抄録
Pulsed laser technologies have been revealing ultrafast phenomena in pico- and femto-second ranges. Combining scanning tunneling microscopy (STM), atomic-scale dynamics has been extensively studied. Two types of time-resolved STM have been performed which utilizes non-linear optical supportively [1-3] and light-field-driven bias modulation [4-6]. The light-field-driven STM has an advantage to measure density of states of electrons in scanning tunneling spectroscopy (STS) [5]. Subcycle (less than monocycle) terahertz (THz) field has been used to modulate bias voltage so that the temporal resolution of around 1 pico-seconds. Here, we developed light-field-driven STM using subcycle mid-infrared (MIR) pulses with the center frequency of 30 THz to reveal photo-induced atomic-scale dynamics in femtosecond range.
A laser source of optical parametric chirped pulse amplifier (OPCPA) with the pulse duration of 8.2 fs was used to generate subcycle MIR pulses using a nonlinear crystal of GaSe. Time-resolved STM was conducted by using near-infrared (NIR) pump pulses as shown in Fig. 1(a) which induces interband transition in a layered compound of MoTe2. Ultrafast modulation of the tunneling current as shown in Fig. 1(b) was succeeded to be measured with the scale of 29 fs which is considered to be contributed by hot-electrons. This technique will pave a new way for studying non-equilibrium electrons in atomic scale. Combining other pump conditions and detection schemes, unambiguous understanding of surface dynamics can be expected [2, 3, 8].
[1] S. Liu et al., Sci. Adv. 8, 5682 (2022). [2] H. Mogi et al., npj 2D Mat. Appl. 6,72 (2022). [3] H. Mogi, et al., Jpn. J. Appl. Phys. 61, SL1011 (2022). [4] T. L. Cocker, et al., Nat. Photon. 7, 620 (2013). [5] S. Yoshida et al., ACS Photon. 6, 1356 (2019). [6] S. Yoshida et al., ACS Photon. 8, 315 (2021). [7] Y. Arashida et al., ACS Photon. 9, 3156 (2022). [8] Y. Arashida et al., Appl. Phys. Exp. 15, 092006 (2022).
