SUPPLEMENTARY INFORMATION
doi: 10.1038/nphoton.2010.235

Supplementary Information
Real-space imaging of transient carrier dynamics
by nanoscale pump-probe microscopy

Yasuhiko Terada, Shoji Yoshida, Osamu Takeuchi, and Hidemi Shigekawa*
Institute of Applied Physics, CREST-JST, University of Tsukuba, Tsukuba 307-8573
Japan

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doi: 10.1038/nphoton.2010.235

OPPR and laser-combined STM spectra

Fig. S1 OPPR and laser-combined STM spectra offor a, b LT-GaAs, c, d

GaNAs and e, f n-GaAs. The mechanism of SPPX measurement is clearly
confirmed for a wide range of lifetimes. The derived decay constants are a 1.5 ps, b
2.4 ps, c 410 ps, d, 440 ps, e 3.3 ns and f 4.8 ns.

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doi: 10.1038/nphoton.2010.235

Mechanism
There may be other possible effects that appear in time-resolved signals, for example, thatof photoemission, and those related to displacement current, rectification current, and thermaleffects such as thermal expansion of the STM tip and the increase in sample temperature. As
described below, however, these processes are ruled out on the basis of the experimental data.
First, it is clarified that the measured signals originate from the tunnelling current but notfrom other possible sauces such as photoemission and displacement currents. This isexperimentally confirmed; the tip-sample distance dependence of signals corresponds to that oftunnelling current, that is, the magnitude of signals is proportional to the tunnelling current (Fig.
3c). Photoemission or displacement current, or currents generated by other possible processes
have a much weaker dependence on the tip-sample distance than the tunnel current; thus, neither
can be the origin of the measured signals.
Second, it is clarified that tunnelling-current signals originate from the change in thebarrier height due to transient SPV, rather than from any other mechanisms irrelevant to SPV,
such as the thermal response of the tip. The magnitude of signals has the same spatial and bias
dependences as SPV. The magnitude of signals is almost zero under a forward bias condition,
which is the same characteristic as SPV. It is important to note that although the signal is related
to SPV, it is not a displacement current but the tunnelling current induced by the change in SPV,
which is related to the carrier dynamics such as recombination and capture rate.
Third, among the possible mechanisms that may change the tunnel current throughtransient SPV, those irrelevant to carrier dynamics can be ruled out on the basis of thecharacteristics of the measured

curves for various samples. The values of !bcorrespond to the lifetimes obtained by the optical pump-probe technique (Fig. S1), and thedecrease in !s when tunnelling current is injected to Co nanoparticle/GaAs can be explained
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doi: 10.1038/nphoton.2010.235

only by the increase in the recombination rate (Fig. 4).
� � Fourth, when the lattice temperature rises owing to laser illumination, the absorptioncoefficient varies and hence the effect of absorption bleaching also changes, which results in thechange in SPV. Therefore, !I may reflect the time evolution of the lattice temperature. The risein the lattice temperature is calculated to be 0.5 K for a laser intensity of 0.1 mJ/cm2 (Fig. 50 inref. H2). Under our measurement conditions, the typical laser intensity is 2.4 mJ/cm2. If weassume the temperature rise to be proportional to the laser intensity for simplicity, the rise in thelattice temperature is roughly estimated to be 12 K.
All experiments were performed at room temperature and we measured, for example,!I(td)=I(td)"I(td=500 ns). Therefore, the change above is not considered to produce the spatial
variations we have observed for our samples, such as Co particles on GaAs. The
tunnelling-current dependence of decay time observed for the Co/GaAs system also cannot be
produced by the thermal effect.
To confirm that there is no thermal expansion effect of the tip and sample, we changed thesign of the bias voltage. When a forward bias voltage is applied, no signal should appear if thethermal expansion effect can be neglected. As expected, no signal was observed under a forward
bias voltage.

Reference
S1 Othonos, A. Probing ultrafast carrier and phonon dynamics in semiconductors.
J. Appl. Phys. 83, 1789-1830 (1998).

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doi: 10.1038/nphoton.2010.235

supplementary information

Fitting procedures
We performed the numerical fitting ofspectra using the model function

developed as follows: The sample response was assumed to be an exponential or
double-exponential decay. Then, the sample response was convoluted with an opticalpulse profile and the impulse response of low-pass filtering by the lock-in amplifier[S2].
A fitting example performed on the spectrum in Fig. 2a is shown in Fig. S2.
SPPX-STM signals were obtained by repeated upward (red line) and downward (blueline) scans of delay time. Owing to the finite lock-in time constant, the peak position
shifts toward the scan direction of delay in each spectrum. The best fit curves are
obtained for the two spectra using the model function with the same parameters, asshown by solid lines in Fig. S2. The fitting error (upper part in the figure) is small andcomparable to the noise level. The value of the decay constant for the spectrum wasobtained to be 2.4 ps. Only the upward spectrum is shown in Fig. 2a.

Fig. S2 Spectra obtained by repeated upward and downward scans of delay time for anLT-GaAs sample and the best fit curves (lower part). The fitting error is plotted in the upper part.
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For the other spectra shown in Figs. 2b to 2d, delay ranges are much wider thanthe optical pulse width and the delay-time scan speed was set at a sufficiently low value.
In such cases, decay constants are not strongly affected by the pulse width and thelock-in time constant, and the fitting of a spectrum using a simple exponential or double
exponential function without any convolution is sufficient. The accurate analysis gives
the same decay constant.

S2. Takeuchi, O., Aoyama, M. & Shigekawa, H.
Analysis of time-resolved tunnel current signal in sub-picosecond range observed byShaken-Pulse-Pair-Excited scanning tunnelling microscopy. Jpn. J. Appl. Phys. 40,
7B, 5354-5357 (2005).

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Time resolution

As described in the method section, when two synchronized lasers are used as thelight source, the time resolution is limited by the timing jitter of the synchronization.
Instead, when we require the higher resolution to observe ultrafast dynamics, we use on
laser and beam splitter to produce pump and probe pulses. With this setup, the time
resolution is only limited by optical pulse width, as discussed below. In general, thetime resolution of the developed microscopy is determined by the following factors:
optical pulse width, accuracy of delay time, and time constant of a lock-in amplifier.
First, the optical pulse width is 140 fs in our setup, which provides the time resolution
determined by the overlap of two optical pulses, that is, the autocorrelation width T0, tobe ~ 200 fs. Second, the accuracy of delay time is mainly determined by that of themirror position in the optical line and is on the order of 10 fs, which is negligibly small
compared with the others. Third, the low-pass filtering with the lock-in amplifier alsodegrades the time resolution. The spectra shown in Fig. S2 were obtained at the lock-intime constant Tc of 300 ms and the delay-time scan speed v of 0.54 ps/s, and thecorresponding degradation in the time resolution is estimated to be vTc~160 fs. Thisvalue is comparable to T0. We carefully adjusted this value to optimize both thesignal-to-noise ratio and the time resolution.
On the basis of the above considerations, the time resolution of this microscopy islikely to be determined by the first factor, which is the optical pulse width. This isconfirmed using the experimental data; as shown in Fig. S2, the fitting error (upper partin the figure) is small and comparable to the noise level. If there are any other possible
mechanisms that result in time resolution degradation, the signal profile may be further
deformed and will not give a good fit.
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These results indicate that the time resolution of this microscopic technique islimited only by the optical pulse width, which is 140 fs in the present case.
� �

For comparison, the spatial and temporal resolutions simultaneously realized bytime-resolved scanning near-field optical microscopy, which is a promising method forthe study of nanoscale properties, are ~100 nm and ~100 fs, respectively [S3-S5]. Other
microscopy techniques based on different methods, such as time-resolved photoelectron
emission microscopy (PEEM, 60 nm and 10 fs) [S6, S7] have also been developed.

References
S3. Nechay, B. A., Siegner, U., Achermann, M., Bielefeldt, H. & Keller, U.
Femtosecond pump-probe near-ﬁeld optical microscopy. Rev. Sci. Instrum. 70,2758-2764 (1999).
S4. Guenther, T. et al. Femtosecond near-ﬁeld spectroscopy of a single GaAs quantum
wire. Appl. Phys. Lett. 75, 3500-3502 (1999).
S5. Nagahara, T., Imura, K. & Okamoto, H. Time-resolved scanning near-ﬁeld optical
microscopy with supercontinuum light pulses generated in microstructure ﬁber. Rev.
Sci. Instrum. 75, 4528-4533 (2004).
S6. Kubo A., Onda K., Petek H., Sun Z., Jung Y., and Koo Kim H.
Femtosecond imaging of surface Plasmon dynamics in a nanostructured silver film.
Nano Lett., 5, 1123-1127 (2995).
S7. Kubo A., Pontius N. and Petek H.
Femtosecond microscopy of surface Plasmon polariton wave packet evolution at thesilver/vacuum interface, Nano Lett., 7, 470-475 (2997).

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doi: 10.1038/nphoton.2010.235

supplementary information

Spectra obtained from GaAs/AlGaAs/LT-GaAs heterostructure.
Figure S3 shows the results for the spatial dependence of carrier recombination in asemiconductor heterostructure consisting of materials with different lifetimes,
GaAs/Al0.5Ga0.5As/LT-GaAs, where LT-GaAs denotes low-temperature-grown GaAs. Alayer of LT-GaAs (1 µm) was grown at 250 °C by molecular beam epitaxy (MBE) on abarrier layer of AlGaAs (1 µm) on an undoped GaAs substrate, which was annealed at
700 °C for 60 s in H2(5 %)/Ar ambient, and contained high-density defects that act asrecombination sites for carriers. Thus, the bulk carrier lifetime of LT-GaAs is much
shorter than that of GaAs. The AlGaAs barrier layer has a band gap larger than theoptical excitation energy (1.55 eV). Figure S3b shows an STM image of the sample
cleaved in ultrahigh vacuum. The interfaces of the three layers can be clearlyrecognized. Figure S3c shows the time-resolved spectra, !I(td), taken at certain points in
the GaAs, AlGaAs and LT-GaAs regions. As expected, the LT-GaAs region exhibits anultrafast decay component with a time constant of 4.0 ps, while the GaAs region
exhibits a decay component with a time constant of 4.8 ns. These values are consistentwith the recombination lifetimes determined by the OPPR measurement, 1.5 ps and 2.7
ns for the LT-GaAs and GaAs samples, respectively. For AlGaAs, there is nodelay-time dependence in !I(td), because no photocarrier generation occurs in this area.

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Fig.

S3

Laser-combined

STM

measurement

doi: 10.1038/nphoton.2010.235

ofGaAs/AlGaAs/LT-GaAs

heterostructure. a Schematic of measurement. b STM image of cleaved surface. cTime-resolved signals observed in each region.

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