Colour centres in KBr induced by femtosecond laser pulses

Full text: Colour centres in alkali halides have many practical applications in high density memory devices, tunable lasers, single molecule trapping, and super-continuum sources. For these applications a simpler femtosecond laser direct write method can be used. It is the most efficient and controllable 3D coloration as compared to electron, ion bombardment and X or γ ray irradiation. One layer of exposed laser regions was coloured in pure KBr crystal at a 100 μm depth with direct laser writing (DLW). KBr become dark blue in locations irradiated by 1030 nm wavelength, 220 fs duration pulses focused by a numerical aperture NA = 0.7 objective lens. The 5 mm long lines with 2 μm separation were fabricated with 5 - 20 nJ (at focus) energy pulses at pulse density of 10 pulses per 1 μm and 1 mm/s scan speed. The absorption spectrum with five distinct bands designated as V, F, R1, R2 and M bands, which were in good accordance with the theoretical prediction for the alkali halides. Each band has a particular charge pattern. From the absorption spectrum, the density of colour centres was calculated using Smakula formula. The density of the F centre was estimated as high as 5.3 x 10¹⁸cm^-3, which colored KBr in dark blue. Under the red(white) light illumination centered at F-band the coloration faints. When structural damage was created with 20 nJ pulses (irradiance 3.6 TW/cm²) the coloration becomes permanent at the focal region where structural changes of crystal matrix are created. Polarization effects of the writing beam on the transmission of a linearly polarized light at the colour center absorption band were investigated. (author)