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The production of brighter coherent XUV radiation by intense laser pulses through the process of high-harmonic generation (HHG) is one of the central challenges in contemporary nonlinear optics. We study the generation and spatial propagation of high harmonics analytically and via ab initio simulations. We focus on the length scales defining the growth of the harmonic signal with propagation distance and show that the well-known coherence length limits HHG only for relatively low driving intensities. For higher intensities, the photoionisation of the medium, naturally accompanying HHG, leads to essentially transient phase matching and laser frequency blue shift. By systematically taking both of these factors into account, we demonstrate that the behaviour of the harmonic signal at higher intensities is defined by another length scale—the blue-shift length. In this generation regime the XUV intensity at a given frequency first grows quadratically and then saturates passing the blue-shift length, but the total harmonic efficiency continues growing linearly due to the linear increase of the harmonic line bandwidth. The changeover to this generation regime takes place for all harmonic orders roughly simultaneously. The rate of the efficiency growth is maximal if the atomic dispersion is compensated by photoelectrons near the centre of the laser pulse. Our theory offers a robust way to choose the generation conditions that optimise the growth of the harmonic signal with propagation.

Context The intermolecular interactions of ethyl acetate (EtOAc)-water (H 2 O)/ethanol (EtOH) mixtures were investigated using a combination of Raman spectroscopy and quantum chemical calculations. The computational approach was used to analyze the structure of hydrogen-bonded complexes of ethyl acetate with water/ethanol molecules, based on density functional theory (DFT). The calculated frequencies closely matched the experimental Raman values, with differences being under 4%. Experimental data show that when the concentrations of ethyl acetate in the ethyl acetate/water/ethanol solutions were reduced, almost all Raman spectral bands are blue-shifted. The AIM analysis reveals that all the given complexes possess a positive energy density, indicating that the molecules interact electrostatically. The energy and bond length indicate that the methyl group forms relatively weak hydrogen bonds. Analysis indicates that EtOAc forms weak H-bonding C = O∙∙∙H and C-H∙∙∙O, which are recognized as van der Waals interactions. As the amount of ethyl acetate decreases in the complex, the interaction forces also decrease. This could also explain why the bands are blue-shifted. It was discovered that the title complexes’ hydrogen bond energy decreased exponentially as bond length increased. Methods The geometries of the molecular complexes were optimized using the Gaussian 09W program and the B3LYP/6–311 +  + G(d,p) set of functions. The potential energy distribution (PED) analysis was performed using VEDA 4.0 software. Raman spectra were drawn using the Origin 8.5 software. The Multiwfn 3.8 software was used to calculate topological parameters of electron density in molecular systems. GaussView 6.0 and Visual Molecular Dynamics (VMD) 1.9.3 tools were used to visualize all computational results. Graphical Abstract

We consider a free-falling observer who crosses the event horizon in the Schwarzschild background. In the course of this fall, he/she can receive signals from an object (like a star surface) that emits radiation. We study how the frequency received by an observer changes depending on the proper time on his/her trajectory. The scenarios are classified depending on whether the frequency is infinite, finite or zero near the singularity and the horizon. This depends crucially on the angular momenta of an observer and a photon. In this work we consider also emission process, and, as we show, conditions of emission strongly influence parameters of a photon, and thus received frequency. As one of our main results, we present numerical calculations showing evolution of the received frequency during the process of diving into a black hole, depending on parameters of an observer and emitter. We also analyze how a falling observer will see a night sky as he/she approaches the singularity. We show that there appear several blind zones, which were not analyzed previously.

Radiation science has a wide range of applications, from industrial uses and medical treatments to space research and national defense. Optical fiber technology offers a flexible and sensitive alternative solution for accurate radiation measurements in various applications. This study presents a new sensor design based on Yb-doped optical fibers for gamma radiation detection. The behavior of Yb-doped fibers under gamma irradiation was investigated, and a sensor was developed based on the red (or blue) shift in fluorescence emission. Three homemade and two commercial Yb-doped optical fibers were irradiated to different total doses of gamma radiation, then excited with UV light to record their spectra. A red-shift (or blue-shift in doped with Al and without P) was observed with increasing radiation dose, attributed to the formation of color centers in the fiber. These spectral shifts depend on the fiber composition and radiation dose. The highest sensitivity was observed in Fiber-3HM and Fiber-5HM which are highly radiation-sensitive, and online radiation monitoring has sensitivity as high as 7.56 nm kGy−1 and 3.65 nm kGy−1. For the first time, we have demonstrated a radiation sensor based on the principle of fluorescence emission shift induced by the applied radiation dose, as described in the existing literature.

The size-dependent photoluminescence (PL) blue shift in organometal halide perovskite nanoparticles has traditionally been attributed to quantum confinement effects (QCEs), irrespective of nanoparticle size. However, this interpretation lacks rigor for nanoparticles with diameters exceeding the exciton Bohr radius (rB). To address this, we investigated the PL of MAPbBr3 nanoparticles (MNPs) with diameters ranging from ~2 to 20 nm. By applying the Brus equation and Burstein–Moss theory to fit the PL and absorption blue shifts, we found that for MNPs larger than rB, the blue shift is not predominantly governed by QCEs but aligns closely with the band filling effect. This was further corroborated by a pronounced excitation-density-dependent PL blue shift (Burstein−Moss shift) at high photoexcitation densities. Additionally, trap-state filling was also found to be not a negligible origin of the PL blue shift, especially for the smaller MNPs. The time-resolved PL spectra (TRPL) and excitation-density-dependent TRPL are collected to support the coexistence of both filling effects by the high initial carrier density (~1017–1018 cm−3) and the recombination dynamics of localized excitons and free carriers in the excited state. These findings underscore the combined role of the band filling and trap-state filling effects in the size-dependent PL blue shift for solution-prepared MNPs with diameters larger than rB, offering new insights into the intrinsic PL blue shift in organometal halide perovskite nanoparticles.

The investigation of the preparation of polystyrene (PS) nanosphere monolayers for the fabrication of carbon nanotube (CNT) forest fishnet metamaterial structures is studied in this paper, as a cheap alternative for top-down patterning methods. The precise control of dry etching conditions resulted in a highly controlled diameter of PS nanobeads, which were then used as a shadow mask for CNT fishnet preparation. The change of the size of the holes from 370 nm to 665 nm resulted in a gradual change of the CNT morphology from multi-walled to single-walled CNTs. The ultraviolet-visible (UV-Vis) reflectance spectra showed that the variation of the hole diameter resulted in the nonlinear light absorption in CNT fishnets that caused the change of the resonance frequency. The change of the fishnet wire width (inductance) and the hole size (capacitance) resulted in the blueshift of the broadband resonance frequency peak. The presented work has a significant potential to allow for the large-scale fabrication of CNT-based fishnet metamaterial structures for applications in energy harvesting, energy storage, solar cells, or optoelectronic devices, such as neuromorphic networks.

Transparent nanocrystalline Zn ( 1 - x ) Ca x O ( 0 ≤ x ≤ 0.20 ) thin films were deposited on glass substrates by sol–gel dip coating method. The X-ray diffraction (XRD) pattern revealed the polycrystalline nature of the films with hexagonal wurtzite structure and confirmed the non-existence of the secondary phase corresponding to CaO indicating the monophasic nature of the deposited films. The crystallinity of the films deteriorated with higher dopant concentration due to the segregation or separation of dopant ions in grain boundaries. The lattice parameters and the unit cell volume increased to a higher Ca-dopant concentration. This was due to the successful incorporation of Ca 2 + ions with larger ionic radius in the host zinc oxide (ZnO) lattice. The optical transmittance spectra of the samples showed transmittances above 60% in the visible spectral range and the absorption edge in the near ultra-violet region got blue-shifted with cation substitution. The estimated optical energy gaps confirmed the band gap widening with increase in Ca-dopant concentration. The calculated values increased from 3.30 eV for undoped ZnO to 3.73 eV for Zn 0.8 Ca 0.2 O thin films giving 13.03% enhancement in the energy gap value due to the electronic perturbation caused by cation substitution as well as deterioration in crystallinity.

Highly efficient and stable blue organic light-emitting diodes (OLEDs), although required for display and lighting applications, remain rare. Here we report a molecular perdeuteration strategy to stabilize blue thermally activated delayed fluorescence (TADF) emitters. Perdeuterated sky-blue TADF emitters exhibit higher efficiencies and doubled device lifetime in OLEDs compared with protonated emitters, owing to suppressed high-energy vibrations. Perdeuteration also leads to blue-shifted and narrowed spectra in the solid state, which in turn improves the Förster energy transfer to the deep-blue final emitter in TADF-sensitized fluorescent OLEDs. These devices exhibit a maximum external quantum efficiency of 33.1% and a lifetime to reach 80% of the initial luminance of 1,365 h with a Commission Internationale de l’Eclairage y coordinate of 0.20 at a luminance of 1,000 cd m −2 , even outperforming blue phosphorescent OLEDs. Our perdeuteration strategy improves the device performance of blue OLEDs, paving the way for their broader applications in displays and lightings. Molecular perdeuteration of thermally activated delayed fluorescence emitters improves the performance of blue organic light-emitting diodes, enabling a peak external quantum efficiency of 33.1% and a device lifetime to reach 80% of initial luminance of over 1,300 h.

Semiconductor mixed-halide perovskites featured with a tunable energy bandgap are ideal candidates for light absorbers in tandem solar cells as well as fluorescent materials in light-emitting diodes and nanoscale lasers. These device advancements are currently hindered by the light-induced phase segregation effect, whereby ion migration would yield smaller-bandgap domains with red-shifted photoluminescence. Here we show that upon laser excitation all-inorganic mixed-halide nanocrystals unexpectedly exhibit a blue shift in the photoluminescence peak that can revert back in the dark, thus depicting the processes of ion migration out of and back to the originally excited nanocrystals. Interestingly, this reversible photoluminescence shift can also be induced by electrical biasing of mixed-halide nanocrystals without the injection of charge carriers. The above findings suggest that it is the local electric field that breaks the ionic bonds in mixed-halide nanocrystals, which could be a universal origin for light-induced phase segregation observed in other mixed-halide perovskite materials. Mixed-halide perovskites possess excellent semiconductor properties but suffer severely from notorious light-induced phase segregation effect. Here Zhang et al. employ simple photoluminescence measurements to link the effect to the local electric field induced ion migration process.

The outstanding structural, electronic, and optical properties of inorganic perovskite materials have gained significant attention in the field of solar technology in recent times. This particular Perovskite demonstrates exceptional optoelectronic properties, such as strong light absorption, extended carrier diffusion distances, and efficient charge transfer. Sr 3 NCl 3 is a material that belongs to the family of inorganic metal halide perovskites and possesses a cubic perovskite crystal structure, which is classified under the space group Pm-3m (No. 221). This study extensively examined the impact of biaxial compressive and tensile strain on the structural, optical, and electronic properties of the inorganic cubic perovskite Sr 3 NCl 3 using density functional theory (DFT) based on first-principles calculations. The unstrained planar Sr 3 NCl 3 molecule exhibits a direct bandgap of 1.252 eV at the Γ point. However, when accounting for the relativistic spin-orbital coupling (SOC) effect in the calculations, the bandgap of the Sr 3 NCl 3 perovskite is reduced to 1.247 eV. When subjected to compressive strain, the bandgap of all structures decreases, but under tensile strain, it increases. The properties of this material, such as its dielectric function, absorption coefficient, and electron loss function, indicate that it can strongly absorb visible light because of its band properties. Moreover, the photon energy spectrum shows a redshift (blueshift) in the absorption coefficient and dielectric function with increasing compressive (tensile) strain. Consequently, investigating the strain-dependent optical and electronic properties of Sr 3 NCl 3 in this study could provide valuable insights into its potential applications in optoelectronics and solar cell design.