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Ultra-high-Q whispering gallery mode resonators provide very
long photon storage lifetimes through many round trips within a circular
geometry. (1, 2). The exterior of the whispering gallery also provides
a convenient surface on which molecules can interact with the circulating
lightwave. This interaction can modify either the optical path length of
the light or its loss. The ability to the detect the resulting changes in
resonant properties depends on the intrinsic loss of the resonator as given
by its Q factor (3,4). In this project, we are investigating the use of ultra-high-Q
microtoroid resonators as a tool for sensitive detection of bio-molecules.
Like other detection techniques, such as Surface Plasmon Resonance (SPR)(5)
or total internal reflection fluorescence microscopy (TIRF),(6)
the interaction which leads to detection occurs at the surface of the resonant
cavity. Unlike in a waveguide sensor, where the molecule is sampled a single
time, in an ultra-high-Q toroidal microresonator with a quality factor of
100 million, the molecule is sampled over 100,000 times.
Figure
1. A scanning electron micrograph (SEM) of an array of ultra-high-Q
silica microtoroid reosnators. |
Ultra-high-Q toroidal microcavities can be fabricated in large
planar arrays using lithographic techniques (Figure 1). These silica
microcavities have demonstrated quality factors greater than 100 million
when immersed in an aqueous environment. (1, 2) Additionally,
the silica microtoroid surface can be functionalized to target a specific
molecule, endowing the toroidal resonator with both sensitivity and specificity.
This improvement in sensitivity was demonstrated by detecting
heavy water. (7) Because the optical absorption of H2O
is larger than D2O, the quality factor of the microtoroid immersed
in H2O
is lower than in D2O at 1300nm. By monitoring the quality
factor, reversible detection of D2O was demonstrated (Figure 2a). Additionally,
the lower bound on detection was determined to be .0001% (1ppmv) which represents
a 30-fold improvement over previous D2O detection methods Figure
2b).
Figure
2. a) As the concentration of heavy water in normal water is decreased
(red circles) the quality factor of the microcavity decreased. As
the concentration of heavy water increases (green triangles), the
Q is recovered. b) At 1ppmv (0.0001%) of heavy water in normal
water, there is a detectable change in quality factor. |
In biological
detection experiments, both specificity and sensitivity are important. While the ultra-high-Q optical resonator is inherently sensitive,
specificity is achieved by functionalizing the surface of the microtoroid. Several
different surface functionalization techniques have been used (antibody,
biotin), each one specific to the target molecule.
A. M. Armani,
D. K. Armani, B. Min, K. J. Vahala, S. M. Spillane, Applied Physics Letters 87,
151118 (Oct 10, 2005).
D. K. Armani, T. J. Kippenberg,
S. M. Spillane, K. J. Vahala, Nature 421, 925 (Feb
27, 2003).
R. W. Boyd, J. E. Heebner, Applied
Optics 40, 5742 (Nov 1, 2001).
S. Arnold, M. Khoshsima,
I. Teraoka, S. Holler, F. Vollmer, Optics Letters 28,
272 (Feb 15, 2003).
K. Nakatani, S. Sando,
I. Saito, Nature Biotechnology 19, 51 (Jan, 2001).
A. Hoshino et al., Microbiology
and Immunology 49, 461 (2005).
A. M. Armani, Vahala,
K. J., Optics Letters 31 (2006).
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