Optical microresonators, such as spherical or toroidal whispering gallery mode (WGM) resonators, have been successfully applied in a variety of precision measurements that could ultimately lead to practical applications in the fields of bio-medical sensing, environmental monitoring, and metrology. Examples include single-virus detection,1 single-nanoparticle characterization,2 ultralow magnetic field sensing,3 and position readout enabling the detection of quantum mechanical zero-point fluctuations.4
WGM resonators can be regarded as miniaturized, spherical versions of the Fabry-Pérot resonator widely used in diode lasers and telecommunication networks. Due to their extraordinary low absorption and scattering losses, WGM resonators can have ultrasmall linewidths down to the kilohertz range. Part of the resonant light field extends beyond the boundaries of the resonator and samples the surrounding area. When a nanoparticle or a molecule approaches the surface of the WGM resonator, it is polarized by the light field.1 This interaction leads to an effective refractive index change in the resonator and a shift in the resonance frequency proportional to the particle's polarizability and the local electric field (see Figure 1).
Figure 1. Scanning electron microscopy image of a toroidal microresonator. The schematic inset shows a cylindrical nanoparticle attached to the rim of the toroid.
In a WGM resonator the ability to resolve a signal of interest boils down to the minimal detectable frequency shift, which is commonly limited by laser frequency noise. Indeed, employing a shot-noise-limited laser source could enhance sensitivity, but these sources are expensive. Moreover, the shot-noise-limited region is generally constrained to the megahertz frequency range.
An alternative way to enhance sensitivity consists of increasing the frequency shift per particle or molecule. As proposed recently by us and others,5, 6 this can be achieved by locally enhancing the electric field at the position of the particle. A locally enhanced field increases the energy required to polarize the particle relative to the total energy in the resonator and ultimately leads to a larger frequency shift.5 Metallic nanoparticles are known to show plasmonic resonances that lead to large field enhancements at or close to their surface. The resonance frequency depends on the size of the particle and the material. A recent experiment involving plasmonic nanoshells with an outer diameter of 140nm, attached to the surface of a microsphere, showed a fourfold enhancement for the detection of polystyrene beads with diameters of 110nm.5
Further enhancement is possible using nonspherical nanoparticles, where the rod-lightning effect strongly amplifies the electric field at the ends. We have theoretically investigated gold nanorods with a diameter of 10nm and a length-to-diameter aspect ratio of 4.6 Using a boundary element method7 we calculated the electric field distribution around the nanorod for excitation at a resonance wavelength of 803nm (see Figure 2). An electric field intensity enhancement in excess of 1000 is theoretically achievable, leading to a frequency shift increase by a factor of 870 for detection of the single-protein molecule bovine serum albumin (BSA) (see Figure 3). Note that this enhancement is only achieved when the BSA molecule attaches to a nanoparticle that is located in the evanescent field of the WGM resonator.
Figure 2. Electrical field (E) intensity enhancement around a 10×40nm nanorod.
Frequency shift enhancement for the single-protein molecule bovine serum albumin (BSA) with the plasmonic nanorod shown in Figure 2
. λ: Wavelength
To put this result in perspective, we compare the expected frequency shift of a single BSA molecule with and without plasmonic enhancement to the detection limit of a typical toroidal microresonator with 70μm diameter and a quality (Q) factor of 10.7 Assuming a diode laser source with frequency fluctuations similar to its linewidth of 100kHz, the minimal detectable frequency shift is ∼100kHz. Without plasmonic enhancement a single BSA molecule causes a frequency shift of only 5kHz and is therefore outside the detection range (see Figure 4). With plasmonic enhancement the frequency shift per molecule becomes wavelength-dependent and achieves a maximum value of 4MHz at the plasmonic resonance. This shift can be comfortably detected.
Figure 4. Frequency shift of a single BSA molecule with and without plasmonic enhancement.
In conclusion, we have shown that, using cylindrical nanorods, the frequency shift caused by a single BSA molecule in a WGM resonator can be enhanced by a factor of 870 at the plasmon resonance frequency. This result is likely to put single-molecule detection within reach, and we are currently setting up an experiment to test it. In the future, plasmon-enhanced sensing could enable detection of tumor markers at ultralow concentrations for clinical diagnosis of tumors at a very early stage. Optical microresonators are amenable to low-cost mass production, which could make this technique suitable for routine medical screening.
This research was funded by the Australian Research Council grant DP0987146.
Joachim Knittel, Jon D. Swaim, Warwick P. Bowen
The University of Queensland
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