Flexoelectric Polarization in a Nematic Liquid Crystal Enhanced by Dopants with Different Molecular Shape Polarities

Funding Information: We would like to acknowledge the great support which we received from Prof. I. Muševič, JSI, Ljubljana, Slovenia, Dr M. Klasen-Memmer, Merck, Germany, Dr Santanu Kumar Pal and Dr Golam Mohiuddin, Indian Institute of Science Education and Research (IISER) Mohali, India, and Prof. P. Kula and K. Garbat, MUT, Warsaw, Poland. This work has been partially supported by SeeReal Technologies and MUT Research Grants 13-843/WAT/2022.Peer reviewedPublisher PD

Neural networks determination of material elastic constants and structures in nematic complex fluids

Supervised machine learning and artificial neural network approaches can allow for the determination of selected material parameters or structures from a measurable signal without knowing the exact mathematical relationship between them. Here, we demonstrate that material nematic elastic constants and the initial structural material configuration can be found using sequential neural networks applied to the transmmited time-dependent light intensity through the nematic liquid crystal (NLC) sample under crossed polarizers. Specifically, we simulate multiple times the relaxation of the NLC from a random (qeunched) initial state to the equilibrium for random values of elastic constants and, simultaneously, the transmittance of the sample for monochromatic polarized light. The obtained time-dependent light transmittances and the corresponding elastic constants form a training data set on which the neural network is trained, which allows for the determination of the elastic constants, as well as the initial state of the director. Finally, we demonstrate that the neural network trained on numerically generated examples can also be used to determine elastic constants from experimentally measured data, finding good agreement between experiments and neural network predictions

Optical gain and photo-bleaching of organic dyes, quantum dots, perovskite nanoplatelets and nanodiamonds

Optical gain and resistance against photo-bleaching are key material parameters for practical applications in microlasers and microphotonics. Here, we present studies of optical amplification and photo-bleaching in a wide range of optical gain materials, including fluorescent dyes, quantum dots and rods, organic and inorganic room temperature polaritons, and nanodiamonds with emission range from 430 to 680 nm. We used amplified spontaneous emission (ASE) for each material to measure the optical gain. Robustness against photo-bleaching was determined by measuring the intensity of spontaneous emission of the material as a function of the number of excitation femtosecond pulses (10$^6$–10$^{10}$) at various excitation energy densities. We show that pyrromethene laser dyes are the best organic emitters in terms of low excitation energy and good optical gain, whereas solid-state polariton materials are better in terms of high optical gain and stability. We found that all organic and inorganic optical gain materials bleach completely after 10$^6$–10$^9$ illumination pulses with typical 0.1–0.6 GW/cm$^2$ peak power density, with an exception of nanodiamonds. We show that nanodiamonds are the only optical gain material that shows no photo-bleaching beyond the 10$^{10}$ excitation pulses of 300 fs pulse duration and with 0.16 GW/cm$^2$ peak power density

Direct evidence of the molecular switching in electrically commanded surfaces for liquid crystal displays

This work, performed by means of time-resolved high-resolution birefringence measurements, establishes the switching mechanism of electrically commanded surfaces _ECS_ for liquid-crystal displays. A distinct polar electro-optic response, due to the field-induced in-plane switching of the molecules of the 200-nm ferroelectric liquid crystalline polysiloxane alignment layer representing ECS, was detected in a cell filled with isotropic liquid _hexadecane_. The similarity between this response and the one reported recently in cells containing the same ECS but with a nematic liquid-crystal bulk with negative dielectric anisotropy (Delta-epsilo

Mapping the local elastic properties of pharmaceutical solids using atomic force microscopy

Elastic modulus of commonly used excipient, i.e. microcrystalline cellulose (MCC) and an active pharmaceutical ingredient (API), i.e. clarithromycin has been measured by atomic force microscopy. Moderate forces applied with AFM cantilever caused small deformations (a few nanometers) of investigated samples and under such conditions the Hertz model was successfully applied. Wide distributions of the measured elastic modulus imply on heterogeneous surface properties of the investigated samples. This could be explained by polycrystalline attributes of clarithromycin and coarse structure of MCC agglomerates. AFM-based nanoindentation method was compared to a permanent indentation technique performed by nanoindenter. Results obtained by applying larger forces using nanoindenter showed a significant indentation depth dependence that could be the consequence of materialćs non-ideally elastic surface or viscoelastic/plastic properties

A dual-wavelength photothermal aerosol absorption monitor

There exists a lack of aerosol absorption measurement techniques with low uncertainties and without artefacts. We have developed the two-wavelength Photothermal Aerosol Absorption Monitor (PTAAM-2λ), which measures the aerosol absorption coefficient at 532 and 1064 nm. Here we describe its design, calibration and mode of operation and evaluate its applicability, limits and uncertainties. The 532 nm channel was calibrated with ∼ 1 µmol mol−1 NO2, whereas the 1064 nm channel was calibrated using measured size distribution spectra of nigrosin particles and a Mie calculation. Since the aerosolized nigrosin used for calibration was dry, we determined the imaginary part of the refractive index of nigrosin from the absorbance measurements on solid thin film samples. The obtained refractive index differed considerably from the one determined using aqueous nigrosin solution. PTAAM-2λ has no scattering artefact and features very low uncertainties: 4 % and 6 % for the absorption coefficient at 532 and 1064 nm, respectively, and 9 % for the absorption Ångström exponent. The artefact-free nature of the measurement method allowed us to investigate the artefacts of filter photometers. Both the Aethalometer AE33 and CLAP suffer from cross-sensitivity to scattering – this scattering artefact is most pronounced for particles smaller than 70 nm. We observed a strong dependence of the filter multiple scattering parameter on the particle size in the 100–500 nm range. The results from the winter ambient campaign in Ljubljana showed similar multiple scattering parameter values for ambient aerosols and laboratory experiments. The spectral dependence of this parameter resulted in AE33 reporting the absorption Ångström exponent for different soot samples with values biased 0.23–0.35 higher than the PTAAM-2λ measurement. Photothermal interferometry is a promising method for reference aerosol absorption measurements