Transient THz conductivity of silicon with optical pumping above and below the second indirect transition

TL;DR

This study investigates the transient terahertz conductivity of silicon after optical pumping near the second indirect gap, revealing significant changes in terahertz absorption linked to carrier relaxation pathways.

Contribution

It provides new insights into carrier dynamics and phonon interactions in silicon when excited above and below the second indirect transition.

Findings

Terahertz absorption decreases by a factor of 2.7 when pumped above the second indirect gap.

Carrier relaxation involves inter-band scattering from L to X points.

Different phonon populations influence the transient conductivity.

Abstract

Here we perform a series of time-resolved experiments where a 100 fs pump pulse is tuned between 528 nm and 555 nm, across the second indirect gap of intrinsic silicon at ~540 nm which involves electrons in a higher-lying conduction band with minimum at the L point. The photo-injected carriers, after inter- and intra-band relaxations are complete, are subsequently probed with high-field single-cycle terahertz radiation. When the energy of the pump pulses exceeds the second indirect gap, the probed terahertz absorption decreases by a factor 2.7$\pm$0.2. We suggest that this dramatic change could be due to the different phonon populations obtained when the carriers undergo the L to X inter-band scattering, instead of just cooling within the X-valley.