Death and rebirth through nonlinear control

doi:10.1126/science.abg5441

GSTDTAP > 气候变化

DOI	10.1126/science.abg5441
	Death and rebirth through nonlinear control
	Piotr Roztocki; Roberto Morandotti
	2021-04-02
发表期刊	Science
出版年	2021
英文摘要	Nonlinear systems, characterized by outputs that are not proportional to their inputs, form the bulk of real systems in nature and applied science. Nonlinearity in a platform can substantially extend the range of its accessible functionalities, as seen in digital electronics and artificial neural networks, for example. However, there are many systems for which the impact of nonlinearity is not yet clear. This includes a complex class of systems that exhibits both topological and non-Hermitian aspects—i.e., a set of interesting properties invariant under continuous deformations and unconventional operators that describe the total system energy, respectively. In such systems, nonlinear control is, thus far, underexplored . On page 72 of this issue, Xia et al. ([ 1 ][1]) bridge the investigation of non-Hermitian topological physics with nonlinearity, leading to implications for complex systems in acoustics, plasmonics, polaritonics, and ultracold atoms. The field of topological photonics was founded from developments in condensed-matter physics, particularly from ideas relating to the celebrated quantum Hall effect ([ 2 ][2]) and topological insulators ([ 3 ][3]). Analogous effects were shown to be accessible in a variety of photonics platforms, which enabled major fundamental and engineering advances, including the demonstration of electromagnetic states protected from scattering ([ 4 ][4], [ 5 ][5]), topological insulator lasers ([ 6 ][6], [ 7 ][7]), and others. In turn, studies of non-Hermitian physics and parity-time (PT) symmetry countered the conventional belief that quantum Hamiltonians had to be Hermitian. Indeed, a weaker condition (commutation with the PT operator) was determined sufficient to enable real spectra ([ 8 ][8]). Such systems interact with their environments through complex potentials, the imaginary part of which determines whether a system is gaining or losing energy. The exploration of analogous systems in optics, with photons experiencing well-designed gain and loss, has enabled rich investigations of fundamental physics ([ 9 ][9]–[ 12 ][10]) while also establishing new engineering paradigms through PT-based devices and sensors ([ 12 ][10]). ![Figure][11] Controlling states in a complex system An optical platform uses waveguides to explore nonlinear control of a complex system. The ability to use local control to change the global character of the system makes it a flexible strategy for investigating complex phenomena. GRAPHIC: C. BICKEL/ SCIENCE However, although the interplay of topology and non-Hermitian physics is already subject to active and productive study ([ 6 ][6], [ 7 ][7], [ 13 ][12], [ 14 ][13]), nonlinear effects have been underexplored in the joint context of these disciplines. Investigations dedicated specifically to the interplay of all three properties are rare to nonexistent. Xia et al. aim to address this research gap and explore a direction in nonlinear non-Hermitian topological photonics. Toward this end, the authors introduce an easy-to-access, versatile platform for the exploration of nonlinearity in such systems, based on the reconfigurable writing of an optical waveguide lattice into a biased photorefractive crystal (see the figure). Structures of this type, called Su-Schrieffer-Heeger lattices ([ 15 ][14]), are an established tool for topological studies in photonics. Using a bias field, Xia et al. are able to modify the waveguide nonlinear response, causing optical self-focusing and defocusing effects that change how light propagates throughout the lattice. Although this nonlinearity affects only the real part of the bulk refractive index, this equivalently tunes both the real and imaginary parts of the waveguide potential function. This degree of freedom enables Xia et al. to demonstrate destruction and restoration of non-Hermitian topological states through nonlinear control. In particular, the authors show that a state initially PT-symmetric in the linear regime can be destroyed by introducing optical nonlinearity. The inverse also holds, as an initially non–PTsymmetric state can be nonlinearly tuned to restore PT symmetry. Counterintuitively, these results show that topology and PT symmetry, which broadly describe global properties of complex systems, can be tuned using optical nonlinearity, which is a local effect. This demonstrates, in a sense, an interplay of local and global effects. Xia et al. also study another pair of seemingly antagonistic effects—the sensitivity of their system near exceptional points (EPs) versus the robustness of topologically protected states. EPs are degenerate points specific to the operation of non-Hermitian systems and characterized by enhanced sensitivity to perturbation ([ 12 ][10]). EP physics are thus especially interesting when combined with the protection from perturbations that topological systems can offer ([ 3 ][3], [ 7 ][7]). Xia et al. find that although the nonlinear system “inherits” topological protection from its linear counterpart, this stability wears off away from the protected mode in a way that is dependent on how close the system is to the EP. Xia et al. open the door for investigating this overlap of disciplines, but many questions remain. There is a clear need for a general theoretical framework to describe the nonlinear driving of non-Hermitian topological systems, as well as for an extension of current symmetry classification methods to such systems. The potential impact of the authors' work on the development of new devices is also exciting. 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/321125
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Piotr Roztocki,Roberto Morandotti. Death and rebirth through nonlinear control[J]. Science,2021.
APA	Piotr Roztocki,&Roberto Morandotti.(2021).Death and rebirth through nonlinear control.Science.
MLA	Piotr Roztocki,et al."Death and rebirth through nonlinear control".Science (2021).