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Ultrathin nonlinear metasurface for optical image encoding Felicitas Walter, Guixin Li, Cedrik Meier, Shuang Zhang, and Thomas Zentgraf Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00676 • Publication Date (Web): 14 Apr 2017 Downloaded from http://pubs.acs.org on April 15, 2017
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Ultrathin nonlinear metasurface for optical image encoding Felicitas Walter1, Guixin Li2, Cedrik Meier1, Shuang Zhang3, and Thomas Zentgraf1 1
Department of Physics, University of Paderborn, Warburger Str. 100, 33098 Paderborn, Germany
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Department of Materials Science and Engineering, Southern University of Science and Technology, 1088 Xueyuan Ave, Shenzhen, 518055, China
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School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, United Kingdom
ABSTRACT Security of optical information is of great importance in modern society. Many cryptography techniques based on classical and quantum optics have been widely explored in the linear optical regime. Nonlinear optical encryption, in which encoding and decoding involve nonlinear frequency conversions, represents a new strategy for securing optical information. Here, we demonstrate that an ultrathin nonlinear photonic metasurface, consisting of meta-atoms with three-fold rotational symmetry, can be used to hide optical images under illumination with a fundamental wave. However, the hidden image can be read out from second harmonic generation (SHG) waves. This is achieved by controlling the destructive and constructive interferences of SHG waves from two neighboring meta-atoms. In addition, we apply this concept to obtain grey-scale SHG imaging. Nonlinear metasurfaces based on space variant optical interference open new avenues for multi-level image encryption, anticounterfeiting and background free image reconstruction. KEY WORDS: metasurface, plasmonics, nonlinear, second harmonic generation, Pacharatman-Berry-phase
Common optical components such as prisms, lenses and diffractive elements shape the wavefront of light through amplitude and phase changes along different optical 1 ACS Paragon Plus Environment
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paths. In comparison, metasurfaces, consisting of spatially variant meta-atoms, enable abrupt phase changes of light and thus can manipulate the wavefront of light at the subwavelength length scale1–6. With this technique various functional metasurfaces for anomalous refraction and reflection, beam shaping, and optical holography were successfully demonstrated7–11. Such optical metasurfaces consist of plasmonic or dielectric meta-atoms with anisotropic optical response. Assuming each meta-atom has an in-plane orientation angle of , the spin angular momentum for some incident photons along the normal direction will be converted from ℏ to – ℏ whereas ∈ −1; 1 stands for the circular polarization state of the incident photon. During the conversion a geometric Pancharatnam-Berry (P-B) phase of 2 is acquired8,9. Based on the concept of geometric P-B phase12,13, dual polarity metalenses, high efficiency metasurface holography, high numerical aperture metalenses, optical spin Hall effects and so on have been successfully demonstrated7–10,14,15.
Recently, the concept has been successfully extended to optical metasurfaces operating in the nonlinear regime16–18. It was shown that the nonlinear geometric P-B phase of each meta-atom is determined by the spin of fundamental wave, the spin of the nonlinear signal, and the order of the nonlinear process16. Based on nonlinear PB phase elements, nonlinear optical spin-orbit interaction and holography have been recently demonstrated19,20. Actually, both linear and nonlinear holography represent possible optical encryption schemes. However, the holography method usually involves complex encoding algorithms and sophisticated read-out procedures at the Fourier plane8,19–22. Here, we propose a grey-scale image encroding by manipulating the interferences of SHG waves from two meta-atoms in one pixel. By locally controlling the phase difference of the nonlinear polarization between neighboring 2 ACS Paragon Plus Environment
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meta-atoms, we successfully demonstrate spatially variant SHG for real space image generation directly from the metasurface interface. With this controllability, both the image encryption and reconstruction are easily performed in the real space by an ultrathin metasurface device. The easy and direct control of nonlinear processes directly on the nanoscale might become very important for the future development of nonlinear optoelectronic devices. Furthermore, optical encryption of information on ultrathin metasurfaces that can only be recovered at certain optical conditions promise great applications in information security.
Figure 1 shows the working principle of real space image encryption of the character ‘META’ by using a nonlinear metasurface. Such a metasurface would not show any particular information if one observes it under incoherent and unpolarized white light illumination in the visible spectral domain (Figure 1a). The metasurface would be only homogeneously brighter than the substrate due to the slightly higher reflectivity (lower row in Figure 1). Under the illumination with intense coherent near infrared (NIR) laser light at a wavelength near the localized plasmon resonance of the gold meta-atoms, an image in the reversed circular polarization state will become visible for the SHG and thus the encrypted information can be read out as optical image in the VIS (Figure 1b). However, if the metasurface is illuminated by incoherent and unpolarized light at the SHG wavelength of the NIR light, the encrypted image information cannot be retrieved (Figure 1c). This confirms that the optical information is only coded in the nonlinear optical process but not in the linear optical processes since each meta-atom exhibits isotropic linear optical property and therefore its orientation angle does not affect the linear scattering process (for more details supplementary information). An image encryption in the SHG signal was already obtained by using nonlinear circular dichroism of chiral meta-atoms23,24 . However, 3 ACS Paragon Plus Environment
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the linear circular dichroism of such chiral meta-atoms provides access to the image information for the fundamental light.
Figure 1. Working principle of the nonlinear metasurface for different illumination conditions. a. If the metasurface is illuminated with incoherent and unpolarized visible light no image in the visible spectral range can be observed. b. Under pumping of circularly polarized near-infrared laser at the regime of 1240 nm, an image with the character appears at the SHG wavelength (620 nm). c. However, for a direct illumination with incoherent and unpolarized light at a wavelength of 620nm, no information of the ‘META’ characters is observable. The bottom row shows the corresponding measured real space microscopy images of the metasurface observed under different conditions (scale bar 20 µm). Note that the experiment is performed in transmission mode and that the META image cannot be resolved with this optical setup
To experimentally verify the working mechanism of the real space encoding, we fabricated metasurfaces that consist of gold meta-atoms with three-fold (C3) rotational symmetry (Figure 2a). For a circularly polarized () fundamental wave propagating along the rotational axis of the C3 meta-atom, only SHG with opposite optical spin (−) is generated, which acquires a nonlinear geometric P-B phase of exp 3, whereas is the in-plane orientation angle of the meta-atom16. By placing two meta-atoms with different orientation angles into one unit-cell (i.e. one virtual 4 ACS Paragon Plus Environment
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pixel), the SHG signal from the two meta-atoms will interfere in the far field and lead to a rotation angle dependent SHG intensity. For the case that both meta-atoms have the same orientation angle , the interference of SHG waves is constructive and the maximum SHG signal can be obtained. For all other cases where the angle between them is different will result in a reduced SHG intensity. Therefore, the spatially variant control of the SHG signal can be realized on the metasurface by controlling the orientation angles of two neighboring meta-atoms. To simplify the encoding process of a nonlinear image, we use two C3 meta-atoms with angles and – relative to the vertical line as one virtual pixel (Figure 2b). As the electric field of the SHG wave of each meta-atom is proportional to exp3, the total SHG intensity from the virtual pixel is then given by: ~|exp ∙ 3 + exp− ∙ 3| ~ cos 3. Based on the above design, we fabricate meta-atom arrays with area size of 100x100 µm² on an ITO/glass substrate by using electron beam lithography and liftoff technique. The orientation angle of the C3 meta-atom in each metasurface array varies from 0° to 60° in steps of 7.5°. Figure 2c shows scanning electron microcopy images of the meta-atom pairs with various angles. The sizes of these meta-atoms are designed to show a localized plasmon resonance in the NIR regime. The arm length, arm width and thickness of each meta-atom are 165 nm, 60 nm and 30 nm, respectively. The period was chosen with 500 nm in both x- and y- directions.
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Figure 2. Metasurface design and linear optical properties. a. Schematic of the metaatoms with C3 rotational symmetry. The meta-atoms are arranged in a periodic pattern with 500 nm period in both x- and y-directions. b. The rotation angle of two neighboring metaatoms in one virtual pixel varies from 0° to 60° in steps of 7.5°. c. Scanning electron microscope image of the fabricated meta-atoms for various rotation angles (scale bar: 200 nm). d. Measured transmission spectrum of a metasurface made of C3 meta-atoms with unpolarized light. The pronounced resonance dip at wavelength of 1240 nm corresponds to the localized plasmon resonance. The vertical lines mark the spectral position of the used laser wavelength and the resulting SHG.
The linear transmission of the fabricated metasurfaces is characterized in the range 1000 nm - 2000 nm with a Fourier Transformation Infrared spectrometer and in the range 450 nm – 1000 nm with a home-build white light transmission setup. Figure 2d shows the measured transmission normalized to that of the bare substrate. The
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transmission exhibits a pronounced dip at wavelength around 1240 nm, which results from the localized plasmon resonance of the gold C3 meta-atoms. Next, we experimentally verify the capability of controlling the SHG intensity from nine different meta-surfaces, where the orientation angle between the metaatoms within the unit-cell is varied. Under illumination with a circularly polarized fundamental wave, the SHG from the metasurface was collected in transmission direction by a 20x microscope objective. After passing a circular polarization analyzer, the SHG wave was analyzed by a spectrometer with EMCCD detector. As shown in Figure 3a, the SHG intensity from the virtual pixels of nine different metasurfaces was experimentally characterized at a wavelength of 1250 nm. As expected from our theoretical prediction, the rotation angle dependent measured SHG intensity exactly follows the function of cos 3 . If equals to zero and 60°, the SHG intensity reaches the maximum value, whereas for = 30°, the SHG intensity reaches the minimum. As expected from the selection rule for SHG on C3 metaatoms16,25 ， the circular polarization of the SHG wave is opposite to that of the fundamental wave ( → " and " → ). The SHG with same polarization ( →
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" → ") as the fundamental wave is forbidden. The selection rule is verified by the experimental results (Figure 3a) where only for the converted polarization a pronounced SHG signal is observed. Regarding linear polarized light as a superposition of right and left circular light, the SHG intensity of the metasurface follows the same function of cos 3 under illumination with light of any linear polarization (supplementary information figure S2). Hence, the polarization of the fundamental light will not influence the nonlinear encoding. Next, we measure the rotation angle dependent SHG intensity for different excitation wavelengths from 1250 nm to 1400 nm in steps of 50 nm (Figure 3b). To 7 ACS Paragon Plus Environment
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simplify the measurement procedure, only the left circularly polarized fundamental wave is used. We find that the rotation angle dependent SHG modulation is independent of the employed wavelength, i.e. it shows a dispersionless behavior for all wavelengths. The highest SHG signal is observed for the excitation wavelength of 1250 nm as it is closest to the localized plasmon resonance frequency of the metaatoms. When the fundamental wavelength increases from 1250 nm to 1400 nm, the SHG intensity continuously drops. Nevertheless, we always obtain the minimum of the SHG signal at the rotation angle of 30°.
Figure 3. SHG intensities of the nonlinear metasurfaces. a. Angle dependent SHG intensity for excitation with a fundamental wave at 1250 nm wavelength for various polarization schemes. The first letter in the legend describes the circular polarization state of the fundamental wave, the second letter the polarization state for SHG detection (L - left circularly polarized, R - right circularly polarized). For a periodic metasurface ( # 0°), SHG with opposite circular polarization state compared to the fundamental wave (LR and RL) is much stronger than the SHG with same polarization state (LL and RR). The lines show the expected behavior based on the interference effect. b. Measured right circularly polarized SHG intensity in dependence of the orientation angle of meta-atoms for left circularly polarized fundamental wave with wavelength of 1250 nm to 1400 nm. 8 ACS Paragon Plus Environment
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The measurements demonstrate that the SHG intensity for homogeneous and periodic arrangement of meta-atom pairs can be precisely controlled. In the following, we apply this concept to encode the character ‘META’ into the metasurface. The background and the letters consist of C3 meta-atoms with orientation angle of # 30° and # 0°, respectively (Figure 4a). If illuminated with a circularly polarized fundamental wave in the NIR, we expect that the letter areas where # 0° show bright SHG emission, whereas the other areas with # 30° should give no SHG signal and therefore represent a dark background. In this way, an image of the encoded letters ‘META’ should become visible through the SHG process. However, the ‘META’ will be hidden in the metasurface if no SHG process from the meta-atoms is utilized. Figure 4b shows that a SHG image with bright letters ‘META’ can be detected on a low signal background. If we replace the short pass filter in front of the detection camera with a long pass filter to directly detect the fundamental wave at wavelength of 1250 nm, the letters ‘META’ disappear and a nearly homogenous background signal is observed (Figure 4c).
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Figure 4. Real space images of the metasurface. a. Spatial map of the rotation angle for the meta-atom pairs. The color indicates the angle between the C3 structures (blue: # 0°, red: # 30°). b. Obtained image at the SHG wavelength for excitation at the resonance wavelength of the meta-atoms showing the character ‘META’. c. Image of the metasurface for imaging the fundamental pump wavelength where no character can be seen. d. SHG image of the character ‘META’, the corresponding orientation angles of meta-atoms for the four letters are: θ=0o, 7.5o, 15o, 22.5o, respectively, leading to an intensity modulation.
To demonstrate the potential of our approach for continuously tailoring the encoded SHG images, we fabricated another metasurface sample, in which the ‘META’ letters have different brightness. We start with the letter ‘M’ as the brightest letter and end with the letter ‘A’ with the lowest SHG intensity. The chosen orientation angles of the meta-atoms in one virtual pixel are # 0°, 7.5°, 15°, 22.5° for the four letters. To obtain a dark background with no SHG signal, the orientation angle of the corresponding meta-atoms is # 30°. The measured SHG image is shown in Figure 4d. Due to the Gaussian intensity profile of the fundamental beam, the SHG intensity of the letters appear brighter closer to the center of the image. Nevertheless, a clear change of the SHG intensity of the four letters is observable. 10 ACS Paragon Plus Environment
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In conclusion we have proposed and demonstrated a simple way to encode information of an optical image into an ultrathin nonlinear metasurface. The encoded image can only be readout through the SHG signal of nonlinear meta-atoms and will be not observable for the fundamental illumination wavelength. The concept is based on a local interference effect of two neighboring meta-atoms exhibiting different geometric Pancharatnam-Berry phases for the SHG waves. By changing the orientation angle of two meta-atoms in one virtual pixel, continuous tailoring of the local SHG intensity can be obtained. Combined with highly nonlinear materials such as semiconductor multiple quantum wells, which can drastically enhance the SHG efficiency26, the real space image encoding with nonlinear metasurfaces opens new avenues for optical information security, nonlinear imaging and information encryption.
Methods Fabrication: The samples were fabricated via electron beam lithography. Before the 30 nm thick gold film a 2 nm thick chromium layer was evaporated onto the ITO/glass substrate which serves as a sticking layer. Then a lift-off procedure finished the fabrication.
Nonlinear optical experiment (Fig. 3): The different metasurface patterns were illuminated with coherent circular polarized light in the NIR from a femtosecond Titanium-sapphire laser pumped optical parametric oscillator (repetition frequency, 80 MHz; pulse duration, ∼200 fs;). The beam polarization was set to either left or right circular state. The light was slightly focused onto the metasurfaces. The emitted SHG signal in transmission direction was collected by a 20x microscope objective. Before coupling the light into a spectrometer it passed a polarization analyzing setup, to 11 ACS Paragon Plus Environment
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ensure that only the SHG light with a certain circular polarization state is measured (supplementary information Fig. S4). For all measurements the average excitation power was fixed to 10 mW and the measured signal was integrated over 1 s at the detector.
Measurement (Fig. 4): All SHG measurements were done with left circularly polarized light at the excitation wavelength of 1240 nm. The signal was integrated over 5 s and the power of the excitation beam was set to 52 mW. The excitation beam size was chosen such that the beam spot was large enough to cover the entire metasurface field (100x100 µm²). Then the metasurface was imaged through a short pass filter to a CCD-camera. As the fundamental beam itself is far more intense than the SHG signal, the image was taken at 0.8 mW integrated over 1 s.
AUTHOR INFORMATION Corresponding authors *Email:
[email protected] *Email:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGEMENTS This work was financially supported by the DFG Research Center TRR142 ‘Tailored nonlinear photonics’ (Grant No. TRR142/A05). G. Li is financially supported by China “Recruitment Program of Global Experts” and Peacock program of Shenzhen.
ADDITIONAL INFORMATION Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. 12 ACS Paragon Plus Environment
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