Supporting information
Subcycle mid-infrared electric-field-driven scanningmicroscopy with a time resolution higher than 30 fs

tunneling

Yusuke Arashida, Hiroyuki Mogi, Masashi Ishikawa, Ippo Igarashi, Akira Hatanaka,Naoki Umeda, Jinbo Peng, Shoji Yoshida, Osamu Takeuchi, and Hidemi Shigekawa*
Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan

Analysis of thermal expansion problem

Fig. S1 Oscilloscope data (65536 integrations). Orange: MIR signal obtained when the light isturned on and off with an optical chopper. Blue: voltage proportional to the z displacement of thepiezo element, measured using the change in orange signal as a trigger. Green: corresponding
change in tunnel current.

Since the signal is weak in time-resolved STM, the lock-in detection method is used. Note that
if the excitation light is in the visible or NIR region, the modulation of this intensity causes thermalexpansion problems1. In THz-STM, the photon energy is small and, the therefore, thermalexpansion problem does not occur, which is an advantage of this method2. In this study, the time
resolution was improved by using MIR light whose frequency is about 30 times higher than THz;
however, it was not obvious whether there would be thermal expansion due to the MIR lightirradiation. Therefore, the MIR light intensity was modulated at a frequency of 120 Hz by an
optical chopper, and the expansion and contraction of the piezo element and the change in tunnelcurrent were measured with an oscilloscope. The change in the piezo element was 1 pm and theeffect on the tunnel current was small. In fact, the measured MIR-induced current remained almost
unchanged. It was thus confirmed that the thermal expansion problem could be ignored for thissystem.

Evaluation of MIR far-fieldFig. S2 (a) Temporal waveform of the MIR far-field measured by EO sampling in air. A GaSe crystal
with the thickness of 30 μm was used. (b) Temporal waveform of the MIR far-field measured by usinga photoconductive antenna (PCA) in the vacuum chamber. (c) and (d) the Fourier spectra of (a) and(b), respectively.

To carry out time-resolved MIR-STM measurement correctly, it is first necessary to confirm thatthe incident MIR far-field is reproduced at the STM tip. Therefore, the electric field waveformsof the incident far-field and the far-field at the STM tip were measured by EO sampling andphotoconductive antenna (PCA) methods, respectively (Fig. S2). The PCA is an element in which
an antenna structure such as a dipole antenna is patterned with a metal on a semiconductor
substrate. The dipole antenna has two convex structures of gold electrodes facing each other witha gap of 6 5 to 10 µm between them. When the PCA is irradiated with NIR light, electrons areexcited from the valence band to the conduction band in the LT-GaAs (Low temperature grownGaAs) substrate, thereby generating optical carriers. Without an electric field, no current flows

because the circuit is closed owing to the gap, but when an electric field is applied in this state, acurrent proportional to the electric field strength flows. Therefore, the MIR waveform can beobtained by measuring the change in current while changing the delay time between the MIR lightand the NIR light similarly to the EO sampling method. This element was placed at the positionof the STM tip in the STM measurement setup.

Reconstruction of the near field voltage VNF, MIR (t) from INF, MIR (t)Fig. S3 (a) Relationship between the applied electric field (voltage) ENF, MIR(t) (VNF, MIR(t)) andthe current I(t) obtained from I- VDC curve shown in Fig. 2(b). (b) VNF, MIR(t) calculated using
this method from the INF, MIR(t) waveform for VS = 6 V shown in Fig. 2(c).

FDTD calculation of MIR near-field waveform

Fig. S4 To evaluate the MIR near-field waveform experimentally obtained by the evaluation

method using photoelectrons, the near-field electromagnetic field was calculated in the time
domain by the finite-difference time-domain (FDTD) method. (a) Shape of the tip and thearrangement of the tip and sample used in the calculation. A Pt cone with a length of 40 μm anda bottom diameter of 1 nm was used as the tip model for the calculation. (b) Magnified image ofpart of (a). A gold substrate was placed 200 nm from the tip. A subcycle MIR pulse was irradiatedat an angle of 35 ° with respect to the gold sample. The component of the electric field in thedirection perpendicular to the gold substrate was calculated at the position 0.5 nm from the tip

shown by a red point in (b). The minimum mesh size was 0.6×0.6×0.13 nm3. (c) Result obtainedby the calculation (blue). The far-field electric field waveform used in the calculation wasmeasured using the PCA shown in Fig. S3 and is reproduced in the figure (green).

Reference
1.

You, A., Be, M. A. Y. & In, I. Laser-induced thermal expansion of a scanning tunnelingmicroscope tip measured with an atomic force microscope cantilever. 2521, 1‒4 (2002).

2.

Cocker, T. L. et al. An ultrafast terahertz scanning tunnelling microscope. Nat. Photonics
7, 620‒625 (2013).