We have designed and implemented a novel acoustic lens based focusing technology into a prototype photoacoustic imaging camera. All photoacoustically generated waves from laser exposed absorbers within a small volume get focused simultaneously by the lens onto an image plane. We use a multi-element ultrasound transducer array to capture the focused photoacoustic signals. Acoustic lens eliminates the need for expensive data acquisition hardware systems, is faster compared to electronic focusing and enables real-time image reconstruction. Using this photoacoustic imaging camera, we have imaged more than 150 several centimeter size ex-vivo human prostate, kidney and thyroid specimens with a millimeter resolution for cancer detection. In this paper, we share our lens design strategy and how we evaluate the resulting quality metrics (on and off axis point spread function, depth of field and modulation transfer function) through simulation. An advanced toolbox in MATLAB was adapted and used for simulating a two-dimensional gridded model that incorporates realistic photoacoustic signal generation and acoustic wave propagation through the lens with medium properties defined on each grid point. Two dimensional point spread functions have been generated and compared with experiments to demonstrate the utility of our design strategy. Finally we present results from work in progress on the use of two lens system aimed at further improving some of the quality metrics of our system.
Two-dimensional Janus transition metal dichalcogenides exhibit intrinsic out-of-plane structural symmetry breaking, which facilitates their applications in spintronics through the enhanced Rashba spin–orbit coupling. Fabrication of Janus structures in a deterministic fashion is essential for their practical heterogeneous integration, which, however, remains challenging in the aspect of material synthesis technology. Here, we demonstrate a synthetic strategy to fabricate Janus WSSe structures at definite positions on its tungsten disulfide (WS2) host through the local electron beam irradiation followed by controllable selenization. We show that the energetic electron flux can significantly modulate the WS2 lattice properties, which locally increase the energy barrier between the WS2 and its Janus counterpart WSSe, thus preventing the irradiated WS2 from being selenized and creating well-defined hetero-boundaries. Moreover, by using monolayer and bilayer WS2 as the hosts, both lateral and vertical heterostructures of WSSe–WS2 can be achieved in a patternable manner. Our work provides a viable route toward the controllable fabrication of the Janus structures for multi-functional spintronics.
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