Femtosecond Laser Induced Metallization in Silicon via Photon Momentum Mediated Band Transition

Abstract

Silicon-based technology has been at the forefront of electronic and photonic research since the dawn of the electronic revolution. However, silicon is fundamentally an indirect-bandgap semiconductor: the conduction band minimum and valence band maximum occur at different points in momentum space, so optical transitions require phonon assistance to conserve crystal momentum. This three-body electron-photon-phonon interaction renders silicon an inefficient material for light-absorbing applications such as solar cells and optoelectronic devices. In this work, we build on an established nanoscale photon-momentum confinement mechanism, which broadens the photon momentum distribution and enables phonon-free absorption in silicon, by subsequently applying a high-intensity femtosecond pulsed excitation that injects hot carriers beyond the Mott density, collapsing the bandgap and inducing a reversible semiconductor-to-metal transition. While nanoscale photon-momentum confinement itself has been previously established, the central contribution of this work is this subsequent, carrier density-driven stage, which drives the momentum-enhanced region into a reversible transient metallic phase. Our simulations report highly negative permittivity, enhanced optical power absorption, a large absorption coefficient, and low skin depth, with carrier densities reaching a notable value, far exceeding the Mott threshold. We have shown carrier driven metallization with stable operation below melting threshold of silicon. These results establish a general mechanism for dynamically inducing metallic states in silicon, offering a potential pathway toward monolithically integrated active photonic components and dynamically reconfigurable electro-optical devices within conventional CMOS platforms.