All-optical injection of ballistic electrical currents in unbiased silicon

Abstract

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 <hω < 0.87 eV (1,420-1,800 nm wavelength). Both single-photon absorption at 2ω and two-photon absorption at ω occur across the indirect bandgap 13 , 1.1 eV, but below the direct bandgap, 3.5 eV. Spatially and temporally overlapped collinearly polarized ω and 2ω pulses are focused at normal incidence on (001)-oriented crystalline silicon samples of thickness between 0.5 and 500 µm. The different samples produce similar results and here we mainly consider results from a 125-µm-thick wafer. Withhω = 0.8 eV and a fundamental (second harmonic) pulse energy of 120 nJ (4 nJ), the near-surface carrier densities generated by each pulse are 5 × 10 15 cm −3 (6 × 10 16 cm −3) for a singlephoton absorption coefficient 13 of 1.3 × 10 3 cm −1 and two-photon absorption coefficient 14,15 of 0.45 cm GW −1. The transient electrical current, directed in the plane of the sample, emits THz radiation that is detected by electro-optic sampling. Figure 1a shows the THz field as a function of time delay between 0.8/1.6 eV pump pulses and an electro-optic sampling 1.53 eV probe pulse; traces are shown for values of φ separated

Second-Harmonic Generation Induced by Electric Currents in GaAs

“…Here N 2D is the areal carrier density. We note that although current injection by the coherent control technique has been demonstrated in many materials by steady-state electric measurement, terahertz detection, and spatially resolved pump-probe techniques, [11,19,20,[23][24][25] the current-induced SHG demonstrated here allows us to time-resolve the ultrafast dynamics of these currents.…”

“…With both pulses being linearly polarized along an arbitrarily chosenx direction, electrons are excited to the conduction band with an average velocity v 0 sin(∆φ)x, where ∆φ is the relative phase of the two transition amplitudes, and v 0 is on the order of 30 nm/ps. [11,[17][18][19][20][21] With a carrier density on the order of 10 17 − 10 18 /cm 3 ,…”

Coherent control of injection currents in high-quality films of Bi2Se3

“…One exception is the investigation of carrier dynamics by differential reflection. 13 Here we demonstrate the use of the two-color excitation scheme 14 to inject ballistic photocurrents in high-quality Bi 2 Se 3 films. We also show that the currents can be controlled by the relative phase between the fundamental and frequency doubled excitation pulses.…”

Ultrafast Control of Strong-Field Electron Dynamics in Solids

“…Furthermore, n(k) is, in general, an asymmetric function of k because the transition amplitudes for the one-and two-photon channels have different symmetries with respect to the transformation k → −k [59]. Injecting currents through interfering photoexcitation pathways was investigated in theory and experiments for semiconductors [59][60][61][62][63][64][65][66] and molecular wires [67].…”