An ultrafast terahertz scanning tunnelling microscope

The ability to directly image ultrafast phenomena with nanometer spatial resolution is essential to our understanding of local excitation dynamics in nanomaterials and devices. We have developed a new approach to ultrafast scanning tunneling microscopy (STM) that couples terahertz (THz) pulses to the scanning tip of a STM. We have used THz-STM under ambient lab conditions to image ultrafast charging dynamics of a single InAs nanodot on GaAs with 0.5 ps time resolution and 2 nm spatial resolution. We are currently developing THz-STM for operation in ultrahigh vacuum with the goal of imaging ultrafast dynamics on surfaces with atomic resolution.

An ultrafast terahertz (THz) pulse coupled to the tunnel junction of a scanning tunnelling microscope (STM) is found to produce a subpicosecond rectified current pulse that can be detected by the STM electronics. We use these THz-induced current pulses to develop a new type of ultrafast STM and demonstrate its unprecedented simultaneous 2 nm spatial resolution and 500 fs temporal resolution under ambient laboratory conditions [1]. The terahertz-pulse-coupled STM (THz-STM) is used to image the ultrafast capture of photoexcited carriers into a single InAs nanodot to show its unique capabilities (Fig. 1).

We detail a new ultrafast scanning tunneling microscopy technique called THz-STM that uses terahertz (THz) pulses coupled to the tip of a scanning tunneling microscope (STM) to directly modulate the STM bias voltage over subpicosecond time scales [1]. In doing so, THz-STM achieves ultrafast time resolution via a mode complementary to normal STM operation, thus providing a general ultrafast probe for stroboscopic pump-probe measurements. We use THz-STM to image ultrafast carrier trapping into a single InAs nanodot and demonstrate simultaneous nanometer (2 nm) spatial resolution and subpicosecond (500 fs) temporal resolution in ambient conditions. Extending THz-STM to vacuum and low temperature operation has the potential to enable studies of a wide variety of subpicosecond dynamics on materials with atomic resolution.

We couple terahertz (THz) pulses to the tip of a scanning tunneling microscope (STM) and observe THz-pulse-induced tunneling in an STM, enabling a new ultrafast technique called THz-STM that allows for direct imaging of sub-picosecond dynamics on surfaces with nanometer spatial resolution. Imaging of sub-picosecond carrier capture dynamics into a single semiconductor nanodot is demonstrated. The potential of THz-STM for imaging ultrafast phenomena with atomic resolution is discussed.

Exciton dynamics is responsible for the photophysical and photochemical functions, such as energy harvesting, light emission, and photosynthesis. In particular, energy conversions, transfers, and dissipations at interfaces and nanostructures govern the dynamics of excitons. Nevertheless, the spatial resolutions of conventional optical methods are limited to several hundred nanometers due to the diffraction limit of light. Luminescence measurements combined with scanning tunneling microscopy (STM) enable to investigate optical properties with sub-nanometer spatial resolution in various systems such as metals[1], semiconductors[2], 2D materials[3], and molecules[4-5]. In this method, the scanning tunneling luminescence (STL) spectroscopy, photons induced by the tunneling current of STM are detected. Since the tunneling current can be controlled with atomic spatial resolution, it is possible to investigate optical phenomena in localized regions. While STL spectroscopy is a powerful tool for investigating exciton dynamics with atomic-scale spatial resolution, the time resolution of STL spectroscopy is limited because it uses “steady-state” tunneling current for the excitation. Therefore, this method cannot be applicable to track the ultrafast exciton dynamics in real-time.For overcoming the limitation of time resolution of STM, a single-cycle terahertz (THz) electric field has recently been installed to drive tunneling electrons[6-9]. By coupling the THz pulse to an atomically sharp metal tip of STM, the electric field is confined to the tip, and it enables to manipulate the tunneling electrons with extremely high spatiotemporal resolution. This method, THz-STM, has been utilized to track the ultrafast carrier dynamics[6-8] and motion of a molecule[9] with sub-picosecond time resolution. In this work, we combined a photon detection system with THz-STM to establish THz-STL spectroscopy (Fig. 1a). We measured visible photon emission from the radiative decay of a localized plasmon as the first demonstration of THz-STL spectroscopy[10]. We anticipate that the THz-STL spectroscopy will open new avenues for “real-time” and “real-space” investigations of exciton dynamics in near future.In this work, we generated single-cycle THz pulses via optical rectification of laser pulses from a Yb fiber laser in a LiNbO3 prism using a tilted-pulse-front configuration (Fig. 1b). The generated THz pulses were guided into a low-temperature STM and focused to the STM junction (Fig. 1a). Figure 1c shows a current trace on Ag(111) surface, where the current induced by the THz pulses was measured as 2.65 pA. Based on the simulation of electron tunneling probability, the maximum magnitude of the voltage applied by the THz pulse was estimated to be ~6.5 V. Figure 1d shows STL spectra when the THz pulse was either introduced to the STM (red) or blocked (grey). Whereas no peak is seen in the grey spectrum, a broad peak ranging from 1.3 eV to 2.3 eV appears in the red spectrum, which is originated from the radiative decay of a plasmon localized in the gap.In this presentation, we would like to discuss the detailed mechanism of plasmon excitation by THz-field-driven electrons as well as the challenges of applying THz-STL to the real-time investigation of exciton dynamics in a molecule.[1] R. Berndt et al., Phys. Rev. Lett. 67, 3796 (1991). [2] J. K. Gimzewski et al., Z. Phys. B Condensed Matter 72, 497 (1988). [3] Schuler et al., Sci. Adv. 6, eabb5988 (2020). [4] H. Imada et al., Nature 538, 364 (2016). [5] K. Kimura et al., Nature 570, 210 (2019). [6] T. L. Cocker et al., Nat. Photon. 7, 620 (2013). [7] K. Yoshioka et al., Nat. Photon. 10, 762 (2016). [8] V. Jelic et al., Nature Physics 13, 591 (2017). [9] D. Peller et al., Nature 585, 58 (2020). [10] K. Kimura et al., ACS photon. 8, 982 (2021). Figure 1

Exciton dynamics governs versatile functions of molecules such as photon emission, photovoltaic and photo chemical reactions. For clarifying the dynamics of excitons in a molecule, an optical measurement with high spatiotemporal resolution is required because the excitons have short lifetimes and their behavior in the local region is important for optical properties. Luminescence spectroscopy based on a scanning tunneling microscope (STM) enable to investigate optical properties with sub-nanometer spatial resolution[1-2]. In this method, scanning tunneling luminescence (STL) spectroscopy, a molecule is excited by the tunneling current of an STM, which can be controlled with atomic spatial resolution. In contrast, the time resolution of STL spectroscopy is limited, because the tunnel current is controlled by electrical circuits. Therefore, this method cannot be used to track the picosecond dynamics of excitons. For overcoming the time-resolution of STL, we have combined a single-cycle terahertz (THz) pulse with STL spectroscopy. By coupling the THz pulse to a tip of an STM, the photoelectric field of THz pulse with sub picosecond time width is confined to the tip apex, which enables to manipulate the tunneling electrons with high spatiotemporal resolutions[4-9]. In this work, we succeeded in forming an exciton in a single molecule by using THz-field-driven current and clarifying the mechanism of the exciton formation.Single-cycle THz pulses were generated by using optical rectification of near infrared (NIR) laser pulses from a Yb fiber laser using a tilted-pulse-front configuration (Fig. 1a). The generated THz pulses were guided into the STM chamber and focused by using a lens near the tip. We prepared a clean Ag(111) surface and deposited NaCl films. Then, we deposited Pd phthalocyanine (PdPc) molecules on the NaCl/Ag(111) surface. Figure 1b shows THz-STL spectra of a PdPc on NaCl/Ag(111). Several sharp peaks appeared at around 1.9 eV in the red spectrum (with THz irradiation) and no peak appeared in the gray spectrum (without THz). The peak energy of the main peak is identical to the fluorescence peak of a PdPc measured by conventional STL[3], thus we concluded that the singlet excited state was formed by THz-field-driven tunneling. In this presentation, we would like to discuss the detailed mechanism for the exciton formation in a molecule by THz-field-driven electrons.[1] H. Imada et al., Nature 538, 364 (2016). [2] K. Kimura et al., Nature 570, 210 (2019). [3] S. Cao et al., Nat. Chem. 13, 766 (2021). [4] T. L. Cocker et al., Nat. Photon. 7, 620 (2013). [5] K. Yoshioka et al., Nat. Photon. 10, 762 (2016). [6] V. Jelic et al., Nature Physics 13, 591 (2017). [7] D. Peller et al., Nature 585, 58 (2020). [8] L. Wang 376, 401 (2022). [9] K. Kimura et al., ACS photon. 8, 982 (2021). Figure 1

Coupling phase-stablesingle-cycle terahertz (THz) pulses to scanningtunneling microscope (STM) junctions enables spatiotemporal imagingwith femtosecond temporal and Ångstrom spatial resolution. Thetime resolution achieved in such THz-gated STM is ultimately limitedby the subcycle temporal variation of the tip-enhanced THz field actingas an ultrafast voltage pulse, and hence by the ability to feed high-frequency,broadband THz pulses into the junction. Here, we report on the couplingof ultrabroadband (1–30 THz) single-cycle THz pulses from aspintronic THz emitter (STE) into a metallic STM junction. We demonstratebroadband phase-resolved detection of the THz voltage transient directlyin the STM junction via THz-field-induced modulation of ultrafastphotocurrents. Comparison to the unperturbed far-field THz waveformreveals the antenna response of the STM tip. Despite tip-induced low-passfiltering, frequencies up to 15 THz can be detected in the tip-enhancednear-field, resulting in THz transients with a half-cycle period of115 fs. We further demonstrate simple polarity control of the THzbias via the STE magnetization and show that up to 2 V THz bias at1 MHz repetition rate can be achieved in the current setup. Finally,we find a nearly constant THz voltage and waveform over a wide rangeof tip–sample distances, which by comparison to numerical simulationsconfirms the quasi-static nature of the THz pulses. Our results demonstratethe suitability of spintronic THz emitters for ultrafast THz-STM withunprecedented bandwidth of the THz bias and provide insight into thefemtosecond response of defined nanoscale junctions.

Recently, (1 0 0) highly textured diamond thin films were successfully synthesized on Si(1 0 0) [1], but complete heteroepitaxial diamond films have not yet been synthesized. This is because the mechanisms of nucleation and growth of diamond by chemical vapour deposition (CVD) are not fully understood. To clarify the mechanism of the nucleation process at the various nucleation sites, continuous observation of the nucleation and growth process in the same area, ideally in situ observation, is required. Scanning tunnelling microscopy (STM) is one of the candidates for this purpose, since STM uses a microscope with atomic resolution which can be operated in various environments, even in CVD conditions. Moreover, STM can also be used for atomic scale spectroscopy (scanning tunnelling spectroscopy) [2] and for the fabrication of nanometre scale structures [3]. For these reasons, STM is expected to be applied not only to the observation of the nucleation process but also to the fabrication of artificial nucleation sites. STM observations of nucleation in diamond CVD have been reported [4, 5]; however, roughened Si wafers scratched by diamond powder have been used exclusively as the substrates. In addition, different areas were observed before and after the depositions. Therefore, it has been impossible to correlate the nucleation process and nucleation sites so far, although information about growth on the nanometre scale have been reported. In this letter, we report the observation of diamond nucleation in a hot-filament assisted CVD process at the nanometre scale holes fabricated artificially on highly orientated pyrolytic graphite (HOPG) by STM. Fig. 1 shows a schematic diagram of the apparatus used in this study. A commercial STM (NanoScope II, Digital Instruments Inc.) was in the STM part, and the deposition was performed by a hot-filament assisted method in the CVD part. To transfer a substrate from the STM part to the CVD part, and vice versa, an Invar rod fixed on the end of a micrometre head was used with a guiding rail and a stopper. The deviation of the substrate position before and after transferring the substrate was less than 3 im, which was enough to access the same area of the substrate surface by the STM. To minimize the deviation of the position, the distance between the STM part and the CVD part should be as short as possible. However, the piezoelectric elements of STM do not function well in a high temperature environment, so they were separated by 10 mm with a thermal shield mounted between them. Deposition time was controlled by a shutter inserted between the filament and the substrate. All of the STM studies were carried out in the constant-current mode at room temperature after exposure to air with etched Pt–Ir tips (Materials Analytical Services). The tunnelling current (It) and the bias voltage (Vb) were 0.5 nA and 100 mV, respectively. HOPG was used as a substrate, since large atomically flat and inert surfaces can be obtained easily by cleaving. However, nucleation of diamond hardly occurs on the as-cleaved (0 0 0 1) graphite surface [6]. Prior to deposition in this study, therefore, 3 3 3 holes at intervals of 200 nm were fabricated on an HOPG substrate by STM in air by raising Vb to 4 V with It ˆ 0.5 nA for 1 s, with the aim of forming preferential nucleation sites [7]. Fig. 2a shows an STM image of the nine holes, of about 40 nm diameter. After transferring the substrate to the CVD part, the chamber was evacuated by a rotary pump, and then CVD was performed on the substrate without auxiliary heating. The experimental conditions were as follows: gases: 2% CH4 in H2 (CH4 1 sccm and H2 49 sccm); total pressure: 6.7 3 103 Pa (50 torr); Ta filament temperature: 2100 8C (measured by an optical pyrometer); fila-

The fundamental understanding of quantum dynamics in advanced materials requires precise characterization at the limit of spatiotemporal resolution. Ultrafast scanning tunneling microscopy is a powerful tool combining the benefits of picosecond time resolution provided by single-cycle terahertz (THz) pulses and atomic spatial resolution of a scanning tunneling microscope (STM). For the selective excitation of localized electronic states, the transient field profile must be tailored to the energetic structure of the system. Here, we present an advanced THz-STM setup combining multi-MHz repetition rates, strong THz near fields, and continuous carrier-envelope phase (CEP) control of the transient waveform. In particular, we employ frustrated total internal reflection as an efficient and cost-effective method for precise CEP control of single-cycle THz pulses with >60% field transmissivity, high pointing stability, and continuous phase shifting of up to 0.75 π in the far and near field. Efficient THz generation and dispersion management enable peak THz voltages at the tip-sample junction exceeding 20 V at 1 MHz and 1 V at 41 MHz. The system comprises two distinct THz generation arms, which facilitate individual pulse shaping and amplitude modulation. This unique feature enables the flexible implementation of various THz pump-probe schemes, thereby facilitating the study of electronic and excitonic excited-state propagation in nanostructures and low-dimensional materials systems. Scalability of the repetition rate up to 41 MHz, combined with a state-of-the-art low-temperature STM, paves the way toward the investigation of dynamical processes in atomic quantum systems at their native length and time scales.

Efficient operation of electronic nanodevices at ultrafastspeedsrequires understanding and control of the currents generated by femtosecondbursts of light. Ultrafast laser-induced currents in metallic nanojunctionscan originate from photoassisted hot electron tunneling or lightwave-inducedtunneling. Both processes can drive localized photocurrents insidea scanning tunneling microscope (STM) on femto- to attosecond timescales, enabling ultrafast STM with atomic spatial resolution. Femtosecondlaser excitation of a metallic nanojunction, however, also leads tothe formation of a transient thermalized electron distribution, butthe tunneling of thermalized hot electrons on time scales faster thanelectron–lattice equilibration is not well understood. Here,we investigate ultrafast electronic heating and transient thermionictunneling inside a metallic photoexcited tunnel junction and its rolein the generation of ultrafast photocurrents in STM. Phase-resolvedsampling of broadband terahertz (THz) pulses via the THz-field-inducedmodulation of ultrafast photocurrents allows us to probe the electronictemperature evolution inside the STM tip and to observe the competitionbetween instantaneous and delayed tunneling due to nonthermal andthermal hot electron distributions in real time. Our results revealthe pronounced nonthermal character of photoinduced hot electron tunnelingand provide a detailed microscopic understanding of hot electron dynamicsinside a laser-excited tunnel junction.

A walking, concentric-tube STM has been designed to interface with an F a . The outer tube, which stands upright on 3 spherical feet, supports the inner tube and serves as an inertial stepper, allowing the STM to move laterally relative to a sample. The ion can pass down through the center of the inner tube, which operates as a conventional tube scanner with the STM tip at its lower end. The STM walks distances of mm with pm-scale resolution, resolves the pyrolytic graphite lattice, and can image regions of up to 3.5 x 3.5 ym. Introduction The scanning tunneling microscope (STM) and the focused ion (FIB) are both versatile instruments. The FIB has found many uses as a tool for micr~fabrication.~ Its small diameter enables it to perform micron-scale modifications, and its microscopic capability can be used to locate features of interest. The STM is known primarily for its atomic resolution images of s ~ r f a c e s , ~ but it is also attracting increasing attention as a possible tool for nanofabrication.3 We have designed an STM to interface with a FIB in a system that will permit us to image regions ranging in size from mm2 to nm2, and to induce modifications at the pm and nm levels. STM AND NANOFABRICATION The ultimate goal of nanofabrication is to be able to arrange individual atoms as desired. Few tools can address a region of only nanometers in extent, let alone precipitate a controlled response within that region. Primary among the tools suggested for nanofabrication are lens-focused electron beams, which are routinely formed into subnanometer probes.4 The diameter of the is not a true measure of the area that is influenced, however, since energy is dissipated throughout a larger region (a manifestation of this is the effect witnessed in electron-beam lithography). It is difficult to conceive how electrons with energies in the keV regime could be used to controllably influence the actions of an atom, or small cluster of atoms. The STM beam is focused by proximity; that is, the source is so close to the target that the electrons cannot diverge to any great degree during transit between the two. The most important attribute of proximity focusing is that the electrons can have much lower energies than those in available lens-focused beams of comparable diameter. Low particle energy is of critical concern when it is considered that many of the events that are of interest for nanofabrication (e.g. migration, bond breaking, chemical reactions) have activation energies of less than ten electron volts per atom.5 The STM is the only instrument that can presently provide a sub-nanometer of such lowenergy particles. Another method by which an STM might exert influence at the nanometer scale is by direct interatomic force. The atomic force microscope (AFM) has exhibited the ability to convert detected changes in the interaction forces between a sharp tip and a substrate into sub-nanometer resolution images.6 If the region of interaction is assumed to be comparable in size to the spatial resolution, then it should be possible to exert forces on nanometer-scale regions using the sharp tip of an STM. This direct-contact mode may prove to be useful for nanomanipulation, perhaps to push atoms around. The STM may be able to exert human influence upon a single atom? but before it becomes possible to arrange atoms at will, the practical problems involved in moving small particles from point A to point B must first be addressed. We plan to study the use of STM for nanomanipulation-to investigate the influence that the STM might exert over nanometer-scale atom clusters.

The carrier-envelope phase (CEP) of terahertz (THz) pulses is a critical parameter in ultrafast THz science, as it directly governs the temporal waveform and peak field strength of the pulse-quantities that are particularly important in THz scanning tunneling microscopy, where the pulse shape determines the transient potential at the nanotip, and in nonlinear THz experiments that require reproducible peak fields. Here, we report the demonstration of progressive CEP control achieved through pure near-field reflection by simply tuning the electrode width of a photoconductive antenna (PCA), without the need for complex metasurfaces or additional optical components in the beam path. By varying the electrode width, a frequency-independent universal phase shift is induced through near-field reflection of THz waves from the electrode surface, enabling a progressive transformation of the THz waveform from half-cycle (symmetric) to single-cycle (antisymmetric). Fourier analysis confirms that the relative phase between carrier waves is tunable up to 0.3π and remains consistent across the 0.2-2 THz frequency range. Crucially, both half-cycle and single-cycle pulses maintain nearly identical peak amplitudes, overcoming a key limitation of prior CEP control methods. The near-field reflection origin of the phase shift is corroborated by finite-difference time-domain (FDTD) simulations and independently confirmed by a control experiment using metallic reflectors physically separated from the active electrode gap, which reproduces the same waveform evolution. Our approach requires no complex fabrication beyond standard photolithography, making it directly compatible with existing THz time-domain spectroscopy systems and offering a practical route toward waveform-tailored THz sources for ultrafast spectroscopy, THz-STM, and nonlinear THz studies.

Phenomena on the nanometer scale are dominated by quantum effects. They can be visualized in a fascinating way either by the scanning tunneling microscope (STM), which itself is based on a quantum‐mechanical effect, electron tunneling, or by related scanning probe methods. These so‐called near‐field microscopies allow the direct observation of the wave nature of electrons in solids and the determination of their effective mass as well as their spin state. Atomic resolution is obtained by STM on an energy scale which is a million times smaller than that of X‐rays. Therefore, quantum effects in solids, which usually occur on energy scales of several millielectronvolts, become accessible while operating at high spatial resolution down to the atomic level. The detailed understanding of quantum phenomena at the nanometer scale will constitute the foundations of the nanotechnology age, which will be dominated by the digital control of matter at the level of single atoms and molecules, single electrons, as well as single spins.

Nano-scale studies of quantum phenomena by scanning probe spectroscopy

Phenomena on the nanometer scale are dominated by quantum effects. They can be visualized in a fascinating way either by the scanning tunneling microscope (STM), which itself is based on a quantum-mechanical effect, electron tunneling, or by related scanning probe methods. These so-called near-field microscopies allow the direct observation of the wave nature of electrons in solids and the determination of their effective mass as well as their spin state. Atomic resolution is obtained by STM on an energy scale which is a million times smaller than that of X-rays. Therefore, quantum effects in solids, which usually occur on energy scales of several millielectronvolts, become accessible while operating at high spatial resolution down to the atomic level. The detailed understanding of quantum phenomena at the nanometer scale will constitute the foundations of the nanotechnology age, which will be dominated by the digital control of matter at the level of single atoms and molecules, single electrons, as well as single spins.