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Photogalvanic Study of the Structure/Topology Relationship in Chiral Multifold Semimetals and Development of Novel Broadband Ultrafast Probes
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2025-05
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Physics
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https://doi.org/10.34944/qkrv-qv56
Abstract
We report on a spectroscopic, enantiomer-dependent study of the linear (LPGE) and circular photogalvanic effect (CPGE) in the structurally chiral multifold fermion material PdGa using THz emission. Both the bulk and surface responses were measured in right and left-handed enantiomers over the $0.47-1.1$~eV incoming photon energy range. The effect of structural handedness was to invert the polarity of the emitted THz transients, yielding spectra roughly equal in magnitude but opposite in sign for all responses measured. All spectra showed evidence of a previously-reported interband resonance deriving from interband transitions midway between the $\Gamma$ and R points of the Brillouin zone. The CPGE spectra deriving from the helicoidally dispersing Fermi arc surface states closely matched that of the bulk, while the preferred axis of radiation for both LPGE and CPGE surface responses rotated in opposite directions in the opposite enantiomers, revealing the complementary helicoidally dispersing nature of their surface states.
Turning our focus to the technical development of the spectroscopy of condensed phase systems, we present a novel time-resolved, optical pump/NIR supercontinuum probe spectrometer suitable for oscillators. A NIR supercontinuum probe spectrum ($850 - 1250$~nm) is generated in a photonic crystal fiber, dispersed across a DMD and then raster scanned into a single element detector at a 5~Hz rate. Dual modulation of pump and probe beams at disparate frequencies permits simultaneous measurement of both the bare reflectance $R$ and its photoinduced change $\Delta R$ through lock-in detection, allowing for continuously self-normalized measurement of $\Delta R/R$. Example data are presented on a germanium wafer sample that demonstrate for signals of order $\Delta R/R\sim 10^{-3}$, 2.87~nm spectral resolution and $\lesssim 400$~fs temporal resolution pre-recompression and comparable sensitivity to standard time-resolved, amplifier-based pump-probe techniques.
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