OPTOMECHANICAL DEVICES FOR AFM APPLICATIONS WITH A LARGE RANGE OF CANTILEVER STIFFNESS

 

Yuxiang Liu, Houxun Miao, Vladimir Aksyuk, Kartik Srinivasan

 

Atomic force microscopy (AFM) has been an irreplaceable tool in physics, chemistry, and biology, thanks to its ability of investigating individual atoms. Although conventional AFM has been well-developed, its bulky size, cantilever design, and detection scheme hinder AFM from being miniaturized and integratable. Recent progress on cavity optomechanics has shed light on an alternative technique to fulfill the AFM’s tasks in a more compact and possibly more sensitive manner.

We have recently demonstrated sensitive transduction of the motion of an integrated nanocantilever with a Si-based optomechanical device. The device consists of a microdisk optical resonator and a semicircular cantilever curving around the disk with a separation of ~100 nm. When the cantilever moves toward or away from the disk, an optical wave circulating around the disk edge feels the resulting change in the peripheral refractive index. This produces an effective path length change for the disk’s resonant optical modes, allowing the mechanical motion of the cantilever to be transduced by monitoring the optical signal with a high sensitivity due to the high optical quality factor (105 to 106) of the disk resonator. Both the signal read-out and optical power input are achieved by either an integrated on-chip waveguide or a thinned optical fiber, resulting in a monolithic device with a small size (tens of micrometers) and high displacement sensitivity (~10-15 m/Hz1/2).

Here, we explore the cantilever stiffness range that can be effectively transduced by our device architecture, since various AFM applications require different stiffness values. By changing the disk diameter (2.5 mm to 50 mm) and cantilever width (100 nm to 250 nm), we experimentally demonstrated the cantilever stiffness ranging from below 0.01 N/m to above 300 N/m, which covers the common stiffness range of conventional AFM cantilevers (0.01 N/m to 100 N/m). Due to the small mass (subpicogram) of the cantilever, the natural frequencies of cantilever mechanical modes in our devices (200 kHz to 110 MHz) are much higher than those of conventional cantilevers (50 kHz to 300 kHz). The higher mechanical frequency may enable not only a higher image scanning speed, but also help decrease the thermal drift during image acquisition. The typical radius of curvature of the cantilever tip is ~10 nm, comparable to that of the conventional AFM. These results indicate that our optomechanical disk-cantilever devices have a potential to serve as an on-chip version of AFM with possibly better performance, in applications ranging from interrogating biological samples (on the soft side of the stiffness range) to imaging with a sub-atomic resolution (on the hard side of the stiffness range).