We demonstrate that the optical response of twisted photonic bilayers, photonic counterparts of van der Waals structures, is sensitive to both spin angular momentum (SAM) and orbital angular momentum (OAM) of light. A beam of unpolarized light with zero angular momentum acquires SAM in transmission and OAM in reflection. The developed analytical theory and numerical calculations show that the SAM and OAM arise from distinct microscopic mechanisms and depend differently on the interlayer distance. The predicted phenomena do not require light absorption and are caused by the photon-helicity-dependent light diffraction by the moiré pattern, which inevitably occurs in the twisted structure, and the SAM-OAM conversion processes. We also reveal strong SAM and OAM in the moiré-diffracted beams. Our findings uncover a profound connection between the emergent SAM and OAM in twisted photonic systems offering new possibilities for angular-momentum-resolved light-matter interactions.

Edges in two-dimensional structures are the source of nonlinear transport and optical phenomena which are particularly important in small-size flakes. We present a microscopic theory of the edge photogalvanic effect, i.e., the formation of DC electric current flowing along the sample edges in response to AC electric field of the incident terahertz radiation, for two-dimensional Dirac materials including the systems with massive and massless charge carriers. The edge current direction is controlled by the AC field polarization. The spectral dependence of the current is determined by the carrier dispersion and the mechanism of carrier scattering, as shown for single-layer and bilayer graphene as examples.

We discuss experimental and theoretical studies of the generation of the third terahertz (THz) frequency harmonic in thin films of Bi2Se3 and Bi2-xSbxTe3-ySey (BSTS) topological insulators (TIs) and the generation of THz radiation in photoconductive antennas based on the TI films. The experimental results, supported by the developed kinetic theory of third harmonic generation, show that the frequency conversion in TIs is highly efficient because of the linear energy spectrum of the surface carriers and fast energy dissipation. In particular, the dependence of the third harmonic field on the pump field remains cubic up to the pump fields of 100 kV/cm. The generation of THz radiation in TI-based antennas is obtained and described for the pump, with the energy of photons corresponding to the electron transitions to higher conduction bands. Our findings open up possibilities for advancing TI-based films into THz photonics as efficient THz wave generators and frequency converters.

Driving a 2D electron gas by AC electric field leads to a DC electric current flowing along the edge of the system. The effect is caused by the local breaking of space inversion symmetry at the edge. The current is generated in a narrow stripe determined by the screening length of the AC electric field and the mean free path of carriers. The developed microscopic theory of the effect shows that the excitation spectrum of the edge current and even the current direction depend on the mechanism of electron scattering. In an external magnetic field, the excitation spectrum of the current features the cyclotron resonance.

Excitation of a topological insulator by a high-frequency electric field of a laser radiation leads to a dc electric current in the helical edge channel whose direction and magnitude are sensitive to the radiation polarization and depend on the physical properties of the edge. An overview of theoretical and experimental studies of such edge photoelectric effects in 2D topological insulators based on semiconductor quantum wells is presented. First, a phenomenological description is given of edge photocurrents, which may originate from the photogalvanic effects or the photon drag effects, for edges of all possible symmetry. Then, microscopic mechanisms of photocurrent generation for different types of optical transitions involving helical edge states are discussed. They include direct and indirect optical transitions within the edge channel and edge-to-bulk optical transitions.

We show that the stacks of two-dimensional semiconductor crystals with the chiral packing exhibit optical activity and circular dichroism. We develop a microscopic theory of these phenomena in the spectral range of exciton transitions that takes into account the spin-dependent hopping of excitons between the layers in the stack and the interlayer coupling of excitons via electromagnetic field. For the stacks of realistic two-dimensional semiconductors such as transition metal dichalcogenides, we calculate the rotation and ellipticity angles of radiation transmitted through such structures. The angles are resonantly enhanced at the frequencies of both bright and dark exciton modes in the stack. We also study the photoluminescence of chiral stacks and show that it is circularly polarized.

We study optical forces acting upon semiconductor quantum dots and the force driven motion of the dots in a colloid. In the spectral range of exciton transitions in uantum dots, when the photon energy is close to the exciton energy, the polarizability of the dots is drastically increased. It leads to a resonant increase of both the gradient and the scattering contributions to the optical force, which enables the efficient manipulation with the dots. We reveal that the optical grating of the colloid leads to the formation of a fluid photonic crystal with spatially periodic circulating fluxes and density of the dots. Pronounced resonant dielectric response of semiconductor quantum dots enables a separation of the quantum dots with different exciton frequencies.

We show that for lattice-mismatched zinc-blende-type (110)-grown quantum wells a significant contribution to the zero-magnetic-field spin splitting of electron subbands comes from strain-induced spin-orbit coupling. Combining envelope function theory and atomistic tight-binding approach we calculate spin-orbit splitting constants for realistic quantum wells. It is found that the strain due to lattice mismatch in conventional GaAs/AlGaAs structures may noticeably modify the spin splitting while in InGaAs/GaAs structures it plays a major role and may even change the sign of the spin splitting constant.

The study of electron transport and scattering processes limiting electron mobility in high-quality semiconductor structures is central to solid-state electronics. Here, we uncover an unavoidable source of electron scattering which is caused by fluctuations of nuclear spins. We calculate the momentum relaxation time of electrons in quantum wells governed by the hyperfine interaction between electrons and nuclei and show that this time drastically depends on the spatial correlation of nuclear spins. Moreover, the scattering processes accompanied by a spin flip are a source of the backscattering of Dirac fermions at conducting surfaces of topological insulators.

To date, optical orientation of free-carrier spins and spin currents have been achieved by circularly polarized light, while the linearly polarized light has been used for optical alignment of electron momenta. Here we show that, in low-dimensional structures, absorption of the linearly polarized light also leads to the spin polarization and spin photocurrent, and, thus, the electron and hole spins can be manipulated by light of zero helicity. The microscopic description of the both effects is developed for interband optical transitions in undoped quantum wells (QWs) as well as for direct intersubband and indirect intrasubband (Drude-like) transitions in n-doped QW structures.

The generation, manipulation and detection of spin-polarized electrons in low-dimensional semiconductors are at the heart of spintronics. Pure spin currents, that is, fluxes of magnetization without charge current, are quite attractive in this respect. A paradigmatic example is the spin Hall effect, where an electrical current drives a transverse spin current and causes a non-equilibrium spin accumulation observed near the sample boundary1,2. Here we provide evidence for an another effect causing spin currents which is fundamentally different from the spin Hall effect. In contrast to the spin Hall effect, it does not require an electric current to flow: without bias the spin separation is achieved by spin-dependent scattering of electrons in media with suitable symmetry. We show, by free-carrier absorption of terahertz (THz) radiation, that spin currents flow in a wide range of temperatures. Moreover, the experimental results provide evidence that simple electron gas heating by any means is already sufficient to yield spin separation due to spin-dependent energy-relaxation processes.