Laser-induced periodic surface structures (LIPSS) (ripples) with different spatial characteristics have been observed after irradiation of single-crystalline zinc oxide surfaces with multiple linearly polarized femtosecond pulses (150–200 fs, 800 nm) in air. For normal incident laser radiation, low spatial frequency LIPSS (LSFL) with a period (630–730 nm) close to the wavelength and an orientation perpendicular to the laser polarization have been found in the fluence range between ∼0.7 and ∼0.8 J/cm2 and predominantly for pulse numbers up to N=100. For lower fluences (0.5–0.7 J/cm2), a sharp transition from the LSFL features toward the formation of high spatial frequency LIPSS (HSFL) appears at any given pulse number below N=100. The HSFL are always parallel to the LSFL, exhibit spatial periods between 200 and 280 nm, and completely substitute the LSFL for pulse numbers N>100. Additionally, the influence of the angle of incidence has been studied experimentally for both LIPSS types revealing a different behavior. Experimental evidence for surface scattered second harmonic generation is presented in the regime of HSFL formation. Moreover, we will show that the HSFL structures on ZnO surfaces can be fully explained by an extension of the existing LIPSS theories if the photoexcitation of the dielectric material (affecting its transient optical properties) is considered in the frame of a simple Drude model along with the second harmonic generation at the irradiated surface. Based on our analysis, the current models of femtosecond laser-induced LIPSS are revisited and an explanation is proposed why HSFL are observed predominantly in the subpicosecond range for below band-gap excitation of dielectrics and semiconductors.

No abstract

High-spatial frequency, quasiperiodic structures ͑HSFL, Nanoripples͒ of 170 nm feature size were induced in rutile-type titanium dioxide surfaces by focused 150 fs Ti:sapphire laser pulses at wavelengths around 800 nm. The ripple formation is distinctly visible for numbers of pulses of N = 100-1000. At lower number of pulses ͑N =10͒, a significant surface roughening appears instead of ripples which is characterized by randomly meandering nanostructures. These observations confirm an essential contribution of early stage irregular material modifications to the dynamics of quasiperiodic ripple formation. The threshold fluence for ripple generation is estimated on the basis of the conventional theory of laser-induced surface structuring. The decrease in the threshold fluence from 0.34 to 0.24 J / cm 2 , as it was found for an increase in the number of pulses from N = 100 to N = 1000, is attributed to a damage accumulation effect. Nanostructuring of spatially extended regions was enabled by utilizing a controlled sample translation at optimized energy and repetition rate of the laser pulses. A significant enhancement of the periodicity of the nanostructures was confirmed by two-dimensional Fourier transform of scanning electron microscopy data. At second harmonic wavelengths around 400 nm, the generation of nanoripples even on the sub-100 nm scale was demonstrated.

We have developed thin film Fabry-Perot filters directly coated on optical fibers to archive a high level of integration with a reduction of optical elements. Such band-pass filters can be used in fiber optical sensor systems, and for fiber communication, e.g. CWDM applications. The filters cavities consist of a single spacer and two dielectric mirrors. The dielectric mirrors are deposited by PVD directly on end-faces of single-mode optical fibers. Dielectric as well as polymeric materials were applied as the spacer layer. Polymeric spacer layers were deposited by dip coating. The influence of the mirror reflectivity on the transmission band of the Fabry-Perot filters was investigated. Furthermore, the optical performance of filters with first order (λ/2) as well as higher order spacers was analyzed. The experimental results are compared with numerical analysis of Fabry-Perot cavities on the end-face of cylindrical waveguides. The spectral characteristic of the filters are calculated using a software solving Maxwell´s equations by a FDTD method. The layer design of the filters and the deposition process were optimized for maximum transmission and narrow bandwidth of the transmission peak. Passive band-pass filters on fiber end-faces were designed, fabricated and characterized for transmission wavelengths of 945 nm, 1300 nm, as well as 1550 nm. Bandwidths as narrow as 1 nm could be achieved for 945 nm.

No abstract