A femto- and picosecond laser assisted periodic nanostructuring of hydrogenated amorphous silicon (a-Si:H) is demonstrated. The grating structure with the subwavelength modulation of refractive index shows form birefringence (Δn ≈ −0.6) which is two orders of magnitude higher than commonly observed in uniaxial crystals and femtosecond laser nanostructured silica glass. The laser-induced giant birefringence and dichroism in a-Si:H film introduce extra dimensions to the polarization sensitive laser writing with applications that include data storage, security marking, and flat optics.

A one-dimensional surface relief with a 1.20  ±  0.02 µm period was formed in amorphous hydrogenated silicon films as a result of irradiation by femtosecond laser pulses (1.25 µm) with a fluence of 0.15 J cm−2. Orientation of the formed structures was determined by the polarization vector of the radiation and the number of acting pulses. Nanocrystalline silicon phases with volume fractions from 40 to 67% were detected in the irradiated films according to the analysis of Raman spectra. Observed micro- and nanostructuring processes were caused by surface plasmon–polariton excitation and near-surface region nanocrystallization, respectively, in the high-intensity femtosecond laser field. Furthermore, the formation of Si-III and Si-XII silicon polymorphous modifications was observed after laser treatment with a large exposure dose. The conductivity of the film increased by three orders of magnitude at proper conditions after femtosecond laser nanocrystallization compared to the conductivity of the untreated amorphous surface. The conductivity anisotropy of the irradiated regions was also observed due to the depolarizing contribution of the surface structure, and the non-uniform intensity distribution in the cross-section of the laser beam used for modification.

We demonstrate a comprehensive modification study of silica glass, crystalline silicon, and amorphous silicon film, irradiated by tightly focused cylindrical vector beams with azimuthal and radial polarizations. The evidence of the longitudinal field associated with radial polarization is revealed by second harmonic generation in z-cut lithium niobate crystal. Despite the lower threshold of ring-shaped modification of silicon materials, the modification in the center of single pulse radially polarized beam is not observed. The phenomenon is interpreted in terms of the enhanced reflection of longitudinal component at the interface with high-index contrast, demonstrating that the longitudinal component is inefficient for the flat surface modification. Enhanced interaction of the longitudinal light field with silicon nanopillar structures produced by the first pulse of double-pulse irradiation is also demonstrated.

1 of 7) 1600575 direct laser writing, as opposed to longer pulses, is that they can rapidly deposit energy in solids with high precision. The light is absorbed and the optical excitation ends before the surrounding lattice is perturbed, which results in highly localized nanostructuring without collateral material damage. [19,20] First observation of laser-induced periodic surface structures dates back to the 1960s, when Birnbaum reported ripple formation on the surface of semiconductors. [21] Since then, this phenomenon was observed on virtually any type of media including metal, semiconductor, dielectric solids, and thin films. [22][23][24][25][26][27][28][29] Processing conditions occurred to be broad with wavelengths ranging from the mid-infrared to visible spectrum and from continuous wave operation to femtosecond laser systems. A vast number of applications, including coloration, [30] control of surface chemical and mechanical properties, [31,32] have been proposed.More than a decade ago, Bricchi and Kazansky demonstrated that femtosecond laser pulses focused inside silica glass can lead to self-assembled nanogratings, which exhibit birefringence comparable to quartz crystals. [33] Later, the tailored surface nanogratings were introduced showing the threefold birefringence increase. [34] Thus, a functional birefringent layer should be at least several tens of micrometers thick. For most applications such a thickness is appropriate. However, this limits the design and integration of miniaturized elements.To realize the laser-induced nanogratings as functional metasurfaces, the intermediate case between volume and surface periodic structuring has to be implemented when the modification of high-index thin films along its depth would significantly enhance the resulting anisotropy. Here we demonstrate that the interaction of femtosecond laser pulses with hydrogenated amorphous silicon (a-Si:H) thin films induces self-assembled periodic lamellae structures oriented perpendicular to the incident beam polarization. These films with the induced subwavelength modulation of refractive index behave as a uniaxial birefringent material with the slow axis oriented parallel to the imprinted nanogratings. Thus, the form birefringence of two orders of magnitude higher than in silica glass can be achieved. [18,[33][34][35] As a result, large retardation is realized within hundreds of times thinner layers (Figure 1a) that can be deposited on various substrates with different textures.We leverage the realization of laser-induced periodic thinfilm structures as a highly birefringent metasurface to design controllable and high precision GPOEs. Here we report on demonstration of various geometric phase designs including arrays of polarization microconverters and microlenses, polarization gratings (PGs) and computer-generated holograms with phase gradients reaching up to ≈1 rad µm −1 .In order to identify the maximum birefringence of the laser-induced periodic thin-film structures, 80 µm long lines

Halide perovskites are widely studied due to their potential applications in solar cells. Despite the remarkable success in increasing perovskite solar cell efficiency, the underlying photophysical processes remain unclear. To cover this gap, we studied temperature, spectral, and light intensity dependence of photoconductivity of CH3NH3PbI3 films in the planar contact configuration. We observed non-monotonic behavior of the photoconductivity temperature dependence: a power-law decrease with increasing temperature at the temperatures below 185 K and close to exponential growth above this temperature. Spectral and light intensity dependences of photoconductivity allowed us to postulate that phase transition between tetragonal and orthorhombic structures and a change in the recombination channel are unlikely to be the reasons for abrupt change in photoconductivity behavior. Charge carrier mobility is proposed to be responsible for unusual photoconductivity changes with temperature.