Brian A. Ruzicka

Ultrafast and spatially resolved studies of charge carriers in atomically thin molybdenum disulfide

Atomically thin molybdenum disulfide is emerging as a new nanomaterial with potential applications in the fields of electronic and photonics. Charge carrier dynamics plays an essential role in determining its electronic and optical properties. We report spatially and temporally resolved pump-probe studies of charge carriers in atomically thin molybdenum disulfide samples fabricated by mechanical exfoliation. Carriers are injected by interband absorption of a 390-nm pump pulse and detected by measuring differential reflection of a time-delayed and spatially scanned probe pulse that is tuned to an exciton transition. Several parameters on charge carrier dynamics are deduced, including carrier lifetime, diffusion coefficient, diffusion length, and mobility.

Synthesis and optoelectronic properties of two-dimensional FeS2 nanoplates.

There is a growing interest in the earth abundant and nontoxic iron disulfide (FeS(2)) photovoltaic materials. Here, we report the synthesis of FeS(2) nanoplates with different spectral features which we have associated with thicknesses and crystallization. The structure and crystalline order of ultrathin FeS(2) nanoplates have a strong influence on the carrier lifetime, electronic and optical properties. We demonstrate that two-dimensional FeS(2) nanoplates show great promise for fabrication of hybrid bulk heterojunction solar cells. This opens up a host of applications of these materials as inexpensive solar cells and photocatalysts.

Spatially resolved femtosecond pump-probe study of topological insulator Bi2Se3

Carrier and phonon dynamics in Bi2Se3 crystals are studied by a spatially resolved ultrafast pump-probe technique. Pronounced oscillations in differential reflection are observed with two distinct frequencies, and are attributed to coherent optical and acoustic phonons, respectively. The rising time of the signal indicates that the thermalization and energy relaxation of hot carriers are both sub-ps in this material. We found that the thermalization and relaxation time decreases with the carrier density. The expansion of the differential reflection profile allows us to estimate an ambipolar carrier diffusion coefficient on the order of 500 square centimeters per second. A long-term slow expansion of the profile shows a thermal diffusion coefficient of 1.2 square centimeters per second.

Ambipolar diffusion of photoexcited carriers in bulk GaAs

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Hot carrier diffusion in graphene

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Femtosecond pump-probe studies of reduced graphene oxide thin films

The dynamics of photocarriers in reduced graphene oxide thin films is studied by using ultrafast pump-probe spectroscopy. Time dependent differential transmissions are measured with sample temperatures ranging from 9 to 300 K. At each sample temperature and probe delay, the sign of the differential transmission remains positive. A fast energy relaxation of hot carriers is observed, and is found to be independent of sample temperature. Our experiments show that the carrier dynamics in reduced graphene oxide is similar to other types of graphene, and that the differential transmission is caused by phase-state filling effects of carriers.

Spatially resolved pump-probe study of single-layer graphene produced by chemical vapor deposition [Invited]

Carrier dynamics in single-layer graphene grown by chemical vapor deposition (CVD) is studied using spatially and temporally resolved pump-probe spectroscopy by measuring both differential transmission and differential reflection. By studying the expansion of a Gaussian spatial profile of carriers excited by a 1500-nm pump pulse with a 1761-nm probe pulse, we observe a diffusion of hot carriers of 5500 cm2/s. We also observe that the expansion of the carrier density profile decreases to a slow rate within 1 ps, which is unexpected. Furthermore, by using an 810-nm probe pulse we observe that both the differential transmission and reflection change signs, but also that this sign change can be permanently removed by exposure of the graphene to femtosecond laser pulses of relatively high fluence. This indicates that the differential transmission and reflection at later times may not be directly caused by carriers, but may be from some residue material from the sample fabrication or transfer process.

Optical injection and detection of ballistic pure spin currents in Ge

Ballistic pure spin currents are injected into Ge at 295 K using quantum interference between one and two photon absorption processes for 1786 and 893 nm, 200 fs optical pulses. The spin currents are spatially and temporally detected using polarization- and phase-dependent differential transmission techniques with nanometer spatial and femtosecond temporal resolution. We interpret the dynamics in terms of the fast spin relaxation of the holes and intervalley transfer of electrons.

Second-harmonic generation induced by electric currents in GaAs.

We demonstrate a new, nonlinear optical effect of electric currents. First, a steady current is generated by applying a voltage on a doped GaAs crystal. We demonstrate that this current induces second-harmonic generation of a probe laser pulse. Second, we optically inject a transient current in an undoped GaAs crystal by using a pair of ultrafast laser pulses and demonstrate that it induces the same second-harmonic generation. In both cases, the induced second-order nonlinear susceptibility is proportional to the current density. This effect can be used for nondestructive, noninvasive, and ultrafast imaging of currents. These advantages are illustrated by the real-time observations of a coherent plasma oscillation and spatial resolution of current distribution in a device. This new effect also provides a mechanism for electrical control of the optical response of materials.

Exciton diffusion in semiconducting single-walled carbon nanotubes studied by transient absorption microscopy

This is the publishers version, also available electronically from http://journals.aps.org/prb/abstract/10.1103/PhysRevB.86.205417.