2025 Theses Doctoral

Cryogenic Ultrafast Nanoscopy of Complex Materials

In this dissertation, we address the longstanding challenge of directly probing electronic and structural dynamics in complex quantum materials at length scales far below the diffraction limit. To do so, we have developed a versatile tabletop beamline capable of generating and detecting light from the visible through the mid-infrared (MIR) and into the terahertz (THz) range which we then couple into our home built cryogenic scattering-type scanning near-field optical microscope (Cryo-SNOM). Our custom beamline employs a high-power, ultrafast Yb-doped laser and nonlinear optical phenomena – most notably tilted-pulse-front pumping of LiNbO₃ for broadband THz generation and electro-optic sampling in ZnTe for sensitive time domain detection. This beamline delivers more than 10 mW of THz radiation spanning 0.1–3 THz with >80 dB dynamic range and a temporal resolution of 20 fs.

This THz light is coupled into the near field via an atomic force microscope tip, thus providing ~100 nm spatial resolution.Using a room temperature microscope coupled to our ultrafast source, we first investigate nanoscale femtosecond dynamics of the Mott insulator Ca₂RuO₄ in the MIR. Temperature-cycling experiments reveal surface-confined stripe domains of coexisting insulating and metallic phases, whose depth and volume fraction evolve predictably across the insulator-metal transition. Ultrafast pump-probe SNOM then shows that above-gap photoexcitation injects free carriers that rapidly become trapped, likely as polarons, producing a transient mid-gap absorption and phonon renormalization on a sub-picosecond timescale.

Next, we turn to van der Waals (vdW) heterostructures. We employ our custom built Cryo-SNOM coupled to our newly developed THz beamline to investigate propagating plasmon polaritons – a coupled light-matter mode encoded with the electronic properties of the host medium. First, we visualize charge-transfer plasmons propagating in bespoke graphene/α-RuCl₃ lateral cavities. These measurements represent a new paradigm for quantitative characterization of long-wavelength polaritons in any vdW heterostructure while also providing insight into the nature of plasmons in graphene/α-RuCl₃ heterostructures. Second, using space-time mapping metrology we study propagating plasmon polaritons in graphene/hBN/Bi₂Sr₂CaCu₂O₈₊δ cavities.

In these preliminary measurements, we demonstrate long lived, gate tunable plasmons in graphene separated by a ~20 nm hBN spacer from optimally doped Bi₂Sr₂CaCu₂O₈₊δ. This proximity can lead to coupling between the graphene plasmons and the hyperbolic polaritonic modes in the high-temperature superconductor. Then, using magnetic-force microscopy, we measure the local Meissner effect in the Bi₂Sr₂CaCu₂O₈₊δ embedded in these vdW cavity heterostructures. From these measurements, we extract the local the superfluid density in the Bi₂Sr₂CaCu₂O₈₊δ flake and any potential modification related to plasmonic coupling with the proximal graphene layer.

Finally, we demonstrate space-time duality in plasmon polariton propagation in graphene. By tailoring ultrafast complex-frequency excitations, we sustain polaritonic fields over unprecedented distances, revealing new pathways for spatio-temporal control of light–matter interactions at the nanoscale.

Together, these studies establish Cryo-SNOM as a multi-messenger probe that unifies ultrafast spectroscopy, near-field imaging, and local scanning probes, and opens routes to manipulating and understanding emergent phenomena in correlated and low-dimensional materials.