Two-photon absorption in gapped bilayer graphene with a tunable chemical potential.

Despite the now vast body of two-dimensional materials under study, bilayer graphene remains unique in two ways: it hosts a simultaneously tunable band gap and electron density; and stems from simple fabrication methods. These two advantages underscore why bilayer graphene is critical as a material for optoelectronic applications. In the work that follows, we calculate the one- and two-photon absorption coefficients for degenerate interband absorption in a graphene bilayer hosting an asymmetry gap and adjustable chemical potential-all at finite temperature. Our analysis is comprehensive, characterizing one- and two-photon absorptive behavior over wide ranges of photon energy, gap, chemical potential, and thermal broadening. The two-photon absorption coefficient for bilayer graphene displays a rich structure as a function of photon energy and band gap due to the existence of multiple absorption pathways and the nontrivial dispersion of the low energy bands. This systematic work will prove integral to the design of bilayer-graphene-based nonlinear optical devices.

Phonon-induced gaps in graphene and graphite observed by angle-resolved photoemission.

Mapping by angle-resolved photoemission spectroscopy of the spectral functions of graphite and graphene layers at low temperatures reveals a heretofore unreported gap of ~ 67 meV at normal emission. This gap persists to room temperature and beyond, and diminishes for increasing emission angles. We show that this gap arises from electronic coupling to out-of-plane vibrational modes at the K(¯) point in the surface Brillouin zone in accordance with conservation laws and selection rules governed by quantum mechanics. Our study suggests a new approach for characterizing phonons and electron-phonon coupling in solids.

Using electronic coherence to probe a deeply embedded quantum well in bimetallic Pb/Ag films on Si(111).

We report an experiment in which we utilize electronic coherence to probe a deeply embedded thin film as a quantum well. An atomically uniform Ag film prepared on Si(111) was covered by Pb films up to 70 A thick, and the resulting electronic structure was examined by angle-resolved photoemission spectroscopy. Despite a photoemission escape depth of just a few Angströms and an incommensurate Pb/Ag interface, the data reveal a striking Fabry-Pérot-like structure characteristic of an Ag etalon buried deeply under the Pb overlayers. Our simulations clearly illustrate the manifest coherence of the electronic structures, permitting the characterization of the embedded Ag quantum well.