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Abstract
Diffractive grating and plasmonic metasurface have always been developing as two parallel optical domains, which have not met for studying their hybridization to discover new applications and potentials. Here, we proposed a novel meta-grating design, which hybridizes the metasurface interfacial gradient with the blazed grating profile. The unique architecture takes advantage of both grating effect and plasmonic resonances with minimum cross-coupling, thus leading to the polarization-selective behaviors to steer different polarized light to drastically inverse directions (> 90°). Furthermore, the hybridized surface also exhibits angle-dependent broadband absorptive tunability (∼ 5% - 86%) by migrating the strong blazed order and plasmonic order at the far field. We believe that the integrated meta-grating device would suggest various potential applications including polarization beam splitters, high signal-to-noise ratio (SNR) optical spectrometer, high-efficiency plasmonic couplers and filter, etc.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Conventional gratings are ubiquitous optical elements present in almost all imaging systems, which enables a normal paradigm of controlling light by passing it through planar arrays of periodic arrangement to induce a swift change in phase and/or polarization. Traditional optical gratings including blazed grating [1–3], Michelson grating [4,5], concave grating [6,7], sinusoidal grating [8,9], operate based on the traditional principle of geometrical and diffraction optics. Actually, they are quite efficient in adjusting and manipulating the propagation wavefronts through reflection, refraction, or diffraction [9–11]. On the other hand, recently a new optical structure – flat metasurface has attracted much attention for developing novel functionalities and extraordinary optical properties enabled by artificially designed plasmonic resonators [12–15]. The subwavelength scale with optically ultrathin thickness makes them suitable and advantageous for miniature optical device innovations. A wide range of metasurface designs can physically implement high-resolution phase elements in various cases and frequency domains, from microwave to infrared to optical frequencies. Currently, extensive studies and metasurface works have pursued to design architecture and achieve an optical system that converts a certain incident optical wave to an arbitrarily desired output and versatile applications such as light steering [16], beam splitting [17,18], tunability and wavefront-shaping [19–21] functionalities. However, two critical optical domains of diffraction optics and plasmonics were always in parallel research and evolution as two separate areas [22,23]. Conventional grating architecture and plasmonic metasurfaces have not met or been studied for their hybridization for new applications and novel potential functionality. Furthermore, it is also not clear how the interaction between the plasmonic effect [24–27] and grating effect would affect the emerging optical device performance at the nanoscale.
Here, we proposed a new “meta-grating” architecture, which hybridizes nanoantenna width gradient (critical resonant length) with blazed grating profiles at the cross-section. By varying the single “meta-atom” geometry along both parallel and perpendicular to the light propagation direction, the unique design of meta-grating is gifted for owning both grating effect and plasmonic phase gradient across the entire visible frequency range and beyond. The meta-grating surface could impart drastically inverse phase gradient trends to different polarized outgoing waves without any cross-polarization effect, thus producing versatile reflected beam splitting to totally different directions corresponding to the specific light polarizations. Moreover, the hybridized architectural surface also reveals strong angle-sensitivity to enable broadband absorptive tunability by migrating the diffraction plasmonic orders at the far-field reflection. The overall surface absorptivity could be freely tuned to serve as a broadband absorber or reflector by the angular orientation of the ingoing light [28–31]. The mingled architecture enjoys high compatibility with conventional microfabrication techniques and allows for monolithic fabrication of optical systems. We believe that our proposed meta-gratings based on hybridizing both grating effect and plasmonic phase gradient could create and enable potential applications including high signal-to-noise ratio (SNR) optical spectrometer [32], polarization beam splitters [33], high efficiency plasmonic couplers [34,35], directional emitter [36,37] and spectral broadband tunable absorber [38].
2. Results and discussion
A three-dimensional (3D) schematic of the meta-grating array is illustrated in Fig. 1(a), with its “meta-atom” shown in Fig. 1(b). The design of optical meta-grating is composed of a single spatially gradient antenna for both height (along z-axis) and width (along x-axis). Essentially, the meta-grating shows the outline profile of trapezoid-shape at x-y plane by width variation, while the blazed grating profile at x-z plane. Specifically, the unit cell of meta-grating with short-side width (W1 = 30 nm) and long-side width (W2 = 200 nm). In the cross section, the silver-trapezium antenna’s height varies from H1 = 30 nm to H2 = 130 nm. The pitch of meta-grating pattern is 1000 nm along the x direction and 300 nm along the y direction. The meta-grating array is placed on top of an optically thick (100 nm) Ag film to prevent light transmission at the bottom and a SiO2 thin film with the thickness of 50 nm in the middle. The inclined architecture patterns can be potentially fabricated using Nanoimprint or 3D printing techniques. Alternatively, DUV (Deep-Ultraviolet)/EUV (Extraordinary-Ultraviolet) photolithography combines with oblique directional deposition technique can also possibly provide a fabrication solution. Since the Ag film at the bottom is optically thick (100 nm) to prevent light transmission T, the absorptivity A is reduced to A = 1 – R, where R represents the all-directional reflectivity.
Fig. 1. Metasurfaces consisting of meta-grating array and the optical schematic diagram. (a) Schematic view of meta-grating array. (b) The unit cell of meta-grating with short-side width (W1 = 30 nm) and long-side width (W2 = 200 nm). In the cross section, the silver-trapezium antenna’s height varies from H1 = 30 nm to H2 = 130 nm. The pitch of meta-grating pattern is 1000 nm along the x direction and 300 nm along the y direction. (c) The circularly polarized light is incident normally onto the meta-gratings array, which produces reflected light in two opposite directional modes according to light polarizations (TE/TM). The angle of the reflection light θ1 and θ2 for the above two cases.
When a circularly polarized light (λ = 650 nm) is normally incident to the meta-gratings array, it can majorly scatter the reflected light into two diffraction components – TE and TM to totally different directions, as depicted in Fig. 1(c). The meta-gratings enable to achieve the versatile functions for TE/TM light inverse-steering because it depends on the combination design of both blazed grating effect and plasmonic phase gradient.
Figures 2(a)–2(b) show that the meta-gratings array creates drastically different reflections under the two orthogonal polarization states (TE and TM polarizations). Under TM polarization (E-field excitation along x-direction, as shown in Fig. 2(a), the meta-grating shows up in conventional blazed grating mode, and the optical path difference along light propagation direction would redirect the diffracted TM component to left-hand side. In the contrast of under TM polarization, the meta-grating would behave as a plasmonic gradient resonator, which produces anomalous reflected light. According to the generalized Snell law, such gradient meta-antenna design would redirect the diffracted TE light component to the counterintuitive opposite direction, the right-hand side.
Fig. 2. Meta-gratings exhibit two reflection modes at the far-field distribution - grating mode and plasmonic mode. (a) Configuration schematics for different polarized incident light with TE-polarization and (b) TM-polarization, respectively. (c) The far-field reflective power distributions for the grating mode and (d) the plasmonic mode as a function of reflection angle θr (x-axis) and wavelength (y-axis).
For the two orthogonal polarization states, we can simply define and divide it as two reflection modes based on the working principles: grating mode and plasmonic mode. Figures 2(c)–2(d) show the simulated angle-dependent reflectance spectra illuminated by the visible-frequency broadband sources with two orthogonal polarization states (TE and TM). Three-dimensional (3D) finite-difference time-domain (FDTD) simulation is conducted by using commercial software (Lumerical). From the calculated the far-field reflectance power distribution, it demonstrates that the meta-grating design could yield broadband dual-directional reflection spectra for grating and plasmonic modes, respectively. The dotted lines in Figs. 2(c)–2(d) represent three diffraction beam directional modes, normal reflection mode (m = 0), anomalous plasmonic reflection mode (m = 1) and the first-order blazed grating mode (m = -1). The higher-order diffraction modes can be neglected. For the blazed grating mode (Fig. 2(c)), most of the incident power is reflected to the m = -1 mode direction (left-hand side), while the reflective power to the m = 0 and m = 1 modes is significantly suppressed. On the other hand, for the plasmonic mode (Fig. 2(d)), most of the reflected power would be guided to form the m = 1 mode (right-hand side). Because the reflected photons with two orthogonal polarization states (TE and TM) of different frequencies are imparted with varied interfacial wavevectors due to the dispersion of the gradient phase shifts, they would be redirected to distinct reflection angles over - 30° – - 60° or 30° – 60° at the far-field distribution spectra, respectively.
Under TM polarization, the beam is guided to the left-handed orientation due to the tilted phase retardation along the interface, as the outgoing wavefront is depicted in Fig. 3(a). In this case, the metasurface design behaves like the blazed grating at the subwavelength scale. Conversely, under TE polarization, the plasmonic phase gradient from the resonant nano-antenna width variation leads the anomalous rainbow reflection light to propagate along the positive reflection angles (the right-hand side), as shown in Fig. 3(b). It is worth noting that the optical performance is featured with broadband reflection spectra in visible-frequency regime, where the grating mode shows high reflection efficiency in the wavelength range of 500 - 750 nm, while the plasmonic mode has a good performance in the wavelength range of 650 - 850 nm. The power contrast between the major outgoing mode to the minor diffraction mode is calculated to be on the order of 100. It is worthy of noting that the total conversion efficiency is determined by the mode power contrast as well as the surface optical absorption. Here the total conversion efficiency from numerical calculation reaches ∼ 88% when considering the surface optical loss.
Fig. 3. Two orthogonal polarization states would induce different reflective wavefront patterns as well as distinctive interfacial phase shift trends. (a) Reflective wavefront patterns at the grating mode (λ = 582 nm) with TM-polarized light and (b) at the plasmonic mode (λ = 735 nm) with TE-polarized light. The arrows denote the angle of the reflection wave θ1 and θ2 for the above two cases, respectively. (c) Simulated 2D map for the phase shift of the meta-grating as a function of nanorod width along the interface and (d) the nanorod height at the cross-section.
The spectral versatile performance to different diffraction directions fundamentally comes from the combination of both conventional flat metasurfaces and blazed gratings with triangular, sawtooth-shaped grooves. The plasmonic mode is based on the phase discontinuities from the localized surface plasmonic resonance of anisotropic resonators perpendicular to the incidence plane. The grating mode is based on accumulating sluggish linear phase delay through different length of light propagation path by changing the groove depth, which is similar to oblique reflection from an inclined plane. Therefore, the meta-grating could benefit from both grating and plasmonic effects to enable the versatile performances for light steering. Such structure has spatially gradient surface along both cross-section and interface could provide new possibilities for integration into other photonic devices.
To gain more insight on how the interfacial phase gradient trends are impacted from different polarization excitations, we have retrieved the local interfacial phase trending (Figs. 3(c)–3(d)) as a function of nanorod width and height scaling for the above two cases of different TM/TE polarizations, respectively. It is observed that the interfacial phase reveals a monotonically increasing trend for broadband visible regime for TM polarization as the nanorod width scales up (Fig. 3(c)). The major component of outgoing light to right-hand side (rather than left-hand side) is due to the plasmonic effect from phase gradient at Fig. 3(c). Here the grating effect could set up the diffractive deflection modes, but the plasmonic effect determines the outgoing direction of the strongest light component.
Fundamentally, due to the trapezoidal geometric shape, the TE-mode light can excite the phase gradient from the localized plasmonic resonance along the interface (Figs. 3(c)-(d)). By linearly changing its width from W1 = 30 nm to W2 = 200 nm, the unit cell of meta-grating’s relative phase shift along the metasurface is continuously modulated and successfully achieve drastic phase change covering from 0 to 2π for the broadband visible regime. According to the generalized Snell’s law adjusted in our condition,
the reflection angle of the strongest outgoing light component is majorly determined by the phase gradient along the interface. Therefore, the visible light consist of different frequencies is redirected to different reflection angles instead of a single reflection angle, thereby forming a rainbow anomalous reflection mode.On the other hand, the interfacial phase gradient shows a drastically different trend – monotonically decreasing as the nanorod height rises (Fig. 3(d)). Therefore, according to the generalized Snell law (1), the different phase change trending determines the orientation of the broadband outgoing wave as well as the titled wavefront shapes. To clarify the correlation between the architectural design and the light outgoing direction, here for the grating mode, the height gradient (at the cross-section) of the blazed grating determines the TM light outgoing direction; for the plasmonic mode, the width gradient of the plasmonic trapezoid-shaped antenna (top-view) determines the TE light outgoing direction.
Furthermore, such reflection power of the anomalous outgoing beam is capable of being tuned by changing the incident angle of light of the meta-gratings array. Figure 4(a) shows the revolution of diffraction spectra at far-field distribution for the case of TE polarization light incidence. As the angle of incident light decrease or increase, the reflection angle/mode migrates accordingly. Specifically, when the incident angle increases from 0° to 50° (incident from right-hand side), the strongest anomalous reflection mode gets closer to normal refection angle. Moreover, the overall reflection power at omni-directional far field also increases as the incident angle rises, as shown in Fig. 4(a). On the hand side, when the incident angle reduces from 0° to - 50° (incident from left-hand side), it is observed in Fig. 4(b) that the anomalous propagation wave gradually moves out of the far field space, thus leaving minor diffraction power in the far field spectra. Therefore, as the incident angle decrease further to - 50°, the total reflection spectrum decreases significant from ∼ 65% to ∼ 13% (λ = 541 nm), as shown in Fig. 4(b). The meta-grating show particularly broadband high absorption when the incident angle decreases to - 30° or further.
Fig. 4. Angular-dependent characteristics at far-field distribution for the plasmonic mode with TE polarization. (a) Far-field diffraction pattern when the incident angle varies from - 50° to 50° using line polarization in the TE polarization. (b) Simulated reflection spectra evolution as a function of wavelength (x-axis) and incident angle (y-axis) for TE polarization mode (schematically shown in the inset figure).
Similarly, when excited with TM polarization to enable the grating mode, the reflected light spectra at far field also migrates according to the incident angle variation. For the case of increasing the incident angle from 0° to 50°, the strong diffraction mode is gradually moving out of the all-directional far field space, which makes the all-directional reflection to be quite low (reflection dip < ∼ 5%) and forms optically high absorptive surface (Fig. 5(b)). Conversely, as the angle of incident light decreases from 0° to - 50°, the evolution of the strong diffraction order is getting closer to the normal reflection angle, which maintains the high reflectivity (as high as ∼ 81%) at far field. Figure 5(b) shows that that the grating mode is almost equally sensitive to the incident angle in the wavelength range of 500 - 700 nm. Such versatile meta-grating design show the particular spectral sensitivity and tunability (peak absorption = ∼ 5% - 86%, λ = 573 nm) as a function of incident angle changes, where the surface absorptive characteristics can be actively engineered by modulating its incident angle.
Fig. 5. Angular-dependent characteristics at far-field distribution for the grating mode with TM polarization. (a) Far-field diffraction pattern when the incident angle varies from - 50° to 50° using line polarization in the TM polarization. (b) Simulated reflection spectra evolution as a function of wavelength (x-axis) and incident angle (y-axis) for TM polarization mode (schematically shown in the inset figure).
3. Discussion
In summary, we proposed a new meta-grating design for visible wavelengths by hybridizing the grating effect with plasmonic phase gradient to enable light steering and beam splitting functionality. By varying the geometry of nano-antenna along both the interface and cross-section direction, it forms a new architecture of quasi-continuous gratings with width variation. Distinctive from either conventional grating or typical metasurfaces, the unique design combines both grating effect and plasmonic phase resonance with minimum mutual cross coupling, thus leading to polarization-sensitive optical behaviors to split different polarizations (TE/TM) to totally different directions (> 90°). Thus, it leads to the function of splitting the broadband circular polarization light into two linear independent polarizations and steering each polarization components respectively to different diffraction angles. Moreover, the strong diffraction mode or plasmonic anomalous mode could be tuned to migrate at far field direction by changing the incident angle. Therefore, such meta-grating architecture is capable to exhibit actively tunable absorptive characteristics by dynamically modulating the incident angle and polarizations. Our proposed meta-gratings based on hybridizing both grating effect and plasmonic phase gradient suggests practical optoelectronic devices and applications for polarization beam splitters, high efficiency plasmonic couplers, directional emitters, spectrally tunable absorber, etc.
4. Methods
Numerical Simulation. Full-field electromagnetic wave calculations were performed using Lumerical, a commercially available finite-difference time-domain (FDTD) simulation software package. 3D simulations for were performed for the proposed metasurfaces design with a unit cell area of 300×1000 nm2 at x-y plane using periodic boundary conditions. Perfectly matched layers (PML) conditions are utilized along the propagation of electromagnetic waves (z-axis). Plane waves were normally incident to the nanostructures along the + z direction, and reflection and transmission is collected with power monitors placed behind the radiation source and after the structure, respectively. The reflected powers at a full range of angles are calculated by the far-field calculation option of the reflection power monitor. Electric and magnetic field distributions are detected by 2D field profile monitors in x-z plane. The complex refractive index of Ag for simulation is utilized from the data of Palik (0-2 µm) and SiO2 is from the data of Palik.
Funding
Wuhan University; Recruitment Program of Global Experts.
Disclosures
The authors have no competing financial interests.
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