We introduce a compact submicron structure consisting of multiple optical microcavities at both the entrance and exit sides of a subwavelength plasmonic slit filled with an absorbing material. We show that such microcavity structures at the entrance side of the slit can greatly enhance the coupling of the incident light into the slit, by improving the impedance matching between the incident plane wave and the slit mode. In addition, the microcavity structures can also increase the reflectivities at both sides of the slit, and therefore the resonant field enhancement. Thus, such structures can greatly enhance the absorption cross section of the slit. An optimized submicron structure consisting of two microcavities at each of the entrance and exit sides of the slit leads to ~9.3 times absorption enhancement at the optical communication wavelength compared to an optimized slit without microcavities.