Resonance energy transfer (RET), the transport of electronic energy from one atom or molecule to another, has significant importance to a number of diverse areas of science. Since the pioneering experiments on RET by Cario and Franck in 1922, the theoretical understanding of the process has been continually refined. This review presents a historical account of the post-Förster outlook on RET, based on quantum electrodynamics, up to the present-day viewpoint. It is through this quantum framework that the short-range, R⁻⁶ distance dependence of Förster theory was unified with the long-range, radiative transfer governed by the inverse-square law. Crucial to the theoretical knowledge of RET is the electric dipole-electric dipole coupling tensor; we outline its mathematical derivation with a view to explaining some key physical concepts of RET. The higher order interactions that involve magnetic dipoles and electric quadrupoles are also discussed. To conclude, a survey is provided on the latest research, which includes transfer between nanomaterials, enhancement due to surface plasmons, possibilities outside the usual ultraviolet or visible range and RET within a cavity.

The multipolar Hamiltonian of quantum electrodynamics is extensively employed in chemical and optical physics to treat rigorously the interaction of electromagnetic fields with matter. It is also widely used to evaluate intermolecular interactions. The multipolar version of the Hamiltonian is commonly obtained by carrying out a unitary transformation of the Coulomb gauge Hamiltonian that goes by the name of Power-Zienau-Woolley (PZW). Not only does the formulation provide excellent agreement with experiment, and versatility in its predictive ability, but also superior physical insight. Recently, the foundations and validity of the PZW Hamiltonian have been questioned, raising a concern over issues of gauge transformation and invariance, and whether observable quantities obtained from unitarily equivalent Hamiltonians are identical. Here, an in-depth analysis of theoretical foundations clarifies the issues and enables misconceptions to be identified. Claims of non-physicality are refuted: the PZW transformation and ensuing Hamiltonian are shown to rest on solid physical principles and secure theoretical ground.

Circular dichroism is the differential rate of absorption of right- and left-handed circularly polarized light by chiral particles. Optical vortices which convey orbital angular momentum (OAM) possess a chirality associated with the clockwise or anticlockwise twisting of their wave front. Here it is highlighted that both oriented and randomly oriented chiral particles absorb photons from twisted beams at different rates depending on whether the vortex twists to the right or the left through a dipole coupling scheme. This is in contrast to previous studies that investigated dipole couplings with vortex modes in the paraxial approximation and showed no such chiral sensitivity to the vortex handedness: only in oriented media where electric quadrupole coupling contributes to optical activity effects due to absorption does such a mechanism exist for paraxial vortices. The distinct difference in the scheme highlighted in this work is that longitudinal fields are taken into account. Due to the vortex dichroism persisting in randomly oriented collections of chiral particles, the mechanism has a distinct advantage in its potential applicability in chemical and biochemical applications where the systems under study are invariably in the liquid phase. Additionally, the result is put into context in terms of the quantifiable optical chirality, highlighting that optical OAM can in fact increase the optical chirality density of an electromagnetic field.

Augmented reality (AR) has the capacity to afford a virtual experience that obviates the reliance on using two-dimensional representations of 3D molecules for teaching stereochemistry to undergraduate students. Using a combination of quantitative instruments and qualitative surveys/interviews, this study explored the relationships between students’ attitudes, perceived cognitive load, spatial ability, and academic performance when engaging in an asynchronous online stereochemistry activity. Our activity was designed using elements of game-based learning, and integrated AR technologies. The control group was provided with a copy of our activity that used two-dimensional drawings, whereas the AR group completed an activity using the AR technologies. For this cohort of students, results indicated significant improvement in academic performance in both the control and AR groups. The introduction of AR technologies did not result in the AR group outperforming the control group. Participants from both groups displayed significant improvements in spatial ability throughout the research period. Further, a moderate correlation (r_s = 0.416) between students’ spatial ability and academic performance was found. No significant intergroup differences in the perceived cognitive loads of students were observed. A significant difference was observed on one item of the Intellectual Accessibility subscale of the ASCI (V2), Complicated–Simple. We found no correlation for student attitude or cognitive load with academic performance. The findings of this study provide insights for future AR-related studies to explore the role of spatial ability, student attitude, and cognitive load in learning performance.

Longitudinal electromagnetic fields generally become comparable with the usually dominant transverse components in strongly focused, non-paraxial beams. For paraxial optical vortex modes it is highlighted here how their angular momentum properties produce longitudinal fields that in general must be accounted for. First-order longitudinal components of quantized Laguerre–Gaussian modes are derived and numerically studied with respect to the paraxial parameter, highlighting light-matter and spin-orbit interactions that stem from the longitudinal fields of paraxial beams in free space. New restrictions are cast on the validity of neglecting longitudinal fields for paraxial optical vortices interacting with atoms, molecules and other nanostructures.