Coherent multi-dimensional widefield microscopy

Abstract

We present a widefield two-dimensional electronic spectroscopy microscope (2DESM) that integrates multidimensional coherent spectroscopy with optical imaging, enabling femtosecond temporal and micrometer spatial resolution. The broadband coverage (1.4-1.8 eV) allows the direct acquisition of spatially resolved two-dimensional electronic spectroscopy (2DES) maps of relevant near infrared excitations without the need for spatial scanning. By capturing both spectral and spatial domains simultaneously, 2DESM overcomes limitations of pump-probe microscopy and scanning 2DES, providing access to decoherence dynamics, inhomogeneous broadening, and coherent coupling in heterogeneous systems. As a proof-of-concept we performed 2DESM measurements on bilayer WSe2 encapsulated in hBN, revealing distinct spatial variations in excitonic dynamics. These results validate the ability of 2DESM to link local environments with ultrafast coherent processes and establish 2DESM as a versatile platform for probing quantum coherence, many-body interactions, and non-local energy transfer in two-dimensional materials, heterostructures, and micrometer-scale optoelectronic devices.

Figures (6)

• Figure 1: (a) Schematic of the 2DESM setup – The light source is a home-built non-collinear optical parametric amplifier (NOPA). The NOPA produces broadband light tunable from 1.4 to 2.1 eV, generating spectrally identical pump and probe pulses. A pair of pump pulses is generated by the Interferometric Collinear Pulse Generator (ICPG), which creates two phase-locked replicas with a controllable time delay $t_{pu}$. The pump–probe delay $t_{del}$ is introduced via a mechanical delay stage, and the chopper provides the timing reference for synchronizing acquisition. After interaction with the sample, a wide-field objective and tube lens image the transmitted probe onto the camera, while a polarizer suppresses residual pump scattering to enhance the SNR. The Interferometric Collinear Spectrometer (ICS) is placed after the tube lens and generates two temporally delayed probe pulses with delay $t_{pr}$. The black double arrow in both ICPG and ICS indicates the wedge translation direction. (b) NOPA spectrum – The NOPA output is centered at 1.70 eV, covering a range from 1.41 to 1.82 eV. (c) Timing diagram of the triggering system – The chopper serves as the master timing reference, while the camera acquires two frames per chopper duty cycle. Each camera acquisition triggers the laser to emit a defined number of pulses, set by the camera exposure time. In this scheme, the chopper blocks the pump beam for one of the two frames, yielding one frame with the pump on and one with the pump blocked.
• Figure 2: (a) Time-domain hypercube – Schematics of the construction of the $S(x,y;t_{pu},t_{pr},t_{del})$ time-domain hypercube at the fixed pump-probe delay $t_{del}$. Each image, represented in the black squares, corresponds to a specific ($t_{pu}$,$t_{pr}$) coordinate. The top sketches illustrate the ICPG and ICS devices with black double arrows indicating the direction of translating wedges to control the $t_{pu}$ and $t_{pr}$ coordinates. The red and blue interferograms represent the typical signals obtained for fixed ($x$,$y$) spatial coordinates upon scanning the $t_{pu}$ and $t_{pr}$ axes. (b) Frequency-domain hypercube – A two-dimensional FT is applied for each image pixel, thus converting the time-domain $S(x,y;t_{pu},t_{pr},t_{del})$ hypercube into the frequency-domain hypercube $S(x,y;\omega_{pu},\omega_{pr},t_{del})$.
• Figure 3: Ultrafast 2DESM temporal dynamics – Illustration of the full 2DESM acquisition process. For each pump-probe delay $t_{del}$ a full frequency-domain hypercube $S(x,y;\omega_{pu},\omega_{pr},t_{del})$ is acquired. For each spatial coordinate ($x$,$y$) a time-dependent 2DES spectrum,as reported in the top right panel, is obtained.
• Figure 4: (a) Pulse duration – The main panel displays the temporal profile obtained by integrating the PG-FROG signal (shown in the inset). (b) Spatial resolution – The main panel displays the line profile extracted from the diffraction pattern of a point-like defect. The spatial resolution of the system is estimated as half of the distance between the first two minima of the Airy disk. The diffraction pattern from the point-like defect is shown in the top-right panel. The red dashed line indicates the line along which the intensity profile reported in the main panel is taken.
• Figure 5: (a) Bilayer WSe$_2$ – An optical image of the sample, taken with a 50x objective, is shown. The bilayer WSe$_2$ flake, indicated by the green contour, was mechanically exfoliated and deposited on a SiO$_2$ substrate covered by a hBN layer (light blue contour). The top side of WSe$_2$ has been protected by an additional hBN overlayer, indicated by the white contour. The black and red rectangles indicated the two regions that are taken as representative of fully encapsulated and unprotected WSe$_2$ bilayer. (b)Photoluminescence – Photoluminescence measurements from the black rectangular area of panel (a) is shown. The spectrum displays two distinct peaks at 770 nm and 810 nm (see dashed red lines), consistent with emission from bilayer WSe$_2$Zhao2013Tonndorf2013.