Imaging quantum coherence in 2D semiconductors
Researchers at Università Cattolica del Sacro Cuore in Brescia, Italy, in collaboration with KU Leuven, the University of British Columbia in Vancouver and Politecnico di Milano, have built a widefield microscope that records a 2D electronic spectrum at every pixel of a camera image in parallel, without raster scanning.
The system, validated on the transition-metal dichalcogenide (TMD) WSe₂, reveals where excitons in semiconductors lose their quantum coherence, with femtosecond temporal and micrometre spatial resolution.
2D semiconductors, such as the TMDs, are leading candidates for atomically thin optoelectronic and quantum technologies. However, their optical response is governed by local conditions that vary across µm-scale structures, including dielectric environment, strain, interfaces and defects. Conventional ultrafast spectroscopy averages over this heterogeneity and so obscures the microscopic origin of the measured dynamics.
The new instrument, termed 2DESM, resolves exciton energies, homogeneous linewidths, disorder-induced inhomogeneous broadening and relaxation dynamics simultaneously and in real space. This establishes a direct link between a material's local structure and the coherent electronic processes that ultimately determine device performance.
Until now, spatially resolved ultrafast spectroscopy has faced a fundamental trade-off. Pump–probe microscopy maps excited-state populations in space but is insensitive to quantum coherence, and it cannot distinguish the intrinsic (homogeneous) linewidth of an optical transition from disorder-induced inhomogeneous broadening. Two-dimensional electronic spectroscopy (2DES) makes this distinction by correlating excitation and detection frequencies, but adding spatial resolution has so far required tight focusing and slow point-by-point raster scanning.
The key advance of 2DESM is to combine 2DES with widefield imaging. The transmitted probe is collected by a widefield microscope, and both frequency axes are encoded by ultrastable common-path birefringent interferometers. Because no focusing objective is placed in the excitation path, the pulses remain short and spectrally broad, so high temporal resolution and broadband coverage are preserved across the entire field of view.
At the heart of 2DESM are two ultrastable common-path interferometers based on translating birefringent wedges. The first generates a pair of phase-locked pump pulses, and the second, placed in the detection path, spectrally resolves the transmitted probe while preserving the full image. A high-speed scientific camera, synchronised with the pump modulation, acquires around 3,000 frames per second. A two-dimensional Fourier transform along both delay axes then yields a complete 2D spectrum at every pixel of the field of view.
The system achieves a temporal resolution of 38 fs, a spatial resolution of 1.05 µm and a spectral resolution of 50 meV, and it operates from room temperature down to 4 K in a custom cryogenic chamber.
To benchmark the technique, the team imaged hBN-encapsulated WSe₂ flakes. The 2DES maps directly revealed spatial variations in the excitonic response across the sample, distinguishing regions with different dielectric environments. They also allowed the intrinsic exciton decoherence time, of a few tens of femtoseconds, to be separated from disorder-induced broadening at each location.
Measurements at 300 K and 20 K further captured the temperature-driven narrowing of the excitonic linewidth and the corresponding increase in coherence time. These results confirm that 2DESM can track coherent exciton dynamics in real space under cryogenic operating conditions.
A complete hyperspectral data cube currently takes around 10 hours to acquire, so faster acquisition is a primary objective for the next generation of the instrument. T
he team now plans to apply 2DESM to van der Waals heterostructures such as WSe₂/MoS₂, where interlayer and moiré-trapped excitons vary across the sample, and to extend the technique to other quantum materials, including MoSe₂ and NiPS₃. More broadly, 2DESM offers a general platform for mapping disorder across device architectures, tracking where many-body complexes such as trions and biexcitons form, and following coherent energy transport in low-dimensional semiconductors and hybrid photonic structures.
Reference
M. Azarm et al., "Coherent multi-dimensional widefield microscopy," Optica 13, 1861 (2026)
