Abstract

Previous femtosecond experiments have shown that a silicon surface becomes highly reflective^1,2. and microscopically disordered³ in less than one picosecond following femtosecond pulsed excitation above a threshold fluence of approximately 0.1 J/cm². The present experiment investigates the temperature rise of crystalline silicon excited below the threshold for melting through time-resolved measurements of refractive index changes in a submicron silicon film, and resolves a delay of tens of picoseconds in semiconductor lattice heating⁴. This technique^5,6 relies upon the well characterized temperature dependence⁷ of the refractive index n + ik of silicon in the wavelength range between the indirect (1.15 eV) and direct (3.0 eV) band gaps. The temperature dependence of the index results from a downward shift of the direct band edge of silicon caused by renormalization of the band energy by the electron-phonon interaction, with a small contribution from lattice thermal expansion.⁸

© 1986 Optical Society of America