Ultrafast Dynamics Revealed with Time-Resolved Scanning Tunneling Microscopy: A Review

A scanning tunneling microscope (STM) capable of performing pump–probe spectroscopy integrates unmatched atomic-scale resolution with high temporal resolution. In recent years, the union of electronic, terahertz, or visible/near-infrared pulses with STM has contributed to our understanding of the atomic-scale processes that happen between milliseconds and attoseconds. This time-resolved STM (TR-STM) technique is evolving into an unparalleled approach for exploring the ultrafast nuclear, electronic, or spin dynamics of molecules, low-dimensional structures, and material surfaces. Here, we review the recent advancements in TR-STM; survey its application in measuring the dynamics of three distinct systems, nucleus, electron, and spin; and report the studies on these transient processes in a series of materials. Besides the discussion on state-of-the-art techniques, we also highlight several emerging research topics about the ultrafast processes in nanoscale objects where we anticipate that the TR-STM can help broaden our knowledge.


INTRODUCTION
The invention of scanning tunneling microscopy (STM) has allowed for unprecedented spatial resolution down to the atomic scale because of the exponential relationship between quantum tunneling probability and the distance between the tip and a substrate. 1 It was soon developed into a versatile tool that provides molecular-level insights into various physical chemistry problems, including chemical structure, 2−15 electronic orbital, 16−20 vibration, 21−29 atomic manipulation, 30−37 potential energy surface characterization, 38 and spin detection. 39−56 Additional functions with enhanced time resolution, such as video-rate faster scanning 57−59 and spatial atomtracking, 60 were later developed for direct visualization of the dynamical motions of adsorbates. 61−64 However, in most cases, the time resolution of STM is limited to the microsecond scale by the response time of its feedback electronics and, thus, can hardly contribute to our understanding of the chemical dynamics that often happen at the time scale of picosecond or femtosecond. There has been a desire to turn STM into a fast-responding camcorder with subpicosecond sensitivity to capture ultrafast processes with atomic-scale details.
The development of the pump−probe technique with either laser or electron pulses has made it possible to induce dynamic transitions and follow their temporal evolution. It has contributed to a greater understanding of the intermediate processes of various quantum degrees of freedom, such as vibrations, 65−70 electronic orbitals, 71−77 and electron or nuclear spins in various materials. 78−82 Moreover, in optical pump−probe measurements, it is possible to manipulate the interference between different excited states by adjusting the phase, frequency, or intensity of the driving laser pulses and consequently control the reaction's pathway coherently to maximize the yield of the desired product. 83 It becomes especially powerful in tracking the movement of electrons between molecular states to understand and control chemical transitions. However, because of the diffraction-limited spatial resolution, 84 most of these studies can only assess the homogeneous properties of an ensemble and cannot account for the nanoscale variation of the local chemical environment.
The exotic chemical and physical phenomena that occur at the nanometer scale have boosted the development of nanoscience and technology in the past two decades. As the size of a material is reduced, the quantum mechanical principles dominate the material behaviors and lead to distinct properties compared with the bulk counterparts. Character-ization of the dynamic processes within the local chemical environment is therefore especially essential in the studies of nanoscale physics or chemistry, such as quantum dots and single-atom catalysis. The combination of STM and pump− probe measurement enables the visualization of ultrafast dynamics at the single-atom or single-molecule level. It provides a unique platform where the inhomogeneous activities of individual molecules or low-dimensional materials can be tracked in both space and time, which otherwise are embedded in the ensemble average. 85,86 In this review, we investigate the approaches that integrate the pump−probe scheme with STM and discuss the recent studies revealing key insights into the dynamics of three types of quantum states: vibration, 87−92 orbital, 93−97 and spin. 43,98,99 2. THREE SCHEMES OF TIME-RESOLVED SCANNING TUNNELING MICROSCOPY

All-Electronic Pump−Probe STM
Traditionally, STM in the constant current mode scans the sample surfaces with the feedback circuit to keep the tunneling current at a constant set point. This is similar to a laser operating in continuous wave mode. However, to achieve higher temporal resolution, the electronic pump−probe scheme can be applied, which uses two short pulses of electrons ( Figure 1). The first electron pulse, usually with relatively higher energy, excites the surface adsorbate under the tip into a transition state. The second pulse arrives after an adjustable time delay and interacts with the excited adsorbate. The delay-dependent tunneling current induced by the second pulse contains information directly related to the time evolution of the transition state. In 2010, Loth et al. first combined STM with an electron pulse generator to realize this all-electronic pump−probe scheme 98 and applied it to probe the spin relaxation of a series of magnetic adsorbates in an external magnetic field. The spin orientation is detected with a spin-polarized tip, which is often made by attaching a magnetic atom, such as Mn, to a nonmagnetic STM tip. The spin orientation of the magnetic atom on the tip aligns nearly parallel with the external magnetic field. Because of the Pauli exclusion principle, preferable tunneling occurs when the orientation of the spin on the tip aligns with the one on the surface. Therefore, when the surface spin relaxes from an excited state triggered by the pump pulse, the time evolution can be traced with the spin-polarized current generated by the probe pulse. This all-electronic pump−probe approach has readily achieved a time resolution in the nanosecond to millisecond range and made great impacts in studying the electron and nuclear spin evolution of material surfaces and absorbates. 70,100−102

Terahertz Pump−Probe STM
The terahertz (THz) techniques support a new development area of time-resolved STM (TR-STM). Free space THz pulses can be generated as short as a single optical cycle, thereby giving a subpicosecond time resolution. Suboptical cycle resolution can be achieved with two identical THz pulses in autocorrelation. 89,93 Thanks to the strong electric field and low photon energy, the coupling of THz light pulses with STM avoids the limitation from tip thermal expansion, electronic microstrip bandwidth, and electrostatic coupling. 93,103,104 Another advantage of THz STM is that it can provide a stable carrier−envelope phase (CEP) in a relatively cheaper and easyaccess approach. This is essential for studying coherent interference between light and quantum states. 96 The working principle of THz STM is detailed in Figure 2. When THz light is focused on the tip apex, the STM junction acts like an antenna that enhances the evanescent THz field, which modulates the Fermi level alignment between sample and tip as ultrafast voltage transients. The enhanced THz field can generate tunnel electrons to either excite the sample underneath the tip or probe its time evolution. 93 Electron tunneling is commonly involved in both the pump and probe processes, which guarantee a sub-Angstrom-level spatial resolution. 105 The CEP stability can also help reach a time resolution below the duration of one THz pulse. For example, Cocker et al. demonstrated an access-state-selective tunneling mechanism where the light-induced tunneling only occurs at the peak of the THz pulse, thereby allowing for a temporal resolution shorter than one oscillation period of a THz wave. 89 However, because of the natural constraint from the optical period, the achievement of a time resolution below a few hundred femtoseconds is still not straightforward. 93

Visible or Near-Infrared Pump−Probe STM
While the time resolution of THz STM is jammed near ∼0.2 ps, which is nearly 2 orders of magnitude longer than the achievable pulse duration in the state-of-art femtosecond laser technology, 89,106 another laser STM setup can make up for this regret. The visible/near-infrared (vis/NIR) ultrafast laser can readily provide extremely short femtosecond light pulses and has recently been applied in the pump−probe measurement with STM. In most cases, the sample interacts with the electron or THz pulses by coupling with the local electric field at the STM junction, while the vis or IR light can easily excite the sample through photon absorption. For example, in the setup used by Terada et al., pulse trains were generated by two synchronized Ti/Sapphire lasers to illuminate the sample beneath the STM tip with a tunable time delay. 107 The pump− probe measurement tracks the time correlation between the transient state initiated by the pump pulse and the variation in a local STM measurable induced by the probe pulses ( Figure  3). The pump pulses can excite the entire area illuminated by the beam, but the probe processes rely on the detection of photoexcited electron tunneling and, therefore, break the diffraction limit. Nevertheless, short IR pulses can also couple the sample through the field-driven mechanism. Garg et al. advanced the time resolution into a few femtoseconds by focusing a CEP of two-cycle long (<6 fs) optical pulses on the tunneling junction via off-axis parabolic mirrors and capturing the generation of excited electrons on the basis of the interference process of the two pulses. 96 Recently, femtosecond scale resolution has also been reported in the midinfrared region through a light-wave-driven mechanism. 108 So far, the vis/NIR or Mid-Infrared coupled STM has provided the highest time resolution in TR-STM studies. The challenge of this approach often comes from the thermal fluctuations of the STM junction. Unlike the THz case where most materials have a high reflection, the absorption of vis/NIR by either tip or sample can cause thermal expansion, hence interfering with the tunneling measurement. This has posed a great challenge for pump−probe studies where power modulation often needs to be applied on the probe pulse for lock-in measurement.
Increasing the laser repetition rate to GHz can partially solve this issue since it reduces the energy of single pulses while still generating a detectable signal. 88 Other approaches, such as modulating the duration, 109 frequency, 109,110 delay time, 107 or polarization 109 of the pulses, are applied to avoid change to the laser power. Alternatively, the detection of a signal that is less sensitive to thermal fluctuations, such as molecular motion/ reaction and photoexcited current, can also loosen the requirement of thermal stability. 88,106 Besides, the use of vis/ NIR light as a pump and THz light as a probe could potentially unify the advantages of both techniques. 93,95 Another newly developed approach to detect ultrafast dynamics at the nanoscale is the time-resolved tip-enhanced Raman spectroscopy (TR-TERS) with ultrashort vis/NIR laser pulses. 111−113 Instead of tunneling electrons detection, TERS collects the scattered photons localized by the filed enhancement at the STM junction. 25,113−115 Plasmonic materials, such as Ag and Au, are often used for the tip to promote strong light−matter interaction at the STM junction. A submolecular spatial resolution has been reported in STM TERS measurement because of the strongly localized nature of surface plasmons. Meanwhile, the high-energy resolution of TERS makes it ideal to investigate low-energy vibrations. Over the past decade, many studies, such as the vibrational mapping, 25 structural and chemical changes of single molecules, 28,115,116 and energy transfer between molecules, have been done with CW light-coupled TERS. 26 Recently, the combination of ultrafast pump−probe spectroscopy with STM-TERS measure-  ment has provided a new route to capture vibrational dynamics with joint spatial−temporal resolution. 92

Nucleus Dynamics
The chemical processes often involve the dynamical rearrangement of atoms in materials, which occurs through different mechanisms, such as vibration, 117,118 electronic excitation, 96,110,119 and proton tunneling. 120,121 Molecular vibrations refer to the periodic motions of atoms relative to one another within a molecule, such that the center position of the mass of the molecule is unchanged. The collective nucleus motion in a periodic lattice can travel across a large-scale sample, which is also known as phonon propagation. The absorption of energy by a vibrator in its ground state can trigger the transitions to a higher vibrational level, a process that can be described with a semiclassical harmonic oscillator model. The vibrational populations can either be read by STM observables, such as tunneling electrons and molecular transition rate, or scattered photons through Raman scattering. The lifetime of molecular vibration is typically at the femtosecond to picosecond level because of the fast energy transfer to the environment�the socalled "bath" coupling. 122 This time scale exceeds the inherent limitations posted by the electronic bandwidth in traditional STM measurements. In a less common case, nuclei motion can also be due to quantum tunneling. 123,124 For small atoms like hydrogen, the spatial tunneling of atoms between different energy wells results in the rearrangement of chemical structure. 125 In this section, we will survey the recent applications of TR-STM on the nucleus dynamics related to vibration, phonon, or quantum tunneling.
In 2016, Cocker et al. for the first time detected the femtosecond-scale single electron tunneling from a surfaceadsorbed molecule and recorded the vertical molecular vibration excited by the THz pulses. 89 In this experiment, ultrafast electron tunneling from the highest occupied molecular orbital (HOMO) of the pentacene to NaCl/ Au (110) was triggered by the enhanced THz electric field at the STM junction. It was revealed via the THz pump−probe measurement that the tunneling current induced by the probe pulses oscillates at a frequency of ∼0.5 THz, which was explained as the result of vertical molecular vibration due to temporary ionization by the pump pulses.
Vis/NIR pump−probe STM can also be used to extract the ultrafast nucleus motion of a single molecule. In 2017, Li et al. reported the reversible conformational transition of the pyrrolidine molecules adsorbed on a Cu(001) surface combined with Angstrom−femtosecond resolution. 88 They found that the transition rate of the molecule under laser illumination shows decaying oscillatory behavior by varying the delay time between the two optical pulses (800 nm, 35 fs pulse width). The peaks in the frequency spectrum at ∼6.9 and ∼2.7 THz were attributed to the bending (27.2 meV) and bouncing (11.3 meV) motions of the pyrrolidine, respectively. It was also demonstrated that with another molecule in proximity, the ∼6.9 THz mode downshifts to ∼6 THz, thereby showcasing the influence of intermolecular interaction on the nucleus dynamics.
In 2020, Peller et al. accomplished the detection and manipulation of ultrafast in-plane vibration-mediated molecular rotation with a THz STM pump−probe scheme. 87 In the experiment, the bistable magnesium phthalocyanine (MgPc) on NaCl/Cu(111) acted as the single-molecule switch that could be triggered by electron tunneling into the lowest unoccupied molecular orbital (LUMO) of MgPc. In the pump−probe measurement, the MgPc was first perturbed by a weak pump THz pulse and then charged by the strong probe THz-pulse-induced tunneling electron from the tip. The switching probability unambiguously oscillates at ∼0.3 THz when varying the pump−probe delay time, rationalized as the mediation through in-plane rotation excited by the pump pulse.
A recent report by Sheng et al. highlights the application of THz pump−probe STM in exploring and controlling the coherent acoustic phonon (CAP), 91 which is the collective motions of atoms in a lattice. In the experiment, they performed pump−probe measurement on the Au thin film grown on mica with paired, delayed, but identical THz pulses (Figure 4a). The tunneling current induced by the tipenhanced probe THz electric field shows long-period oscillation at ∼10.1 GHz and decays after several hundreds of picoseconds, which strongly suggests the periodic out-ofplane motion of the Au surface atoms (Figure 4b,d). The observed oscillation was assigned to the CAP wave packets propagating between the Au/mica and Au/vacuum interfaces, whose dispersion relation in a lattice with a thickness of d, f CAP = v/2d, fits well with the oscillation frequencies measured over an Au film of different thicknesses, from 151 nm down to 6.4 nm (Figure 4c). The CAP was rationalized as a consequence of the Au atoms' displacement by the local ultrafast Coulomb force between the charges on the tip and Au surfaces, which were induced by the strong THz field at the tunnel junction ( Figure 4a). It was further shown that the displacement of the surface atoms increased from 2.8 to 5.1 pm when the tip-Au distance decreased by 2.7 Å, which demonstrated a way to manipulate the CAP wave packets. More recently, the coherent phonon modes in another system, ZnO thin films on Ag(111), were revealed with a NIR pump−probe STM. 106 The CAP can also be excited in graphene nanoribbons (GNRs) through stimulated Raman scattering. Luo et al. demonstrated the observation and control over the CAP in single GNR on Au(111) with a TR-TERS setup. 92 In this experiment, the GNR under a Au tip was pumped to a superposition of different vibrational states via tip-enhanced impulsively stimulated Raman scattering (ISRS) with two ∼100 fs broadband laser pulses that centered at 780 nm (pump pulse) and 850 nm (Stokes pulse), respectively. The frequency difference between the pump and Stokes pulses (∼2464 cm −1 ) covered several phonon modes to trigger the coherence phonon. A narrowband (∼500 fs and centering at 728 nm) pulse was sequentially used as a probe to interact with the phonon wave packet. The anti-Stokes Raman scattered light of the probe pulse contained the dynamic information on the CAP. By keeping the pump and Stokes pulses temporally overlapped and by varying the delay time (τ 23 ) between the Stokes and probe pulses, the dephasing time of the impulsively excited phonons was found to be ∼440 fs. Furthermore, the initial phase of the coherent photon could be manipulated by temporally separating the pump and Stokes pulses (τ 12 ) and detecting immediately (τ 23 = 0) with the probe pulse. The scattered light showed an oscillatory pattern as a function of τ 12 . Fourier transforming at different energies further revealed several beat frequencies, which were assigned to the quantum beating between different vibrational levels. Besides the dynamic motion of atoms associated with vibration or phonon, the small atoms in a molecule can also rearrange through quantum tunneling. In a recent work by Wang et al., ultrafast THz-coupled STM was adopted to detect the transition between two quantum levels of H 2 in the STM junction. 90 The H 2 could be confined in an asymmetric double-well potential defined by the tip and CuN/Cu(100) substrate. The energy profile of the double-well, as well as the proton tunneling rate across the barrier, codefined two nondegenerate quantum levels in the two spatially separated wells. The transition between these two levels, accompanied by the structural change of H 2 by hydrogen tunneling, occurred when the molecule was excited with either photons or electrons. In a pump−probe scheme, the authors drove the transition between these two levels with THz pump pulses and extracted its evolving superposition state with delayed probe pulses. The rectification current by the probe pulses, which they showed to be proportional to the second derivative of tunneling current over bias voltage (d 2 I/dV 2 ), exhibited obvious oscillating features, which were attributed to the coherent superposition of the two wave functions.

Electron Dynamics
The excitation and relaxation of electrons between molecular orbitals or semiconductor bands is the origin of many energy and mass transfer processes, including chemical reactions, 126,127 electrical conduction, 94,99 and photon emission. 96,128 The atomic details about the electronic dynamics are vital to understanding the effect of the local chemical environment on these dynamics, such as the effects of nanocatalysis in chemical transformation or atomic dopants in a semiconductor. 117 In the following section, we will review the recent TR-STM studies on electron dynamics in molecules, 95 semiconductors, 94,97 and metals. 96 In 2017, the work by Jelic et al. unveiled the subpicosecond scale electron tunneling process between the Si(111)-(7×7) surface states and the bulk states of Si. 94 In this study, the free-space THz pulses with a strong electric field (−200 V/cm) could induce a tunneling current that depended exponentially on tip−sample separation at the STM tunnel junction ( Figure  5b). Under illumination with single-cycle THz pulses ( Figure  5a), the spatial mapping of the Si(111)-(7×7) surface at the constant-current mode without a DC bias could resolve the Si adatoms and well reproduce the conventional STM topographic image taken over the same area. The agreement in image appearance suggests the localized nature of the THzdriven tunneling electrons. The subpicosecond time window for electron tunneling was revealed in the autocorrelation measurement with paired identical but weaker THz pulses (−100 V/cm), which are not strong enough to induce a detectable current individually (Figure 6e). Surprisingly, an extreme field-induced transient current of ∼160 μA through the Si(111)-(7×7) surface was derived from the −20 pA tunneling current set point used in the THz constant-current imaging after the width and repetition rate of the THz pulses were taken into account. This unusually large current was attributed to the opening of additional tunneling channels between the surface and bulk states. The authors suggested that a large amount of THz-induced tunneling electrons into (or from) the Si(111)-(7×7) transiently saturate (Figure 5c) (or deplete) ( Figure 5d) the surface states because of the poor lateral conductivity, which consequentially splits the originally aligned Fermi levels of the bulk and surface and introduces a new tunneling pathway. This mechanism was supported by the observation of faulted−unfaulted asymmetry (Figure 5e and Figure 6a,c,d) in the unit cell taken with either a negative (Figure 6c) or positive (Figure 6d) THz field. Such a topographic asymmetry is usually absent in conventional STM images taken with a positive bias (Figure 6b). However, in the case of THz-induced tunneling, since the surface states leading to the faulted−unfaulted asymmetry are close to the surface Fermi level, they always contribute to the electron tunneling, either from the surface to the tip at the negative half   on Au (111) and showcased the unique capability of TR-STM to map the spatial inhomogeneity in the ultrafast charge transfer process. 95 The experimental setup is depicted in Figure  7a, where a 1035 nm, 309 fs laser pulse pumps the Au conduction electrons into the LUMO of C 60 , while a following THz pulse probes the instant population of LUMO electrons by modulating the tunnel barrier and inducing a net tunneling current I THz . The authors believed that the I THz mostly originates from the electrons tunneling from the LUMO of C 60 to the tip. This mechanism was supported by the ∼2 eV energy barrier of the tunnel junction derived from the I THz versus z measurement (Figure 8a), which agrees with electron tunneling from the C 60 LUMO to tip on the basis of the ∼3.1 eV HOMO and LUMO energy gap of C 60 and the measured 5.48 eV energy barrier with −3 V DC bias where most electrons tunnel from C 60 HOMO (Figure 8b). The spatial mappings of I THz at various delay times clearly show a longer electron trapping time near the lower level of the C 60 step edge (Figure 7b) and at the defect sites, including both C 60 vacancy and misorientation (Figure 7c). The scanning tunneling spectroscopy (STS) measurement reveals a nanoscale local energy minimum of LUMO at both the step edge (Figure 8c,d) and the defect sites (Figure 8e,f), either because of the structural discontinuity at the step edge or the formation of in-gap defect states (Figure 7d). Such a local potential minimum is believed to be the origin of the prolonged electron lifetime. The measured I THz maximizes across the surveyed areas within 1 ps, followed by a gradual decay in several tens of picoseconds, which indicates that horizontal electron diffusion occurs much faster than the vertical relaxation back into the Au substrate (Figure 7b,c). The horizontal charge redistribution process might be resolved in the future with a shorter laser pulse.
The electronic dynamics related to surface plasmons resonance often occur within a few femtoseconds and, therefore, require a higher time resolution to visualize. Garg et al. compressed the pulse width of near-infrared pulses to less than 6 fs and succeeded in resolving the very fast collective electron oscillation and its decaying process with STM. 96 In this experiment, localized surface plasmon resonances (LSPRs) in a Au nanorod on n-doped 6H-SiC were excited with CEPstable pump laser pulses. The probe pulses delayed by a time Δt triggered the electron tunneling from the Au to the tip. The tunneling current from the pump−probe measurement oscillated at a period of ∼2.5 fs, which corresponded to the LSPR with an ∼750 nm wavelength. The light-induced current signal decayed in ∼40 fs, which was assigned to the decay of the LSPR because of the electron−electron scattering. This study demonstrated the capability of using TR-STM to study ultrafast electronic dynamics at the attosecond scale.
Guo et al. applied the TR-STM to investigate the dynamics of electrons captured by the surface oxygen vacancies (V o ) on rutile TiO 2 (110). 97 The STS measurement identified an in-gap defect state localized around V o . The authors suggested that polarons form near the V o because of the transfer of the excess electrons trapped by the V o to the nearby Ti to form Ti 3+ ions, which is supported by density-functional theory calculations. The pump−probe measurement with paired nanosecond 532 nm laser pulses revealed an exponentially decaying photoninduced tunneling current (I ph ) close to a V o site, which was assigned to the retrapping dynamics of the photoexcited electrons from the polaron states. The I ph measured between two V o defects decayed much faster, which was attributed to the higher trapping efficiency because of the extra V o .

Spin Dynamics
As a natural binary system, electron spin in a solid-state environment is of both scientific and industrial interest for its potential application as the one of the building blocks for quantum computation and quantum information. 129−133 The spin lifetime and dephasing time, two important parameters for assessing the quality of the qubit, are susceptible to the local environment, such as the impurities nearby and the electron bath. 134−136 Therefore, it is important to investigate the spin relaxation and dephasing processes at the spatial limit. In the following section, we will summarize the recent advancements of TR-STM on the spin dynamics in atomic/molecular magnets and semiconductors.
The groundbreaking experiment by Loth et al. in 2010 first revealed the atomic-scale electron spin relaxation dynamics. 98 In this experiment, they excited the spin state of a Fe−Cu dimer on the CuN/Cu(100) in an external magnetic field with a strong pump voltage pulse. The evolving spin state was sequentially read out by detecting the spin-polarized current generated by a weaker probe voltage pulse after a pump−probe delay time Δt. The spin relaxation lifetime was extracted to be ∼87 ns from the exponential decay of the spin-polarized current as a function of Δt. In the following decade, this allelectronic pump−probe scheme was extended to the detection and control of the spin dynamics of few-atom/molecule systems. In 2015, Yan et al. succeeded in manipulating the microsecond-scale spin lifetime of a linear antiferromagnetic Fe trimer on Cu 2 N/Cu(100) in an external magnetic field with the Heisenberg exchange interaction. 101 They found that the exchange interaction between the paramagnetic tip and Fe atoms modifies the state mixing of the two lowest-lying spin states of the Fe trimer as an effective magnetic field and, hence, either increases or decreases the lifetime of the excited higherlying spin state depending on the antiferromagnetic or ferromagnetic alignment of the magnetic moments between the tip and Fe atoms. In 2017, Paul et al. showcased the control over the millisecond-scale spin lifetime of single Fe atoms on MgO/Ag(001). 137 Since the spin relaxes through the exchange of energy and angular momentum with either the tip electrons or the metal substrate electrons, the spin lifetime varies in response to the tip-Fe distance and the thickness of MgO, which both influence the population of electrons available for inelastic scattering with the excited spin. More recently, the integration of all-electronic pump−probe STM with the electron spin resonance technique has allowed the characterization and manipulation of the coherent spin evolution in several atomic 40,42,43,102 and molecular 138 systems.
The temporal resolution of the STM pump−probe spectroscopy with electronic pulses remains limited to nanoseconds, thereby restricting its applications on capturing faster spin dynamics, such as the processes involving the strong spin−orbit coupling (SOC) in semiconductors. Alternatively, circularly polarized (CP) light carries an angular momentum that can alternate the spin state and, therefore, be used to probe the fast spin processes. In 2014, Yoshida et al. extended the vis/NIR pump−probe measurement with STM to the spin relaxation and precession processes in GaAs. 99 In GaAs, photo absorption of either +1 or −1 spin angular momentum induces a 50% spin polarization in the conduction band because of the characteristic 3:1 ratio between the photoexcited electrons ACS Applied Optical Materials pubs.acs.org/acsaom Review  from the m s = ±3/2 heavy-hole and m s = ±1/2 light-hole valence bands (Figure 9b). The population decay and the precession of the spin-polarized electrons excited by a CP pump NIR pulse can be detected by a probe CP NIR pulse after a delay time of t d . The handedness of the pump and probe pulses can be either the same or opposite, which is referred to as co-CP and counter-CP excitation, respectively. The co-CP mode generates a lower photoinduced current than the counter-CP mode at a small delay time because of the depletion of the valence band electrons by the pump pulse (Figure 9c). In the experiment, the authors modulated the polarization of the 90 MHz NIR pump/probe pulse trains with a pocket cell and one waveplate to generate a series of alternating co-CP and counter-CP pulse pairs. The difference between the tunneling current induced by the co-CP and counter-CP excitations (ΔI S ) was used to characterize the spin dynamics.
Yoshida et al. studied the spin dynamics in three different GaAs samples. 99 First of all, they reported the temperaturedependent spin relaxation in p-type GaAs(110) as a proof-ofconcept experiment. The spin lifetime (τ S ) at five different temperatures was extracted by fitting ΔI S as a function of t d (Figure 10a). The dominant relaxation channel through scattering by impurities at a high temperature (Dyakonov− Perel mechanism) is confirmed by the relationship of τ S = ∼T −3 , while τ S is limited by the electron−hole coupling (Bir− Aronov−Pikus mechanism) in the hole-doped GaAs, as shown by the tendency of saturation at a low temperature ( Figure  10b). In the following measurement, they showcased the joint spatial−temporal resolution on GaAs/AlGaAs quantum wells at room temperature. The measured ΔI S at t d = 2.3 ps increased both near the GaAs/AlGaAs interface and inside the 6 nm quantum well, thereby demonstrating a ps−nm resolution across the sample surface (Figure 10c). The τ S obtained at the 6 nm (τ S = 68 ± 6 ps) and 8 nm (τ S = 112 ± 6 ps) wide quantum wells agreed with the Dyakonov−Perel mechanism where the τ S relates to the width-dependent confinement energy E 1e as τ S = ∼E 1e −2 . Last but not least, the pump−probe measurement on the n-type GaAs in an external magnetic field at 2.5 K revealed the coherent dynamics of spin. As shown in Figure 10d, the measured ΔI S exhibits a clear oscillation whose frequency depends on the magnitude of the external magnetic fields. These oscillations serve as the signatures of the spin precession of the conduction band electrons. Here, although not explicitly stated by the original authors, we attribute the mechanism of the measured oscillations in ΔI S to the magneto-optic Kerr effect. In the classical picture, the Larmor procession of the spin excited by the pump pulse leads to a time-dependent spin magnetic moment rotating around the magnetic field direction. This alternating magnetic moment effectively changes the absorption of the probe pulse and, therefore, leads to an oscillatory ΔI S following the pace of the spin procession.

CONCLUSIONS AND OUTLOOK
In this review, we have discussed the recent application of TR-STM in the studies of ultrafast dynamics beyond the typical electronic response time. The STM gains a temporal resolution at different time scales from its union with short pulses of electrons, THz wave, and/or vis/NIR photons. The usage of short electron pulses allows us to resolve the activities at the order of nanoseconds and has been applied to investigate the spin dynamics. The introduction of free-space THz pulses further brings the resolution down to the subpicosecond scale, which can catch many of the electronic and vibrational dynamics. The use of ultrashort vis/NIR laser pulses can not only provide finer femtosecond scale details of the abovementioned dynamics but also enable the atomic-scale study of even faster processes, such as the excitation and relaxation of surface plasmons. Furthermore, time-resolved TERS opens new avenues of tracing and control of coherence, thereby groundbreakingly expanding the potential applications of TR-STM. These advances have allowed for the nanoscale visualization of many nonequilibrium states, as well as their response to the local inhomogeneous environment. The energy, intensity, polarization, and phases of the driving pulses serve as a series of easily accessible tuning knobs for us to access and control these transient processes at the atomic and molecular levels.
The TR-STM provided a new window to view the transient motion of the nucleus from the stretching of a single molecular bond to the collective motion of atoms in a lattice. A THz pulse focused at the tunneling junction can excite the vibrations in small and large molecules by distinct mechanisms from the transient ionization of the molecule to the impulsive force because of the focused electric field near the tip. It can also generate standing acoustic phonon waves in thin films. These nuclear dynamics can also be triggered by either the direct adsorption of infrared photons or by the Raman scattering of visible or NIR light. The temporal evolution of these processes is traced by either the photoinduced current or the transition rate of a molecular reaction. The measured lifetime ranges from picoseconds of individual molecules adsorbed on conductors to several hundred picoseconds for phonons in a thin metal film. Many of these processes exhibit dramatic variation in response to even saddle changes in the local environment, such as the variations in van der Waals forces between adsorbates and the substrate or the Coulomb interaction between molecules, which is an inaccessible area for other experimental approaches without the simultaneous spatial and temporal resolution.
The TR-STM has also shed light on the charge carrier dynamics in systems ranging from a single dopant or defect in a semiconductor to LSPRs confined near the tip and a conducting substrate. The THz pulses impinging on the organic or inorganic semiconductors can alter the band bending near the surface, thereby leading to nonequilibrium electron tunneling occurring within a picosecond. The local detection of THz-induced current signal with STM helps discriminate the spatial inhomogeneous dynamics resulting from local structural disorders. At the attosecond time scale, the ultrashort CEP-stable NIR pules can stimulate and detect the collective electron oscillation whose oscillation direction depends on the polarity of the pump pulse. These studies have deepened our understanding of how the local electronic or chemical structures can change the lifetime of excited charge carriers in different materials that serve as guidance for the future design of optical electro devices.
The spin dynamics of the charge carriers in solid-state materials or an isolated magnetic surface impurity can be excited and traced using electron or photon pulses. Photons with different angular momentum can control the spin orientation of the conducting electron in semiconductors. The transitions between spin levels split by an external magnetic field can also be excited by electron pulses. Manipulation of the spin population can be achieved by ACS Applied Optical Materials pubs.acs.org/acsaom Review adjusting either the pulse intensity or duration. The characterization and manipulation of the spin degree of freedom in both space and time domains are especially impactful in quantum information science, where spin-based quantum sensing or computing algorithms have exhibited advantages compared with their classical counterparts. Besides the demonstrated success in the aforementioned areas, TR-STM promises to make unique contributions in many other fields in physics, chemistry, and quantum information science. As a versatile approach, we expect that it readily adapts itself to the investigation of emerging materials. Among others, low-dimensional van der Waals heterostructures have exhibited rich and exotic physics originating from the nonequilibrium charge carriers. 139−143 The TR-STM can visualize the excitation, spatial distribution, and temporal decay of these charge carriers to provide unparalleled, detailed, and comprehensive information in both space and time. In chemistry, this new technique can broaden our knowledge of nanocatalysis where the interplay between local quantum confinement effects and the chemical transition pathway can be visualized for the first time. Moreover, the coherent manipulation of light−matter interactions can now occur with atomic accuracy, 144 which may provide an alternate approach to switch the quantum state population of a qubit. Even though TR-STM is currently a sophisticated technique only available in a limited number of research laboratories, recent advances in technology have made T R -S T M m o r e a c c e s s i b l e a n d u s e rfriendly. 92,97,106,108,109,145−152 Because of its versatility, we highly anticipate its future as a widely used technique that is accessible to users in a diverse array of fields.