Abstract
Periodic high spatial frequency ripples structures (HSFL) have been generated in silicon (Si) and in germanium (Ge) at very low fluence below or close to the melting fluence threshold, at different wavelengths and at high repetition rate femtosecond laser pulses (80 MHz, 700–950 nm, 170 fs). HSFL initiation, formation, and arrangement combine structural modification of the surface initiated by heat accumulation of successive pulses with second harmonic generation. HSFL are wavelength dependent and the refractive index plays a central role on their periodicities. HSFL spacing follows quite well a law of Λ=λ/2nλ*, where nλ* is the modified femtosecond laser excited refractive index as a function of the wavelength for Si and Ge.
Highlights
We have demonstrated a refinement of the model fitting very well the experimental results, showing that transient changes in material properties as the complex refractive index have to be taken into account in the formation of high spatial frequency ripples structures (HSFL) under fs laser irradiation
Structural modifications at the surface of semiconductors are first initiated by a thermal process, heat accumulation of successive pulses occur up to the melting temperature after multi pulses (MHz) laser irradiation at low fluence below or close to the melting threshold
Subsequent laser pulses will interact with a modified material, leading to incubation effects where the nonlinear absorption of the laser energy is affected by the subsurface alteration
Summary
The experimental setup, which has been already described in detail elsewhere, comprises a compact ultrafast Ti:sapphire tuneable laser system (80 MHz, Emax $ 40 nJ/ pulse, s 1⁄4 140 fs (FWHM), kEX 1⁄4 690–1060 nm) coupled to a laser workstation specially designed and developed for accurate micro- and nano-processing. The low energy fs-laser beam was focused by the use of a 20Â focusing objective with a relatively high numerical aperture NA of 0.5 to reach the fluence threshold of HSFL generation in Si and Ge in air. For both semiconductors, the fluences were estimated hereinafter by using the relation F 1⁄4 2Ep=px20 1⁄4 2Ep=ðpD2=4Þ, where EP is the average energy per pulse and D % 2lm is the Gaussian beam diameter at 1/e2 at the surface of the sample. A controlled etching process using ammonium fluoride was necessary to remove these undesirable debris to obtain a homogeneous structured field of HSFL
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
