Dual-Mode Chiral Meta-Holography through All-Dielectric Nano-Surfaces in Visible Regime
Item Type Conference Paper
Authors Khaliq, Hafiz Saad;Naeem, Taimoor;Riaz, Kashif;Zubair, Muhammad;Mehmood, Muhammad Qasim;Massoud, Yehia Mahmoud
Citation Khaliq, H. S., Naeem, T., Riaz, K., Zubair, M., Mehmood, M. Q., &
Massoud, Y. (2022). Dual-Mode Chiral Meta-Holography through All-Dielectric Nano-Surfaces in Visible Regime. 2022 IEEE 22nd International Conference on Nanotechnology (NANO). https://
doi.org/10.1109/nano54668.2022.9928670 Eprint version Post-print
DOI 10.1109/NANO54668.2022.9928670
Publisher IEEE
Rights This is an accepted manuscript version of a paper before final publisher editing and formatting. Archived with thanks to IEEE.
Download date 2023-11-29 18:42:32
Link to Item http://hdl.handle.net/10754/685682
Dual-Mode Chiral Meta-Holography through All- Dielectric Nano-Surfaces in Visible Regime
Hafiz Saad Khaliq1, Taimoor Naeem2,3, Kashif Riaz3, Muhammad Zubair3, Muhammad Qasim Mehmood3, Yehia Massoud1
1Innovative Technologies Laboratories (ITL), Electrical and Computer Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia.
2Department of Electrical and Computer Engineering, COMSATS University Islamabad, Lahore Campus, Lahore 54000, Pakistan
3MicroNano Lab, Department of Electrical Engineering, Information Technology University (ITU), Lahore 54600, Pakistan.
Abstract— Chiro-optical effects have numerous applications in optical displays, chiral sensing, and chiral imaging. The simultaneous control of amplitude, phase, and polarization with chirality in metasurfaces provides a compact and cost-effective solution for these applications. Here, we propose a planar all- dielectric metasurface based on anisotropic nano-bars instead of conventional chiral structures to avoid the fabrication complexities and the narrow bandwidth at the nanoscale. The fundamental meta-atom of the proposed metasurface contains a pair of nano-bars introducing giant chiro-optical effects in dual transmission mode at wide bandwidth of visible regime. The proposed meta-nano-surface provides the maximum asymmetric transmission of ~77%. For the proof of concept, the phase mask of an image is encoded into the meta-nano-surface to reproduce chiral meta-hologram for circularly polarized light at RGB wavelengths. The demonstrated design provides a route to realize giant chiro-optical effects for promising application in optical displays, circular dichroism spectroscopy, circular polarizers, security, and imaging.
Keywords—All-dielectric; Chiral Metasurfaces; Chiro- Optical effects; Asymmetric Transmission; Chiral Sensing; Chiral Imaging
I. INTRODUCTION
Metasurfaces, a planar version of metamaterials containing an array of subwavelength featured meta-atoms, have been extensively used due to their ability to manipulate the properties (amplitude, polarization, and phase) of electromagnetic (EM) waves. These extraordinary controls have been utilized to demonstrate numerous exciting applications such as meta-lensing [1], polarization optics and communication [2], [3], reflective holography [4], light- structuring [5]–[10] etc. Moreover, the chirality inclusion in the metasurfaces introduces the extra degree of freedom and has been utilized to demonstrate a wide range of real-life applications. Chirality can be found in any object or molecule whole mirror image cannot be superimposed on itself [11].
Chiral molecules also termed “enantiomers,” have different handedness but can have the same energy levels and plays a crucial part in the pesticides and medicine industry for enantiomers separation and detection [12].
Previously, several research groups investigated chiral metasurfaces and unveiled their applicability in countless applications. Conversely, the absorption losses in the
plasmonic chiral nanostructures and their small thickness involved in light-matter interaction limit their potentia applicability in next-generation optical systems [4]. Apart from absorption losses in the metals, it is recently used to design photonic devices for various applications [13]–[19]. To mitigate these limitations in chiral metasurfaces, the general research trend shifts toward the implementation using high contrast all-dielectric materials. The compact chiral system with on-chip integration can have possible applications in advanced optical displays, chiral imaging, CD spectroscopy, data encryption, security systems, etc. However, chiral metasurfaces at nano-scale in visible regime with fabrication ease remain a challenge. The fabrication of chiral geometries at a nano-scale required advanced fabrication techniques.
Therefore, we adopt a novel way of designing a planar chiral meta-nano-surface based on a simplified chiral structure using anisotropic nano-bars. Along with the easy fabrication of these nano-bars, they also provide extra degrees of design freedom. The proposed meta-atom optimized for dual transmission mode and provides giant chiro-optical effects at wide-bandwidth in visible regime. The meta-nano-surface has the ability to reproduce the meta-hologram for one handedness of circularly polarized (CP) light whereas no visible information for the opposite handedness.
II. DESIGN MEHTODOLOGY AND RESULTS
The working principle of the proposed meta-nano-surface is demonstrated in Fig. 1. In forward propagation direction (+𝑧), the nano-surface reproduces meta-hologram for cross- polarized light of RCP illumination in visible regime at RGB (red (633 nm), green (532 nm) and blue (488 nm)) wavelengths whereas no visible information for LCP incident light. In contrast, for backward propagation direction (−𝑧) the nano-surface will reproduce meta-hologram for LCP illumination whereas no visible information for RCP incident light.
The fundamental optimized meta-atom of the proposed nano-surface is shown in Fig. 2. It contains a pair of anisotropic nano-bars with distinct geometrical parameters.
Hydrogenated amorphous silicon (a-Si:H) [5] is utilized to design meta-atom with silicon dioxide (SiO2) as a substrate.
𝑝𝑥= 580 𝑛𝑚 and 𝑝𝑦= 255 𝑛𝑚 is the periodicity of the meta-atoms in x and y-direction, respectively. 𝑙1= 200 𝑛𝑚,
𝑙2= 205 𝑛𝑚, 𝑤1= 100 𝑛𝑚 and 𝑤2= 75 𝑛𝑚 are the lengths and widths of both nano-bars with optimal values.
The central displacement is denoted by 𝑑 whereas ∆𝜙 is relative rotation between both nano-bars. The simultaneous control of geometrical parameters, central displacement, and relative rotation between nano-bars provides full control on amplitude, polarization, and phase to induce chiro-optical effects in terms of asymmetric transmission.
Fig. 1. The working principle of proposed diatomic meta-nano-surface for wideband chiral holography.
Fig. 2. (a) Perspective view of designed chiral meta-atom (b) Top view of chiral meta-atom with geometrical parameters.
The semi-analytical analysis of the proposed diatomic meta-atom using Jones calculus demonstrates the transmission matrices for both nano-bars which can be expressed as in equations (1) and (2),
𝑇1= 𝑅(−𝜙1) [𝑒𝑖𝜏1 0
0 𝑒𝑖(𝜏1+𝜋)] 𝑅(𝜙1) (1)
𝑇2= 𝑅(−𝜙2) [𝑒𝑖𝜏2 0
0 𝑒𝑖(𝜏2+𝜋)] 𝑅(𝜙2) (2)
where 𝑇1 and 𝑇2 are the Jones matrices in transmission for nano-bar 1 and 2, respectively. The spatial rotations for nano- bars can be denoted as 𝜙1 and 𝜙2 and 𝑒𝑖𝜏1 and 𝑒𝑖𝜏2 are the propagation phases.
After the conversion of matrices into circular basis followed by the addition of both transmission matrices and the inclusion of coupling effects, the final transmission matrix can be written as [5]
𝑇𝑇𝑐𝑖𝑟= 𝑒𝑖𝜏[0 𝑒−𝑖2𝜙1
0 0 ] (3)
By multiplying the final transmission matrix for diatomic meta-atom with CP light, it demonstrates the maximum transmission for RCP illumination whereas the maximum absorption for LCP incident light which proves the inclusion of chiro-optical effects in the proposed structure. However, for the optimal value selection of geometrical parameters to realize chiro-optical effects, the structure is optimized using the Finite Difference Time Domain (FDTD) method.
Fig. 3. Transmittance parameters for the optimized meta-atom when the CP light is illuminated in the forward direction. Maximum transmittance for the RCP incident light.
Fig. 4. Transmittance parameters for the optimized meta-atom when the CP light illuminated in the backward direction. Maximum transmittance for the LCP incident light.
Fig. 5. Asymmetric transmission (AT) parameters in both forward and backward propagation direction for the diatomic chiral meta-atom.
In forward propagation direction, the optimized chiral diatomic chiral structure introduces giant asymmetric transmission as depicted in Fig. 3 where 𝑇𝐿𝑅 denote the transmittance for RCP incident and LCP transmitted light. It
describes the maximum transmittance for the cross-polarized parameter of RCP illumination at a wide range of visible wavelengths whereas the minimum transmittance for LCP incident light. In contrast, the opposite results were obtained when the light was illuminated in the backward direction as shown in Fig. 4. This figure demonstrates the maximum transmittance for the cross-polarized parameter of LCP incident light which described the dual-mode operation of the designed chiral structure with the polarization-dependent transmission.
Fig. 6. Full phase coverage for the chiral structure using the geometric phase modulation technique at RGB wavelengths for wavefront shaping.
Fig. 5 depicts the asymmetric transmission (AT) parameters for the designed chiral meta-atom in the dual- mode propagation direction. AT can be defined as the difference between cross-polarized parameters in transmission for CP incident light. For the designed structure, it shows the giant AT at a wide range of visible wavelengths.
The maximum obtained efficiency of AT is ~ 77% at the wavelength of 567 nm. To use a meta-atom for wavefront engineering, it is essential to optimize it for full phase modulation. Fig. 6 demonstrates the full phase (0-2π) coverage for designed meta-atom at RGB wavelengths.
III. WIDEBAND CHIRAL META-HOLOGRAPHY
As proved from the previous section that the proposed chiral meta-atom provides the simultaneous control on amplitude, polarization, and phase to induce the giant chiro- optical effects in terms of asymmetric transmission at wideband visible wavelengths. Hence, the optimized meta- atom can be utilized to design a meta-nano-surface for chiral holography. Fig. 7a depicts the computer generated hologram (CGH) of an image of the moon. The CGH image includes 100 pixels in x-direction whereas 215 pixels in the y- direction. The phase mask of that CGH is implemented in a meta-nano-surface using optimized diatomic meta-atom.
The total size of the simulated meta-nano-surface is 55 × 55 µ𝑚2 . The meta-nano-surface simulated at RGB wavelengths to reproduce chiral meta-holograms. Fig. 7b &
7c depicts the simulated results for the meta-nano-surface at the wavelength of 488 nm for RCP and LCP illumination, respectively. It reproduces the meta-hologram for RCP
incident light and no visible information for LCP illuminations. Similarly, Fig. 7d-7g depicts the reproduced chiral meta-holograms for RCP and LCP incident light at the wavelength of 532 nm and 633 nm. In contrast, Fig. 8 depicts the reproduced chiral meta-holograms when light illuminated in backward propagation direction and visible information obtained for LCP incident light. These reproduced meta- hologram results verify the wideband giant chiro-optical effects in the proposed compact chiral meta-atoms and outperforms the state of the art chiral metausurfaces[20].
Fig. 7. (a) Computer Generated Hologram. Simulated reproduced meta- hologram in forward propagation direction for (b), (e), (f) RCP and (c), (e), (g) LCP incident light at the wavelength of 488 nm, 532 nm and 633 nm, respectively.
Fig. 8. Simulated reproduced meta-hologram in backward propagation direction for (a), (c), (e) LCP and (b), (d), (f) LCP incident light at the wavelength of 488 nm, 532 nm and 633 nm, respectively.
IV. CONCLUSION
A planar chiral all-dielectric meta-nano-surface is demonstrated based on a unique chiral structure manifesting giant asymmetric transmission realized by reproduced meta- hologram at wide bandwidth in the visible regime. The proposed chiral structure contains a pair of anisotropic nano- bars instead of conventional chiral structures to avoid the fabrication complexities and the narrow bandwidth at the nanoscale. The maximum achieved asymmetric transmission is ~77%. The reproduced chiral meta-hologram for circularly polarized light at RGB wavelengths proves the simultaneous conversion of amplitude, polarization, and phase. The demonstrated meta-platform has promising application in optical displays, circular dichroism spectroscopy, security, and imaging.
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