
- Photonics Research
- Vol. 10, Issue 4, 1031 (2022)
Abstract
1. INTRODUCTION
As a technique to fully engineer the wavefronts of light, holography has demonstrated remarkable modulation abilities for both free-space beams [1–4] and surface waves [5–7], and facilitated applications of wavefront shaping, data storage, three-dimensional display, and so on. However, for optical holographic video display and complex optical encryption, the bandwidth is still limited, and undesired diffraction orders exist in using traditional approaches. The metasurface, proposed as a novel artificial planar element with subwavelength units, has become a powerful platform for hologram recording in recent years. It can overcome the above limitations and also provide unprecedented spatial resolution and a large field of view (FOV). Over the past decade, delicately designed meta-atoms have shown flexible light manipulation properties, such as amplitude [6,8–13], phase [8–15], polarization [14,16–21], orbital angular momentum (OAM) [22–26], and frequency [12,25,27–31]. Through coding the meta-atoms with diverse holograms in different optical channels, holographic information capacity can be further augmented, and multiplexed functions are available [8,10–14,17–27,31–33].
Among the fundamental light properties, polarization records the vectorial nature of light containing rich invisible information to human eyes. Due to the unique advantages of metasurfaces in designing anisotropic optical response, polarization multiplexed holography based on various metasurface design strategies has emerged [8–12,14,18–21,25,34–37], and greatly promotes potential applications such as optical document security and optical switching devices. However, most schemes are dependent on orthogonal polarization states [14,19,21,24], rather than using all states on the full Poincaré sphere, which decreases the versality and information capacity of polarization multiplexed holography. As one of the most widely used strategies, the spatial multiplexing method [8,20,34,35] relying on supercells suffers from low conversion efficiency and undesired diffraction orders. Especially for supercell metasurfaces tailored by the detour phase [8,14,36], oblique incidence becomes necessary and does not adapt to applications working under normal illumination. So far, progress has put forward challenging requirements for an advanced encoding method and more delicate metasurface design to achieve greater polarization modulation possibilities, more information capacity, and high conversion efficiency.
Here, we propose a novel metasurface encoding method to achieve polarization multiplexed holography, which can realize diverse holographic mappings from one full-Stokes space to another efficiently with subwavelength units. As shown in Fig. 1, based on metasurfaces encoded with vectorial holograms with unbounded possibilities, we can selectively address the intensity distributions in the transmitted field according to input and output polarization states, which greatly improves the availability of polarization modes and information capacity. The delicately designed metasurfaces have high polarization conversion in the circular polarization channels, and can achieve independent phase modulations that combine dynamic phase and geometric phase. With the help of a hybrid genetic algorithm, we generated phase-only holograms to synthesize multiple vectorial fields based on the designed meta-atoms. By loading the time-varying polarization channels in sequence, a holographic video and complex optical encryption with double secret keys have been experimentally demonstrated in real time. Such a scheme opens new avenues for multi customizable polarization modulations, and is expected to be used in dynamic display, dynamic optical manipulation, optical encryption, and anticounterfeiting.
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Figure 1.Principle of polarization multiplexed holography for time sequence holographic video and optical encryption based on metasurfaces. (a) Time-dependent image frames encoded in diverse linear polarization states ranging from zero to
2. RESULTS AND DISCUSSION
A. Principle of Polarization Multiplexed Holography
Mathematically, at a given input polarization
Next, we describe the recording rules of vectorial holography to achieve dynamic modulations by diverse input/output polarization channels. Since arbitrary
When we use another polarized input
B. Metasurface Design
Next, we introduce the design strategy of metasurface digitalization. First, we developed an efficient approach to design meta-atoms with separated and sufficient phase modulations in
Figure 2.Metasurface design and digitalization based on hybrid genetic algorithm. (a) Schematic illustration of phase modulation by combining dynamic phase and geometric phase. (b) Simulated results for the amplitude and dynamic phase of
Then we designed a hybrid genetic algorithm to calculate the phase-only holograms of RCP and LCP channels based on the structural database. As shown in Fig. 2(c), we first generated the initial metasurface by randomly setting the lengths, widths, and rotations based on the above nanofins. The constructed fields of both circularly polarized channels
C. Experimental Characterizations
We designed and fabricated two metasurfaces to demonstrate our method. As described in Fig. 1, the first is the time sequence holographic display of a prose poem, and the second is an optical encryption composed of a metasurface and two separate keys. Nano fabrication depends on the standard electron beam lithography and inductively coupled plasma reactive ion etching process. Scanning electron microscopy images of sample 1 (dynamic display) and sample 2 (optical encryption) are shown in Fig. 3(a) and Fig. 3(b). Both metasurfaces have a footprint of
Figure 3.Scanning electron microscopy images of the fabricated samples and the experimental setup. (a), (b) Scanning electron microscopy images of sample 1 (time sequence holographic video) and sample 2 (optical encryption), composed of diverse cross sections and rotations. (c) Experimental setup for vectorial holography, which works at various input/output polarization channels.
1. Time Sequence Holographic Video
To facilitate practical application, we encoded the time-dependent frames with linearly polarized input that orbits around the equator of the Poincaré sphere, and the analyzer in the output space was fixed without any adjustment. For the
Here, we make
Figure 4.Design principle of the time sequence holographic video of Tagore’s poem. (a) Time-dependent frames encoded in the linearly polarized input, which round the equator of the Poincaré sphere. (b) The preset phase modulation
Above all, the dynamic display of holographic videos with high performance has been experimentally realized. It does not require adjustment of the analyzer at the output end, which is more practical for system integration. By increasing the multiplexed number, the movie can be encoded with more information. Furthermore, different display requirements are also available by the polarization multiplexed strategy. For example, for videos played in linearly polarized input/output channels, the display time of sub-images can be precisely customized according to the ellipticity, and real-time editing, rewinding, and inserting of the movie can be easily realized based on diverse polarization selections. Compared with other multiplexed schemes, such as utilizing incident wavelength, OAM, and nonlinear frequencies as multiplexing parameters, the polarization multiplexed strategy shows the advantage of simple manipulation. Various functionalities can be realized by modulating the input/output polarizations. In contrast, nonlinear frequency multiplexing needs high incident intensity, large nonlinear susceptibility, and a complex design to meet phase matching conditions. OAM multiplexing relies on a spatially distributed phase profile generated from spiral plates or spatial light modulators. Furthermore, polarization multiplexing based on our method can facilitate applications of compact devices with the integration of liquid crystal.
2. Optical Encryption with High Complexity
The first design mainly depends on linear polarization multiplexing of input/output channels. In the second design, we extended the polarization modulation range to the full-Stokes space, and a demonstration of optical encryption with high security is provided as follows. The secret information is hidden in multiple polarization multiplexed images, which has many possibilities due to distinct input/output polarization selections. As shown in Fig. 5(b), the designed pattern of a dove with five wings and leaves has seven polarization states, which can convert an arbitrary incident polarization state (except for circular polarization states) into different polarization states in the output full-Stokes space. Upon
Figure 5.Design principle of the optical encryption with double secret keys. (a) First secret key provided by input polarization states. The blue dots represent some deceptive keys, and the blue star marked at
Figure 6.Experimental results of sample 1 at diverse input and output polarization selections. The blue arrows and orange arrows are polarization states of input and output, respectively.
For the use of optical encryption, we set the first secret key as
Figure 7.Experimental results of sample 2 at six diverse input and output polarization selections. The blue arrows and orange arrows are polarization states of input and output, respectively.
Figure 8.Theoretical results of sample 2 at six diverse input and output polarization selections. The blue arrows and orange arrows are polarization states of input and output, respectively.
3. CONCLUSION
In conclusion, we proposed and realized polarization multiplexed holography based on metasurfaces optimized by a hybrid genetic algorithm, which demonstrated the viability and versatility of time sequence dynamic display and optical encryption. Through full-Stokes polarization transformations and delicate metasurface design, multiple and independent input/output polarization modulations can be customized flexibly in two full-Stokes spaces. The subwavelength feature of the nanofin without coherent pixels or any spatial multiplexing not only decreases fabrication difficulties, but also ensures a larger FOV. Based on the efficient and independent phase encoding of a circular polarization basis, the output vectorial wavefront can be freely customized within an observable numerical aperture that reaches 0.80 (see Appendix A, Note 2) and conversion efficiency reaches 40.24%. The designed holographic videos with large information capacity have more displayed possibilities by using different polarization combinations. The optical encryption shows significantly improved performance compared with other polarization encryption schemes in security and data capacity. This method is expected to be used in dynamic display and beam shaping, polarization detection, holographic tweezers, optical encryption/anticounterfeiting, and so on.
4. METHODS
APPENDIX A
This section includes the following.
Figure
Figure
Figure
Note 1. Conversion efficiency of metasurfaces (including Figs.
Figure 9.Conversion efficiency and broadband behavior of sample 1. (a) Data processing of holographic construction efficiency, using ROIs generated by the algorithm to extract effective information. (b) Polarization conversion efficiencies of four circularly polarized channels. (c) Conversion efficiencies
Figure 10.Conversion efficiency and broadband behavior of sample 2. (a) Data processing of holographic construction efficiency, using ROIs generated by the algorithm to extract effective information. (b) Polarization conversion efficiencies of four circularly polarized channels. (c) Conversion efficiencies
Note 2. Numerical aperture of vectorial holography.
Note 3. Dynamic display of the designed vectorial patterns.
Other materials for this paper include Visualizations 1 and 2.
The conversion efficiency of polarization multiplexed holography is defined as the ratio of effective diffracted power with circular polarization conversion to incident power. It equals the product of polarization conversion efficiency and holographic reconstruction efficiency. That is,
First, we measured and calculated the polarization conversion efficiency of circularly polarized channels based on the optical setup shown in Fig.
Furthermore, to get accurate efficiency for each vectorial component of the holographic image, we calculated the holographic reconstruction efficiency of circularly polarized channels based on an image recognition algorithm. Through such data processing, we defined the region of interest (ROI) of collected images and extracted effective information. ROIs at different wavelengths were generated according to comparisons between collected images with the designed patterns, which serve as a mask to extract holographic imaging. By calculating the ratio of extracted information to the initial images as shown in Figs.
The method we used ensures that each subwavelength unit is connected with all vectorial fields, which is totally different from the spatial multiplexing scheme and guarantees a larger FOV. The numerical aperture (NA) of metasurfaces is defined as
Here, we used varied linearly polarized input and
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