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Silicon nanowires (SiNWs) attached to a wafer substrate are converted toinversely tapered silicon nanocones (SiNCs). After excitation with visiblelight, individual SiNCs show a 200-fold enhanced integral band-to-bandluminescence as compared to a straight SiNW reference. Furthermore, the reversetaper is responsible for multifold emission peaks in addition to the relativelybroad nearinfrared (NIR) luminescence spectrum. A thorough numerical modeanalysis reveals that unlike a SiNW the inverted SiNC sustains a multitude ofleaky whispering gallery modes. The modes are unique to this geometry and theyare characterized by a relatively high quality factor ($Q = 1300$) and a lowmode volume ($0.2 < ({\lambda}/n_{eff})^3 < 4$). In addition they show avertical out coupling of the optically excited NIR luminescence with anumerical aperture as low as 0.22. Estimated Purcell factors $F_p \proptoQ/V_m$ of these modes can explain the enhanced luminescence in individualemission peaks as compared to the SiNW reference. Investigating the relationbetween the SiNC geometry and the mode formation leads to simple design rulesthat permit to control the number and wavelength of the hosted modes andtherefore the luminescent emission peaks.

Observation of strongly enhanced photoluminescence from inverted cone-shaped silicon nanostuctures