# Integrated Hybrid Tweezer for Particle Trapping with Combined Optical and Acoustic Forces
## 5 Discussion  
This device is a hybrid integrated optical tweezer with several valuable characteristics, and it possesses a few unique advantages compared to conventional integrated optical tweezers. First, for the optical wave, one can design facet tapers at the input and output ports of optical waveguides to reduce reflection and avoid standing waves [53]. For the acoustic wave, the device was completely immersed in water using a millifluidic channel with a 5-mm-thick water layer. Considering the absorption coefficient of water for ultrasonic waves, which is \(25.6\times 10^{-17}\) s\({}^{2}\)/cm [41], a 200-MHz acoustic wave would quickly attenuate when it propagated in the water. Since the acoustic wave was well confined in LN, and the thickness of the silicon substrate exceeded one hundred micrometers, the acoustic wave was very weak on the boundaries of the silicon substrate. Therefore, no standing waves were formed. Due to no formation of standing waves or field enhancement during capturing particles, the light intensity felt by particles was relatively weak, which can mitigate the impact of phototoxicity on cells. Second, it could capture micron-sized particles, such as cells, and control their movements. Continuous particle movement could be achieved by adjusting the optical power or voltage. Third, it could capture particles with different refractive indices, enabling the separation of particles with significantly different refractive indices by simply applying different voltages required for trapping these particles.  
Compared with conventional free-space optical tweezers, this device has the potential to reduce the size and cost of the optical system. It also enables the parallel trapping and transporting of multiple particles on a chip without using lenses [54].  
For possible applications, the proposed tweezer can be used as the fixed center for yeasts and other small biological objects in chip laboratories within the fields of biology and medicine, including applications in drinking water technology and drug delivery [3; 31]. At the same time, the device can separate particles with significantly different refractive indices, which can be used for particle sorting. Additionally, it enables the construction of complete analysis systems by integrating various functional modules on the chip [55].  
To avoid adhesion during the strong contact between cells and the surface of the device, it can be achieved by washing cells in isotonic sucrose solution [11] or washing the surface of the device in some specific solutions [56].  
Our design is based on mature fabrication processes. First, thin-film lithium niobate with a thickness of 20 \(\upmu\)m is commercially available from NANOLN [57], the leading vendor of providing thin-film lithium niobate with a thickness ranging from hundreds of nanometers to tens of microns. Second, IDT made of aluminum can be patterned by magnetron sputtering followed by lift-off [58]. Third, silicon nitride can be deposited by ICPCVD (Oxford PlasmaPro 100), and the upper cladding silicon can be deposited by APCVD [59; 60; 61; 62]. Finally, the silicon waveguide (Part B of the hybrid tweezer in Figure 1) is patterned by dry etching [63]. The millifluidic channel can be fabricated by soft lithography based on Polydimethylsiloxane [40].