  Nanowires are perfect transducers for ultra-sensitive detections of force and
mass. For small mass sensing, recent advances in detecting more properties than
their masses with high sensitivity for absorbed particles onto a nanowire rely
on identifications of two orthogonal flexural vibration modes of the nanowire.
The directions of these orthogonal flexural vibrations with respect to
measurement direction are crucial parameters of vibration modes. However,
previous method, which determines a nanowire's vibration direction using
thermal vibrations, requires simultaneously detecting of two orthogonal
flexural vibrations of the nanowire with sufficient sensitivity. In this work,
we propose and realize a method for the determination of directions of a
nanowire's flexural vibrations by micro-lens optical fiber interferometer. Our
method combines the light interference and light scattering of the nanowire. It
does not require detecting a pair of degenerated orthogonal flexural vibration
modes. Therefore, our method is expected to have wide usages in characterizing
nanowire's vibrations and their applications.
