Multiple Fano Resonances in Symmetry-Breaking Silicon Metasurface

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Article Cite This: ACS Photonics 2018, 5, 4074−4080

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Multiple Fano Resonances in Symmetry-Breaking Silicon Metasurface for Manipulating Light Emission Chengcong Cui,† Chaobiao Zhou,† Shuai Yuan, Xingzhi Qiu, Liangqiu Zhu, Yuxi Wang, Yi Li, Jinwen Song, Qingzhong Huang, Yi Wang, Cheng Zeng,* and Jinsong Xia* Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

ACS Photonics 2018.5:4074-4080. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/04/18. For personal use only.

S Supporting Information *

ABSTRACT: A resonant metasurface with high quality factor can not only localize light at the nanoscale but also manipulate the far-field radiation. In this work, we experimentally demonstrate an active Fano-resonant metasurface that combines an asymmetric silicon nanorod array with embedded germanium quantum dots. The collective resonance of the nanorods results in strong near-field confinement, and the nanorods also lead to directional emission. This gives rise to 3 orders of magnitude enhancement of the photoluminescence intensity with respect to the unpatterned area. Besides, due to the symmetry-breaking property of the structure, the light emission is of specific polarization. Moreover, by varying the geometric parameters of the nanorods, different resonances are spectrally overlapped, which can be utilized to manipulate the far-field radiation pattern. The metasurface shows enormous potential in manipulating light emission and provides a route for high-directionality, high-efficiency LEDs and potentially functional dielectric metasurface lasers. KEYWORDS: silicon nanorod, germanium quantum dot, Fano resonance, photoluminescence enhancement, radiation manipulation

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of quantum dots (QDs) using silicon nanodisks, respectively.31,32 In their works, the light emission intensity and the radiation property are controlled by the distribution of the radiative Mie resonance in the nanodisk structure. Yuan et al. used silicon nanohole structures for light emission enhancement of Ge QDs.33 Note that, by using sharp Fano-type resonances in dielectric metasurfaces, one can achieve strong field confinement as well as light manipulation,16,33−35 and this may offer advantages in low-threshold dielectric metasurface lasers. In this work, by combining embedded Ge QDs with a symmetry-breaking silicon metasurface, we experimentally demonstrate strong PL enhancement and radiation property manipulation. Multiple Fano resonances are excited through introducing proper symmetry breaking to the otherwise highly symmetric silicon nanorods. The Q factors of the resonances are in the range of a few hundreds and even thousands (reaching up to ∼1946), which is extremely large among metamaterials and metasurfaces.19−25 Up to 3 orders of magnitude enhancement of the PL intensity is achieced, which we attribute to the enhanced spontaneous emission as well as the modified radiation pattern. Moreover, by simply varying the location of the cut air hole, the spectral positions of different resonances can be tuned to overlap, which can result in shaping of the radiation pattern and directional emission.26−30 Our demonstrations may pave the way toward

ecent progress in nanophotonics is often related to the study of resonant metamaterials and metasurfaces that localize light at the nanoscale or manipulate the far-field scattering.1−3 Nanoscale light confinement can lead to applications such as slow-light devices,4,5 optical sensors,6,7 harmonic generations,8,9 and lasers,10−12 and far-field manipulation can be used in flat optics such as subwavelength focusing lens,13 waveplates,14 and holograms.15 Further, one can take advantage of both the aspects mentioned above by combining nanostructures with active materials.16 This means one can realize flat light sources with strong light emission enhancement as well as radiation pattern optimization. In plasmonics, for example, Zheludev et al. developed the spaser comcept by combining coherently resonant plasmonic resonators with a dielectric gain medium.10,11 This so-called “lasing spaser” can act as a planar source of spatially and temporally coherent radiation. On the other hand, high-index dielectric nanoparticles show advantages over their plasmonic countparts in reducing dissipative losses and enhancing both the electric and the magnetic near-field in the optical region.16−18 By choosing low-loss dielectric materials such as Si, Ge, and GaAs and through careful structural design, one can achieve much higher resonant quality factors (Q factors) than in plasmonics.19−25 The coexistence of electric and magnetic resonances with equivalent strengths enables effective mode interactions to realize the Kerker effect,26 which allows manipulating the directionality of the light radiation.27−30 Isabelle Staude et al. and Rutckaia et al. demonstrated the photoluminescence (PL) spectra shaping and emission driving © 2018 American Chemical Society

Received: June 5, 2018 Published: September 19, 2018 4074

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Figure 1. (a) Schematic drawing of the designed structure. The inset shows the details of the nanorod. The radius R and height H of the cylinder are set to be 300 and 290 nm, the radius of the air hole r is 100 nm, and the distance between the air hole and the nanorod centers is 240 nm (Δd = 240 nm). The lattice constant of the metasurface is 820 nm. (b) Top-view and cross-sectional-view SEM images of the fabricated nanorod array. The inset illustrates the details.

Figure 2. (a) Simulated transmission spectra with/without symmetry breaking, respectively. Four modes named M0, M1, E0, and M2 are located at 1365−1600 nm. (b) Experimental transmission spectrum with four high-Q Fano resonances under unpolarized incident light excitation. The inset shows a detailed Fano line shape for resonance M0 at y-polarized incident light, and the extinction ratio is ∼20 dB. (c, d, e, f) Simulated electromagnetic vortex field distributions with vector arrows of modes M0, M1, E0, and M2 in the x−y plane, respectively. (c, d, f) The magnetic fields and (e) the electric field. The arrows represent the direction of the field, and the color scale represents the corresponding normalized field intensity distribution.

near-field of the bright and dark resonances.3 To obtain a high Q factor, the coupling efficient should be particularly appropriate to not only minimize the radiative losses induced by the bright resonance but also overcome the material losses to efficiently excite the dark resonance. As for the array effect, when placing the isolated resonator in two-dimensional array, the radiative loss is further suppressed. Thus, the overall Q factor of the collective resonance becomes much larger than the individual resonator.21,24,25 Herein, we bring in a nanohole at the edge of the rod to introduce appropriate structural symmetry breaking while taking the fabrication tolerance into consideration and then arrange the asymmetric rods into a periodic structure to finally form the metasurface. Figure 1a shows the schematic drawing and the geometry parameters of the metasurface. The designed structure is fabricated on a silicon-on-insulator (SOI) substrate (with Ge QDs pregrown) using molecular beam epitaxy (MBE), electron-beam lithography (EBL), and inductively coupled plasma (ICP) etching techniques (see the Methods for details of Ge QD growth, EBL, and ICP processes). Figure 1b exhibits the top-view and cross-sectional-view SEM images of the fabricated sample.

high-power and high-efficiency LEDs, displays, and perhaps novel dielectric metasurface lasers.

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FANO RESONANCE AND STRUCTURE DESIGN Several approaches for enhancing the Q factors in nanoparticles have been demonstrated in previous works.36 Herein, we consider two aspects to design the high-Q metasurface. The first is by bringing in proper symmetry breaking to a symmetric nanorod to access Fano-type resonances.19,23−25 The second is periodically arranging the nanorod to further suppress the radiative losses, called the array effect.21,23−25 As is known, a Fano resonance in nanophotonic structures arises from the constructive and destructive interference of discrete resonance states with broadband continuum states.37,38 The discrete resonances refer to the “dark” mode, which is subradiated, is spectrally sharp, and cannot be directly addressed, while the broadband continuum state is super-radiant and can be directly excited, called the “bright” mode.4,5,12,23−25,39 One can achieve Fano resonances in nanophotonic structures with a varity of methods, for example, by bringing in structural asymmetry4,5,7,8,19,20,22−25 or by obliquing the incident plane wave.40,41 Overall, these methods make the eigenmodes in the structures nonorthogonal and provide a coupling channel between the 4075

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Figure 3. (a) Comparison of PL intensities between the patterned and the unpatterned area and the dark counts of our spectrometer. The PL spectrum of the Ge QDs in the unpatterned area is inhomogeneously broadened due to the variation of the QD sizes. (b) Enhancement factors for the four peaks. (c) Simulated angular scattering intensity for M2 in the y−z and x−z planes, respectively.

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Fano fitting for E0 is exhibited in Figure S3. The experimentally obtained Q factors are lower than those of the FDTD simulations and can be attributed to fabrication discrepancies. It is noted that the Q factors, for modes E0 and M2, are much higher than early studies such as asymmetric double bars,22 asymmetry Dolmen-type,23 ring bars,24 and symmetry-breaking cubes,25 since those structures couple stronger to free space, leading to larger radiative losses. In order to simultaneously excite the four resonances, which are of different polarization properties, unpolarized incident light is used. Under specific polarized incidence, the extinction ratio is much higher and the Fano line shape is more obvious, as seen in the inset of Figure 2b and Figure S4 in the Supporting Information. A slight mismatch in the resonant wavelengths between the experiment and simulation mainly arises from the imperfections such as the roughness of the nanorods as well as the randomly distributed Ge QDs (Figure S1 in the Supporting Information).

TRANSMISSION SPECTRA AND RESONANCE CHARACTERIZATION The designed metasurface is modeled using a commercial software (Lumerical Solutions) implementation of the finitedifference time-domain (FDTD) method. As shown in Figure 2a, the simulated transmission spectrum exhibits four Fano resonances (named M0, M1, E0, M2) when the symmetry breaking is introduced; otherwise in symmetric structures, no sharp resonance can be found. To gain further insight into the nature of the four Fano resonances, in Figure 2c−f, simulated magnetic and electric field distributions of the modes in the x− y plane are illustrated. Figure 2c,e indicate the out-of-plane electric and magnetic dipole resonances (modes M0 and E0) with circular magnetic and electric fields distributing in the broken rods, respectively. For mode M1, as illustrated in Figure 2d, the circular magnetic field within each unit cell is divided into two parts with antiphase oscillations, which originates from the strong inter-resonator interaction. Mode M2, shown in Figure 2f, is a higher-order resonance, which can be treated as an electric quadrupole similar to the symmetrybreaking Ag nanodisk.42 It is worth noting that due to the specific size and period of the nanorods, the collective effects also have an impact on the field distributions of the resonances. Additionally, as shown in Figure 2c−f and Figure S2 in the Supporting Information, the electromagnetic fields are mainly confined inside the dielectric resonator, thus making it possible to spatially overlap with the embedded Ge QDs. Linear optical transmission measurements are carried out by a home-built system (see the Methods). As illustrated in Figure 2b, the experimental transmission spectrum of the sample agrees well with the simulation. Four Fano resonances with high Q factors are excited within the wavelength range of 1365−1600 nm. The Q factors are estimated to be 170, 193, 1946, and 723 for modes M0, M1, E0, and M2, respectively. The four resonances located in the experimental transmission spectrum are fitted to a Fano line shape given by T = a1 + ja 2 +

b ω − ω0 + jγ

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PL ENHANCEMENT PL measurements are carried out at different temperatures, and the metasurface exhibits enormous PL enhancement at both low temperature and room temperature. In particular, at about 25 K, the light emission from the Ge QDs reaches the highest value and the PL between the unpatterned area and the dark counts is well distinguished (see the Supporting Information for temperature dependence of PL intensity).44,45 The enhanced PL spectrum from the metasurface sample, combined with the spectra from the unpatterned area and the dark counts (both enlarged by 150 times), is illustrated in Figure 3a. Four strong PL peaks are observed in the spectra, agreeing well with the simulated and measured transmission spectra. This proves that the four optical resonances are excited by the spontaneous emission from the embedded Ge QDs and the output light emission is modified by the metasurface. The enhancement factor, which is defined as the ratio of the resonant peak intensity to the PL intensity of the unpatterned area at the resonance wavelength,46 reaches an ultrahigh value of about 1472 for resonance M2 and 222, 125, and 622 for resonances M0, M1, and E0, as shown in Figure 3b, respectively. Such large PL enhancement can mainly be attributed to two reasons. First, the spontaneous emission rate of the photoexcited carriers is enhanced owing to the

,23,24,43 where a1, a2, and b are

constant real numbers; ω0 is the central resonant frequency; and γ is the overall damping rate of the resonance. The Q ω factor is then determined by Q = 2γ0 . The fitting method is

identical to previous works.23,24 Herein, as an example, the 4076

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Purcell effect.47,48 Second, the extraction efficiency and the radiation pattern are modified by the scattering of the nanorods.27−30 We first consider the Purcell effect of our metasurface.47−50 However, due to the complex field distributions of the resonances, the random size and position distributions of the QDs, and the different moment directions, it is difficult to estimate accurate Purcell factors. Here, we roughly estimate the Purcell factors following the treatment of a similar situation in previous work.20 (A general approach for calculating the Purcell factor is presented in some previous works.49,50) By assuming that the emitters and the resonances are spatially and spectrally matched, and ignoring the collective nature of the modes, we can roughly calculate the Purcell factor (F) by F = (3/4π2)(Q/Vm)(λ/n)3.20 Here, Q is the overall quality factor of the Fano mode, Vm is the mode volume, n is the refractive index, and λ is the resonant wavelength. The Purcell factors for modes M0, M1, E0, and M2 are found to be about 101, 31, 330, and 123, respectively. Next, we study the emission properties driven by the metasurface. In the simulation, a dipole source is placed inside the nanorod to function as an internal source. Take M2 for example; the simulated angular scattering intensity in the x−z plane and y−z plane is shown in Figure 3c, which indicates that the upward radiation is dominant and efficiently contributes to the extraction and collection of the spontaneous emission. In other words, the nanorods can serve as nanoantennas for directive light emission.51

the y-direction are in antiphase but have the same amplitude (Figure 2e). This results in the complete cancellation of the electric far-field in the y-direction, and the output light becomes x-polarized.21 As for mode M2, the electromagnetic resonance leads to a weak in-plane polarization characteristic. The measured dependence of PL intensities on polarization angles agrees well with the properties of the modes.

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OVERLAP OF DIFFERENT RESONANCES It is known that the interference of different resonances that are spectrally overlapped can result in shaping of the far-field radiation.26−30 For example, the full interaction of an electric dipole and a magnetic dipole exhibits a directive radiation pattern with zero backscattering, known as the Kerker effect.26 Basically, this kind of phenomenon relies on the constructive/ destructive interference of different multipole moments in the forward/backward direction.28 In our situation, we find that the spectral positions of the resonances are able to shift while varying the location of the air hole. In Figure 5, the dependence of the measured PL peaks and the simulated Fano resonant peaks on the air hole location is illustrated, and the results are well agreed. The distance between the air hole and the nanorod center (Δd) varies from 20 to 300 nm. Within the variation range, overlapping of different Fano resonances is obtained. For instance, electric and magnetic dipole modes are tuned to overlap with each other at a distance Δd of about 70 nm, and electric resonances M0 and M1 are overlapped at a Δd of about 180 nm (red-circled). We suggest that our designs provide a method to control and shape the scattering patterns of the radiation and may finally obtain directional emission enhancement or suppression.26−30

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POLARIZATION PROPERTIES It is noticed that the asymmetric shape of the nanorods leads to a specific distribution of the resonant modes and finally controls the far-field polarization properties of the light emission. We set a linear polarizer into the collection side of the PL measurement path to measure the polarizationdependent PL intensities of the four resonant peaks. Figure 4 and Figure S7 illustrate the PL intensities at different

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CONCLUSION In summary, we experimentally demonstrate spontaneous emission enhancement and radiation manipulation in a highQ silicon metasurface combined with Ge QDs. The designed metasurface consists of an array of nanorods that are under appropriate symmetry breaking. This structural asymmetry offers a route to the near-field interaction of the collective bright and dark mode, leading to the appearance of four highQ Fano resonances in the spectral range of 1365−1600 nm. A large PL enhancement up to 1472 times is achieved and can be attributed to two resonances: first, the Q factors of the corresponding resonances are high and the field distributions are mainly inside the nanorods, which offers a large resonance enhancement as well as fine spatial overlap to the Ge QDs. Second, the nanorods can serve as nanoantennas that modify the radiation pattern; thus the extraction and the collection efficiency are improved. The far-field polarization properties of the light emission agree well with the characteristics of the resonant modes. Furthermore, the spectral location of the resonances can be flexibly tuned and overlapped by simply modifying the location of the air hole, which may lead to directional scattering and high efficiency radiation. Our designs may pave the way toward scalable fabrication of silicon-based on-chip light sources, high-directionality, high-efficiency LEDs, displays, and potentially functional metasurface lasers such as specific polarization pattern and optical vortex generation.

Figure 4. Dependence of the PL intensities of the four resonant peaks on the polarizer angles, indicating that the Fano resonant modes are of specific polarization properties.

polarizer angles. For mode M0, the major part of the electric field oscillation is along the z-axis direction, thus leading to weak polarization dependence in the x−y plane. Differently, modes M1 and E0 are strongly polarized in the x−y plane, since the electric fields of these two modes mainly oscillate in this plane, along the y- and x-directions, as the vector arrows show in Figure 2d,e and Figure S2b,c in the Supporting Information, respectively. Take mode E0 for example; the field distribution exhibits that the two nearby electric oscillations in

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METHODS MBE Growth. Four layers of self-assembled Ge QDs are grown under the Stranski−Krastanov (S−K) growth mode

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Figure 5. (a) SEM images of the nanorods with cut air holes at different locations (Δd). Here we give four nanorods with Δd values of about 20, 100, 180, and 260 nm, respectively. (b, c) Experimental PL peaks and simulated transmission spectra at different Δd. Different resonances are tuned to overlap as circled.

intensity (from 4.5 μW to 10.5 mW) using a neutral density filter. The laser beam is focused to a ∼100 μm diameter spot on the sample by a microscope objective (Olympus, NA = 0.1). The upward μ-PL signals are collected by the same objective, dispersed by a monochromator with a focus length of 500 mm, and finally recorded by a liquid-nitrogen-cooled InGaAs linear detector array. The PL measurements are performed with a resolution of 0.32 nm. Also, a linear polarizer is set into the collection side of the setup for polarization characteristic measurements.

using the MBE technique. In detail, an SOI wafer with a 220 nm top Si and a 2 μm burried oxide layer is prepared using the standard RCA cleaning technique and H+ passivation process before loading into a solid source MBE chamber (Omicron EVO-50). After a thermal degassing at 900 °C, an 80 nm buffer layer of Si and four layers of self-assembled Ge QDs are grown. Each of the four layers are formed by a 7.5 monolayer Ge deposition and separated by a 15 nm thick Si spacer. The sample is then capped with a 100 nm thick Si layer in order to prevent the Ge dots from being exposed to the air. EBL and ICP. The grown sample is spin coated with a 340 nm ZEP-520A resist and prebaked. Then the symmetry-broken nanorod array patterns are defined into the resist by EBL (Vistec EBPG 5000 Plus). After being carefully developed and fixed, the patterns are etched down to the Ge QD layers by an ICP (Oxford Plasmalab System 100 ICP180) dry etching process using a SF6/C4F8 mixture. The remaining resist is ultrasonically removed by dipping into the N-methyl-2pyrrolidone (NMP). Numerical Simulations. The simulations of the transmission spectra and the electromagnetic field distributions are carried out based on the FDTD method. In the simulation setup, the periodical boundary conditions are set in the x- and y-directions, and perfectly matched layers are set in the zdirection. The direction of the light source propagates toward the positive z-direction. Transmission Spectroscopy Measurements. The linear optical transmission spectra of the fabricated metasurface sample are measured by a home-built setup, where the light from a picosecond pulsing laser (YSL Photonics SC-5-FC, 480−2200 nm) is incident onto the fabricated pattern with a fiber collimator. The size of the fabricated nanorod array (410 μm × 410 μm) is large enough to ensure that the whole light beam is passing through the patterned area. The light transmitted through was then collected on the backside of the sample using another fiber collimator and directed to a spectrometer (YOKOGAWA AQ-6370). Measurements are referenced to the transmission of the unpatterned area next to the silicon nanorod array. PL Measurements. A micro-photoluminescence (μ-PL) measurement system associated with a temperature-controlled liquid-helium cryostat is introduced to characterize the temperature- and polarization-dependent performances of the fabricated sample. After being set into the cryostat chamber and cooled to the target temperature, the sample is optically pumped by a 532 nm diode laser with variable pumping

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b00754.

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Details for Ge QD distribution inside the nanorods; detailed electromagnetic field patterns for the four Fano modes in the x−y and x−z planes; Fano fitting for mode E0; transmission spectrum with y-polarized incident light; temperature-, power-, and polarization-dependent properties of PL (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jinsong Xia: 0000-0002-9650-7839 Author Contributions †

C. Cui and C. Zhou contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Grant Nos. 61335002 and 11574102 and the National High Technology Research and Development Program of China under Grant No. 2015AA016904. We thank the Center of Micro-Fabrication and Characterization (CMFC) of WNLO and the Center for Nanoscale Characterization & Devices (CNCD), WNLO of HUST for the facility support. 4078

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