Morphological Study of Nanostructures Induced by Direct Femtosecond Laser Ablation on Diamond

Abstract

High spatial frequency laser induced periodic surface structure (HSFL) morphology induced by femtosecond laser with 230 fs pulse duration, 250 kHz repetition rate at 1030 nm wavelength on CVD diamond surface is investigated and discussed. The spatial modification was characterized and analyzed by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and 2D-Fast Fourier Transform (2D-FFT). We studied the effect of pulse number and laser power on the spatial development of nanostructures, and also deduced the impact of thermal accumulation effect on their morphology. A generalized plasmonic model has been used to follow the optical evolution of the irradiated surface and to determine the periodic value of the nanostructures. We suggest that non-thermal melting and plasmonic excitation are the main processes responsible for the formation of HSFL-type nanostructures.

Optical and Thermal Behavior of Germanium Thin Films under Femtosecond Laser Irradiation

“…Now, the question arises: what is the mechanism responsible for this thermal effect, leading to different types of modification under fs laser irradiation? By looking at Figure 6 , we can observe that, when the laser fluence reaches 0.028 J cm −2 (point “a” in the figure) the free electron density exceeds the critical density , whereas the bulk material is hardly heated, indicating that the optical behavior of Ge is metal-like, and thus the surface morphology can be modified with minimum thermal effect [ 8 , 16 ]. When the laser fluence reaches 0.12 J cm −2 (point “b”), we observe that the kinetic energy of the free electrons exceeds the bandgap energy; electrons have now enough energy to excite valence electrons by impact ionization (avalanche ionization process), transferring more thermal energy to the lattice.…”

“…Phase explosion is the most dominant mechanism in metals [ 4 , 5 ] and in semiconductors as well, whereas the Coulomb Explosion (CE) is predominant in dielectrics [ 6 ]. Conversely, in multi-pulse femtosecond laser interactions, the thermal accumulation effect becomes predominant [ 7 , 8 ] regardless of the type of material; the ablation threshold can be easily reached, and liquid and/or aggregates can be observed on the irradiated surface [ 9 ].…”

Quantum technologies in diamond enabled by laser processing

“…The high temperature annealing mobilizes the vacancies, which eventually are captured by substitutional nitrogen impurities to form NV centers. 22,26 Yurgens et al 46 recently proposed an alternative focusing method relying on a solid-immersion lens (SIL), which enabled laser writing at pulses energies as low as 5.8 nJ per pulse, where lattice vacancies were created by tunneling breakdown rather than multiphoton ionization. These low pulse energies are fivefold lower than the first report by Chen et al and also offer the advantage of using a lower-cost femtosecond laser oscillator instead of an amplified system.…”

“…63 In agreement with previous studies, high density NV À center ensembles have been written in HPHT diamond through single static exposures followed by thermal annealing, and their integration within a waveguide has been characterized. 22 Arrays of "empty" waveguides together with "static exposure" ones (containing arrays of nine single 100-nJ static exposures uniaxially separated by 2 lm) were written in HPHT diamond at a depth of 18 lm. During annealing, NV À s are generated throughout the waveguide mode due to the diffusion of vacancies from the modification lines, as can be seen from the confocal microscope images of Figs.…”

“…A solution to such problems has recently been found in femtosecond laser writing of photonic structures, 17,18 benefiting from the localized bulk modification due to nonlinear absorption of focused ultrashort pulses. 19,20 In this way, direct writing of 3D integrated optical circuit has been demonstrated, 21,22 with the possibility to create NV À color centers in a controlled manner, while retaining their high-optical and spin coherence. 23 In this Perspective, first, the physical mechanism behind the fabrication of optical waveguides in bulk diamond will be explained.…”