  Achievement of all-optical ultrafast signal modulation and routing by a
low-loss nanodevice is a crucial step towards an ultracompact optical chip with
high performance. Here, we propose a specifically designed silicon dimer
nanoantenna, which is tunable via photoexcitation of dense electron-hole plasma
with ultrafast relaxation rate. Basing on this concept, we demonstrate the
effect of beam steering up to 20 degrees via simple variation of incident
intensity, being suitable for ultrafast light routing in an optical chip. The
effect is demonstrated both in the visible and near-IR spectral regions for
silicon and germanium based nanoantennas. We also reveal the effect of
electron-hole plasma photoexcitation on local density of states (LDOS) in the
dimer gap and find that the orientation averaged LDOS can be altered by 50\%,
whereas modification of the projected LDOS can be even more dramatic: almost
500\% for transverse dipole orientation. Moreover, our analytical model sheds
light on transient dynamics of the studied nonlinear nanoantennas, yielding all
temporal characteristics of the proposed ultrafast nanodevice. The proposed
concept paves the ways to creation of low-loss, ultrafast, and compact devices
for optical signal modulation and routing.

Optics