Download Conditional Anti-Bunching of Photons Generated in a Cold Atomic

Biosensing with silicon chip
based microcavities
Warwick Bowen
Co-workers
PhD Students
Jacob Chemmannore
Matthew McGovern
Terry McRae
Jian Wei Tay
Collaborators
Tobias Kippenberg (Max Planck)
Jeff Kimble (Caltech)
Kerry Vahala (Caltech)
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.
Motivation
• Great need for highly sensitive biosensing
techniques
• Fundamental contribution to the understanding of:
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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…
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.
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.
Current biosensing systems
• Many biological imaging and manipulation systems
based on such enhancements:
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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
…
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.
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)]
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)]
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.
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)]
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)]
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)]
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 20-120 m 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)]
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)]
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)]
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)]
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.
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.
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.
Where we are currently
• Developed:
– 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!
– ...
Cavity quantum electrodynamics 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)]
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.
systems.
• Complimentary to DNA microarrays/
SPR arrays (Biacore).
Photonics and optical
microresonators
• Q-V
Q: 107
Q: 5×108
V: 300 m3
Q: 5×108
V: 75 m3
[Vahala et al., Nature 424 839 (2003)]