R.-Y. Yuan

Broadband polarization-insensitive terahertz absorber based on heavily doped silicon surface relief structures

Design, simulation, and measurement of a broadband polarization-insensitive terahertz absorber are presented. The absorber utilizes subwavelength surface relief grating structures on a heavily phosphorous-doped silicon substrate surface. Experimental results indicate that the absorber achieved over 90% absorption in a broad frequency range from 3 to 5.1 THz due to destructive interference of waves. Simulation results indicate that the design can be extended to a wider absorption bandwidth by optimizing the grating parameters, doping types, and doping concentration. This broadband polarization-insensitive absorber has potential applications in anti-reflection coatings and imaging systems.

Coulomb interaction effects on the terahertz photon-assisted tunneling through an InAs quantum dot

Recently, the terahertz (THz) photon-assisted tunneling (PAT) through a two-level InAs quantum dot (QD) has been successfully realized in experiment [Phys. Rev. Lett. {\bf 109}, 077401 (2012)]. The Coulomb interaction in this device is comparable with the energy difference between the two energy levels. We theoretically explore the effects of Coulomb interaction on the PAT processes and show that the main peaks of the experiment can be well derived by our model analysis. Furthermore, we find additional peaks, which were not addressed in the InAs QD experiment and may be further identified in experiments. In particular, we show that, to observe the interesting photon-induced excited state resonance in InAs QD, the Coulomb interaction should be larger than THz photon frequency.

Temperature-dependent of Nonlinear Optical Conductance of Graphene-based Systems in High-intensity Terahertz Field

For multi-photon processed with the linear dispersion in the high-intensity terahertz (THz) field, we have systematically investigated the temperature-dependent nonlinear optical response of graphene-based systems, including single layer graphene, graphene superlattice and gapped graphene. In the intrinsic single layer graphene system, it demonstrates that, at low temperature, nonlinear optical conductivities of the thirdand fifth-order are respectively five and ten orders of magnitude larger than the universal conductivity with high-intensity and low frequency THz wave.In the graphene superlattice and gapped graphene systems, the optical responses enhanced because of the anisotropic massless and massive Dirac fermions.

Terahertz photon-electron pump effect and Fano-like resonance in the two-level InAs quantum dot

In this work, with the completely asymmetric terahertz (THz) irradiation, electron transport through a two-level InAs quantum dot is theoretically discussed. The Coulomb interaction in this system is compared with the energy difference between the two energy levels and is finite contrasted with the Microwave Field system. We investigate the average current trend with the change of Coulomb interaction and the THz irradiation strength. Our results reveal that there exists the photon-electron pump effect at the Coulomb interaction related energy level, and demonstrate Fano-like resonance in the low THz field strength due to the presence of the photon induced excited state resonance. We also find that the Fano-like resonance can be suppressed with increasing the temperature.

Spin and valley transport in the ferromagnetic MoS2 junctions subjected by the gate voltage

In this paper, we have studied valley and spin-polarized transport with different gate voltage in ferromagnetic MoS2 junctions. The results show that, the valley and spin transport through the junctions has a large oscillation. In particular, a fully spin polarized current can be put out by effectively tuning the gate voltage U. Moreover, with increasing of U, the scope of the valley and spin polarization can be greatly expanded. These findings indicate the structure is a considerable candidate for the spintronics or valleytronics device.

Electric controlled spin and valley transport of massive electrons in graphene with spin-orbit coupling

In this work, we have explored the influence of an external electric field on the spin and valley transport of massive electrons in a graphene system with spin-orbit coupling. Both the strength and width of the spin- and valley-polarization are greatly dependent on the external electric field. As the external electric field increases, the spin/valley polarization can be enhanced, even up to 100%. In addition, the presence of a gap resulting from the interplay of massive electrons and spin-orbit coupling can occur in the direction of the spin polarization being changed. Without the gap, spin-down electrons can be filtered at the low-energy Fermi level. However, with the gap, the effect is just the opposite; spin-up electrons are filtered. These findings may open an avenue for the electric control of valley and spin transport in graphene-based electronic devices.

Spin-dependent and photon-assisted transmission enhancement and suppression in a magnetic-field tunable ZnSe/Zn1–xMnxSe heterostructure

Using the effective-mass approximation and Floquet theory, we theoretically investigate the terahertz photon-assisted transport through a ZnSe/Zn1−xMnxSeheterostructure under an external magnetic field, an electric field, and a spatially homogeneous oscillatory field. The results show that both amplitude and frequency of the oscillatory field can accurately manipulate the magnitude of the spin-dependent transmission probability and the positions of the Fano-type resonance due to photon absorption and emission processes. Transmission resonances can be enhanced to optimal resonances or drastically suppressed for spin-down electrons tunneling through the heterostructure and for spin-up ones tunneling through the same structure, resonances can also be enhanced or suppressed, but the intensity is less than the spin-down ones. Furthermore, it is important to note that transmission suppression can be clearly seen from both the spin-down component and the spin-up component of the current density at low magnetic field; at the larger magnetic field, however, the spin-down component is suppressed, and the spin-up component is enhanced. These interesting properties may provide an alternative method to develop multi-parameter modulation electron-polarized devices.

Nonlinear optical response in Kronig Penney type graphene superlattice in terahertz regime

The terahertz nonlinear optical response in Kronig–Penney (KP) type graphene superlattice is demonstrated. The single-, triple- and quintuple-frequencies of the fifth-order nonlinear responses are investigated for different frequencies and temperatures with the angle φ along the periodicity of the superlattice toward the external field tuning from 0 to π/2. The results show that the fifth-order nonlinear optical conductance of graphene superlattice is enhanced in the terahertz regime when φ = 0, i.e. an external field is applied along the periodicity of the superlattice. The fifth-order nonlinear optical conductances at φ = 0 for different frequencies and temperatures are calculated. The results show that the nonlinear optical conductance is enhanced in low frequency and low temperature. Our results suggest that KP type graphene superlattices are preferred structures for developing graphene-based nonlinear photonics and optoelectronics devices.