Letter Cite This: ACS Photonics 2018, 5, 2568−2573
Visible Metasurfaces for On-Chip Polarimetry Pin Chieh Wu,*,† Jia-Wern Chen,‡ Chih-Wei Yin,§ Yi-Chieh Lai,‡ Tsung Lin Chung,‡ Chun Yen Liao,‡ Bo Han Chen,‡ Kuan-Wei Lee,‡ Chin-Jung Chuang,§ Chih-Ming Wang,*,§ and Din Ping Tsai*,†,‡ †
Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan Department of Physics, National Taiwan University, Taipei 10617, Taiwan § Department of Optoelectronic Engineering, National Dong Hwa University, Hualien 97401, Taiwan ‡
ACS Photonics 2018.5:2568-2573. Downloaded from pubs.acs.org by ST FRANCIS XAVIER UNIV on 08/09/18. For personal use only.
S Supporting Information *
ABSTRACT: Measuring the polarization state of light and determining the optical properties of chiral materials are inherently complex issues because of the requirement of consequential measurements between different orthogonal states of polarization. Here, we introduce an on-chip polarimetry based on the visible metasurfaces for addressing the issue of polarization analysis with compact components. We demonstrate integrated metasurface chips can effectively determine a set of Stokes parameters covering a broad wave-band at visible light. For the proof of concept, the optical properties of chiral materials are measured using our proposed device, while experimental verifications are convincing by comparing with the data obtained from commercial ellipsometry.
KEYWORDS: visible metasurfaces, aluminum plasmonics, on-chip polarimetry, polarization analysis light control and profit the development of flat optical components and systems.12−20 Particularly, the optical response of metasurfaces are mostly polarization-dependent,21−24 therefore, they can be promising for probing the polarization states after careful designs. For example, previous works have utilized space-variant metasurfaces (also refers to geometric or Pancharatnam−Berry phase based metasurfaces) to distinguish states of polarization.25,26 The dipolar-resonant based metasurfaces have also been proposed for detecting linearly polarized light with related intensity change of anomalous beams. Although successful, to completely interpret the states of polarization, one must obtain the whole set of Stokes parameters.4,27 For this purpose, there have been demonstrations of the measure of the Stokes parameters at nearinfrared and partial of the visible spectrum using reflective antennas based metasurfaces.28 The compact dimensions of metasurfaces even provide a flexible channel for integrating several required chips, hence, extending the functionality of metasurfaces.15,29−32 However, these lectures only suggest the polarization analysis with metasurfaces from probing a laser light beam with a known state of polarization. Nevertheless, there is still lack of measuring the properties of real materials with on-chip metasurfaces for practical applications. Here, a state-of-the-art demonstration is presented for realizing practical applications of determining Stokes parame-
P
olarization is one of the key properties of an electromagnetic wave for information delivery and signal measurement. Analyzing the polarization state of light therefore can provide details regarding to the physical systems, even material properties.1 For instance, chiral materials, which are common in biological compounds, can be classified from consequential analyses of circular polarizations.2,3 Typically, experimental realization of the polarization states requires sequential measurements (such as polarimeter or ellipsometer) with properly arranged optical components like polarizers and wave-plates in the optical system (especially in front of the detectors). It allows us eventually retrieving the information on Stokes parameters, which enable completely describing the polarization of light.4 Beside single detection for each orthogonal polarization state, one can also measure all states simultaneously by splitting the incident light beam into a couple of light beams with multiple sets of optical components.5,6 However, such approaches significantly increase the sizes, cost, and complexity of the entire optical system. The recent desire for miniaturizing the optical systems is subsequently obvious, that will benefit the development of an innovate class for flat optical components and on-chip integration. To address above-mentioned issue, it is necessary to introduce strong polarization-dependent interactions between light and interested matters at a small spatial scale. Metasurfaces are ultrathin artificially designed optical scatterers, which enable abrupt changes to the optical features such as electromagnetic amplitude and phase of scattered light within a subwavelength spatial region.7−11 These properties provide a great flexibility in © 2017 American Chemical Society
Special Issue: Recent Developments and Applications of Plasmonics Received: December 12, 2017 Published: December 14, 2017 2568
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nanoantenna with correspondingly optimized structural dimensions for different polarizations (see Supporting Information, part 1). The polarization-dependent beam steering can be realized using gradient metasurfaces with various design principles. For linear polarizations (LP), one can optimize the structural parameters of nanoantennas to accomplish 2π phase shift with reliable reflection efficiency, as shown in Figure 2a,b. In terms of circular polarization (CP), geometric phase provides a promising design rule for achieving 2π phase shift, which decouples from the amplitude modulation because of the space-invariant structural sizes,39,40 as shown in Figure 2c. For simplicity, the length of supercell (which means the minimal length where the 2π phase gradient distribution is achieved) of six metasurface chips keep as the same at 1600 nm, while each corresponding anomalous reflected beam is separated via spatially arranging the metasurface chips. Figure 2d shows the angle of anomalous reflection as a function of incident wavelength (see Supporting Information, part 2, for detailed information). The variation of reflected angle is within 20° when the wavelength changed across the whole range of visible light. It is worth mentioning that the level number of each gradient metasurface can be changed at will, and one can obtain the relative intensity of each polarization component at arbitrary observation angle. Because of the introduction of Al plasmonics, the working wave-band can be pushed to the blue light with acceptable efficiency.41 Figure 2e shows the simulated efficiency of the anomalous deflection of six designed metasurface chips. The operation efficiencies generally show good performance over the entire range of visible light. Although the efficiency drops at the spectrum boundaries, our proposed device still works well for the polarization analysis because of the use of self-reference (the detailed discussions will be given in the last section).
ters with visible metasurfaces. To make them working over the entire visible spectrum, aluminum (Al) is employed as the metallic material of metasurfaces.33,34 To obtain the related intensity between different polarization states at one single snapshot, six metasurface chips are integrated onto one single sample (so-called on-chip polarimetry, see Figure 1). One
Figure 1. Schematic illustration of the on-chip polarimetry with integrated metasurfaces working at visible light. The light beam with unknown polarization state, which is generated from the sample pending for test, is analyzed by the related intensity of anomalous beams. Due to the on-chip integration of metasurfaces, intensity information on six polarization states are obtained simultaneously. The inset shows the structural configuration of metasurface unit cell: Al nanoantennas with 50 nm thickness on a SiO2/Al/Si substrate. The thicknesses of SiO2 and Al mirror is 30 and 150 nm, respectively.
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SAMPLE PREPARATIONS Based on the above discussions, six metasurface chips are fabricated and subsequently integrated into one single sample for realizing an on-chip polarimetry. Figure 3a shows the scanning electron microscope (SEM) images of the on-chip polarimetry consisting of six metasurface chips. The supercell of each metasurface chip composed of 8 unit elements, in which 4level and 8-level phase gradients are employed for linear and circular polarizations, respectively. For the proof of concept, Stokes parameters of the light beam after interacting with chiral samples are measured through our device. The testing samples are different layers of commonly used commercial-grade packing tape, also called biaxially oriented polypropylene (BOPP) films. It is commonly used in manufacture, especially for package because of its high flexibility and good adhesion.42 Such BOPP film exhibits strong optical birefringence (which means the optical response is different when it interacts with xor y-polarized light) because its polymer chains are aligned along the long axis, leading to the chiral molecule chains (see in Figure 3b). As shown in Figure 3c, when the single layer BOPP film is sandwiched by a set of linear cross polarizer, it shows light yellow color under white light illumination. The color changes to blue, pink, and light green when the layer number of BOPP films is two, three, and four, respectively. These images not only verify the chiral property of BOPP films, but also reveal such chiral property highly depending on the wavelength of incidence. Therefore, an on-chip polarimetry working in a wide range of visible wavelength is required for determining the optical properties of such highly chiral materials.
metasurface chip assists in detecting the intensity of one specific polarization state, therefore, one can attain a set of Stokes parameters simultaneously with the absent of optical components in front of the detector (a sCOMS camera in our case). To further evaluate our proposed device, samples with chiral properties35−37 (which will rotate/change the polarization state of the output light beam when the incident linear polarization is kept unchanged) are measured using both on-chip polarimetry and commercial ellipsometry. The fair agreement of these results shows that the on-chip polarimetry can yield accurate determination of optical properties of unknown samples, with a more compact track compared to the commercial systems. Comparing with previously reported lecture,38 which only uses Pancharatnam−Berry (P−B) phase based metasurfaces as the building blocks, our metasurfaces is able to completely decompose all polarization states on the Poincaré sphere. Our work therefore provides a practical way for innovating flat devices in optical communication, signal detection, quantum optics, and so on.
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DESIGN OF METASURFACE CHIPS To completely characterize the reflected light with unknown polarization state, it can be achieved with analysis of the relative intensity between distinct polarization states. For practical applications with compact devices, six metasurface chips are monolithically integrated onto a single sample. Each metasurface is responsible for steering one eigen-polarization component of incidence into distinct spatial position, as shown in Figure 1. All metasurfaces are designed via Al 2569
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Figure 2. Simulated reflection (black dots) and phase shift (blue stars) for (a) LP-H, (b) LP+45°, and (c) RCP at λ = 521.74 nm. The wavelength choice here is purely random and one can also observe the similar result as the linear correlation of phase shift for arbitrary wavelength at visible light. More details of optimized unit elements for all six metasurface chips can be found in Supporting Information. (d) Prediction for the anomalous reflected angles from numerical (blue dots) and theoretical (olive curve) calculations. Since the length of the supercell of each metasurface chip is identical, the angles of deflection are all the same. (e) Simulated diffraction efficiency of each polarization state from corresponding metasurface chips. LP-H, LP-V, LP+45°, and LP-45° represent the LP along horizontal (x-direction), vertical (y-direction), +45°, and −45°, respectively. RCP and LCP depict the right- and left-hand circular polarization, respectively.
Figure 3. (a) Scanning electron microscope (SEM) images of on-chip polarimetry with six metasurface chips. Each metasurface chip has footprint of 100 × 100 μm2 with a 50 μm gap between its neighboring chips. (b) Chemical structural formula of the BOPP films. Due to its chiral property, the polarization state is changed after the incident light passes through the films. (c) Photography of the BOPP films with different number of layers. The BOPP films are sandwiched between two linear polarizers, which are orthogonal to each other. Each BOPP film is 25 μm thick.
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EXPERIMENTAL RESULTS AND DISCUSSIONS To analyze the chirality of the BOPP films, a y-polarized light is firstly generated using a half wave-plate and a linear polarizer, and goes through the transparent BOPP films (see Figure 4a). Due to the chiral property of BOPP films, a light beam with unknown polarization state consequently interact with the onchip polarimetry, and then generate six deflected light beams from gradient metasurfaces. Based on the diffraction pattern
from the CCD images (see Supporting Information, part 3), the powers of the deflected beams from each polarization states are individually integrated. The Stokes parameters are consequently obtained based on the integrated power of each polarization state. To suppress the influence from the different efficiency of metasurfaces, each Stokes parameter is normalized to the corresponding power of the polarization state (so-called I −I I −I self-reference). That is, the S′1 = ILPV + ILPH , S′2 = ILP+45° + ILP−45° , LPV
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LPH
LP + 45 °
LP − 45 °
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Figure 4. (a) Optical setup for experimentally evaluating the performance of on-chip polarimetry. (b) Measured Stokes parameters of single layer BOPP film using the on-chip polarimetry (solid curves) and ellipsometry (dots). (c) Experimental results of ellipticity angle (top panel) and azimuth angle (bottom panel) from commercial ellipsometry (navy dots) and on-chip polarimetry (orange dots) at λ = 633 nm. All olive symbols denote the measured polarizations, which are realized from measured ellipticity and azimuth angles.
and S′3 =
IRCP − ILCP . IRCP + ILCP
with the BOPP films, can be comprehended through eq 2. Take wavelength λ = 633 nm as an example, according to the measured results, we found that the single layer of BOPP film acts as a half wave-plate because the measured final polarization is close to an x-polarization. For the case of four layers of BOPP films, it can be regarded as a quarter wave-plate because of the circular polarization state of the scattered light (see orange dots and olive symbols in Figure 4c). To evaluate the performance of our on-chip polarimetry, we compare the measured results from on-chip polarimetry with the ones measured using commercial ellipsometry. As shown in Figure 4c, these measured results show fair agreements with each other, indicating the proposed on-chip polarimetry indeed offer a promising method for analyzing the chiral materials with compact dimensions. It is worth mentioning that the resolved property from on-chip polarimetry and commercial ellipsometry for two layers of BOPP films actually matches very well in between (see olive symbols at the bottom panel in Figure 4c). The counterfeit difference is mainly from the sing of the azimuth angle, which is very sensitive when the long-axis of polarization ellipse is roughly aligned along the y-axis. In summary, an on-chip polarimeter with visible metasurfaces is proposed. Due to its broad working bandwidth, the complete Stokes parameters can be resolved across entire visible spectrum. The demonstrations with metasurfaces make them for feasible applications of analyzing the optical properties of unknown chiral matters. For future perspective, one can illuminate the incident light (which is to be measured) onto the metasurface chip individually, which enables resolving both polarization property and spectral information together using one single device. Compared with commercial ellipsometry, our device shows reliable performance on polarization analysis as well as spectral measurement with more compact dimensions. We envisage that the on-chip polarimeter will pave an innovate
It means that the absorption and
depolarization of the sample are not involved in the analysis, which is reliable because the testing samples are transparent and optically flat. Under these assumptions, stable measurement results with minimized alignment tolerance can be attained. Figure 4b shows the measured Stokes parameters as a function of wavelength from our on-chip polarimetry. Here, the Stokes parameters are normalized for making the total light intensity as unity: S1 =
S1′ S1′ 2 + S2′ 2 + S3′ 2
S3 =
, S2 =
S2′ S1′ 2 + S2′ 2 + S3′ 2
,
S3′ 2
S1′ + S2′ 2 + S3′ 2
(1)
where S0 = S12 + S22 + S32 = 1. According to our measured results, one can obviously analyze the birefringence and dichroism of the testing samples over the entire visible spectrum. Indeed, the Stokes parameters can be described as following for further evaluating the polarization state with ellipticity and azimuth angles: ⎡ S0 ⎤ ⎤ ⎡ 1 ⎢ ⎥ ⎥ ⎢ ⎢ S1 ⎥ cos 2θ cos 2ε ⎥ ⎢ S = ⎢ ⎥ = I0 ⎢ cos 2θ sin 2ε ⎥ ⎢ S2 ⎥ ⎥ ⎢ ⎢S ⎥ ⎦ ⎣ sin 2θ ⎣ 3⎦
(2)
where I0, θ, and ε is the total intensity, ellipticity angle, and azimuth angle, respectively. The polarization state of the light beams, which is the outcome of the y-polarized light interacts 2571
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class for the development of flat optical devices in polarization control, chiral molecule detections, and material analysis, just named a few.
METHODS Fabrication of On-Chip Polarimeter. All metasurface chips are fabricated by standard electron beam lithography and lift-off process. A 30 nm thick SiO2 is deposited using plasmaenhanced chemical vapor deposition (PECVD) on a 150 nm thick aluminum mirror followed by a bare silicon substrate. The 495 K PMMA photoresist was then deposited by spin coating and prebaked at 180 °C for 3 min. After electron beam writing with 30 pA current, the sample was developed in a solution of isopropyl alcohol (IPA) and methyl isobutyl ketone (MIBK) of IPA/MIBK = 3:1 for 60 s. The sample was subsequently rinsed with IPA for 60 s and blow-dried with nitrogen gun. Aluminum with the desired thickness was then deposited via electron beam evaporator. The aluminum metasurfaces were consequently defined on the preprepared substrate after the lift-off process. Numerical Simulation. All simulation results were performed with the commercial software CST Microwave Studio. For the case of single cell design, a unit cell boundary condition is employed for the simulation of reflection and phase shift in an array structure. The refractive index of SiO2 is obtained from ref 43. The permittivity of aluminum in the visible regime is described by the Drude model, with bulk plasma frequency of 14.4 eV and damping constant of 1.1 eV. It has been shown that the oxidation layer of the Al barely affects its optical properties under off-resonant condition. Therefore, the oxidation layer of the Al is not considered. ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b01527.
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Additional analytical details (PDF).
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Pin Chieh Wu: 0000-0002-5781-9696 Din Ping Tsai: 0000-0002-0883-9906 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from Ministry of Science and Technology, Taiwan (Grant Nos. MOST-1062745-M-002-003-ASP and 104-2221-E-259-028-MY3) and Academia Sinica (Grant No. AS-103-TP-A06). They are also grateful to National Center for Theoretical Sciences, NEMS Research Center of National Taiwan University, National Center for High-Performance Computing, Taiwan, and Research Center for Applied Sciences, Academia Sinica, Taiwan for their supports. 2572
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