Polarity Reversal of Terahertz Electric Field from Heavily p-Doped Silicon Surfaces

Above-band-gap optical excitation of electron-hole pairs screens the doping-induced surface electric field and generates terahertz(THz)pulses via free-carrier transport.THz emission from a heavily doped silicon surface is much weaker than that of lightly doped samples.A polarity reversal of the THz electric field is observed in heavily doped p-type silicon,indicating that the doping related and carrier induced surface electric fields oppose each other.By comparing the penetration depth of the excitation laser with the thickness of the depletion layer for the doped silicon,it is shown that competition between diffusion and drift current causes the polarity reversal.

Real-Time Observation of Electron-Hole Coherence Induced by Strong-Field Ionization

We introduce and demonstrate a new approach to measure the electron-hole dynamics and coherence induced by strong-field ionization using hole-assisted high-harmonic spectroscopy.The coherent driving of the infrared and XUV pulses correlates the dynamics of the core-hole and the valence-hole by coupling multiple continua,which leads to the otherwise forbidden absorption and emission of high harmonics.An analytical model is developed based on the strong-field approximation by taking into account the essential multielectron configurations.The emission spectra from the core-valence transition and the core-hole recombination are found to modulate strongly as functions of the time delay between the two pulses,suggesting that the coherent electron wave packets in multiple continua can be utilized to temporally resolve the core-valence transition in attoseconds.

Ultrafast Hole Deformation Revealed by Molecular Attosecond Interferometry

Understanding the evolution of molecular electronic structures is the key to explore and control photochemical reactions and photobiological processes.Subjected to strong laser fields,electronic holes are formed upon ionization and evolve in the attosecond timescale.It is crucial to probe the electronic dynamics in real time with attosecond-temporal and atomic-spatial precision.Here,we present molecular attosecond interferometry that enables the in situ manipulation of holes in carbon dioxide molecules via the interferometry of the phase-locked electrons(propagating in opposite directions)of a laser-triggered rotational wave packet.The joint measurement on high-harmonic and terahertz spectroscopy(HATS)provides a unique tool for understanding electron dynamics from picoseconds to attoseconds.The optimum phases of two-color pulses for controlling the electron wave packet are precisely determined owing to the robust reference provided with the terahertz pulse generation.It is noteworthy that the contribution of HOMO-1 and HOMO-2 increases reflecting the deformation of the hole as the harmonic order increases.Our method can be applied to study hole dynamics of complex molecules and electron correlations during the strong-field process.The threefold control through molecular alignment,laser polarization,and the two-color pulse phase delay allows the precise manipulation of the transient hole paving the way for new advances in attochemistry.