Qingli Zhou

Van der Waals heterostructures with negligible lattice mismatch and flexible fabrication provide a platform to modulate the photoactive properties and extend the applications of two-dimensional materials. In addition to the band alignment, the substrate-introduced effective electric field is also indispensable for manipulating the carrier dynamics at the heterointerface. Here, we have proposed a heterostructure composed of tellurium (Te) and bismuth selenide (Bi2Se3) nanofilms to regulate the influence of the substrate field via the stacking order. Our experimental results demonstrate that the terahertz transient photoresponses of the two types of heterojunctions exhibit distinct modulation depths and carrier lifetimes. Notably, unlike the Te/Bi2Se3 configuration, where the Bi2Se3 interlayer serves as a spacer to screen the substrate field, the Bi2Se3/Te heterostructure possesses faster relaxation due to hot carrier transfer promoted by the substrate-introduced effective field. This work provides in-depth physical insights into substrate engineering, which is crucial for further exploration of terahertz optoelectronic devices.

The human retina capable of extracting key feature information is a crucial sensory element in the visual system. Constructing bionic devices with multitype feedback to emulate the retina behaviors in complex environments has been a persistent pursuit to broaden the visual range. However, the definition and regulation of synaptic weights persist as a bottleneck problem in the terahertz (THz) devices to achieve neuromorphic function. Here, we have proposed bismuth‐based THz photonic neuromorphic devices with picosecond short‐term plasticity and constructed the THz‐optical neural network (THz‐ONN) to imitate retina function. Crucially, the weight embodied by THz photoresponse can be precisely regulated via incremental optical pulses, delivering an incredibly simple yet powerful approach that heralds systems with a continuously variable plasticity. Further development of diverse neuromorphic devices for various scenarios could be realized through the combination of band alignment engineering and substrate effects to control photocarrier transport. The corresponding neuromorphic computing based on THz‐ONN indicates the high recognition accuracy of hardware. The present study provides an exciting paradigm for the realization of THz neuromorphic devices and opens an avenue for mimicking biological sensory system.

Negative terahertz photoconductivity is a rare effect that holds great significance for numerous applications. However, the maintenance and enhancement of negative photoconductivity response still face a huge challenge. Here, combined with material selection and band alignment engineering, we have proposed a mixed-dimensional heterostructure comprised of 1D tellurium (Te) nanowires and 2D monolayer graphene. Intriguingly, the Te/graphene heterojunction could enhance negative terahertz photoconductivity by twice compared with graphene, following a transition from negative to positive photoconductivities. We have further performed first-principles calculations and characterizations to elucidate the type-I band alignment at the heterointerface, revealing the phenomena attributed to interfacial charge transfer. Our results provide in-depth physical insights into the carrier transport mechanism, which is crucial for further exploration of optoelectronic devices based on mixed-dimensional heterostructures.

High-performance active terahertz modulators as the indispensable core components are of great importance for the next generation communication technology. However, they currently suffer from the tradeoff between modulation depth and speed. Here, we introduce two-dimensional (2D) tellurium (Te) nanofilms with the unique structure as a new class of optically controlled terahertz modulators and demonstrate their integrated heterojunctions can successfully improve the device performances to the optimal and applicable levels among the existing all-2D broadband modulators. Further photoresponse measurements confirm the significant impact of the stacking order. We first clarify the direction of the substrate-induced electric field through first-principles calculations and uncover the unusual interaction mechanism in the photoexcited carrier dynamics associated with the charge transfer and interlayer exciton recombination. This advances the fundamental and applicative research of Te nanomaterials in high-performance terahertz optoelectronics.

Mixed-dimensional van der Waals systems could improve terahertz modulators’ performance by utilizing the advantages of different dimensional materials. However, the reported available mixed-dimensional heterojunctions using two-dimensional (2D) and three-dimensional materials usually sacrifice the modulation speed to realize a higher modulation depth. Here, we creatively integrate one-dimensional (1D) nanowires with 2D nanofilms to construct the novel mixed-dimensional tellurium (Te) homojunction and achieve optimal indices with an ultrahigh modulation depth and a shorter carrier lifetime. In addition, a Te-based large-array imaging element was fabricated to successfully reproduce the painting colors under specific pump conditions as well as the dynamic multicolor display. Further measurements with the introduction of metamaterials prove that the required energy consumption can be significantly reduced by one order of magnitude. Our proposed 1D/2D integration strategy opens a new way to build high-performance terahertz functional devices and greatly expands the application fields of Te nanomaterials.

We numerically and experimentally demonstrate a terahertz metadevice consisting of split-ring resonators (SRRs) present within square metallic rings. This device can function as a dual-band polarization converter by breaking the symmetry of SRRs. Under x-polarized incidence, the metastructure is able to convert linearly polarized (LP) light into a left-hand circular-polarized (LCP) wave. Intriguingly, under y-polarized incidence, frequency-dependent conversion from LP to LCP and right-hand circular-polarized (RCP) states can be achieved at different frequencies. Furthermore, reconfigurable LCP-to-LP and RCP-to-LP switching can be simulated by integrating the device with patterned graphene and changing its Fermi energy. This dual-band and multi-state polarization control provides an alternative solution to developing compact and multifunctional components in the terahertz regime.

Dynamical control of terahertz metadevices and integration of versatile functions are highly desired due to increasing practical demands. Here, we propose a multifunctional photosensitive Si hybrid metastructure consisting of twisted split-ring resonator pairs that could empower multi-state polarization switching and object-recognized imaging. The theoretical and simulated results show that inverse complete modulation of cross-polarized components could be realized via tuning the conductivity of the Si-bridge. The calculated ellipticity indicates that our metadevices possess the ability to convert linearly polarized light into left-hand circular-polarized or right-hand circular-polarized waves, as well as left-hand circular-polarized or right-hand circular-polarized into linearly polarized states. Combined with these properties, mono-parameter amplitude imaging and amplitude-phase synergistic encryption imaging are accomplished. Our research provides a new strategy to develop reconfigurable and multifunctional components in the terahertz regime.

A terahertz hybrid metamaterial incorporated with active media VO₂ holds great promise for the realization of a new generation of reconfigurable and multifunctional devices. However, for the electrical control, many efforts on reducing high working threshold are usually based on the utilization of patterned VO₂ patches or additional insulation layers, which will increase the complexity of the fabrication procedure. Here, we have proposed an effective strategy only by combining the surface microstructure and the unpatterned VO₂ film to realize the tunability of working current and uncover its highly dependent correlation with the structural resonance responses. It is shown the fully modulated current in our hybrid metastructures can be reduced with the prominently separated hysteresis loops. Further developed binary encoders can perform not only the information transformation of the fixed code symbols but also the arbitrary encoding with the programmable current pulse. Additionally, the dynamic color display can be accomplished to illustrate the intriguing function of the information encryption and multi-image reappearance with the current as the decryption key. Our work provides an approach to reduce the operating current and paves a pathway for the development of photonic memory information processors.