Displaying items by tag: Photonics

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.

Nanocoax ( metal- dielectric- metal) structures are potential devices for nanoscale photonics systems such as polarization- preserving optical waveguides for optical communication. It is very important to know the propagating properties of such structure for different excitation wavelengths.

NSOM is the best tool for study of such plasmonic nanodevices since the nSOM measurement is not diffraction-limited, and it provides both topographic and optical data with high resolution.

A Nanonics MV-4000 system was used for the AFM/NSOM measurements. This system enables scanning with probe as well as with the sample. The tip scanning capability is absolutely necessary for the study of objects requiring any form of constant optical alignment. The feedback in this system is based on the tuning fork, hence there no interference between the beam bounce laser and the light propagating through the sample.

The sample with the nanocoax array was specially prepared. The sample was illuminated from the bottom with different light sources: 473 nm, 532 nm, 660nm 850 nm. The light was transmitted through the nanocoax structures, collected via cantilevered NSOM probe (aperture diameter 100 nm), and detected with an avalanche photodiode.

Experimental results together with numerical calculations showed that the propagated modes have a different nature depending on the excitation wavelength, i.e, plasmonic TE₁₁ and TE₂₁ modes in the near infrared and photonic TE₃₁, TE₄₁ and TM₁₁ modes in the visible. Far field transmission out of the nanocoaxes is dominated by the superposition of Fabry-Perot cavity modes resonating in the structures, consistent with theory. 

In summary, propagation of the plasmonic and photonic modes in nanocoax structures was experimentally observed for the first time in this work.

Fig. 1    NSOM-measured (a - d) and calculated (f - i) images of the propagated modes in the nanocoax structure. The wavelengths were 850 nm (a, f), 660 nm (b, g), 532 nm (c, h) and 473 nm (d, i). In all cases, the polarization is in the vertical direction, the scale bars represent 200 nm and circles represent the inner and outer radii of the coax annuli. (e) Three-dimensional representation of the nanocoax topography (lower, via AFM) and the corresponding near-field intensity (upper) for 532 nm wavelength. 

Published:  Optics Express June 16, 2014 Vol 22, No. 12

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This paper demonstrates the detection of whispering–gallery mode (WGM) distribution in a fused silica micro disk resonator with a highly sensitive Nanonics Imaging cantilevered Pt/Au micro thermocouple probe and a Nanonics Imaging MultiView 4000 Scanning Probe Microscope.

Whispering gallery modes were excited by an evanescent tapered fiber with wavelengths in the 1550nm-1560nm range.  The thermocouple probe enabled detection of thermal distribution simultaneously with the topographic data.  As the tip penetrates the evanescence field, the light absorption leads to heating measured by the thermal tip, giving a thermal image of the light intensity distribution.  The thermal distribution was compared with NSOM results on the sample in transmission, reflection and collection modes.

In Brief: This paper elegantly shows that the thermocouple probe has great potential for detection in the near field in visible and near-IR optical ranges and can be used as a tool to study different photonics devices.  

Published:  March 1, 2014/Vol.29, No.5/Optics Letters

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