半导体中电流的亚周期光学控制

Nonlinear interactions between ultrashort optical waveforms and solids can be used to induce and steer electric current  on a femtosecond (fs) timescales, holding promise for electronic signal processing at PHz (1015 Hz) frequencies [Nature  493, 70 (2013)]. So far, this approach has been limited to insulators, requiring extremely strong peak electric fields (> 1  V/Å) and intensities (> 1013 W/cm2 ). Here, we show all-optical generation and control of directly measurable electric  current in a semiconductor relevant for high-speed and high-power (opto)electronics, gallium nitride (GaN), within an  optical cycle and on a timescale shorter than 2 fs, at intensities at least an order of magnitude lower than those required  for dielectrics. Our approach opens the door to PHz electronics and metrology, applicable to low-power (non-amplified)  laser pulses, and may lead to future applications in semiconductor and photonic integrated circuit technologies.

Here, we demonstrate ultrafast, direct-field control of current at substantially lower fields, in a material with a smaller  bandgap (Eg). We have chosen GaN (Eg ≈ 3.4 eV), which  has attracted much interest for applications in optoelectronics and high-frequency and high-power electronics due to its  high electron mobility, mechanical stability and heat capacity. We show that charge displacement results from the  interference between multiphoton excitation channels  in  presence of field-induced intraband carrier motion and dynamic screening of the optical field. With increasing intensity, we observe a gradual transition from the multiphoton to  the tunneling regime, supporting a unified quantum mechanical picture valid in both limits.

Figure 1(b) shows the CEP-dependent fraction QP of the  charge per pulse collected by the unbiased Au electrodes as a  function of ∆l and ∆ϕCE. Here, FL(t) was applied perpendicular to the electrodes, along the x-axis; Fig. 1(a). The measured signal QP reverses its sign periodically with CEP. Inverting the optical field ( CE ∆ = ϕ π ) reverses the direction of  the measured charge displacement: the instantaneous electric  field of the laser pulse generates and controls QP, similar to  the case of an insulator.

Fig1

To clarify the physical mechanism behind the generated  current, we compared our experimental data with the results  of quantum-mechanical (QM) simulations based on the numerical solution of the time-dependent Schrödinger equation (Supplement 1, Sections 3–5). We considered optical  transitions between three valence (VB) and two conduction  bands (CB) for crystal momenta kx along the Γ–M direction  of the Brillouin zone (BZ) [Figs. 4(a) and S1 in Supplement  1]. The electrodes orientation relative to the crystalline axes  of the (0001) surface does not play an important role, since bands are isotropic in the vicinity of the Γ-point [20] and the  considered field amplitudes are too low for most charge carriers to reach the BZ boundaries.