Engineering Transistorlike Optical Gain in Two-Dimensional Materials with Berry Curvature Dipoles

Lattice-mismatched and twisted multi-layered materials for efficient solar cells

We argue that alternating-layer structures of lattice mismatched or misaligned (twisted) atomically-thin layers should be expected to be more efficient absorbers of the broad-spectrum of solar radiation than the bulk material of each individual layer. In such mismatched layer-structures the conduction and valence bands of the bulk material, split into multiple minibands separated by minigaps confined to a small-size emerging Brillouin zone due to band-folding. We extended the Shockley-Queisser approach to calculate the photovoltaic efficiency for a band split into minibands of bandwidth ΔEand mini-gaps δGto model the case when such structures are used as solar cells. We find a significant efficiency enhancement due to impact ionization processes, especially in the limit of small but non-zero δG, and a dramatic increase when fully concentrated Sun-light is used.

Metallic Electro-optic Effect in Gapped Bilayer Graphene

Electro-optic (EO) modulation is a critical device action in photonics. Recently, the non-Drude dynamics induced by the Berry curvature dipole (BCD) in metals have attracted attention as a potential candidate for terahertz EO modulation. However, such BCD-induced EO effects can be challenging to realize, often requiring flat bands and complex materials such as a strained magic-angle twisted bilayer graphene on hexagonal boron nitride. Here, we argue that metallic EO can be achieved with a much simpler material, gapped bilayer graphene, with EO coefficients comparable to that in flat-band materials in the terahertz range. In particular, we find metallic EO can be realized without a Berry curvature dipole; we identify skew-scattering and a "snap" (third-order derivative of velocity) can readily produce pronounced Pockels and Kerr EO effects in clean metals. These yield nonreciprocal and field-activated birefringence and field-induced modulations to transmission and reflection, essential components for terahertz EO modulators.

Tunable optical bistability of two-dimensional tilted Dirac system

We study the phenomenon of controlling the light by light known as the optical bistability for the two-dimensional tilted Dirac system. Using the Boltzmann approach under relaxation time approximation, we find that the optical bistability can be controlled by the nonlinear response of the system. For the prototype, we consider an inversion symmetry broken system. We find that the optical bistability associated with the nonlinear response is tunable with the strength of the tilt, gap and chemical potential. The resulting features suggest the inputs for the development of future-generation optical devices.