Cryogenic scanning photocurrent spectroscopy for materials responses to structured optical fields

Circular dichroism spectroscopy is known to provide important insights into the interplay of different degrees of freedom in quantum materials, and yet spectroscopic study of the optoelectronic responses of quantum materials to structured optical fields, such as light with finite spin and orbital angular momentum, has not yet been widely explored, particularly at cryogenic temperature. Here, we demonstrate the design and application of a novel instrument that integrates scanning spectroscopic photocurrent measurements with structured light of controlled spin and orbital angular momentum. For structured photons with wavelengths between 500 and 700 nm, this instrument can perform spatially resolved photocurrent measurements of two-dimensional materials or thin crystals under magnetic fields up to ±14 T, at temperatures from 400 K down to 3 K, with either spin angular momentum ±h or orbital angular momentum ± ℓh (where ℓ = 1, 2, 3… is the topological charge), and over a (35 × 25) μm² area with ∼1 μm spatial resolution when coupling with a f = 75 mm objective lens at 3 K. These capabilities of the instrument are exemplified by magneto-photocurrent spectroscopic measurements of monolayer 2H–MoS₂ field-effect transistors, which not only reveal the excitonic spectra but also demonstrate monotonically increasing photocurrents with increasing |ℓ| and excitonic Zeeman splitting and an enhanced Landé g-factor due to the enhanced formation of intervalley dark excitons under magnetic field. These studies thus demonstrate the versatility of the scanning photocurrent spectrometry for investigating excitonic physics, optical selection rules, and optoelectronic responses of novel quantum materials and engineered quantum devices to structured light.