  Gold (Au) nanoparticles particularly nanorods are actively exploited as
imaging probes because of their special nonblinking and nonbleaching
absorption, scattering and emitting properties that arise from the excitation
of surface plasmons. Herein, we report a novel orientation sensor at nanoscale
by defocused imaging of single Au nanorods (AuNRs) through an ordinary
wide-field optical microscope. By simultaneously recording defocused images and
two-photon luminescence intensities for a large number of single AuNRs, we
correlate their defocused images with their three-dimensional spatial
orientations. As many AuNRs can be monitored in parallel, defocused imaging of
single AuNRs is a high throughput sensing technique that allows us to obtain
their spatial orientations within one frame in situ and real-time. The probe
size can be down to several nanometers, which is highly desirable in order to
minimize any potential interference from the probe itself. Furthermore, the
sensing property is insensitive to the excitation polarization and the
distribution of the probe aspect ratio, which allows AuNRs of any length in a
proper regime to be used as orientation sensors without changing the laser
frequency and polarization. These unique features make the orientation probes
proposed here outstanding candidates for optical imaging and sensing in
material science and biology.
