Figure 9

Morphology evolution of the sample surface exposed by (a–e) a femtosecond laser single pulse; (f–j) femtosecond laser double pulses at different stages of the etching process, the pulse delay is 350 fs. The SEM images have varying scale bars. (k–n) Simulation of the free electron density distributions and (o–r) center laser intensity distributions in fused silica irradiated by femtosecond double pulses at different pulse delays. (s) Schematic diagram of the manufacturing and Si-O bond structure. (t) Normalized Raman spectra of modified regions irradiated using femtosecond laser single and double pulses (the pulse delay is 350 fs) in fused silica. Dashed lines below the D2 peaks are baselines used in the peak area measurement in u. Inset is the schematic diagram of 4- and 3-membered ring structures. (u) Percent area of the total reduced Raman spectrum under the D2 line versus different pulse delays. The femtosecond laser with wavelength of 800 nm, duaration of 50 fs and repetition rate up to 1 KHz. The laser fluence is fixed at 9.46 J cm⁻² in all experiments and the energy distribution ratio is 1:1. Reproduced from Ref. 121 (with the permission of NPG).