The Dirac semimetal cadmium arsenide (Cd3As2), a 3D electronic analog of graphene, has sparked renewed research interests for its novel topological phases and excellent optoelectronic properties. The gapless nature of its 3D electronic band facilitates strong optical nonlinearity and supports Dirac plasmons that are of particular interest to realize high‐performance electronic and photonic devices at terahertz (1 THz = 4.1 meV) frequencies, where the performance of most dynamic materials are limited by the tradeoff between power‐efficiency and switching speed. Here, all‐optical, low‐power, ultrafast broadband modulation of terahertz waves using an ultrathin film (100 nm, λ/3000) of Cd3As2 are experimentally demonstrated through active tailoring of the photoconductivity. The measurements reveal the photosensitive metallic behavior of Cd3As2 with high terahertz electron mobility of 7200 cm2 (Vs)−1. In addition, optical fluence dependent ultrafast charge carrier relaxation (15.5 ps), terahertz mobility, and long momentum scattering time (157 fs) comparable to superconductors that invoke kinetic inductance at terahertz frequencies are demonstrated. These remarkable properties of 3D Dirac topological semimetal envision a new class of power‐efficient, high speed, compact, tunable electronic, and photonic devices.
Rapid fabricating and harnessing stimuli-responsive behaviors of microscale bio-compatible hydrogels are of great interest to the emerging micro-mechanics, drug delivery, artificial scaffolds, nano-robotics, and lab chips. Herein, we demonstrate a novel femtosecond laser additive manufacturing process with smart materials for soft interactive hydrogel micro-machines. Bio-compatible hyaluronic acid methacryloyl was polymerized with hydrophilic diacrylate into an absorbent hydrogel matrix under a tight topological control through a 532 nm green femtosecond laser beam. The proposed hetero-scanning strategy modifies the hierarchical polymeric degrees inside the hydrogel matrix, leading to a controllable surface tension mismatch. Strikingly, these programmable stimuli-responsive matrices mechanized hydrogels into robotic applications at the micro/nanoscale (<300 × 300 × 100 μm3). Reverse high-freedom shape mutations of diversified microstructures were created from simple initial shapes and identified without evident fatigue. We further confirmed the biocompatibility, cell adhesion, and tunable mechanics of the as-prepared hydrogels. Benefiting from the high-efficiency two-photon polymerization (TPP), nanometer feature size (<200 nm), and flexible digitalized modeling technique, many more micro/nanoscale hydrogel robots or machines have become obtainable in respect of future interdisciplinary applications.
Platinum diselenide (PtSe 2 ) has been exploited for visible and infrared photodetectors due to its tunable electrical and photoelectric properties determined by a layer-dependent band gap. We investigated the photoconductive mechanism of the fewlayer (3 nm) PtSe 2 field-effect phototransistor (FEPT) under illumination of visible and infrared lasers. The few-layer PtSe 2 FEPT shows ambipolar characteristics modulated by a gate voltage with a mobility of 1496.7 cm 2 v −1 s −1 (electron) and 1410 cm 2 v −1 s −1 (hole) at V DS = 0.16 V. Both positive photoconductivity (PPC) and negative photoconductivity (NPC) were observed in the PtSe 2 photodetector. The responsivity (R) of the device at V G = 4 V was 0.14 A/W, 0.63 A/W, and 0.086 A/W, and the specific detectivity (D*) was 4.34 × 10 8 Jones, 1.95 × 10 9 Jones, and 2.4 × 10 8 Jones under 532, 808, and 405 nm laser irradiation, respectively. The R of the device at V G = −2.4 V was 0.525 A/W, 0.34 A/W, and 0.087 A/W, and theD* was 1.84 × 10 9 Jones 1.06 × 10 9 Jones, and 2.19 × 10 8 Jones. The change of the photoconductivity is attributed to a combination of three mechanisms: (1) photoinduced desorption of oxygen molecules from the surface, which is related to the NPC; (2) the photoinduced carriers, which leads to PPC; and (3) the photogating effects related to the photoinduced electrons in the interface of PtSe 2 /SiO 2 . NPC dominates in the device at a high gate voltage due to the photogating effect. Through the study of the P/NPC mechanism in PtSe 2 , understanding of the photoconductive mechanism in the field of photoelectric detection is further deepened.
In this study, an optical setup for generating terahertz (THz) pulses through a two-color femtosecond laser filament was carefully designed to achieve a precise overlap of two-color laser pulses in space and time. β-barium borate (BBO), α-BBO, and a dual-wavelength plate were used to compensate the phase delay of the two-color lasers. Tilting of α-BBO could further realize the precise spatial overlap of the two beams by counteracting the walk-off effect. The maximum output THz pulse energy reached 21 μJ in argon gas when using a commercial Ti:sapphire laser with a pulse energy of 6 mJ at a 1 kHz repetition rate. The corresponding conversion efficiency exceeded 0.35%.
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