Recent developments in near field optical probes allow an unprecedented view on nanoscale spectroscopic and dynamic properties in materials. Key technology is combining the spatial resolution of an AFM with the spectroscopic power of optical probes in near field optical microscopes. They allow imaging phase separation at phase transitions; local vibrational mode mapping or probing for spatial extension of excitations. Prominent playgrounds in the field in our research cover van-der-Waals heterostructures to strongly correlated electron systems like superconductors or materials with strong orbit interactions. The project brings together technical expertise of near-field optics in the groups of Z. Ye (UBC) and S. Kaiser (MPI-FKF) that is linked to their instrumentation for broad-band tunable laser spectroscopies and time-resolved studies. The investigated quantum materials within the project are in the core of the centers interest with strong scientific overlap to the research of J. Folk (UBC), S. Burke (UBC), B. Keimer (MPI-FKF), or H. Takagi (MPI-FKF/U Tokyo).
As a prominent example for spectroscopic imaging we are interested in applying nearfield optical techniques to directly probe the intrinsic optical property of two-dimensional (2D) materials and their van-der-Waals (vdW) heterostructures. Recent experiments show that the moiré pattern in the vdW heterostructure can significantly alter the band structure of the constituent material and leads to emerging phenomena such as superconductivity and magnetism. Such progress allows us to design and create novel functional materials that do not exist in nature. We are interested in studying the impact of the moiré pattern on the optical property of these materials, particularly focused on the excitonic effect.
Excitonic effect is enhanced in the 2D material due to the strong quantum confinement and reduced Coulomb screening. As a result, room-temperature exciton and even higher many-body complexes have been observed to be stable with orders-of-magnitude larger binding energy . In many vdW heterostructures, the electronic band of the constituent material staggers and can separate the electron and hole of an exciton into different layers, forming so-called interlayer excitons. Besides phenomenal tunability, these interlayer excitons have a new valley degree of freedom which has a potential for quantum information applications.
On the other hand, when the crystal angle of the substitute layer in a heterostructure is not perfectly aligned or there is a lattice constant mismatch, moiré pattern with periodic potential arises. These potential have periods ranging from a few nm to tens of nm with tens of meV in amplitude, and can significantly alter the dispersion of the interlayer exciton. Since the moiré period is deeply below the diffraction limit, all optical measurements so far are made on the ensemble level. We are currently developing a new nearfield optical technique based on optical induced force to quantitatively probe the optical response at the exciton resonance with a sub-diffraction resolution. Our technique is promising in directly revealing the intrinsic optical property at different locations of the moiré pattern, thus providing new understanding of the vdW heterostructure and its intrinsic response.
Extending these spatially resolved spectroscopic probes into the time domain is triggered by recent achievements in optical control of phase transitions with ultra-short light pulses that trigger the quantum many body dynamics in solids.
Scientifically one of the most fascinating emergent phenomenon of such dynamics is high temperature superconductivity; most prominent in cuprate superconductors. One region of their phase diagram that has attracted much attention is the so-called pseudogap phase. Its origin has been attributed to a variety of nanoscopic phenomena, ranging from a precursor superconducting state with pre-formed Cooper pairs without phase coherence, to a scenario where electronic correlations compete with superconductivity.
Transport and tunneling measurements, far field optical spectroscopy and scattering experiments have been used so far to investigate the superconducting and normal-state phase in order to identify the interactions and excitations to which these phases are coupled. Dynamically ultrashort laser pulses allow probing the excitation of hot electron carriers in the normal phase as they relax back to the superconducting ground state.
Going beyond such photo-doping dynamics, mid-infrared pulses have shown to dynamically induce non-thermal phase transitions directly by triggering low frequency excitations in complex oxides photo-inducing superconductivity . THz time-domain spectroscopy fully characterizes not only the optical properties of the dynamically driven superconducting state but also its time evolution whilst the driving phonon-modulation and its decay. The results point to a scenario of dynamical stabilization of superconducting coherence within spatial localized areas. To prove this picture and finding ways to improve the stabilization one needs to access the complex transient response with spatial resolution that allows mapping the localized islands of Cooper-pairs. To achieve the required spatial subwavelength resolution for these measurements we want to combine the successful ultrafast optical control and THz-probe spectroscopy with scattering-type scanning near-field optical microscopy (s-SNOM).
We already have successfully shown the measurement of static tunable VIS-MIR near field spectra on such a setup as well as time-domain THz data at equilibrium and also are able to trace phase separation at thermally driven phase transitions (Fig. 1) or mapping of plasmons. We will use this technique to probe various novel phase transitions (temperature, strain, current, etc. driven) of interest within the center. Now we will push to extend these experiments into a transient pump-probe setup to observe light-induced dynamics and investigate metastable novel non-equilibrium states.
Ziliang Ye (UBC) email@example.com
Stefan Kaiser (MPI-FKF) firstname.lastname@example.org
 Z. Ye, Nature Communications 9, 3718 (2018).
 S. Kaiser, Physics Scripta 92, 103001 (2017).