Energy localization in three-dimensional self-rolled photodetector

Photothermoelectric (PTE) effect involves two energy conversion processes: photothermal conversion and thermoelectric effect. When the incident light is locally absorbed, a temperature difference is generated in thermoelectric material, and thus a voltage potential difference is established as an output electrical signal for photodetection. 
To achieve higher detection performance, efficient approach for localizing both optical and thermal energies is crucial. However, due to the influence of substrate, it is a great challenge to promote the device performance and to study the coupling mechanism of multi-physical fields in micro/nano scale devices. With the tendency of on-chip integrated devices towards three-dimensional (3D) structure, the structure-performance relationship also needs to be explored. 
In this work, we used the self-rolled technology to separate tellurium (Te) as the PTE active nanomembrane from the substrate, and constructed an isolated 3D micro/nano structure. In the tubular structure, photon energy is trapped in the Te nanomembrane with a higher refractive index. The heat generated by optical absorption is localized in the isolated 3D tube wall to generate a larger temperature difference, thus creating a remarakbe potential difference in thermoelectric conversion. The experimental results further verify that the energy localization effect in the isolated 3D tubular structure improves the photodetection performance: the self-driven photovoltage of the tubular detector is 307 times higher than that of the planar detector.

Figure 1. Structure and energy localization in self-driven PTE detector. a) Schematic diagram of structure and working principle of self-rolled PTE detector. b) Simulated electric field distribution in the device under light illumination. c) Photovoltage-time plots of self-rolled detector and planar detector illuminated by the same pulsed laser.
We verified the PTE effect and its position dependence, which helps to disclose the PTE coupling and conversion in the 3D structure. The mapping results of the incident light position and the photocurrent was depicted. For light spot moving perpendicular to the axial direction, the best photo response appears when the tubular detector was irradiated from the top along the tube diameter. 
Since the PTE detector outputs an electrical signal induced by the local temperature difference caused by incident light absorption, the response spectrum of PTE detector is theoretically not limited by the bandgap of the active material. The self-rolled PTE detector in this work also demonstrates broad-band photodetection from visible light to long-wave infrared region. The modulation of the rolling rotations of the tubular device is used to optimize the performance of the self-driven detector.
Furthermore, the unique 3D structure of the tubular detector is capable of conducting multi-dimensional detection. The self-rolled detector demonstrated a wide-angle detection capability, and it has a better response to the polarized light with electric field parallel to the tube axis induced by the cylinder symmetry. In addition, polarimetric imaging with high resolution through single-pixel sensing is achieved. The results demonstrate the multi-dimensional detection ability of self-rolled PTE detector to get the information of both intensity and polarization.
In summary, the realization of geometry-induced light and heat energy localizations provides an ideal platform for studying the energy conversion in micro-/nano-devices and the output paves the way for the practical application in the on-chip photodetection.