It is now well established that a laser pulse can demagnetize a ferromagnet. However, for a long time, it has not had an analytic theory because it falls into neither nonlinear optics (NLOs) nor magnetism. Here we attempt to fill this gap by developing a nonlinear optical theory centered on the spin moment, instead of the more popular susceptibility. We first employ group theory to pin down the lowest order of the nonzero spin moment in a centrosymmetric system to be the second order, where the second-order density matrix contains four terms of sum frequency generation and four terms of difference frequency generation (DFG). By tracing over the product of the density matrix and the spin matrix, we are now able to compute the light-induced spin moment. We apply our theory to FePt and FePd, two most popular magnetic recording materials with identical crystal and electronic structures. We find that the theory can clearly distinguish the difference between those two similar systems. Specifically, we show that FePt has a stronger light-induced spin moment than FePd, in agreement with our real-time ultrafast demagnetization simulation and the experimental results. Among all the possible NLO processes, DFGs produce the largest spin moment change, a manifestation of optical rectification. Our research lays a solid theoretical foundation for femtomagnetism, so the light-induced spin moment reduction can now be computed and compared among different systems, without time-consuming real-time calculations, representing a significant step forward.

It is now well established that a laser pulse can demagnetize a ferromagnet. However, for a long time, it has not had an analytic theory because it falls into neither nonlinear optics (NLOs) nor magnetism. Here we attempt to fill this gap by developing a nonlinear optical theory centered on the spin moment, instead of the more popular susceptibility. We first employ group theory to pin down the lowest order of the nonzero spin moment in a centrosymmetric system to be the second order, where the second-order density matrix contains four terms of sum frequency generation and four terms of difference frequency generation (DFG). By tracing over the product of the density matrix and the spin matrix, we are now able to compute the light-induced spin moment. We apply our theory to FePt and FePd, two most popular magnetic recording materials with identical crystal and electronic structures. We find that the theory can clearly distinguish the difference between those two similar systems. Specifically, we show that FePt has a stronger light-induced spin moment than FePd, in agreement with our real-time ultrafast demagnetization simulation and the experimental results. Among all the possible NLO processes, DFGs produce the largest spin moment change, a manifestation of optical rectification. Our research lays a solid theoretical foundation for femtomagnetism, so the light-induced spin moment reduction can now be computed and compared among different systems, without time-consuming real-time calculations, representing a significant step forward.

The metal halide perovskite nanocrystals exhibit a remarkable tolerance to midgap defect states, resulting in high photoluminescence quantum yields. However, the potential of these nanocrystals for applications in display devices is hindered by the suppression of biexcitonic emission due to various Auger recombination processes. By adopting single-particle photoluminescence spectroscopy, herein, we establish that the biexcitonic quantum efficiency increases with the increase in the number of facets on cesium lead bromide perovskite nanocrystals, progressing from cube to rhombic dodecahedron to rhombicuboctahedron nanostructures. The observed enhancement is attributed mainly to an increase in their surface polarity as the number of facets increases, which reduces the Coulomb interaction of charge carriers, thereby suppressing Auger recombination. Moreover, Auger recombination rate constants obtained from the time-gated photon correlation studies exhibited a discernible decrease as the number of facets increased. These findings underscore the significance of facet engineering in fine-tuning biexciton emission in metal halide perovskite nanocrystals.

All-optical spin switching (AOS) is a new phenomenon found in a small group of magnetic media, where a single laser pulse can switch spins from one direction to another, without assistance of a magnetic field, on a time scale much shorter than existing magnetic technology. However, despite intensive efforts over a decade, its underlying working principle remains elusive. Here through manganese-based Heusler ferrimagnets, we show that a group of flat bands around the Fermi level act as gateway states to form efficient channels for spin switching, where their noncentrosymmetry allows us to correlate the spin dynamics to the second-order optical response. To quantify their efficacy, we introduce the third-rank Pancharatnam-Berry tensor (PB tensor), eta((3)) = < i vertical bar p vertical bar m > < m vertical bar p vertical bar f > < f vertical bar p vertical bar i >, where vertical bar i >, vertical bar m > and vertical bar f > are initial, intermediate and final band states, respectively, and p is the momentum operator. A picture emerges: Those which show AOS, such as the recently discovered Mn2RuGa, always have a large PB tensor element but have a small sublattice spin moment ratio, consistent with the prior experimental small remanence criterion. This does not only reveal that the delicate balance between the large PB tensor element and the small sublattice spin ratio plays a decisive role in AOS, but also, conceptually, connects the nth-order nonlinear optics to (n + 1)th-rank PB tensors in general.

The phenomenon of excited-state symmetry breaking is often observed in multipolar molecular systems, significantly affecting their photophysical and charge separation behavior. As a result of this phenomenon, the electronic excitation is partially localized in one of the molecular branches. However, the intrinsic structural and electronic factors that regulate excited-state symmetry breaking in multibranched systems have hardly been investigated. Herein, we explore these aspects by adopting a joint experimental and theoretical investigation for a class of phenyleneethynylenes, one of the most widely used molecular building blocks for optoelectronic applications. The large Stokes shifts observed for highly symmetric phenyleneethynylenes are explained by the presence of low-lying dark states, as also established by two-photon absorption measurements and TDDFT calculations. In spite of the presence of low-lying dark states, these systems show an intense fluorescence in striking contrast to Kasha's rule. This intriguing behavior is explained in terms of a novel phenomenon, dubbed "symmetry swapping" that describes the inversion of the energy order of excited states, i.e., the swapping of excited states occurring as a consequence of symmetry breaking. Thus, symmetry swapping explains quite naturally the observation of an intense fluorescence emission in molecular systems whose lowest vertical excited state is a dark state. In short, symmetry swapping is observed in highly symmetric molecules having multiple degenerate or quasi-degenerate excited states that are prone to symmetry breaking.

The surface domains of self-assembled amphiphiles are well-organized and can perform many physical, chemical, and biological functions. Here, we present the significance of chiral surface domains of these self-assemblies in transferring chirality to achiral chromophores. These aspects are probed using l- and d-isomers of alkyl alanine amphiphiles which self-assemble in water as nanofibers, possessing a negative surface charge. When bound on these nanofibers, positively charged cyanine dyes (CY524 and CY600), each having two quinoline rings bridged by conjugated double bonds, show contrasting chiroptical features. Interestingly, CY600 displays a bisignated circular dichroic (CD) signal with mirror-image symmetry, while CY524 is CD silent. Molecular dynamics simulations reveal that the model cylindrical micelles (CM) derived from the two isomers exhibit surface chirality and the chromophores are buried as monomers in mirror-imaged pockets on their surfaces. The monomeric nature of template-bound chromophores and their binding reversibility are established by concentration- and temperature-dependent spectroscopies and calorimetry. On the CM, CY524 displays two equally populated conformers with opposite sense, whereas CY600 is present as two pairs of twisted conformers in each of which one is in excess, due to differences in weak dye-amphiphile hydrogen bonding interactions. Infrared and NMR spectroscopies support these findings. Reduction of electronic conjugation caused by the twist establishes the two quinoline rings as independent entities. On-resonance coupling between the transition dipoles of these units generates bisignated CD signals with mirror-image symmetry. The results presented herein provide insight on the little-known structurally induced chirality of achiral chromophores through transfer of chiral surface information.

The optical properties of non-toxic indium phosphide (InP) quantum dots (QDs) are impinged by the existence of characteristic deep trap states. Several surface engineering strategies have been adopted to improve their optical quality, which has promoted the use of InP QDs for various technological applications. An antithetical approach involves the effective utilization of the deep trap states in InP QDs to modulate back electron transfer rates. Here, we explore the influence of the core-size of InP on their In-to-P stoichiometry and charge transfer dynamics when bound to an acceptor molecule, decyl viologen (DV2+). The mechanism of interaction of InP and DV2+ based on the quenching sphere model established the presence of (i) a 1:1 complex of DV2+ bound on InP and (ii) immobile quenchers in the quenching sphere, depending on the concentration of DV2+. While the forward electron transfer rates from photoexcited InP to bound DV2+ does not substantially vary with an increase in core size, the back electron transfer rates are found to be retarded. Findings from inductively coupled plasma-optical emission spectroscopy (ICP-OES) and X-ray photoelectron spectroscopy (XPS) reveal that the In to P ratio is higher for QDs with larger core size, which further brings about increased carrier trapping and a decreased rate of charge recombination. Furthermore, long-lived charge-separated states in DV2+ bound to InP, extending to hundreds of milliseconds, are obtained by varying the number of DV2+ in the quenching sphere of the QDs.

In this review, we discuss the fabrication, optimization and applications of liquid crystal microcapsules in detail. The current challenges and potential development directions are also introduced briefly.

The fascinating optoelectronic properties of semi-conductor quantum dots (QDs) originate from the quantum confinement effect; i.e., the band gap increases as the size decreases. Another significant parameter dictating the photophysics of QD is the dynamics of trapping and detrapping from the trap-states, but it is nonetheless less understood. To understand these aspects, herein we investigate the photoluminescence (PL) fluctuations in CdSe QDs using time-resolved PL spectroscopy by systematically varying its core size, maintaining a constant shell thickness by coating with CdS followed by ZnS. The probability density distribution of ON- and OFF-events of QDs is constructed from the PL trajectories and fitted with the truncated power-law from which the trapping (k(t)) and detrapping (k(d)) rate constants are estimated. The increase in phi(PL) observed with the increase in core size of CdSe at the ensemble level is related to the enhanced k(d)/k(t) and charge carrier wave function localization in the core. Indeed, the band gap decreases as the core size increases, bringing the trap-states close to the band edge positions, leading to an efficient detrapping of carriers. The fluorescence lifetime-intensity distribution plots revealed the presence of a high-intensity and high-lifetime component due to neutral exciton recombination and a low-intensity and low-lifetime component due to trap-induced Auger recombination. The decay kinetics in a single QD is further modeled using a pre-equilibrium approximation.

We present a study of the propagation of three-dimensional (3D) bipolar electromagnetic ultrashort pulses in an inhomogeneous array of semiconductor carbon nanotubes (CNTs) in the presence of a control high-frequency (HF) electric field. The inhomogeneity is present in the form of a layer with an increased concentration of conduction electrons, which acts as a barrier for the propagation of ultrashort electromagnetic pulses through the CNT array. The dynamics of the pulse is described by a nonlinear equation for the vector potential of the electromagnetic field (it takes the form of a 3D generalization of the sine-Gordon equation), derived from the Maxwell's equations and averaged over the period of the HF control field. By means of systematic simulations, we demonstrate that, depending on the amplitude and frequency of the HF control, the ultrashort pulse approaching the barrier layer either passes it or bounces back. The layer's transmissivity for the incident pulse is significantly affected by the amplitude and frequency of the HF control field, with the reflection coefficient nearly vanishing in intervals that make up a discrete set of transparency windows, which resembles the effect of the electromagnetically induced transparency. Having passed the barrier, the ultrashort pulse continues to propagate, keeping its spatiotemporal integrity. The results may be used for the design of soliton valves, with the transmissivity of the soliton stream accurately controlled by the HF field.

In this article, the detailed photoexcitation dynamics which combines nonadiabatic molecular dynamics with electronic transitions shows the occurrence of amplified spontaneous emission (ASE) in conjugated polymers, accompanied by spontaneous electric polarization. The elaborate molecular dynamic process of ultrafast photoexcitation can be described as follows: Continuous external optical pumping (laser of 70 µJ/cm2) not only triggers the appearance of an instantaneous four-level electronic structure but causes population inversion for ASE as well. At the same time, the phonon spectrum of the conjugated polymer changes, and five local infrared lattice vibrational modes form at the two ends, which break the original symmetry in the system and leads to charge accumulation at the ends of the polymer chain without an external electric field. This novel phenomenon gives a brand-new avenue to explain how the lattice vibrations play a role in the evolution of the stimulated emission, which leads to an ultrafast effect in solid conjugated polymers.

The handedness of a supramolecular chiral aggregate is often assigned based on the sign of circular dichroism spectra, adopting the exciton chirality method. However, the method does not properly account for the nature of intermolecular interactions. We introduce a generalized picture on the use of the sign of chiral signals in determining the helicity of chiral aggregates, rooted in the exciton model, supported by TD-DFT results.