  Cavity optomechanics provides high-performance sensor technology, and the
scheme is also applicable to liquid samples for biological and rheological
applications. However, previously reported methods using fluidic capillary
channels and liquid droplets are based on fixed-by-design structures and
therefore do not allow an active free-space approach to the samples. Here, we
demonstrate an alternate technique using a probe-based architecture with a
twin-microbottle resonator. The probe consists of two microbottle
optomechanical resonators, where one bottle (for detection) is immersed in
liquid and the other bottle (for readout) is placed in air, which retains
excellent detection performance through the high optical-Q (~107) of the
readout bottle. The scheme allows the detection of thermomechanical motion of
the detection bottle as well as its optomechanical sideband drive. This
technique could lead to in-situ metrology at the target location in arbitrary
media, and could be extended to ultrasensitive biochips and rheometers.
