ARTICLESPUBLISHED ONLINE: 24 OCTOBER 2010 | DOI: 10.1038/NPHOTON.2010.235

Real-space imaging of transient carrier dynamics
by nanoscale pump–probe microscopy
Yasuhiko Terada, Shoji Yoshida, Osamu Takeuchi and Hidemi Shigekawa*
Smaller and faster are key concepts underlying the progress of current nanoscience and nanotechnology. The developmentof a method of exploring the transient carrier dynamics in organized nanostructures with pinpoint accuracy is therefore
highly desirable. Here, we present a new microscopy that enables real-space measurement of the spatial variation ofultrafast dynamics. It is a pulse-laser-combined scanning tunnelling microscopy with a novel delay-time modulation method
based on a pulse-picking technique. A non-equilibrium carrier distribution is generated with ultrashort laser pulses, and its
relaxation processes are observed by scanning tunnelling microscopy using a pump–probe technique. We have directly
analysed the recombination of excited carriers via the gap states associated with a cobalt nanoparticle/GaAs structure in
real space. Through the site dependence of the decay time on the tunnelling current injection from the scanning tunnelling
microscopy tip, the hole capture rate at the gap states has been imaged on the nanoscale for the ﬁrst time.

T

he understanding and control of quantum dynamics, such as
carrier transitions and transport in nanoscale structures, are
key factors for continuing the advancement of nanoscalescience and technology. However, with the miniaturization of functional devices consisting of composite materials, carrier dynamics,
which has been analysed by techniques providing spatially and/or
temporally averaged information, does not provide a sufﬁcientdescription for the analysis and design of macroscopic functions.
For example, atomic-scale defects have signiﬁcantly changed theentire approach. Defects, which were once considered a problem
to be avoided, are now actively designed and controlled to realize
desired functions1. Fluctuation in the distribution of dopantmaterials in nanostructural components governs the characteristic
properties of the macroscopic functions of the total system.
Therefore, evaluation of the transient carrier dynamics on thenanoscale is of great importance.
The recent development of ultrashort-pulse laser technology has
enabled the observation of ultrafast dynamics in the femtosecond
range. A prominent method used for this is optical pump–probe
reﬂectivity (OPPR) measurement2,3. Its spatial resolution,
however, is generally limited by wavelength, and information isinevitably averaged over the nanoscale components, despite theircareful organization to produce desired functions. In contrast, thereal-space observation of atomic-scale structures by scanning tunnelling microscopy (STM) has addressed various longstanding problems and is extending the frontiers of science and technology4–9.
However, because the temporal resolution of STM is limited toless than 100 kHz because of the circuit bandwidth, the target
carrier dynamics has been beyond its ﬁeld of vision. Since the invention of STM, one of the most challenging goals has been to combine
it with ultrashort-pulse laser technology.
In their pioneering work, Hamers and colleagues determined therecombination lifetime of photoexcited carriers in silicon by analysing the observed surface photovoltage (SPV) by simulation10. Other
methods that involve the combined use of an optical pulse pair and
STM have also been proposed11–18. However, the spatial and temporal resolutions of the former method were limited to the 1 mm
scale and the laser-pulse repetition rate, respectively, and the lattermethods probe, for example, spatial variation in dI/dV rather

than the transient dynamics itself18. With shaken-pulse-pairexcited STM (SPPX-STM)19,20, which was designed to detect the
weak tunnelling current of a transient signal under optical excitation, the time-resolved tunnelling current in the subpicosecond
range was successfully probed. However, its temporal range wasvery narrow and it is still difﬁcult to reliably measure a very weaktime-resolved STM signal in a short period of time, preventing
SPPX-STM from revealing transient carrier dynamics in nanostructures consisting of composite materials with a wide variety of lifetimes, as the long measurement time interferes with the imagingof dynamics.
Here, we demonstrate a new methodology, that simultaneously
realizes STM spatial resolution together with the temporal resolution of the optical pump–probe technique, enabling the visualization of transient carrier dynamics in nanometre-scale structureswith a wide variety of lifetimes. As an example, through the site
dependence of the decay time on the tunnelling current injection
from the STM tip, the ﬁrst direct analysis of the hole capture rate
via a gap state in a cobalt nanoparticle/GaAs structure is shown.
Pinpoint probing and real-space imaging of a meaningful physicalquantity in nanometre-scale structures have been realized.

Results
Nanoscale pump–probe microscopy and its fundamentals.
Figure 1 shows a schematic of the microscopy technique and its
basic probing mechanism. The surface of a sample beneath an
STM tip is illuminated with a sequence of paired laser pulses witha certain delay time td , and the (temporally averaged) tunnellingcurrent I is measured as a function of td (Fig. 1a). The opticalpulses create an excited state in the sample, resulting in a rapidchange in a physical quantity n such as the photocarrier density
in a semiconductor (Fig. 1c), which often gives rise to changes in
the raw tunnelling current I* (Fig. 1d), reﬂecting the excitation
and relaxation of the sample. When td is sufﬁciently long, paired
optical pulses with the same intensity independently induce two
excitation pulses with the same height in n (Fig. 1c,i). In contrast,
when td is short and the second pulse illuminates the sample inan excited state caused by the ﬁrst optical pulse, the second
excitation in n may have a different intensity, depending on td

Institute of Applied Physics, CREST-JST, University of Tsukuba, Tsukuba 307-8573, Japan. * e-mail: hidemi@ims.tsukuba.ac.jp
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NATURE PHOTONICS DOI: 10.1038/NPHOTON.2010.235ab

Delay time td

td

Pulse
picker

1 MHz

Ti:Sapphirelaser

90 MHzSynchronization
Pulse
pickerVs
STM

Modulation
1 kHz

Reference
I

I*

In-phase signal (ΔI)

Lock-in
amplifierΔI(td) = I(td) − I(∞)

Preamplifier
td

c
n (i)

1st

e

d
I* (i)

2nd

(ii)

I

I(∞)

(ii)
(ii)

(iii)

(iii)
t

Ti:Sapphire
laser

(iii)

t

0

(i)
td

Figure 1 | Schematics of the microscopy system and its probe mechanism. a,b, Optical pulses from the two synchronized lasers are selectively transmittedusing pulse pickers and coaxially aligned to produce a sequence of paired pulses with a certain delay time td. Pulse pairs were focused to less than 10 mm,
and the tunnelling current was measured. c, Relationship between photoexcitation and delay time td. d, Change in transient tunnelling current induced by the
excitation in c. e, Measured tunnelling current I(td), where delay times in (i), (ii) and (iii) correspond to those in (i), (ii) and (iii) in c and d.

SPPX signal from semiconductor. Figure 2 presents sample spectra
obtained using this microscopic technique. For the ﬁrst time, themeasurement of carrier dynamics over a wide range of timescales
has been achieved using a single microscope. A low-temperaturegrown GaAs (LT-GaAs) region formed on a GaAs substrate
870

a

0

b

LTGaAsΔI (pA)ΔI (pA)

GaNAs
0

−1

−0.8
0
500
Delay time (ps)−10 −5 0 5 10
Delay time (ps)
1 ps

c

1 μs

1 ns

0

GaAs

d
ΔI (pA)ΔI (pA)(Fig. 1c,ii,iii). Although the lifetime of the excited state is, in general,
different from the decay time of I*, the change in n strongly affects
the second pulse in I*. Thus, the change in I*, and thereby the signal
I, also depends on td , because the height difference in the second
current pulse in I* changes the temporally averaged value of thetunnelling current (Fig. 1e). Accordingly, the relaxation dynamics
of the excited state in the target material, namely the change in aphysical quantity after excitation by the ﬁrst optical pulse, can beprobed by STM at the pulse-width resolution (that is, in the
femtosecond range). In principle, we can obtain the temporal
resolution of OPPR together with the spatial resolution of STM.
To realize the desired microscopy, we developed a method for thelarge and discrete modulation of delay time using a pulse-picking
technique (Fig. 1b, see Methods). With the digital modulation of
td between t 1d and t2d, the in-phase component obtained by lock-in
detection of the tunnelling current gives DI(t1d,t2d) ; I(t1d)2I(t2d). As
t2d is set to a value larger than the relaxation time of the probed
dynamics, DI(t1d,t2d) can be approximated as DI(t1d) ; I(t1d)2I(1),where I(1) is the tunnelling current for a delay time that is sufﬁciently long for the excited state to be relaxed. Because a large
delay time to satisfy this condition (t2d ¼ 1) can be realized with
the new microscopic technique, DI(t1d) is accurately obtained
through the lock-in detection of I by sweeping t1d. In addition,
because the modulation can be performed at a high frequency(1 kHz in our case), the measurement is hardly affected by low-frequency ﬂuctuations in the laser intensity and tunnelling current.
Accordingly, this method improves the signal-to-noise ratio (SNR)
by a factor of 100 for our system compared with the method of
mechanical modulation using mirrors, and therefore enables thespatial mapping of time-resolved signals, which has long
been desirable.

−0.3
0

5
10
Delay time (ns)

H-Si
0

−0.3
−1

0
1
Delay time (μs)

Figure 2 | Carrier dynamics over a wide range of timescales measured
using the developed microscopy. a–d, DI(td) versus td curves obtained for
LT-GaAs (a), GaNAs (b), undoped GaAs (c) and hydrogen-terminatedSi(111) (n-type, 0.5 V.cm) (d). The decay constants derived by ﬁtting are
2.4 ps (a), 440 ps (b), 4.8 ns (c) and 0.87 ms (d). See ‘Fitting Procedures’ inSupplementary Information for more details.

exhibits an ultrafast decay component tb with a time constant of
2.4 ps, and the GaAs substrate region exhibits a time constant of
4.8 ns. These values are consistent with the recombination
lifetimes determined from OPPR measurements, namely 1.5 ps
and 3.3 ns for the LT-GaAs and GaAs samples, respectively. Suchagreement has been conﬁrmed for various semiconductors with awide range of lifetimes (Supplementary Fig. S1).
When the delay time is changed over a wider range of timescales,DI(td) exhibits a slower component with a time constant of 100 ns
for both the GaAs and LT-GaAs regions, as shown in the typical
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ARTICLES

NATURE PHOTONICS DOI: 10.1038/NPHOTON.2010.235

Supplementary Fig. S1), and the large time constant ts to surfaceside decay (Fig. 3a). We next show the results obtained for acobalt/GaAs system. This is the ﬁrst demonstration that a meaningful physical quantity can be directly derived from the analysis of ts
on the nanoscale, which also provides more speciﬁc evidence forour mechanism.

ΔI (pA)

a
GaAs

0

−0.1

Time constant (ns)

b

c

0

Amplitude (fA)

−200
0
200
Delay time (ns)

−50

200
100
10
5

−100

As

Ab

−150

0
0

50
100
Tunnelling current (pA)

0
50
100
Tunnelling current (pA)

Figure 3 | Bulk-side and surface-side carrier decay reﬂected in DI(t d).
a, Typical spectrum obtained for GaAs over a wider range of timescales(Vs ¼ 5.5 V, It ¼ 40 pA). The components with small and large time
constants tb and ts represent bulk-side and surface-side decay, respectively.
b,c, Dependence of the decay constants on set-point tunnelling current It(Vs ¼ 5 V).

spectrum obtained for GaAs in Fig. 3a. Although the decay process
for a semiconductor is complex, a possible mechanism can be
described as follows (see ‘Mechanism’ in Supplementary
Information). In STM of a semiconductor, a nanoscale metal–insulator–semiconductor (MIS) junction is formed by the STM tip, tunnelling gap and sample. Thus, when a reverse bias voltage is applied
to the junction, tip-induced band bending (TIBB) occurs in thesurface region owing to leakage of the electric ﬁeld into thesample9,18,21. Upon optical illumination, the redistribution of photocarriers reduces the electric ﬁeld and changes the surface potential(SPV), thus increasing the effective bias voltage applied to thetunnel junction. Consequently, the illumination increases the raw
tunnelling current I*. The excited state subsequently relaxes to theoriginal state through two processes. One is the decay of photocarriers on the bulk side (bulk-side decay) by means of recombination,
drift and diffusion. The other is the decay of carriers trapped at thesurface (surface-side decay) through recombination and thermionicemission. Because of the lack of counterpart carriers near thesurface, the surface-side decay constant is larger than the bulkside decay constant.
The bulk-side decay is probed through the mechanism of absorption bleaching, which is a typical mechanism observed in OPPR
measurement. The carriers excited by the ﬁrst optical pulse
remain in the excited state, but the absorption of the second
optical pulse is suppressed. The total number of carriers excitedby the two optical pulses changes with td , resulting in the total tunnelling current depending on td. On the other hand, the surface-side
decay is probed through the mechanism related to SPV. If surface
carriers remain when the second optical pulse arrives, the electricﬁeld in the surface region remains low. Therefore, the excited photocarriers are less efﬁciently trapped at the surface, leading to adecrease in SPV caused by the second optical pulse, and therefore
a reduction in the height of the second current pulse in I*.
Accordingly, DI(td) is a measure of surface-side carrier density at
the delay time td , as well as bulk-side carrier density.
The decay constants for both components are derived from theﬁtting of DI(td) by a double exponential function; the small time
constant tb is attributed to bulk-side decay (Fig. 2 andReal-space probing of gap-state-enhanced carrier recombination.
If a gap state exists in a bandgap, minority carriers are captured at
the gap state with a high probability, and then rapidly recombinewith majority carriers. The modulation of carrier dynamics due togap states signiﬁcantly affects the properties of a device, andtherefore a new method that enables the direct analysis of gapstate-related processes at the nanoscale is strongly desired. As an
example, we demonstrate the direct analysis of the hole capture
rate through a cobalt-derived gap state.
Figure 4a shows an STM image of cobalt nanoparticles on an
n-type GaAs surface. Cobalt particles grow in the Volmer–Weber
mode22 on GaAs, and their size is controllable. Nanoparticles are
typically 2 nm in size (400 atoms), which is sufﬁciently large toform a gap-state level23,24.
Figure 4b,c shows DI(td) curves measured at positions corresponding to bare GaAs and cobalt, respectively. The peak ratio of
the bulk-side decay component to the surface-side decay component varies with the excitation intensity; the bulk-side decay
decreases as the excitation intensity increases. Here, we adjusted
the excitation intensity to observe only surface-side decay. As
expected, the decay constant at cobalt, 42 ns, is shorter than thatat the bare GaAs surface, 223 ns.
A depletion layer is formed beneath the tip because of TIBB (Fig. 4d)
as well as the band bending induced by the gap states in the present case.
The holes (minority carriers) photogenerated in the depletion layer
drift to the surface and are rapidly captured at the gap states.
However, the electrons (majority carriers) are depleted near thesurface, and therefore no counterpart carriers exist that recombinewith the holes captured at the gap states. The carrier decay time isnevertheless much shorter at the cobalt because the electrons are
injected from the STM tip to the cobalt site by means of tunnelling.
This is conﬁrmed by the dependence of carrier decay time atcobalt on tunnelling current (Fig. 4e). The decay time markedly
decreases as tunnelling current increases. In addition, the decay
time is almost constant (Tcap ¼ 6.9 ns) above a threshold current
Ic (120 pA). From the analysis below, the hole capture probability
Ph was derived as Ph ¼ 1/Tcap ¼ 1.5 × 108 (s21).
The hole capture rate Jh is written as Jh ¼ PhNh , where Ph is theprobability per second that a hole in the depletion layer is captured
at a gap state, and Nh is the number of holes trapped at the surface
after photoexcitation. The recombination process is governed by thetunnelling current directly injected into the gap state localized at thecobalt site. The rate of injection electrons to a gap state Je is proportional to the tunnelling current It: Je ¼ It/e. Assuming that therate of carrier recombination at the gap state is high25, the recombination rate is equal to the lower of Jh and Je. Thus, the recombination
rate is Je-limited when It is low, but is Jh-limited and independent of
It when It is high. In the Jh-limited regime, Jh varies with Nh , and thetime evolution of Nh is written asdNh= −Jh = −Ph Nh ,dt

Nh = Nh0 exp(−Ph t)

Here, the hole decay rate is equal to Ph.
For Ic , It (Jh-limited regime), the gap state is ﬁlled with tunnelling electrons, and even if Nh is large immediately after photoexcitation, Jh , Je and the recombination rate is Ph , independent of It.
Accordingly, the hole capture rate Ph can be directly derived fromthe decay time in this regime.

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ARTICLES

NATURE PHOTONICS DOI: 10.1038/NPHOTON.2010.235
a

b

c

GaAs

0

Co

4.0 nmVs = 3.0 V
It = 100 pA
223 ± 11 ns−0.5−1,000ΔI (pA)ΔI (pA)0
Vs = 2.0 V
It = 80 pA
42 ± 5 ns

−0.5

1,000

0

−100

Delay time (ns)Low VsNo tunnellingeHigh Vs

Depletion

Je

Co

100

Je-limited

Decay time (ns)d0Delay time (ns)Jh60

Jh-limited
0
IC

400

It (pA)1 × 10−18g

0ΔI (pA)f

Trapped charge (C)

0

A

Co

Vs = 2.5 V
It = 80 pA
232 ± 17 ns

−1

B
0
0

20

−500

Time (ns)0500Delay time (ns)Figure 4 | Carrier recombination via cobalt nanoparticle/GaAs gap states. a, STM image of cobalt nanoparticles on a GaAs(110) surface. b,c, SPPX signals
measured above bare GaAs and a cobalt nanoparticle. d, Schematics of surface-side recombination. e, Tunnelling current dependence of carrier decay time atcobalt (Vs ¼ 2 V). f, Simulation of the SPPX signal with two decay modes when It , Ic. g, SPPX-STM signal at a high bias voltage.

872

line in Fig. 5d is the ﬁt to the data with the simpliﬁed function
Tcap ≈ 1/S (ref. 26). The discrepancy in the ﬁtting is considered tobe due to the use of a simpliﬁed model.
abTopography
ns
150

0

2 nm

c

Overlayd

10
Tcap (ns)

For It , Ic , on the other hand, Nh is large and Jh . Je immediately
after photoexcitation, and Nh initially decays linearly with time (A inFig. 4f ). Subsequently, when Nh becomes small and Jh , Je , Nh starts
to decay exponentially (B in Fig. 4f ). The time evolution of Nh is
therefore represented as a black line in Fig. 4f. Because the SNR
was not sufﬁcient to distinguish between linear and exponential
decay, the ﬁt to the data was carried out with a single exponentialfunction. Therefore, the apparent decay time derived from theﬁtting should be larger than P21
h , which was indeed observed asshown in Fig. 4e.
When It ¼ Ic , Je is equal to Jh at the excitation time; thisrelation gives the number of holes Nh0 photogenerated in thedepletion layer and trapped beneath the tip. From Fig. 4e, Ic ¼
120 pA (Je ¼ 7.5 × 108 s21) and Nh0 is derived to be 5.0. Thisvalue is comparable to the number of charges N0 estimated toexist between a sphere of radius R and an inﬁnite plane placed at
a distance of 1 nm from the sample; N0 is 15.5 for R ¼ 10 nm and40.7 for R ¼ 20 nm.
Next we show probing of hole capture rate in real space (Fig. 5). The
decay time is short directly above a cobalt nanoparticle and markedly
increases when the tip is placed at a subnanometre distance away fromit. This feature is consistent with the fact that the recombination ismarkedly accelerated only when tunnelling electrons are injectedfrom the STM tip to the gap states formed by the cobalt nanoparticle.
Furthermore, the result shown in Fig. 5 conﬁrms that the holecapture rate for each cobalt nanoparticle is distinguishable when
the nanoparticles are at least 1 nm apart. Accordingly, we measured
Tcap for cobalt nanoparticles with different sizes (Fig. 5d). As thecobalt base area (S) increases, the capture probability is expected
to increase and thus Tcap decreases, which corresponds well to theobtained result shown in Fig. 5d.
Thus, the new microscopy method enables the measurementof the carrier capture rate at a surface in real space. The solid

5

0
0

10
S (nm2)20

Figure 5 | Real-space analysis of the hole capture rate at cobalt
nanoparticle/GaAs gap states. a,b, STM image (a) and two-dimensional
mapping of time-resolved signal (b) obtained for a cobalt nanoparticle/
GaAs(110) system. c, Image of a superimposed on b. d, Size dependence ofhole capture rate.
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ARTICLES

NATURE PHOTONICS DOI: 10.1038/NPHOTON.2010.235
Discussion
Here, we discuss the processes occurring in SPPX-STM measurement in more detail. The faster of the two decay components
appearing in DI(td) is considered to reﬂect the decay of bulk-side
carriers, which are probed on the basis of absorption bleaching.
This is well supported by the fact that tb is consistent with the
decay time obtained by the OPPR measurement of various
samples (Supplementary Fig. S1). On the other hand, the slower
decay component ts has a complex dependence on tunnellingcurrent and sample bias voltage, which is consistent with the characteristic of surface-side carrier decay.
Because recombination at the surface is enhanced for a large setpoint tunnelling current It , ts decreases as It increases. In contrast,
the bulk-side carrier decay is not affected by the magnitude of It.
Therefore, tb is independent of It. These expectations were conﬁrmed, as shown in Fig. 3b; ts decreases with It , while tb remains
almost constant. The linear relationship between signal amplitude
and set-point tunnelling current (Fig. 3c) indicates that the timeresolved signal does not originate, for example, from photoelectrons
and the displacement current, but from the tunnelling current (see‘Mechanism’ in Supplementary Information).
The mechanism for the cobalt nanoparticle/GaAs is conﬁrmed
by its dependence on the sample bias voltage Vs. For high Vs , theFermi level of the tip is higher than the conduction band
minimum of the sample. Because the tunnelling probability for anelectron is higher when its energy is closer to the vacuum level, electrons directly tunnel to the conduction band (Fig. 4d). In this case,accelerated recombination via the gap state is less probable. This
expectation was experimentally conﬁrmed. For Vs ¼ 2.5 V(Fig. 4g), the surface-side decay constant was 232 ns, comparable
to that for the bare GaAs (Fig. 4b).
As the device size decreases, the nanoscale modiﬁcation of carrier
dynamics, for example by atomic-scale defects, will strongly affectthe macroscopic device properties, so an urgent task is to understand and control the correlation between structural ﬂuctuations
and carrier dynamics. The microscopic process of intrinsic,
surface and bulk carrier dynamics is complex, particularly in an
inhomogeneous sample. Although further analysis is required, it isof great signiﬁcance that our new microscopic technique provides
data that can be used to discuss the dynamics in nanosystems.
By combining advanced ultrashort-pulse laser technology with
STM, we have developed a new microscopy technique that achieves
the spatiotemporal imaging of ultrafast dynamics. The time resolution of this microscopy is limited only by the optical pulsewidth,
which is 140 fs in the present case (see ‘Fitting Procedures andTime Resolution’ in Supplementary Information). As a demonstration of this microscopy technique, real-space probing of gap-stateenhanced carrier recombination for a cobalt nanoparticle/GaAs
structure has been shown. Through the site-dependence of the
decay time on the tunnelling current injection from the STM tip,
the hole capture rate at the gap states associated with a cobalt
nanoparticle/GaAs structure has been imaged on the nanoscale forthe ﬁrst time.
SPPX-STM is also applicable to the systems in which the responseof the tunnelling current has a nonlinear dependence on opticalintensity. With the use of selected-wavelength and circularly polarized
illumination, quantum transitions including excited and/or spin states
may be probed. The direct observation of such dynamics is expected toenable further advances in nanoscale science and technology.

Methods
As shown in Fig. 1b, pulse trains were generated by two synchronized Ti:Sapphirelasers (Mira and Chameleon, Coherent Inc.; central wavelength, 800 nm; averageintensity, 1 W) at a repetition rate of 90 MHz with a pulse width of 140 fs. The delay
time between the two pulse trains could be continuously varied from zero to thepulse interval (11 ns) by the synchronizing circuit (Synchrolock, Coherent Inc.),
which had a time jitter of 600 fs. Each train was guided to a pulse picker, which

consisted of an ultrafast Pockels cell and polarizers, and was controlled by alaboratory-built timing generator. The pulse picker selectively transmitted one pulse
out of typically every 90 pulses, resulting in a pulse train with a reduced repetition
rate of fp (typically 1 MHz), while reducing the intensity of other pulses by a factor of300. When the lifetime to be measured was longer than 0.5 ms (¼1/(2fp)), fp was set
to lower than 1 MHz to optimize the measurement in accordance with the lifetime.
By changing the relative timing of pulse picking, an additional longer delay time was
generated, which could be controlled to multiples of the original pulse interval(11 ns). Combining the short delay time generated by the synchronizing circuitand the longer delay time generated by the pulse pickers, the total delay time could
be adjusted continuously from zero to as large a value as required. Note that the
reduced repetition rate could be set as low as required, reducing the average laser
intensity. The two pulse trains were arranged onto the same light axis and guided
into the STM (VT-STM, Omicron NanoTechnology GmbH). The light spot, which
had a diameter of ,10 mm and an average intensity of up to several milliwatts, wasfocused on a sample surface below the STM tip using two lenses placed outside theultrahigh vacuum chamber (base pressure, ,1 × 1028 Pa). The light spot was
precisely positioned to maximize the SPV effect appearing in the tunnelling current.
All experiments discussed in this paper were carried out at room temperature.
Delay-time modulation was achieved by changing the pulse-picking timing
periodically at a frequency of 1 kHz. In the anterior half of the period, pulse
picking was carried out at almost the same time for the pump and probe pulse trains,and in the posterior half of the period it was carried out with a difference of 1808. As
a result, the delay time was modulated in a rectangular form between a relatively
small value td (1 ps–1 ms) and a very large value (0.5 ms to several microseconds), bywhich the ultrafast phenomena completely reached the steady state. Synchronouslywith the delay-time modulation, the tunnelling current signal from the STM
preampliﬁer with a bandwidth of 10 kHz was phase-sensitively detected by a lockin ampliﬁer. By virtue of the rectangular modulation of the delay time, the output of
the lock-in ampliﬁer was proportional to the difference between the tunnellingcurrent at the two delay times. In particular, when the larger delay time was set to be
sufﬁciently long for the transient phenomena to reach the steady state, the signal
became proportional to DI ¼ I(td)2I(1). This novel technique of using pulse
pickers to modulate the delay time in a rectangular form with a large amplitude anda high modulation frequency is essential for maximizing the SNR to achieve
nanometre spatial resolution simultaneously with subpicosecond time resolution.
The time resolution of the system described above is mainly limited by thetiming jitter between the two lasers, which is controlled by the synchronizing circuit.
A higher resolution can be achieved by replacing the two lasers with an alternative
set-up comprising one laser, a beamsplitter and a mechanical optical delay line,
which is compatible with the novel delay-time modulation by pulse-picking.
Although the measurable delay-time range with this set-up is typically limited to
1 ns, the time resolution is almost the same as the pulse width of the laser, whichallows us to observe the carrier dynamics in a few-picosecond range (see ‘Time
Resolution’ in Supplementary Information).

Received 2 April 2010; accepted 14 September 2010;
published online 24 October 2010

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Acknowledgements
The authors thank Y. Hirayama of Tohoku University in Japan for stimulating discussions.

Author contributions
Y.T. performed the SPPX-STM experiment and data analysis. S.Y. assisted in the
SPPX-STM experiment and laser operation. O.T. provided technical and theoretical advice.
H.S. organized and supervised the project and edited the paper. All authors carried out
extensive analysis of the results.

Additional information
The authors declare no competing ﬁnancial interests. Supplementary informationaccompanies this paper at www.nature.com/naturephotonics. Reprints and permission
information is available online at http://npg.nature.com/reprintsandpermissions/.
Correspondence and requests for materials should be addressed to H.S.

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