Julien Rioux

Coherent Control of Ballistic Photocurrents in Multilayer Epitaxial Graphene Using Quantum Interference

We report generation of ballistic electric currents in unbiased epitaxial graphene at 300 K via quantum interference between phase-controlled cross-polarized fundamental and second harmonic 220 fs pulses. The transient currents are detected via the emitted terahertz radiation. Because of graphenes special structure symmetry, the injected current direction can be well controlled by the polarization of the pump beam in epitaxial graphene. This all optical injection of current provides not only a noncontact way of injecting directional current in graphene but also new insight into optical and transport process in epitaxial graphene.

Current injection by coherent one- and two-photon excitation in graphene and its bilayer

Coherent control of optically-injected carrier distributions in single and bilayer graphene allows the injection of electrical currents. Using a tight-binding model and Fermi’s golden rule, we derive the carrier and photocurrent densities achieved via interference of the quantum amplitudes for two-photon absorption at a fundamental frequency, w, and one-photon absorption at the second harmonic, 2w. Strong currents are injected under co-circular and linear polarizations. In contrast, opposite-circular polarization yields no net current. For single-layer graphene, the magnitude of the current is unaffected by the rotation of linear-polarization axes, in contrast with the bilayer and with conventional semiconductors. The dependence of the photocurrent on the linear-polarization axes is a clear and measurable signature of interlayer coupling in AB-stacked multilayer graphene. We also find that single and bilayer graphene exhibit a strong, distinct linear-circular dichroism in two-photon absorption.

Evidence for interlayer electronic coupling in multilayer epitaxial graphene from polarization-dependent coherently controlled photocurrent generation

Most experimental studies to date of multilayer epitaxial graphene on C-face SiC have indicated that the electronic states of different layers are decoupled as a consequence of rotational stacking. We have measured the third order nonlinear tensor in epitaxial graphene as a novel approach to probe interlayer electronic coupling, by studying THz emission from coherently controlled photocurrents as a function of the optical pump and THz beam polarizations. We find that the polarization dependence of the coherently controlled THz emission expected from perfectly uncoupled layers, i.e. a single graphene sheet, is not observed. We hypothesize that the observed angular dependence arises from weak coupling between the layers; a model calculation of the angular dependence treating the multilayer structure as a stack of independent bilayers with variable interlayer coupling qualitatively reproduces the polarization dependence, providing evidence for coupling.

Full band structure calculation of two-photon indirect absorption in bulk silicon

Degenerate two-photon indirect absorption in silicon is an important limiting effect on the use of silicon structures for all-optical information processing at telecommunication wavelengths. We perform a full band structure calculation to investigate two-photon indirect absorption in bulk silicon, using a pseudopotential description of the energy bands and an adiabatic bond charge model to describe phonon dispersion and polarization. Our results agree well with some recent experimental results. The transverse acoustic/optical phonon-assisted processes dominate.

Photoinduced pure spin-current injection in graphene with Rashba spin-orbit interaction

We propose a photoexcitation scheme for pure spin-current generation in graphene subject to a Rashba spin-orbit coupling. Although excitation using circularly polarized light does not result in optical orientation of spins in graphene unless an additional magnetic field is present, we show that excitation with linearly polarized light at normal incidence yields spin-current injection without magnetic field. Spins are polarized within the graphene plane and are displaced in opposite directions, with no net charge displacement. The direction of the spin current is determined by the linear polarization axis of the light, and the injection rate is proportional to the intensity. The technique is tunable via an applied bias voltage and is accessible over a wide frequency range. We predict a spin-current polarization as high as 75% for photon frequencies comparable to the Rashba frequency. Spin-current injection via optical methods removes the need for ferromagnetic contacts, which have been identified as a possible source of spin scattering in electrical spin injection in graphene.

Ultrafast optical measurement of hole and electron spin dynamics in germanium

Spin-dependent carrier dynamics in Ge, including hole spin relaxation, intervalley scattering, many-body effects, cooling, and phase space filling, are selectively investigated and analyzed for the first time using spectrally, temporally and polarization resolved pump-probe techniques.

Interference of stimulated electronic Raman scattering and linear absorption in coherent control

We consider quantum interference effects in carrier and photocurrent excitation in graphene using coherent electromagnetic field components at frequencies w and 2w. The response of the material at the fundamental frequency w is presented, and it is shown that one-photon absorption at w interferes with stimulated electronic Raman scattering (combined 2w absorption and w emission) to result in a net contribution to the current injec- tion. This interference occurs with a net energy absorption ofw and exists in addition to the previously studied interference occurring with a net energy absorption of 2¯w under the same irradiation conditions. Due to the absence of a bandgap and the possibility to block photon absorption by tuning the Fermi level, graphene is the perfect material to study this contribution. We calculate the polarization dependence of this all-optical effect for intrinsic graphene and show that the combined response of the material at both w and 2w leads to an anisotropic photocurrent injection, whereas the magnitude of the injection current in doped graphene, when transitions at w are Pauli blocked, is isotropic. By considering the contribution to coherent current control from stimulated electronic Raman scattering, we find that graphene offers tunable, polarization sensitive applications. Coherent control due to the interference of stimulated electronic Raman scattering and linear absorption is relevant not only for graphene but also for narrow-gap semiconductors, topological insulators, and metals.

One- and two-photon indirect injection of carriers and spins in silicon

Using an empirical pseudopoential description of electron states and an adiabatic bond charge model for phonon states, a full band structure calculation is performed for the one- and two-photon indirect optical injection of carriers and spins in bulk silicon. The calculated one- and two-photon absorption coefficients are in agreement with experiments. For σ- light propagation direction along 001, the carrier and spin injection rates and the degree of spin polarization show strong valley anisotropy. The carrier injection rates in the Z valleys are larger than that in the X valley. Furthermore, the two photon indirect carrier injection shows anisotropy and linear-circular dichroism with respect to the light propagation direction.

Two-photon indirect optical injection and two-color coherent control in bulk silicon

Using an empirical pseudopotential description of electron states and an adiabatic bond charge model for phonon states in bulk silicon, we theoretically investigate two-photon indirect optical injection of carriers and spins and two-color coherent control of the motion of the injected carriers and spins. For two-photon indirect carrier and spin injection, we identify the selection rules of band edge transitions, the injection in each conduction band valley, and the injection from each phonon branch at 4 K and 300 K. At 4 K, the TA phonon-assisted transitions dominate the injection at low photon energies, and the TO phonon-assisted at high photon energies. At 300 K, the former dominates at all photon energies of interest. The carrier injection shows anisotropy and linear-circular dichroism with respect to light propagation direction. For light propagating along the