Bidirectional nanoprinting, has received significant attention in image display and on-chip integration, due to its superior advantages. By manipulating the amplitude in a narrow- or broad-band wavelength range of forward and backward incident light, different spatially varied intensities or color distributions can be generated on the structure plane. However, the current scheme cannot fully decouple the bidirectional light intensity due to the limitation of design degree of freedom, and it would hinder the development of asymmetric photonic devices. In this paper, we propose and demonstrate bidirectional nanoprinting based on an all-dielectric bilayer metasurface, which can independently control the intensity of forward and backward incident light, resulting in two different continuous grayscale meta-image displaying in the visible region. This asymmetric but still bidirectional optical response is introduced by stacking two layers of nanostructures with different functionality in space, in which the first- and second-layer nanostructures act as a half-wave plate and a polarizer, respectively. Interestingly, these bidirectional nanoprinting metasurfaces have flexible working modes and may bring great convenience for practical applications. Specifically, two different meta-images generated by a bidirectional nanoprinting metasurface can be displayed not only on two sides of the metasurface (working mode in transmission or reflection), but on the same side due to the forward transmitted light and backward reflected light also having asymmetric optical properties. Similar phenomena also exist for forward reflected light and backward transmitted light. Our work extremely expands the design freedom for metasurface devices and may play a significant role in the field of optical display, information multiplexing, etc.
The circular dichroism effect characterized by different optical responses between left-handed and right-handed circularly polarized lights is widely applied for biological monitoring, analytical chemistry, and plasmonic sensing. Despite the fact that circular dichroisms are achieved by many conventional chiral and anisotropic metamaterials, dynamic and efficient modulation of circular dichroisms is still challenging. In this paper, we demonstrate a VO 2 embedded metamaterial enabling tunable chirality by taking advantages of the VO 2 phase transitions between insulators and metals. Specifically, by changing the laser power and the irradiated position on the metamaterial, the VO 2 phase transition takes place at the irradiated region and induces a tunable circular dichroism effect. This work provides a strategy for the active control and modulation of circular polarizations, which pays the way for applications in terahertz and microwave regions.
Recently, the non-Hermitian optical system draws much attention due to their peculiar optical properties. In this paper, a non-parity-time symmetric muti-layer metasurface is proposed. The polarization-dependent unidirectional reflectionless (UR) effect associated with the exceptional point (EP) is investigated. By adjusting the geometric parameters of the metasurface, the EP based UR phenomenon can be observed at 2360 nm, and the simulation results are in good agreement with the theoretical work. Unlike other isotropic non-Hermitian platforms, the proposed UR phenomenon has a strong dependence on the incident polarization state, which guarantees its great application potential in the fields of near-field imaging and optical encryption, etc.
A chiral metamaterial based on vanadium dioxide (VO2) is proposed to realize the ultrafast active control of an electromagnetically induced transparency-like (EIT-like) effect. The design of the tunable metamaterial is based on the heat-induced insulator-to-metal transition of VO2 and a tunable transmission peak with circular polarization selectivity is achieved. For the right circular polarization (RCP) laser incidence, the sharp EIT-like peak is generated and rises with the increasing signal powers, which has not happened in the left circular polarization (LCP) incident condition. The spin-dependent phenomenon in time domain is also observed and the slow light effect in the metamaterial yields a tunable time delay up to 4 ns. Therefore, the tunable EIT-like effect with chiral selectivity is presented and highlights its practical applications in programmable metamaterial regions and slow light devices, etc.
Electromagnetically induced transparency (EIT) based on dielectric metamaterials has attracted attentions in recent years because of its functional manipulation of electromagnetic waves and high refractive index sensitivity, such as high transmission, sharp phase change, and large group delay, etc. In this paper, an active controlled EIT effect based on a graphene-dielectric hybrid metamaterial is proposed in the near infrared region. By changing the Fermi level of the top-covered graphene, a dynamic EIT effect with a high quality factor (Q-factor) is realized, which exhibits a tunable, slow, light performance with a maximum group index of 2500. Another intriguing characteristic of the EIT effect is its high refractive index sensitivity. In the graphene-covered metamaterial, the refractive index sensitivity is simulated as high as 411 nm/RIU and the figure-of-merit (FOM) is up to 159, which outperforms the metastructure without graphene. Therefore, the proposed graphene-covered dielectric metamaterial presents an active EIT effect in the near infrared region, which highlights its great application potential in deep optical switching, tunable slow light devices, and sensitive refractive index sensors, etc.
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