On the progress of ultrafast time-resolved THz scanning tunneling microscopy

Scanning tunneling microscopy combined with terahertz (THz) electromagnetic pulses and its related technologies have developed remarkably. This technology has atomic-level spatial resolution in an ultrahigh vacuum and low-temperature environment


I. INTRODUCTION
Since its invention in 1982, 1 scanning tunneling microscopy (STM) has been used to image the electronic states of material surfaces with atomic resolution. 2,3In STM, electrons, driven by an applied bias voltage, pass through a tunnel junction that forms when the metallic tip is set in very close proximity to a conductive sample [Figs.1(a) and (b)].The resulting tunneling current varies exponentially with respect to the width of the gap w between the tip and the surface.Scanning a sample with the tip under constant bias allows us to map the topology and conductivity of the surface.When the tip is sharpened to an atomic-scale apex like in Fig. 1(a), STM is able to produce two-dimensional images of individual atoms of solid surfaces and map the local density of state (LDOS) on metallic surfaces. 4,517 An enormous amount of research and development has been conducted on STM in the last 40 years; today, STM is one of the premiere tools of materials science. An imortant challenge has been how to obtain a fine enough time resolution to map the dynamics of materials. 18 Thtemporal resolution of STM had been limited by the bandwidth of the microscope's electronics (typically kHz-MHz).As a way around this limitation, ultrashort optical pulses can be irradiated on the tunnel junction as an alternative to application of an electronic bias.This method has recently been used to modulate the I-V curves transiently and measure carrier and spin dynamics with femtosecond time resolution and nanoscale spatial resolution.[19][20][21][22][23][24][25][26] Meanwhile, the advances in the femtosecond laser have led to extensive development of spectroscopy techniques for studying various materials in the terahertz (THz) frequency region.The term "terahertz (THz) light" refers to electromagnetic waves with frequencies ranging from 0.1 to 10 THz, energies from 0.4 to 41 meV, wavelengths from 30 μm to 3 mm, and wavenumbers from 3.3 to 333 cm −1 .[27][28][29] Many elementary excitations that are distinct physical properties of solids exist in this frequency: for example, superconducting gap, 30,31 phonons, [32][33][34] spin resonances, [35][36][37] plasma frequencies, 38,39 electron binding energies of impurities, 40 and binding energies of excitons in semiconductors.41,42 This makes the THz frequency region very attractive from the viewpoint of materials science.
3][54][55][56][57] This modulation does not cause a lot of thermal energy, which would otherwise be a problem with the photoexcitation bias method.In this Perspective, we review recent progress in research and development of STM based on THz pulses.

II. FEMTOSECOND-RESOLVED DYNAMICAL IMAGING BY SCANNING TUNNELING MICROSCOPY
The energy diagram of the tunneling junction with a gap of length w under a positive bias voltage V is shown in Fig. 1(b).In this model, called the Fowler-Nordheim model, 58,59 electrons in the tip that are in energy states of the Fermi level tunnel through the barrier of height ϕ to the unoccupied energy states of the sample.Quantum mechanics tells us that the tunnel current I strongly depends on the gap length w and applied bias V: I(V) ∝ exp(−Cw) [Fig.1(b)].The tunnel current changes exponentially with respect to the gap width w under a constant bias voltage V.The factor C in the exponential term is typically ∼10 nm −1 , so the tunneling current varies drastically when the gap is changed by atomic-size distance.This is the fundamental principle behind STM's ability to resolve minute asperities of surfaces.This dependency is also the basis for STM's ultrahigh lateral resolution because the tunneling current is concentrated on the atom closest to the tip.
In THz-STM, the bias applied to the tunnel junction is simply from a THz pulse [Fig.1(a)] rather than a DC voltage.The voltage arising from the irradiation varies with time [Fig.1(c)].The strong enhancement and spatial confinement of the THz electric field just underneath the metal tip allow the THz irradiation to be used as a local transient bias though the spot size of the THz pulse is much larger than the metal tip.In addition, because of the nonlinearity of the bias-current (I-V) relation of the tunnel junction, such a pulse causes the tip to emit a bunch of electrons that appear as a tunneling current I when the electric field reaches around the peak electric field.Thus, the temporal duration of a bunch of electrons becomes less than that of the temporal duration of the THz pulse and can reach a few hundred femtoseconds.If the applied THz pulses are single cycle or multi-cycle, the net tunnel current will be zero or very small.This is not a suitable situation for a measurement because the injected electrons generate a lot of heat that distorts the state of the sample even though the detected electric signal from these electrons may be very small.On the other hand, an asymmetric temporal shape of the THz pulse can generate a tunneling current in one direction of the electric field; because the tunneling current is unidirectional, almost all of the electrons contribute to the electrical signal.In fact, a (quasi) half-cycle THz pulse is an ideal bias for ultrafast time-resolved STM. 45he principle of time-resolved measurements of THz-STM is the same as in pump-probe spectroscopy.A temporally separated pair of pulses that is a combination of THz-THz 20,22,23 or THz-optical pulses 20,26 is focused on the tip-sample gap.In both cases, the tunnel current induced by THz pulse excitation is monitored as a function of the temporal separation τ between the pulses, which is varied by using an optical delay line.The THz pulse induces a tunnel current that is used as a probe of the response of the material to the optical (or THz) pump pulse.The spatial resolution of THz-STM is determined in the same way as static STM.An atomic level of resolution can be realized by appropriately setting the gap width, bias voltage, and sharpness of the apex of the tip under ultralow temperature and ultrahigh vacuum conditions.

III. DEMONSTRATIONS OF THz-STM
THz-STM was first reported by Cocker et al. in 2013, where it was used for imaging of highly ordered pyrolytic graphite (HOPG) and semiconductor nanodots under ambient conditions. 20The nonlinear characteristic of the current-voltage relation (I-V curve) in the tunnel junction and bias that is the sum of the electric field of the THz pulse and DC component made an asymmetric directional tunneling current.Therefore, the THz-induced ultrashort pulsed The contrast of the THz-STM image shown in Fig. 3(a) could be explained by a model of the tunnel junction including band bending, screening, and non-equilibrium carrier occupation.A key point in this model is that the electric field of a THz pulse causes the electric band to bend across the subsurface depletion region, resulting in a shift of the surface state whose Fermi level is temporarily shifted from the bulk Fermi level.As a result, a new tunneling path through the surface states appears, and it transiently enhances the tunnel current.
As an example, let us consider a situation in which a bipolar electrical pulse with an asymmetric electric field strength is applied to the tunnel junction.The positive half cycle of the THz pulse, i.e., the first half of the pulse shown in Fig. 3(b), causes ultrafast charging of surface states by electrons tunneling from the tip.This, in turn, causes the band to bend upward and shift the surface states upward.Therefore, electrons tunnel from the tip to unoccupied states of the surface concurrently with electrons tunneling from the surface states into the unoccupied bulk states [Fig.3(c)].In the negative half cycle, i.e., the last half of the pulse shown in Fig. 3(b), ultrafast electron depletion of the surface states occurs until the transport of electrons from the bulk into the depletion region recovers from the band bending.During the electron depletion of the surface states, electrons on the surface tunnel to the tip and electrons in bulk move to the surface [Fig.3(d)].This electron flow creates more tunneling current than in the steady state because the electrons lost by the surface states due to the tunneling into the tip are continuously replenished from the bulk.
The experimental tunneling junction modulated by a picosecond transient electric field of the THz pulse was well described by a model shown above that includes the bending of the electrical band and cascading electron flow via surface states.This means that THz-STM works in the same way as static STM, even though the tunneling current induced by the THz pulse forms an ultrashort bunch of electrons.The experimental results and their interpretation showed that THz-STM is a versatile inspection apparatus with subpicosecond temporal resolution and atomic-scale spatial resolution that can be applied to various interesting materials.

B. Ultrafast carrier dynamics in C 60 films
Spatiotemporal imaging of the diffusion dynamics of carriers was demonstrated using a C 60 multilayer film on an Au (111) substrate. 26The energy diagram of the sample and tunneling junction is shown in Fig. 4(a).Without optical excitation under a negative bias condition (dark condition), the tunneling current to the tip comes from the highest occupied molecular orbital (HOMO) of C 60 that is below the Fermi surface of the Au substrate.The optical pulse (1064-nm wavelength) excites electrons in the tip and Au substrate,

FIG. 1 .
FIG. 1. Schematic diagram of tunnel junction (a) of STM biased static voltage (left half) and THz-STM (right half).(b) A potential barrier of height ϕ across an insulating layer (vacuum) of width w between the tip and sample forms the tunnel junction.The dashed line indicates the actual potential under a constant bias V. (c) The electric field accompanying the THz pulse transiently modulates the tunneling junction.
FIG. 3. Constant-height STM images of Si(111)-(7 × 7) at a THz-pulse peak elecfield of −200 V/cm (a) without DC bias.Image size 9 × 9 nm 2 ; a 7 × 7 unit cell is shown in the red rhombus.(b) Waveform of THz pulse.(c) and (d) Band diagrams and electron flow for the first half (c) and last half (d) of the THz pulse shown in (b).The figures are reproduced from Ref. 23 with permission for the reuse of Springer Nature content.

FIG. 5 .
FIG. 5. (a) Molecular structure of pentacene.The THz-STM image of an isolated pentacene molecule with a maximum THz voltage set to −2.05 V (b) and +1.3 V (c).HOMO and LUMO of free pentacene simulated by DFT are illustrated at the bottom.(d) Measurement of a single pentacene molecule's dynamics in a pump and probe experiment.The tunneling current due to the second pulse (probe pulse) shows coherent oscillations as a function of delay time τ.(e) Schematic diagram of the pump and probe experiment.The first THz pulse excites a vertical vibration of the pentacene molecule by removing an electron from HOMO.The second THz pulse detects the temporal change in the heights of the molecules.The figures are reproduced from Ref. 22 with permission for the reuse of Springer Nature content.