Meta-Holograms with Full Parameter Control of Wavefront over a 1000

Sep 4, 2017 - Metasurfaces offer promising structures for controlling the wavefront of light. The development of such structures is evidence for numer...
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Letter pubs.acs.org/journal/apchd5

Meta-Holograms with Full Parameter Control of Wavefront over a 1000 nm Bandwidth Zhenwei Xie,†,‡ Ting Lei,†,‡ Guangyuan Si,§ Xianyou Wang,† Jiao Lin,†,§,∥ Changjun Min,*,† and Xiaocong Yuan*,† †

Nanophotonics Research Center, Shenzhen University and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, Guangdong, China § Melbourne Centre for Nanofabrication, Australian National Fabrication Facility, Clayton, VIC 3168, Australia ∥ School of Engineering, RMIT University, Melbourne, Victoria 3001, Australia S Supporting Information *

ABSTRACT: Metasurfaces offer promising structures for controlling the wavefront of light. The development of such structures is evidence for numerous ways to alter on demand light properties such as amplitude, phase, and polarization. However, the simultaneous control of all parameters of light over a wide bandwidth is still a great challenge. With polarization multiplexing, we have achieved the lesser goal of simultaneous control of phase and amplitude over a 1000 nm bandwidth using a plasmonic nanoslit array associated with the traditional detour phase. In a proof-of-concept experiment, we demonstrate 3D object reconstruction and polarization multiplexing images at various prescribed wavelengths from 473 to 1550 nm using a specially designed meta-hologram. Benefiting from high controllability of amplitude, phase, and polarization, metaholograms offer great potential in future applications such as 3D displays, optical communications, and beam shaping. KEYWORDS: meta-hologram, metasurface, wavefront control, full parameter control, holography

T

powerful method, it is usually converted to phase-only modulation using a phase retrieval algorithm.30,40 However, for certain applications, such as complex laser-beam shaping or three-dimensional (3D) holography, the phase retrieval algorithm would have low efficiency and be time-consuming; therefore, controlling the phase and amplitude has become a priority. Attempts to manipulate all aspects of electromagnetic waves have been made over recent years.26,29,41,42 Instead of designing new geometric forms of optical antennas to realize specific phase and amplitude control, many researchers simply use binary amplitude modulation to add an extra degree of freedom in meta-hologram generation.26,29 However, two-level amplitudes and eight-level phases impose severe constraints in practical applications. An alternative solution for tailoring both phase and amplitude by combining multilayer metasurfaces was proposed,41 but the complexity in fabrication rapidly increases with layer alignment being a major hurdle. An approach that uses the geometrical configuration of the antenna to control the antenna phase and orientation in order to tailor the amplitude has been demonstrated in the terahertz regime.42 However, transferring the scheme to the visible regime is very difficult because of the complexity in fabrication. Moreover, the

he concept of metasurface continues to attract enormous interest for the fantastic and unlimited ways of tailoring the wavefront of light at subwavelength scale by altering their phase, amplitude, or polarization.1−3 Originally, the metasurface was introduced by demonstrating an instant phase jump at subwavelength thickness scale using V-shaped optical antennas.4 Subsequently, numerous types of optical antennas were proposed that realized phase modulation using optical resonance or geometric orientation.5−8 Through phase gradients, metasurfaces inducing arbitrary phase profiles from the visible and infrared to terahertz wave bands were reported with diverse applications as ultrathin meta-lens,9−14 in orbital angular momentum generation and detection,15−17 in the photonic spin Hall effect,18 in versatile polarization generation and detection,19,20 in chiral-dependent multifunctional devices,20−23 in producing meta-holograms,22,24−31 in generating multiholograms for the broadband visible light,32 in spin and wavelength multiplexed nonlinear meta-holograms,33 and as high-efficiency dielectric metasurfaces.34−38 Most of the optical antennas demonstrated were designed only for manipulating phase profiles of electromagnetic waves, regardless of the phase tailoring mechanism, for example, symmetric and asymmetric resonance,4 Pancharatnam−Berry phase,6 and magnetic and electric dipoles.37,39 Although complex amplitude and phase modulations is still the most © XXXX American Chemical Society

Received: July 3, 2017 Published: September 4, 2017 A

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Figure 1. Schematics illustrating principle and structural design of the meta-holograms. (a) Phase and amplitude control of the two typical unit cells. The cell is composed of several nanoslits in an aluminum film on a glass substrate. P denotes the size of a single unit cell, m and n denote number of slits in regard to the amplitude modulation of the cell. The two cells induce a phase shift Δφ at diffraction angle θ that depends on cell separation d. The diffraction angle is obtained from θ = arcsin(λ/P). (b) Typical 3D object reconstructed from a 3D meta-hologram. The enlarged unit cell shows that it contains m vertically oriented nanoslits that only respond to horizontal polarization. For a single unit cell, its phase φ is defined as a function of the distance between the center of the nanoslit array and the center of the unit cell. If the meta-hologram is illuminated with horizontally linear polarized light, the corresponding first diffraction order with a diffraction angle of θ shows the desired 3D object. (c) Scheme for the polarization multiplexed meta-hologram. The enlarged unit cell shows that it is composed of two sets of nanoslits with orthogonal orientations. Similarly, horizontally polarized light reconstructs one object at the horizontal diffraction order, and vertically polarized light reconstructs the other object in the vertical direction.

of light passing through these nanoslits can be controlled by the number of slits. According to the detour effect,43,44 the phase difference Δφ along a given direction θ between these two adjacent cells is

operation bandwidth would also be limited because of the intrinsic nature of the symmetric and asymmetric resonance modes. Here, we propose and demonstrate a simple and effective approach to simultaneously control phase and amplitude over a 1000 nm bandwidth, from visible to near-infrared (NIR). The proposed meta-hologram relies on a unit cell design that combines the detour phase and a plasmonic nanoslit array in an aluminum film on a glass substrate. It provides continuous phase modulation and 10-level amplitude control with two orthogonal polarization multiplexing and is easily fabricated. In comparison with other pure phase tailoring approaches, the proposed meta-hologram may open the way for precise, continuous, and complete complex wavefront engineering.

Δφ =

2πD sin θ λ

(1)

where D is the distance between the centers of the two sets of nanoslit arrays and λ the wavelength of incident light. Therefore, the phase shift of the light diffracted from two adjacent unit cells along direction θ can be controlled by adjusting distance D (for more detail discussion about the phase modulation of a unit cell, please see Supporting Information, section 1). If D sin θ = nλ, with n an integer, the diffracted light from these two cells are in-phase; if D = 0 ∼ λ/sin θ, the corresponding phase shift changes from 0 to 2π. The diffraction angle θ is determined by the period length of the unit cell P (see Figure 1a), with typically θ = arcsin(λ/P). Note that the maximum diffraction angle is 90°, and therefore, in theory, the operation wavelength for the meta-hologram is in the range from 0 to P. Here, the meta-hologram is designed for visible and NIR light, so the corresponding period size P is chosen as 2 μm. Hence, the induced detour phase would work with a broad band of wavelengths from 0 to 2000 nm. Figure 1b shows a schematic of a 3D object reconstruction at a diffraction



DESIGN OF THE META-HOLOGRAM Our approach combines the traditional detour phase and a plasmonic nanoslit array on a metal film. Whereas phase difference is induced by the detour effect, amplitude is controlled by the number of nanoslits, and the polarizationmultiplexing functionality is enabled by the polarizationdependent transmission of the plasmonic nanoslits. The specific unit cell design (Figure 1a) incorporates two-unit cells, each having several plasmonic nanoslits and the transmittance is proportional to the number of nanoslits. Hence, the amplitude B

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angle of θ by the proposed meta-holograms. One important advantage of this meta-hologram technique is that, as the amplitude and phase information on the 3D object is encoded simultaneously, images from meta-holograms are generated through an inverse Fourier transformation without needing any phase retrieval algorithm. Given that a unit cell of the meta-hologram is composed of several subwavelength plasmonic apertures, the transmittance of the cell is polarization-dependent.45,46 For a plasmonic nanoslit, TM polarized light excites surface plasmon polaritons (SPPs) that propagate through the deep-subwavelength aperture, whereas TE polarized light is reflected completely by the opaque metal without exciting SPPs. Therefore, we can use the polarization-dependent characteristics to design a polarization multiplexing meta-hologram. For this purpose, each unit cell is composed of two sets of plasmonic nanoslit arrays (Figure 1c). One set with a horizontal orientation diffracts only vertically polarized light, which forms the portrait of Tesla in the far field; the other set with vertical orientation diffracts only horizontally polarized light, which reconstructs the portrait of Einstein in the far field. When the incident light is polarized at 45°, both images are constructed. The complexity of fabrication is determined by the size of the unit cell, the size of the nanoslit, and the maximum number of nanoslits in a single cell. Given a fabrication feature size, the maximum levels for amplitude modulation are limited by the unit size. To choose the proper level, we numerically reconstructed several images of certain 3D objects from holograms with continuous phase changes and different levels of amplitude modulation, specifically, 1-level, 2-level, 4-level, 10-level, and continuous (see Supporting Information, section 1). From the reconstructed images, we observe that the image quality increases with increasing amplitude modulation level. The quality of the reconstructed image from the 4-level amplitude is already moderate, and that with 10-level amplitude is almost comparable to the image reconstructed from the continuous amplitude hologram. With a fixed slit width, the total transmittance is proportional to the number of slits. In considering transmittance and fabrication complexity, we adopted the 10-level amplitude modulation for imaging for the rest of the study. For a single plasmonic nanoslit, there is a trade-off between transmission efficiency and polarization selectivity. Efficiency increases with increasing width of slit; nevertheless, the polarization extinction ratio decreases. To optimize efficiency and polarization performance, the width of the slit is set at 100 nm, and the slit length set slightly smaller than the size of the cell (1.9 μm). The period of the nanoslit array is an important parameter which could affect the performance of a unit cell. We also investigate the transmittance and the polarization extinction ratio of a single unit cell with respect to different periods of the nanoslit array at prescribed wavelength from 473 to 1550 nm. Each unit cell contains four nanoslits with a width of 100 nm. From the simulation illustrated in Figure 2a, we can see that the corresponding period of the nanoslit array at the peak transmittance is varying with respect to the wavelength, due to the direct coupling between adjacent slits. Therefore, if it is for a narrow band design, one may choose the nanoslit array period depending on the operation wavelength. From Figure 2b, we can also find that the polarization extinction ratio almost remains the same as the period of the nanoslit varying. Considering that we aim for a broadband application, also taking the fabrication complexity in to account, we chose the

Figure 2. Broadband investigation for amplitude modulation and polarization extinction ratio: (a) transmittance and (b) polarization extinction ratio with respect to the period of the nanoslit array; (c) transmittance and (d) polarization extinction ratio with respect to the number of the nanoslits at various wavelengths.

period of the nanoslit array as 200 nm throughout the paper. We chose aluminum (Al) for the metal film instead of gold or silver because the substantial loss expanding from UV to NIR associated with the interband transitions in Al is smaller than that in the noble metals.19,24 To investigate the properties of the amplitude modulation and the polarization selectivity, we computed the transmittance and polarization extinction ratio against the number of nanoslits at six prescribed wavelengths between 473 and 1550 nm by using FDTD (Figure 2c). The amplitude transmittance is nearly linear with slit number for each wavelength, verifying the linearity of amplitude modulation over a bandwidth of more than 1000 nm. With 10 slits, the maximum transmittance is 89% at 532 nm, and above 75% at other wavelengths. In plotting the polarization extinction ratio versus slit number at various wavelengths from 473 to 1550 nm (Figure 2d), the polarization extinction ratio is defined as ⎛T ⎞ γ = 10· log⎜ TM ⎟ ⎝ TTE ⎠

(2)

where TTM and TTE are the transmittances for TM and TE polarized light. The trend shows a slight decrease in the extinction ratio initially that subsequently flattens to a constant value with increasing slit number. However, even the lowest extinction ratio at 473 nm wavelength is large at 78 dB; hence, the meta-hologram would be excellent in polarization multiplexing over a very wide band ranging from 473 to 1550 nm.



RESULTS AND DISCUSSION As proof of simultaneous amplitude and phase control with this meta-hologram technique, we designed a hologram of a 3D object containing the letter “E”, a portrait of Einstein, and the letters “mc2” projected onto different planes (z1, z2, and z3, respectively) in 3D space (Figures 1b and 3a). Letter “E” is projected behind the hologram into the plane 2.5 mm, and similarly for the portrait (2 mm) and “mc2” (1.5 mm). The hologram has an area of 0.4 mm × 0.4 mm with 200 × 200 pixels (unit cells). The complex amplitude distribution at the hologram plane is simply obtained using the inverse Fresnel transformation; then, its amplitude is “quantized” to the 10level (Figure 3b), along with the corresponding continuous C

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Figure 5. Polarization multiplexed hologram design: (a, e) Portraits of Einstein and Tesla from the original images of the holograms for horizontally polarized light and vertically polarized light, respectively. (b, f) and (c, g) Corresponding 10-level quantized amplitude distribution and continuous phase distribution in the hologram recording plane, respectively. (d, h) Numerically reconstructed images at 633 nm a wavelength with 0° and 90° polarizations, respectively.

Figure 3. Hologram generation for a 3D object. (a) Original 3D object comprises letter “E”, a portrait of Einstein, and letters “mc2”, located at three different planes (z1, z2, and z3, respectively). (b) The 10-level quantized amplitude distribution in the hologram recording plane. (c) The corresponding continuous phase distribution. (d−f) Numerically reconstructed images at 633 nm wavelength in the planes z1, z2, and z3, respectively.

trans-formed into a meta-hologram by encoding its amplitude into the slit number and its phase depending on the separation of the slit array. The transformed meta-hologram scheme is illustrated in Figure 4a, and the corresponding scanning electron microscope (SEM) images of the fabricated metahologram are presented in Figure 4b,c. Here, the nanoslits are

phase distribution (Figure 3c). From the numerically reconstructed images at 633 nm wavelength (Figures 3d−f), the letter “E”, the portrait, and letters “mc2” are clearly seen in their respective planes z1, z2, and z3. The designed hologram is

Figure 4. Experimental measurements for the 3D meta-hologram: (a) Scheme for the desired meta-hologram; (b) SEM image of the fabricated 3D meta-hologram (scale bar: 100 μm); (c) enlargement of framed area in (b); the width of the nanoslit is 100 nm (scale bar: 2 μm); (d) Measured reconstructed images from the 3D meta-hologram at several wavelengths from 473 to 1550 nm in the three different planes (z1, z2, and z3). D

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Figure 6. Experimental measurements for the polarization multiplexed meta-hologram: (a) Scheme of the polarization multiplexed meta-hologram comprising two meta-holograms with perpendicularly oriented nanoslits. (b) SEM image of the fabricated meta-hologram (scale bar: 100 μm), (c) a small area in (b), the width of the nanoslit is 100 nm (scale bar: 1 μm). (d) Measured reconstructed images from the meta-hologram at prescribed wavelengths from 473 to 1550 nm with 0°, 45°, and 90°, polarizations, respectively. White arrows denote polarization direction of the illumination.

two orthogonal polarizations. For horizontal polarization, the Einstein portrait is reconstructed (Figure 5a); for vertical polarization, the Tesla portrait is reconstructed (Figure 5e). The total size of the holograms is set as 0.32 mm × 0.32 mm, with 160 × 160 pixels. The reconstruction plane is 2 mm behind the hologram at 633 nm wavelength. The corresponding 10-level quantized amplitude distributions in the hologram plane are presented in Figure 5b,f, the corresponding phase in Figure 5c,g, and the numerical reconstructions at 633 nm wavelength in Figure 5d,h. The complex amplitude distribution in the hologram plane of the Einstein portrait is then encoded into a meta-hologram with vertically oriented nanoslits, and the Tesla portrait is encoded into another meta-hologram with horizontally oriented nanoslits. Combining these two metaholograms, we obtained a polarization multiplexing metahologram; the scheme of this combined meta-hologram is illustrated in Figure 6a; Figure 6b,c shows SEM images of the corresponding fabricated meta-hologram, in which we can see the two sets of nanoslits perpendicularly oriented in each unit cell that enable polarization multiplexing. From the experimentally reconstructed images at six prescribed wavelengths (Figure 6d), only the Einstein portrait is reconstructed if the incident light is horizontally polarized (Figure 6d, first column; here the white arrows denote the polarization of the illumination). If the incident light is polarized at 45°, both portraits appear (Figure 6d, second column). Finally, if the

vertically oriented, implying that they only respond to horizontally polarized light; therefore, all of the reconstructed 3D objects are horizontally polarized, regardless of their incident polarization. For the experimentally reconstructed 3D objects from the meta-hologram (Figure 4d), the incident light is horizontally linear polarized at the six prescribed wavelengths. Because the diffraction angle θ = arcsin(λ/P) increases with increasing wavelength of incident light, the reconstructed images are projected at different angles for different wavelengths. For each wavelength, we choose three different planes to clearly record the letter “E”, portrait of Einstein, and letters “mc2” (Figure 4d, first, second, and third row, respectively). This verifies the efficacy and the validity of the meta-hologram over a 1000 nm bandwidth. The small defects in the reconstructed images are primarily ascribable to separation inaccuracies of the nanoslits incurred in fabrication. The experimentally measured total efficiency ranges from 8%− 18%, with the highest efficiency at 1550 nm and the lowest at 473 nm. The efficiency of the meta-hologram is highly dependent on the average amplitude of the object and the diffraction orders. In theory, for an object with uniform amplitude, the efficiency can be increased up to 70% by adding a blazed phase that blazes the other diffraction orders into first order. To investigate the polarization selectivity and multiplexing of the proposed meta-holograms, we designed two holograms for E

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illumination is vertically polarized, only the Tesla portrait is reconstructed (Figure 6d, third column). Note that, at the diffraction order, Tesla’s portrait is vertical whereas Einstein’s portrait is horizontal (cf. Figure 1c; see also Supporting Information, section 3). Here, for convenience in making comparisons, we only show the portrait areas; the full images are given in Supporting Information. The measured polarization extinction ratios for all six prescribed wavelengths are in the range from 29.6 to 54.1 dB. Compared with the numerically simulated polarization extinction ratios (Figure 2b), the experimentally measured values are smaller because the crossing of the perpendicular sets of nanoslits reduces the polarization selectivity. From SEM images (Figure 6c), we see that the crossings of the nanoslits are slightly larger than the desired size, which would decrease the polarization extinction ratio. In conclusion, we have demonstrated a polarization multiplexing meta-hologram with simultaneous control of phase and amplitude covering a wide bandwidth ranging from 473 to 1550 nm. In the proof-of-concept demonstration, we designed, fabricated, and characterized two nanoslit meta-holograms. One meta-hologram reconstructed images of 3D objects under simultaneous phase and amplitude control; the other metahologram generated portrait images of Einstein and Tesla using polarization selectivity and multiplexing. The measured efficiency of the proposed meta-hologram is as high as 18% at 532 nm wavelength, and the measured polarization extinction ratio is up to 54.1 dB at 1550 nm wavelength. By adding a blazed phase in the unit cell design which could blazes the other diffraction orders into first order, the efficiency could be further improved. In comparison with the traditional hologram, the proposed meta-hologram is ultrathin, lightweight, and easily fabricated. The major advantage of the proposed meta-hologram over the existing meta-holograms is that it can simultaneously control phase and amplitude over a 1000 nm bandwidth, from visible to near-infrared (NIR). Therefore, there is only an inverse Fourier transformation needed in the hologram generation instead of a timeconsuming phase retrieval algorithm. It provides a new method to simultaneously alter on demand the phase and amplitude with polarization multiplexing and ultrabroadband response. It may pave the way for 3D broadband displays, beam shaping, and other short-pulse or broadband applications.



Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhenwei Xie: 0000-0002-4526-9746 Jiao Lin: 0000-0002-3943-5947 Xiaocong Yuan: 0000-0003-2605-9003 Author Contributions ‡

Z.X. and T.L. contributed equally to this work. X.Y., C.M., and Z.X. developed the concept presented in this paper. Z.X. performed analytical and numerical modeling and designed the device. G.S., J.L., and X.W. fabricated the device. Z.X. and T.L. conducted measurements. X.Y. and C.M. supervised the entire project. Z.X., T.L., and C.M. wrote the manuscript. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 61427819, 61422506, 61675136, 61405121, 11604218), the Science and Technology Innovation Commission of Shenzhen (Grant No. KQCS2015032416183980), the Fundamental Research Foundation of Shenzhen (Grant No. JCYJ20140418091413543), and the Natural Science Foundation of SZU (Grant Nos. 000011 and 000075). G.S. thanks the National Natural Science Foundation of China (Grant No. 61405031) for financial support. X.Y. appreciates the support given by the leading talents of Guangdong Province Program No. 00201505. T.L. acknowledges the support by Shenzhen Peacock Plan No. KQTD2015071016560101 and KQJSCX20160226193555889. K.Z. and F.D. (National Center for Nanoscience and Technology, Beijing, China) are acknowledged for discussions about the sample fabrication.



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00710. The design of the meta-holograms; Fabrication of the meta-holograms; Experimental methods; Figures S1−S4; and Table S1 (PDF). Video S1: Holograms switching at wavelength of 473 nm (AVI). Video S2: Holograms switching at wavelength of 532 nm (AVI). Video S3: Holograms switching at wavelength of 633 nm (AVI). Video S4: Holograms switching at wavelength of 785 nm (AVI). F

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DOI: 10.1021/acsphotonics.7b00710 ACS Photonics XXXX, XXX, XXX−XXX