss feature of B nanospheres. In our study, B nanospheres were fabricated by two-step laser ablation in liquid; the detailed structural characterization of the nanospheres is presented in Figure S2 in

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the Supporting Information. The scattering antenna behavior of the nanospheres was studied by dark-field scattering spectroscopy (see detail in Experimental Section). The scattering from three B nanospheres with diameter of about 120 nm is shown in Figure 2a, and Figure S3a and b. The forward and backward scattering spectra both contain one resonance peak. However, the forward scattering intensities are all much stronger than the backward scattering ones, especially in the spectral range from 450 to 600 nm. Such distinct directional scattering cannot be observed for dielectric materials with high refractive indices.17 To determine the origin of the directional scattering, the optical field that scattered by a sphere is carefully decomposed into a multipole series using Mie theory. Notably, it is demonstrated that the fabricated B nanospheres are clearly polycrystalline structure, whose dielectric function was reported in previous work.35 For a B nanosphere ( d ≤ 200nm ), we mainly considered the dominant ED (Mie coefficient a1 ) and MD (Mie coefficient b1 ) plotted in Figure 2b. Meanwhile the forward and backward scattering spectra calculated by the FDTD method are also plotted in Figure 2b for comparison with the contributions from MD and ED. The line shape of the forward scattering spectrum is similar to that of the ED contribution, which is much stronger than the MD one, while the backward scattering spectrum is different and shows an asymmetrical lineshape at 470 nm. As depicted in Figure S4, the shallow dips in the backward scattering spectra are good evidence for the destructive interference between overlapping ED and MD resonances.28 Furthermore, to compare the scattering intensities in the forward and backward directions clearly, the calculated scattering spectra are replotted in Figure 2c.

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Obviously the forward scattering is much stronger than the backward scattering over a broad spectral range, which is in accordance with the experimental results. The mechanism behind directional scattering can be understood by considering Kerker's theory.34,36,37 The changes of ED ( a1 ) and MD ( b1 ) as a function of wavelength determine the unidirectional scattering properties of the B nanosphere, and the unidirectional scattering is obvious when the ED and MD are overlapped in spectrum. Noted that, the spectra in Figure S5a calculated using Kerker's theory are similar to both FDTD-simulated and experimental results. In addition, the calculated scattering patterns presented in Figure S5b-d also exhibit an asymmetry in the backward and forward directions. The unidirectional behavior is always preserved while the size of the B nanosphere is changed. As shown in Figure 2d and Figure S3c, two typical large B spheres with a diameter of 200 nm exhibit obvious unidirectional scattering in the visible region. Furthermore, compared with the small B spheres in Figure 2a, larger spheres can scatter light over a broader spectral range (from 400 to 800 nm). This factor should be beneficial to construct a broadband unidirectional nanoantenna. In addition, the Mie coefficients that represent MD and ED can be calculated, as shown in Figure 2e. And further, the backward and forward scattering spectra simulated by the FDTD method are provided in Figure 2e to investigate how the synergistic effect between the ED and MD responses influences the direction of scattering. Compared with small B nanospheres, the MD in the B spheres with a diameter of 200 nm is stronger and broader, so the interaction between ED and MD are more prominent than

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those for a smaller nanosphere. When the MD approaches the same strength as the ED, there is almost no backscattering and most light scatter is forward. Noted that, the simulated spectra in Figure 2f shows the difference between the forward scattering and backward scattering for a large B nanosphere. Similar as the measured spectra, the scattering intensity in the forward direction is about five times higher than the backscattering intensity over a broad spectral range from 300 to 800 nm. At some specific wavelengths, which are marked in Figure 2f, the ratio of forward to backward scattering exceeds 25. Lower quantum efficiencies of the charge coupled device (CCD) below λ = 500nm produce an obvious drop close to λ = 400nm leading to the differences between simulated and measured results. Assembling two B nanospheres to form a dimer greatly improves the unidirectional scattering behavior, as shown in Figure S3d and e. Two nearly touching B spheres can generate strong field enhancements in the gap between them, and this gap mode strengthens the existing EM coupling and leads to superior unidirectional scattering. Furthermore, to confirm the features of MRMs, we demonstrated that three other materials, ZnO (n=2.1), SiC (n=2.6) and AlN (n=2.2), also possess this unique broadband unidirectional scattering property, as illustrated in Figure S6. Since all-dielectric materials have become a hot topic in recent years, for example, the Si nanospheres as the most common dielectric nanostructures with high refractive-index show strong and detached electric dipole resonance and magnetic dipole resonance. While for the investigated boron nanospheres, the lower refractive index brings new physics in nanophotonics. For instance, the lower refractive index can produce strong

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and broad magnetic dipole mode and electric dipole mode. If further decreasing the refractive index like AlN (n=2.2), although the electric and magnetic dipole modes would become even broader, and their scattering intensities are also greatly weakened. Obviously, this class of materials broadens our range of technological choices when designing nanophotonic devices. To further understand how n influences the mode evolution and directional scattering behavior of MRM nanospheres, we used B as a standard candidate and adjusted its n and extinction coefficient (k) to calculate the scattering behavior using Mie theory theoretically. It is found that the relationship between n and the mode changes is important to aid the design and selection of suitable materials for different radiative functions. The scattering spectra of 200-nm B spheres are replotted in Figure 3e and f. Firstly, the quadrupole resonances (Mie coefficients a2 and b2 ) are weak and located below 400 nm (see Figure 3e), so we only need to focus on the directional scattering originating from dipole modes. As n is increased from 3.2 to 4.4 as displayed in Figure 3e, g, i and k, the dipole resonances become stronger and exhibit red shifts, especially the MD mode. The narrowing and red shift of the MD resonance lead to a separation between ED and MD. When n=4.4 (see a detailed dielectric function in Figure S7), the scattering spectra containing contributions from ED and MD modes are narrow and detached, resembling those of Si. Therefore, the increasing n enhances the backward scattering and renders the MD and ED peaks more obvious and detached in the forward direction, as shown in Figure 3f, h, j and l. These changes hinder the application of the nanosphere as a broadband unidirectional antenna.

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Although the directivity can be further improved by finding a material with lower n (

Midrefractive Dielectric Modulator for Broadband Unidirectional

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