Tracing the mechanism that enhances the signal from a single molecule

• P. Alonso-Gonzalez, P. Albella, M. Schnell, J. Chen, F. Huth, A. Garcia-Etxarri, F. Casanova, F. Golmar, L. Arzubiaga, L.E. Hueso, J. Aizpurua, and R. Hillenbrand

• Researchers at DIPC and CFM in collaboration with scientists from the Center for Nanoscience and Nanotechnology of the Basque Country nanoGUNE have provided the first unambiguous experimental evidence of the electromagnetic mechanism that amplifies enormously the signal coming from a molecule or nanoparticle placed in the proximity of a metal nanostructure. The mechanism, although commonly accepted, had never been experimentally traced with this level of accuracy quantitatively. The understanding of this mechanism opens the door to develop more sensitive and accurate techniques to explore the nanoscale.
  
  Nanotechnology is the technology that arises from the manipulation of a few atoms or molecules, sometimes even a single one. The development of nanotechnology requires very sensitive characterization and analysis techniques that allow measuring the properties of very small samples, even a single atom or molecule in the limit case. However, the intensity of the signal to be measured is proportional to the size of the sample, what means that the signal coming from a nano-object could be too small to be detected. Some kind of amplifier of the signal would be required if one wishes to measure the properties of a single molecule.
  
  Nanoantennas to focus and enhance radiation

  
  An optical amplifier can be provided at the nanoscale by a metal nanostructure. Acting as a nanoantenna analogously to their counterparts in the macroscale, metal nanostructures can collect incoming radiation from a relatively large area and enhance and concentrate it into small spots (the so-called hot-spots). Being the enhancement factor of this signal h, the energy deposited at the hot spot will be then h² times larger, as energy scales with the square of the field.
  
  The probing signal will be therefore h² times more intense by the effect of the nanoantenna but, this is not the end of the story. As the signal is scattered by the molecule or nanoscale object exactly at the same hot spot, the nanoantenna will act again for the second time as an amplifier increasing the emitted signal from the molecule by an extra factor h². This double amplification effect by the nanoantenna gives a total amplifying factor of h⁴, what eventually results in an enormous signal enhancement factor of more than 10 orders of magnitude, allowing the measurement of the properties of a single molecule which were not possible without this amplification.
  
  In the experiment developed at nanoGUNE, the researchers have substituted the nanosample by a vibrating tip that is located at a hot-stop induced at a metal rod. A modulation technique applied to the position of the sample allows for the split of the amplified and the unamplified signal thus tracing experimentally the whole amplification process in the nanoscale with exquisite quantitative detail. The experimental dependence of the molecular signal by a factor of h² has been corroborated quantitatively by calculations of the field enhancement produced at the antennas by researchers of DIPC and CFM.
  
  The experimental verification of this electromagnetic enhancement mechanism in the most general scenario completes the understanding of antenna-enhanced spectroscopic studies that with great impact have been strongly applied during the last 30 years”.
  

   

  

   

  Fig. 1: Schematics of a surface-enhanced light scattering process. (a) Inelastic scattering process (ω₁≠ω₂) from an object (O) in the presence of a metal nanostructure that acts as an optical antenna (A). (b) Elastic scattering process (ω₁=ω₂=ω) . Ε_A(ω) denotes the field directly radiated by the antenna and Ε_AOA(ω) denotes the field radiated by the object via the antenna. To select Ε_AOA, the antenna-object distance d is modulated at frequency Ω and the detector signal is demodulated at the higher harmonic frequency nΩ.
  
  

  
  
  Fig. 2: Elastic light scattering in the hot spot of infrared gap antennas. (a) A Si tip (vertically oscillating at frequency Ω) is scanned across a Au rod antenna (A). Both antenna and tip (the latter mimicking the scattering object O) are illuminated with s-polarized light. The s-polarized light backscattered from the tip-antenna configuration is detected interferometrically (not shown). Demodulation of the detector signal at nΩ yields the amplitude |Ε_n| and phase shift Δφ_n at each position of the tip. (b) Zoom-in amplitude |Ε_n| and phase shift Δφ_n images of the gap region in a resonant gap antenna. A full width at half maximum (FWHM) of the hot spot of about 50 nm is measured. The scale bar denotes 100 nm. (c) Topography, amplitude |Ε_n| and phase shift Δφ_n images of a resonant gap antenna. The images were recorded at an excitation wavelength of and demodulation was done with n = 4. The scale bar denotes 2 μm. (d) Parametric representation of log(Ι_n (L)] and log(f (L)] (red symbols). A linear least-square fitting of the data points (red solid line) yields a slope of 4.56. (e) Parametric representation of Δφ_n (L) and Δφ_f (L) (red symbols). Linear least-square fitting of the data yields the relation Δφ_n = 1.99 ⋅ Δφ_f .
  
  Original publication:
  Resolving the electromagnetic mechanism of surface-enhanced light scattering at single hot spots,
  P. Alonso-Gonzalez, P. Albella, M. Schnell, J. Chen, F. Huth, A. Garcia-Etxarri, F. Casanova, F. Golmar, L. Arzubiaga, L.E. Hueso, J. Aizpurua, and R. Hillenbrand,
  Nature Communications 3, 684 (2012)
  

   

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