Reflectionless dual standing-wave microcavity resonator units for photonic integrated circuits

Area of Science:

• Photonics
• Integrated Optics
• Nanophotonics

Background:

• Traveling-wave resonators are crucial in integrated photonics but can suffer from back-reflection.
• Existing solutions often involve complex designs or compromise performance.
• Photonic crystal (PhC) nanobeam cavities offer high quality factors and compact footprints.

Purpose of the Study:

• To propose and demonstrate a novel photonic circuit element that emulates a traveling-wave resonator's through-port response.
• To achieve reflectionless transmission using standing-wave resonant cavities.
• To enable lower-energy, smaller footprint active photonic devices.

Main Methods:

• Theoretical analysis of a novel photonic circuit topology with two evanescently coupled PhC nanobeam cavities.
• Finite-difference time-domain (FDTD) simulations to verify reflection cancellation.
• Experimental fabrication and characterization using a 45 nm silicon-on-insulator (SOI CMOS) process.

Main Results:

• Demonstrated a "reflectionless resonator unit" using two PhC nanobeam cavities with opposite mode symmetries.
• Achieved symmetry-induced destructive interference, eliminating reflection when cavities are wavelength-aligned.
• Experimental results showed transmission dips reduced from -16/-17 dB to -4.2 dB and reflection peaks reduced by 10 dB.
• Measured intrinsic Q factors exceeding 200,000 for the PhC nanobeam cavities.

Conclusions:

• The proposed photonic circuit topology successfully emulates traveling-wave resonator behavior without back-reflection.
• This approach offers a pathway to highly efficient, low-energy active photonic devices.
• The demonstrated quasi-traveling-wave behavior is suitable for cascaded wavelength-division multiplexed (WDM) links.