Teraherz Field Effects Overview

High-precise calculation of near-infrared absorption and transient spectra in bulk semiconductors irradiated by an intense terahertz (THz) laser shows that, the ionization rate of the ground-state exciton, probed through the dynamic Fano resonance, may decrease with the increase of the THz laser intensity. This counterintuitive e.ect indicates the excitons are stabilized against the .eldionization. The much lower Rydberg energy and "atomic unit" of laser intensity for excitons, and the possibility of creating excitons from the "vacuum state" allow observing the excitonic stabilization in experiments, in contrast to the case of atomic stabilization.
In accordance to Einstein's theory of the photoelectric effect, an electron can be stripped from its atom by light beams with sufficiently high frequency. As the intensity of a light beam is increased, the stripping probability of the electron increases owing to the increasing number of photon impacts. Surprisingly, since the late 1980's, theoretical investigations and numerical simulations [8,9] have predicted that when the electric field, associated with a laser of sufficiently high intensity and frequency, approaches or exceeds the electrostatic field between the electron and the ion in an atom, the wave function of the irradiated atom may be distorted adiabatically into a distribution with two well separated peaks. The peak spacing increases with increasing the field intensity, thus the atomic electron spends more time far away from the nucleus, and the ionization rate slows down dramatically till almost totally suppressed. In other words, the adiabatic stabilization of the atom occurs. The observation of this atomic stabilization e.ect is, however, extremely difficult. Firstly, for the atomic ground state, the high-frequency and high-intensity condition requires an extreme ultraviolet laser with intensity larger than an atomic unit (~ 3.51 x 10¹⁶ W/cm²), which seems not available in the near future. Secondly, and more vitally, sufficiently slow turn-on is demanded for the laser pulse to adiabatically drive the atomic ground state into a stable dressed state; during the rise time of the super-intense laser pulse, however, substantial ionization has already been inevitable [10], which prevents the e.ect from observation. Some researchers even argued that it was impossible to observe such an adiabatic stabilization of atoms [11,12]. In fact, except for a few disputable indications for the stabilization of high Rydberg states [13], so far there is no unambiguous experimental evidence for the atomic stabilization.
It is compelling to settle the debate whether this e.ect exists or not. Apart from the pure curiosity in basic research, the atomic stabilization is also of importance in potential application, such as the high harmonic generation [14] and quantum computation [15], as it can quench the undesired ionization and dephasing caused by the intense laser that is used to drive electrons or to manipulate a qubit in an atom.
The excitonic stabilization (XS), in contrast to its atomic counterpart, is de.nitely an observable e.ect, owing to the basic characteristic of excitons that they are created from "vacuum" of carriers by the interband optical excitation. In our study, the dressed exciton states are prepared by weak near-infrared (NIR) laser pulses in semiconductors in the presence of a quasi-continuous terahertz (THz) field, as in most recent experiments on the dynamical Franz-Keldysh effect [16,18]. The ionization rate or lifetime of the dressed excitons can be directly extracted by monitoring either the linewidth of absorption peaks or the decay time of transient signals. Thus the "turn-on" problem is safely circumvented as long as the pulsed interband excitation and the following dephasing process of excitons are well covered by the attop region of the THz laser pulse.
Moreover, the present laboratory gear is already ready for observing the XS effect. Due to the small effective mass µ and large dielectric constant E, an exciton has much smaller binding energy E_B than the hydrogen atom does, and the "excitonic unit" for laser intensity in a typical semiconductor like GaAs is about 10 orders of magnitude smaller than the atomic unit. So, the "high-intensity and high-frequency" requirement for the XS can readily be fulfilled by the free-electron lasers operating with MW/cm² power and THz frequency [19]. Our investigation is based on the high-precise calculation of interband optical spectra, which includes properly the Coulomb interaction, the nonperturbative ac-field, and the contribution of continuum states. To avoid unnecessary complexity, the excitonic state is treated within