1. Biosensing with silicon chip based microcavities
Warwick Bowen

2. Co-workers PhD Students Collaborators Jacob Chemmannore
Matthew McGovern
Terry McRae
Jian Wei Tay
Collaborators
Tobias Kippenberg (Max Planck)
Jeff Kimble (Caltech)
Kerry Vahala (Caltech)

3. Aims of research Broad goal: apply experience in
quantum/atom optics to current biophotonics problems.
Aim: implement novel and effective solutions.
Specific short and medium term goals in two areas:
Biophotonic applications of ultrahigh Q optical microcavities used in cavity QED experiments.
Quantum limits of particle position measurement with optical tweezers.

4. Motivation Great need for highly sensitive biosensing techniques
Fundamental contribution to the understanding of:
DNA binding
Protein conformational changes
Molecular motors
Cellular processes
Ion channels…
Pharmacological and biological diagnosis applications:
Enhance control and understanding of biochemical processes leading to greater yields
Small molecule aspects of drug design
Detect biological pathogens, drugs, chemicals…

5. Light-matter interaction
Interaction of light and matter primarily due to optical electric field coupling to electric dipoles in matter.
Determines all major atom-light phenomena (refraction, absorption, Rayleigh scattering, Raman scattering, fluorescence…).
In biophotonic sensing systems, typically want to maximise interaction strength
Especially for single molecule detection.

6. Light-matter interaction
Strength of interaction determined by:
Increase by enhancing either d or E.
Typically:
For E confine optical field to small volume, and increase intensity (e.g. high NA lens, femtosecond pulses).
For d label the molecule with a fluorophore or metallic nano or micro-scale sphere.

7. Current biosensing systems
Many biological imaging and manipulation systems based on such enhancements:
Scanning near-field optical microscopes (SNOMs)
Surface enhanced Raman spectrometers (SERS)
Surface plasmon resonance imaging systems (SPR)
Evanescent wave induced fluorescence spectrometers
Confocal fluoresence microscopes
Optical tweezers …

8. Current biosensing systems
However, in terms of the long standing goals of single small molecule detection, observation, and manipulation the usefulness of such techniques still relatively limited.
Techniques with resolution capable of single molecule detection currently:
Rely on molecular labels which can be difficult to attach in practice, and can affect observed behaviour.
Are not real-time, or have temporal resolution in the seconds to milliseconds regime, and therefore cannot capture the fast dynamics of molecules such as molecular motors, and of molecular binding.

9. Optical microcavity based biosensing
New techniques needed to provide further insight into single molecule dynamics.
Interaction strength can be enhanced beyond what is presently possible by confining light not only spatially, but also temporally.
Achieved in optical microcavities
used in cavity quantum
electrodynamics.
Preliminary investigations into
molecular detection by
Vollmer et al.
[Arnold et al., Opt. Lett. 28, 272 (2003)]
[Vollmer et al., Appl. Phys. Lett. 80, 4057 (2002)]

10. Optical microcavity based biosensing
Focus on microsphere cavities:
Light resonates via total internal
reflection in WGMs.
Part of the WGM located outside
microsphere in exponentially
decaying evanescent field.
Optical taper coupling.
Sharp spectral resonances when
optical path length equals integer
number of optical wavelengths.
[Arnold et al., Opt. Lett. 28, 272 (2003)]
[Vollmer et al., Appl. Phys. Lett. 80, 4057 (2002)]

11. Optical microcavity based biosensing
Interaction of protein molecule
with evanescent field polarises
molecule, alters local refractive
index experienced by WGM.
Causes optical path length
change.
Detected as shift in optical
resonance frequencies.
No molecular labels are required.
The surface of microsphere
sensitisable – adsorbs only
specific proteins.

12. Optical microcavity based biosensing
Minimum detectable molecule size determined by polarisability of molecule and optical electric field strength.
Optical electric field maximised by:
Maximising Q of optical resonance (hence “ultrahigh Q”).
Minimising V of optical field (hence “microcavity”).
Vollmer:
Silica microspheres immersed in water.
Q~106, V~3000 m3.
[Vollmer et al., Appl. Phys. Lett. 80, 4057 (2002)]

13. Optical microcavity based biosensing
They:
Experimentally demonstrated bulk detection of specific proteins (BSA).
Predicted adsorption of as few as 6000 BSA protein molecules was detectable.
Larger protein molecules (typically) have larger induced dipoles.
Detection of smaller numbers possible.
However, rare to find proteins with molecular weight > 15 BSA.
[Vollmer et al., Appl. Phys. Lett. 80, 4057 (2002)]

14. Optical microcavity based biosensing
To achieve single molecule detection need better microcavities.
Vollmer’s V limited by:
Microsphere geometry.
Optical wavelength (1300 nm).
Fabrication issues.
Vollmer’s Q limited primarily by optical absorption of water
High at 1300 nm.
Overcome these limits with new type of optical microcavity, the microtoroid.
[Armani et al., Nature 421, 925 (2003)]

15. Microtoroids
WGM type ultrahigh Q optical microcavities similar to microspheres.
As the name suggests, the geometry is toroidal rather than spherical.
Reproducibly lithographically fabricated:
Etch mm diameter circular SiO2 pad on silicon wafer.
Etch away Silicon with XeF2 to produce a SiO2 disk on a pedestal.
Produce toroid by melting disk
with a CO2 laser.
Surface tension causes the
surface of the resulting
microtoroid to be exceptionally
smooth.
[Armani et al., Nature 421, 925 (2003)]

16. Microtoroids Smaller mode volumes due to azimuthal mode compression.
For large compression, toroid mode identical
to mode of single mode fiber.
Very efficient coupling
achievable using tapered fibers
(>99.5%).
[Armani et al., Nature 421, 925 (2003)]

17. Microtoroids Smaller mode volumes due to azimuthal mode compression.
For large compression, toroid mode identical
to mode of single mode fiber.
Very efficient coupling
achievable using tapered fibers
(>99.5%).
[Kippenberg et al., Appl. Phys. Lett. 83, 797 (2003)]

18. Microtoroids Smaller mode volumes due to azimuthal mode compression.
For large compression, toroid mode identical
to mode of single mode fiber.
Very efficient coupling
achievable using tapered fibers
(>99.5%).
[Kippenberg et al., Appl. Phys. Lett. 83, 797 (2003)]

19. Microtoroids for biosensing
V’s as small as 75 m3 and Q‘s as high as 5·108 (finesse > 106) routinely achievable with 1550 nm light in air.
40 reduction in V and a 200 increase in Q c.f. microspheres studied by Vollmer et al..
However, when immersed in water, the quality is predicted to drop to around 106 as a result of optical absorption.

20. Microtoroids for biosensing
Use 532 nm light.
Minimum absorption wavelength of water.
Absorption coefficient four orders of magnitude smaller than at 1550 nm.
Should not limit Q.
Furthermore, microcavity dimensions ultimately limited by the optical wavelength used.
Reduction from 1550 to 532
nm should allow (1550/532)3 
 25 times reduction in V.
In principle 1000 times total
mode volume reduction
possible.

21. Microtoroids for biosensing
Optical microcavity based biosensor sensitivity proportional to ratio Q/V.
Therefore potential for 1000  200 = 200,000 times sensitivity improvement c.f. Vollmer experiments.
Should easily facilitate the detection of single molecules.
Aim of the microcavity research programme at Otago:
Fabricate microtoroids with this
sort of sensitivity
Use to detect single unlabeled
molecules
Study dynamics.

22. Where we are currently Developed: In development: For the future:
Laser reflow stage of microtoroid fabrication
Optical fibre taper pulling setup
Toroid/taper coupling setup
In development:
Remaining steps of
microtoroid fabrication
Water immersion bath for
bulk protein detection
Laser frequency/taper
position control systems
For the future:
Single molecule detection!
...

23. Cavity quantum electro-dynamics with microtoroids
First demonstration of strong coupling between a single atom and a single photon in a monolithic optical resonator.
Single atom detection events
[Aoki et al., Nature 443, 671 (2006)]

24. Conclusion
Microtoroid based optical biosensors have potential to facilitate detection and monitoring of single biomolecules.
New insight into the dynamics of motor molecules, and molecular binding processes.
Array of lithographically fabricated microtoroids, each surface activated for a particular biomolecule can be envisaged.
Such a system could be used to monitor the concentration of multiple proteins/molecules in real time:
Quality control in water treatment
systems.
Early detection systems for biotoxins
and biological warfare agents.
Complimentary to DNA microarrays/
SPR arrays (Biacore).

25. Photonics and optical microresonators
Q-V
V: 75 mm3
Q: 5×108
Q: 107
V: 300 mm3
[Vahala et al., Nature (2003)]