Optomechanical interaction occurs when light waves and mechanical waves are localized and couple with each other, exchanging energy through the inelastic light scattering [1,2]. This interaction comes from the photoelastic effect caused by the strain effect and the moving-boundary effect that is induced by the mechanical mode distortion of the optical boundary [3,4]. Optomechanical coupling could be manipulated in many sophisticatedly designed microstructures such as optomechanical crystal [5,6], whispering gallery mode cavities [[7], [8], [9]], and membrane resonators [10]. Utilizing optomechanical coupling could tailor photon-phonon interaction to promote the research progress in fundamental quantum theory [[11], [12], [13]], information process [[14], [15], [16]], sensing [17], and so on.

Large optomechanical interaction requires great confinement of both optical and mechanical fields. Various novel and complex structures have been designed and investigated to maximize the overlap between optical modes and mechanical modes. Devices of wavelength scale can provide considerable confinement not only for optical waves in the telecom band but also for acoustic waves of Gigahertz Hertz (GHz) frequency, which has already been demonstrated in previous works [[18], [19], [20], [21], [22]]. For many designed structures, the effective single-photon optomechanical coupling rates g0/2π are far below the level of MHz [6,[23], [24], [25], [26], [27]]. Recently, we have proposed and demonstrated that chain-like nanorod arrays can realize about 2 MHz optomechanical coupling rates by providing a significant overlap between light waves and acoustic waves [21,22]. Nevertheless, the active region is still constrained within a small area, hindering the practical applications. It should be noted that in addition to traditional microcavities and waveguides, metasurfaces can afford flexible manipulation for optical waves with artificial planar arrays of subwavelength electromagnetic structures [[28], [29], [30], [31]]. Particularly, they can be designed to support the bound states in the continuum (BIC), realizing tight confinement of optical modes with an infinite Q factor in principle [32,33]. The exotic property of BIC has attracted various studies on its mechanism and applications [[34], [35], [36]]. The great confinement of light with BIC can benefit the interaction between optical and acoustic waves. Therefore, it is a promising strategy to employ metasurfaces to enhance the optomechanical interaction of a large active area, facilitating practical applications.

In this work, we propose and demonstrate the possibility that metasurface heterostructures consisting of electron-beam resist nanowire arrays on a homogeneous chalcogenide thin film can realize a large optomechanical coupling rate above MHz level. The introduced chalcogenide GeSbS thin film has a large photoelastic coefficient, thus facilitating optomechanical interaction via the electrostriction effect. The metasurface heterostructure is designed and optimized to confine optical waves with BIC modes of high-quality factors at the wavelength of about 1500 nm, while the large velocity mismatch between the resist, GeSbS, and silica materials benefits the confinement of mechanical waves at the frequency of 7.36 GHz. As a result, the optomechanical coupling rate can approach 1160 kHz, which surpasses the performance of most microstructures. Because the confined optical waves and mechanical waves are over the whole metasurfaces, the active area of enhanced optomechanical interaction can be arbitrarily extended in principle. Meanwhile, the metasurfaces are realized by structuring the electron-beam resist other than the chalcogenide thin film, releasing the fabrication difficulty with a simpler receipt. Furthermore, we have studied the effects of size parameters of metasurfaces including the period, width, thickness, and tilted wall of the nanowires, and the results show a robust performance for the enhanced optomechanical interaction.