Topological Photodetection

Recently, researchers from International Center for Quantum Materials of Peking University have made progress on high-performance photodetection based on topological properties of Weyl semimetal TaIrTe₄. Topological effect on nonlinear optical response of Type-II Weyl semimetal has been explored in this work, which greatly boosts the responsivity of semimetal based photodetector at mid-infrared wavelength. This is a representative work of a series of recent research progress on photodetection based on topological semimetals by Dong Sun’s group and has been published on Nature Materials with the title of “Nonlinear photoresponse of type-II Weyl semimetals” (volume 18, page 476) and selected as cover article of current issue with the title “a topological shift”.

Since graphene, semimetal based photodetectors have been extensively studied for their attracting perspectives on detecting mid/far IR wavelength range at room temperature and further pushing the operation speed of photodetector down to picosecond. Graphene turns to be a great success in solving the above issues. However, despite the superior properties on many aspects, the final success of semimetal based photodetection was hindered by the extremely low responsivity which inherits from the intrinsic gapless bandstructure of semimetals. It was an impossible mission before to promote the responsivity of semimetal based photodetector to practically applicable level while keeping the broadband response and ultrafast operation speed at the same time.

During the past few years, Sun’s group has pioneered in introducing topological semimetallic materials to solve the above issue and made a great success recently. The concept of using topological semimetals for photodetection started with introducing three dimensional Cd₃As₂ for such purpose in Dr. Sun’s research article published on Nano Letters (17, 834) in 2017, this work has successfully promoted the responsivity by an order of magnitude over previously reported two dimensional solutions, while has kept very broad wavelength and ultrafast temporal response at the same time. Late on, Sun’s group further introduces topological Weyl semimetals for photodetection in their research articles published on Advanced Materials (30, 1707152) and ACS Nano (12,4055) in 2018. However, all these effort has limited boost on the responsivity and cannot meet the requirement of many practical applications.

In this most recent breakthrough published on Nature Materials (volume 18, page 476) in 2019 (selected as cover image), which is a collaborative success between Dong Sun’s group, Ji Feng’s group (theoretical calculation) and Jian-Hao Chen’s group (device fabrication) all from International Center for Quantum Materials in Peking University, and Zheng Liu’s group (crystal growth) in Nanyang Technological University, finally boosted the responsivity by three orders of magnitude by incorporating authentic topological effect of Weyl semimetals into photodetection. This work finally solved the long lasting responsivity problem of semimetal based photodetector from the fundamental physical principle level, which enables semimetals for low energy photon detection with ultrafast response time and practically applicable sensitivities. Divergence of Berry curvature in the vicinity of Weyl nodes of Weyl semimetals is the main topological effect that was applied in this work. Shift current is related to the geometric effects associated with Berry curvature of the Bloch bands and will be enhanced accordingly, especially when the optical transition happens near the Weyl nodes with low energy photon excitation. Although there are some detail technical problems remain to be solved in the future, topological semimetals become one of the most promising solution for photodetection in mid-/far-infrared wavelength range as result of the great boost of responsivity by the topological effects. Furthermore, this work marks that photodetection is established as the direction that is most closed to real applications for topological materials.

This work was supported by the National Natural Science Foundation of China together with some other funding agents. Second year Ph.D student Junchao Ma in Sun’s group from International Center for Quantum Materials is the first author and Dong Sun, Ji Feng, and Jian-Hao Chen share the corresponding authors of this work.

Figure: a-d, Band structure of TaIrTe₄ at the vicinity of four Weyl nodes in Brillouin zone, with a (d) and b(c) corresponding to opposite chiralities. k_a and k_b are reciprocal lattice vectors. e, Excitation power-dependence of the photocurrent response under 4-mm linear polarized along a-axis (LP-a), b-axis (LP-b), right circular polarized (RCP) and left circular polarized (LCP) excitations, respectively. The gray and red dashed lines show the linear fittings of responsivity of 130.2 mA/W and 42.1 mA/W of linear regions at low excitation power with LP-a (from 155.33 nW to 206.1 nW) and LP-b excitation (from 192.35 nW to 246.54 nW) respectively. The solid lines are the guide to the eye. The shadow region from -0.4 nA to 0.4 nA marks the maximum noise fluctuation range. f, Fix energy contour plot of the integrand of effective third-order optical conductivity tensor at fixed energy contour. The radius of the disc indicates the numerical value of matrix elements and the purple (cyan) color marks the positive (negative) sign. The red (+1) and blue (-1) crosses are the Weyl Points with opposite chirality. The tensor is boosted near the Weyl points.