We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photogenerated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (<5  ps) interdot tunneling is shown to occur over a surprisingly wide bandwidth, up to ∼8  meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system.

Silicon has been the dominant material in electronics since the invention of the integrated transistor. In contrast, silicon's indirect bandgap and vanishing second-order optical nonlinearity limit its applications in optoelectronics 1. Although all-optical components such as Raman lasers 2 , parametric amplifiers 3 and electro-optic modulators 4,5 have recently been reported, control over charge motion in silicon has only ever been achieved electronically. Here, we report all-optical generation of ultrafast ballistic electrical currents in clean, unbiased, bulk silicon at room temperature. This current injection, which provides new insights into optical processes in silicon, results from quantum interference between one-and twophoton absorption pathways across the indirect bandgap despite phonon participation and the multi-valley conduction band. The transient currents induced by 150 fs pulses are detected via the emitted THz radiation. The efficiency of this third-order optical process is surprisingly large for fundamental wavelengths in the 1,420-1,800 nm range. The use of phase-related optical beams has advanced the application of light in a variety of processes such as controlling photochemical reactions 6 , forming attosecond pulses 7 and producing high-intensity THz pulses in air 8. In directbandgap semiconductors, both charge 9,10 and pure spin currents 11,12 have been generated through quantum interference of singleand two-photon interband absorption of light at frequencies 2ω and ω with the current dependent on the phase parameter φ = 2φ ω − φ 2ω ; here φ ω,2ω is the phase of the optical beam. For electrical-current generation in Si, we use 150 fs pulses with 0.69

The effective nonlinear optical absorption coefficient ␤ eff is measured for 20-nm-thick Au films at 630 nm as a function of pulse width. The z-scan measurements show that ␤ eff increases from 6.8ϫ 10 −7 to 6.7ϫ 10 −5 cm W −1 as the pulse width is varied from 0.1 to 5.8 ps. To help interpret this ϳ100ϫ increase, differential transmission and reflectivity measurements are performed using 775 nm pump and 630 nm probe pulses. All experiments are simulated with a two-temperature model for electrons and lattice. The pulse width dependence of ␤ eff is consistent with thermal smearing of d-band to conduction-band transitions, with ␤ eff arising from changes in the linear ͑Im ͑1͒ ͒ absorption coefficient.

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We employ ultrafast pump-probe spectroscopy with photocurrent readout to directly probe the dynamics of a single hole spin in a single, electrically tunable self-assembled quantum dot molecule formed by vertically stacking InGaAs quantum dots. Excitons with defined spin configurations are initialized in one of the two dots using circularly polarized picosecond pulses. The time-dependent spin configuration is probed by the spin selective optical absorption of the resulting few Fermion complex. Taking advantage of sub-5 ps electron tunneling to an orbitally excited state of the other dot, we initialize a single hole spin with a purity of > 96 %, i.e., much higher than demonstrated in previous single dot experiments. Measurements in a lateral magnetic field monitor the coherent Larmor precession of the single hole spin with no observable loss of spin coherence within the ∼ 300ps hole lifetime. Thereby, the purity of the hole spin initialization remains unchanged for all investigated magnetic fields. arXiv:1204.5899v1 [cond-mat.mes-hall]