Plasmon-enhanced Single Molecule Detection with Microcavities

Label-free detection of single molecules has been a dream of biologists and biotechnologists. We have come one step closer towards achieving this goal by coupling optical resonators to nanoplasmonic structures. We use whispering gallery modes (WGM) in optical microsphere resonators to excite plasmon resonances in 55 nm gold nanoparticles. Strong electromagnetic field enhancements (hotspots, in red) are observed at the nanoparticle site without significant losses to the quality factor of the resonator. When a molecule binds to the hotspot location it tunes the resonance frequency in proportion to the encountered field strength. The hotspots can provide large sensitivity enhancements – bringing label-free single molecule detection within reach, see right. Such single molecule detection capability is essential for designing the next generation of biosensors and to elucidate intricate mechanisms of molecular machines.  We demonstrate this new sensing concept in collaboration with Pennsylvania State University and MIT.
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Whispering Gallery Mode Biosensing

Optical resonance is created by localizing coherent light within a micro or nanoscale structure so that it interferes constructively. Examples for such miniature optical resonators are silica microspheres and silicon photonic crystals. Because these optical resonators are almost immune to damping in a liquid, they are ultra sensitive biosensors: for example, single virus particles can be detected from discrete resonance frequency-shifts without requiring any chemical or fluorescent labeling of the particles. Sensitivity on the single particle level is possible due to the high quality (Q-) factor and the small size of the resonator.

Read the Nature Methods Perspective

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Optical Resonator Physics

Characterizition optical wave propagation along line defects in two-dimensional arrays of air-holes in free-standing silicon slabs. The fabricated waveguides contain random variations in orientation of the photonic lattice elements which perturb the in-plane translational symmetry. The vertical slab symmetry is also broken by a tilt of the etched sidewalls. We discuss how these lattice imperfections affect out-of-plane scattering losses and introduce a mechanism for high-Q cavity excitation related to polarization mixing.

See "Out-of-plane scattering from vertically asymmetric photonic crystal slab waveguides with in-plane disorder" for the full article.

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Anderson Localization & Photonic Quasicrystals

Fabrication disorder is perceived as impediment to ideal performance of optical resonators, and disorder ultimately limits the performance of any engineered device. However, when superimposed on a photonic band gap material, disorder itself can induce light localization. We show such strong photon localization by disorder in photonic crystal waveguides. In this design concept, disorder and variability does not limit device performance, but instead is the basis for high-Q resonance. This new approach to engineering circumvents limitations posed by disorder, and illustrates a bioinspired design principle found throughout nature.

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Optical Trapping and Accumulation Of Nanoparticles

A recent paper in Nature Biotechnologl has pointed out an unexplained discrepancy in miniature biosensors between theoretical binding rates and experimental results. For extremely dilute (sub-femtomolar) solutions the binding delay time calculated based on diffusive and convective transport of target molecules to miniature sensors are impractically long in contrast to recently measured experimental times. A particular example of this discrepancy is the large binding rates of bioparticles to an optical Whispering Gallery Mode (WGM) micro-toroidal bio-sensor.

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Surface Plasmon Resonance

Since their introduction nearly 30 years ago, biosensors based on surface plasmon resonance (SPR) have become one of the most popular tools used interrogate bimolecular interactions. Despite this, and somewhat surprisingly, there are still many phenomena in this system which are not well understood.

In our research, we look at the influence of focussed beams on plasmon excitation using the Kreschmann attenuated total reflection (ATR) configuration. The effect of such a focussed beam is many-fold.

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