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Surface-mediated spin dynamics probed by optical-pump-probe scanningtunneling microscopy
Zi-han Wang,a Cheul-hyun Yoon,a Shoji Yoshida,a Yusuke Arashida,a Osamu Takeuchi,a Yuzo
Ohnoa and Hidemi Shigekawaa*
a Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573,

Japan* Corresponding address: Hidemi@ims.tsukuba.ac.jp

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STM tip and sample preparations
An electrochemically etched tungsten tip was used for the STM and OPP-STM measurements.
A GaAs (001) wafer was cleaved (~7.5 × 10-7 Pa) in a load-lock cell to expose a clean (110) surface.
STM topography measurements including a cleavage quality check were carried out in the mainchamber of the STM system (~1 × 10-8 Pa). Then, Mn was deposited in a preparation chamber (~4× 10-8 Pa) by evaporation. Deposition procedures were conducted with identical evaporation
parameters (voltage, flux, etc.), except for the deposition time, which was controlled using amechanical shutter. After each evaporation, the sample was immediately transferred to the main
STM chamber for measurement. Two types of GaAs wafer were used. Sample A was n-type Sidoped GaAs (001) with a doping concentration of ~ 1 × 1018 cm-3, which was used to confirm thedensity of Mn deposited for different times via the STM topography, because a higher doping
concentration makes STM imaging easier. Sample B was n-type Si-doped GaAs (001) with a doping
concentration of (3.5-6.8) × 1016 cm-3, on which OPP-STM and OPPR experiments were performed.
A lower doping concentration causes larger tip-induced band bending (TIBB), increasing theintensity of the OPP-STM signal. All experiments were conducted at RT (300 K) using a VT-STM
system (Scienta Omicron, Inc.)

OPP-STM and OPPR measurement system
As presented in Fig.1 in the main text, two tunable femtosecond oscillators (Chameleon Ultra andMira-HP, Coherent, Inc, 80 MHz) were synchronized (Synchrolock-AP, Coherent, Inc) in a typicalmaster-slave configuration. OPP-STM and OPPR measurements were carried out using the samesetup. Each laser can deliver ~150 fs pulses with an average power of 3.5 W. Two ultrafast Pockels
cells (Leysop Ltd., RTP-3-20 crystal) combined with λ/4 wave plates were used to modulate thepolarization states of the optical pulse trains. Pump and probe pulses were colinearly guided into
the STM chamber, the diameter of the laser spot on the sample surface was estimated to be ~50 μm,
and the laser spot position was precisely adjusted using a mirror on a piezo-driven mirror mountplaced before a viewport of the chamber. For all OPP-STM measurements, a positive sample bias
voltage in the range of 0.8 to 1.3 V was applied with the tunneling current set at values from 6.5 to125 nA. The laser wavelength was set at 814 nm (1.52 eV) with an average power of ~100 mW,
measured outside the STM chamber. In the OPPR experiment, the optical conditions were adjusted2

to be identical to those for OPP-STM. Instead of using the STM tip to collect the time-resolved
tunneling current, a photodetector was placed outside the STM chamber to measure a reflected light.

Working principle of OPP-STM for probing electron spin dynamics
To realize the observation of local time-resolved spin dynamics, we must add a measurement
scheme to detect the very weak spin-related tunnel current to OPP-STM. Electron spin
dynamics has been intensively studied, particularly in the ultrafast time domain by using opticalmethods such as time-resolved optical Kerr (Faraday) rotation spectroscopy1,2. For instance, by
making use of the magneto-optical Kerr (Faraday) effect, in a typical pump-probe scheme,
specific electron spins are excited using a circularly polarized pump pulse, which is followed
by a time-delayed, linearly polarized probe pulse to measure the spin dynamics by measuring
the rotation of the probe pulse from its initial direction under linear polarization. However,these magneto-optic-effect-based techniques cannot easily be implemented in an OPP-STM
scheme. That is, to achieve fully operational OPP-STM to probe electron spin dynamics, a newelectron spin excitation and detection approach is necessary.
From this perspective, the OPP-STM system presented here has a uniquely designed“polarization modulation” technique, in which laser polarization states are carefully modulated
for OPP-STM to carry out nanoscale spin dynamics measurement. The system layout is
schematically shown in Fig. 1 in the main text. Two ultrafast Pockels cells are precisely
controlled by a laboratory-built, complex programmable logic device (CPLD)-based logic
circuit to drive both of them in a well-defined timing sequence. To be more specific, using thetwo Pockels cells and λ/4 waveplates, laser pulses can be modulated between two distinctpolarization states: right-handed circularly polarized (R-light) and left-handed circularlypolarized (L-light) states, respectively.
This laser polarization modulation technique is crucial in functionalizing OPP-STM, whichis in contrast to conventional optical pump-probe spectroscopy, in which the laser intensity iscommonly modulated using optical choppers. In the OPP-STM scheme, the modulation of thelaser intensity will severely distort the STM tunneling current owing to the thermal effectapplied to the tip3,4. Therefore, for the spin detection, there is no active modulation of the laserintensity. Additionally, although the intensity of the R-light is produced equally to that of the3L-light in this high-speed polarization modulation technique, polarization can beunintentionally modulated owing to the polarization dependence of the reflectance on theoptics. This passive laser intensity modulation must also be minimized. We have introduced anew modulation technique to overcome the above problems.
It is necessary to highlight that, for the two Pockels cells, a slight frequency difference isintentionally introduced, for example, polarization modulation is carried out on one Pockels
cell at 1 MHz and on the other Pockels cell at 0.999 MHz, as shown in Fig. 1 in the main text.
Here, this difference in the modulation frequency of Δf = 1 MHz – 0.999 MHz = 1 kHz isessential for obtaining a time-resolved, spin-related tunneling current in this OPP-STM scheme.
Specifically speaking, by providing a modulation frequency difference Δf, consecutive phaseshifts in the relative circularly polarization states will occur between pump and probe pulses.
Consequently, this modulation frequency difference Δf gives rise to two distinct polarization
modes: one polarization mode is named “co circular polarization (co-CP)”, in which pump andprobe pulses have identical polarization states; the other polarization mode is named “countercircular polarization (counter-CP)”, in which pump and probe pulses always have opposite
polarization states. These two polarization modes switch back and forth at Δf = 1 kHz. In fact,
these modes contribute differently to the photocarrier density nex as a function of delay time,
and the differential photo-carrier density Δnex can be reflected in the STM tunneling current via
polarization modulation. If we define the co- (counter-) CP-determined tunneling current asIco(td) (Icounter(td)), as the STM tunneling current I is proportional to nex, the relaxation of electron
spin can be reflected in the lock-in detected differential tunneling current, which can bequalitatively represented as ΔI(td) = Icounter(td) - Ico(td). See ref. 5 for more details.

Fitting procedures and definition of lifetime
For both OPP-STM and OPPR results (Figs. 3 and 4 in the main text), fitting was carried out
using the following single exponential function:f (t )  A exp{

(t  t0 )} B(1)where A = amplitude; t0 = zero-delay position; τ = decay constant; B = background (baseline).
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In the experiment, t0 was well calibrated using the autocorrelation method by adjusting theoptical delay time td between the pump and probe pulses to have the value at which the strongest
optical interference occurred.
It is also important to state that our OPP-STM (and OPPR) method is a spin-dependent,differential-absorption-based method, as previously explained. In other words, this approach
itself probes the overall mixture of “up-spin” and “down-spin” electron, with consideration ofthe rate equationsdN NNN     dt
r s s(2)dN NNN     dt
r s sHere, N  is the density of electrons with an “up-spin” orientation, whereas N  is the densityof electrons with a “down-spin” orientation, respectively. τr is the carrier recombination lifetimeand τs is the spin relaxation time, or spin lifetime. By solving Eq. (2), we obtainN N02tt{1  exp()}exp( )2srN2ttN   0 {1  exp()}exp( )
2sr

(3)

Here, N0 is defined as the initial value of N  . From Eq. (3), it is clear thatN   N   N 0 exp(2ts) exp(tr)(4)Note that in the n-type GaAs sample we used for experiments, at room temperature (300 K) thecarrier lifetime (τr ~5 ns9) is significantly longer than the spin lifetime (τs < 500 ps). Therefore,
τ in Eq. (1) can be expressed as τ = τs/2.

Penetration depth determination
A simple calculation was carried out as follows to obtain the penetration depth in our case.
The optical penetration depth of ~ 800 nm light in GaAs with normal incidence geometry is~1 μm10,11. The incident angle in our experimental configuration is shown in Fig. S4(a) and a5

schematic illustration is shown in Fig. S4(b). On the basis of well-known Snell’s Law, with anincident angle of 31°, the refracted light angle with respect to the sample normal becomes 13.5°,
where refraction indexes of the vacuum and GaAs of nvac = 1 and nGaAs = 3.67 were used,
respectively. In general, the optical penetration depth δ, which is the depth at which the laser
intensity of the transmitted (refracted) light drops to 1/e of its initial value at the incidence
interface, is defined as12

=1



(5)

Here, α is the absorption coefficient of the material at a certain wavelength and temperature. In
the case of our GaAs sample excited by 814 nm pulses at 300 K, α ~12764 cm-1 (see Ref. 13).
Thus, by using Eq. (5) under an oblique incidence geometry (refracted angle of 13.5°), weestimated the penetration depth to be  * =1

1~ 800 nm .
 cos(13.5o)

6Fig. S1 Two spin relaxation mechanisms involved in the system under investigation. (a)Dyakonov-Perel (DP) spin relaxation mechanism6. Small red balls represent electrons, whereas
larger gray balls represent scattering centers. Ωn (n = 1, 2, 3) are microscopic magnetic fields
with different magnitudes and directions, which force electrons to precess around eachmagnetic field axis between the two subsequent scattering events, leading to spin relaxation.
(b) Bir-Aronov-Pikus (BAP) spin relaxation mechanism6. Red and blue balls represent
electrons and holes, respectively. In this case, electron spins relax owing to the electron-holespin exchange interaction. The effect of potential modulation by Mn acceptor (2 x 1018 cm-3) ina dark condition works in the range of 10 nm7. When surface Mn density is further increased,
it may be necessary to consider the effect that the potential becomes smoother in addition to theBAP effect. Details including the screening effect under photo illumination. Details while photoexcitation process are left for future work.

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Fig. S2 A model for producing spin-related OPP-STM signal and results of simulations ofTIBB. (a) TIBB under dark condition. (b) Excitation by pump pulse. The electric field and thus
the surface potential are reduced, increasing the effective bias voltage applied to the tunneljunction (surface photovoltage: SPV). Consequently, the photoexcitation increases the transienttunnel current. For simplicity spin orientation was presented as 100 %. (c) to (e), The excitedstate subsequently relaxes to the original state via three processes: the randomization of spinorientation (c), the decay of photocarriers via recombination (d), diffusion and drift, and thedecay of minority carriers transiently trapped at the surface during the photoexcitation via
thermal emission or recombination at the Mn site with the tunnel current from the STM tip (e)8.
When a probe pulse arrives during the relaxation processes, an additional SPV is induced,
which changes the total magnitude of the tunnel current depending on the delay time, td,
providing time-resolved spectra. (f) Simulated TIBB. Calculations were carried out using
SEMTiP Fortran Package provided by Feenstra (CMU) for the three typical applied bias
voltages used in this experiment. For a GaAs sample we used the doping level of 3.8-6.2 x 1016
cm-3, and the TIBB depth is ~40 nm even in a dark condition and becomes close to flat underphoto illumination, which is much shorter than the optical penetration depth of ~800 nm.
Detailed analysis while excitation and relaxation are left for a future work.
8Fig. S3 Results of simulations for the electric fields modulated by the STM tip. (a) Schematic
illustration of the experimental setup used for the simulation. (b)-(d), Maps of the strength ofthe electric fields of the x-, y- and z-components of circularly polarized light. Time-domain
calculations of electromagnetic wave propagation were carried out using CST Studio Suite. Atungsten (W) tip and a GaAs sample were arranged at a distance of 1 nm. The W tip was atruncated cone and its apex was spherical shape with a diameter of 10 nm. Circularly polarized
light with a center frequency of 370 THz (λ = 814 nm) was used as the incident electromagnetic
wave with an angle of 31° from the horizontal plane. The minimum mesh of the space was 2.5nm. The black line in the maps indicate the position of the sample surface. In the case of a Wtip, the tip-induced electric-field enhancement in the z-direction is about 400 times the incident lightintensity around the tip apex, which sharply decreases inside the sample with increasing distancefrom the surface. Therefore, the excitation by the circularly polarized light condition below the STM
tip is initially disturbed around the tip apex but then recovers at a depth of a few nm.
9Fig S4. (a) Experimental setup. Both OPP-STM and OPPR measurements were conductedusing an STM UHV chamber, at a fixed laser incident angle of ~31°. (b) Schematic illustrationof the geometry of the oblique incidence light and the refracted light that penetrates into thesample, which determines the optical penetration depth.

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