Ultrafast terahertz microscopy: from near fields to single atoms

A new experimental frontier has recently emerged with the potential to significantly impact
physics, chemistry, materials science, and biology: the regime of ultrafast time resolution and ultrasmall spatial resolution. This is the domain in which single atoms, molecules, and electronic orbitals move. It also corresponds, on larger length scales, to the territory of low-energy elementary excitations such as plasmons, phonons, and interlevel transitions in excitons. These processes are of particular importance for nanomaterial functionality and typically survive for only femtoseconds to picoseconds after photoexcitation. In this talk, I will show how these diverse dynamics can be studied with new techniques that combine terahertz technology with scanning probe microscopy. First, I will describe how ultrafast near-field microscopy has been employed to perform sub-cycle spectroscopy of single
nanoparticles [1], reveal hidden structure in correlated electron systems [2], and resolve transient interface polaritons in van der Waals heterostructures [3]. Then I will discuss the development of a related technique: lightwave-driven terahertz scanning tunneling microscopy [4,5]. In this novel approach, the oscillating electric field of a phase-stable, few-cycle light pulse at an atomically sharp tip can be used to remove a single electron from a single molecular orbital within a time window faster than an oscillation cycle of the terahertz wave. I will show how this technique has been used to take ultrafast snapshot images of the electron density in single molecular orbitals (e.g. Figure 1) and watch the motion of a single molecule for the first time [5].

Figure 1: Ultrafast snapshot of the electron density in the highest molecular orbital of a single pentacene molecule resolved with lightwave-driven terahertz scanning tunneling microscopy [5].

References:
[1] M. Eisele et al., Nature Photon. 8. 841 (2014).
[2] M. A. Huber et al., Nano Lett. 16, 1421 (2016).
[3] M. A. Huber et al., Nature Nanotech. 12, 207 (2017).
[4] T. L. Cocker et al., Nature Photon. 7, 620 (2013).
[5] T. L. Cocker et al., Nature 539, 263 (2016).

Bright Lights, Big Opportunities for Quantum Materials Research at NSLS-II

Abstract: The realization of quantum materials for energy science and quantum information applications requires an understanding of competing electronic phases spanning multiple scales of energy, time and length. Operating since 2014, the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory has become a nexus for X-ray based studies of the electronic properties to advance knowledge of these key issues. Following an overview of NSLS-II and its beamlines tailored for quantum materials research, this talk will review the capabilities and current status of the soft resonant inelastic X-ray scattering (RIXS) beamline, called SIX. Recent research examples involving transition-metal oxides and 4f Kondo systems will be showcased.

Condensed Matter Physics and Materials Science Division, Brookhaven National Laboratory, Upton, New York 11973

Abstract

Low energy, laser-based ARPES with variable light polarization, including both linear and circularly polarized, is used to examine the Fe-based superconductor family, FeTe_1-xSe_x.  At the center of the Brillouin zone we observe the presence of a Dirac cones with helical spin structure as expected for a topological surface state and as previously reported in the related FeTe_0.55Se_0.45.  These experimental studies are compared with theoretical studies that take account of the disordered local magnetic moments related to the paramagnetism observed in this system.  Indeed including the magnetic contributions in the theoretical description is necessary to bring the chemical potential of the calculated electronic band structure into alignment with the experimental observations.  In the bulk superconducting state for FeTe_0.7Se_0.3 the system appears to reflect the presence of some level of orbital selectivity in the pairing even though the system is in the tetragonal phase above and below the transition temperature T_c.  At the same time the topological state appears to acquire mass at the superconducting transition, possibly indicative of time reversal symmetry breaking.  These observations are discussed in detail.