  A one-dimensional photonic-crystal (PC) cavity with nanoholes is proposed for
extremely enhancing the THz electric fields by utilizing the electromagnetic
(EM) boundary conditions, where both slot effect (for the perpendicular
component of the electric displacement field) and anti-slot effect (for the
parallel component of the electric field) contribute to the considerable field
enhancement. The EM energy density can be enhanced in the high refractive index
material by a factor of ({\epsilon}h/{\epsilon}l)^2, where {\epsilon}h and
{\epsilon}l are the permittivities of the high and low refractive index
materials, respectively. Correspondingly, the mode volume can be enhanced by a
factor of 288 as compared with the regular THz PC cavity and is three orders of
magnitude smaller than the diffraction limitation. While the proposed THz
cavity design also supports the modes with high Q > 10^4, which lead to strong
Purcell enhancement of spontaneous emission by a factor exceeds 10^6. Our THz
cavity design is feasible and appealing for experimental demonstrations, since
the semiconductor layer where the EM is maximized can naturally be filled with
a quantum engineered active materials, which is potential for developing the
room-temperature coherent THz radiation sources.
