Abstract
Whispering gallery mode biosensors allow selective unlabelled detection of single proteins and, combined with quantum limited sensitivity, the possibility for noninvasive real-time observation of motor molecule motion. However, to date technical noise sources, most particularly low frequency laser noise, have constrained such applications. Here we introduce a new technique for whispering gallery mode sensing based on direct detection of back-scattered light. This experimentally straightforward technique is immune to frequency noise in principle, and further, acts to suppress thermorefractive noise. We demonstrate 27 dB of frequency noise suppression, eliminating frequency noise as a source of sensitivity degradation and allowing an absolute frequency shift sensitivity of 76 kHz. Our results open a new pathway towards single molecule biophysics experiments and ultrasensitive biosensors.
Highlights
Whispering gallery mode biosensors allow selective unlabelled detection of single proteins and, combined with quantum limited sensitivity, the possibility for noninvasive real-time observation of motor molecule motion
An intrinsic scattering rate g0 is included between the forwards- and backwards-propagating whispering gallery modes, which in experiments typically results from surface roughness and Rayleigh scattering
The frequency shift experienced by the standing-wave mode that interacts with the biomolecule or nanoparticle is exactly equal to twice the scattering rate g it introduces between counter-propagating modes[12]
Summary
Whispering gallery mode biosensors allow selective unlabelled detection of single proteins and, combined with quantum limited sensitivity, the possibility for noninvasive real-time observation of motor molecule motion. Direct laser frequency noise cancellation has been achieved using an independent reference interferometer to characterise the laser frequency noise in real-time as experiments proceed[10] This allowed the detection of 12.5 nm radius nanoparticles with a frequency shift noise floor of 133 kHz. Self-referencing provides an elegant alternative approach to suppress frequency noise, and is well suited to WGM sensors since they inherently exhibit frequency degenerate pairs of forwards- and backwardspropagating optical modes. Measuring the splitting provides a self-referenced method to sense the presence of the scatterer that is in principle frequency noise immune This approach was first demonstrated in Ref. 12, where nanoparticles of 30 nm radius were detected in air by scanning laser spectroscopy of the frequency splitting. The approach is directly compatible with enhanced WGM sensing approaches using both plasmonic field hot-spots[5,6,7,8] and thermo-optic heating[9]
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