Date/Time: 04-04-2018 - Wednesday - 05:00 PM - 07:00 PM
Fatemeh Rezaeifar1 Rehan Kapadia1

1, University of Southern California, Los Angeles, California, United States

Metallic, III-V semiconductor, graphene and carbon-based photocathodes have been extensively investigated both from material perspective as well as structures that enabling higher quantum efficiency. One of the major challenges that result in poor quantum efficiency is the small optical absorption via directly illuminating photons. While metallic emitter reflects major portion of the illuminating light and optical absorption is small in monolayer graphene, III-V semiconductor has higher optical absorption. However, this interaction is limited to beam size and alignment of laser beam to appropriately focus on emitter. In addition to material, researchers proposed structured emitter such as sub-wavelength grating emitter as a means to increase optical absorption. This design requires nm scale precision for grating in metallic or semiconductor photocathodes which is a serious challenge in fabrication. Here, for the first time, we introduce an integrated photonic waveguide assisted photocathodes in which evanescent optical mode from the waveguide will be gradually coupled in to emitter layer. This method provides greater chance of interaction for photon absorption in emitter layer. Our demonstration consisting of graphene as an electron emitter layer on top of silicon nitride optical waveguide. We fabricated V-groove for coupling 445 nm CW laser from 200 µm core optical fiber in to silicon nitride waveguide. The fabricated silicon nitride waveguide has a height of 5 µm and its width tapered down from 200 µm at the opening of V-groove to 50 µm where a continuous layer of graphene is transferred using wet transfer technique on waveguide and extended on two Ti/Au contacts on two sides of the optical waveguide. We present the first demonstration of waveguide coupled laser induced electron emission and show comparative result with free space laser induced electron emission. Photocurrent observed at E-field as small as 0.3V/µm and the IV curves under different laser power indicates strong power dependency of the emission current. The current versus laser power curve indicates two photon process that matches with our expectation given graphene work function is higher than photon energy (2.78eV) in our laser source. Through experimental results, it is shown that up to 50X higher photocurrents can be obtained from graphene photocathode utilizing integrated photonic approach due to efficient absorption of photons. This approach can be used with other materials as well and with the recent advances in developing integrated laser this proposed device will be a solution for obtaining higher quantum efficiency photocathodes.

Meeting Program

5:00 PM–7:00 PM Apr 4, 2018 (America - Denver)

PCC North, 300 Level, Exhibit Hall C-E