Chiral interactions are prevalent in nature, driving a variety of bio-chemical processes. Discerning the two non-superimposable mirror images of a chiral molecule, known as enantiomers, requires interaction with a chiral reagent with known handedness. Circularly polarized light beams are often used as a chiral reagent. Here, we demonstrate efficient chiral sensitivity with linearly polarized helical light beams carrying an orbital angular momentum of ±lh, in which the handedness is defined by the twisted wavefront structure tracing a left-or right-handed corkscrew pattern as it propagates in space. By probing nonlinear optical response, we show that helicity dependent nonlinear absorption occurs even in achiral molecules and can be precisely controlled. We model this effect by considering induced multipole moments in light-matter interactions. Design and control of light-matter interactions with helical light opens new opportunities in chiroptical spectroscopy, light-driven molecular machines, optical switching, and in-situ ultrafast probing of chiral systems and magnetic materials.

An intuitive and complete understanding of the underlying processes in high harmonic generation (HHG) in solids will enable the development and optimization of experimental techniques for attosecond measurement of dynamical and structural properties of solids. Here we introduce the Wannier quasi-classical (WQC) theory, which allows the characterization of HHG in terms of classical trajectories. The WQC approach completes the single-body picture for HHG in semiconductors, as it is in quantitative agreement with quantum calculations. The importance of WQC theory extends beyond HHG; it enables modeling of dynamic processes in solids with classical trajectories, such as for coherent control and transport processes, potentially providing better scalability and a more intuitive understanding.

A three step model for high harmonic generation from impurities in solids is developed. The process is found to be similar to high harmonic generation in atomic and molecular gases with the main difference coming from the non-parabolic nature of the bands. This opens a new avenue for strong field atomic and molecular physics in the condensed matter phase. As a first application, our conceptual study demonstrates the feasibility of tomographic measurement of impurity orbitals.

We present a detailed study of many-body effects associated with the intraband 1s-2p transition in two-and three-dimensional photoexcited semiconductors. We employ a previously developed excitonic model to treat effects of exchange and phase space filling (PSF). In this work, we extend the model to include intraband transitions and static free-carrier screening. The exciton transition energies are renormalized by many-body interactions, and the excitonic dynamical equations provide simple expressions for the individual contributions of screening, PSF and exchange. The excitonic model correctly predicts the blue shift and bleaching of the 1s exciton resonance due to exchange and PSF. Free-carrier screening is found to enhance these effects by lowering the binding energy of the 1s exciton. In contrast, the effects of free-carrier screening on the 1s-2p transition energy are subtler. For a coherent exciton system, in the absence of free-carrier screening, exchange and PSF lead to a blue shift of the transition energy. However, screening decreases the 1s binding energy faster than the 2p binding energy, which in turn decreases the transition energy. Thus screening effects oppose exchange and PSF, and the overall magnitude and sign of the 1s-2p transition energy shift depends on the free-carrier density. Specifically, for low to moderate excitation densities, exchange and PSF can be dominated by screening, leading to a net redshift of the transition energy. The results for two-and three-dimensional systems are qualitatively similar, although the magnitude of the shift is much smaller in three dimensions. the formalism: the effects of screening, exchange, and PSF on the response to terahertz fields.We start from the familiar semiconductor Hamiltonian 13,25where H 0 represents the one-body energies of electrons and holes, V e−e , V h−h , and V e−h are electron-electron, holehole, and electron-hole Coulomb interactions, and H I is the interaction Hamiltonian between the carriers and the optical and terahertz electric fields. We project this Hamiltonian onto a basis of pair operators using the Usui transformation. 13,28,37,43 This transformation is not unitary, and must be augmented with a suitable electron-hole pairing operator. For optically excited direct-gap semiconductors, it is natural to employ a pairing scheme matching electrons and holes with opposite momenta, such that the total pair crystal momentum is zero. This transformation introduces the electron-hole-pair creation (annihilation) operator, B † k (B k ), which creates (destroys) an electron with crystal momentumhk and a hole with crystal momentum, −hk. The commutation relation for these operators is 13,37which is identical to the canonical bosonic commutation relation, [B k ,B † k ] = 0, when k = k but is given by a fermionic anticommutation relation, {B k ,B † k } = 1, when k = k . We thus refer to the electron-hole pairs as quasibosons (qbosons). We note that the fermionic nature of the qbosons is also reflected in the identity

We present a theoretical investigation of the effect of quantum confinement on high harmonic generation in semiconductor materials by systematically varying the confinement width along one or two directions transverse to the laser polarization. Our analysis shows a growth in high harmonic efficiency concurrent with a reduction of ionization. This decrease in ionization comes as a consequence of an increased band gap resulting from the confinement. The increase in harmonic efficiency results from a restriction of wave packet spreading, leading to greater recollision probability. Consequently, nanoengineering of one and two-dimensional nanosystems may prove to be a viable means to increase harmonic yield and photon energy in semiconductor materials driven by intense laser fields.