Tyler L. Cocker

Controlling electronic quantum motion on subcycle and atomic scales

Atomically strong multi-terahertz waves drive novel subcycle quantum motions of electrons, generating high-harmonics, dynamical Bloch oscillations, quasiparticle collisions etc. Lightwave-driven scanning tunneling microscopy allows us to take the first femtosecond movie of a single-molecule orbital.

Ultrafast photo-activation of interface polaritons in black phosphorus heterostructures

Van der Waals layered materials, such as graphene, hexagonal boron nitride, and transition metal dichalcogenides, have redefined the perspectives of future ultra-compact electronics and optics on the nanoscale. Specifically, the possibility of hybridizing collective electronic motion with mid-infrared photons in so-called surface polaritons has allowed for extreme light confinement and represents a key ingredient for tailored nanophotonics [1].

Terahertz subcycle control: From high-harmonic generation to molecular snapshots

Atomically strong THz fields accelerate electrons in bulk semiconductors, new 2D materials, and atomically sharp tunneling junctions. By tracking this lightwave-driven charge transport with subcycle resolution, we explore dynamical Bloch oscillations as well as quasiparticle collisions and record the first single-molecule femtosecond movie.

Terahertz imaging with ultimate resolution

Summary form only given. Field-resolved detection of ultrafast pulses in the THz (0.1-10 THz) and multi-THz (10-100 THz) spectral range has provided key insights into the dynamics of low-energy collective excitations in condensed matter systems. However, the spatial resolution of these far-field studies is intrinsically limited to the scale of the probing wavelength by diffraction. Thus, the measured optical response is an average over sub-wavelength structures such as nanoparticles, nanocrystals, nanodomains, and microscopic defects. Apertureless scattering-type near-field scanning optical microscopy (s-NSOM) bypasses this fundamental limit by utilizing the strong confinement of the optical near-field at the apex of a sharp metal tip. We have combined ultra-sensitive, field-resolved multi-THz spectroscopy with s-NSOM to access dynamic complex conductivities on the surfaces of nanostructures with 10 nm spatial resolution. Electro-optic sampling of the scattered near-field pulses enables sub-cycle detection (10 fs temporal resolution) of waveforms consisting of less than one coherent photon per pulse. We have applied our versatile microscope to two nanostructures of similar shape but vastly different composition. First, carrier dynamics were studied in indium arsenide nanowires with sub-cycle temporal resolution, revealing the ultrafast (<; 50 fs) formation of a carrier depletion layer at the nanowire surface. Second, we studied heterogeneous local dynamics in vanadium dioxide nanowires. Vanadium dioxide is a model system for insulator-to-metal phase transitions and is promising for technological applications. In our study, we found that substrate-induced strain drives a periodic modulation of the ultrafast photoconductivity along the nanowire. Finally, we have explored the ultimate limits of THz imaging resolution using a new technique called THz scanning tunnelling microscopy, where THz pulses drive femtosecond local currents across an atomic-scale tunnel junction.

Ultrafast field-resolved multi-THz spectroscopy on the sub-nanoparticle scale

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.

Ultrafast Infrared Nanoscopy with Sub-Cycle Temporal Resolution

Ultrafast microscopy of surfaces with simultaneous nanometer spatial resolution and femtosecond temporal resolution can be achieved by coupling ultrafast optical pulses to sharp metal tips [1-10]. In the case of scattering-type near-field scanning optical microscopy (s-NSOM), evanescent fields scattered from the tip apex provide a window into a drastically subwavelength world. Moreover, since these scattered fields can be measured using established far-field technologies, ultrafast infrared pulses emerging from a nanoscale volume can be detected with the best possible time resolution: faster than a single optical oscillation cycle [1].