No abstract

Graphene, a monolayer sheet of carbon atoms, exhibits intriguing electronic properties that arise from its massless Dirac dispersion of electrons. A striking example is the half-integer quantum Hall effect, which endorses the presence of Dirac cones or, equivalently, a non-zero (p) Berry's (topological) phase. It is curious how these anomalous features of Dirac electrons would affect optical properties. Here we observe the quantum magneto-optical Faraday and Kerr effects in graphene in the terahertz frequency range. Our results detect the quantum plateaus in the Faraday and Kerr rotations at precisely the quantum Hall steps that hallmark the Dirac electrons, with the rotation angle defined by the fine-structure constant. The robust quantum Hall plateaus in the optical regime, besides being conceptually interesting, may open avenues for new graphene-based optoelectronic applications.

Combining the superior optical properties of their bulk counterparts with quantum confinement effects, lead halide perovskite nanocrystals are unique laser materials with low-threshold optical gain. In such nonlinear optical regimes, multiple excitons are generated in the nanocrystals and strongly affect the optical gain through many-body interactions. Here, we investigate the exciton-exciton interactions in CsPbI nanocrystals by femtosecond transient absorption spectroscopy. From the analysis of the induced absorption signal observed immediately after the pump excitation, we estimated the binding energy for the hot biexcitons that are composed of an exciton at the band edge and a hot exciton generated by the pump pulse. We found that the exciton-exciton interaction becomes stronger for hot excitons with greater excess energies and that the optical gain can be controlled by changing the excess energy of the hot excitons.

Lead halide perovskite nanocrystals (NCs) are one of the most anticipated and promising materials for light-emitting diodes and lasers because of their high photoluminescence quantum yields (PLQYs). However, the formation of trions (charged excitons) in the NCs reduces their PLQYs. Here, we clarify the trion formation mechanism in perovskite CsPbBr3 NCs by analyzing the excitation fluence dependence of transient absorption signals. Under weak photoexcitation, trions are formed by charge carrier trapping at surface states. In contrast, biexciton Auger recombination dominates the trion formation under strong photoexcitation. We found that the postsynthetic surface treatment suppresses the extrinsic surface-related formation of trions. The thorough understanding of the trion formation mechanisms is essential for the PLQY improvement of perovskite NCs and helps to reduce ionization of NCs in solid-state devices.