In conventional APDs, the charge amplification mechanism is based on a one-carrier cascade impact ionization process involving only one type of carriers. [9] To achieve pronounced charge amplification, that is, high avalanche gain, a large breakdown voltage is required to provide enough energy for each injected carrier to produce multiple cascade ionizations in an avalanche region defined by a length of multiple mean-free paths. This leads to a grand challenge that the ultrahigh avalanche gain and the low breakdown voltage cannot be realized simultaneously in the conventional APD materials. Moreover, the breakdown voltages reported to date in the experimental works have never approached the theoretical limit of breakdown voltage of 1.5 E g /e with high gain, [10] hindering the development of APDs with both low energy consumption and high sensitivity. Searching for novel APD materials with alternative mechanisms to realize charge amplification represents a highly promising solution for addressing such challenges.Recently, the emerging family of 2D materials and van der Waals (vdW) heterostructures has prompted a revolution in developing high-performance avalanche photodetectors due to their unique properties. [3,[11][12][13] In particular, the enhanced Coulomb interaction resulting from the quantum confinement in vdW layered materials could boost the ionization rate [12,14] during the process of impact ionization. Here, we propose a new type of APDs based on the vdW Schottky junction, and realize both intrinsic threshold breakdown voltage of 1.5 E g /e and ultrahigh avalanche gain up to ≈3 × 10 5 . Such an excellent performance of the vdW Schottky APD can be well explained by a 2D avalanche model. In addition, we find the temperature dependence of the breakdown voltage and the gain relies not only on the ionization process but also on the thermally assisted carrier collection process. Our work highlights the potential of the vdW Schottky junction for developing next-generation high-performance APDs. Results and DiscussionAs schematically shown in Figure 1a, vdW Schottky APD was fabricated based on graphite/InSe vdW heterostructures, Realizing both ultralow breakdown voltage and ultrahigh gain is one of the major challenges in the development of high-performance avalanche photodetector. Here, it is reported that an ultrahigh avalanche gain of 3 × 10 5 can be realized in the graphite/InSe Schottky photodetector at a breakdown voltage down to 5.5 V. Remarkably, the threshold breakdown voltage can be further reduced down to 1.8 V by raising the operating temperature, approaching the theoretical limit of 1.5 E g g /e, with E g g the bandgap of semiconductor. A 2D impact ionization model is developed and it is uncovered that observation of high gain at low breakdown voltage arises from reduced dimensionality of electron-phonon scattering in the layered InSe flake. These findings open up a promising avenue for developing novel weak-light detectors with low energy consumption and high sensitivity.
Ultrashort electron bunches are useful for applications like ultrafast imaging, coherent radiation production, and the design of compact electron accelerators. Currently, however, the shortest achievable bunches, at attosecond time scales, have only been realized in the single-or very fewelectron regimes, limited by Coulomb repulsion and electron energy spread. Using ab initio simulations and complementary theoretical analysis, we show that highly-charged bunches are achievable by subjecting relativistic (few MeV-scale) electrons to a superposition of terahertz and optical pulses. We provide two detailed examples that use realistic electron bunches and laser pulse parameters which are within the reach of current compact set-ups: one with bunches of >240 electrons contained within 20 as durations and 15 μm radii, and one with final electron bunches of 1 fC contained within sub-400 as durations and 8 μm radii. Our results reveal a route to achieve such extreme combinations of high charge and attosecond pulse durations with existing technology.
Unlike conventional semiconductor platforms, 3D Dirac semimetals (DSMs) require relatively low input laser intensities for efficient terahertz (THz) high harmonic generation (HHG), making them promising materials for developing compact THz light sources. Here, we show that 3D DSMs’ high nonlinearity opens up a regime of nonlinear optics where extreme subwavelength current density features develop within nanoscale propagation distances of the driving field. Our results reveal orders-of-magnitude enhancement in HHG intensity with thicker 3D DSM films, and show that these subwavelength features fundamentally limit HHG enhancement beyond an optimal film thickness. This decrease in HHG intensity beyond the optimal thickness constitutes an effective propagation-induced dephasing. Our findings highlight the importance of propagation dynamics in nanofilms of extreme optical nonlinearity.
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