Metastable carbon allotropes in picosecond-laser-modified

In this paper, we report on the bulk modifica- tions of type IIa single-crystal diamond with visible 10-ps pulses (at k = 532 nm) and microstructural changes char- acterized by the appearance of several 'unidentifiable' vibrational modes in the frequency range of 1000-1400 cm -1 in the Raman spectra of laser-modified diamond. It is found that the new Raman modes are strongly pronounced in the spectra of high-stress regions in immediate proximity to the bulk microstructures in the absence of the G mode at *1580 cm -1 characteristic of the sp 2 phase. The high internal stresses are determined from the splitting of the triply degenerate diamond Raman line. The revealed structure transformation is localized within a narrow bulk layer near the bulk microstructures formed, and the stress relaxation is found to result in dis- appearance of the detected vibrational modes in the spec- tra. It is suggested that the formation of bulk regions with a sp 3 carbon structure consisting of Z-carbon and hexagonal diamond is responsible for the appearance of new Raman modes in the spectra of laser-modified diamond. These findings evidence that the stress-assisted formation of novel metastable carbon phases or defect structures occur in the course of bulk modification of diamond with ps-laser pulses. In addition, we report the results of simulations of internal stresses in the system 'graphitized cylinder-in- diamond' to show (1) the effect of the mechanical prop- erties of laser-modified diamond on the resulting stresses and (2) formation of bulk microscopic regions with high stresses of (10 GPa, i.e., the conditions at which various sp 3 carbon allotropes and defect structures become more stable than graphite.