Fabrication of a nanofiber Bragg cavity with high quality factor using a focused helium ion beam

Abstract

Nanofiber Bragg cavities (NFBCs) are solid-state microcavities fabricated in an optical tapered fiber. NFBCs are promising candidates as a platform for photonic quantum information devices due to their small mode volume, ultra-high coupling efficiencies, and ultra-wide tunability. However, the quality (Q) factor has been limited to be approximately 250, which may be due to limitations in the fabrication process. Here we report high Q NFBCs fabricated using a focused helium ion beam. When an NFBC with grooves of 640 periods is fabricated, the Q factor is over 4170, which is more than 16 times larger than that previously fabricated using a focused gallium ion beam.

Fabrication of optical nanofibre-based cavities using focussed ion-beam milling: a review

“…Milled holes were ∼ 100 nm × 100 nm Scanning ion microscope (SIM) images of optical cavities with Bragg grating structures on nanofibres fabricated using FIB. a ∼ 560 nm diameter fibre and grooves of ∼ 100 nm deep and ∼ 150 nm wide, reprinted with permission from [40], copyright The Optical Society; b scale bar of 1 μm , ∼ 270 nm diameter fibre and grooves of 45 nm, 300 nm pitch and 450 nm defect length, reprinted from [52] with permission, Creative Commons license; c scale bar of 1 μm , ∼ 310 nm diameter fibre and grooves of 30 nm, 320 nm pitch and 840 nm defect length, reprinted with permission from [53], copyright The Optical Society with a 310 nm pitch. Finite-difference time-domain (FDTD) simulation of the structure predicted a reflectivity higher than 80% with only 30 periods, resulting in a very compact device.…”

“…As discussed earlier, deposition of Ga ions can be a source of loss in the fibre cavity. To overcome this limitation, ion milling using a different ion was demonstrated by Takashima et al [53]. A helium (He) ion beam was used to mill Bragg gratings and to generate a nanofibre cavity with higher Q (Fig.…”

“…it can be considered contamination free. In this work [53], the authors compared cavities produced with He and Ga ions. They showed that the Q-factor was higher in the cavity milled with He (450 versus 250 for the Ga FIB milling).…”

“…Despite advantages of using a He ion source [53], Ga is still the most accessible ion type in FIB systems. The system we use is a gallium ion source FIB integrated with an SEM (FEI system, model Helios NanoLab G3 UC), in which the ion beam is tilted at 52 • in relation to the electron beam.…”

“…In the literature, there are several different techniques describing how to produce cavities directly in optical nanofibres [42][43][44][47][48][49][50][51]. One method that is proving particularly successful is the use of FIB milling to fabricate Bragg mirrors for fibre-based optical cavities [40,[51][52][53]. This technique uses a highly focussed ion beam to cut patterns into the fibre with very high resolution.…”