Observation of Asymmetric Transport in Structures with Active Nonlinearities

Abstract

A mechanism for asymmetric transport based on the interplay between the fundamental symmetries of parity (P) and time (T ) with nonlinearity is presented. We experimentally demonstrate and theoretically analyze the phenomenon using a pair of coupled van der Pol oscillators, as a reference system, one with anharmonic gain and the other with complementary anharmonic loss; connected to two transmission lines. An increase of the gain/loss strength or the number of PT -symmetric nonlinear dimers in a chain, can increase both the asymmetry and transmittance intensities.PACS numbers: 42.25.Bs, 11.30.Er Directed transport is at the heart of many fundamental problems in physics. Furthermore it is of great relevance to engineering where the challenge is to design on-chip integrated devices that control energy and/or mass flows in different spatial directions. Along these lines, the creation of novel classes of integrated photonic, electronic, acoustic or thermal diodes is of great interest. Unidirectional elements constitute the basic building blocks for a variety of transport-based devices such as rectifiers, pumps, molecular switches and transistors.The idea was originally implemented in the electronics framework, with the construction of electrical diodes that were able to rectify the current flux. This significant revolution motivated researchers to investigate the possibility of implementing this idea of "diode action" to other areas. For example, a proposal for the creation of a thermal diode, capable of transmitting heat asymmetrically between two temperature sources, was suggested in Ref.[1]. Another domain of application was the propagation of acoustic pulses in granular systems [2].A related issue concerns the possibility of devising an optical diode which transmits light differently along opposite propagation directions. Currently, such unidirectional elements rely almost exclusively on the Faraday effect, where external magnetic fields are used to break space-time symmetry. Generally this requires materials with appreciable Verdet constants and/or large size non-reciprocal devices -typically not compatible with on-chip integration schemes or light-emitting wafers [3]. To address these problems, alternative proposals for the creation of optical diodes have been suggested recently. Examples include optical diodes based on second harmonic generation in asymmetric waveguides [4] and nonlinear photonic crystals [5], photonic quasi-crystals and molecules [6], or asymmetric nonlinear structures [7]. Most of these schemes, however, suffer from serious drawbacks making them unsuitable for commercial or smallscale applications. Relatively large physical sizes are often needed while absorption or direct reflection dramatically affects the functionality leading to an inadequate balance between figures of merit and optical intensities. In other cases, cumbersome structural designs are necessary to provide structural asymmetry, or the transmitted signal has different characteristics than the incident one.In this Letter ...

PT-Symmetric Acoustics

“…However, when wave incidents come from the gain side, the nonlinearity kicks in and bends the "backbone" of resonance, resulting in strong asymmetric wave propagation and nonreciprocity at the same frequency without harmonic generation. Such asymmetric transport has already been demonstrated in electronics and optics [36][37][38][39][40]; however, acoustics remains an open area to be explored.…”

“…Several interesting physical features have been explored, such as power oscillations [28], unidirectional invisibility [29][30][31][32], the reconfigurable Talbot effect [33], and coherent perfect laser absorbers [34,35]. Moreover, in the nonlinear domain, PT symmetry has been used to realize potential optical isolators and circulators [36][37][38][39][40].…”

Interaction of sine-Gordon kinks and breathers with a parity-time-symmetric defect

“…This mathematical discovery has initiated numerous studies of open systems with balanced gain and loss even though the generality of this construction is under discussion [3]. Experimental setups have been offered to create PT -symmetric physical systems in optics [4][5][6][7][8], electronic circuits [9][10][11], as well as in mechanical systems [12].…”

Nonreciprocal wave scattering on nonlinear string-coupled oscillators

“…11 symmetry-breaking in two nonlinear microcavities has been described. Other proposals include left-handed metamaterials 12 , quasiperiodic systems 13 , coupled nonlinear cavities 14 or PT −symmetric waveguides [15][16][17] and transmission lines 18 . Extensions to the quantum systems 19,20 and nonlinear Aharanov-Bohm rings 21 have been also considered.…”