We have established phase-resolved THz-THG probes at beamline and table-top experiments that allow for systematic studies of different superconductors. We focus on unconventional superconductivity in high-Tc cuprates for which Higgs modes are expected to show a rich spectrum and potential couplings to multiple orders based on the cuprate phase diagrams. Besides that, we also investigate other textbook examples for the interplay of superconductivity with CDWs. Experimentally we can successfully show the interplay of the Higgs mode with CDWs in these systems. It shows up as a Fano-like resonance in the amplitude and phase response of the THG signal. Most strikingly we can identify two distinguishable contributions to the THG directly in the time-domain signal and assign them to the corresponding orders. Here the interplay between the orders shows up as interference pattern in the time-domain. Analyzing the phase relation between the two THG contributions reveals the dynamical interplay between the orders. Further the phase relation/interference pattern can characterize the type of interaction (e.g. competing or coexisting) and show if upon external parameter changes (temperature, doping, magnetic field, THz-field strength) there is a change of the amplitude of the orders or a change in the coupling between the orders.
For light-induced states we have developed a pump-drive technique where the pump pulse induces non-equilibrium changes to the superconducting state and the drive uses the Higgs oscillations to characterize the transient state. Using methods from 2D spectroscopy on this scheme allows us to distinguish other excitations in the system from the transient Higgs response and thus revealing the intrinsic order parameter dynamics of the system.
Information on Higgs in different symmetry channels of the d-wave order parameter in cuprates we gain by populating the anti-Stokes channel that we read out with a time resolved Raman probe. That complements ongoing developments to reveal Higgs modes with full momentum resolution in future time resolved ARPES measurements.
We also merge ultrafast impulsive stimulated Raman probes with frequency domain Raman to identify local order dynamics of "puddles" that shape the global response of orders. This becomes important for incommensurate and coexisting orders. As first prime example we investigate such dynamics in NbSe2 where we find hybridisation of Higgs- and CDW order.
As new development we also probe collective Higgs-equivalent amplitude modes in excitonic insulators, a condensate of excitons. Here we started investigating novel materials systems that do not show CDW order or structural phase transitions that occur in parallel to the condensation.
As novel technical development we start using near field THz emitters to exfoliate thin flakes of samples right onto the emitter. This will allow spectroscopic probes on sub wavelength sized samples, near-field optical probes of in-plane superconducting plasmons, and in the future probes of twisted cuprates (then key road to topological superconductivity.)
