Huygens’ Metadevices for Parametric Waves

Abstract

Huygens' metasurfaces have demonstrated almost arbitrary control over the shape of a scattered beam, however its spatial profile is typically fixed at fabrication time. Dynamic reconfiguration of this beam profile with tunable elements remains challenging, due to the need to maintain the Huygens' condition across the tuning range. In this work, we experimentally demonstrate that a time-varying meta-device which performs frequency conversion, can steer transmitted or reflected beams in an almost arbitrary manner, with fully dynamic control. Our time-varying Huygens' metadevice is made of both electric and magnetic meta-atoms with independently controlled modulation, and the phase of this modulation is imprinted on the scattered parametric waves, controlling their shapes and directions. We develop a theory which shows how the scattering directionality, phase and conversion efficiency of sidebands can be manipulated almost arbitrarily. We demonstrate novel effects including all-angle beam steering and frequency-multiplexed functionalities at microwave frequencies around 4 GHz, using varactor diodes as tunable elements. We believe that the concept can be extended to other frequency bands, enabling metasurfaces with arbitrary phase pattern that can be dynamically tuned over the complete 2π range. arXiv:1807.08873v3 [physics.app-ph] 2 Dec 2018 AUTHOR CONTRIBUTION M. Liu conceived the idea, performed the theoretical, numerical and experimental studies, with support from

Revealing the Buildup Dynamics of Harmonic Mode‐Locking States in Ultrafast Lasers

“…Note that, in the raised relaxation oscillation, the second laser spike is stronger than the first one, contrary to the damped relaxation oscillation where the second one is weaker than the first one. [15,57] Each laser spike contains a large number of sub-nanosecond pulses, as shown in Figure 3a. The obvious beating behavior is observed from 1.9 × 10 4 to 2.08 × 10 4 RTs.…”

“…Figure 5a illustrates the Fourier transform of each single-shot spectrum in Figure 2b, corresponding to a field autocorrelation of the momentary pulses, which traces the evolution of temporal separation between pulses. [15,51] Figures 5b and 5c are enlarge-ments of the A and B regions in Figure 5a Figure 4b,c), and in succession, weaker modulation instability is induced by SPM. [58] The SPM-induced instability gradually increases until 1.06 × 10 5 RTs (an example is shown as black curves in Figure 4b,c), finally leading to the pulse splitting which gives birth to five pulses, as shown in Figure 5c.…”

“…[1][2][3][4][5][6][7][8][9] Usually, fiber lasers have a fundamental repetition rate of tens to hundreds of MHz due to the difficulty of shortening the laser cavity. [10][11][12][13][14][15] To increase the pulse repetition rate of fiber lasers, a less technically challenging and more convenient way is the harmonic mode-locking (HML) scheme, where multiple pulses are evenly spaced in the cavity. [16] Stable HML operations of fiber lasers have been successfully achieved by means of active mode-locking, [17,18] passive mode-locking, [19][20][21] and subloops.…”

Active Anisotropic Coding Metasurface with Independent Real‐Time Reconfigurability for Dual Polarized Waves

“…To solve the problem of fixed wave-manipulations by passive metasurfaces, more efforts have been devoted to realize active control of EM behaviors of metasurfaces upon the external stimulations by incorporating with tunable materials or components, [30,31] such as diodes, [32][33][34][35] graphene, [36,37] phase-change materials, [38,39] etc. Recently, coding metasurfaces have been proposed to control the wave functionalities using unique particles with, for example, either the "0" or "1" state in 1-bit coding case.…”

“…linearly polarized excitation, [32][33][34][35][36][37][38] leaving much potentials unexploited. Undoubtedly, the active anisotropic metasurfaces for independent dual-polarized manipulations of EM waves will further increase their information capabilities, bringing new degrees of freedom in achieving versatile tunable functionalities that are highly desirable from application aspects.…”