The photon-avalanche effect, which involves both absorption from a metastable intermediate level and cross-relaxation energy transfer, introduces a new trend in the achievement of efficient up-conversion lasers. In this paper, we report a quite general theoretical treatment of this process which is successfully applied to the case of ${\mathrm{LiYF}}_4$:${\mathrm{Nd}}^{3+}$. This model shows clearly that the avalanche effect may occur, above a pumping threshold, only if the cross-relaxation energy-transfer probability is higher than the relaxation rate from the up-converted excited state to the levels located below the metastable intermediate state.

The electronic structure of a chiral Yb(III)-based complex is fully determined by taking advantage of experimental magnetic, luminescence, and chiroptical (NIR-ECD and CPL) characterizations in combination with ab initio wavefunction calculations. The combined use of these techniques allows determining with high resolution the electronic structure diagram as well as the nature of the different states involved in the magnetic and chiroptical properties of the investigated complex. The crystal-field picture deduced from spectroscopic measurements (absorption and emission) is used to reproduce the magnetic properties. Subsequently, advanced ab initio calculations demonstrate that global chiroptical spectra correspond to the sum of entangled transitions with similar or opposite polarizations.

Starting from a microscopic description, we show the analogy between photon avalanche and second-order phase transition. This justifies the validity of the mean-field approximation. Then we show that the rate equations reproduce quantitatively the experimental results as soon as the Gaussian intensity profile of the input beam is taken into account. Moreover, we apply the general Landau theory to the photon avalanche: the absorption between the excited state is the control parameter of the transition whereas the ground-state absorption is an external field. By this way the impact of the residual nonresonant ground-state absorption is clarified.

Circularly polarized luminescence (CPL) is a fast growing research field as a complementary chiroptical spectroscopy alternative to the conventional circular dichroism or in the quest of devices producing circularly polarized light for different applications. Because chiroptical signals are generally lower than 0.1%, conventional chiral spectroscopies rely on polarization time modulation requiring step-by-step wavelength scanning and a long acquisition time. High throughput controls motivated the development of CPL spectrophotometers using cameras as detectors and space polarization splitting. However, CPL measurements imposes careful precautions to minimize the numerous artifacts arising from experimental imperfections. Some previous work used complex calibration procedure to this end. Here we present a rigorous Mueller analysis of an instrument based on polarizations space splitting. We show that by using one camera and combining spatial and temporal separation through two switchable circular polarization encoding arms we can record accurate CPL spectra without the need of any calibration. The measurements robustness and their fast acquisition times are exemplified on different chiral emitters.