The non-conservative nature of optical forces has been explored many times previously. Non-conservative optical forces occur in many guises and include lateral forces due to shape asymmetry, polarisation dependant optical torques and spin-dependant effects. When considering the design of optical force actuators, one recent approach has been to exploit the periodic structure of 2-dimensional metamaterial surfaces. In some cases, extremely large optical forces have been reported (Zhang et al, Optics Letters, 39, 4883 (2014)). Most recently macroscopic forces and torques have been demonstrated (Magallanes and Brasselet, Nature Photonics, 12, 461 (2018)). Here we consider refractive-index patterning of a substrate, achieved through a photo-lithographic approach. In this paper we will explore the use of different types of patterning, and show how the use of a periodic structure can enhance the optical forces and torques that may be generated. Using computational electromagnetic techniques we will demonstrate how enhancements of two orders of magnitude in the optical force are available in specific cases. We will examine the influence of such surface structuring on the resultant forces and torques, with a view to optimising such 2-dimensional materials for applications as light-driven actuators. Further, we will demonstrate the sensitivity of the forces generated to variations in the local environment, opening up possibilities for optical sensing applications.
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