Efficient and ultrafast optical modulation of on-chip thermionic emission using resonant cavity coupled electron emitters

Here, the authors explore microscale optical cavities coupled to thermionic emitters as a means to enable a class of efficient and ultrafast optically modulated, on-chip, thermionic electron emitters. They term this class of devices optical cavity thermionic emitters (OCTET). The devices consist of a microfabricated optical cavity, such as Fabry–Perot or ring resonator, and a heterostructured thermionic emitter with a small bandgap or metallic thermionic emitter (e.g., LaB₆) deposited on a wider bandgap electrical and thermal conductor (e.g., doped Si). By tuning the resonant wavelength of the optical cavity, the authors can ensure photons are efficiently and selectively absorbed by the small bandgap/metallic emitter, enabling design of gigahertz–terahertz regime on-chip electron emission sources. The work here focuses on elucidating the properties of single cavity-single emitter OCTETs, but may be applied to more complex cavity-tip structures. First, the authors establish fundamental design rules based solely on the cavity optical properties and emitter optical and thermal properties. Next, detailed device simulations are carried out using optical and thermal three dimensional numerical simulations that accurately account for both geometry as well as temperature and wavelength dependent materials properties. The authors illustrate that devices with highly efficient photon to thermal conversion efficiencies >60% can be achieved despite small emitter active absorption volumes <0.01 μm³ and moderate Q optical cavities. Critically, OCTETs may be designed with ultrafast subnanosecond thermal response time, and sub-10 ps current response times, or efficient steady state excitation—with <10 μW of power required to achieve nanoampere level current emission per tip. Importantly, due to the recent advances in integrated photonics and electronics, the structures explored here may be fabricated using standard microfabrication techniques.