We show how lightwave-driven terahertz scanning tunneling microscopy can be used as a platform for atomic-scale terahertz time-domain spectroscopy. We apply our new technique to silicon-vacancy centers at the surface of GaAs and discover a single-atom resonator with features reminiscent of the technologically important DX centers.
Terahertz scanning tunneling microscopy (THz-STM) is an emerging technique that provides simultaneous ultrafast temporal and Angstrom spatial resolution through lightwave control of an atomic tunnel junction. THz-STM can further accesses extreme tunneling regimes thanks to the low duty cycle and ultrafast duration of the THz bias. Here, we use low-temperature, ultrahigh vacuum THz-STM to explore the local density of electronic states in 7-atom-wide armchair graphene nanoribbons (7AGNRs) at tip heights that are inaccessible to conventional STM. Using a new procedure for THz scanning tunneling spectroscopy (THz-STS), we determine the differential conductance above the 7AGNR with atomic resolution in three dimensions.
Tip-based nanoscopy techniques have emerged as powerful tools for probing the exceptional optoelectronic properties of van der Waals crystals (vdW) on deeply sub-wavelength scales. Based on two sets of experiments, we demonstrate how bound electron–hole pairs – so-called excitons – can be interrogated with near-field microscopy. First, we build on terahertz nanoscopy with subcycle temporal resolution to access the separation of photo-carriers via interlayer tunneling and their subsequent recombination in transition metal dichalcogenide bilayers. By tracing the local polarizability of electron–hole pairs with evanescent terahertz fields, we reveal pronounced variations of the exciton dynamics on the nanoscale. This approach is uniquely suitable to reveal how ultrafast charge transfer processes shape functionalities in a variety of solid-state systems. Second, we image waveguide modes (WMs) in thin flakes of the biaxial vdW crystal ReS2 across a wide range of near-infrared frequencies. Resolving the dependence of the WM dispersion on the crystallographic direction, polarization of the electric field and sample thickness, enables us to quantify the anisotropic dielectric tensor of ReS2 including the elusive out-of-plane response. The excitonic absorption at ~1.5 eV induces a backbending of the dispersion and increased losses of the WMs as fully supported by numerical calculations. Thus, we provide crucial insights into the optical properties of ReS2 and explore light-matter coupling in layered, anisotropic waveguides. Our findings set the stage for probing ultrafast dynamics in biaxial vdW crystals on the nanoscale.
As conventional electronics approaches its ultimate limits, novel concepts of fast quantum control have been sought after. Lightwave electronics – the foundation of attosecond science – has opened a new arena by utilizing the oscillating carrier wave of intense light pulses to control electrons faster than a cycle of light. We employ atomically strong terahertz electromagnetic pulses to accelerate electrons through the entire Brillouin zone of solids, drive quasiparticle collisions, and generate high-harmonic radiation as well as high-order sidebands. The unique band structures of topological insulators allow for all-ballistic and quasi-relativistic acceleration of Dirac quasiparticles over distances as large as 0.5 μm. In monolayers of transition metal dichalcogenides, we switch the electrons’ valley pseudospin, opening the door to subcycle valleytronics. Finally, we show that lightwave electronics can be combined with ultimate atomic spatial resolution in state-selective ultrafast scanning tunneling microscopy.
Terahertz spectroscopy plays a key role in understanding ultrafast carrier dynamics in nanomaterials. Diffraction, however, limits time-resolved terahertz spectroscopy to ensemble measurements. By combining time-resolved terahertz spectroscopy in the multi-terahertz range with scattering-type near-field scanning optical microscopy, we show that we can directly trace ultrafast local carrier dynamics in single nanoparticles with sub-cycle temporal resolution (10 fs). Our microscope provides both 10 nm lateral resolution and tomographic sensitivity, allowing us to observe the ultrafast build-up of a local surface depletion layer in an InAs nanowire.
We have recently developed an ultrafast terahertz-pulse-coupled scanning tunneling microscope (THz-STM) that can
image nanoscale dynamics with simultaneous 0.5 ps temporal resolution and 2 nm spatial resolution under ambient
conditions. Broadband THz pulses that are focused onto the metallic tip of an STM induce sub-picosecond voltage
transients across the STM junction, producing a rectified current signal due to the nonlinear tunnel junction currentvoltage
(I-V) relationship. We use the Simmons model to simulate a tunnel junction I-V curve whereby a THz pulse
induces an ultrafast voltage transient, generating milliamp-level rectified currents over sub-picosecond timescales. The
nature of the ultrafast field emission tunneling regime achieved in the THz-STM is discussed.
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.
Nonlinear dynamics of free-carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using
intense few-cycle pulses. Techniques as Z-scan, THz-pump / THz-probe, and optical-pump/ THz-probe are employed to
explore nonlinear interactions in both n-doped and photoexcited systems. The physical mechanism that gives rise to such
interactions is found to be intervalley scattering.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.