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.	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.

Physicists have found a new way to confine electromagnetic energy without it leaking using nonradiating anapole modes.  Nonradiating electromagnetic sources continue to be of interest as a model for stable atoms and to understand why orbiting electrons do not radiate, and have potential applications for combatting energy losses and explaining dark matter.   The radiationless anapole mode is achieved by dividing the current between two different components, a conventional electrical dipole and a toroidal dipole.  The radiation or far-field scattering is cancelled out if these two configurations are out of phase rendering the feature invisible.

Scientists Miroshnickenko and colleagues tested this theory with near-field characterization of single silicon nanodisks of 50nm height and 200nm-400nm diameter, which were made effectively invisible by cancelling the disc’s scattering of visible light.  The existence of an optical anapole mode in these nanodisks was investigated in both the far field and near field; the near-field characterization was critical for the device becomes invisible and thus undetectible in the far-field at the mode's excitation.   A spectral dip in the far-field spectrum was observed corresponding to the dark anapole mode excitation, while near-field distribution of the disks was mapped at different wavelengths.  The image on the right shows (a) far-field scattering spectra and (b) near field map for a 310nm diameter disk, where the far-field spectrum dip is most pronounced at 620nm.

Near field characterization was done with a CryoView MP that provides optical access from the top and from the bottom.  It enables easy integration with all conventional optical microscopes for near field measurements in transmission, true reflection and collection modes.  Both tip scanning and sample scanning are possible in the same scanning head which is especially important for the optical measurements described in this paper.  The sample was illuminated in the far field using a supercontinuum source while transmitted light was collected in near field with a Nanonics cantilevered NSOM probe.  with aperture diameter 50 nm during tip scanning.   The Nanonics MV 4000 scanning head together with unique Nanonics cantilevered NSOM probes are the best tools for optical characterization of nanodevices in the near field with nanometric resolution.

Published:  NATURE COMMUNICATIONS 2015

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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.	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.

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.

 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.

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

The need for periodic metallic nanoparticle arrays is driven by a wide variety of applications including plasmonic waveguides, nanoscale lenses, and catalyzing the growth of ordered carbon nanotubes.  Current methods to form such organized, periodic structures usually suffer from high cost or poor quality structures.  Researchers Yadavali et al. from Oak Ridge National Labs and Univ of Tennessee reveal a new way to spontaneously make low cost, high quality periodic arrangements of gold nanoparticles.   

Laser interferences processes are known to induce periodic surface structures where a pattern forms due to the interference phenomenon between incident light and scattered light. The interference pattern produces periodic thermal gradients that induces mass transport and rearrangement. However, these types of patters often poor quality with defects. 

This work describes a new method involving irradiation of the surface by a single laser beam while apply a DC electric field to the underlying substrate.   When an electric field parallel to the laser polarization is applied, single-crystal like periodic ordering was formed covering a large area and with very low defect density.  In addition to high resolution studies by SEM, a Nanonics Multiview 1000 was used to analyze the topography and quality of the resulting patterns.  The resulting AFM images below show the gold periodic structures at difference conditions of a) no E field  b) E field perpendicular to the polarization and c) E field parallel to the polarization.

Published: Yadavali et al., Nanotechnology 25 (2014) p. 465301

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Plasmonics waveguides have unique, attractive features such as 1) allowing co-propagation of optical and electrical signals and 2) tight confinement of the electromagnet mode.  A recently published work describes a new configuration of plasmonic waveguides that tries to overcome the device’s well-known obstacle of a limited propagation length, where the loss of light signal occurs due to light absorption in the metal.  Authors Uriel Levy and colleagues use a Nanonics Multiview 4000 in this paper [Optics Express, 2014] to characterize the properties of the waveguide and to measure the propagation loss of the waveguide modes.  They find comparable propagation loss in their devices, and they suggest significant improvements to the propagation losses with changes to the fabrication process of the device.

Published:  Optics Express, Sep 3 2014

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Surface Plasmon polariton waves are well-known to suffer from high loss where, with a result of this loss causing localized heating of the SPP structure.  However, directly measuring optically-induced heating with nanoscale resolution has always been very challenging.  Now, using Scanning Thermal Microscopy and NSOM, researchers Boris Desiatov, Ilya Goykhman, and Uriel Levy have characterized both the electromagnetic field and thermal distribution in silicon plasmonic nanotips.   The authors used a Multiview 4000 equipped with a Nanonics thermocouple tip - to measure thermal signal as a function of tip position - and a metal-coated NSOM tip to measure the electromagnetic intensity distribution.   The use the silicon integrated plasmonic nanoptip device to confine the electromagnetic energy at the silicon tip/metal boundary, resulting in a localized thermal hotspot that they then measure.   They find that coupling 10mW of optical power into the waveguide results in a temperature rise of about 15C at the apex of the nanotip compared to ambient temperature; these experimental findings were confirmed with simulations.

This study is important in helping understanding the localized heating of this structure, a major consequence of ohmic loss in these devices.    

Published:  Nano Letters  2014 14(2) p. 648-652

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