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.

Hydrothermal synthesis of KNbO₃ leads to crystallite sizes down to 30 nm. They exhibit significant SHG-signals throughout the visible range.

Hydrothermal synthesized NaNbO₃ nanoparticles exhibit a second harmonic signal compareable to KNbO₃. Insertion of cations is a crucial step in hydrothermal synthesis.

Soft chemical methods for the synthesis of alkali niobates are so far very limited due to the insufficient knowledge of the underlying niobium chemistry and the influences of the solvents used. With the aim of synthesizing particles of high quality without the use of water as an additional solvent, a set of glycols was used here. Glycols combine several benefits as they possess a large dipole moment, a relatively high boiling point, are inexpensive and pose little toxicological risk. We show that solvothermal synthesis in glycols without the addition of water or surfactants produces small particles of alkali niobates in the size range from above 100 nm down to about 5 nm, depending on the glycol used, with a relatively narrow size distribution. NaNbO₃ and KNbO₃ crystallize as orthorhombic nanocrystals. The KNbO₃ obtained, however, can also be interpreted as monoclinic, especially if the particle size is small. While there is no obvious size dependence in the corresponding band gaps, smaller particles show a lower harmonic response than larger ones.