Near-infrared (NIR) marker-based imaging is of growing importance for deep tissue imaging and is based on a considerable reduction of optical losses at large wavelengths. We aim to extend the range of NIR excitation wavelengths particularly to values beyond 1.6 $\mathsf{\mu }$m in order to profit from the low loss biological windows NIR-III and NIR-IV. We address this task by studying NIR-excitation to NIR-emission conversion and imaging in the range of 1200 up to 2400 nm at the example of harmonic Mg-doped lithium niobate nanoparticles (i) using a nonlinear diffuse femtosecond-pulse reflectometer and (ii) a Tunable hIGh EneRgy (TIGER) widefield microscope. We successfully demonstrate the existence of appropriate excitation/emission configurations in this spectral region taking harmonic generation into account. Moreover, NIR-imaging using the most striking configurations NIR-III to NIR-I, based on second harmonic generation (SHG), and NIR-IV to NIR-I, based on third harmonic generation (THG), is demonstrated with excitation wavelengths from 1.6–1.8 $\mathsf{\mu }$m and from 2.1–2.2 $\mathsf{\mu }$m, respectively. The advantages of the approach and the potential to additionally extend the emission range up to 2400 nm, making use of sum frequency generation (SFG) and difference frequency generation (DFG), are discussed.

The tuning of second (SHG) and third (THG) harmonic emission is studied in the model system LiNb${ }_{1-x}$Ta${ }_x$O${ }_3$ ($0\le x\le 1$, LNT) between the established edge compositions lithium niobate (LiNbO${ }_3$, $x=0$, LN) and lithium tantalate (LiTaO${ }_3$, $x=1$, LT). Thus, the existence of optical nonlinearities of the second and third order is demonstrated in the ferroelectric solid solution system, and the question about the suitability of LNT in the field of nonlinear and quantum optics, in particular as a promising nonlinear optical material for frequency conversion with tunable composition, is addressed. For this purpose, harmonic generation is studied in nanosized crystallites of mechanochemically synthesized LNT using nonlinear diffuse reflectometry with wavelength-tunable fundamental femtosecond laser pulses from 1200 nm to 2000 nm. As a result, a gap-free harmonic emission is validated that accords with the theoretically expected energy relations, dependencies on intensity and wavelength, as well as spectral bandwidths for harmonic generation. The SHG/THG harmonic ratio ≫1 is characteristic of the ferroelectric bulk nature of the LNT nanocrystallites. We can conclude that LNT is particularly attractive for applications in nonlinear optics that benefit from the possibility of the composition-dependent control of mechanical, electrical, and/or optical properties.

In the context of experimental optics- and photonics-research, motorized, high-precision rotation stages are an integral part of almost every laboratory setup. Nevertheless, their availability in the laboratory is limited due to the relatively high acquisition costs in the range of several 1000 and is often supplemented by manual rotation stages. If only a single sample is to be analyzed repeatedly at two different angles or the polarization of a laser source is to be rotated, this approach is understandable. Yet, in the context of automation and the associated gain in measurement time, cost-effective and precise rotation stages designed for the use of optics are lacking. We present a low-cost alternative of a motorized high precision rotation stage system. The design is based on a combination of 3D-printed components, which form the monolithic mechanical framework, and a stepper motor controlled by an ESP32 based microcontroller. By coupling the motor and rotation unit via a toothed belt, backlash is minimized and at the same time high positioning accuracy can be achieved. Finally, the implementation of remote procedure calls for serial communication and the utilization of a physical home switch and incremental encoder complete the desired feature set of an integrated system for laboratory setups. The total costs can thus be reduced to less than 100 without significantly restricting the performance criteria.