Yujie Liu

Optical cable fault locating using Brillouin optical time domain reflectometer and cable localized heating method

A novel optical cable fault location method, which is based on Brillouin optical time domain reflectometer (BOTDR) and cable localized heating, is proposed and demonstrated. In the method, a BOTDR apparatus is used to measure the optical loss and strain distribution along the fiber in an optical cable, and a heating device is used to heat the cable at its certain local site. Actual experimental results make it clear that the proposed method works effectively without complicated calculation. By means of the new method, we have successfully located the optical cable fault in the 60 km optical fiber composite power cable from Shanghai to Shengshi, Zhejiang. A fault location accuracy of 1 meter was achieved. The fault location uncertainty of the new optical cable fault location method is at least one order of magnitude smaller than that of the traditional OTDR method.

Graphene-carbon nanotube hybrid films for high-performance flexible photodetectors

Graphene is being actively explored as a candidate material for flexible and stretchable devices. However, the development of graphene-based flexible photonic devices, i.e. photodetectors, is hindered by the low absorbance of the single layer of carbon atoms. Recently, van der Waals bonded carbon nanotube and graphene hybrid films have demonstrated excellent photoresponsivity, and the use of vein-like carbon nanotube networks resulted in significantly higher mechanical strength. Here, we report for the first time, a flexible photodetector with a high photoresponsivity of ~ 51 A/W and a fast response time of ~ 40 ms over the visible range, revealing the unique potential of this emerging all-carbon hybrid films for flexible devices. In addition, the device exhibits good robustness against repetitive bending, suggesting its applicability in large-area matrix-array flexible photodetectors.

Bandgap renormalization in single-wall carbon nanotubes

Single-wall carbon nanotubes (SWNTs) have been extensively explored as an ultrafast nonlinear optical material. However, due to the numerous electronic and morphological arrangements, a simple and self-contained physical model that can unambiguously account for the rich photocarrier dynamics in SWNTs is still absent. Here, by performing broadband degenerate and non-degenerate pump-probe experiments on SWNTs of different chiralities and morphologies, we reveal strong evidences for the existence of bandgap renormalization in SWNTs. In particularly, it is found that the broadband transient response of SWNTs can be well explained by the combined effects of Pauli blocking and bandgap renormalization, and the distinct dynamics is further influenced by the different sensitivity of degenerate and non-degenerate measurements to these two concurrent effects. Furthermore, we attribute optical-phonon bath thermalization as an underlying mechanism for the observed bandgap renormalization. Our findings provide new guidelines for interpreting the broadband optical response of carbon nanotubes.

Photoresponsivity of an all-semimetal heterostructure based on graphene and WTe 2

Heterostructures based on two-dimensional (2D) materials have sparked wide interests in both fundamental physics and applied devices. Recently, Dirac/Weyl semimetals are emerging as capable functional materials for optoelectronic devices. However, thus far the interfacial coupling of an all-semimetal 2D heterostructure has not been investigated, and its effects on optoelectronic properties remain less well understood. Here, a heterostructure comprising of all semi-metallic constituents, namely graphene and WTe2, is fabricated. Standard photocurrent measurements on a graphene/WTe2 phototransistor reveal a pronounced photocurrent enhancement (a photoresponsivity ~8.7 A/W under 650 nm laser illumination). Transport and photocurrent mapping suggest that both photovoltaic and photothermoelectric effects contribute to the enhanced photoresponse of the hybrid system. Our results help to enrich the understanding of new and emerging device concepts based on 2D layered materials.

Observation of bimolecular recombination in high mobility semiconductor Bi2O2Se using ultrafast spectroscopy

Bi2O2Se is emerging as a high mobility functional material for optoelectronics, but its fundamental optical properties remain less well studied. Here, ultrafast photocarrier dynamics in single crystal Bi2O2Se is investigated by pump fluence-dependent, broadband ultrafast spectroscopy. Our results reveal that bimolecular recombination plays an important role in the photocarrier relaxation process, and a room-temperature bimolecular recombination constant of (1.29 ± 0.42) × 10−9 cm−3 s−1 is obtained for Bi2O2Se. Such a level of the recombination constant combined with a high mobility (∼1006 cm2 V−1 s−1 at 200 K for Bi2O2Se) suggests that Bi2O2Se can be a promising material for photovoltaic applications.Bi2O2Se is emerging as a high mobility functional material for optoelectronics, but its fundamental optical properties remain less well studied. Here, ultrafast photocarrier dynamics in single crystal Bi2O2Se is investigated by pump fluence-dependent, broadband ultrafast spectroscopy. Our results reveal that bimolecular recombination plays an important role in the photocarrier relaxation process, and a room-temperature bimolecular recombination constant of (1.29 ± 0.42) × 10−9 cm−3 s−1 is obtained for Bi2O2Se. Such a level of the recombination constant combined with a high mobility (∼1006 cm2 V−1 s−1 at 200 K for Bi2O2Se) suggests that Bi2O2Se can be a promising material for photovoltaic applications.

Weak Anti-Localization and Quantum Oscillations in Topological Crystalline Insulator PbTe

Topological crystalline insulators (TCIs) have attracted worldwide interest since their theoretical predication and have created exciting opportunities for studying topological quantum physics and for exploring spintronic applications. In this work, we successfully synthesize PbTe nanowires via the chemical vapor deposition method and demonstrate the existence of topological surface states by their 2D weak anti-localization effect and Shubnikov–de Haas oscillations. More importantly, the surface state contributes ~61% of the total conduction, suggesting dominant surface transport in PbTe nanowires at low temperatures. Our work provides an experimental groundwork for researching TCIs and is a step forward for the applications of PbTe nanowires in spintronic devices.

Light-activated artificial synapses based on graphene hybrid phototransistors

A novel light-activated artificial synapse based on graphene/nanotube hybrid phototransistor is reported. Tunable synaptic weight and short-term synaptic plasticity behaviors are for the first time demonstrated using optical laser pulses.