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Title: Biosensing with silicon chip based microcavities


1
Biosensing with silicon chip based microcavities
  • Warwick Bowen

2
Co-workers
PhD Students 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.
  • Q106, V3000 ?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 ?gt 15? BSA.

Vollmer et al., Appl. Phys. Lett. 80, 4057
(2002)
14
Optical microcavity based biosensing
  • To achieve single molecule detection need better
    microcavities.
  • Vollmers V limited by
  • Microsphere geometry.
  • Optical wavelength (1300 nm).
  • Fabrication issues.
  • Vollmers 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 20-120 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
  • (gt99.5).

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
  • (gt99.5).

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
  • (gt99.5).

Kippenberg et al., Appl. Phys. Lett. 83, 797
(2003)
19
Microtoroids for biosensing
  • Vs as small as 75 ?m3 and Qs as high as 5108
    (finesse gt 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
  • 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.
  • systems.
  • Complimentary to DNA microarrays/
  • SPR arrays (Biacore).

25
Photonics and optical microresonators
  • Q-V

Vahala et al., Nature 424 839 (2003)
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