Intrapulse in situ Raman probing of electron, phonon and structural dynamics in synthetic diamond excited by ultrashort laser pulses: Insights into atomistic structural damage

Abstract

Synthetic Ib-diamond plate with a minor concentration of substitutional atomic nitrogen C-centers was exposed in its bulk by tightly focused 0.2-ps, 525-nm laser pulses, coming at 80-MHz repetition rate and nJ-level variable pulse energies. Stokes first-order Raman scattering line related to 1332-cm−1 optical phonon was observed in the transmitted laser spectra and its pulse energy-dependent intensity, position wavenumber and half-width were analyzed. The Raman intensity, along with the second-order Raman band (2200–2700 cm−1), demonstrated quadratic dependence on pulse energy due to three-photon inter-band absorption, transforming at higher energies into a linear dependence for saturated absorption of opaque dense electron-hole plasma. Under these conditions, first-order Raman scattering was related to a non-resonant spontaneous process, enhanced by plasma emission of seeding coherent optical phonons. The Raman wavenumber decreased versus pulse energy via photo-injection of plasma and then slowly increased due to the “electronic” stress, until its final drop upon the onset of self-accelerated decay of optical phonons into acoustic ones (quasi-heating). The Raman half-width increased versus pulse energy due to the optical-phonon decay during the 0.2-ps laser pulse. An additional Stokes Raman band of radiation-induced defects was observed in the range of 900–1200 cm−1, exhibiting its yield scaling with generation of energetic Auger electrons. At the maximal pulse energies (stronger intra-pulse quasi-heating) all Raman lines disappeared, implying ultrafast structural disordering in the diamond without graphitization.