Doppler cloak restores invisibility to objects in relativistic motion

Abstract

Although cloaks are effective at suppressing the observability of static objects, they can be defeated when in motion. Here we discuss a general technique to cloak the motion of objects from static observers, based on compensating the Doppler shift associated with their motion with frequency conversion sustained by a spatiotemporally modulated cover. The concept is theoretically and numerically demonstrated in a system composed of a planar reflector covered by a spatiotemporally modulated slab. It is shown that, for properly selected modulation frequency, the composite system can appear to an external observer as stationary, even though it is actually moving. This concept may pave the way to the minimization of clutter produced by moving objects as well as to new directions in the science of cloaking

A Dynamically Modulated All‐Dielectric Metasurface Doublet for Directional Harmonic Generation and Manipulation in Transmission

“…Dynamic modulation of metasurfaces adds a new dimensionality to the design space which can potentially overcome several limitations associated with the quasi‐static tunable metasurfaces. Furthermore, it can give rise to entirely novel functionalities such as Doppler cloaking, extreme energy accumulation, and nonreciprocal isolation and circulation . In order to realize time‐modulated metasurfaces, a high‐speed dynamic metasurface with continuous tunability is required such that its optical parameters can be modulated in time by applying a time‐varying biasing signal.…”

“…It is noteworthy that the pure frequency mixing in the time‐modulated metasurface mimics the Doppler shift of a moving object at the velocity of v = ( f 0 2 − ( f 0 + f m ) 2 ) c /( f 0 2 + ( f 0 + f m ) 2 ) with c being the velocity of light which leads to a velocity illusion. Moreover, the up‐ or downconversion of frequency can also be used to cancel out the Doppler shift of a moving object at this speed depending on its travelling direction realizing a Doppler cloak, that is, cloaking the motion from an stationary observer …”

“…This creates a new paradigm for designing dynamic metasurfaces with 2π phase span and uniform amplitude. Furthermore, time‐modulated metasurfaces give rise to several exotic space‐time scattering phenomena and can be envisioned for a variety of novel applications such as magnetless nonreciprocal components, Doppler cloaking and illusion, pulse shaping, extreme energy accumulation, and localization of light . Development of all‐dielectric time‐modulated metasurfaces is of particular interest as they can enable directional harmonic generation and manipulation in transmission mode by establishing a Huygens' operation regime, eliminating the bidirectionality of harmonics generated by time‐modulated metasurfaces with single isolated resonances.…”

Progress and perspective on advanced cloaking metasurfaces: from invisibility to intelligent antennas

“…Furthermore, we have shown that electronic components loading the antenna coating device may introduce even more degrees of freedom by tailoring the antenna response to the waveform. The antenna capabilities may be enriched even more through the use of cloaking devices integrating time-modulated metasurfaces [123], further paving the way to a new generation of intelligent antennas.…”

Metamaterials with high degrees of freedom: space, time, and more

“…The research on time-varying electromagnetic platforms has a long history dating back to 1950s, when one of the early works, by Morgenthaler [17], explored theoretically what would happen to a monochromatic electromagnetic wave in a medium when the medium's phase velocity is rapidly changed in time. In the past decades, there have been numerous efforts in investigating various phenomena related to the temporal variation of electromagnetic media, providing temporal boundaries [18], temporal holography [19], Doppler cloaking [20], temporal band gaps [21][22][23], temporal effective medium concept [24], temporal impedance matching [25,26], Fresnel drag in spatiotemporal metamaterials [27], spacetime cloaks [28,29], modeling time and causality with metamaterials [30,31], rapidly growing plasma and accelerating reference frame [32], and exceptional points in timevarying media [33], just to name a few.…”