Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 4994–4997
Part 1, No. 7B, July 2002c
2002
The Japan Society of Applied Physics

Development of Time-Resolved Scanning Tunneling Microscopy in Femtosecond Range
Osamu TAKEUCHI, Ryuji M ORITA1 , Mikio YAMASHITA1 and Hidemi S HIGEKAWA∗
Institute of Applied Physics, CREST, University of Tsukuba, Tsukuba 305-8573, Japan
1 Department of Applied Physics, CREST, Hokkaido University, Sapporo 060-8628, Japan(Received April 8, 2002; accepted for publication May 7, 2002)

A promising design for achieving the ultimate spatial and temporal resolution is proposed. With modulation of the time delay
between two femtosecond laser pulses repeatedly illuminating a scanning tunneling microscope junction, the induced component of the tunneling current is detected as a function of the delay time. With this method, a high signal-to-noise ratio, stablemeasurement free from fluctuation of laser intensity, and removal of the thermal expansion effect can be achieved. Using thephotovoltage effect of GaAs, the high performance of the newly developed system is demonstrated in the femtosecond range.
[DOI: 10.1143/JJAP.41.4994]
KEYWORDS: scanning tunneling microscopy, temporal resolution, femtosecond laser

The emerging fields of nanoscale science, engineering, andtechnology—the ability to work at the molecular level, atom
by atom, to create large structures with fundamentally newproperties and functions—are leading to unprecedented understanding and control over the basic building blocks andproperties of all natural and man-made things. However, as
the size of the components in the integrated structures shrinks
to the nanoscale order, there appears a growing and urgent
need to measure the local electronic structures and their dynamics, which determine the major device characteristics aswell as fundamental functions.
The recent improvement of measurement techniques has
resulted in extremely high spatial and temporal resolutions.
On the one hand, scanning tunneling microscopy (STM) allows us to observe individual molecules and atoms withangstrom resolution. On the other hand, currently, the stateof the art in quantum optics offers ultrashort laser pulses on
the order of several femtoseconds, which enables terahertz
bandwidth measurement based on the pump-probe technique.
However, these spatial and time resolutions were not established simultaneously; the measurement bandwidths of scanning probe microscopes are typically limited by the external
electronics to ∼ 100 kHz and the spatial resolution of the laser
technique is generally limited to its wavelength. Therefore,
one of the most challenging and exciting tasks is to combine these two techniques to achieve unprecedented simultaneous spatial and temporal resolution, i.e., femtosecondangstrom technology. In fact, since the development of STM,
many researchers have been making efforts to accomplish thistask.1–6) The ultimate goal is to analyze and control the electronic and structural dynamics of materials with atomic resolution on a femtosecond time scale. There have been two
major concepts for achieving this goal.
One is to utilize ultrafast photoconductive gates to adjoin
the time resolution to STM. Weiss et al. presented photoconductively gated STM with a time resolution of 1 ps, in which
an ultrafast photoconductive gate was inserted on the tunneling current detection line.1) This was based on the idea thatthe photoconductive gate enables the sampling of instantaneous tunneling current immediately after laser pulse excitation of the sample. The sampled current is then recorded as afunction of the delay time between excitation and sampling,

just as in other pump-probe techniques. If this were indeed
the case, the system would be a femtosecond STM. However,Groeneveld and van Kempen pointed out that the detected signal was primarily due, instead of to tunneling current, to thedisplacement current generated by the coupling of two stray
capacitances, one at the tunneling junction and the other at
the photoconductive gate.7) When the signal arises from thedisplacement current, the lateral resolution of measurement
is given by the geometric dimension that defines the capacitance at the junction and is much worse than that of STM;
it is typically about 1 µm. Up to now, several attempts havebeen made to improve this type of STM.2, 3) Junction mixing
STM has exhibited 1 ps time resolution with 1 nm spatial resolution. However, this spatial resolution originates from thenanoscale variation of the current–voltage dependence of thesample while the ultrafast phenomenon itself does not havean atomic-scale structure. Thus, it is of rather limited application. Moreover, the time resolution of this type of STM, inprinciple, cannot overcome the response time of the photoconductive gate, which is typically about 1 ps.
The other approach relies on the direct excitation of the tunneling junction by a sequence of laser pulses and the detectionof the induced tunneling current as a function of interpulse
spacing. Hamers and Cahill, in their pioneering work, determined the carrier relaxation time at the Si(111)-7 × 7 surface
with about 10 ns time resolution and 1 µm spatial resolution
by this approach.4) In order to measure the small current variation due to surface photovoltage, they used a lock-in amplifier with the laser intensity modulated by a mechanical chopper. The difference in the time-averaged tunneling current
with and without illumination of an optical pulse train wasrecorded as a function of the repetition rate of laser pulses. To
obtain a time resolution better than the repetition rate, pulsepair-excited STM is also proposed, where the illuminationis composed of a sequence of pulse pairs and the tunnelingcurrent is measured as a function of delay time of the pulse
pair. The signal from this kind of setup is determined by individual physical systems; thus it is complicated compared tomore straightforward approaches such as photoconductively
gated STM, but is free from the drawback of slow responseof the electronic circuit. In addition, the time resolution is determined by the time delay between the two pulses, instead of
the repetition rate of the original laser, and can be improved∗ http://dora.ims.tsukuba.ac.jp/4994

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 7B

O. TAKEUCHI et al.

without limit by shortening the laser pulse width. Thus, inprinciple, pulse-pair-excited STM has the potential of achieving femtosecond and angstrom resolution. Although several
attempts to realize this concept have been made, its applicability has not yet been demonstrated.
In this study, we reconsider the difficulty in the development of a pulse-pair-excited STM and propose a promising
new design to surpass the critical requirements. The basic
performance of the system is clearly demonstrated. Temporal resolution is achieved up to the femtosecond range withthe spatial resolution of STM remaining unchanged.
In the development of pulse-pair-excited STM, we must
devise a way of detecting the small component of tunneling current that depends on delay time. For the purpose, thedetection must be sensitive. It is common to use lock-in detection with light intensity modulation. For instance, Hamers
and Cahill used a mechanical chopper,4) while Gerstner et al.
mentioned locking to the repetition rate of the laser pulse.6)
In both cases, the output from the lock-in amplifier corresponds to the illumination-induced tunneling current difference, Idiff (td ), as
Idiff (td ) = Iillum (td ) − Idark .

(1)Here, Iillum (td ) is the tunneling current under illumination,
which is delay time dependent, and Idark is that under dark
condition. On account of the lock-in detection, the broadband noise from the current–voltage converter of STM is
reduced. Thus, the delay time dependence in the inducedcurrent difference can be obtained precisely. In practice, however, the delay-time-dependent component is expected to bequite small compared to the large background of the delaytime-independent component. In addition, when the laser intensity is modulated at low frequency such that the STM
current–voltage converter can respond, the change in the heatdelivered by laser pulses causes the thermal expansion andshrinking of the STM tip at the modulation frequency, which
increases the background. Other undesirable signals, such as
the displacement current due to the photovoltage effect in thecase of the semiconductor sample, also increase the background. Thus, an unrealistic dynamic range will be required
for the lock-in amplifier. What is worse is that this large background fluctuates with the unavoidable small fluctuation oflaser intensity at low frequency. The lock-in amplifier does
not eliminate the fluctuation. When each of the two pulses in
the pulse pair is chopped independently at different frequencies and the amplifier is locked at the difference frequency,
the signal proportional to
Isignal = (I12 + Idark ) − (I1 + I2 )(2)would be obtained, where Idark , I1 , I2 and I12 are the tunneling currents with no laser pulse, with laser pulse 1, withlaser pulse 2, and with both laser pulses, respectively. Thus,
only the nonlinearity of the tunneling current with respect
to the laser intensity can be retrieved. This method reduces
the dynamic range required to the lock-in amplifier as wellas the fluctuation of the background component. However,since it also modulates the laser intensity, thermal expansion
and shrinking of the STM tip cannot be avoided. Measurement with high intensity of the exciting laser, which generally
gives larger nonlinearity in the tunneling current, will crash
the STM tip into the sample.

Delay circuit

Ti:Sapphire
Pulse Laser
shake

Time
Delay td
STM

amplitude td
frequency
Trig
D
C
C
Ref

Fig. 1.

800 nm, 25 fs
80 MHz, 100 mW

Sig

td

Lock-in
AmplifierCurrent

UHV LHe
Chamber

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Sig

Schematic of our shaken-pulse-pair-excited STM system.

Here, we summarize the requirements that have to be met
for the development of time-resolved STM.
(1) High signal-to-noise ratio.
(2) Reduction of noise from the fluctuation of laser intensity.
(3) Removal of the thermal expansion effect of tip and sample.
These requirements are difficulties that had not been surpassed until now.
To overcome the difficulties listed above, we propose
the shaken-pulse-pair-excited STM system. Figure 1 shows
the schematic. The optical source is a Ti:sapphire laserthat routinely generates 25 fs pulses centered at 800 nm at
80 MHz repetition rate. The pulse pairs are generated by aninterferometer-type delay circuit. A half-mirror splits the laser
beam into two delay arms with slightly different path lengths.
Those beams are reflected by retro-reflectors at the end of thedelay line and remixed by the half-mirror. The delay time isdetermined by the difference between the path lengths of thedelay arms, which can be precisely adjusted using piezoactuators. Finally, a sequence of pulse pairs is focused onto the
STM tip-sample junction, and the tunneling current is measured using an STM electronic circuit with 10 kHz bandwidth.
In our system, the repetition rate of the excitation laser ischosen to be high in order to minimize the thermal expansion
effect of the STM tip. Furthermore, we do not use the mechanical chopper. Instead, what is noteworthy is that it modulates
the delay time between two laser pulses by periodically shaking one of the two end reflectors on delay arms and detects
the component of the corresponding tunneling current using a
lock-in amplifier. Our two delay lines are capable of shaking
the end reflector at a maximum frequency of 10 Hz or 100 Hz
up to 150 ps or 0.7 ps, respectively.
When the time delay of the two pulses td is modulated witha small value td around the center position td at frequency
ω, the tunneling current can be represented as
It (td + td sin ωt) = It (td ) + td sin ωt

d It+ O(td2 ).
dtd(3)With the use of a lock-in amplifier, the coefficient of the termsin ωt, d It /dtd , can be obtained. In this system, when thetunneling current is independent of the delay time, the output is zero. There is no background at all. Thus, the signalto-background ratio is maximized. The fluctuation of laser

O. TAKEUCHI et al.

Tunnel Current [pA]

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 7B

Illuminated
Dark

1

800pA

0

600
400
200

(a)

-1
-2

-1

0

1

2

Sample bias voltage [V]Fig. 2. I –V curves obtained for n-type GaAs(100) with and without laser
illumination.

intensity can be reduced by lock-in detection. Finally, since
the laser intensity is not modulated, thermal expansion andshrinking of the STM tip does not occur. All the requirements
listed above can be satisfied by this technique.
In order to confirm the validity of the idea, the performance of our system was examined using a surface photovoltage effect. A nondoped n-GaAs(001) was used as the sample(carrier density ∼ 1 × 1015 cm−3 ). Figure 2 shows the I –V
curves obtained with and without laser illumination (set point
Vs = −2.5 V, Is = 0.10 nA). Because of the low doping level,
when a positive sample bias voltage is applied, the wide depletion layer formed at the tunneling gap prevents the tunneling current under a dark condition. With illumination, however, the photoinduced carriers decrease the depletion layer,allowing the tunneling current to flow. The tunneling current
increases with the intensity of the excitation laser. Since thesample is nondoped, small changes in the photoillumination
intensity at the tunneling junction can be sensitively detected
as changes in tunneling current. According to the independence of the current difference between illuminated and dark
conditions on the frequency of switching, the displacement
current component is determined to be negligible. In addition,no current difference at negative sample bias voltages indicates the thermal expansion effect of the tip is also negligible
in our experiments. Thus, almost all of the current change at
positive sample bias voltage is attributed to a change of thetunneling current.
Figure 3 shows an experiment for evaluating the temporal
resolution of our system. The tunneling current was recorded
during sweeping of the temporal delay of the paired laser
pulses with a static tip-sample separation. When the delay
time is small compared to the pulse width and the two pulses
are temporarily overlapped, sweeping the delay time causes
the oscillation of the effective laser intensity at the periodicity of the carrier wave owing to the interference between thetwo pulses. The oscillation decreases as the portion of overlap
decreases, and when the two laser pulses separate, the oscillation vanishes. The oscillation of illumination intensity at thetunneling junction can be detected as an oscillation in tunneling current as shown in Fig. 3. The wavelength of the laser
pulse is centered at 800 nm, and the pulse width is 25 fs. As
is shown in Fig. 3(a), the tunneling current oscillates at 2.6 fs,
which corresponds to the periodicity expected from the wavelength of 800 nm.
Similar measurement was performed by Gerstner et al.6)

Current Difference [pA]

Tunnel current [nA]

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100pA
0
-100
-80fs -60

-40

-20
0
20
Delay Time [fs]

40

60

80

(b)
Fig. 3. Results of the temporal resolution of our system; (a) delay-time-dependent tunneling current, (b) shaken-pulse-pair-excited STM
signal.

However, in their case, thermal expansion of the STM tip
was used for the analysis. In our case, by using a nondoped
n-GaAs(001) as the sample, the laser intensity could be kept
low so that the thermal expansion effect could be avoided. As
a result, the laser intensity precisely at the tunneling junction
could be obtained.
Now let us examine the performance of the shaker method
which we introduced. Figure 3(b) shows the lock-in amplifier signal (time constant = 30 ms) plotted versus delay time,
where the delay time was modulated by 0.5 fs around the center value at 400 Hz. The derivative of the graph in Fig. 3(a)was successfully obtained with a lower noise level, indicating
the expected performance of our time-resolved STM. Its temporal resolution is limited by the pulse width, in the range offemtoseconds. In the actual experiment, one has to maintaina delay time longer than the pulse width to avoid the interference effect shown here.
So far, we have demonstrated that our method can measure
the dependence of tunneling current on the delay time of an
excitation pulse pair with good sensitivity and precision. The
question “which system causes the dependence of tunnelingcurrent on the delay time?” or “to which system can your microscope be applied?” may arise. One of the easiest cases ofusing this method is when the dependence of the tunnelingcurrent on laser intensity in the specific system is nonlinear.
In this case, the average tunneling current with excitation by
the sequence of pulse pairs with long delay time is different
from that with excitation by a sequence of completely overlapped pulse pairs, in other words, a single pulse with double
intensity. Therefore, when the delay time is swept, the decay
time of the state excited by the first pulse can be obtained. For
example, let us assume that each of the laser pulses generates
a change in the tunneling current of 10 nA during 1 ps, andthe tunneling current due to the second pulse changes 10%
depending on the delay time. The resultant change in the av-

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 7B

O. TAKEUCHI et al.

eraged tunneling current is
10 (nA) × 1 (ps) × 80 (MHz) × 10(%) = 0.08 (pA).

(4)

This is in the range of realistic values.
There is another issue to be considered for spatial resolution. Owing to the exponential dependence of tunneling current on tunneling distance, when the tunneling current is detected by precision electronics and mechanics of STM apparatus, acquisition of localized information in the angstrom orderis guaranteed. However, when the detected signal arises fromphotoexcited electron emission, displacement current or fieldemission, the spatial resolution can be much worse. Thus, as
discussed with photoconductively gated STMs,1–4) the origin of the signal must be examined carefully for individual
systems. Thermal drift of the STM tip relative to the sample surface can also affect the spatial resolution, especiallywhen the time-resolved tunneling current detection takes along time. Our method discussed above keeps the averaged
laser power constant so that the thermal drift is minimized.
However, current detection with an extremely long acquisition time might require active drift correction combined withthe atom-tracking technique,8) where the velocity and direction of thermal drift are determined by the atom-tracking technique and compensated.

4997

In conclusion, a promising new design for achieving ultimate spatial and temporal resolution, by combining a shortpulse laser and STM, is proposed. Using the photovoltage effect of GaAs, high performance of the newly developed system is demonstrated in the femtosecond range. Further experiments are now in progress.
This work was supported in part by a Grant-in-Aid forscientific research from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.

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