Ultra-high-Q free-space coupling to microtoroid resonators

Area of Science:

• Optics and Photonics
• Nanotechnology
• Biochemical Sensing

Background:

• Whispering gallery mode (WGM) microtoroid resonators offer high sensitivity for single-molecule detection.
• Current laboratory use is limited by fragile, vibration-sensitive optical fiber tapers for light coupling.
• Precise alignment is required for evanescent coupling, hindering practical applications.

Purpose of the Study:

• To eliminate the need for optical fiber tapers in WGM microtoroid resonators.
• To enable robust and practical implementation of WGM microtoroid resonators outside laboratory settings.
• To enhance biochemical sensing capabilities through improved light coupling methods.

Main Methods:

• Developed a free-space coupling technique using a long working distance objective lens and a digital micromirror device (DMD).
• Light is injected and scattered light is collected via free-space optics.
• Integrated with the frequency locked whispering evanescent resonator (FLOWER) for sensing experiments.

Main Results:

• Achieved high Q-factors (quality factors) comparable to fiber-coupled systems.
• Observed electromagnetically induced transparency (EIT)-like and Fano resonances in a single cavity.
• Demonstrated robust temperature sensing and examined thermal nonlinear optical effects.

Conclusions:

• Free-space coupling eliminates the need for fragile optical fibers, improving device robustness.
• The large effective coupling area simplifies alignment and enhances positioning tolerance.
• This approach provides a foundation for real-world applications of WGM microtoroid resonators.