Fig. 1: Overview of select collective excitations in quantum materials and corresponding probing techniques.
From: Time-domain study of coupled collective excitations in quantum materials
a Schematic of four different collective excitations in solids and their respective coupling with light arranged by their approximate timescale in order of magnitude. The blue sinusoidal wave represents a light wave. The gray shadow represents the hybridization between light and each collective excitation to form different polaritons. b Top: Schematic setup of ultrafast optical experiments. Middle: Schematic of optically-induced ultrafast demagnetization of a ferromagnet. Bottom: Representative data for the oscillatory component of time-resolved optical polarimetry, showing a sudden decrease of the probe laser polarization angle θ due to ultrafast demagnetization followed by a periodic modulation due to a coherent magnon mode. The phase ϕ of the waveform is labeled. c Top: Schematic setup of time-resolved electron or X-ray diffraction. Bottom: Representative data for electron diffuse scattering, showing a specific momentum space distribution of diffuse intensity due to the nonthermal population of certain phonon branches. d Top: Schematic setup of time- and angle-resolved photoemission spectroscopy. Bottom: Representative transient photoemission data for probing unoccupied states in a semiconductor, showing new features in the measured band dispersion, such as the conduction band (red curve) and excitons (green curve). e Top: Schematic setup for ultrafast near-field electron microscopy and spectroscopy. Alternating red and blue regions represent the photoinduced field distribution due to a plasmon-polariton. Green ovals on the sample represent the electron scanning positions. Bottom: Representative data for probing the interaction between free electrons and photons. When the probe electron packet overlaps spatiotemporally with the photoinduced plasmon-polariton, electrons undergo quantized gain or loss of the quasiparticle energy (ℏω), resulting in discrete energy peaks in the electron energy loss spectrum. Panel c adapted from ref. 117, Springer Nature Ltd.
