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Externally triggerable optical pump-probe scanning tunneling microscopy
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LETTER

Applied Physics Express 12, 025005 (2019)https://doi.org/10.7567/1882-0786/aaf8b2

Externally triggerable optical pump-probe scanning tunneling microscopy
Hiroyuki Mogi†, Zi-han Wang†, Ryusei Kikuchi, Cheul Hyun Yoon, Shoji Yoshida, Osamu Takeuchi, and Hidemi Shigekawa*
Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, 305-8573, Japan*

E-mail: hidemi@ims.tsukuba.ac.jp
These two authors contributed equally to this work.

†Received November 30, 2018; revised December 10, 2018; accepted December 13, 2018; published online January 18, 2019
Optical pump-probe scanning tunneling microscopy (OPP-STM) has enabled the measurement of ultrafast dynamics in real space. However, theuse of a pulse picker to extract selected laser pulses to realize delay-time modulation, which efficiently suppresses the thermal expansion
problems, limits the availability of time-resolved measurement. Here, we present a more applicable type of OPP-STM that we have developed. Twoexternally triggerable pulse lasers were used to produce pump and probe pulses, and wide-range delay-time modulation was simply realized byadjusting the timing of the pulses. The performance of this new type of OPP-STM was demonstrated by measuring the carrier dynamics in WSe2.
© 2019 The Japan Society of Applied Physics

It is very important to measure and evaluate the dynamics
in nanoscale materials and devices for further advances inthe development of desired functions. Optical pumpprobe (OPP) methods1) have excellent time resolution forsuch purposes. However, for an in-depth understanding of thedynamics, simultaneous analysis in the spatial domain withthe time-resolved measurement is a key factor. For thispurpose, time-resolved techniques for simultaneous use withmethods such as electron microscopy,2) photoemission
electron microscopy,3) and diffraction methods4) have beendevised. Scanning tunneling microscopy/spectroscopy (STM/STS) is a promising technique for visualizing, characterizing,and manipulating solid surfaces/interfaces at the atomicscale.5) However, its temporal resolution is in the submillisecond range owing to the limitation imposed by thepreampliﬁer bandwidth, and considerable effort has been
made to improve the temporal resolution since its
invention.6–9)
Recently, by combining STM with optical pump-probe(OPP) methods, STM methods with a higher time resolution
have been developed.10–12) In OPP methods, to accurately
measure small changes in a signal, usually the pump lightintensity is modulated and the change in the signal corresponding to the modulation is detected by the lock-in
method. When OPP methods are combined with STM,
however, thermal expansion of the tip and sample occurs
when the light intensity changes. Since a 0.1 nm change inthe tunnel gap (tip-sample distance) causes a one-orderchange in tunnel current, this has impeded the combinationof STM with OPP methods (OPP-STM). Therefore, a
modulation technique different to those used in the conventional OPP methods has been desired. One of the promisingtechniques is the delay-time modulation.13)
In OPP-STM with delay-time modulation, the pulse widthof the laser determines the time resolution, and the modulation amplitude of the delay time determines the measurable
physical phenomena. If only small amplitudes can berealized, only ultrafast physical phenomena can be measured,but if both small and large amplitudes can be realized, it
becomes possible to measure a wide range of physicalphenomena with ultrafast to relatively slow dynamics.
Also, the frequency of the delay-time modulation affects
the measurement accuracy. Generally, a signal from ameasuring device contains a noise component whose amplitude is inversely proportional to the frequency f (1/f noise).

When the modulation frequency is too low, a large noise
remains in the signal after lock-in detection. Therefore, it isdesirable that the modulation frequency is set to 1 kHz orhigher.
We previously introduced the following two types ofpump-probe generator capable of delay-time modulation.
(1) A type that mechanically moves a mirror to change theoptical path length.14) (2) A type that uses speciﬁc pulse pairs
extracted from a pulse train by a pulse picker such as aPockels cell.15,16) The ﬁrst type can be realized relatively
easily when the modulation amplitude is small. However, torealize an amplitude exceeding 100 ps, an optical path length
on the order of 10 cm must be instantaneously changed, andit becomes impossible to achieve a fast delay-time modulation mechanically. On the other hand, the second type caneasily realize delay-time modulation by a large amplitude ofabout 1 μs with a modulation frequency of 10 kHz or more.
However, to achieve a high time resolution, it is necessary tosynchronize the oscillations of the two lasers, and a pulse
picker, a circuit for driving it, a cooling system and othervarious optical elements are required, making the systemrather complicated and not so easy to use. In the case of OPPSTM, a stability much higher than the case of the conventional OPP method is required. In addition, since an ultrafast
pulse picker used in our system, integrated with an RTP
crystal (LEYSOP, RTP-3-20) operating at 1 MHz, generally
has a limited extinction ratio of 250:1,17) some of the lightother than the selected (picked) and transmitted light also
passes through, which has an adverse effect on the measurement of some object.18)
With the above perspective, here we present a newly
developed OPP-STM system that utilizes externally triggerable pulse lasers, which we call externally triggerable OPPSTM (ET-OPP-STM). Figure 1 shows a schematic illustration of the designed system. The system simply consists of
two lasers, which can generate optical pulses immediately
after an electric pulse is input at an arbitrary timing, and aspeciﬁcally designed signal generator. This simple setup withfewer optical elements makes the system stable, compact, andeasy to use.
It is desirable that the repetition period from one pulse to
the next be short and that the ﬂuctuation of the delay timefrom the input of the electric signal to the output of the lightpulse (timing jitter) is small. An FPGA, for example, can beused to control the signal generation. It is possible to operate

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Fig. 1. (Color online) (a) Schematic illustration of the developed OPPSTM system. The system consists of two lasers with a nanosecond pulse
width of ∼8 ns, which can generate an optical pulse immediately after theinput of an electric pulse at an arbitrary timing, and a signal generator. A
ﬁeld-programmable gate array (FPGA) was used to control the signalgeneration. A high-resolution optical microscope (∼1 μm spatial resolution)with an optical access port was attached outside the main UHV chamber of
the STM system, which was oriented perpendicular to the sample plate. Thisparticular design not only provides a way of checking the locations of thesample surface and tip, but has excellent laser focusability. The STM tip wasattached diagonally, so that its apex can be more clearly seen from above.
The light spot size observed by the optical microscope and the form of thelaser pulse produced are shown together. (b) Schematic of delay-timemodulation. The delay time between the pump and probe pulses wasmodulated between td and td + Δtd at ∼1 kHz. Here, a simple type ofmodulation realized by adjusting the timing of the probe pulse generation isshown. M, mirror; H.M., half mirror; td, delay time between the pump andprobe pulses; Δtd; difference between two delay times used for modulation;
Vs, sample bias voltage; fmod, delay-time modulation frequency; I, raw tunnel
current; I*, signal current averaged by preampliﬁer.

the internal circuit with a clock frequency of about 200 MHz–
2 GHz using a recently produced commercial FPGA, and it ispossible to control the timing of the generation of the electricpulse with a time resolution of 0.5–5 ns by only using theembedded clock synchronization circuit. Furthermore, depending on the FPGA, the output section includes an output
delay circuit with a resolution of the order of 10 ps, and bycombining this section with a clock synchronization circuit,the timing of the generation of the electric pulse can becontrolled with a resolution of the order of 10 ps. If
necessary, it is also possible to add an analog delay circuitthat can be adjusted with a ps-order resolution. Thus, bychanging this delay time from the FPGA, it is possible togenerate electric pulses with ps-order resolution.
Although it is possible to have a ns or ps optical pulse
width, the case of a ns pulse width is described here to simply
enable a comparison with the previous result obtained using
the original OPP-STM system.19) Here, two thermallystabilized diode systems (Thorlabs, NPL52B) with a central
wavelength at 520 nm and nanosecond pulse width (∼8 ns)
were used. By taking advantage of the external triggerability,
one can determine the repetition rate frep of each laser solely
by the frequency of the input electrical trigger pulse. frep forthe laser was electrically adjusted from 1 kHz to 10 MHz in
the current case. On this basis, the two lasers were easily
synchronized in terms of frep by feeding them with twotrigger pulses (pump trigger and probe trigger) with anidentical frequency. This synchronization scheme provides
a simple and efﬁcient way of freely tuning the optical delay
time td between the pump and probe pulses. The probe trigger
is intentionally (electronically) delayed with respect to thepump trigger by up to td = 1/(2 × frep). The sweeping of td
back and forth at a designated speed is also achievable. In
addition, the delay-time modulation is also easily realized:
the delay time can be modulated between td and td + Δtd(<1/(2 × frep)) at a desired frequency fmod (∼1 kHz) without
the pulse-picking technique.
Next, time-resolved experiments were carried out by
measuring the carrier dynamics in a WSe2 sample. Notethat an STM system with special design was adopted: a highresolution optical microscope (∼1 μm spatial resolution) with
an optical access port was attached outside the main UHVchamber of the STM system, which was oriented perpendicular to the sample plate. This particular design not only
provides a way of checking the locations of the sample
surface and tip, but has excellent laser focusability (∼3 μm inthis case). The STM tip was attached diagonally, as shown inFig. 1, so that its apex can be more clearly seen from above.
WSe2, a typical indirect-bandgap semiconductor with arelatively long carrier recombination lifetime (∼2 μs),20) waschosen as a sample. Since OPP-STM measurement has
already been carried out on this material, it is suitable fordemonstrating the performance of ET-OPP-STM, by comparison with the results obtained by OPP-STM. In theprevious OPP-STM measurement, two components withlifetimes of ns and 100 ns order were observed. Since therecombination lifetime was ∼2 μs, these components wererespectively attributed to rapid tunneling of excess minority
carriers transiently trapped at the surface and the photocurrent
dynamics reﬂecting the ﬂow of excited photocarriers at thesurface, whose lifetime is determined by the balance between
the diffusion/drift and tunneling rates. In OPP-STM withthe pulsed light selected by pulse picking (1 MHz) from apulse train of 90 MHz, the maximum delay time was1/(2 × 1 MHz) = ∼500 ns. Although the excess electrons
trapped at the surface, which are difﬁcult to measure byother methods, were successfully detected, it was not possible
to obtain information on the slow recombination lifetime ofelectrons (∼2 μs). In fact, when the light intensity was high,relaxation had not been completed even at 400 ns, namely
because although the existence of a long-lifetime component
was indicated, it could not be observed. Therefore, thissample was used as a measurement target to demonstrate theperformance of ET-OPP-STM.
A fresh WSe2 surface was prepared by exfoliation in airand then immediately transferred to the STM UHV chamber.
First, tunnel current-bias voltage (I–V ) characteristics weremeasured using the light-modulated scanning tunnelingspectroscopy (STS) method.21,22) As shown in Fig. 2(a),025005-2© 2019 The Japan Society of Applied Physics

Appl. Phys. Express 12, 025005 (2019)

H. Mogi et al.

Fig. 2. (Color online) (a) I–V curves obtained by light-modulated STS
without (blue) and with (red) laser illumination. Measurements were carriedout with a sample bias voltage of Vs = −3 V and a setpoint current ofIset = 100 pA. (b) Typical ET-OPP-STM spectrum obtained for WSe2.

the I–V curve under laser illumination indicated that theWSe2 sample exhibited typical p-type semiconductor behavior. Namely, the current ﬂow was enhanced by laser
illumination under a reverse-bias condition (negative samplebias).
Next, the ET-OPP-STM experiment was carried out. In allmeasurements, the intensities for the pump and probe lasers
were set to be identical and attenuation was realized using
natural-density optical ﬁlters when necessary. When the laser
repetition rate frep was set to 100 kHz an optical delay time up
to 5 μs was provided. The STM bias voltage Vs was set tosatisfy the reverse-bias condition (Vs < 0). Figure 2(b) shows atypical spectrum obtained by ET-OPP-STM. Two components
with lifetimes of 100 ns and μs order were observed. Since thetime resolution was ∼8 ns, no initial tunneling of extra
electrons trapped at the surface was observed, but as expected,
we were able to measure the recombination process.
To explain the entire process, a schematic model is shownin Fig. 3. The time-resolved tunneling current measurement isbased on the surface photovoltage (SPV) effect.23) With nolaser illumination, downward tip-induced band bending(TIBB)24,25) occurs in a p-type semiconductor sample undera reversed bias condition (Vs < 0), as shown in Fig. 3(a).
When the sample is illuminated, namely, a pump pulse
arrives at the sample, and the TIBB is reduced owing to
the redistribution of the photocarriers. That is, the transient
SPV effect consequently increases the effective bias voltage
across the tip-sample junction.
After the excitation by the pump pulse, three processes are
considered to occur. (1) Initial immediate tunneling of theexcess electrons trapped at the surface [Fig. 3(b), ns order].
(2) Minority carrier (electron) diffusion, drift and tunnelingon the surface side, where the lifetime is determined by theFig. 3. (Color online) Schematic illustration of band structures of p-type
WSe2 samples. (a) Without laser illumination. When the sample is reverselybiased (Vs < 0), TIBB occurs at the sample surface. (b) Upon laser
illumination (here, with a pump pulse), TIBB is reduced owing to charge
redistribution, and the tunneling of electrons trapped at the surface occurs.
(c) Minority carriers (electrons) that diffuse and drift towards the surface
tunnel into the STM tip. Decay process is determined by the balance between
the diffusion/drift and tunneling rates. (d) Recombination. EFT, Fermi level ofthe tip; EFS: Fermi level of the sample; Ev, energy of the valance band edge;
Ec, energy of the conduction band edge; Jc: tunneling current contribution
from the conduction band; Jv: tunneling current contribution from thevalance band.

balance between the tunneling and diffusion/drift rates[Fig. 3(c), 100 ns order]. (3) Recombination process[Fig. 3(d), μs order).19) In OPP-STM, the sample surface
below the STM tip is excited by a sequence of paired pulses,
similar to a conventional OPP method, and the change intunneling current is measured as a function of the delay time
td between the pump and probe pulses. The delay time wasmodulated at 1 kHz for the lock-in detection of a weak signal.
The detected signal current is ΔI = I(td)−I(∞).26) It isconsidered that the former two components were observedin the previous OPP-STM, whereas the latter two components were observed in the current ET-OPP-STM measurement.
To further validate this model, we measured the tunnelingcurrent dependence of the spectrum. Figure 4(a) shows aseries of spectra obtained with different tunneling current
setpoints from 10–100 pA. The lifetimes obtained by ﬁtting
the data with a double exponential function are presented as afunction of setpoint tunneling current in Fig. 4(b) (fast decaycomponent) and 4(c) (slow decay component). It can be seenthat when the tip-sample distance was reduced (increasing
tunneling current setpoint), the lifetime of the fast component
decreased, whereas the lifetime of the slow component wasalmost constant, which is in good agreement with theproposed model. Namely, a smaller tip-sample distanceincreases the tunneling rate in the system, which accelerates
the fast decay process at the surface, whereas the recombination rate, which determines the decay of the slow component,
remains constant regardless of the tunnel current rate.

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Fig. 4. (Color online) (a) Setpoint tunneling current dependence of thespectrum obtained by ET-OPP-STM (Vs = −3 V, 12 μW). Black lines show
the ﬁtting curves, where a double exponential function was used. (b) Lifetimeof the fast decay component. (c) Lifetime of the slow decay component.

In conclusion, we have developed an applicable type ofOPP-STM. Two externally triggerable pulse lasers were used toproduce pump and probe pulses, and wide-range delay-time
modulation was realized by simply adjusting the timing ofelectric pulses. The performance of the OPP-STM we havedeveloped was demonstrated by measuring the carrier dynamics in WSe2. In addition to the processes of minority carrier(electron) diffusion and drift, recombination lifetime, which
could not be observed by the former OPP-STM measurement,
was successfully detected. The easy applicability of the system
is expected to lower the barriers to the use of time-resolved
STM and play an important role in analyzing local electronic/
structural dynamics at nanoscale. For example, in semiconductors such as organic solar cells27–29) the evaluation of the longer
recombination lifetime (μs to ms) is important for bandengineering. Also, using externally triggerable lasers withpicosecond optical pulse width increases the time resolutionof the system, and combined with the polarization modulation
technique30) is highly promising for the realization of spin
dynamics measurement by OPP-STM.
Acknowledgment Support from the Japan Society for the Promotion ofScience (Grants-in-Aid for Scientiﬁc Research: 17H06088) is acknowledged.

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