Abstract
We compare supercontinuum generation in hbox {CaF}_2 crystal under tight and loose focusing of 150 fs, 515 nm second harmonic pulses from an amplified Yb:KGW laser at a repetition rate of 10 kHz. It is demonstrated that supercontinuum generation geometry applying loose focusing (hbox {NA}=0.004) of the pump beam into a long (25 mm) hbox {CaF}_2 sample is advantageous in terms of supercontinuum spectral extent and durability of damage-free operation of the nonlinear material as compared to a commonly used supercontinuum generation setup which employs tight focusing (hbox {NA}=0.012) into a short (5 mm) sample and to setup which uses tight focusing into a long (25 mm) sample. More specifically, loose focusing into a long sample showed remarkably longer (20 min) damage-free operation of the nonlinear material, which was not translated with respect of the pump beam, while in tight focusing condition the sample is damaged just within 2 min of operation, leading to a complete extinction of the supercontinuum spectrum. The evolution of optical degradation of the nonlinear material in time and its impact to supercontinuum spectrum is studied in terms of filament-induced luminescence due to self-trapped exciton emission and light scattering at the pump wavelength indicating the onset of optical damage. Our findings are supported by the numerical simulations which compare relevant parameters related to filament propagation in tight and loose focusing conditions.
Highlights
We compare supercontinuum generation in CaF2 crystal under tight and loose focusing of 150 fs, 515 nm second harmonic pulses from an amplified Yb:KGW laser at a repetition rate of 10 kHz
We examine the case of loose focusing into a thick sample in more detail by performing simultaneous measurements of filament-induced luminescence trace, which provides the information on the shape of the light filament, and light scattering at the pump wavelength, which indicates the onset of the optical damage
We demonstrate that loose focusing ( NA = 0.004 ) of the pump beam into a long (25 mm) CaF2 sample produces a nearly symmetrically broadened supercontinuum spectrum which remains stable in untranslated crystal for 20 minutes of operation
Summary
We compare supercontinuum generation in CaF2 crystal under tight and loose focusing of 150 fs, 515 nm second harmonic pulses from an amplified Yb:KGW laser at a repetition rate of 10 kHz. The largest spectral broadenings and the largest spectral blue-shifts in the UV, in particular, are obtained when SC is generated in dielectric materials featuring large energy bandgap, i.e. materials having wide transparency in the UV and small chromatic d ispersion[4,5] These criteria are fulfilled in alkali metal fluorides: CaF2 , BaF2 , MgF2 , LiF and LiSAF, which produce SC spectra with the largest spectral blue-shifts extending into deep UV, and show good performance with femtosecond near and mid infrared laser pulses[6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]. The critical power for self-focusing, and in part due to the reduced contribution of impact ionization, whose rate decreases with decreasing the pump wavelength
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.
