Abstract
Recently, spatiotemporally modulated metamaterial has been theoretically demonstrated for the design of Doppler cloak, a technique used to cloak the motion of moving objects from the observer by compensating for the Doppler shift. Linear Doppler effect has an angular counterpart, i.e., the rotational Doppler effect, which can be observed by the orbital angular momentum (OAM) of light scattered from a spinning object. In this work, we predict that the spatiotemporally modulated metamaterial has its angular equivalent phenomenon. We therefore propose a technique to observe the rotational Doppler effect by cylindrical spatiotemporally modulated metamaterial. Conversely, such a metamaterial is able to cloak the Doppler shift associated with linear motion by generating an opposite rotational Doppler shift. This novel concept is theoretically analyzed, and a conceptual design by spatiotemporally modulating the permittivity of a voltage-controlled OAM ferroelectric reflector is demonstrated by theoretical calculation and numerical simulation. Finally, a Doppler cloak is experimentally demonstrated by a spinning OAM metasurface in radar system, which the spatiotemporal reflection phase are mechanically modulated. Our work presented in this paper may pave the way for new directions of OAM carrying beams and science of cloaking, and also explore the potential applications of tunable materials and metasurfaces.
Highlights
The possibility of cloaking an object from detection by an external observer has recently become a topic of considerable interest [1,2]
We discretize the device in both space and time, and an 8-element cylindrical spatiotemporally modulated array is proposed, which is termed as a voltage-controlled orbital angular momentum (OAM) ferroelectric reflector
Simulation and calculation results show that the tunable OAM ferroelectric reflector can generate a frequency shift when it is in stationary
Summary
The possibility of cloaking an object from detection by an external observer has recently become a topic of considerable interest [1,2]. We explore the potential applications of rotational Doppler effect from spinning OAM metasurface, we demonstrate the Doppler cloak concept by an OAM metasurface with both linear moving and spinning in Doppler radar system, a zero-frequency shift is experimentally observed. It means that the Doppler radar detects the OAM metasurface as stationary but the fact that it is moving. Different from the time-varying metasurface generating linear Doppler effect [7,8,9], the proposed cylindrical spatiotemporally modulated metamaterial generating OAM beam with rotational Doppler effect can mimic the motion spinning, which may find more applications for detection of spinning objects. The proposed approach is expected to broaden the applications of OAM carrying beams, explores the potential application of ferroelectric materials and metasurfaces, and promises important advantages in scenarios such as Doppler cloak, Doppler radar deception and Doppler cancellation in wireless communication [20]
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