**6. Conclusions**

In summary, we report a gain-assisted plasmonic MDM system with a superior slow light performance based on PIT effect. Both the transmission and optical delay of slow light can be dramatically enhanced by the gain power. Finally, a double-channel, enhanced slow light can be achieved by introducing another disk cavity. After applying the gain material, the device performance can be enhanced and we can also realize the active control of the PIT peak and delay time. To propose such a structure, Ag film can be deposited on the Ni surface by thermal evaporation method [31], and the bus waveguide and cavities can be etched on the Ag layer by focused ion beam (FIB) [9]. Such a system also has potential applications for bio-sensing, nanolaser and optical switching.

**Author Contributions:** Conceptualization by Z.Z.; data curation by Z.Z.; formal analysis by Z.Z.; investigation by Z.Z.; resources by J.Z.; software by J.H. and D.C.; validation by J.Y., X.H., and Y.H.; visualization by S.X.; writing of original draft by Z.Z.; writing review and editing by J.Y.

**Funding:** This research was funded by the National Natural Science Foundation of China (60907003, 61671455), the Foundation of NUDT (JC13-02-13, ZK17-03-01), the Hunan Provincial Natural Science Foundation of China (13JJ3001), and the Program for New Century Excellent Talents in University (NCET-12-0142).

**Conflicts of Interest:** The authors declare no conflict of interest.
