Characterisation of femtosecond laser structured cubic-BN

Full text: Cubic-BN has mechanical properties similar to diamond however it is not oxidizing at high temperatures in air ambient. Its low-pressure hexagonal phase has a 2D graphene structure with appealing optical, mechanical, and electrical properties; bandgap of c-BN is 6.4 eV and bulk modulus 400 GPa. Laser pulses of 230 fs duration at 1030 nm and 515 nm wavelengths were focused at a 20 μm depth below the surface of a facet plane inside c-BN crystals. Millimeter-sized grains of c-BN were formed at high pressure conditions using belt-type high pressure apparatus. Tight focusing with numerical aperture NA = 1.42 was implemented to form arrays of damage sites recorded at different pulse energies E_p at a single pulse conditions. Strong contrast changes were observed at threshold values of E_p = 4.5 nJ (515 nm) and 80 (1030 nm) estimated at the focus. Separation between irradiation spots was 10 μm to eliminate a cross talk for void-formation and optical characterisation. For comparison, dense sub-surface array of void-structures were fabricated with 800 nm /150 fs pulses focused with NA = 0.95 at a 10 μm depth. Raman scattering and photoluminescence (PL) was measured from the laser modified void-structures in c-BN. Characteristic TO and LO phonon modes were observed with small broadening at the high-energy side of the modes. Back-scattered Raman signal excited by 785 nm irradiation was collected with NA = 0.4 lens averaging response from area with several void-structures. Wide peak at 520 cm^-1 was the largest modification observed from sub-surface void-structures (800 nm/150 fs). Presence of a lower density h-BN was not confirmed in the void-structures which would be recognisable by its 1350 cm^-1 E_2g mode. Under 405 nm excitation, PL from the void regions has recognizable features which can be identified as atomic neutral nitrogen N emission at 455, 556, 577 nm. This implies that photolysis of c-BN occurred under high intensity driven by nano-/micro-explosion at the focus. N atoms should be trapped inside the laser shocked region of the void-structure. Molecular N₂ PL which occurs in 300-400 nm window was out of the range of observation in this first test. The PL excited at 405 nm/30 ps illumination showed close to a single exponential decay with time constant of 4 ns; under the 532 nm/100 ps excitation the decay was similar 4 ns (not shown here). The PL decay times from the laser structured regions and pristine c-BN were similar; only intensity was different. Formation of voids in materials with the highest bulk modulus exceeding pressures at the Earth's center can bring fundamental understanding how high- pressure/temperature materials are formed. Conditions at the focal region and energy deposition are discussed. (author)