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Abstract
Traditional grating lenses can accumulate phase for adjusting wavefronts, and plasmonic resonances can be excited in metasurfaces with discrete structures for optical field modulation. Diffractive and plasma optics have been developing in parallel, with easy processing, small size, and dynamic control advantages. Due to theoretical hybridization, structural design can combine advantages and show great potential value. Changing the shape and size of the flat metasurface can easily produce light field reflections, but changes in height are rarely cross-explored. We propose a graded metasurface with a single-structure periodic arrangement, which can mix the effects of plasmonic resonance and grating diffraction. As for solvents of different polarities, strong polarization-dependent beam reflections are produced, enabling versatile beam convergence and deflection. Dielectric/metal nanostructures with selective hydrophobic/hydrophilic properties can be arranged by the structural material specification to selectively settle the location of the solution in a liquid environment. Furthermore, the wetted metasurface is actively triggered to achieve spectral control and initiate polarization-dependent beam steering in the broadband visible light region. Actively reconfigurable polarization-dependent beam steering has potential applications in tunable optical displays, directional emission, beam manipulation and processing, and sensing technologies.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Traditional optical components, such as gratings, lenses, and wave plates, are based on geometric and diffractive optics principles for light field regulation. Due to the diffraction limit of physical optics, they are relatively bulky and have limited beam manipulation performance at wavelength dimensions. With the research of nano-optics, the development of metasurfaces has facilitated substantial control of light fields beyond the diffraction limit. Periodic two-dimensional (2D) metasurface arrays generate position-dependent phase changes along the surface of the medium. As a result, the beam can be regulated by almost flat optical elements at the subwavelength scale [1]. Traditional metasurfaces are periodically arranged with discrete plasmonic or dielectric nano-resonators of different shapes or geometric sizes [2–8]. Each discrete structure corresponds to a specific resonance phase retardation or amplitude tuning [9]. Anomalous reflection and refraction [4,6], beam splitting, beam focusing [8], and other wavefront shaping functions [7,10,11] have been achieved in the visible, infrared, terahertz, and microwave bands. However, metal nano-resonators with discrete structures have limited operating bandwidth and strong optical losses in the visible band. In addition, interference between regular reflected/transmitted beams and other diffraction modes occurs between discrete units, resulting in limited conversion efficiency of incident light to the anomalous reflected light, with the sacrifice of reflected/transmitted intensity. As a result, light field modulation of metasurface with broadband properties is still complicated [12].
With the development of nanofabrication [13–15], plasmonic metasurfaces have provided a new scheme for light steering, arbitrary wavefront shaping, and spectral splitting at the nanoscale [2,16,17]. The interfacial phase shift of plasmonic modes simulates beam-splitting functions similar to blazed gratings for broadband anomalous reflection [18–22]. Adding 3D spatial height variation on a flat metasurface can be combined with the diffraction grating effect and plasma phase gradient. In the absence of a cross-polarization effect, strong polarization-dependent phase gradients generate to form a multifunctional beam splitting of a specific band. The future trend will be to promote combining traditional devices and plasma metasurface.
In recent years, the focus of metasurface-based phase regulation has shifted from fixed to tunable. Typical active tuning mechanisms include electrical [23–28], optical [29,30], mechanical [31–34], thermal [35–37], or chemical [38,39] schemes. However, tunable phase modulation at the nanoscale has rarely been achieved, especially in the visible light band, which makes realizing active large-angle optical tuning extremely challenging [40]. The spectral performance, including resonance wavelength, amplitude, and bandwidth [41,42], is usually adjusted by changing the ambient refractive index. Moreover, dropping liquid to realize plasma coloring [41], sensing [42,43], full-color dynamic display [44], and spectral selective [45] on the metasurface. Even by microfluidic channels [46] and nanoscale solid-state proton switch [47] to manipulate their optical behavior. Liquid immersion would be an effective control scheme in the liquid working environment. However, demonstrating phase-shift tuning, beam control, and immersion schemes are rare to achieve differentiation of liquids with different affinities. Compared with Near-infrared band tunable light field control [26], droplet type control [22], and simple light switch [36], we use a visible light source for convenient detection. The droplet can be standard distribution by materials and has the potential to be realized by large area array cells without complex nanoscale electrical contacts or standardized circuit layouts. It can form a large angle span, broadband rainbow spots. The reflection direction shows strong polarization and affinity correlation. Potential applies in contact-free optical directional emission [48], high signal-to-noise ratio (SNR) spectrometers [49], polarization beam splitters [50], efficient plasma couplers [17,51], directional emitters [7,52], tunable absorbers [53] and related environmental monitor. It shows strongly polarized and affinity-dependent beam deflections. Even have good applications in light detection and light sensing related to the proportion of environmental concentration.
We propose a novel metasurface array for affinity selective liquid-wetting on immersion metasurfaces (ALIM) to explore the effects of different liquid immersion environments. Spatially mixing gradient widths and heights can effectively combine localized surface plasmon resonance (LSPR) and grating diffraction benefits. It can produce an intense polarization-dependent beam-dispersion reflection of broadband visible light, with different beam reflections for liquids with different affinities. The proposed ALIM design that the graded polyimide (PI) (hydrophobic and oleophilic) trapezoids embed the constructing framework polytetrafluoroethylene (PTFE) (hydrophobic and oleophobic) arrays distributed on metal (hydrophilic) substrates. The ALIM platform controls the interface liquid distribution and realizes the beam steering behavior with intense polarization and affinity. Specifically, by actively adjusting the polarization of the incident beam and changing the wetting conditions of different affinity solutions, the metasurface will exhibit different beam reflection directions and reflection angles. Structural functions include switching the reflected light from the metasurface to two opposite directions, reflecting beams in specific near-infrared wavelengths, and realizing beam convergence in some wavelengths. There is excellent development potential for affinity sample identification, beam polarization identification, and light sensing for specific light detection.
2. Introduction reflection principle
Conventional phase gradient metasurfaces generate position-dependent phase discontinuities along the interface. And the accumulation phase is negligible due to thickness at deep subwavelength scales (∼λ/20), as shown in Fig. 1(a). With discontinuities in the abrupt phase, the reflected beam is redirected to an arbitrary direction determined by the phase gradient, leading to the generalized Snell's law of reflection [3]. Therefore, adjust orient of the structure so there is no phase shift along the Y-axis. Furthermore, when the incident is normal white light, it can simplify Snell's law for the reflected beam [see Supplement 1 for details]:
Here θi is the incident angle, θr is the reflection angle, φ and φ+dφ are the abrupt phases of the two paths (red and black), respectively. And dx is the structural period, λ0 is the incident wavelength, ni is the refractive index of the incident medium. According to Eq. (1), the anomalous reflection angle is related to the phase gradient and incident wavelength. Different frequency components of visible light will be redirected to different angles, showing anomalous reflections in a wide bandwidth band and forming anomalous dispersive rainbow light.
Fig. 1. (a) Reflection diagram of flat plasmonic nanoresonator. (b) Reflection diagram of 3D plasmonic nanoresonator.
The metasurface was adjusted to a gradual structure in height (Z-axis) direction. An accumulated phase varying with thickness was added along the light propagation direction, as shown in Fig. 1(b). The phase of the reflected beam is continuously affected by the discontinuity of the abrupt phase and the accumulated phase. The Snell's law of the reflected beam can be translated into [see Supplement 1 for details]:
Among them, θt is the transmission angle, θ is the slope angle of the trapezoidal table, and nt is the refractive index of the trapezoidal table. The reflected beam will be accompanied by the abrupt phase change, which is caused by the metasurface, and the phase retardation caused by the height change. According to Fermat's principle, it can be known that the phase retardation of the actual optical path of the infinitely close two beams is zero. Compared to conventional ultrathin metasurfaces, the phase retardation due to the height creates an additional phase term. When the slope angle of the trapezoidal table is fixed, as shown in Equations (2) (3), the final reflection angle is only related to the phase gradient and the incident wavelength, which will produce different reflection phenomena in different wavelength bands. Notably, this metasurface-based anomalous reflection differs from conventional blazed gratings with triangular zigzag grooves [see Supplement 1 for details].
3. Results and discussion
A typical phase metasurface periodically arranges a series of discrete resonator elements with different structures and sizes to form a directional light field-regulated wavefront. Consequently, the periodic discrete structure requires pattern design and lithography processing. In contrast, the processing of visible-band subwavelength metasurfaces is more challenging. The quasi-continuous building block with a high duty ratio can effectively suppress the regular reflection of discrete blanks. Regularly arranged single structures can reduce processing difficulty, improve conversion efficiency, realize light field adjustment, and realize the realization of metasurfaces in the visible band.
Different from the periodic discrete structure of conventional phase metasurfaces, we propose an isolation belt (PTFE, refractive index n = 1.35, extinction coefficient k = 0, conductivity δ = 0.33 × 10−10 S/m) and embedded guides trapezoidal structures (PI, n = 1.87, k = 0, δ = 0.1 × 10−10 S/m) deposited on Aluminum (Al) substrates. The width of the guide trapezoidal structure is gradually changed along the X-axis, resulting in a position-dependent abrupt phase at the interface, as shown in Fig. 2. The structural parameters of the periodic resonator unit are width w = 500 nm, length L1 = 1100 nm, and the height of the bottom Al mirror h1 = 1000 nm, more than five times the skin depth, which can effectively prevent the transmission of light. The thickness of the trapezoidal guide structure is h2 = 50 nm, and the length L2 = 1000 nm. Keeping the long side of the guiding trapezoid structure unchanged, the width of the short side changes with a linear gradient from w1 = 350 nm to w2 = 50 nm, and the thickness of the annular isolation belt is t = 50 nm. Extensive numerical simulations of the resonator element were performed using the finite element method (COMSOL MULTIPHYSICS) to analyze the phase shift under incident light sources of different wavelengths and polarizations.
Moreover, building a wavelength-reflection phase library for graded structures (see Supplement 1 for details). The normal incident light source corresponds to two orthogonal polarization states. Here the TE(TM) broadband source with the electric field direction along the X-axis (Y-axis), respectively. Due to the phase discontinuity of the plasmonic metasurface, the outgoing photon will produce a broadband gradient phase shift without any cross-polarization effect. Then results in anomalous reflected rainbow light propagating along the positive reflection angle (right side, -X-axis). When excited along the long side of the trapezoidal guide structure, the resonator typically exhibits a strong optical response of LSPR. However, while excited along the short side, it exhibits a less responsive plasmonic interaction with external fields. Furthermore, the outgoing electric field of the linearly graded structure shows a complete phase shift covering more than 2π.
The incident light source covering the 400 - 1000 nm band is perpendicular to the metasurface (the same as below), which induces abrupt phases along the planar 2D surface. Interface wave vectors are provided to achieve linear/non-linear phase discontinuities, steering the “normal” transmitted beam into anomalous directions determined by phase gradients. The simulation yields a single uniform nanorod phase that merges into a nanoresonator unit whose short side varies linearly with a continuously modulated phase shift (see Supplement 1 for details). The short side length functions and simulated phase shift diagrams under different polarization modes are arranged in sequence along the long sides of the trapezoidal, as shown in Fig. 3 (a1) - (d1) (a2) - (f2). The phase gradient is relatively flat at shorter wavelengths and steeper at longer wavelengths corresponding to larger reflection angles. Orthogonally polarized light excitation generates phase gradients in the same direction. Due to the dispersion of the gradient phase change, the reflected photons of different frequencies are endowed with different interface wave vectors, and the anomalous reflected light emerges as a rainbow light at a regular reflection angle (right). Furthermore, Fig. 3 (i1) and (m2) correspond to the reflection diagrams in TE and TM modes, respectively. The high-bandwidth rainbow light shows polarization dependence. The anomalous reflection occurs in the violet (400 - 450 nm) and yellow (560 - 580 nm) bands under TE modes, while TM modes violet (400 - 450 nm), green (530 - 590 nm), and red-orange (620 - 650 nm). The calculated reflection angle ranges are: TE polarized violet light (23.4°-26.95°) and red light (34.76° - 36.18°), TM polarized violet light (23.4° - 26.95°), green light (32.63° - 36.18°) and red-orange light (39.02° - 41.15°). In addition, in the frequency domain between the anomalous reflection bands, no obvious beam reflection is observed (see Supplement 1 for details). The wavefront shape of the reflected light exhibits a strong interference pattern above the metasurface structures, and the anomalous reflection is caused by strong interference effects between light and normally reflected light.
Fig. 2. Structure diagram of the flat plasma resonator. The structure is divided into an aluminum mirror substrate, an annular isolation belt, and a trapezoidal guide structure.
Fig. 3. Reflection diagram of flat metasurface array. (a1) - (d1), (a2) - (f2) Simulated phase shift diagram at the corresponding incident wavelength with the length of the short side function, (e1) - (h1), (g2) - (l2) Far-field electric pattern diagram at the corresponding incident wavelength, (i1) and (m2) Schematic diagram of the reflection of the beam, TE polarization footnote 1, TM polarization footnote 2 (the same as the full text below).
The flat subwavelength metasurface can excite the local plasmon mode of the anisotropic resonator perpendicular to the incident plane. As a result, planar phase discontinuities can be realized and form polarization-dependent anomalous beam reflections. The regular arrangement of a single structure can improve the processing difficulty of discrete structures. Furthermore, we achieved the combination of traditional diffractive optics and plasmonic optics by introducing a new degree of freedom (spatial height variation), as shown in Fig. 4. The bottom Al mirror assembles the unit structure, height h1 = 1000 nm, length L1 = 1100 nm, and width w = 500 nm. The annular isolation belt height is h2 = 100 nm, and the thickness is t = 50 nm. The guide trapezoidal block length is L2 = 1000 nm. The variation of the wideband phase shift with the width of the short side is numerically inverted by the finite element method. The graded structure under different polarizations establishes the wavelength-reflection phase library. It is worth noting that the non-integrated fusion between the width-related LSPR effect and the diffraction effect caused by the height can achieve strong polarization selectivity and wavelength correlation. Moreover, produce opposite reflection effects for beams in different bands (See Supplement 1 for details).
Unlike the conventional grating or typical metasurface, the spatial stereo structure can achieve both accumulated phase and phase gradients in the visible light range, resulting in a strong polarization correlation. In TE mode, the simulated phase shift diagram shown in Fig. 5(a1) (b1) can see the phase gradient presents a monotonically decreasing distribution. In the range of orange light (560 - 620 nm), the anomalous reflection caused by LSPR excitation is in the right positive direction, and the far-field electric pattern diagram shown in Fig. 5 (e1) (f1) is obtained by simulation. The angle range of reflection is (27.01° - 30.05°). On the contrary, in the range of red light (640 - 730 nm), the phase gradient of the simulated phase shift diagram shows a monotonically increasing trend, as shown in Fig. 5 (c1) (d1). The X-Z plane's stereo structure displays the blazed grating's outline. When the central maximum of diffraction of a single groove surface coincides with the first-order principal maximum (spectrum of first order) of interference between the groove surfaces. Then according to the grating equation calculate the blaze wavelength λB = 2L1sinγ ≈ 660 nm. Under the influence of plasma optics and the grating diffraction effect, the beam in the vicinity of the blaze wavelength has anomalously reflected. Resulting in the electric field reflection diagram in the left and opposite direction, as shown in Fig. 5(g1) (h1). The incident TE mode white light is split into two beams of rainbow light in opposite directions, resulting in the effect diagram shown in Fig. 5(i1). In TM mode, the simulated phase shift diagram (Fig. 5(a2) (b2)) indicates a monotonically decreasing phase gradient in the range of orange light (540 - 580 nm). The incident light propagates to the right along the positive reflection angle under the influence of a more substantial LSPR effect. As shown at the bottom of Fig. 5(e2) (f2), the stronger electric field intensity appears at the interface between the bottom metal and the upper medium, indicating an intense LSPR effect.
Fig. 5. Reflection diagram of gradient metasurface array. (a)-(d) Simulated phase shift diagram at the corresponding incident wavelength with the length of the short side function, (e)-(h) Far-field electric pattern diagram at the corresponding incident wavelength, (i) Schematic diagram of the reflection of the beam.
On the contrary, in Fig. 5(g2) (h2), the electric field intensity of the contact surface between the bottom metal and the upper medium and the LSPR effect is weaker. The diffraction effect enhances the TM polarized light, and the dominant diffraction effect will make the beam oriented to the left. The simulated phase shift diagram (Fig. 5(c2) (d2)) corresponding to the near-infrared (800 - 830 nm) shows a monotonically increasing phase shift pattern, with the light beam reflecting from -22.5° to -22.61° (negative sign indicates left). Furthermore, it is interesting that the beam convergence phenomenon occurs in the TM mode's red light (730 - 750 nm) range. Due to the LSPR-induced near-field hot spots and their coupling between adjacent units at different wavelengths, the planar reflecting metasurface is used to achieve the convergence effect of concave mirrors in several bands (see Supplement 1 for details).
In our proposed metal-dielectric metasurface, the interfacial phase shift originates from the accumulated phase caused by the diffraction grating effect under the influence of medium height and the phase gradient caused by the plasmon resonance of the trapezoidal groove. At the same time, the spatial arrangement of the solution can be realized through the selection of top-structure materials [22], showing strong affinity sensitivity. Utilize the properties of a metal surface (hydrophilic and lipophilic), top layer PTFE (hydrophobic and oleophobic), and PI (hydrophobic and oleophilic), selectively wetting with different affinity solutions. When the ALIM sample (Fig. 6(a)) statically immerse in the analysis solution, the top kept the liquid surface flat and uniform. It accumulated into a complex spatial distribution at the nanoscale according to the affinity of the solution. Among them, the hydrophilic droplets tend to fill the groove surface of the metal. Due to the hydrophobicity of the PI material, it is finally displayed as a templated hydrophilic solution above the protruding sample array, as shown in Fig. 6. (b). The droplets tend to fill the entire groove on the metasurface and remain aggregated over the PI material, eventually appearing as a protruding array of lipophilic solution on the whole sample, as shown in Fig. 6. (c).
Fig. 6. Schematic diagram of the hypothetical interaction between ALIM and analyte. (a) Empty structure, (b) hydrophilic attachment diagram, (c) lipophilic attachment diagram.
The simulation used different immersion liquids to analyze the effect and characterize the affinity sensitivity of ALIM. The dropping water (obtained by using the optical parameter of temperature 22°, which has been well verified by the experiment of S. Kedenburg [54]) on the surface of the ALIM sample forms a complex spatial water aggregation distribution. Keep the water level flat, and the orthogonal polarization light source is used to carry out continuous beam scanning in the visible band. In TE mode, yellow light (560 - 570 nm) and orange light (600 - 620 nm) is reflected in a narrow band range to obtain the far-field electric pattern diagram as shown in Fig. 7 (a1) - (d1). The diffraction grating effect enhances the TE polarized light, and the spectrum of first-order shining energy is the largest. And the zero-order principal maximum and other minor diffraction orders are significantly suppressed.
Fig. 7. Reflection diagram of the active beam steering tuning of the aqueous solution wetted metasurface. (a1) - (d1) (a2) (b2) Far-field electric pattern diagram at the corresponding incident wavelength, (e1) (c2) Reflection diagram of beam.
Meanwhile, under the influence of the LSPR effect, the phenomenon of high electric field intensity appears on the thinner side of the bottom metal and the guide trapezoidal block, as well as the interface between the guide trapezoidal block and water. As a result, the electric fields of the two narrow band bands were reflected along the left side in the opposite direction, as shown in Fig. 7 (e1). Quite the contrary, in TM mode, anomalous reflection occurs in the range of orange light (550–620 nm), resulting in the far-field electric pattern diagram shown in Fig. 7 (a2) (b2). The intense electric field strength originates from the contact surface between water and polymer dielectric. The reflected beam typically deviates to the right and positive direction under the influence of the LSPR effect, the final effect is shown in Fig. 7(c2).
After adjusting the drop of the oily solution, the liquid appears in a differential distribution, then occupying the protruding part and being isolated from each other. Here we simulate the common organic solvent Dimethylformamide (DMF, n = 1.43, δ = 6 × 10-8 S/m) under orthogonally polarized light. Compare the effect on beam reflection of the dried empty model and the structure wetted by oily solution. When the sample dries, the beams of the different wavelength bands under the orthogonally polarized light source are split into two reflections in opposite directions. When ALIM is immersed in the solution, it may gradually grow from a trapezoidal air groove to a protruding dielectric block with a change in the surrounding refractive index (from 1.35 to 1.87). The topographic change makes the resonant electric field confinement within the groove more powerful. Corresponding to the incident of the TE polarized light source, as shown in Fig. 8(a1) (b1), the regular reflection in the range of yellow light (560–590 nm) is in the right positive direction. Under the TE light source, the near-field hot spot effect induced by LSPR occurs on the contact surface between the organic solvent and the guiding block and the contact surface between the guiding trapezoid block and the underlying metal. Therefore, coupling of the end-to-end near-field hot spots between adjacent cells results. The reflection diagram is Fig. 8(c1). In the TM mode, this near-field hot spot almost disappears, and the reflected beam is order reflected like a blazed grating. The maximum light energy is ordered and reflected in the spectrum of first-order direction. The excitation of TM polarized light broadens the reflection frequency band (560–620 nm), and the far-field electric pattern diagram of the reflection range is reflected in the right positive direction as shown in Fig. 8(a2) (b2), resulting in Fig. 8(c2) reflection schematic shown.
Fig. 8. Reflection diagram of the active beam steering tuning of the oily solution wetting metasurface. (a) (b) Far-field electric pattern diagram at the corresponding incident wavelength, (c) Reflection diagram of beam.
4. Conclusion
In this work, we have designed a flat metasurface array to generate polarization-dependent anomalous rainbow speckles. It can solve the problems of complex structure, complicated processing, and low efficiency of traditional phase metasurface. A single resonator periodic arrangement distribution simplifies the design without the cross-polarization effect. To realize the combination of diffractive optics and plasmon resonance by exploring the variation of space height. Finally, a multifunctional metasurface generates beam reflection and other functions. The spatial stereo structure realizes the LSPR phase discontinuity along the optical surface in the visible band, produces polarization-related optical reflections, and realizes the beam convergence in some bands. In addition, the metasurface with a strong polarization correlation shows different beam reflection functions in a liquid environment with different affinities. The metasurface reflection array has wider bandwidth and higher conversion efficiency than the traditional shining diffraction grating and provides a new scheme for integrating other photonic and plasma devices. The structural functions show great promise in light absorbers, efficient plasma couplers, directional emitters, planar lenses, and mirrors.
Funding
National Natural Science Foundation of China (62175021, 51902033); Chengdu Technology Innovation and Research and Development Project (2021-YF05-02420-GX, 2021-YF05-02422-GX, 2021-YF08-00159-GX); Open Project Program of State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization (2021P4FZG08A); Sichuan Science and Technology Program (2020YFQ0040, 2021YFG0020).
Acknowledgments
This work is supported by Natural National Science Foundation of China (NSFC) (62175021 and 51902033), Chengdu Technology Innovation and Research and Development Project(2021-YF08-00159-GX, 2021-YF05-02422-GX, 2021-YF05-02420-GX), Sichuan Science and Technology Program (2020YFQ0040 and 2021YFG0020) and Open Project Program of State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization (2021P4FZG08A)
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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