Free-carrier absorption (FCA) in semiconductor is a process which carriers within conduction or valence band absorbs near-bandgap photon energies to transfer themselves into a higher energy state without generating electron-hole pairs. The energy absorption activates phonon scattering and is generally considered to have undesirable impact on the performance of electronic or optical devices. Here we find, in an electrochemical processing of silicon, the FCA not only has a significate impact on chemical reaction but also contributed to the formation of nanocrystals. By the effect of FCA, nanocrystals form directly on the bulk surface via a rate-limited photo-electrochemistry processing. The formation mechanism of silicon quantum dots as follows: phonon scattering is generated at the photo-absorption site with its nearby atoms from absorbing laser as an hole-countercurrent to reduce redox reaction for dissolving silicon and caused amphoteric defects around the point. To proving the concept, we employed a 0.9 W/cm², 1310 nm laser that can penetrate through silicon but be absorbed by free-carriers. After anodizing, a brilliant non-etching spot irradiated by the laser appeared on the black surface of P⁺⁺ silicon substrate. Under He-Cd laser photoexcitation, we found the photoluminescence in the spot was enhanced to 450% than the around porous silicon forming in dark in the vicinity of the spot.