LETTERS
PUBLISHED ONLINE: 29 JUNE 2014 | DOI: 10.1038/NNANO.2014.125

Probing ultrafast spin dynamics with optical
pump–probe scanning tunnelling microscopy
Shoji Yoshida1, Yuta Aizawa1, Zi-han Wang1, Ryuji Oshima2, Yutaka Mera3, Eiji Matsuyama1,
Haruhiro Oigawa1, Osamu Takeuchi1 and Hidemi Shigekawa1 *Studies of spin dynamics in low-dimensional systems areimportant from both fundamental and practical points ofview1,2. Spin-polarized scanning tunnelling microscopy allows
localized spin dynamics to be characterized and plays important roles in nanoscale science and technology3–5. However,nanoscale analysis of the ultrafast dynamics of itinerant magnetism, as well as its localized characteristics, should be
pursued to advance further the investigation of quantum
dynamics in functional structures of small systems. Here, we
demonstrate the optical pump–probe scanning tunnelling
microscopy technique, which enables the nanoscale probingof spin dynamics with the temporal resolution corresponding,in principle, to the optical pulse width. Spins are opticallyoriented using circularly polarized light, and their dynamics
are probed by scanning tunnelling microscopy based on the
optical pump–probe method. Spin relaxation in a single
quantum well with a width of 6 nm was observed with aspatial resolution of ∼1 nm. In addition to spin relaxation
dynamics, spin precession, which provides an estimation of
the Landé g factor, was observed successfully.
In nanoscale science and technology, the addition of high temporal resolution to scanning probe microscopy (SPM) has attracted
considerable attention since its invention4–9. An approach to achieving such microscopy is to combine the techniques of quantum optics
with SPM9–15, and even single-atomic-level analysis has been realized
by laser-combined scanning tunnelling microscopy (STM) based on
the optical pump–probe (OPP) method15. As spin dynamics havebeen studied extensively by quantum optical methods and the interaction between spins and optics is a major subject in science andtechnology2,16, to combine SPM with optical techniques is expected
to be highly advantageous. Although studies that have employed
optical SPM showed its high potential for characterizing quantum
dynamics in small organized structures, the observation of ultrafast
spin dynamics by optical SPM has not yet been achieved.
Here we present an STM technique realized by the developmentof a new modulation method with circularly polarized (CP) lightthat enables the nanoscale probing of spin dynamics with temporal
resolution corresponding to the optical pulse width in principle (seeMethods). Its performance is demonstrated by three major
examples: (1) the temperature dependence of the relaxation of optically oriented spins in GaAs, which shows the basic spin interaction
mechanisms; (2) the independent probing of spin dynamics insingle GaAs/AlGaAs quantum wells with lifetimes dependent on
conﬁnement energy; (3) the quantum beat of spin precession,
which provides the Landé g factor, a fundamental value in spinscience and technology.
Figure 1a shows a schematic illustration of the OPP-STM technique we have developed. In OPP-STM for spin detection, thesample surface below the STM tip is excited by a train of CP pulse
pairs and the change in tunnelling current is measured as afunction of delay time13. As shown in Fig. 2a for GaAs, when thephoton energy of CP light is adjusted to excite only the upperbands consisting of the heavy-hole and light-hole bands, the spinpolarization becomes 50% (ref. 2). When the circular polarizationsof pump and probe pulses are the same, namely, left-handed (L-)
or right-handed (R-) rotation, we call this co-circularly polarized(co-CP) excitation. However, when they are different, we call it
counter-circularly polarized (counter-CP) excitation. If co-CP or
counter-CP excitation is used for the OPP method, the relaxationof spin orientation can be probed as a function of delay time
because of absorption bleaching. Namely, if up (down) spinsremain in the excited states, the excitation of up (down) spins issuppressed. Therefore, as shown in Fig. 1d, the number of electronscount(td)), increases (decreases)
excited by the probe pulse, n coex (td) (nexwith delay time td for co-CP (counter-CP) excitation dependingon the relaxation of the spin direction oriented by the pumppulse17, which is reﬂected in the tunnelling current IS(td)(refs 13,14). Accordingly, with the new modulation method wehave developed, the difference between the signals measured by the(td) 2 Ico
two excitation modes, (DIS(td) ¼ IcountS
S (td)), can be obtained
as a spin-related spectrum (see Methods).
First, the modulation system we have developed was evaluated bymeasuring the spin dynamics in GaAs (undoped) using the optical
pump–probe reﬂectivity (OPPR) method and OPP-STM. In our
OPPR measurement, a GaAs(100) sample was illuminated by
optical pulse trains in air, as shown in Fig. 2b. The results areshown in Fig. 2c, and two components with decay constants of
0.49 + 0.02 ps and 5.3 + 0.8 ps were observed. Second, excitationby the pump and probe pulses with the combination of CP andlinearly polarized (LP) light (CP þ CP or CP þ LP) was used to
conﬁrm that the signal was spin related: when the CP þ LP
combination was used, the signal disappeared, as expected. The
relaxation lifetimes, which are twice those of the decay constants17,
are consistent with those for electron spins previously observed.
The fast and slow components are attributed to the cooling of theexcited electrons18 and subsequent relaxation of the opticallyoriented spins in the conduction band19,20 (relaxation of holespins is much faster2).
Figure 2d shows a typical spectrum obtained by OPP-STM forthe same GaAs sample using the new modulation technique.
The GaAs(110) surface was prepared by cleavage in a vacuum(5 × 1028 Pa, Omicron VT-STM). A chemically etched W tip wasused. The photoexcitation was carried out at an angle of about
708 from the normal to the surface. The decay of spin orientationconsists of two components with decay constants of 0.51 + 0.01 ps
and 5.8 + 0.8 ps, which is in good agreement with the results obtained

1

Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan, 2 Advanced Industrial Science and Technology, Tsukuba 305-8568,
Japan, 3 Shiga University of Medical Science, Otsu, 520-2192, Japan. * e-mail: hidemi@ims.tsukuba.ac.jp

588

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LETTERS

NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2014.125
a

b

1 MHz
λ/4

L

R

Ti:sapphire
laser

PC

Pump

td1

90 MHz

Probe

STM

1
M

HM

L

R

L

R

L
R

L

1 μs
R

L

R

2

L

Vs

Synchronization

L

1 MHz
λ/4
Ti:sapphire
laser

PC
M

Delay
line

3
R

L
4
L

c

f = 1 kHz
1dCounter-CP

R

td1

nex

Mixing degree

3Counter-CP

count
nex

4

L

R

2
0

L

R

co
nex

1

Co–CP
R

1 Co–CP1

20Time (ms)

R

or

L

L

Delay time, td

Figure 1 | OPP-STM used for spin detection. a, Schematic illustration of the OPP-STM technique. Two titanium:sapphire lasers (0.75 W and 0.7 W,
90 MHz, duration 130 fs) were used with a Synchrolock system to produce pump and probe excitation. L (blue pulses) and R (red pulses) denote circularpolarizations with L- and R-rotation, respectively. Vs is the bias voltage. PC is a Pockels cell. M and HM are a mirror and half-mirror, respectively. l/4 is aquarter-wave plate. The red spot below the STM tip represents the photoexcited area. b, Modulation of circular polarizations of pump and probe pulses. td1 isthe delay time between the pump and probe pulses. The square wave indicates the times when the circular polarization is switched. c, Mixing degree ofcircular polarizations of pump and probe pulses: co-CP (0, green dashed line) and counter-CP (1, red dashed line) excitation at frequency f ¼ 1 kHz. Points 1
to 4 correspond to the states in b. d, Spin-detection signal that can be measured by the two excitation modes of counter-CP excitation (blue line) and co-CP
coexcitation (red line). nex , ncountex (td) and nex(td) indicate the number of excited electrons and the numbers excited by the two excitation modes, respectively.
Using the new modulation method, Dnex(td) ¼ Dncount(td) 2 Dncoexex(td), indicated by the vertical grey arrow, can be measured.

by the OPPR method shown in Fig. 2c. The slower components are
considered hereinafter.
This is the ﬁrst-ever time-resolved STM observation of opticallyoriented spin dynamics in the picosecond range. A new method of
microscopy, which enables the observation of ultrafast spin
dynamics, has been realized successfully. To show the potential ofthis microscopy, we carried out three fundamental experiments,
described as follows.
First, we measured the temperature dependence of the spin
dynamics in GaAs. According to a theoretical study, the temperature(T ) dependence of spin-orientation lifetime (ts) in non-centrosymmetric semiconductors basically follows the relationship ts ≈ T 23
(Dyakonov–Perel mechanism)2,21, in which spin rotates duringcollisions with impurities because of the spin-orbit splitting of
the conduction band. In the case of p-type semiconductors,
however, the spin relaxation of non-equilibrium electrons
caused by the exchange interaction between the electron and holespins produces the relationship ts ≈ T 20.5 at low temperatures(Bir–Aronov–Pikus mechanism)2,21.
A cleaved p-type GaAs(110) surface (Zn-doped, 5.0 × 1016 cm23)
was used as the sample. Figure 3a,b shows the temperature-dependent spectra obtained by OPP-STM and their magniﬁcations,
respectively. The lifetime increased with decreasing temperature,
which is summarized in Fig. 3c. The lifetime satisﬁes ts ≈ T 23 athigh temperatures and tends to saturate at low temperatures

owing to the effect of the Bir–Aronov–Pikus mechanism, as
expected, which indicates that OPP-STM works well.
Second, we carried out measurements on GaAs/AlGaAsquantum wells, as schematically shown in Fig. 4a (see Methods),
using the set-up of the Omicron VT-STM at room temperature(RT). As shown in Fig. 4b, the quantum wells (with a width of
6 nm here) were observed clearly by STM; that is, we could identify
their location. In consideration of the energy levels in the quantum
wells, we adjusted the photon energy to 1.51 eV (819 nm) for the
6 nm quantum well and 1.49 eV (834 nm) for the 8 nm quantum
well, as shown in Fig. 4e.
The results are shown in Fig. 4c,d. In the quantum wells, the spin
lifetimes were observed to be longer and increased with increasing
well width, from 68 + 6 ps ((34 + 3) × 2 ps) for the 6 nm
quantum well to 112 + 6 ps ((56 + 3) × 2 ps) for the 8 nm
quantum well. According to the Dyakonov–Perel mechanism, thespin lifetime ts has a conﬁnement-energy (E1e) dependence ofts ≈ E22
1e (see Methods). We estimated the conﬁnement energy
from the well widths, and the lifetimes are plotted as a function of
the conﬁnement energy in Fig. 4f. Macroscopically obtained
results on multiple quantum wells are also shown for comparison22.
According to Tackeuchi et al.22, the lifetimes were different fromsample to sample. Therefore, although it would be interesting tocompare the results obtained for a single quantum well and multiple
quantum wells, a discussion on the difference in lifetimes is left as

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589

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NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2014.125

1

Heavy hole

1

3

50

ΔIS

a

100

20 pA

b

CP + CP200 K

0CP + LP−4

164 K

Light hole
−20

−10

0

10

20

M
HM

HM
M

ΔIS (pA)

Sample

100

102 K

50
CP + CP0
CP + LP

Delay
Photodetector line

M

80 K
−50

0
Delay time (ps)

−20

−10

0

10

50

150
100
50

80 90 100200

Temperature (K)20

Delay time (ps)Figure 2 | OPP and OPP-STM measurements. a, Schematic diagram oftransitions between heavy-hole/light-hole bands and | . and | .
conduction band states, which indicate the relative transition strengths,
3:1 or 1:3, for light with R-rotation (red lines) or L-rotation (blue lines),
respectively2. b, OPPR measurement set-up. The new modulation techniqueshown in Fig. 1 is used. c, OPPR spectrum with ﬁtting curve (black line
superimposed on blue signal). DR is the change in reﬂectivity. d, OPP-STM
spectrum with ﬁtting curve (black line superimposed on the red signal).
DIs is the change in spin-related tunnelling current (sample bias voltage
Vs ¼ 20.9 V, tunnelling current It ¼ 140 nA, room temperature, laser
intensity ¼ 200 mW).

future work. The single-quantum well-level analysis of spin
dynamics has become possible.
Figure 4g shows a cross-section of time-resolved signal across the
6 nm quantum well and the GaAs/AlGaAs interface obtained fortd ¼ 2.3 ps. As expected, the time-resolved signal rapidly increases(1) at the GaAs/AlGaAs interface and (2) at the interface between
the AlGaAs layer and the quantum well. Both interfaces are adjacentto the AlGaAs region, in which no carriers are excited. A spatial resolution close to 1 nm has been achieved, which is considered to belimited, for example, by the signal-to-noise ratio.
As our ﬁnal example, we demonstrate the results obtained byprobing the quantum beat of spin precession in a magnetic
ﬁeld2,23–25. The Landé g factor for an electronic state in a semiconductor is highly sensitive to the characteristics of local electronic
structures as well as to the associated band structures. Therefore,
nanoscale evaluation of the g factor by OPP-STM is expected toprovide a deeper analysis, enabling the fundamental understanding
and theoretical modelling of quantum dynamics in materials
and devices.
Measurements were carried out using an Omicron TESLA
system, in which a uniform magnetic ﬁeld of resolution better
than 1025 T can be applied to the sample below the STM tip
over the scan area (here less than 1 mm × 1 mm). The experimental
set-up is shown in Fig. 5a. An n-type GaAs sample with adonor concentration of 2 × 1016 cm23, which is near thevalue for a metal–insulator transition that provides a signiﬁcantly
long spin lifetime26, was prepared by cleavage in a vacuum(2 × 1028 Pa).
Figure 5b shows typical results for the magnetic-ﬁeld dependenceof spin precession obtained by OPP-STM at 2.5 K. In a magnetic
ﬁeld B, the precession of spins occurs at the Larmor frequency ofv ¼ gmBB/h, where g is the Landé g factor27, mB is the Bohr
590

Spin lifetime (ps)

125 K

d

4

c 200

Delay time (ps)b

0Delay time (ps)ΔIS

3

ΔR/R (×10−6)

c 150

a

Figure 3 | Temperature dependence of spin dynamics in GaAs.
a, Temperature-dependent OPP-STM spectra (Vs ¼ 21.0 V, It ¼ 10 nA,
excitation wavelength ¼ 813 nm, laser intensity ¼ 200 mW). Black lines are
ﬁtting curves. The decay constants are 5.5+5.5 ps (200 K), 12+3 ps(164 K), 38+7 ps (125 K), 49+7 ps (102K) and 59+5 ps (80 K).
b, Subsection of data in a. The intensity of the fast component decreased toclose to the noise level at low temperatures, which is considered to becaused by the reduction of the excess energy because of the expansion of
the bandgap. Thus, the ﬁtting here was carried out after removing thecentral part shown by the coloured area in b (less than +1.0 ps). We
conﬁrmed that the removal of further data above +1.0 ps did not affect the
lifetime obtained. c, Spin lifetime as a function of temperature. Data areshown by open circles with error bars of ﬁtting in a. The solid red line
represents the theoretical model, A*T 23, which has been validated bymacroscopic experiments.

magneton and h is the Dirac constant. The oscillation frequencyof spin precession increases with increasing magnetic ﬁeld.
However, the oscillation in intensity, depending on the magnetic
ﬁeld, is caused by the repetition of laser pulses. Namely, when therepetition rate of the laser pulses, 11 ns (90 MHz), coincides witha multiple of the Larmor frequency, resonant ampliﬁcation
occurs24,28. This is the ﬁrst successful probing of the precession
dynamics of optically oriented spins by STM.
As shown in Fig. 5c, the signal (DIS) consists of two components(DIS1 and DIS2) with different spin lifetimes (tS1 and tS2).
Considering the lifetimes of the two components, S1 and S2 are,respectively, attributed to the spin relaxations of free conduction
band electrons, which relax to delocalized donor-bound band
electrons and delocalized donor-bound band electrons25.
For S2, with a lifetime that appears to be longer than 11 ns, we
carried out measurements of tS2 using the resonant spin ampliﬁcation (see Methods)24,28. Figure 5d shows the change in DIS as a function of the magnetic ﬁeld (red lines) and ﬁtting curves (black lines).
The upper blue line is the ﬁtting error. Magniﬁcations of some of thepeaks in Fig. 5d are shown in Fig. 5e. For S1, tS1 (,11 ns) was
obtained by the ﬁtting shown in Fig. 5c. The lifetimes of the twocomponents, tS1 and tS2 , are shown in Fig. 5f. Further analysis byvarying the measurement conditions, which was performed to alimited extent here, is necessary to understand the mechanism
in detail.
Although there is a resonant ampliﬁcation, the Larmor
frequencies of S1 and S2 can be obtained accurately from thecurves in Fig. 5b by the ﬁtting shown in Fig. 5c because highquality oscillations could be measured. Larmor frequencies areplotted as functions of magnetic ﬁeld in Fig. 5g. The ﬁtting

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NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2014.125
a

b

GaAs
AIGaAs QW AIGaAsz (nm)GaAs

AIGaAsAIGaAsGaAs

AIGaAsAIGaAsGaAs(001)
substrate

1.6

0
50 nm

c

d

GaAs 6-nm QW

56 ± 3 ps

34 ± 3 ps

1e

2.0

1e

4.0
ΔIS (pA)

ΔIS (pA)e

GaAs 8-nm QW

1.02.18 eV

1.51 eV

1.49 eV

568 nm819 nm834 nm

2.0
1hh

0.0

0

20

40

8 nm
QW

6 nm
QW

0.0
−40 −201hh−40 −20

Delay time (ps)

0

20

40

Delay time (ps)gf

AIGaAs

GaAs

1,000

AIGaAs6-nm QW

ΔIS (pA)

Spin lifetime (ps)

4.0

100

2.0
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td = 2.3 ps

−2.0
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80

120

160

200

ΔIS (pA)

4.0

Confinement energy (meV)

200 nm
6 nm

2.0
0.0
−2.0

5 nm

Figure 4 | OPP-STM measurement on quantum wells. a, Schematic structure of quantum wells with the STM tip. b, STM image of a quantum well(Vs ¼ 3.0 V, It ¼ 100 pA). c,d, OPP-STM spectra obtained for the 6-nm quantum well and the 8-nm quantum well, respectively (Vs ¼ 21.0 V, It ¼ 8 nA, laser
intensity ¼ 200 mW). Solid red lines are ﬁtting curves. e, Band structure of quantum wells. 1e (blue lines) and 1hh (red lines) indicate the ﬁrst excited statesof electrons and heavy holes, respectively. Yellow arrows represent the AlGaAs bandgap energy and the 1e–1hh energies for the 6 nm quantum well and
8-nm quantum well. f, Conﬁnement-energy dependence of the spin lifetime of carriers in a quantum well, with error bars of ﬁtting in c and d. The two red
circles with error bars represent the data. The results experimentally obtained by the OPPR method in Tackeuchi et al.22 are also shown (blue dots on the two
ﬁtting black lines). g, Cross-sections of the time-resolved signal for td ¼ 2.3 ps measured across the 6-nm quantum well and the GaAs/AlGaAs interface(Vs ¼ 4.0 V, It ¼ 10 nA, scan rate ¼ 50 nm s21, time constant of lock-in ¼ 10 ms). High bias voltages were used in b and g to enable measurement over the
insulating AlGaAs area, in which no carriers are excited. Slight differences in measurement conditions are because of the adjustment for optimization, which
depends on factors such as the tip and sample conditions in each case. However, the bias voltage and current did not have any discernible effect on the
lifetime under our range of experimental conditions.

error of the Larmor frequency at each magnetic ﬁeld was less than
+1%, which could not be shown on the graph. Therefore, theelectron g factors deduced from the two slopes in Fig. 5g were
20.404 + 0.002 and 20.435 + 0.003 for S1 and S2, respectively.
The deviation observed for S1 is attributed to the excess-energy
dependence of the g factor for excited electrons25,27. We cannow evaluate the electron g factor by STM through probing a
spin-precession beat.

We have succeeded in developing an OPP-STM technique thatenables the nanoscale probing of optically oriented spin dynamics
with temporal resolution that corresponds to the optical pulse
width. The combination of this technique with spin polarized(SP)-STM may enable further advancement of this ﬁeld. We
believe that this microscopy technique will make an important contribution to the further advancement of nanoscale scienceand technology.

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NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2014.125
B (T)

b

c

10 pA

0.489

B = 0.285 (T)
ΔIS

5 pA

a

0.448
0.407
0.326
0.285

ΔIS

25°

ΔIS1

ΔIS

0.367

ΔIS2

0.245
0.223
0.207

0

0.184
0.163
0

1

2

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4

5

Delay time (ns)Error (pA)

d
0.5

2

3

4

5

Delay time (ns)f 40
Spin lifetime (ns)

B

1

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20

τS2

10
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τS1

2
1

e

−0.5

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1.5

0.30

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Magnetic field (T)1.0Frequency (GHz)ΔIS (pA)

ΔIS (pA)

g 3.0
1.0

0.5

0.5

0.30

S1

1.0
0.0

0.0
0.20

S2
2.0

0.40

0.26

Magnetic field (T)0.28

Magnetic field (T)0.0 0.1 0.2 0.3 0.4 0.5
Magnetic field (T)Figure 5 | Spin precession measurement by OPP-STM. a, Measurement geometry. Photoexcitation was carried out at an angle of 258 from the direction
parallel to the sample surface. Blue and black arrows indicate spin and the externally applied magnetic ﬁeld, respectively. The red spot below the STM tip
represents the photoexcited area. b, Magnetic-ﬁeld dependence of OPP-STM spectra (Vs ¼ 2.5 V, It ¼ 3 nA, excitation wavelength ¼ 826 nm,
laser intensity ¼ 35 mW). c, Two components (blue and red lines) obtained by ﬁtting the observed signal in b (green line) for the case of 0.285 T(DIS ¼ DIS1 þ DIS2 ¼ AS1exp(2td/ts1)cos(vS1td) þ AS2exp(2td/ts2)cos(vS2td)). The black line superimposed on DIs (green line) is the ﬁtting curve. d, DIS as afunction of magnetic ﬁeld for td ¼ 1 ps (Vs ¼ 1.5 V, It ¼ 3 nA). The black line superimposed on the data (red trace) is the ﬁtting curve obtained by assuming
the Lorentz curves, and its error is shown in the upper graph. The blue trace is the error between the data and the ﬁtting. e, Magniﬁcation of some of thespectra in d. The black lines superimposed on the data (red trace) are the sums of Lorentz curves (blue dotted lines) ﬁtted to the peaks (see Methods).
f, Magnetic-ﬁeld dependence of the spin lifetimes of the two components (shown with error bars of ﬁtting). g, Precession frequencies f ¼ v/2p of thetwo components as a function of magnetic ﬁeld, used to obtain the g factors. Solid lines are ﬁtted lines assuming a linear relationship between v and B,
v ¼ gmBB/h. The error bars are too small (less than 1%) to be shown.

Methods
Two titanium:sapphire lasers (Coherent, Chameleon 0.75 W and Mira 0.7 W,
90 MHz, duration 130 fs) were used with the Synchrolock system (Coherent Inc)
to produce pump and probe excitations. The circular polarizations of pump andprobe laser pulses are modulated between L- and R-rotations at 1 MHz bycontrolling the directions of the LP light using Pockels cells (Fig. 1a). Each of the Land R-rotation blocks has 90 laser pulses, and the delay time between pump andprobe pulses is indicated by td ¼ td1 in the top diagram in Fig. 1b.
As the signal is very weak, a lock-in detection method is necessary, in which
modulating the excitation without inducing thermal expansion is the key factor11.
When polarization is modulated, even a slight difference in light intensity between
L- and R-polarizations is critical, and the introduction of a new modulation method
is essential.
When a 1 kHz difference is given between the pump and probe modulation
frequencies, for example, 1.001 MHz for the pump pulse and 1 MHz for theprobe pulse, the relative phase of the two circular polarizations changes at 1 kHz,as shown in Fig. 1b, where the delay time between pump and probe pulses is
ﬁxed at a certain value td1. The difference in frequencies, 1 kHz in this case,is used as the reference frequency for the lock-in detection method.
Accordingly, the co-CP and counter-CP excitations are mixed and their ratiochanges at 1 kHz, as shown in Fig. 1c. With this method, the inﬂuence of thepolarization-dependent change in light intensity, which may be introduced
unintentionally, can be removed.
The number of electrons excited by the co-CP (ncoex (td)) and counter-CP(ncount(td)) excitations provide the two points for a delay time td1 , as shown by theex592

two circles on the signal lines in Fig. 1d, and is reﬂected in the signal tunnellingcurrent Is(td). Therefore, when the delay time is changed simultaneously with themodulation of circular polarizations described here, the OPP-STM signal provides adifference of DIs , that is, the difference between the two signal lines obtained by theco-CP and counter-CP excitations. Here the delay time was changed by the
Synchrolock system used for the synchronization of the pump and probe pulses. The
jitter of the Synchrolock system we used was 200 fs at best, which reduces the
temporal resolution in the subpicosecond range in this case. A system with better
resolution using a laser with a shorter pulse and reduced jitter is necessary for thedetailed analysis of the fast component.
The sample of GaAs/Al0.3Ga0.7As quantum wells was grown on a GaAs(100)(undoped) substrate and two quantum wells with widths of 6 nm and 8 nm wereformed, as shown in Fig. 4a. A 200 nm GaAs layer was placed as a spacer to isolate thetwo quantum wells to reduce the effect of the relative interaction between them. We
cleaved the sample to produce a clean (110) surface, similarly to the ﬁrst example. In
the quantum wells, the energy levels of the upper bands (heavy- and light-hole bands)
shown in Fig. 2a split owing to the strain caused by the lattice mismatch between
GaAs and AlGaAs layers29. Therefore, we can excite fully polarized spins, whichmakes the measurements easier because the signal becomes stronger17. According to
the Dyakonov–Perel mechanism, the spin lifetime in a quantum well can be expressed
2 2
2
as ts ≈ (kBT)21E22
1e , E1e ¼ n h /(8ma ), n ¼ 1, where kB is the Boltzmann constant,h is the Planck constant, m is the electron mass, a is the lattice constant, and E1e is the
conﬁnement energy for n ¼ 1 (ref. 22).
Spectra were analysed basically using a function that consists of two exponential
components and a constant corresponding to the offset caused by the slight

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NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2014.125
difference in the intensities between the right-handed and left-handed lights andthe slight deviation from circular polarization (see the caption to Fig. 3 for the otherspectral analysis procedure shown in Figs 3a,b). All spectra were drawn with thehorizontal axis adjusted to compensate for the offset.
In the case of a spin lifetime longer than the laser repetition rate, the lifetime canbe obtained using resonant spin ampliﬁcation24,28. As shown in Fig. 5d, the carrier
spin has sharp resonances as a function of the magnetic ﬁeld. Here the spectra wereﬁtted by the sum of the Lorentz curves (dotted lines in Fig. 5e), as shown by the blacksolid lines in Fig. 5d,e. The value of tS2 for each peak was obtained from thefull-width at half-maximum of each Lorentz curve.

Received 22 December 2013; accepted 22 May 2014;
published online 29 June 2014

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Acknowledgements
Support from Japan Society for the Promotion of Science (Grants-in-Aid for ScientiﬁcResearch, 22226003) is acknowledged.

Author contributions
S.Y. performed the experiment and data analysis (with H.S.). Y.A., Z.W. and Y.M. assistedwith the measurement. R.O. and H.O. worked on sample preparation. E.M. helped todesign the TESLA system. O.T. provided technical advice and helped to design the TESLA
system. H.S. organized and supervised the project and edited the paper.

Additional information
Reprints and permissions information is available online at www.nature.com/reprints.
Correspondence and requests for materials should be addressed to H.S.

Competing ﬁnancial interests
The authors declare no competing ﬁnancial interests.

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