Strong Photoluminescence Enhancement in All-Dielectric Fano Metasurface with High Quality Factor Shuai Yuan,‡ Xingzhi Qiu,‡ Chengcong Cui, Liangqiu Zhu, Yuxi Wang, Yi Li, Jinwen Song, Qingzhong Huang, and Jinsong Xia* Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China S Supporting Information *
ABSTRACT: All-dielectric metamaterials offer great flexibility for controlling light−matter interaction, owing to their strong electric and magnetic resonances with negligible loss at wavelengths above the material bandgap. Here, we propose an all-dielectric asymmetric metasurface structure exhibiting high quality factor and prominent Fano line shape. Over three-orders photoluminescence enhancement is demonstrated in the fabricated all-dielectric metasurface with record-high quality factor of 1011. We find this strong emission enhancement is attributed to the coherent Fano resonances, which originate from the destructive interferences of antisymmetric displacement currents in the asymmetric all-dielectric metasurface. Our observations show a promising approach to realize light emitters based on all-dielectric metasurfaces. KEYWORDS: all-dielectric metasurface, Fano resonances, high quality factor, symmetry breaking, Ge quantum dots
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promising candidate for the light emitter based on metamaterials. However, the required high pump power and the generation of excessive heat in the nanoplasmonic metamaterials still remain to be solved. Recently, the manipulation of optically induced Mie resonances in low-loss all-dielectric nanostructures has become one of the fastest growing branches of nanophotonics.19−22 The coexistence of strong electric and magnetic resonances in dielectric nanostructures brings diverse applications, including field enhancement23−25 and directional scattering26−29 in nanoantennas, optical sensing30−33 based on high-Q Fano resonances, dynamically tunable or reconfigurable photonic metamaterials,34,35 enhanced nonlinear optical effects,36−38 and wavefront shaping in flat optics.39−44 Importantly, all-dielectric
nhancing light−matter interactions in metallic nanoparticles and nanostructures have been intensively studied in regimes of lasing,1−3 nonlinear optical processes,4,5 and enhanced light emission6−8 in the past decades, due to the deep-subwavelength localization of electromagnetic fields. However, the optical response in these structures is inevitably accompanied by strong dissipative metal losses, which compromise the performance of the device and impede further applications. To overcome the problem of dissipative losses, enormous efforts have been devoted to compensate loss using gain material, such as semiconductor quantum dots (QDs),9−11 organic dyes,12 InGaAs quantum well,13 and so on.14−16 Significant progress has been achieved in understanding the complex interaction between plasmons and gain medium in active nanoplasmonic metamaterial devices.17 Specially, the “lasing spaser”18 consisting of metallic split-ring nanoresonator array and gain medium exhibits spatially and temporally coherent radiation in theory, which makes it a © 2017 American Chemical Society
Received: July 9, 2017 Accepted: October 12, 2017 Published: October 12, 2017 10704
DOI: 10.1021/acsnano.7b04810 ACS Nano 2017, 11, 10704−10711
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Figure 1. Configuration of the Fano metasurface. (a) Schematic of the Fano metasurface. The structure consists of a SOI slab with a periodic lattice of asymmetric holes on the top. Four layers of self-assembled Ge QDs are embedded in the top silicon. The thickness of buried silicon oxide (gray area in the structure) is 2 μm, and the total thickness of the top silicon is 238 nm. A magnified top view is shown in inset. The geometrical parameters are r = 120 nm, ra = 100 nm, rb = 120 nm, p = 560 nm. The depth of the air hole is 238 nm. (b, c) Schematic of the destructive interferences of antiphase dipoles in the metasurface. After cancellation (inside the dashed line frame), only small residual dipole components remain and radiate energy into the free space. (d) Scanning electron microscope image (SEM) of a fabricated sample. Inset: Enlarged image of four unit cells. The average diameter and height of random distributed Ge QDs are 60 and 8 nm, respectively. The overall dot density is around 5.2 × 109 cm−2. (e) A cross section of the sample. The top silicon is almost etched through.
nanoparticles and metamaterials have been applied to modify the photoluminescence (PL) of the coupled nanoemitters.45−51 By placing the nanoemitter in the nanogap of dielectric nanoantennas, strong fluorescence enhancement has been achieved.52,53 Despite that dielectric nanoresonators have much lower absorption losses than their plasmonic counterparts at near-infrared frequencies, the spectral resonances are generally broad in all-dielectric metasurface due to strong radiation loss coupling to the external field. To overcome this drawback, Fano resonances based on the interference between “bright” and “dark” modes or “trapped modes” in the symmetry-broken structure were developed to achieve a high quality factor. This strategy has been proven efficient for plasmonic metasurface54−56 and dielectric resonator-based metasurfaces,31,57−59 where the highest quality factor around 600 at near-infrared frequency has been experimentally demonstrated. Here we propose a high-quality-factor metasurface consisting of asymmetric air-hole array in a silicon slab. In the structure, Fano resonances stem from the destructive interference of antisymmetric displacement currents in asymmetric nanoresonators array. The high quality factor of the metasurface relies on both the coherent collective oscillation in all unit cells and the deliberate asymmetry in each unit cell. A vast majority of high-quality-factor metasurface in the previous reports are composed of bulging-out nanostructures, such as nanodisks,57 nanobars,33,60 nanocrosses,58 corner-missing cubes,31 and oligomers.32 However, here we choose sunken asymmetric air holes in a silicon slab to construct the metasurface structure. This construction is equivalent to the irregular nanorod array (indicated by the black dashed area in Figure 1a) with very large duty factor. We emphasize that the manipulation of light in our metasurface structure is different from photonic crystals, where the collective response of the system is dominated by the local optical properties of diffractive modes. But in the metasurface structure, the propagation direction of light is normal to the metasurface plane, and the first-order diffraction
of light occurs when the wavelength of incident light in air equals to the lattice period. In our case, the wavelength of incident light is much larger than the period, and there is no light diffraction in the range of 1100−1600 nm. So the Mie resonant requirements are relaxed19,44 in the structure, and the optical response in the metasurface is dominated by the nearfield coupling between subwavelength building blocks. Simultaneously, the air-hole design can help to improve the uniformity of the nanoresonators during fabrication process and contribute to a better device performance. In this paper, we begin by presenting the design principle and numerical simulation to reveal the origin of high-Q Fano resonances. Next, we exploit the transmission and PL characteristics of the all-dielectric metasurface with embedded Ge quantum dots (Ge QDs). We observe over 1000-fold emission enhancement of the Ge QDs embedded in the alldielectric metasurface, accompanied by a narrow Fano line shape with record-high quality factor of 1011. Furthermore, the dependence between array dimension and quality factor is also explored to verify the coherent collective oscillation of the electric and magnetic dipoles. Finally, the emission polarization characteristics of Ge QDs embedded in this asymmetric metasurface are studied by the polarization resolved PL measurement. Our observations take further steps toward active metamaterial devices in all-dielectric nanophotonics.
RESULTS AND DISCUSSION Design of High-Q Fano Metasurface. A schematic of the proposed Fano metasurface is displayed in Figure 1a. The structure consists of a periodic lattice of asymmetric air holes in a SOI slab embedded with four layers of self-assembled Ge QDs. The asymmetric air hole is composed of a semicircle and a semiellipse, and the hole is etched through the top layer of the structure. A magnified top view of the studied geometry is present in inset of Figure 1a. Compared with other symmetrybroken structures reported previously,31,33,56,60 this design has a smaller variation of refractive index in a unit cell due to the 10705
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Figure 2. (a) Simulated and experimental relative transmission spectra of the metasurface using FDTD method. Two broad resonances and four sharp resonances are excited under normal incidence of unpolarized plane wave. The extinction ratios of the sharp resonances are small because they are polarization sensitive. The electric resonances ME0 at 1513 nm and ME1 at 1414 nm are excited by the y-polarized electricfield component of the incident light, while magnetic resonances MM0 at 1224 nm and MM1 at 1297 nm are excited by the y-polarized magnetic-field component of the incident light. (b−d) Electric mode profiles and field vector distributions of electric modes ME0, ME1, and ME2 under y-polarized light incidence. Dashed curves in the figure show the location of air holes. The size of white arrow indicates magnitude of field vectors. (e−g) Magnetic mode profiles and field vector distributions of magnetic modes MM0, MM1, and MM2 under x-polarized light incidence.
slowly changing boundary, which can contribute to large fabrication tolerance (see Supporting Information). When Ge QDs embedded in the metasurface are pumped by the external light source, their light emission excites collective oscillations of displacement currents in all unit cells of the metasurface, which we treat as electric and magnetic dipoles by their different orientations. Due to the weak asymmetry of air holes, the antiphased dipoles in each unit cell interfere destructively, and the residual components form the discrete “trapped” resonance modes in the metasurface. The interferences of trapped modes and the inherent Fabry−Perot (FP) resonances in SOI slab finally give rise to Fano resonances. Since each unit cell is identical, the near-field coupling of individual oscillation reinforces the resonance behavior and leads to high-qualityfactor Fano resonances, which can enhance the PL of Ge QDs dramatically. The detailed origin of high-Q trapped mode is schematically shown in Figure 1b,c. When the emission of Ge QDs couples to the asymmetric nanoresonators in the metasurface, a pair of antiphased dipoles are excited in each unit cell, and the moments of the two adjacent dipoles are different due to structural asymmetry, as shown in Figure 1b. Most dipole radiation components are canceled via destructive interference in such displacement current configuration, so the scattered electromagnetic fields generated by residual dipole components are very weak, as shown in Figure 1c. Therefore, the radiation loss of the metasurface structure is dramatically reduced, and the quality factor increases rapidly. Moreover, under normal incidence of unpolarized plane wave, the phase of the scattering wave changes sharply by π around the resonance point. Thus, the interaction between scattering wave and FP resonance will result in constructive and destructive interference phenomena located very close to each other, corresponding to a maximum Tmax and a minimum Tmin of the transmission spectrum, forming the asymmetric transmission shape of the Fano resonance.61,62
In our metasurface structure, the lattice period and thickness of the top layer are fixed, and the residual dipole components in metasurface critically depend on the degree of structure asymmetry. In theory, an infinite quality factor can be achieved in an infinite-size perfect metasurface with such configuration when the asymmetry of air hole is infinitesimal. However, this high-Q Fano resonance will vanish when the air hole in the metasurface becomes symmetric, for example, a perfect circle (see Supporting Information for details). This is understandable that complete destructive interferences of dipoles occur in such a structure, thus no residual dipole components interact with FP resonances to form the Fano resonances. This also demonstrates the connection between the deliberate asymmetry of air-hole and high-Q Fano resonances. Furthermore, the influence of lattice period of the metasurface on the quality factor is also given in the Supporting Information. Note that the high quality factor is also affected by the group effect of the dipole oscillations, and we will discuss the changes of spectral line width with an increasing number of unit cells later. Numerical Simulation. To explore the properties of the resonance modes, we employ the finite difference time domain (FDTD) method to calculate the relative transmission spectrum of the proposed structure under normal incidence of unpolarized plane wave (see Methods section for details). As shown in upper inset of Figure 2a, there are six different modes in the wavelength ranging from 1100 to 1600 nm, and we note them as ME and MM respectively for their different field distributions. Four sharp resonant peaks (Q ∼ 100,000), marked as ME0, ME1, MM0, and MM1 correspond to modes originating from the small relative refractive index difference induced by the weak asymmetry of the hole shape, while the two broad resonant peaks (Q ∼ 100) come from the larger relative refractive index difference induced by the existence of holes in a unit cell. The different origin of each mode can be deduced form their field vector distributions. In addition, the 10706
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Figure 3. PL of the fabricated sample. (a) Experimentally measured relative transmission spectrum (dark cyan) and PL spectra of the metasurface at 293 K (blue) and 5 K (magenta). MM0 in the PL spectra is too weak to see. PL spectrum of bare Ge QDs in pattern-free region at 5 K is indicated by the light gray area and magnified by 700 times. (b) Fano fitting of the peak MM1 (solid line) at 1294 nm. The fwhm of the peak is fitted to be 1.28 nm using Breit−Wigner−Fano fitting. (c) The magnified graph of the PL spectrum at 293 K and corresponding relative transmission spectrum. (d) PL spectra of arrays with different sizes. We present only the resonance peak at 1507 nm for simplicity.
hole and excite the high-Q electric or magnetic modes, respectively. This suggests that for high-Q modes, only half components of incident light can be captured and resonant with the metasurface under normal incidence of unpolarized plane wave, so the extinction ratios of these modes are much lower. Instead, the low-Q modes can be excited under arbitrary polarization of incident light. Due to the asymmetric shape of the hole shown in Figure 1a, the effective refractive index of the metasurface varies in different directions, and the resonance wavelength will change slightly with the incident beam polarization state. It is expected that emission enhancement can be achieved by taking advantage of these high-Q oscillations of dipoles coupled with quantum emitters (see Supporting Information for details). Experimental Results. The experimentally measured relative transmission spectrum plotted in Figure 2a is acquired by illuminating the sample with normal incident unpolarized light. The shape of the measured spectrum agrees well with the simulation, although the quality factor is reduced. This may be caused by the following factors: First, the imperfections within the fabricated sample can introduce additional scattering loss and break coherence among the unit cells, and thus the quality factor decreases. This is the main reason. Second, the test area in our experiment is finite-sized because of the limited size of pumping spot. The array size plays an important role in determining the quality factor of the metasurface structure, which we will discuss later. Note that the measured extinction ratios of high-Q modes are much lower than the low-Q ones. Because only half of the components of incident light can be captured and resonant with the metasurface under normal
low-Q modes have a larger extinction ratio than the high-Q ones, and this is related to their polarization characteristics. The mode profiles and corresponding field vector distributions of the resonant peaks are shown in Figure 2b−g. In Figure 2b, there are circular displacement currents with the center point of the circle located around the center of the unit cell (the hole is not symmetric). We can assume that there are two adjacent electric dipoles at the two sides of the center point, and the opposite directions of their oscillation give rise to the clear circular displacement current configuration. Notice that the local effective refractive index of left side and right side in a unit cell is slightly different because of the asymmetric shape of air hole, and the strength of the two antiphased electric dipole oscillations is slightly different. Thus, only very weak dipole radiation components remain after the oscillation cancellation.18,33 At high-order mode such as ME1 displacement currents form four small semirings at the edge of unit cell, as shown in Figure 2c. Nevertheless, the displacement current configuration is a bit different at low-Q mode, as shown in Figure 2d, and the adjacent electric dipoles align along the hole region (the edge of unit cell) and no-hole region (the center of unit cell). Because the size of the hole is relatively large, the differences between the two dipole moments caused by the effective refractive index difference are much bigger, leading to relatively large residual radiative components, thus the Q-factor of these modes are much smaller. The same goes for magnetic dipole resonances in Figure 2e−g. Notice that the hole is asymmetric along the y-axis, and only y-polarized electric or magnetic field can “feel” the weak difference of effective refractive index between two sides of the 10707
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Figure 4. (a) Polarization dependence of electric modes ME1 and ME2. (b) Polarization dependence of the magnetic modes MM1, MM2. The high-Q resonant PL peaks ME1 and MM1 are linear polarized, and their polarization directions are orthogonal to each other. The low-Q resonant PL peaks ME2 and MM2 are almost polarization independent.
emission range of Ge QDs overlaps with the cavity modes of the metasurface, and the high quality factor and small mode volume finally contribute to the strong PL enhancement of Ge QDs coupled to the metasurface structure. It is very attractive that the array size of the metasurface can be extended scalably, while the effective mode volume and the number of resonant modes remain constant. This feature is completely different from the traditional size-dependent resonant optical microcavity (microdisk, microring, and photonic crystal cavity), and it may benefit the fabrication of large-area high-power light emitter. In addition, considering that the emission of Ge QDs is collected by an objective with small numerical aperture in a confocal microscope PL system, the measured PL enhancement can also be influenced by possible directional effect. Moreover, since the Ge QDs are randomly distributed on the wafer, the PL enhancement can also be affected by the slight fluctuations in the density of Ge QDs. It is notable that the alignment between the transmission and PL peaks in our metasurface systems is very interesting. In Figure 3c, the resonant PL peaks in our metasurface are aligned with a middle point between the top and the bottom of the Fano-like transmission spectrum. As we mentioned above, the broad FP resonance mode in the slab plays the role of continuum state, and its phase change is negligible during the narrow Fano resonance. For the discrete trapped mode, the phase of the scattering wave changes sharply by π around the resonant wavelength. Thus, the interaction of discrete and continuum will result in constructive and destructive interference, corresponding to a maximum and a minimum (point A and point C) in transmission spectrum. It is believed that the actual resonant wavelength may lie somewhere between the maximum and minimum of the asymmetric profile.61 On the other hand, the discrete trapped mode is formed by the destructive interference between antiphased dipoles, corresponding to a smallest residual dipole component at the actual resonant wavelength. In our experiment, the maximum of PL corresponds to this exact resonant wavelength (point B) because of its minimum radiation loss to free space. This alignment between relative transmission and PL spectra in experiment is consistent with the theory of Fano resonance in nanostructure.61 The collective oscillations of the electric and magnetic dipoles in all unit cells play an important role in the high-Q metasurface, so the array size thus becomes an important factor since strong scattering loss occurs at the array’s edge. We place
incidence of unpolarized light, the extinction ratios of high-Q modes are much lower, and this is consistent with our simulation. Then we study the PL of embedded Ge QDs coupled to the metasurface. Microphotoluminescence (μPL) measurement is performed at 293 K and 5 K, respectively, using a confocal microscope photoluminescence system. A diode pumped solidstate (DPSS) laser of 532 nm is used for pumping as shown in Figure 1a. The PL of the metasurface with embedded Ge QDs is presented in Figure 3a. Several major resonant peaks dominate the spectrum over a weak background, and their positions agree well with the simulated relative transmission spectrum as well as measured one. PL spectrum of bare Ge QDs in pattern-free region at 5 K is shown by the light gray area and magnified by 700 times for better viewing. Ge selfassembled QDs exhibit a broad PL spectrum in 1200−1600 nm due to the intermixing of GeSi and size dispersion.63,64 Compared with the PL spectrum at 293 K, the resonance peaks show an obvious blue shift at 5 K. This is caused by the decrease of refractive index when temperature decreases. Note that the peak located at 1278 nm in the PL spectrum at 5 K is not a resonance mode but the defect-related G-line65 induced during the fabrication. As shown in Figure 3a, the presence of the metasurface leads to a great intensity enhancement as well as spectral narrowing of the Ge QDs PL. A rough estimation of the PL enhancement factor is given by the ratio of PL intensity of the QDs coupled into the metasurface to the PL collected from unstructured regions of the wafer (gray shading in Figure 3a). The intensity of peak MM1 is enhanced by a factor up to 1097, and the fullwidth half-maximum (fwhm) of MM1 is fitted to be 1.28 nm using Breit−Wigner−Fano fitting (see Supporting Information for details). The PL line width of our metasurface is much narrower than the reported results for plasmonic and dielectric metamaterials so far.31,32,54−57 One possible reason for the strong PL enhancement is the cavity QED Purcell effect.66 Indeed, the spontaneous emission decay rate is proportional to the density of photon states that the photonic environment provides. It has been reported that the decay rate of a quantum emitter placed in close proximity to a nanoantenna can be modified due to the modified local density of states.52 The metasurface creates an environment equivalent to a microcavity with a quality factor Q and an effective mode volume V within each unit cell.11 The Purcell factor is proportional to the factor Q/V. In our experiment, the 10708
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METHODS
an assembled lens in front of a pumping laser to change the spot size in order to alter the number of unit cells of the metasurface covered by the illumination spot. The PL spectra of different array sizes are shown in Figure 3d, and we observe a rapid increase in quality factor with increasing array size. In the extreme case where the array size is only 4 × 4 unit cells, light scattering into the space at the edges dominants the light− matter interaction. The collective oscillations of dipoles in the structure are very weak, thus the metasurface exhibits a very broad resonance (pink line in Figure 3d). When the array size increases, the portion of light scattered into space at the edges becomes smaller, and strong collective oscillations form and impose an in-phase oscillation of all unit cells, leading to the coherent resonance with high quality factor. We anticipate that a higher quality factor can be achieved with a larger illumination spot from the evolution of line shape in Figure 3d. The metasurface studied here has prominent polarization characteristics. Polarization resolved PL measurement result is shown in Figure 4. We record the corresponding PL intensities of different resonance modes when the linear polarizer is rotated at different angles. As shown in Figure 4, two high-Q resonance modes, ME1 and MM1, show distinct linear polarized characteristics with an extinction ratio of about 6 for Max/Min. Nevertheless, two low-Q resonance modes, ME2 and MM2, are almost polarization independent. This is consistent with our theory and results above. Although the luminescence of individual Ge QD is unpolarized, the PL of embedded Ge QDs in our structure is shaped by the metasurface. Due to the asymmetric shape of holes, only electric or magnetic y-polarized light can “feel” the weak difference of an effective refractive index between two parts in a unit cell, so the residual electric or magnetic dipoles are along the y-axis for electric or magnetic high-Q modes, respectively. As for the two low-Q modes, ME2 and MM2, Fano resonances can be excited in arbitrary polarization, except that the resonance wavelength changes slightly under different polarization, so they are almost polarization independent. Moreover, the polarization directions of ME1 and MM1 are perpendicular to each other as shown in Figure 4a,b. This is reasonable since the electric fields of mode ME1 and MM1 are perpendicular.
Sample Fabrication. A SOI wafer with 160 nm-thick top silicon film and 2 μm-thick buried silicon oxide (BOX) is used as the substrate. The substrate is cleaned by the standard RCA method and passivated with H+ by immersion in dilute HF solution for 80 s before loading into a solid source molecular beam epitaxy chamber (Omicron EVO-50). First, a 40 nm silicon buffer layer is grown, and then four layers of Ge self-assembled QDs with 15 nm-thick Si spacer layers are grown in S−K mode and finally capped by a 20 nm silicon layer. Ebeam lithography (Vistec EBPG 5000 Plus) is used to pattern the asymmetric hole array on the ZEP520A resist. The Ge/Si layer is etched by inductively coupled plasma etching using SF6 and C4F8 gases. After etching, oxygen plasma is used to remove the remaining resist. Experiments Setup and Measurements. The low-temperature microphotoluminescence (μ-PL) measurement is performed in a temperature-controlled liquid-helium cryostat with internal x−y nanopositioners. The sample is pumped with a 532 nm diode laser, which is focused by a microscope objective (NA = 0.1). The pump power density is 3W/cm2. The μ-PL signal is collected by the same objective and then dispersed by a monochromator with a 500 mm focus length and recorded by a liquid-nitrogen-cooled InGaAs detector array. Numerical Calculations. We employ the finite difference time domain (FDTD) method to perform transmission simulations. The geometrical model of the metasurface is built up in the commercial software FDTD Solutions. The materials applied in simulations are air and silicon. Periodic boundary condition and unpolarized incident light are imposed in the simulation. To simulate an unpolarized plane wave source, two simulations with orthogonally polarized plane wave need to be performed. The fields from each simulation can then be added incoherently. The largest mesh element size is set lower than 1/ 10 of the lowest wavelength, and finer meshes are applied at the domain with abrupt changes of geometry.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04810. The detailed fitting of Breit−Wigner−Fano line shape; transmission spectrum measurement setup; evolution of Q-factors when the asymmetry of the holes changes; measured relative transmission spectrum under different polarization states; polarization characteristics of the two broad resonance modes; numerical simulation of Purcell factor with excitation of point dipole source; numerical simulation of metasurface with symmetric hole; influence of different x−y grid periodicities to the Q-factor (PDF)
CONCLUSIONS In conclusion, we have developed a symmetry-broken alldielectric metasurface based on the Fano resonances to achieve a high quality factor. Over three orders of PL enhancement with a typical Fano line shape is experimentally demonstrated in the metasurface embedded with Ge QDs, accompanied by a record-high quality factor of 1011. The destructive interferences of antiphased electric or magnetic dipoles and collective coherent oscillations finally lead to high-quality-factor Fano resonances, which enhance the PL of Ge QDs dramatically. It is notable that the resonant PL peak aligns with a middle point of the Fano-like dip in the transmission spectrum, where the best cancellation of antiphased dipole radiation is achieved. The dependence between array dimension and quality factor confirms that the collective oscillations of the electric and magnetic dipoles contribute to the high quality factor of the metasurface. Polarization resolved PL measurement reveals the different origins of the resonant modes and is consistent with the theoretical analysis. The results of our work hold great promise to exploit the active all-dielectric metamaterial devices such as lasers and modulators.
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Jinsong Xia: 0000-0002-9650-7839 Author Contributions ‡
These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under grant no. 61335002, the Major State Basic Research Development Program of China under grant no. 2013CB632104, and the National High Technology 10709
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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 and Devices (CNCD), WNLO of HUST for the facility support.
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