Nonreciprocal thermal metamaterials:Methods and applications

Nonreciprocity of thermal metamaterials has significant application prospects in isolation protection,unidirectional transmission,and energy harvesting.However,due to the inherent isotropic diffusion law of heat flow,it is extremely difficult to achieve nonreciprocity of heat transfer.This review presents the recent developments in thermal nonreciprocity and explores the fundamental theories,which underpin the design of nonreciprocal thermal metamaterials,i.e.,the Onsager reciprocity theorem.Next,three methods for achieving nonreciprocal metamaterials in the thermal field are elucidated,namely,nonlinearity,spatiotemporal modulation,and angular momentum bias,and the applications of nonreciprocal thermal metamaterials are outlined.We also discuss nonreciprocal thermal radiation.Moreover,the potential applications of nonreciprocity to other Laplacian physical fields are discussed.Finally,the prospects for advancing nonreciprocal thermal metamaterials are highlighted,including developments in device design and manufacturing techniques and machine learning-assisted material design.

Spatiotemporal Fourier transform with femtosecond pulses for on-chip devices

On-chip manipulation of the spatiotemporal characteristics of optical signals is important in the transmission and processing of information.However,the simultaneous modulation of on-chip optical pulses,both spatially at the nano-scale and temporally over ultra-fast intervals,is challenging.Here,we propose a spatiotemporal Fourier transform method for on-chip control of the propagation of femtosecond optical pulses and verify this method employing surface plasmon polariton(SPP)pulses on metal surface.An analytical model is built for the method and proved by numerical simulations.By varying space-and frequency-dependent parameters,we demonstrate that the traditional SPP focal spot may be bent into a ring shape,and that the direction of propagation of a curved SPP-Airy beam may be reversed at certain moments to create an S-shaped path.Compared with conventional spatial modulation of SPPs,this method offers potentially a variety of extraordinary effects in SPP modulation especially associated with the temporal domain,thereby providing a new platform for on-chip spatiotemporal manipulation of optical pulses with applications including ultrafast on-chip photonic information processing,ultrafast pulse/beam shaping,and optical computing.

Quantized vortices in spinor Bose–Einstein condensates with time–space modulated interactions and stability analysis

The three-component Gross–Pitaevskii equation with an angular momentum rotational term can be served as a model to study spinor Bose–Einstein condensates (BECs) with time–space modulated interactions. Vortex solutions of the spinor BECs with spatiotemporally modulated interactions are worked out by similarity transformation. Theoretical analysis and numerical simulation of vortex states are demonstrated. Stable vortex states are obtained by adjusting the frequency of the external potential and the spatiotemporally modulated interaction.

Non-reciprocal Sound Transmission in Electro-acoustic Systems with Time-Modulated Circuits

Non-reciprocal sound transmission is demonstrated in an electro-acoustic system consisting of two shunt loudspeakers with time-modulated circuits.The shunt circuit is modulated by periodically varying the resistance in time sequence,and a phase difference of time modulation is set between two sets of loudspeakers to produce a spatial bias.The spatiotemporal modulation of acoustic properties is thus formed to break the reciprocity.The theoretical model based on the transfer-matrix method is developed to predict acoustic scatterings of electro-acoustic systems.Acoustic asymmetric transmission in opposite directions is disclosed by the model and is verified by time-domain simulation results based on the finite-difference time-domain method.Asymmetric transmission in multiple frequency bands can be created by tuning circuit parameters,showing their ability in regulating acoustic non-reciprocal behavior.This study may provide a platform for the design of compact and non-reciprocal acoustic devices with applications to efficient noise control.