  High index dielectric nanostructure supports different types of resonant
modes. However, it is very challenging to achieve high-Q factor in a single
subwavelength dielectric nanoresonator due to non-hermtian property of the open
system. Here, we present a universal approach of finding out a series of high-Q
resonant modes in a single nonspherical dielectric nanocavity by exploring
quasi-bound state in the continuum. Unlike conventional method relying on heavy
computation (ie, frequency scanning by FDTD), our approach is built upon leaky
mode engineering, through which many high-Q modes can be easily achieved by
constructing avoid-crossing (or crossing) of the eigenvalue for pair leaky
modes. The Q-factor can be up to 2.3*10^4 for square subwavelength nanowire
(NW) (n=4), which is 64 times larger than the highest Q-factor (Q=360) reported
so far in single subwavelength nanodisk. Such high-Q modes can be attributed to
suppressed radiation in the corresponding eigenchannels and simultaneously
quenched electric(magnetic) at momentum space. As a proof of concept, we
experimentally demonstrate the emergence of the high-Q resonant modes (Q=380)
in the scattering spectrum of a single silicon subwavelength nanowire.
