Article Cite This: ACS Photonics 2018, 5, 1359−1364
Directional and Spectral Shaping of Light Emission with MieResonant Silicon Nanoantenna Arrays Aleksandr Vaskin,*,† Justus Bohn,† Katie E. Chong,‡ Tobias Bucher,† Matthias Zilk,† Duk-Yong Choi,§ Dragomir N. Neshev,‡ Yuri S. Kivshar,‡ Thomas Pertsch,† and Isabelle Staude† †
Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena, 07745 Jena, Germany Nonlinear Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia § Laser Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia ‡
S Supporting Information *
ABSTRACT: We study light emission from square arrays of Mie-resonant silicon nanoantennas situated on a fluorescent glass substrate. When the spectral positions of the silicon nanoantennas’ resonances overlap with the intrinsic emission from the glass, the emission is selectively enhanced for certain spectral and spatial frequencies detemined by the design of the nanoantenna array. We measure the emission spectra of the coupled system for a systematic variation of the nanoantenna geometry, showing that the spectral maximum of the emission coincides with the antenna resonance positions observed in linear-optical transmittance spectra. Furthermore, we study the directionality of the emission by back focal plane imaging and numerical calculations based on the Fourier modal method and the reciprocity principle. We observe that the nanoantenna array induces a reshaping of the resonantly enhanced emission in the air half-space into a narrow lobe directed out of the substrate plane. This reshaping is explained by coherent scattering of the emitted light in the nanoantenna array. Our results demonstrate that combining emission enhancement by magnetic dipolar Mie-type resonances of silicon nanoantennas with diffractive coupling in the periodic arrangement allows for the creation of flat light sources with tailored spectral and directional emission properties. KEYWORDS: all-dielectric nanophotonics, dielectric nanoantennas, spontaneous emission, back focal plane imaging
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effect was recently employed to demonstrate directional emission from a monolayer of MoS2 using a silicon nanoantenna.24 Additional opportunities are offered by interfering the scattering contributions from several all-dielectric nanoparticles arranged in a chain or an array.3,25 A prominent example is Yagi-Uda nanoantennas, which were previously studied for both plasmonic26 and dielectric3 building blocks. However, for a planar arrangement of their elements, Yagi-Uda nanoantennas direct the light sideways, such that collection requires the use of high numerical aperture (NA) liquid immersion objectives. As previously demonstrated for active plasmonic nanoantenna architectures, more general angular distributions of the emitted light can be accessed by twodimensional nanoantenna arrays.27,28 In particular, it was shown theoretically29 that the emission of a single electric point dipole can be strongly modified by a periodic array of plasmonic nanoantennas sustaining surface lattice resonances.30 By tuning the emission wavelength with respect to the lattice period, the
ptical nanoantennas provide comprehensive opportunities for enhancing and controlling the spontaneous emission of nanoscale sources located in their vicinity.1,2 While research so far has mainly focused on plasmonic nanoantennas, all-dielectric nanoantennas have recently attracted an increasing amount of attention.3−10 Owing to their low absorption losses, they can offer high radiation efficiencies, which is of interest for many possible applications of nanoantennas such as quantum light sources or displays. At the same time they allow for strong enhancement5 as well as spectral6,11,12 and directional shaping3,4,10,11,13 of emission. Nanoantennas composed of high-index dielectric nanoparticles are of particular interest, as they exhibit localized Mie-type14 resonances.8,15−17 Based on the Purcell effect,18 Mie-resonant dielectric nanoparticles can exert a strong influence on the radiative decay rate of emitters.5,9,20 Furthermore, interference of scattering contributions from different Mie-type resonances supported by a single dielectric nanoparticle can result in directional emission patterns.4,13,21,22 Most prominently, the Kerker effect23 allows for controlling directional scattering from a nanoparticle exhibiting both electric and magnetic polarizabilities. This © 2018 American Chemical Society
Received: November 14, 2017 Published: February 1, 2018 1359
DOI: 10.1021/acsphotonics.7b01375 ACS Photonics 2018, 5, 1359−1364
Article
ACS Photonics emission pattern can be structured into a single lobe, two lobes, or multiple lobes.29 Here we experimentally demonstrate spectral reshaping and out-of plane directional emission enhancement by coupling of the intrinsic fluorescence from a glass substrate in the 600−900 nm spectral range to square arrays of Mie-resonant silicon nanocylinder antennas. To achieve the desired emission spectrum and pattern, we combine, for the first time to our knowledge, emission enhancement by the Mie-resonant dielectric nanoantennas with coherent scattering in a tailored periodic arrangement. By choosing the lattice period to match the wavelength of the magnetic dipolar Mie-type resonance of the nanocylinders, we shape the air half-space emission into a single lobe out of the substrate plane. Note that this mechanism is different from previous experimental demonstrations of directional emission enhancement by dielectric nanoantennas, which relied on waveguiding10 or the Kerker effect.24 A sketch of our experiment is depicted in Figure 1. Intrinsic fluorescence
Figure 2. (a) Experimentally measured and numerically calculated transmittance of the a-Si:H nanocylinder array with a diameter of 286 nm. (b) Calculated electric field distribution in a plane through the center of the nanocylinder of the electric dipole mode at 730 nm and the magnetic dipole mode at 844 nm wavelength, excited by a linearly polarized, normally incident plane wave.
software package COMSOL Multiphysics (see Methods for details on the numerical calculations). The results are presented alongside the experimental spectra in Figure 2(a). In accord with experimental values, the lattice constant was fixed to 560 nm. The silicon nanocylinder diameter and height were allowed to vary within the experimental accuracy limits. A diameter of 210 nm and a height of 160 nm were found to provide the best agreement with experimental data. In accordance with a previous work,35 the cylinders appear smaller in optical measurements as compared to SEM images, which is likely due to the formation of a low-refractive-index layer on the nanocylinder surface with a thickness of several tens of nanometers during the fabrication. Indeed, a low-contrast layer was observed in focused ion beam cross-section images. The measured and calculated transmittance spectra exhibit three pronounced minima located at wavelengths of 620, 730, and 844 nm. In order to gain further insight on the multipolar order of the Mie-type modes associated with these transmittance minima, we also calculated the electric field distribution in the vertical cross-section through the center of the nanocylinder for the resonance wavelengths. These results are shown in Figure 2(b) for 730 and 844 nm. The field profiles indicate that the electric dipole (ED) resonance of the nanocylinder is excited at 730 nm, while the magnetic dipole (MD) resonance occurs at 844 nm, as evidenced by the circulating electric field. Note that inside the glass substrate, the MD resonance features an electric near-field enhancement of almost 10 immediately below the nanoresonators. The minimum at 620 nm originates from the excitation of a higher-order mode and was not considered in the present work. As a next step, we collect emission spectra of our samples for a range of nanocylinder diameters using a commercial confocal microscope setup (Picoquant, MicroTime-200) with an NA = 0.25 objective (Olympus, Plan N) coupled to a spectrometer (Horiba, iHR320). As excitation source, we used a 532 nm laser with an average excitation power of 1.4 mW at 100 ps pulse length and 80 MHz repetition rate. Under these excitation conditions, we estimate that the focus diameter is 2r = 2λ/(NA × π) = 1.355 μm, leading to an excitation power flux of 1 × 105 W/cm2. The results of the photoluminescence (PL) measurements are shown in Figure 3. For reference, we also measured the emission from the bare glass substrate next to the nanoantenna arrays (gray areas). Clearly, the substrate fluorescence spectrum is strongly reshaped by the presence of
Figure 1. (a) Artist’s impression of Mie-resonant a-Si:H nanocylinders placed on a fluorescent glass substrate. (b) Scanning electron micrograph of a typical sample.
in optical glass originates for instance from point defects and certain impurities, which can be influenced by process technology and the choice of raw materials.31−34 While often considered a nuisance in low-light measurements, in our experiment the intrinsic fluorescence of the glass substrate provides a convenient broadband light source that can be efficiently enhanced by coupling to the dielectric nanoantennas. Our experimental approach furthermore avoids the use of potentially toxic emitters such as colloidal quantum dots. Finally, its simplicity prospectively allows for additional hybridization with other functional materials, such as for active control, while keeping the complexity of the entire system manageable.
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RESULTS AND DISCUSSION We fabricate several nanocylinder arrays with a footprint of 100 μm × 100 μm and nanocylinder diameters varying between 186 and 286 nm (see Methods for details on the fabrication process). The lattice constant and the cylinder height are fixed to 560 and 182 nm, respectively. A scanning electron micrograph (SEM) of a typical sample is shown in Figure 1(b). As a first step, we measured the near-normal-incidence linearoptical transmittance spectra of the fabricated nanocylinder arrays using a home-built white-light spectroscopy setup. We inserted an iris aperture in the illumination path to limit the angles of incidence to the range from −3° to 3°, corresponding to an NA of approximately 0.05. Figure 2(a) shows an exemplary measured spectrum for nanocylinders with a diameter of 286 nm. To compare our experimental results with theory, we numerically calculated the normal-incidence transmittance spectra of the system using the commercial 1360
DOI: 10.1021/acsphotonics.7b01375 ACS Photonics 2018, 5, 1359−1364
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imaging36 of the emission from the nanoantenna array with a nanocylinder diameter of 286 nm, using a 0.85 NA objective (see Methods for details on the BFP measurements). A bandpass filter with a center wavelength of 850 nm and a passband width of 10 nm is used to selectively probe the emission into the magnetic dipolar Mie-type resonance of the nanocylinder arrays. The measured BFP image is depicted in Figure 4(a). The observed emission pattern shows 4-fold
Figure 3. Experimentally measured emission spectra (solid lines) of our samples for a systematic variation of the nanocylinder diameter. Spectra for different diameters are vertically displaced by 0.7 × 104 counts for better visibility. The emission spectrum of the bare glass substrate is shown in gray shading as a reference. The dashed lines show the linear-optical transmittance spectra of the corresponding nanocylinder arrays.
Figure 4. (a) Experimental back focal plane image of the emission from the sample with a nanocylinder diameter of 286 nm passed through a 850 nm bandpass filter (k0 = 2π/λ). (b) Corresponding experimental (blue solid line) and calculated (red dashed line) upper half-space emission pattern cross-section for ky = 0. (c) Calculated back focal plane image. (d) Solutions of eqs 1 (blue) and 2 (green). The red circle has a radius of (2π/λ)NA, where λ = 850 nm and NA = 0.85.
the silicon nanoantennas and shows a systematic dependence on the nanocylinder diameter. In particular, the emission signal of the coupled system is enhanced near the spectral position of the dipolar Mie-type resonances of the silicon nanocylinders as observed in the linear-optical transmittance spectra, which we have included in Figure 3 as dashed lines for direct comparison. These observations can be explained by an increase of the radiative decay rate through the Purcell effect,18 caused by an increased number of photonic states available for radiative decay at resonance. This, in turn, leads to an increase of the quantum efficiency2 of the coupled system with respect to the bare substrate. Since the local modification of photonic density of states by the nanoresonators can be linked to the local enhancement of the optical near-fields,19 the significantly enhanced emission around the resonance wavelength will mainly originate from a small fraction of emitters located inside the substrate volume where the electromagnetic near-fields are resonantly enhanced. The emitters in the bulk of the glass substrate, on the other hand, should be almost unaffected by the presence of the nanocylinder array. Away from the spectral positions of the resonances the emission spectra from the nanocylinder sample coincide with the reference spectra. This indicates that no significant additional fluorescence is introduced by the nanocylinders themselves. Also note that the exact enhancement values will depend on an interplay of various factors including resonance properties, wavelengthdependent losses in the silicon, and emitter quantum efficiency. Next, to study the directionality of the resonantly enhanced substrate fluorescence, we performed back focal plane (BFP)
symmetry in accordance with the lattice geometry. Most prominently, a bright spot is observed at the center of the BFP image, showing that the majority of the photons emitted within the probed solid angle as defined by the NA of the collection objective are emitted under small polar angles θ. This is further illustrated in Figure 4(b), showing the corresponding upper half-space emission pattern in a cross section for ky = 0. The emission pattern features a narrow lobe directed perpendicularly to the substrate plane. Such a spatial emission characteristic is ideally suited to enhance the collection efficiency of the emitted light. In our example, the reshaping of the emission pattern can enhance the fraction of photons emitted in the upper half-space that can be collected with an objective with a moderate NA of 0.4 from 22% for an isotropic emission within the considered solid angle to 50%. To compare these experimental findings with theory, we performed numerical simulations of the emission patterns. The chosen approach is based on the reciprocity principle,37,38 allowing us to take into account the periodicity of the structure, the substrate, and the spatial distribution of the emitters. We assumed that the emission of the glass substrate can be represented by point-like electric dipole emitters, which are randomly oriented and homogeneously distributed within the glass substrate. The reciprocity principle states37 that P(θ, ϕ) ∝ ⟨|E(θ, ϕ)|2/E02⟩, where P(θ, ϕ) represents the emitted intensity in the direction defined by the polar angle θ and azimuthal angle ϕ. E0 is the electric field amplitude of the plane wave 1361
DOI: 10.1021/acsphotonics.7b01375 ACS Photonics 2018, 5, 1359−1364
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ing to an improvement of 66% as compared to the situation without nanocylinders. For decreasing NA, while the absolute number of collected photons is further reduced, the improvement with respect to the reference case is further enhanced. The well-known effect of asymmetric emission caused by the substrate could be eliminated and symmetric emission could be restored by embedding the nanocylinders into a homogeneous environment, usually by deposition of a low-refractive-index layer.4,29 To shed light onto the origin of the observed directional emission pattern, we investigated the influence of the periodicity in nanocylinder arrangements. Apart from Mie resonances of the nanocylinders, the system exhibits lattice modes associated with Wood’s anomaly and described by the following set of equations:
incident onto the structure along the same direction, and E(θ, ϕ) is the near-field induced by the incident plane wave. ⟨·⟩ denotes averaging over the sample volume containing the emitters. We consider the absorption (A) as a measure of ⟨|E(θ, ϕ)|2/E02⟩. Consequently, the directionality of emission from the structure is linked to its absorption of a plane wave as a function of the angles of incidence. To calculate the angular-resolved absorption by the periodic structure, we used the Fourier modal method39 and the same structure parameters as for the calculations shown in Figure 2. In our calculations, the plane wave is incident onto the structure from the upper half-space. The proposed method is effective for including all diffraction orders appearing even at high inclination angles. We used a decomposition into 25 × 25 × 2 modes. The wavelength of the incident plane wave was set to 850 nm, ϕ was varied from 0° to 90° making use of the 4fold symmetry, and θ was varied from 0° to 58° corresponding to the NA of the collection objective used in experiments. Both a-Si:H and the material of the substrate exhibit negligible losses at λ = 850 nm. Thus, in order to make use of the reciprocity principle, we added a small loss factor of 0.001i to the refractive index of the glass substrate within the near-field region of the nanocylinders. Thereby we simulate the angular-resolved emission from point defects and impurities located within this layer. Restricting the thickness of the absorbing layer is necessary in order to render the simulation result independent from the size of the computational domain, since the substrate occupies the entire half-space of the domain. The thickness of the absorbing layer is chosen as 50 nm to accommodate the substrate volume where the optical near-fields are enhanced by the MD resonance. At this distance from the interface, the nearfield intensity of the MD resonance in the substrate has decayed to approximately 1/e as compared to its maximum value at the interface (see Supporting Information for details). Thereby, we can simulate the effect of local emission enhancement without the need for considering local dipole sources in our calculations. Our approach also avoids difficulties arising from periodic boundary conditions in simulations modeling the local emitters as point dipole sources. To emulate the experimental BFP image depicted in Figure 4(a), we integrated the absorption for TE and TM polarization of the incident plane wave, yielding the total angle-dependent absorption A(θ, ϕ). We then switched to the coordinates of reciprocal space related to the angles of incidence by kx = (2π/ λ) sin θ cos ϕ and ky = (2π/λ) sin θ sin ϕ and applied the apodization factor36 to the angular-resolved absorption resulting in A(kx, ky) = A(θ, ϕ) cos−1 θ. The calculated BFP image is shown in Figure 4(c). Figure 4(b) also shows the corresponding simulated emission pattern for ky = 0. The experimental and numerically simulated directional emission characteristics show an excellent overall agreement. Most prominently, the preferential emission out of the substrate plane is reproduced well. Differences are observed in the fine features of the emission pattern and can be explained by imperfections of the fabricated sample and the optical measurement system. Note that while our analysis concentrates on emission into the upper half-space, the higher refractive index of the substrate will favor emission into the lower halfspace (see Supporting Information for additional numerical calculations for emission into the substrate half-space). Thus, taking into account the full solid angle of emission, the fraction of emitted photons collected by an NA = 0.85 objective from the air half-space is reduced to approximately 10%, correspond-
2π = λ
n
2π = λ
2 2 ⎛ 2π ⎛ 2π 2π ⎞ 2π ⎞ ⎜ sin θ cos ϕ + p ⎟ + ⎜ sin θ sin ϕ + j ⎟ ⎝ λ ⎝ λ a ⎠ a ⎠ (1) 2 2 ⎛ 2π ⎛ 2π 2π ⎞ 2π ⎞ ⎜ sin θ cos ϕ + l ⎟ + ⎜ sin θ sin ϕ + m ⎟ ⎝ λ ⎝ λ a ⎠ a ⎠ (2)
where p, j, l, and m are integers defining the diffraction orders and n = 1.51 is the refractive index of the substrate. If the wavelength and the inclination angles of the incident plane wave satisfy eq 1 or 2, the light scattered by the nanocylinders will interfere constructively along the array plane. The energy of the incident wave will be trapped in the array,40 which results in the sharp resonant signatures in Figure 4(a and c). The solutions of eqs 1 (blue) and 2 (green) for λ = 850 nm are plotted in Figure 4(d). The coordinate system is the same as in Figure 4(a). The radius of the red circle is given by (2π/ λ)NA, where λ = 850 nm and NA = 0.85 is the NA of the objective used in the BFP experiments. Comparing Figure 4(a), (c), and (d) one can notice the strong influence of the periodic arrangement on the emission pattern. Considering only the air half-space, we observe channeling of the emission into a single lobe when the wavelength of the magnetic dipolar Mie-type resonance λ and thus the spectral maximum of the emission coincide approximately with the optical path length na inside the substrate between two lattice sites. This condition corresponds to the situation in which the green circles in Figure 4(d) are almost touching at the center. We can conclude that while the emission is enhanced by the magnetic dipole mode of the silicon nanocylinders, its directional pattern originates from the coherent scattering in the periodic arrangement. As such, the mechanism is similar to that employed in phased arrays28 and Yagi-Uda nanoantennas.3 We can consider the fluorescence centers of the glass substrate as incoherent emitters. At the operating wavelength of 850 nm, the emission of a single emitter located close to a nanocylinder weakly couples to its magnetic dipole mode. Next, the oscillating magnetic dipole moment of the nanocylinder excites neighboring nanocyliners with a retarded phase. The interference of the emission scattered by different nanocylinders results in a narrow lobe directed in the forward direction.
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CONCLUSION In summary, we have experimentally demonstrated spectral and directional control of spontaneous emission by all-dielectric 1362
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nanoantenna arrays using the intrinsic fluorescence from the glass substrate as a weak and broadband emitter. This is achieved, for the first time to our knowledge, by harnessing the resonant emission enhancement by magnetic dipolar Mie-type resonances of high-refractive-index dielectric nanoparticles in combination with coherent scattering of the emitted light in the tailored periodic arrangement. Our results demonstrate that Mie-resonant silicon nanoantenna arrays offer useful opportunities for enhancing the collection efficiency in fluorescence microscopy and the creation of flat sources of tailored light fields.
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Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b01375. Numerical investigation of the near-field decay length at the MD resonance; simulations of the emission in the full 4π solid angle (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
METHODS
ORCID
Sample Fabrication. For fabrication of silicon disks on the glass substrate, we use the same procedure as described in detail in Arslan et al.35 We first deposit thin films of hydrogenated amorphous silicon (a-Si:H) with a thickness of 182 nm, using plasma-enhanced chemical vapor deposition at a temperature of 250 °C on standard microscope coverslips. Next, the substrates are spin-coated with the negative-tone electron-beam resist maN-2403. The nanocylinders are then defined by electronbeam lithography in combination with inductively coupled plasma etching of the silicon thin film, where the exposed electron-beam resist is used as an etch mask. As etch gases, we used SF6 (1.8 sccm) and CHF3 (50 sccm). Etching was performed at 20 °C with 10 mTorr at 500 W induction power and 15 W bias power. Finally, residual resist and organic solvent residue left on the sample were removed using an oxygen plasma. To render the sample conductive for imaging with an electron microscope, we cover it with a thin (15 nm) transparent layer of indium tin oxide (ITO) using sputter coating at 1.5 mTorr pressure, 20 sccm argon flow, 60 W power, and 8 × 10−7 Torr base pressure. Transmittance Simulations. An elementary unit cell with Floquet periodic boundary conditions and two ports, one at the top and one at the bottom, was considered. The top port acted as a source exciting a normally incident plane wave. The reflected, transmitted, and diffracted light was detected by both ports. The glass substrate was modeled with a constant refractive index of n = 1.51. For the optical material parameters of the a-Si:H, we used experimental data obtained from ellipsometry measurements on unstructured a-Si:H films. Back Focal Plane Imaging. A continuous wave laser emitting at 532 nm wavelength was used for the excitation. The laser was focused onto the sample by a 0.85 NA (Nikon LWD IMSI 100×) objective. An additional lens was placed in the excitation-beam path to increase the size of the laser spot at the sample, leading to a diameter of the laser spot of approximately 24 μm in diameter. The laser power was 0.8 mW, resulting in an excitation power flux of 1.8 × 102 W/cm2. The same objective was collecting the emitted light in reflection, which then propagated through a dedicated lens system to an electron multiplying CCD camera (Andor iXon Ultra 897). The residual laser light was filtered out by a long-pass dichroic mirror (Thorlabs DMLP650R). The lens system was adjusted to form the back focal plane image at the sensor of the camera. A bandpass filter (Thorlabs FB850-10) with center wavelength of 850 nm and a passband width of 10 nm was used to selectively probe the emission in the spectral range of interest.
Aleksandr Vaskin: 0000-0002-3014-1002 Dragomir N. Neshev: 0000-0002-4508-8646 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support by the Thuringian State Government within its ProExcellence initiative (ACP2020), the Australian Research Council, and the German Research Foundation (STA 1426/21) is gratefully acknowledged. The authors also acknowledge their participation in the Erasmus Mundus NANOPHI project, contract number 2013 5659/002-001. Y.S.K. acknowledges a support from the Humboldt Foundation. This research is supported by an Australian Government Research Training Program (RTP) Scholarship. This work was performed in part at the ACT node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and microfabrication facilities for Australia’s researchers.
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DOI: 10.1021/acsphotonics.7b01375 ACS Photonics 2018, 5, 1359−1364
