Institute of Spectroscopy

The photoluminescence (PL) kinetics of semiconductor nanocrystals (quantum dots, QDs) includes a surprisingly wide range of times. The presence of shallow metastable trap states leads to the appearance of a long tail on the PL decay curve, the so-called delayed PL, at times ranging from hundreds of nanoseconds to microseconds. Surprisingly, intensity and lifetime of the delayed PL from a single QD notably fluctuate in time. In this work, delayed PL of single colloidal core/shell CdSeS/ZnS QDs was in depth studied by time-gated PL traces analysis. Observed delayed PL blinking allowed us to study the shallow trap states. We show that single QD PL in active trap state is characterized by mono or bi-exponential decays with times altering from 60 ns to 600 ns, that is much exceed exciton lifetime in bright state ~30 ns. Simulation allowed us to conclude that the absence of an exciton component in the kinetics of delayed PL can be explained by the fast (~100 ps) charge carrier Auger trapping process on a shallow trap state. We associate this state with a localized defect at the inner core/shell QD interface. Also, we demonstrate that the variation in the lifetimes and delayed PL decay shapes can be described entirely by a change in the thermal de-trapping rate.

This work provides new insights into the emission mechanisms of laser-produced lithium plasma in the extreme ultraviolet (EUV) spectral region, with important implications for the development of EUV light sources. High-resolution spectra of the Li III Lyman series were recorded using a custom-built grazing-incidence spectrometer. The analysis of relative line intensities and profiles allowed us to determine key plasma parameters, including the electron density and temperature of the radiating region. Contrary to the conventional understanding that EUV emission primarily originates from high-density, collisionally dominated plasma regions, we demonstrate that the dominant EUV power flux can arise from a recombination-dominated mechanism occurring in a low-density peripheral zone, far from the plasma core. This conclusion is supported by spatially and spectrally resolved imaging of the EUV-emitting plasma, obtained with a Schwarzschild objective, as well as by simulations performed with the RZ-Line code.

Abstract Topological pumping refers to transfer of a physical quantity governed by the system topology, resulting in quantized amounts of the transferred quantities. It is a ubiquitous wave phenomenon typically considered subject to exactly periodic adiabatic variation of the system parameters. Recently, proposals for generalizing quasi-periodic topological pumping and identifying possible physical settings for its implementation have emerged. In a strict sense, pumping with incommensurate frequencies can only manifest over infinite evolution distances, raising a fundamental question about its observability in real-world finite-dimensional systems. Here we demonstrate that bi-chromatic topological pumping with two frequencies, whose ratio is an irrational number, can be viewed as the convergence limit of pumping with two commensurate frequencies representing the best rational approximations of that irrational number. In our experiment, this phenomenon is observed as the displacement of a light beam center in photorefractive crystals induced by two optical lattices. The longitudinal periods of the lattices, that in the paraxial approximation emulate two pumping frequencies, are related as Fibonacci numbers, successively approaching the golden ratio. We observed that a one-cycle displacement of the beam center at each successive approximation is determined by the relation between successive Fibonacci numbers, while the average direction of propagation (emulating average pumping velocity) of the beam is determined by the golden ratio.

We present a doubly resonant nanocavity for second harmonic generation (SHG) in the visible range, consisting of a 590 nm crystalline gallium phosphide (c-GaP) layer, a gold mirror, and a silica spacer to achieve optimal overlap of pump and harmonic resonances. Experimental results show a conversion efficiency spanning 4 orders of magnitude, with a maximum η̃max = 0.014% at λSHG = 600 nm, comparable to best-performing semiconductor metasurfaces, and surpassing a bulk 400 μm GaP crystal by over an order of magnitude. Using nonlinear scattering theory, we explain the spectral response and demonstrate wavelength tunability via angle of incidence or spacer thickness adjustments. We extend this result to a closed-cavity design with an ultrathin gold top layer which we predict can achieve efficiencies up to η̃max ∼ 0.4% when the thickness of the c-GaP layer approaches the coherence length. This design can be scaled up to planar nanocavities with higher Q-factor and used to enhance the performance of metasurfaces, advancing nonlinear conversion efficiency toward practical applications.

The fabrication of a nanohybrid photocatalyst that combines α-Fe2O3 nanoparticles with graphitic carbon nitride (g-C3N4) is reported. The ensuing direct Z-scheme heterojunction greatly boosts the photocatalytic activity of the α-Fe2O3/g-C3N4 nanohybrids. This results in organic dye degradation rates more than two times higher than its individual components, promoted by the efficient charge separation and transfer of the Z-scheme heterojunction mechanism of the nanohybrid photocatalyst. In addition, recyclability tests show an outstanding stability of the nanohybrids spanning five consecutive dye degradation experiments, during which the degradation rate is slightly improved. The origin of the improved photocatalytic performance of the nanohybrid lies in the intimate interaction between α-Fe2O3 and g-C3N4 afforded by the two-step fabrication process, which enables the direct and controlled growth of α-Fe2O3 nanoparticles on g-C3N4. A first ultrasound impregnation step promotes the effective anchoring of stable Fe species via Fe-N and C-N/C-O bonding, while a second microwave phase conversion step induces the subsequent growth of α-Fe2O3 nanoparticles on the g-C3N4 sheets. Careful control of the FeCl3 precursor concentration up to a threshold value of 0.25 M during impregnation enables complete control over their size and phase. This approach clearly highlights the benefits of microwave reactor systems in the fabrication of hematite-based Z-scheme photocatalytic, overcoming the limitations of conventional thermal treatment technology.

In this second installment, we continue examining C.V. Raman’s amazing discovery of the Raman Effect in 1928, a milestone that fundamentally altered the landscape of physics and catalyzed advancements in quantum spectroscopy. Raman’s observation that light scattering could induce wavelength shifts provided empirical support for quantum theory, establishing a foundation for modern spectroscopic methodologies. His pioneering research earned him the 1930 Nobel Prize in Physics and continues to influence disciplines ranging from analytical chemistry to biomedical imaging. This discussion traces Raman’s scientific contributions, emphasizing his transformative role in the revolution of quantum spectroscopy and its contemporary applications. Across this two-part series, we have explored Raman’s quest to characterize what he described as “a new kind of radiation” and its profound impact on scientific progress.

Cesium lead halide nanocrystals and quantum dots are prominent materials for different types of applications because of their remarkable photophysical properties. However, they are also known to exhibit the same...