Non-linear optical properties of coloured diamonds: Observations of frequency up conversion and “whispering gallery-like” modes in photoluminescence

Four coloured diamonds, 1.7 MeV e-beam irradiated to a dose of 1018e per sqcm and annealed around 900–930 °C, were investigated at room temperature using conventional photoluminescence (PL) and excitation spectra, and PL using 514.5 and 785 nm lasers. Interesting new observations are: (i) excitation spectra reveal energy transfer between NV0 and NV− centers (ii) frequency up conversion, blue-green emission on excitation in 740–900 nm range, was observed in blue and lemon yellow samples. This appears to be due to two-photon absorption in GR1 centers and energy transfer to N3/H3 centres. (iii) Excitation with 514 nm line of Ar+ laser, in pink and purple samples resulted in the appearance of an intense and broad emission at 267 nm with FWHM of 32 nm. This is rather close to 2ν of the exciting line. It is identified as second harmonic generation (SHG) due to a synergistic effect of strain induced birefringence and resonant absorption which facilitated phase matching. The 785 nm excited anti-Stokes PL contained a frequency up converted band in 620 nm region having extensive closely spaced structure like in the case of ‘whispering gallery modes’ due to spheroid inclusions.