Ultrafast bandgap photonics in mid-infrared is an exciting area of nonlinear photonics, which shows different ultrafast damage characteristics of solids compared to that in near-infrared fields. It allows periodic surface nano-structures formation in low bandgap materials like germanium. Ultrafast mid-infrared field interaction at 2 micron wavelength with non-linear photonic crystal results in generation of high efficiency harmonic generation up to sixth harmonic.
Single 5 and 40 femtosecond, near IR pulses with fluences varying from 0.4 – 80 J/cm2 from a Ti:Sapphire laser was focused onto a single crystal Cu sample surface with 2.0 μm focal spot at 15 and 45 degree angle of incidence. The surface profiles after interaction were studied with an interferometric depth profiler (Wyko NT9100), and benchmarked against crater size and morphology predicted by 2D Particle-In-Cell (PIC) laser damage simulation model.
Laser induced periodic surface structures (LIPSS) were generated via interaction of multiple 90
femtosecond 1900 - 3600 nm mid IR laser pulses (3 -10,000) on single crystal Ge targets. For specific
laser parameters, both low and high frequency LIPSS are found together, which are oriented
perpendicular to each other. Study of polarization dependence of LIPSS revealed that orientation and
symmetry of interaction could be controlled by rotating polarization of laser pulses. Low frequency
LIPSS formation was consistent with surface plasmon coupling of laser pulses with excited Ge.
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