LETTERS https://doi.org/10.1038/s41566-019-0524-1 1 Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, CA, USA. 2 These authors contributed equally: Fatemeh Rezaeifar, Ragib Ahsan. *e-mail: rkapadia@usc.edu Photoemission plays a central role in a wide range of fields, from electronic structure measurements to free-electron laser sources. In metallic emitters, single-photon 1 , multipho- ton 2–5 or strong-field emission 6–10 processes are the primary photoemission mechanisms. Here, using a sub-work-function 3.06 eV continuous-wave laser, photoemission from wave- guide-integrated monolayer graphene is observed to occur at peak power densities >5 orders of magnitude lower than reported multiphoton and strong-field emission 6,11,12 . The behaviour is explained by the emission of hot electrons in gra- phene. In monolayer graphene, the need for photoelectrons to be transported to an emitting surface is eliminated, dramati- cally enhancing the probability of emission before thermaliza- tion. These results indicate that integrated-photonics-driven hot-electron emission provides a rich new area of exploration for both electron emission and integrated photonics. Metallic photoemission cathodes operate through three pro- cesses: (1) single-photon excitation 1 , where a single photon imparts enough energy to an electron to overcome the material work func- tion; (2) multiphoton emission 2–5 , where a high intensity of pho- tons drives a single electron to undergo multiple absorption events before scattering causes loss of energy; or (3) strong-field emis- sion 6–10 , where the electric field of the photons themselves drive field emission. Multiphoton and strong-field emission processes typically use photon energies that are below the metal work func- tion, but require high-power pulsed laser sources. In this work, we experimentally demonstrate that sub-work-function photons can be used at powers below the threshold for multiphoton emission or strong-field emission to modulate the graphene electron emission current through hot-electron emission. Monolayer graphene is cho- sen as a model material system due to the relatively low optical pow- ers needed to enable large non-equilibrium electron temperatures 13 , long scattering times 14–16 and the ultra-thin geometry, eliminating the need for hot-electron transport to emitting surfaces. Integrated photonic circuits simultaneously enables us to eliminate the need for free-space alignment, and overcome the absorption limitations of a single layer of graphene 17,18 . We integrate monolayer graphene with silicon nitride waveguides on a silicon substrate. While single devices are reported here, an integrated photonics approach lays the foundation for on-chip arrays of photoemitters where each element can be independently controlled. Figure 1a schematically illustrates the structure of the waveguide- integrated graphene electron emitter. A layer of graphene is trans- ferred to a Si/SiO 2 substrate with a Si 3 N 4 waveguide and gold contacts. Photons are coupled onto the chip via an optical fibre fixed to a U-groove etched into silicon and aligned to the waveguide. Detailed fabrication steps are given in the Methods and summarized in Supplementary Fig. 1. The free-space-coupled version of the emitter is shown in Fig. 1b. The samples are measured in vacuum using a high-voltage source to apply the extracting field, an optical fibre for laser excitation and an ammeter for current measurement (Fig. 1c). For these measurements, the gold contacts are grounded, and the anode is held at high voltage (1–10 kV). The optical absorption of graphene in the ultraviolet–visible spectrum range is primarily from interband transitions 17,18 (Fig. 1d), with electrons excited from ½E Ph below the Dirac point to ½E Ph above the Dirac point where E Ph is the energy of the photon. While electrons can be excited by both single-photon and multiphoton absorption processes, only single- photon absorption is considered here due to the low optical excita- tion powers of <5 mW continuous-wave (CW) laser. In this work, 405 nm (3.06 eV) photons are used to excite the graphene. These do not have enough energy to excite electrons above the graphene work function. Thus, if the optical power is below the multiphoton emis- sion threshold, there should be no observed photocurrent unless the graphene is heated to the point that thermionic emission occurs, or photoexcited electrons are emitted before relaxation. However, photoexcited electrons will undergo both electron– electron scattering and electron–phonon scattering, thermalizing on a timescale of less than a picosecond. Figure 1e schematically represents the photoexcited electron and the subsequent energy loss through scattering. In the presence of a vertical electric field, tun- nelling through the vacuum barrier offers another possible pathway for the photoexcited electron. For monolayer graphene, the photo- excited electrons will experience the tunnelling potential over the entire cooling process. Thus, the energy of the absorbed photon, the scattering rate and the rate of tunnelling into the vacuum will all determine the pathway an excited electron takes. Since the photoex- cited electrons experience a smaller tunnel barrier as compared with the thermal population, hot electrons will dominate the emission currents below a certain critical electric field while the large popula- tion of thermal electrons will dominate emission above the critical field. Figure 1f shows the thermal electron distribution of elec- trons and the hot electrons where E Vac is the energy of the vacuum level. This will then give rise to two distinct emission behaviours: (1) emission of hot electrons before thermalization, which should be dependent on photon absorption and dominant below some critical field, and (2) field emission of the cold electrons, which will be independent of photon absorption and dominant above some critical field. Figure 2a shows an optical image of a fabricated integrated pho- tonics device, with the U-groove, tapered Si 3 N 4 waveguide and monolayer graphene visible. Figure 2b shows scanning electron microscope images of the U-groove (top left), plan-view of a Si 3 N 4 waveguide (top right) and cross-sectional view of a Si 3 N 4 wave- guide (bottom). To ensure that the graphene was successfully trans- ferred onto our waveguide, we carried out Raman spectroscopy Hot-electron emission processes in waveguide-integrated graphene Fatemeh Rezaeifar 1,2 , Ragib Ahsan  1,2 , Qingfeng Lin 1 , Hyun Uk Chae 1 and Rehan Kapadia  1 * NATURE PHOTONICS | VOL 13 | DECEMBER 2019 | 843–848 | www.nature.com/naturephotonics 843