Nonlinear Diffuse fs-Pulse Reflectometry of Harmonic Upconversion Nanoparticles

Optical and Electrical Properties of A3[VS4] (A = Na, K) Synthesized via a Straightforward and Scalable Solid-State Method

Two literature-known sulfido vanadates, Na3[VS4] and K3[VS4], were obtained through a straightforward and scalable synthetic method. Highly crystalline powders of both compounds were obtained from the homogeneous molten phases of starting materials via a─comparably rapid─solid-state technique. Low-temperature structure determination, ambient temperature powder diffraction, and solid-state NMR spectroscopy verify previous structural reports and indicate purity of the obtained samples. Both compounds show semiconductivity with the optical band gap values in the range of 2.1 to 2.3 eV. Experimental values of the ionic conductivity and dielectric constants are σ = 2.41·10-5 mS·cm-1, k = 76.52 and σ = 1.36·10-4 mS·cm-1, k = 103.67 at ambient temperature for Na3[VS4] and K3[VS4], respectively. It is demonstrated that Na3[VS4] depicts second-order nonlinear optical properties, i.e., second harmonic generation over a broad wavelength spectrum. The results introduce new aspects of sulfido vanadates as multifunctional candidates for potential optical and electrical applications.

Gap-Free Tuning of Second and Third Harmonic Generation in Mechanochemically Synthesized Nanocrystalline LiNb1-xTaxO3 (0 ≤ x ≤ 1) Studied with Nonlinear Diffuse Femtosecond-Pulse Reflectometry

The tuning of second (SHG) and third (THG) harmonic emission is studied in the model system LiNb 1-xTa xO 3 (0≤x≤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.

Remarkable Infrared Nonlinear Optical, Dielectric, and Strong Diamagnetic Characteristics of Semiconducting K3[BiS3]

The ternary sulfido bismuthate K₃[BiS₃] is synthesized in quantitative yields. The material exhibits nonlinear optical properties with strong second harmonic generation properties at arbitrary wavelengths in the infrared spectral range and a notable laser-induced damage threshold of 5.22 GW cm^-2 for pulsed laser radiation at a wavelength of 1040 nm, a pulse duration of 180 fs, and a repetition rate of 12.5 kHz. K₃[BiS₃] indicates semiconductivity with a direct optical band gap of 2.51 eV. Dielectric and impedance characterizations demonstrate κ values in the range of 6-13 at 1 kHz and a high electrical resistivity. A strong diamagnetic behavior with a susceptibility of -2.73 × 10^-4 m³ kg^-1 at room temperature is observed. These results suggest it is a promising nonlinear optical candidate for the infrared region. The synergic physical characteristics of K₃[BiS₃] provide insight into the correlation of optical, electrical, and magnetic properties.

NIR-to-NIR Imaging: Extended Excitation Up to 2.2 μm Using Harmonic Nanoparticles with a Tunable hIGh EneRgy (TIGER) Widefield Microscope

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 μm 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 μm and from 2.1–2.2 μm, 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 role of cations in hydrothermal synthesis of nonlinear optical sodium niobate nanocrystals

The usability of the alkali niobates with their ferroelectric and photorefractive properties could be expanded if the development of synthesis methods would allow to obtain small, preferably monodispersed, crystals in the sub-μm to nanometer regime. Of all the possible synthesis methods, the most reliable is currently hydrothermal synthesis to generate small crystallite sizes of these materials. Although the products of sodium niobate are polydisperse and partially agglomerated, they show a significant SHG signal that is unexpectedly comparable to that of potassium niobate. A view on the hydrothermal synthesis of sodium niobate reveals that the incorporation of cations in the crystalline lattice of the niobium educt plays a part in the formation of the product. The occurrence of distinct different phases, as in the case of potassium niobate, is not observed. Instead, it is shown that a clear assignment of the crystalline phase cannot be made here. This indicates that crystallization of the alkali niobates in hydrothermal synthesis depends on the stoichiometry, the niobium starting material and the cation used.

Combining photoinduced linkage isomerism and nonlinear optical properties in ruthenium nitrosyl complexes

The complex trans-[RuNO(NH₃)₄F]SiF₆ was synthesized in quantitative yield and the structure was characterized by X-ray diffraction and spectroscopic methods. The complex crystallizes in the non-centrosymmetric space group Pn. Hirshfeld surface analysis revealed that the dominant intermolecular interactions are of types H...F and F...O, which are likely to be responsible for the packing of the molecules in a non-centrosymmetric structure. Irradiation with blue light leads to the formation of Ru-ON (metastable state MS1) and Ru-η²-(NO) (metastable state MS2) bond isomers, as shown by IR and UV-Vis spectroscopy. The structural features of the MS1 isomer were elucidated by photocrystallography. The complex exhibits exceptionally good thermal stability of the metastable state MS1, such that it can be populated by light at 290-300 K, which is important for potential applications. The second harmonic (SH) emission can be generated by femtosecond-pulsed irradiation of the complex. The generated SH is rather efficient and stable under long-term exposure. Finally, since both metastable states and harmonic generation can be generated at room temperature, an attempt to drive the SH response by photoisomerization of the nitrosyl ligand was made and is discussed.

Absorption and Remission Characterization of Pure, Dielectric (Nano-)Powders Using Diffuse Reflectance Spectroscopy: An End-To-End Instruction

This paper addresses the challenging task of optical characterization of pure, dielectric (nano-)powders with the aim to provide an end-to-end instruction from appropriate sample preparation up to the determination of material remission and absorption spectra. We succeeded in establishing an innovative preparation procedure to reproducibly obtain powder pellet samples with an ideal Lambertian scattering behavior. As a result, a procedure based on diffuse reflectance spectroscopy was developed that allows for (i) performing reproducible and artifact-free, high-quality measurements as well as (ii) a thorough optical analysis using Monte Carlo and Mie scattering simulations yielding the absorption spectrum in the visible spectral range. The procedure is valid for the particular case of powders that can be compressed into thick, non-translucent pellets and neither requires embedding of the dielectric (nano-)powders within an appropriate host matrix for measurements nor the use of integrating spheres. The reduced spectroscopic procedure minimizes the large number of sources for errors, enables an in-depth understanding of non-avoidable artifacts and is of particular advantage in the field of material sciences, i.e., for getting first insights to the optical features of a newly synthesized, pure dielectric powder, but also as an inline inspection tool for massively parallelised material characterization.

Mechanochemical Reactions of Lithium Niobate Induced by High-Energy Ball-Milling

Lithium niobate (LiNbO3, LN) nanocrystals were prepared by ball-milling of the crucible residue of a Czochralski grown congruent single crystal, using a Spex 8000 Mixer Mill with different types of vials (stainless steel, alumina, tungsten carbide) and various milling parameters. Dynamic light scattering and powder X-ray diffraction were used to determine the achieved particle and grain sizes, respectively. Possible contamination from the vials was checked by energy-dispersive X-ray spectroscopy measurements. Milling resulted in sample darkening due to mechanochemical reduction of Nb (V) via polaron and bipolaron formation, oxygen release and Li2O segregation, while subsequent oxidizing heat-treatments recovered the white color with the evaporation of Li2O and crystallization of a LiNb3O8 phase instead. The phase transformations occurring during both the grinding and the post-grinding heat treatments were studied by Raman spectroscopy, X-ray diffraction and optical reflection measurement, while the Li2O content of the as-ground samples was quantitatively measured by coulometric titration.

Nonlinear optical organic-inorganic crystals: synthesis, structural analysis and verification of harmonic generation in tri-(o-chloroanilinium nitrate)

The structural and nonlinear optical properties of a new anilinium hybrid crystal of chemical formula (C₆H₇NCl⁺·NO₃^-)₃ have been investigated. The crystal structure was determined from single-crystal X-ray diffraction measurements performed at a temperature of 100 K which show that the compound crystallizes in a noncentrosymmetric space group (Pna2₁). The structural analysis was coupled with Hirshfeld surface analysis to evaluate the contribution of the different intermolecular interactions to the formation of supramolecular assemblies in the solid state that exhibit nonlinear optical features. This analysis reveals that the studied compound is characterized by a three-dimensional network of hydrogen bonds and the main contributions are provided by the O...H, C...H, H...H and Cl...H interactions, which alone represent ∼85% of the total contributions to the Hirshfeld surfaces. It is noteworthy that the halogen...H contributions are quite comparable with those of the H...H contacts. The nonlinear optical properties were investigated by nonlinear diffuse femtosecond-pulse reflectometry and the obtained results were compared with those of the reference material LiNbO₃. The hybrid crystals exhibit notable second (SHG) and third (THG) harmonic generation which confirms its polarity is generated by the different intermolecular interactions. These measurements also highlight that the THG signal of the new anilinium compound normalized to its SHG counterpart is more pronounced than for LiNbO₃.

Nonlinear optical potassium niobate nanocrystals as harmonic markers: the role of precursors and stoichiometry in hydrothermal synthesis

Nanocrystals of alkaline niobates are currently being discussed for various applications because of their diverse and remarkable properties. Although the growth of bulk niobate crystals is well established, little is known about respective nanocrystals and the optical properties of niobates below 100 nm. A systematic view of the hydrothermal synthesis of potassium niobate with respect to the precursor species reveals the sensitive dependence of the resulting crystalline phases and sizes on the educt modifications. With a variation of stoichiometry of the procedure, the product modification and crystallite size can be changed. By means of second harmonic generation, nanocrystalline potassium niobate offers the possibility for use as an optical marker in high resolution nonlinear microscopy. Redispersed particles show a significant second harmonic generation signal throughout the visible spectral range.