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Wednesday, 29 July 2015 14:32

Plasmonics

 

Plasmonics

Case Study: Cherenkov SPPs

 

Patrice Genevet, Daniel Wintz, Antonio Ambrosio, Alan She, Romain Blanchardand Federico Capasso

Nature Nanotechnology

PUBLISHED ONLINE: 6 JULY 2015 | DOI: 10.1038/NNANO.2015.137

When a charged particle travels faster than the phase speed of light in a medium, a photonic shock wave called Cherenkov radiation is emitted. This electromagnetic shock wave is emitted as a cone in the three spatial dimensions. In this paper it was shown that a two dimensional analogue of Cherenkov radiation can be created to control and steer plasmons in one-dimensional metamaterials.

A one-dimensional metasuface was fabricated for the experiment, which consisted of an array of slits in different directions etched in the thin metal film. S-polarized light illuminates the metasurface in far-field and creates running waves of polarization (RWP). The RWP can be understood as a series of dipoles oriented normally to the slit axis
with different phases generated along the metasurface. These dipoles interact with the local distribution of free electrons on the metal surface and radiate SPP waves along the metal–dielectric surface. The RWP propagation speed is always larger than the SPP phase velocity. SEM image of nano- array structure

EM image of nano- array structure

Thus two-dimensional Cherenkov radiation is generated in the metasurface.
Moreover both experimental and theoretical analyses have showed that the direction of the Cherenkov radiation depends on the

angle and spin of the incident polarized light. Thus the propagation direction of the Cherenkov radiation can be controlled and steered by either one of these parameters.The experimental results show that that the direction of the SPP wakes depends on the angle and spin of the incident light. Thus the steering of the SPPs wakes can be achieved by variation of either one these parameters. The propagation direction of the two- dimensional Cherenkov radiation can be steered from forward to backward.

The experimental results are in the good agreement with the theoretical simulation.

The obtained results are very important step toward in understanding of the SPPs wakes propagation. The ability to control and manipulate of the SPPs propagation direction opens new horizons in development of novel plasmonic devices such as plasmonic phase modulators, plasmonic couplers, plasmonic holograms and beam-steering devices.


Experimental results. Forward Cherenkov SPP wakes (left), backward Cherenkov SPP wakes (right). Θ is angle of circular polarized incident light; σ+ and σ- are spin of the polarization, ϒ angle of Cherenkov SPP wakes propagation 

 

Experimental Setup:

The experimental analysis of the SPPs wake propagation was performed with a Nanonics Multiprobe MV 4000 near-field optical microscope in collection mode (with the NSOM probe collecting the light into a detector).  The MV 4000 allowed for such an experiment as a result of the following advantages:

  • Free optical access of the NSOM head from the both the top and the bottom. Allowing for direct illumination of the sample from below and easy visualization of the tip and sample from above.
  • Tip scanning allowing NSOM mapping of the SPP independent to the illumination and without moving the sample.
  • Topographic and near-field optical data are acquired simultaneously by scanning with Nanonics cantilevered NSOM probe.
  • Tuning fork (TF) feedback allows for no optical AFM feedback and no optical interference with the measurement
  • It is important to note that Apertured NSOM is important for such an experiment as an apertureless NSOM configuration would require laser illumination at the tip which can interferes with the Cherenkov radiation and lead to optical artifacts.
Monday, 27 July 2015 10:22

NSOM - a basic overview

We have put together the diagram below to explain the advantage of optical resolution in the near-field over the resolution of diffraction-limited optics.  As shown below, in diffraction limited optics the resolution is limited to approximately half the wavelength of light.  But by operating in the near-field, the resolution improves significantly to less than 100nm

 

Sunday, 05 July 2015 11:47

Nanonics at ICAVS July 2015

Nanonics at ICAVS - International Conference on Advanced Vibrational Spectroscopy 2015

 

Tuning forks offer significant advantages and increased sensitivity over conventional Si probes in large part due to their high quality factors (Q of 10x and higher.)   Tuning forks in force spectroscopy have especially begun to show exciting results that push the measurement's possibilities.  Over the last decade, Nanonics has developed instruments using a NanoToolKit of such probes that are increasingly being used with on-line forcespectroscopy.  A demonstration of the force sensitivity of these probes is the measurement of 1.6 pN for the force of a single photon. [D. C. Kohlgraf-Owens et al "Mapping the mechanical action of light," Phys. Rev. A 84, 011807R (2011)]

Besides force sensitivity, tuning forks offer other advantages over optical beam deflection and conventional Si probes.  Tuning forks have much stiffer (spring constant of ~2600 N/m and greater) than standard silicon cantilevers.  As a result, the problem of "jump to contact" instability that limits the optical beam deflection based feedback methods is eliminated, and this permits the study of forces in the proximal 10-20 nm above a surface.  Smooth approach curves together with lack of adhesion ringing upon withdrawal is combined with additional advantages of no feedback laser interference; these features are important for semiconductor electrical probing and combinations of AFM with Raman spectroscopy that Nanonics has pioneered.  Furthermore, tuning forks in force spectroscopy enable the point of contact with the surface to be accurately measured for the first time.  For all these reasons and its ease of use, tuning forks are becoming an ideal choice for new horizons in experiments requiring the ultimate tip-sample control stability and force sensitivity from areas of bioimaging, to physics of devices, and to single molecule and polymer spectroscopy.

 

Wednesday, 24 June 2015 13:18

Nanonics announces image contest winners

Image contest winners

 As surface plasmon polaritons (SPP) continue to develop potential for controlling light on the nanoscale, the ability to excite and control these optical modes becomes increasingly important.   A new lens design strategy for SPPs is presented by Wintz, Capasso et al. of Harvard University.  The lens consists of a metasurface (nanostructured surface) composed of nanoslits that can steer the SPPs between foci on the surface based on the incident wavelength.

 
Using a  Nanonics MultiView 4000 system equipped with Nanonics NSOM probes, the authors map the surface plasmon polaritons through NSOM experiments at multiple wavelengths and polarizations demonstrating switchable focusing and steering of the SPPs as the wavelength or polarization is changed.  Their experimental setup, shown here, includes an NSOM probe mounted on to a tuning fork with incident laser radiation from below that passes through a polarizer in order to set the polarization.  Some of the laser light is transmitted through the metasurface, and depending on the relative intensities of the SPPs and transmitted light can produce an interference pattern. The NSOM tip is used to collect the light by interacting with the evanescent field of the SPP, converting energy from the SPP Mode into a propagating waveguide mode in the optical fiber. 
 
 
Additionally, they conduct spectrally resolved NSOM measurements by using the same experimental setup as above but now with the optical fiber connected to a spectrometer.  Shown on the left is an NSOM image collected at 580-700nm, where each pixel records the total counts in that wavelength range.  Individual spectra at different points from the image are shown on the right.
 
 
This strategy can be used to overcome coupling and focus issues currently present for SPPs as well as providing both wavelength and polarization tunability of the direction of SPP beam propagation.
 
 

Published: Nano Lett. 2015, 15, 3585−3589

 

 

Wednesday, 27 May 2015 00:50

Nanonics May 2015 newsletter

Nanonics May 2015 newsletter

Sunday, 17 May 2015 23:28

Nanonics at SPP7 conference

Nanonics event at upcoming SPP7 conference

Monday, 11 May 2015 15:54

NanoPhotonics

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