Three-dimensional direct laser writing technology enables one to print polymer microstructures whose size varies from a few hundred nanometers to a few millimeters. It has been shown that, by tuning the laser power during writing, one can adjust continuously the optical and elastic properties with the same base material. This process is referred to as gray-tone lithography. In this paper, we characterize by Brillouin light scattering the complex elastic constant $C_{11}$ of different reticulated isotropic polymers, at longitudinal phonon frequencies of the order of 16 GHz. We estimate the real part of the $C_{11}$ constant to vary from 7 to 11 GPa as a function of laser power, whereas its imaginary part varies between 0.25 and 0.6 GPa. The linear elastic properties are further measured at a fixed laser power as a function of temperature, from $20{}^{\circ }$C to $80{}^{\circ }$C. Overall, we show that our 3D printed samples have a good elastic quality with high Q factors only ten times smaller than fused silica at hypersonic frequencies.

Brillouin light scattering is a versatile measurement technique of the dispersion of bulk acoustic phonons in amorphous and crystalline solids. It allows contactless and non-destructive characterization of the relevant material tensors of optically transparent materials, provided that the optical refraction indices, and in addition the dielectric tensor for piezoelectric materials, are known beforehand. The complete estimation of the anisotropic material tensors is often performed using many different crystal orientations, and hence different samples of the same crystal. In this paper, we consider the problem of measuring those tensors using a single sample of an anisotropic single crystal with trigonal symmetry. A complete measurement requires sufficient experimental diversity in the phonon wavevectors when sampling the phonon velocity surfaces, hence the consideration of a large number of directions away from crystallographic axes. We estimate all six independent elastic constants of non-piezoelectric sapphire (3¯ m point group) with a single X-cut wafer and the six independent elastic constants together with the four independent piezoelectric constants of piezoelectric lithium niobate (3 m point group) with a single Y-cut wafer. The estimated tensors are in close agreement with those reported in the literature based on resonant ultrasonic techniques.

The process of aging is associated with a decline in cell, tissue, and organ function, leading to a range of health problems. Increasing evidence indicates that dietary restriction can counteract age‐dependent effects and improve health and longevity in whole organisms, but less is known about the influence of aging and the impact of nutrition on individual organs of an organism. In this study, we examined the intestine of the very short‐lived aging model system, the African turquoise killifish ( Nothobranchius furzeri ), throughout its lifetime. We investigated the effects of age and nutrition on the preservation of gut tissue at stages corresponding to human neonatal, adolescent, adult, and old age, and integrated morphological measurements, histology, and transcriptomics. The intestinal mucosa is characterized by folds and intervening interfold regions, where intestinal stem cells localize. The stem cells occur in clusters, and the cycle time of stem cells increases with age. We also observed a reduction in intestinal length and volume with age. Age‐dependent transcriptomic profiling revealed significant changes in the expression of peripheral circadian clock genes and stem cell niche markers. Notably, the majority of these genes maintained their adult gene expression levels in old age following intermittent fasting during adulthood. Therefore, our results demonstrate that the decline in structural intestinal tissue homeostasis is associated with a decline in stem cell activity that can be counteracted by intermittent fasting. Since the intestinal mucosa of killifish is similar to that of mammals, the results of this study can be translated to general gut biology.

Brillouin light backscattering from isotropic platelet samples involves the contribution of two phase-matched longitudinal acoustic phonons. A primary, stronger, signal is provided by the usual condition of a phonon wavevector collinear to and twice as large as the incident photon wavevector inside the medium. After reflection of incident light on the backside of the platelet, another bulk phonon propagating along the platelet axis contributes a secondary, weaker, signal that is independent of the index of refraction but dependent on the angle of incidence. Combining contributions from both phonons, the refractive index and the acoustic phonon velocity can be estimated jointly and precisely in the backscattering geometry from experimental data. Experiments with poly(methyl methacrylate), fused silica, and soda-lime glass platelets are used to assess the accuracy of the estimation. It is argued that the secondary Brillouin signal dictates the estimation of velocity whereas the primary Brillouin signal dictates the estimation of the index of refraction.

The anisotropic dependence of the velocity of surface acoustic waves (SAW) on silicon is explored using surface Brillouin light scattering. Measurements of the SAW velocity are compared to a numerical model that takes into account the native thin amorphous oxide layer formed at the top surface of the silicon wafer. The model accounts for material loss and provides a relative estimate for the backscattered intensity resulting from the ripple effect. For the (100) sample considered, a thickness of 4 nm fits well with experimental data, considering material constants of amorphous silica for the oxide. A global phase velocity decrease of −11 m/s per nanometer of silica thickness is predicted for surface phonons at frequencies around 16 GHz.

Synthetic and natural fibers with diameters in the range of a few tens of micrometers can be routinely fabricated. Because of the intricate microstructure of the fibers, however, their elastic properties remain poorly understood. In this study, we employ micro-Brillouin light spectroscopy (micro-BLS) to explore direction-dependent acoustic phonon propagation in amorphous E-glass, synthetic silk, polyamide 11 (PA11), and flax fibers. The technique is.non-invasive and non-destructive, and is an alternative to static mechanical tests. The observable frequency shifts of laser light resulting from Brillouin scattering from hypersonic acoustic phonons of the fiber are in a few 10-GHz range. We determine the full elastic tensors and the optical anisotropy, assuming only transversely isotropic symmetry at the optical wavelength scale. The obtained elastic constants are compared with values reported in the literature for similar materials.