Sensing dispersive and dissipative forces by an optomechanical cavity

We experimentally study an optomechanical cavity that is formed between a mechanical resonator, which serves as a movable mirror, and a stationary on-fiber dielectric mirror. A significant change in the behavior of the system is observed when the distance between the fiber’s tip and the mechanical resonator is made smaller than about 1 μm. The combined influence of Casimir force, Coulomb interaction due to trapped charges, and optomechanical coupling is theoretically analyzed. The comparison between experimental results and theory yields a partial agreement. editor’s choice Copyright c © EPLA, 2016 The study of the interaction between a mechanical resonator and nearby bodies is of great importance for the fields of microelectromechanical systems and scanning probe microscopy. For sufficiently short distances the interaction is expected to be dominated by the Casimir force [1–3], which originates from the dependence of the ground-state energy of the electromagnetic field upon boundary conditions [4–9]. For larger distances, however, other mechanisms such as Coulomb interaction between trapped charges and their image charges [10] and local variations in the work function [11] commonly dominate the interaction. In this study we investigate the effect of the interaction between nearby bodies on the dynamics of an optomechanical cavity [12–18]. In our setup the optomechanical cavity is formed between two mirrors, a stationary fiber Bragg grating (FBG) mirror and a movable mirror made of aluminum in the shape of a trampoline supported by 4 beams (see fig. 1(a)). The tip of the fiber is blown into a dome shape. Piezoelectric motors are employed for positioning the center of the dome above the center of the trampoline and for controlling the distance d between them. The observed response of the optomechanical cavity in the range d 1μm exhibits rich dynamics resulting from the interplay between back-reaction optomechanical effects and the nonlinear coupling between the interacting bodies. In general, such coupling may result in both a static force due to dispersive interaction, and a friction force due to dissipative (or retarded) interaction [19]. Contrary to some other previously employed techniques, in which only the static force can be measured, we find that the observed response of the optomechanical cavity allows the extraction of both static and friction forces [20–22]. The comparison between data and theoretical predictions reveals that some of the experimental findings are not well understood. A photo-lithography process is used to pattern a 200 nm thick aluminum on a high-resistivity silicon wafer, into a mechanical resonator in the shape of a 100 × 100μm trampoline [23] (see fig. 1(a)). Details of the fabrication process can be found elsewhere [24]. Measurements are performed at a temperature of 77K and a pressure well below 5 × 10mbar. A graded index fiber (GIF) having a peak refractive index of nGIF = 1.49 and a pitch of 0.47 is spliced to the end of the single mode fiber (SMF), and its tip is blown into a dome shape of radius R = 90μm. A cryogenic piezoelectric three-axis positioning system having sub-nanometer resolution is employed for manipulating the position of the optical fiber. A tunable laser operating near the Bragg wavelength λB = 1545.7 nm of the FBG together with an external attenuator are employed to excite the optical cavity. The optical power reflected off the cavity is measured by a photodetector (PD), which is connected to both a spectrum analyzer and to an oscilloscope. Two neighboring optical cavity resonances are seen in panel (b) of fig. 1, in which the reflected optical power is plotted as a function of the voltage Vz that is applied to the piezoelectric motor, which is