Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here we demonstrate both theoretically and experimentally the realization of a novel THz source with high peak power performance based on two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm. Our theory predicts that under this scheme sub-cycle THz pulses with multi-millijoule energies and record conversion efficiencies can be produced. Besides, we elucidate the origin of this high efficiency, which is made up of several factors, including a novel mechanism where the harmonics produced by the mid-infrared pulses strongly contribute to the field symmetry breaking and enhance the THz generation. In our experiments we verify the theoretical predictions by demonstrating ultrashort sub-cycle THz pulses with sub-millijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm-1. Moreover, we show that these intense THz fields can drive nonlinear effects in bulk semiconductors (ZnSe and ZnTe) in free space and at room temperature. Our numerical simulations indicate that the observed THz yield can be significantly upscaled by further optimizing the experimental setup leading to even higher field strengths. Such intense THz pulses enable extreme field science, including into other, relativistic phenomena.
We investigate the possibilities offered by tightly focused ultrashort laser pulses at 2-µm wavelength for modifying the bulk of silicon. Results show that the lower the pulse duration, the lower the probability to modify the material, in good agreement with nonlinear propagation simulations. By evaluating the influence of several laser parameters, we have found ideal conditions for successfully initiating modifications systematically in the bulk of silicon with ultrashort laser pulses through plane surface for the first time. This result holds promises for contactless monolithic integration of three-dimensional architectures inside silicon.
Ultrashort ring-Airy laser beams with tunable characteristics are experimentally generated and employed for the advanced fabrication of large 3D structures with high resolution using multiphoton polymerization. These beams can be adjusted to abruptly autofocus over an extended range of working distances while keeping almost invariant their voxel shape and dimensions. This striking property together with the real-time controlled focus tuning, through a spatial light modulator, make them ideal candidates for long range multiphoton polymerization. Moreover, the paraxial ring-Airy beams can approach the wavelength limit when scaling down, while observing a counterintuitive, strong enhancement of their focal peak intensity. Using numerical simulations, we show that this behavior is a result of the coherent constructive action of paraxial and nonparaxial energy flow.