No abstract

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.

AbstractParticle simulation has been widely used in studying plasmas. The technique follows the motion of a large assembly of charged particles in their self-consistent electric and magnetic fields. Plasmons, collective oscillations of the free electrons in conducting media such as metals, are connected to plasmas by very similar physics, in particular, the notion of collective charge oscillations. In many cases of interest, plasmons are theoretically characterized by solving the classical Maxwell’s equations, where the electromagnetic responses can be described by bulk permittivity. That approach pays more attention to fields rather than motion of electrons. In this work, however, we apply the particle simulation method to model the kinetics of plasmons, by updating both particle position and momentum (Newton–Lorentz equation) and electromagnetic fields (Ampere and Faraday laws) that are connected by current. Particle simulation of plasmons can offer insights and information that supplement those gained by traditional experimental and theoretical approaches. Specifically, we present two case studies to show its capabilities of modeling single-electron excitation of plasmons, tracing instantaneous movements of electrons to elucidate the physical dynamics of plasmons, and revealing electron spill-out effects of ultrasmall nanoparticles approaching the quantum limit. These preliminary demonstrations open the door to realistic particle simulations of plasmons.

Prototypical three-dimensional (3D) topological Dirac semimetals (DSMs), such as Cd3As2 and Na3Bi, contain electrons that obey a linear momentum–energy dispersion with different Fermi velocities along the three orthogonal momentum dimensions. Despite being extensively studied in recent years, the inherent Fermi velocity anisotropy has often been neglected in the theoretical and numerical studies of 3D DSMs. Although this omission does not qualitatively alter the physics of light-driven massless quasiparticles in 3D DSMs, it does quantitatively change the optical coefficients which can lead to nontrivial implications in terms of nanophotonics and plasmonics applications. Here we study the linear optical response of 3D DSMs for general Fermi velocity values along each direction. Although the signature conductivity-frequency scaling, σ(ω) ∝ ω, of 3D Dirac fermion is well-protected from the Fermi velocity anisotropy, the linear optical response exhibits strong linear dichroism as captured by the universal extinction ratio scaling law, Λij = (vi /vj )2 (where i ≠ j denotes the three spatial coordinates x,y,z, and vi is the i-direction Fermi velocity), which is independent of frequency, temperature, doping, and carrier scattering lifetime. For Cd3As2 and Na3Bi3, an exceptionally strong extinction ratio larger than 15 and covering a broad terahertz window is revealed. Our findings shed new light on the role of Fermi velocity anisotropy in the optical response of Dirac semimetals and open up novel polarization-sensitive functionalities, such as photodetection and light modulation.

This work presents a general framework for quantum interference between processes that can involve different fundamental particles or quasi‐particles. This framework shows that shaping input wavefunctions is a versatile and powerful tool for producing and controlling quantum interference between distinguishable pathways, beyond previously explored quantum interference between indistinguishable pathways. Two examples of quantum interference enabled by shaping in interactions between free electrons, bound electrons, and photons are presented: i) the vanishing of the zero‐loss peak by destructive quantum interference when a shaped electron wavepacket couples to light, under conditions where the electron's zero‐loss peak otherwise dominates; ii) quantum interference between free electron and atomic (bound electron) spontaneous emission processes, which can be significant even when the free electron and atom are far apart, breaking the common notion that a free electron and an atom must be close by to significantly affect each other's processes. Conclusions show that emerging quantum wave‐shaping techniques unlock the door to greater versatility in light‐matter interactions and other quantum processes in general.

It is shown that three-dimensional Dirac semimetals are promising candidates for highly efficient optical-to-terahertz conversion due to their extreme optical nonlinearities. In particular, it is predicted that the conversion efficiency of Cd 3 As 2 exceeds typical materials like LiNbO 3 by >5000 times over nanoscale propagation distances. Studies show that even when no restrictions are placed on propagation distance, Cd 3 As 2 still outperforms LiNbO 3 in efficiency by >10 times. The results indicate that by tuning the Fermi energy, Pauli blocking can be leveraged to realize a step-like efficiency increase in the optical-to-terahertz conversion process. It is found that large optical-to-terahertz conversion efficiencies persists over a wide range of input frequencies, input field strengths, Fermi energies, and temperatures. These results could pave the way to the development of ultrathin-film terahertz sources for compact terahertz technologies.