High-harmonic generation is a cornerstone of nonlinear optics. It has been demonstrated in dielectrics, semiconductors, semi-metals, plasmas, and gases, but, until now, not in metals. Here we report high harmonics of 800-nm-wavelength light irradiating metallic titanium nitride film. Titanium nitride is a refractory metal known for its high melting temperature and large laser damage threshold. We show that it can withstand few-cycle light pulses with peak intensities as high as 13 TW/cm2, enabling high-harmonics generation up to photon energies of 11 eV. We measure the emitted vacuum ultraviolet radiation as a function of the crystal orientation with respect to the laser polarization and show that it is consistent with the anisotropic conduction band structure of titanium nitride. The generation of high harmonics from metals opens a link between solid and plasma harmonics. In addition, titanium nitride is a promising material for refractory plasmonic devices and could enable compact vacuum ultraviolet frequency combs.

We report ultrafast-laser-induced photochemical, structural, and morphological changes in a polyimide film irradiated at the polymer-glass interface in back-incident geometry. Back-illumination creates locally hot material at the interface leading to a confined photochemical change at the interface and a morphological change through a blister formation. The laser-induced photochemical changes in polyimide resulted in new absorption and luminescence properties in the visible region. The laser-treated polyimide exhibited photoluminescence anisotropy resulting from formation of ordered polymer upon irradiation by linearly polarized ultrashort laser pulses. Confocal fluorescence microscopy resulted in similar observations to the bulk. Reflection-absorption infrared spectroscopy and X-ray photoelectron spectroscopy together indicated confinement of laser-induced chemical changes at the interface.

No abstract

Nonlinear absorption of femtosecond laser pulses provides a unique opportunity to confine energy deposition in any medium to a region that is below the focal diameter of a pulse. Illumination of a polymer film through a transparent high-band-gap material such as glass, followed by nonlinear absorption of 800nm light in polymers, allows us to further restrict absorption to a very thin layer along the propagation direction. We demonstrate this confinement by simulating femtosecond-laser-induced polymer modification by linear, two-photon, and three-photon absorption, and discuss the control over energy absorption in polymers that multiphoton processes offer. Energy deposited in a thin polymer film induces a protruding blister. We present experimental results for blister diameter and height scaling with variation of pulse energy. Using pulse energies of 20-200 nJ and 0.4-NA focusing, we fabricate blisters with diameters of 1-5.5 μm and heights of 75 nm to 2 μm. Using 0.95-NA focusing, we obtain laser-induced blisters with diameters as small as 700 nm, suggesting blister-based laser-induced forward transfer is possible on and below the 1-μm scale. Submicrometer blister formation with use of femtosecond lasers also offers a method of direct, precise laser writing of microstructures on films with single laser pulses. This method is a possible alternative to lithography, laser milling, and laser-based additive machining.