Nonlinear diffuse femtosecond-pulse reflectometry is introduced as a powerful experimental tool for the unambiguous characterization of polar and non-polar point symmetry groups of harmonic upconversion nanoparticles. Using intense ultrashort 40 femtosecond laser pulses and an appropriate figure of merit (FOM), second and third harmonic emission serve for the structural characterization of polar Yb-doped lithium niobate and non-polar titanium dioxide nanoparticles. The tool is capable of differentiating these two samples by FOM values that differ by up to 13 orders of magnitude. The general applicability to harmonic upconversion nanoparticles over a broad range of intensities and wavelength spectrum, is discussed.

The role of chirp on the light–matter interaction of femto- and pico-second laser pulses with functional structured surfaces is studied using drag-reducing riblets as an example. The three-dimensional, periodic microstructure naturally gives rise to a mutual interplay of (i) reflection, (ii) scattering, and (iii) diffraction phenomena of incident coherent light. Furthermore, for femtosecond pulses, the structure induces (iv) an optical delay equivalent to a consecutive temporal delay of 230 fs in places of the pulse. These features enable studying experimentally and numerically the effect of tuning both pulse duration $\tau$ and spectral bandwidth $\mathrm{\Delta }\omega$ on the features of the wide-angle scattering pattern from the riblet structure. As a result, we discovered a significant breakdown of fringes in the scattering pattern with decreasing pulse duration and/or increasing spectral bandwidth. This unique type of chirp control is straightforwardly explained and verified by numerical modeling considering the spectral and temporal interaction between different segments within the scattered, linearly chirped pulse and the particular geometric features of the riblet structure. The visibility of the fringe pattern can be precisely adjusted, and the off-state is achieved using $\tau <230\mathrm{fs}$ or $\mathrm{\Delta }\omega >2.85\times {10}^{13}\mathrm{rad}/\mathrm{s}$.

Beam parameters of a probing laser source in an optical riblet sensor are studied by considering the high demands on a sensors’ precision and reliability for the determination of deviations of the geometrical shape of a riblet. Mandatory requirements, such as minimum intensity and light polarization, are obtained by means of detailed inspection of the optical response of the riblet using ray and wave optics; the impact of wavelength is studied. Novel measures for analyzing the riblet shape without the necessity of a measurement with a reference sample are derived; reference values for an ideal riblet structure obtained with the optical riblet sensor are given. The application of a low-cost, frequency-doubled Nd:YVO₄ laser pointer sufficient to serve as a reliable laser source in an appropriate optical riblet sensor is discussed.