Direct Wave-Vector Excitation in an Indirect-Band-Gap Semiconductor of Silicon with an Optical Near-field

In this work, our first-principles calculations reveal that direct wave-vector excitation (i.e., interband transitions between different wavenumbers without phonon assistance) can occur in the indirect-band-gap semiconductor silicon. The wave-vector excitation is successfully induced by irradiation of a silicon thin film with an optical near-field (ONF). As a result, the absorption-band-edge energy E-edge shifts to a lower photon energy of 1.6 eV for ONF excitation from E-edge of 2.1 eV for the conventional excitation by propagating far-field light. The direct wave-vector excitation is caused by the sufficiently large components of wave vectors inherent in the ONF, and thus does not require phonon assistance. For a realistic silicon system, it is clarified that the wave-vector excitations are determined by the energy difference between the valence and conduction bands and occur irrespective of the initial and final wave vectors.