Chiral meta-atoms rotated by light

Mingkai Liu, David A. Powell, Ilya V. Shadrivov
2012 Applied Physics Letters  
We study the opto-mechanical properties of coupled chiral meta-atoms based on a pair of twisted split-ring resonators. By using a simple analytical model in conjunction with the Maxwell stress tensor, we capture insight into the mechanism and find that this structure can be used as a general prototype of subwavelength light-driven actuators over a wide range of frequencies. This coupled structure can provide a strong and tunable torque, and can support different opto-mechanical modes, including
more » ... al modes, including uniform rotation, periodically variable rotation and damped oscillations. Our results suggest that chiral meta-atoms are good candidates for creating sub-wavelength motors or wrenches controlled by light. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737441] Using light to manipulate objects or even to drive a machine is fascinating. It is well known that the interaction between light and matter is usually accompanied by the transfer of momentum or angular momentum. Many tools and devices based on this principle have been developed, such as optical tweezers, 1,2 optical wrenches, 3,4 and optical motors. 5,6 These light-driven tools were shown to make special contributions in the fields like micro-fluidics 7 and biophysics. 8 Basically, there are two ways to generate an optical torque, one is to introduce the angular momentum directly from the illuminating wave by manipulating its polarization state or phase front 1,9,10 ; another is to use chiral structures, which scatter light into different angular momentum modes with unequal intensity. 11,12 However, due to the small dielectric constant (which also means a weak light-matter interaction) in most dielectric materials, early studies of optical torque often required birefringent or chiral structures that are of the order of (or even orders of magnitude larger than) the working wavelength to generate a sufficiently large torque. 13 Recently, it was demonstrated that by taking advantage of the plasmonic resonance in a metallic chiral structure, a strong radiation torque can be generated with a nanoscale plasmonic motor. 14 It would be inspiring progress to generate strong torque with a sub-wavelength actuator, since this is the key step towards highly integrated light-driven systems. The advent of metamaterials and plasmonic structures paves the way to achieve strong light-matter interaction within a subwavelength scale, and the coupled structures further offer us the chance to tune their performance through near-field interaction. 15, 16 Twisted dimers, such as twisted split-ring resonators 17,18 and cut-wire pairs, 19 are a particularly interesting class of coupled chiral meta-atoms. These chiral structures exhibit strong linear or even nonlinear optical activity near the resonances. 20 Although much work has been done on their electromagnetic properties, their opto-mechanical properties are still largely unexplored. In this letter, we provide an approach to achieve a strong and tunable torque with a subwavelength chiral meta-atom. We study the radiation torque generated by a twisted splitring resonator pair (TSRRP) when illuminated by linearly polarized electromagnetic waves. With a simple analytical model based on near-field interaction, we study the mechanism and show that such structures can offer two rotational frequency bands (RFBs), in which the direction of the torque does not change as the structure rotates. The prediction from this simple model shows good overall agreement with full-wave calculations based on the Maxwell stress tensor. Moreover, we study the dynamics of this structure under the influence of friction and show that such a simple coupled structure can offer rich opto-mechanical modes, which enables the TSRRP to function as an optical motor or an optical wrench at different frequencies. Unlike the chiral structure shown in Ref. 14, the TSRRP does not rely on plasmonic resonance, and it can generate strong torques with opposite directions within a much narrower bandwidth, which further facilitate its implementation and application from microwave to optical frequency regimes. The TSRRP studied here is illustrated in Fig. 1 . The two identical SRRs (denoted as "1" and "2") are offset by a distance s in the z direction, and the twist angle between them is fixed at h. The incident plane wave propagates along the z axis, with a linear polarization in the x direction. Since the structure is chiral, the TSRRP may experience a radiation torque even under the illumination of linearly polarized waves, and the rotation angle is denoted as U. To understand the opto-mechanical properties of the structure, we first need to find its electromagnetic response, then we use its frequency-dependent charge and current distributions to calculate the radiation torque under different rotation angles, and finally we can solve the dynamic equations of the system. As our previous papers showed, 18, 19 under the single mode approximation, we can separate the current and charge of a single SRR into a frequency-dependent mode amplitude Q m ðxÞ (m ¼ 1; 2) and spatial distributions j(r) and q(r). To describe the coupling of the system, the following coupled equations are employed, a)
doi:10.1063/1.4737441 fatcat:vpyljghiafbgdb2g4rfdca5ohq