Terahertz Scanning Tunneling Research Articles

The development of a low-temperature terahertz scanning tunneling microscope based on a cryogen-free scheme.

We have developed a cryogen-free, low-temperature terahertz scanning tunneling microscope (THz-STM). This system utilizes a continuous-flow cryogen-free cooler to achieve low temperatures of ∼25K. Meanwhile, an ultra-small ultra-high vacuum chamber results in the reduction of the distance from sample to viewport to only 4cm. NA = 0.6 can be achieved while placing the entire optical component, including a large parabolic mirror, outside the vacuum chamber. Thus, the convenience of optical coupling is much improved without compromising the performance of STM. Based on this, we introduced THz pulses into the tunnel junction and constructed the THz-STM, achieving atomic-level spatial resolution in THz-driven current imaging and sub-picosecond (sub-ps) time resolution in autocorrelation signals during pump-probe measurements. Experimental data from various representative samples are presented to showcase the performance of the instrument, establishing it as an ideal platform for studying non-equilibrium dynamic processes at nanoscale.

Electrical Manipulation of Quantum Coherence in a Two-Level Molecular System.

We report the manipulation of ultrafast quantum coherence of a two-level single hydrogen molecular system by employing static electric field from the sample bias in a femtosecond terahertz scanning tunneling microscope. A H_{2} molecule adsorbed on the polar Cu_{2}N surface develops an electric dipole and exhibits a giant Stark effect. An avoided crossing of the quantum state energy levels is derived from the resonant frequency of the single H_{2} two levels in a double-well potential. The dephasing time of the initial wave packet can also be changed by applying the electric field. The electrical manipulation for different tunneling gaps in three dimensions allows quantification of the surface electrostatic fields at the atomic scale. Our work demonstrated the potential application of molecules as controllable two-level molecular systems.

Scaling of Time-Dependent Tunneling Current in Terahertz Scanning Tunneling Microscopes

Terahertz scanning tunneling microscopy (THz-STM) has enabled spatiotemporal imaging with femtosecond temporal resolution and angstrom spatial resolution. In THz-STMs, the junction bias is modulated by coupling THz pulses, which results in transient voltage bias and extremely high transient tunneling currents. In order to have efficient imaging of the sample surface, it is important to understand the nonlinear tunneling current response and its parametric dependence. In this work, we theoretically investigate the basic scaling of rectified electrons in a THz-STM junction. We use a self-consistent quantum model that includes both space-charge potential and exchange-correlation potential, which were ignored in previous studies. Since THz-STMs are operated at high transient voltage in field emission regime, the effects of exchange-correlation potential become crucial. We validate our calculation with recently reported experimental data and investigate the rectification property of the tip-sample junction for different parameters. We find that the time-dependent tunneling current and the electron transport can be manipulated by varying the dc bias voltage (polarity, amplitude), incident THz field (polarity, shape, peak amplitude), work functions of STM tip and sample---especially their difference $\mathrm{\ensuremath{\Delta}}W$, and the tip-sample separation. Our study provides an important framework that can be used in the future to characterize, control, and improve THz-induced currents and probing techniques at the nanometer scale over subpicosecond time periods.

Algorithm for subcycle terahertz scanning tunneling spectroscopy

Terahertz scanning tunneling microscopy (THz-STM) enables ultrafast measurements of surfaces, single molecules, and nanostructures with simultaneous subpicosecond temporal resolution and atomic spatial resolution. In pump-probe THz-STM experiments employing femtosecond optical pump pulses, lightwave-driven tunneling by a time-delayed THz probe pulse accesses the evolving differential conductance of the tunnel junction following photoexcitation. However, a general theoretical approach to extract the time- and voltage-dependent differential conductance from THz-STM measurements is lacking. Here, we introduce an algorithm for pump-probe THz scanning tunneling spectroscopy (THz-STS) analysis. Our approach allows us to reliably reconstruct the tunnel junction's differential conductance in steady-state or time-dependent scenarios from simulated THz-STS data. The algorithm achieves subcycle time resolution, which we demonstrate by retrieving dynamics faster than the bandwidth of the input THz voltage transient. Subcycle THz-STS will make lightwave-driven microscopy yet more powerful as a tool for characterizing \aa{}ngstr\"om-scale ultrafast dynamics in novel materials.

Nanoscale terahertz STM imaging of a metal surface

Terahertz scanning tunneling microscopy (THz-STM) has enabled studies of ultrafast dynamics in materials down to the atomic scale. However, despite recent advances, more work is needed to better understand and quantify the subpicosecond THz pulse-induced tunnel currents and corresponding THz-STM images of nanoscale features on surfaces. Here, we perform THz-STM on a metal surface and fully characterize the observed THz pulse-induced tunnel current and nanoscale imaging at atomic steps and defects using a Bardeen tunneling model in a three-dimensional (3D) tip geometry. We show that the measured steady-state STM current-voltage curves can be used in our model to accurately map the observed ultrafast THz-induced tunnel currents and calibrate the magnitude of the near-field peak transient THz voltage bias in the tunnel junction. Peak THz voltage bias transients greater than 10 V across the STM junction are achieved leading to field emission of subpicosecond tunnel currents with current densities exceeding ${10}^{9}\phantom{\rule{0.28em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}^{2}$ in THz-STM imaging of a Cu(111) surface. Our results establish an important benchmark for future studies in THz-STM by quantifying the ultrafast THz-induced currents and bias voltages in the tunnel junction and providing a 3D tunneling model for understanding and accurately simulating THz-STM images of nanoscale features on metal surfaces.

Sub-cycle Manipulation of Electrons in a Tunnel Junction with Phase-controlled Single-cycle THz Near-fields

By utilizing terahertz scanning tunneling microscopy (THz-STM) with a carrier envelope phase shifter for broadband THz pulses, we could successfully control the near-field-mediated electron dynamics in a tunnel junction with sub-cycle precision. Measurements of the phase-resolved sub-cycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms.

Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields.

Light-field-driven processes occurring under conditions far beyond the diffraction limit of the light can be manipulated by harnessing spatiotemporally tunable near fields. A tailor-made carrier envelope phase in a tunnel junction formed between nanogap electrodes allows precisely controlled manipulation of these processes. In particular, the characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the atomic-scale. Here, we demonstrate that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy (THz-STM) with a phase shifter. Measurements of the phase-resolved subcycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms. The phase shift stems from the wavelength-scale feature of the tip-sample configuration. By using a dual-phase double-pulse scheme, the electron tunneling was coherently manipulated over the femtosecond time scale. Our new prescription-in situ tailoring of single-cycle THz near fields in a tunnel junction-will offer unprecedented control of electrons for ultrafast atomic-scale electronics and metrology.