Fig. 1.

Phase-matching methods used for optical microresonators. a Birefringent phase matching. b Cyclic phase matching. c Modal phase matching. d Periodic poling in an LN microresonator. a Reprinted with permission from Ref. [59]. Copyright 2010, American Physical Society. b Reprinted with permission from Ref. [63]. Copyright 2019, American Physical Society. c Reprinted with permission from Ref. [66]. Copyright 2019, American Physical Society. d Reprinted with permission from Ref. [70]. Copyright 2019, Optica