Physical mechanisms for the optical properties of the femtosecond-laser-treated black diamond

Vacancies in diamond can introduce defect bands in the band gap of diamond and lead to the sub-band gap absorption in the visible and infrared regions. At sufficiently high concentrations of vacancy, the defect bands overlap each other to form a single partially filled intermediate band (IB) in the band gap and the sub-band gap absorption in the infrared region is especially strong. Along with the decreasing of the vacancy concentrations, the IB splits into two separate bands and the sub-band gap absorption decreases sharply. The computed results are important for thoroughly understanding of the optical properties of black diamond.