1. Ring Resonators & Optofluidic Applications
Damla Ozcelik
12/02/2010

2. Overview What is Ring Resonator? OFRR Applications Principles
Important Parameters
Fabrication Challenges
Different Types, Optofluidic RRs (OFRR)
OFRR Applications
Sensors
Particle Transportation

3. Whispering Gallery Modes (WGM)
Wave propagation is confined to the inside of a cylindrical/spherical surface and guided by it by repeated reflection
Spheres
Disks
Rings
Domes
John E. Heebner et al., Optical microresonators: theory, fabrication, and applications (Springer, 2008).  

4. Disks vs Rings
Main drawback of microresonators surface roughness due to etching processes low Q factors
Disks:
Less scattering loss (1 edge)
Multi-moded
Rings:
More scattering loss (2 edges)
Single-moded
John E. Heebner et al., Optical microresonators: theory, fabrication, and applications (Springer, 2008).  

5. RR Principles Wavelength Dependent Intensity Build-Up
Resonant Frequency 
Intensity Build-Up
Resonators delay incoming signals via the temporary storage of optical energy within the resonator.
Constructive interference circulating optical intensity is built up to a higher value than that initially injected.
Coherent source
John E. Heebner et al., Optical microresonators: theory, fabrication, and applications (Springer, 2008).  

6. Important Parameters
Quality factor: Sharpness of the resonance. Ratio of the operation wavelength and the resonant line width. Stored energy divided by the power lost per optical cycle. (typically: 104 – 109)
Leff : Effective light-matter interaction length.
Free Spectral Range (FSR): Distance between resonance peaks.
Extinction Ratio (ER): The ratio of the intensity on resonance to the intensity off resonance.
DG Rabus, Integrated Ring Resonators: The Compendium (Springer Series in Optical Sciences) (Springer Series in Optical Sciences) (Springer, 2007)

7. Fabrication Challenges
The construction of microrings smaller diameter is a challenging effort.
It requires;
high index contrast
anisotropically etched pedestal waveguide designs
ultrasmooth sidewalls
The lateral-coupling approach demands strict patterning tolerances typically requiring e-beam lithography followed by advanced etching techniques.
John E. Heebner et al., Optical microresonators: theory, fabrication, and applications (Springer, 2008).  

8. Various Types of RRs Race Track Multiple RR Add-Drop All-Pass
DG Rabus, Integrated Ring Resonators: The Compendium (Springer Series in Optical Sciences) (Springer Series in Optical Sciences) (Springer, 2007)

9. All Pass Add-drop

10. Optofluidic RRs (OFRR)
Bio/Chemical Sensing
Higher sensitivity
Planar integration
Basic Applications:
Sensors
Particle Transportation

11. OFRR Sensors Applications medical diagnosis environmental monitoring
food quality control
Ian M. White, Hesam Oveys, and Xudong Fan, “Liquid-core optical ring-resonator sensors,” Optics Letters 31, no. 9 (May 1, 2006):  

12. OFRR Sensors LCORRs Small sample volume is needed Longer Leff Better
Analyte binds to the resonator surface
Δn Change
WGM Evanescent field detection
Longer
Leff
Better
Light-Matter Interaction
Sensing Performance
Small sample volume is needed
Ian M. White, Hesam Oveys, and Xudong Fan, “Liquid-core optical ring-resonator sensors,” Optics Letters 31, no. 9 (May 1, 2006):  

13. Integrated in liquid-core waveguide
Liquid Core OFRR Sensor
Liquid-core waveguide, MMI coupler
0.11 nl liquid
Sensitivity: Δλ /Δn =260nm/RIU
Integrated in liquid-core waveguide
Optical part
(ring resonator)
Microfluidic part
(deliver the sample)
Yujian Huang et al., “Integrated silicon optofluidic ring resonator,” Applied Physics Letters 97, no. 13 (2010):  

14. OFRR switch for optical particle transport
Particles trapped in the evanescent field of a solid-core waveguide
Switching fraction: 80%
Particle velocity: 250% in the ring
Change λ
actively control the particles’ direction
λ On resonance
high optical intensities
gradient force
ParticlesRing
Allen Yang and David Erickson, “Optofluidic ring resonator switch for optical particle transport,” Lab Chip 10, no. 6 (2010):  

15. OFRR switch for optical particle transport
λr= nm (on resonance)
λn= nm (off resonance)

16. Thank you Any Questions?