Large nonlinear optical rectification in atomic hexagonal layers with broken space inversion symmetry

Abstract

Motivated by possible applications in optoelectronics, we consider nonlinear optical rectification (NOR) in two planar hexagonal lattice structures with broken space inversion symmetry—namely, in graphene epitaxially grown on a SiC substrate and in boronitrene (a monolayer of BN). For both structures, we calculate the second-order nonlinear optical susceptibility χ(2)(0;ω, − ω) relevant to the NOR effect and evaluate a bias voltage V0 appearing at the structure terminals under strong laser irradiation. We show that the reason for the χ(2)(0;ω, − ω) being nonvanishing in the examined structures is their sublattice (inversion) asymmetry combined with the trigonal symmetry of their π-electron energy bands near the corners of the hexagonal Brillouin zone of those structures. In spite of being rather small, the trigonal warping of the energy bands involved is found to provide a remarkably large magnitude of the NOR susceptibility, reaching the order of 5 × 10−4 esu for the graphene/SiC overlayer system when the pump photon energy ħω approaches the bandgap energy EG (≈0.26 eV) of the overlying graphene. For a graphene sample of a few microns length, irradiated by a normally incident laser beam with a relatively moderate power density of 10 kW cm−2, the corresponding optical rectification voltage V0 is estimated to be as large as several millivolts. Moreover, the sign of the voltage (i.e., its polarity) can be sharply reversed by sweeping the photon energy through the inter-π-band resonance condition ħω = EG. This frequency-controlled optical switching, if realized, will be a potent technique for graphene-based photonics and optoelectronics.