Jingwen He

Exciton localization in solution-processed organolead trihalide perovskites

Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium–lead–halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices.

Generation and evolution of the terahertz vortex beam

Based on the complementary V-shaped antenna structure, ultrathin vortex phase plates are designed to achieve the terahertz (THz) optical vortices with different topological charges. Utilizing a THz holographic imaging system, the two dimensional complex field information of the generated THz vortex beam with the topological number l=1 is directly obtained. Its far field propagation properties are analyzed in detail, including the rotation, the twist direction, and the Gouy phase shift of the vortex phase. An analytic Laguerre-Gaussian mode is used to simulate and explain the measured phenomena. The experimental and simulation results overlap each other very well.

An ultrathin terahertz lens with axial long focal depth based on metasurfaces

The plasmonic resonance effect on metasurfaces generates an abrupt phase change. We employ this phase modulation mechanism to design the longitudinal field distribution of an ultrathin terahertz (THz) lens for achieving the axial long-focal-depth (LFD) property. Phase distributions of the designed lens are obtained by the Yang-Gu iterative amplitude-phase retrieval algorithm. By depositing a 100 nm gold film on a 500 μm silicon substrate and etching arrayed V-shaped air holes through the gold film, the designed ultrathin THz lens is fabricated by the micro photolithography technology. Experimental measurements have demonstrated its LFD property, which basically agree with the theoretical simulations. In addition, the designed THz lens possesses a good LFD property with a bandwidth of 200 GHz. It is expected that the designed ultrathin LFD THz lens should have wide potential applications in broadband THz imaging and THz communication systems.

A broadband terahertz ultrathin multi-focus lens.

Ultrathin transmission metasurface devices are designed on the basis of the Yang-Gu amplitude-phase retrieval algorithm for focusing the terahertz (THz) radiation into four or nine spots with focal spacing of 2 or 3 mm at a frequency of 0.8 THz. The focal properties are experimentally investigated in detail, and the results agree well with the theoretical expectations. The designed THz multi-focus lens (TMFL) demonstrates a good focusing function over a broad frequency range from 0.3 to 1.1 THz. As a transmission-type device based on metasurface, the diffraction efficiency of the TMFL can be as high as 33.92% at the designed frequency. The imaging function of the TMFL is also demonstrated experimentally and clear images are obtained. The proposed method produces an ultrathin, low-cost, and broadband multi-focus lens for THz-band application

Wavelength de-multiplexing metasurface hologram.

A wavelength de-multiplexing metasurface hologram composed of subwavelength metallic antennas is designed and demonstrated experimentally in the terahertz (THz) regime. Different character patterns are generated at the separated working frequencies 0.50 THz and 0.63 THz which determine a narrow frequency bandwidth of 130 GHz. The two working frequencies are around the central resonance frequency of the antennas where antennas behave strong wavefront modulation. Each antenna is fully utilized to control the wavefront of the metasurface at different frequencies by an optimization algorithm. The results demonstrate a candidate way to design multi-colors optical display elements.

Generation of terahertz vector beams with a concentric ring metal grating and photo-generated carriers

A scheme for vector terahertz (THz) beam generation is proposed. A subwavelength metal grating is utilized to adjust the polarization of the THz radiation. The amplitude and phase distributions of the THz beam are dynamically regulated by a THz computer-generated hologram (CGH) pattern of the photo-generated carriers. A radially polarized THz beam and a vector THz vortex beam with a topological charge of 1 are generated to demonstrate the validity and the effectiveness of the proposed scheme. Experimental results correspond to the theoretical simulations well. Moreover, the proposed method is applicative for a broadband THz radiation. These results could be applied in the THz sensing, THz imaging, and THz communication in the future.

Terahertz Tunable Metasurface Lens Based on Vanadium Dioxide Phase Transition

Based on the insulator-to-metal phase transition of vanadium dioxide (VO 2), a terahertz (THz) tunable metasurface lens (TML), which consists of a THz metasurface lens and a VO 2 film on Al 2O3 substrate, is proposed and experimentally verified. The focal intensity of the TML can be thermally controlled. The changes of the cross-polarized amplitude spectrum and the focal intensity during the heating and cooling processes are also investigated in detail. Any desired focal intensity can be obtained by adjusting the TML temperature. This TML and its approach will be of great significance for the development of the THz active devices.

Terahertz polarization modulator based on metasurface

Based on the metasurface composed of C-shaped slit (CSS) antennas, a broadband transmitted terahertz (THz) polarization modulator (TPM) is proposed and experimentally verified. By rotating the designed TPM, the amplitudes and phase difference of the two orthogonal electric components can be tuned, and thus the polarization of the transmitted THz beam is modulated. The linear-to-linear and linear-to-elliptical polarization conversion is realized with the proposed TPM. Numerical simulations are also performed and agree well with the experimental measurements. The proposed concept can be applied to other wavebands simply by varying the structure parameters and the material of the substrate.

Abruptly autofocusing terahertz waves with meta-hologram

An abruptly autofocusing ring-Airy beam is demonstrated in the terahertz (THz) waveband with a meta-hologram. The designed meta-hologram is composed of gold C-shaped slot antennas, which can realize both phase and amplitude modulation of the incident THz wave. A THz holographic imaging system is utilized to measure the generated ring-Airy beam; an abrupt focus following a parabolic trajectory is subsequently observed. THz ring-Airy beams with different parameters are also generated and investigated. This method can be expanded to other wavebands, such as the visible band, for which the meta-hologram can replace traditional computer-generated holography to avoid undesirable multiple diffraction orders.

Ultrafast terahertz response in photoexcited, vertically grown few-layer graphene

The terahertz (THz) response from vertically aligned few-layer graphene samples with and without femtosecond optical excitation was investigated. The frequency-dependent optical conductivity of the photoexcited vertically aligned few-layer graphene had a strong free carrier response. Upon photoexcitation, a transient decrease in THz transmission on the subpicosecond timescale was observed. A modulation depth of nearly 16% was demonstrated in the range of the photoexcitation power used. The photoinduced ultrafast response presented here is distinct from previous studies on horizontally grown graphene. The mechanism underlying this photoconductive ultrafast response was investigated by measuring the transmission properties and by calculating the carrier density. The results of these studies are promising for the development of high-performance THz modulators and ultrafast switchable THz photoelectric devices.