Epitaxy-the growth of a crystalline material on a substrate-is crucial for the semiconductor industry, but is often limited by the need for lattice matching between the two material systems. This strict requirement is relaxed for van der Waals epitaxy, in which epitaxy on layered or two-dimensional (2D) materials is mediated by weak van der Waals interactions, and which also allows facile layer release from 2D surfaces. It has been thought that 2D materials are the only seed layers for van der Waals epitaxy. However, the substrates below 2D materials may still interact with the layers grown during epitaxy (epilayers), as in the case of the so-called wetting transparency documented for graphene. Here we show that the weak van der Waals potential of graphene cannot completely screen the stronger potential field of many substrates, which enables epitaxial growth to occur despite its presence. We use density functional theory calculations to establish that adatoms will experience remote epitaxial registry with a substrate through a substrate-epilayer gap of up to nine ångströms; this gap can accommodate a monolayer of graphene. We confirm the predictions with homoepitaxial growth of GaAs(001) on GaAs(001) substrates through monolayer graphene, and show that the approach is also applicable to InP and GaP. The grown single-crystalline films are rapidly released from the graphene-coated substrate and perform as well as conventionally prepared films when incorporated in light-emitting devices. This technique enables any type of semiconductor film to be copied from underlying substrates through 2D materials, and then the resultant epilayer to be rapidly released and transferred to a substrate of interest. This process is particularly attractive in the context of non-silicon electronics and photonics, where the ability to re-use the graphene-coated substrates allows savings on the high cost of non-silicon substrates.

Although several types of architecture combining memory cells and transistors have been used to demonstrate artificial synaptic arrays, they usually present limited scalability and high power consumption. Transistor-free analog switching devices may overcome these limitations, yet the typical switching process they rely on-formation of filaments in an amorphous medium-is not easily controlled and hence hampers the spatial and temporal reproducibility of the performance. Here, we demonstrate analog resistive switching devices that possess desired characteristics for neuromorphic computing networks with minimal performance variations using a single-crystalline SiGe layer epitaxially grown on Si as a switching medium. Such epitaxial random access memories utilize threading dislocations in SiGe to confine metal filaments in a defined, one-dimensional channel. This confinement results in drastically enhanced switching uniformity and long retention/high endurance with a high analog on/off ratio. Simulations using the MNIST handwritten recognition data set prove that epitaxial random access memories can operate with an online learning accuracy of 95.1%.

Although flakes of two-dimensional (2D) heterostructures at the micrometer scale can be formed with adhesive-tape exfoliation methods, isolation of 2D flakes into monolayers is extremely time consuming because it is a trial-and-error process. Controlling the number of 2D layers through direct growth also presents difficulty because of the high nucleation barrier on 2D materials. We demonstrate a layer-resolved 2D material splitting technique that permits high-throughput production of multiple monolayers of wafer-scale (5-centimeter diameter) 2D materials by splitting single stacks of thick 2D materials grown on a single wafer. Wafer-scale uniformity of hexagonal boron nitride, tungsten disulfide, tungsten diselenide, molybdenum disulfide, and molybdenum diselenide monolayers was verified by photoluminescence response and by substantial retention of electronic conductivity. We fabricated wafer-scale van der Waals heterostructures, including field-effect transistors, with single-atom thickness resolution.

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Optical frequency combs, which emit pulses of light at discrete, equally spaced frequencies, are cornerstones of modern-day frequency metrology, precision spectroscopy, astronomical observations, ultrafast optics and quantum information. Chip-scale frequency combs, based on the Kerr and Raman nonlinearities in monolithic microresonators with ultrahigh quality factors, have recently led to progress in optical clockwork and observations of temporal cavity solitons. But the chromatic dispersion within a laser cavity, which determines the comb formation, is usually difficult to tune with an electric field, whether in microcavities or fibre cavities. Such electrically dynamic control could bridge optical frequency combs and optoelectronics, enabling diverse comb outputs in one resonator with fast and convenient tunability. Arising from its exceptional Fermi-Dirac tunability and ultrafast carrier mobility, graphene has a complex optical dispersion determined by its optical conductivity, which can be tuned through a gate voltage. This has brought about optoelectronic advances such as modulators, photodetectors and controllable plasmonics. Here we demonstrate the gated intracavity tunability of graphene-based optical frequency combs, by coupling the gate-tunable optical conductivity to a silicon nitride photonic microresonator, thus modulating its second- and higher-order chromatic dispersions by altering the Fermi level. Preserving cavity quality factors up to 10 in the graphene-based comb, we implement a dual-layer ion-gel-gated transistor to tune the Fermi level of graphene across the range 0.45-0.65 electronvolts, under single-volt-level control. We use this to produce charge-tunable primary comb lines from 2.3 terahertz to 7.2 terahertz, coherent Kerr frequency combs, controllable Cherenkov radiation and controllable soliton states, all in a single microcavity. We further demonstrate voltage-tunable transitions from periodic soliton crystals to crystals with defects, mapped by our ultrafast second-harmonic optical autocorrelation. This heterogeneous graphene microcavity, which combines single-atomic-layer nanoscience and ultrafast optoelectronics, will help to improve our understanding of dynamical frequency combs and ultrafast optics.

Graphene, a unique two-dimensional material of carbon in a honeycomb lattice [1], has brought remarkable breakthroughs across the domains of electronics, mechanics, and thermal transport, driven by the quasiparticle Dirac fermions obeying a linear dispersion [2-3]. Here we demonstrate a counter-pumped all-optical difference frequency process to coherently generate and control THz plasmons in atomic layer graphene with an octave tunability and high efficiency. We leverage the inherent surface asymmetry of graphene for a strong second-order nonlinear polarizability  (2) [4-5], which together with tight plasmon field confinement, enables a robust difference frequency signal at THz frequencies. The counter-pumped resonant process on graphene uniquely achieves both energy and momentum conservation. Consequently we demonstrate a dual-layer graphene heterostructure that achieves the charge-and gate-tunability of the THz plasmons over an octave, from 9.4 THz to 4.7 THz, bounded only by the pump amplifier optical bandwidth. Theoretical modeling supports our single-volt-level gate tuning and optical-bandwidth-bounded 4.7 THz phase-matching measurements, through the random phase approximation with phonon coupling, saturable absorption, and below the Landau damping, to predict and understand the graphene carrier plasmon physics. 2The discovery of graphene spurred dramatic advances ranging from condensed matter physics, materials science to physical electronics, mechanics, and thermal processes. In optics [6][7], the additional chiral symmetry of the Dirac fermion quasiparticles of graphene [8] enables an optical conductivity defined only by the fine structure constant  [9], one that is remarkably charge-density tunable [10][11] and with broadband nonlinearities [12][13][14][15]. The collective oscillations of the two-dimensional correlated quasiparticles in graphene [16] naturally make for a fascinating cross-disciplinary field in graphene plasmonics [17], with applications ranging from tight-field-enhanced modulators, detectors, lasers, polarizers, to biochemical sensors [18][19][20][21][22]. Different from conventional noble metal plasmons, graphene plasmons are dominant in the terahertz and far-infrared frequencies [23]. To excite and detect these plasmons, specialized techniques such as resonant scattering nanoscale antennae near-field microscopy or micro-and nano-scale scattering arrays have been pursued, albeit still using terahertz/far-infrared sources [24][25][26][27][28]. Recently nonlinear optical processes, only with free-space experiments, have proven especially effective in generating graphene plasmons with efficiencies up to 10 -5 [4][5]. However, to date, it is challenging to generate, detect, and control on-chip graphene plasmons all-optically, a key step towards planar integration and next-generation high-density optoelectronics.Concurrently THz generation has recently been revisited by a number of studies for imaging, spectroscopy, and communications [29]. While a wide tunability in THz can provide new g...