Fluorescence spectroscopy enhancement on photonic nanoantennas

Despite the significant progress in single molecule fluorescence microscopy made over the last two decades, the efficient detection of a single molecule remains a major goal with applications in chemical, biochemical and biophysical analysis [1, 2]. The phenomenon of light diffraction appears as a main physical limiting factor. Indeed, there is a huge size mismatch between a single molecule (below 5 nm) and the wavelength of light (around 500 nm). This size mismatch prevents the efficient interaction between an incoming light beam from a conventional optical microscope and a single fluorescent molecule, so the net detected fluorescence signal from a single molecule (ultimately defining the sensitivity and dynamic temporal resolution achievable) remain limited [3]. Additionally, conventional optical microscopes are restricted to conditions of low density (or concentration) of fluorescent molecules [1, 4]. In order to isolate a single molecule in the diffraction-limited volume of a confocal microscope, the concentration range must be typically in the pico to nanomolar range. However, a large majority of enzymes and proteins requires concentrations in the micro to millimolar range to reach relevant reaction kinetics and biochemical stability. Monitoring single molecules at high physiological concentrations thus requires overcoming the diffraction limit to confine light in a nanometer spot of volume in the zepto(10−21) or atto(10−18) liter range, more than three orders of magnitude below the femtoliter volumes achieved with confocal microscopes [5, 6]. Confining light to the nanoscale can be achieved thanks to optical antennas [7]. Optical antennas are generally metallic nanostructures with dimensions much below the wavelength of light [8, 9]. Like their radiofrequency counterparts, optical antennas convert propagating radiation into localized energy and vice-versa [7]. This opens new routes to enhance and control the emission from a single fluorescent emitter by improving the light-matter interaction between a single fluorescent molecule and the incoming beam, leading to the phenomenon of antenna-enhanced fluorescence emission [10, 11, 12]. Over thousand-fold enhancement of the single molecule fluorescence signal was reported