Supporting Information

Terahertz scanning tunneling microscopy forvisualizing ultrafast electron motion innanoscale potential variations
Shoji. Yoshida , Yusuke. Arashida , Hideki Hirori , Takehiro. Tachizaki , Atsushi.
1

12

3

Taninaka , Hiroki Ueno , Osamu Takeuchi , and Hidemi Shigekawa *
1

11

1

1

Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan

2

Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011

3Department of Optical and Imaging Science and Technology, Tokai University, Hiratsuka-shi,Kanagawa, 259-1292, Japan*Correspondence should be addressed to H.S. (hidemi@ims.tsukuba.ac.jp).

Fig. S1. Schematic illustration of electric field strength control system. A LiNbO3
crystal was excited using an optical pulses output (1035 nm, 40 W, 309 fs, 50 MHz
repetition rate) from a femtosecond laser (Coherent Monaco) with the tilted pulse front
technique to generate and THz pulses (1). We used THz CEP shifter (Hamamatsu
photonics) as a half waveplate (HWP) to rotate the polarization angle of THz pulse (2).
The polarization angle of the THz pulse changes by 2α depending on the rotation angle αof the HWP. Therefore, when α is changed from 0° to π/2, the polarization angle of theTHz pulse changes from 0° to π. After that, a wire grid polarizer (WGP) is used to transmit
only the polarized component along the STM probe axis (S polarized light). The CEP ofthe THz pulse after WGP can be set to two values, 0 or π. After that, the THz pulse wasmerged coaxially with the infrared pulse for sample excitation using the THz/Vis beam
splitter (BS), and the THz lens installed in the ultra-high vacuum STM chamber(Tsurupica, f = 100 mm) is used to illuminate the STM tunnel junction.
A simple estimate of the number of photons and excited electrons was made. We
estimated the number of photon and excited electrons. With a laser power of 10 mW anda repetition frequency of 16.6 MHz, the number of photons per pulse is about 3 x 109.
Assuming the spot size of 10 μm diameter, the number of photons in the rage of 10 nm
diameter is 3000. The reflectance (1035 nm) of the Au surface is 96.5%, and if all the loss
is used for electronic excitation, one electron is excited per 1 nm x 1 nm. It is not clear
how many electrons will be injected into LUMO, but if the measured THz current is 1pA,
the number of electrons per pulse becomes 0.4, which is almost of the same order withthe experimental value. The effects of tip enhancement and the lateral diffusion are notconsidered here.

Fig. S2. Method for measuring THz near-field waveform using photoemission. (a)Schematics of the measurement setup and the band diagram near the STM tip. (b) TypicalI-V curve obtained by photoelectron current. (c) Magnified view of (b) for the positiveside. The black line shows the fitting of the linear region. ITHz, 0 and ITHz, π are the THz
currents induced by the THz electric field for Φ = 0 and π, respectively. (d) and (e)Magnifications of ITHz,0 and ITHz,π shown in (c), respectively.
As shown in Fig. S2a, when the STM probe and sample are irradiated by a strong visible(517 nm 10 mJ/cm2, 1 MHz) pulse produced by the second harmonic generation (SHG)of fundamental pulse, some of the hot electrons excited by multiphoton absorption
overcome the barrier and photoemission from both surfaces occurs. The I-V curvemeasured with the tip-sample distance of 1 μm is shown in Fig. S2b. The photoelectron
current changes with the applied bias voltage, and photoelectrons generated on the tip
surface flow into the sample at a high positive voltage. As shown in Fig. S2c, amagnification of b, the I-V curve is almost linear above VDC = 4V. Therefore, when VDC
is set to +8 V, within this linear region, and the THz pulse is applied during the excitationby the IR pulse, the photoelectron current changes linearly with the magnitude of the THz
electric field. Since the lifetime of hot electrons is very short ( 〜 100 fs), ITHz is
proportional to the THz electric field during the IR excitation. ITHz (td) is shown for ΔΦ= 0 and π in Figs. S2d and S2e, respectively. The shape of ITHz(td) is similar to the THz
near-field waveform at the tip, i.e., ITHz = αVTHz. The proportional coefficient α is obtained
from the slope of the linear region in Fig. S2c. Thus, it is possible to convert ITHz to VTHz
using this relationship (3,4).

Fig. S3. Derivation of THz voltage in tunnel region. (a) Typical static I-V curvemeasured over a wide voltage range (setpoint: VS = -3.0 V, It = 10 pA). The relationshipbetween the voltage VTHz produced by the THz electric field and the THz-induced current
ITHz is also shown. (b), (c) Waveforms of VTHz and ITHz calculated using VTHz for (ΔΦ =π, VDC = -2.0 V, VTHz-peak = 1.5 V) and (ΔΦ = π, VDC = -2.0 V, VTHz-peak = -1.5 V),
respectively. (d) Calculated (solid line) and measured (dots) ITHz-VDC curves. ITHz,0 andITHz,π are THz currents for ΔΦ = 0 and π, respectively.
To understand the origin of ITHz in the time-resolved THz-STM experiment, we need toknow the magnitude of VTHz applied between the STM tip and sample in a tunnelingregime. Therefore, we carried out ITHz-VDC measurement to calibrate the magnitude ofVTHz. Figures S3a and S3b show the basic concept of the analysis. The experimental ITHz
is time-averaged as shown by the following equation:
I THz (VDC , td )  f rep td 1/ f reptd[ I {VDC  VTHz (t )}  I (VDC )]dt ,(1)Therefore, in principle, we can calculate ITHz at a particular voltage VDC based on the staticI-V curve and VTHz(t). To obtain the magnitude of VTHz, we first measured ITHz-VDC curves
using VTHz for two CEPs (VTHz,0 and VTHz,π), then we performed curve fitting using thenumerically calculated ITHz-VDC curve with the magnitude of VTHz as an independentvariable. Best fitting was obtained when the peak of VTHz (VTHz-peak) is equal to ± 1.5 V.
as shown in Fig. S3d, The positions of the rising voltage of ITHz and the magnitude of ITHz
show good agreement with each other.

Fig. S4. (a)Time-resolved images of electron motion obtained over a large

area, showing the generality of the observed phenomena. (b) Delay timedependence of the profile along the dotted line in (a). The gradual decreases
in the high-density area can be seen, showing the existence of the horizontal
diffusion process faster than the vertical diffusion. Blue arrows indicate thepositions of the step edges crossing the dotted line.
Movie S1. Movie showing the motion of electrons obtained in the same area of fig.S4.
REFERENCES
(1)
(2)
(3)

(4)

Hirori, H.; Doi, A.; Blanchard, F.; Tanaka, K. Single-Cycle Terahertz Pulses with
Amplitudes Exceeding 1 MV/cm Generated by Optical Rectification in LiNbO3.
Appl. Phys. Lett. 2011, 98 091106.
Kawata, Y.; Yasuda, T.; Takahashi, H. Carrier Envelope Phase Shifter forBroadband Terahertz Pulses. Opt. Lett. 2016, 41, 986–989.
Yoshida, S.; Hirori, H.; Tachizaki, T.; Yoshioka, K.; Arashida, Y.; Wang, Z. H.;
Sanari, Y.; Takeuchi, O.; Kanemitsu, Y.; Shigekawa, H. Subcycle Transient
Scanning Tunneling Spectroscopy with Visualization of Enhanced Terahertz
Near Field. ACS Photonics 2019, 6, 1356–1364.
Müller, M.; Martín-Sabanés, N.; Kampfrath, T.; Wolf, M. Phase-Resolved
Detection and Control of Ultrabroadband THz Pulses Coupled to a Scanning
Tunneling Microscope Junction. ACS Photonics 2020, 7, 2046–2055.