ᐊ O14-5
Open Readings 2021 • O15-1
O15-2 ᐅ

CONICAL THIRD HARMONIC GENERATION FROM FEMTOSECOND LASER INDUCED NANOGRATING IN TRANSPARENT MATERIALS DURING FILAMENTATION AT HIGH REPETITION RATES

Robertas Grigutis1, Vytautas Jukna1, Marius Navickas1, Gintaras Tamošauskas1, Kęstutis Staliūnas1, 2, Audrius Dubietis1

1 Laser Research Center, Vilnius University, Saulėtekio Avenue 10, LT-10223 Vilnius, Lithuania

2 Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig de Lluís Companys, 23, 08010, Barcelona, Catalonia, Spain

[email protected]

Laser-induced periodic surface structures (LIPSS) are a universal physical phenomenon, which occurs on many materials (metals, semiconductors, dielectrics) within focal spot of the linearly polarized laser radiation [1] and find many applications in plasmonics, photonics and microfluidics [2]. Gradual transition from LIPSS to a polarization-independent low spatial frequency structures, called laser-induced periodic annular surface structures (LIPASS) in the center of the damage crater was observed [3] and more importantly, the formation of nanogratings in the volume of various materials was investigated [4]. More recently, conical third harmonic (TH) generation from in-bulk optical damage was reported and its correlation with extinction of supercontinuum radiation was unveiled [5].

In this Study we demonstrate that filamentation of femtosecond laser pulses at high repetition rate inscribes a nanograting in the volume of transparent nonlinear material, that has a certain spectrum of periods and corresponding lattice vectors, necessary to fullfil the phase-matching condition for conical TH generation. These findings are supported by the measurements of TH cone angles inside of various nonlinear crystals and glasses: YAG, sapphire, YLF, LiF, MgF2, LiSAF, LiCAF, fused silica and BK-7 glass, performed with an amplified Yb:KGW laser that delivers 180 fs, 1035 nm pulses at 200 kHz repetition rate, see Fig. 1(a). We also investigated LIPSS and LIPASS morphology (Fig. 1(b) and (c) respectively) and the corresponding spectra of nanograting periods retrieved by 2D Fourier transform (Fig. 1(d)). Our results show that conical TH generation occurs as a noncollinear four-wave mixing process, where the TH cone angle is defined by the longitudinal phase-matching set by the material dispersion, while the transverse phase-matching involves the reciprocal lattice vector of nanograting, which is produced by high repetition rate filamentation of femtosecond laser pulses.

Figure 1
Fig. 1. (a) Measured TH cone angles in various materials. The inset shows a screenshot of conical TH emission in sapphire. (b) SEM image of LIPSS and (c) LIPASS on the surface of sapphire. (d) 2D Fourier transform spectrum of LIPASS, where white circle indicates the nanograting period required to phase-match conical TH generation.

We believe that our study provides a deeper understanding about the nature of conical TH generation and its relation with evolving in-bulk optical damage during femtosecond filamentation and supercontinuum generation in bulk materials at high repetition rates.

[1] J. Bonse, J. Krüger, S. Höhm, and A. Rosenfeld, "Femtosecond laser induced periodic surface structures", J. Laser Appl. 24, 042006 (2012).

[2] R. Buividas, M. Mikutis, and S. Juodkazis, "Surface and bulk structuring of materials by ripples with long and short laser pulses: Recent advances", Prog. Quantum Electron. 38, 119–156 (2014).

[3] Y. Liu, Y. Brelet, Z. He, L. Yu, B. Forestier, Y. Deng, H. Jiang, and A. Houard, "Laser-induced periodic annular surface structures on fused silica surface", Appl. Phys. Lett. 102, 251103 (2013).

[4] A. Rudenko, J.-P. Colombier, S. Höhm, A. Rosenfeld, J. Krüger, J. Bonse, and T. E. Itina, "Spontaneous periodic ordering on the surface and in the bulk of dielectrics irradiated by ultrafast laser: a shared electromagnetic origin", Sci. Rep. 7, 12306 (2017).

[5] R. Grigutis, G. Tamošauskas, V. Jukna, A. Risos, and A. Dubietis, "Supercontinuum generation and optical damage of sapphire and YAG at high repetition rates", Opt. Lett. 45, 4507-4510 (2020).