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Plasmonic Crystals

Venu Gopal Achanta

6 January 2012

Schematic of magneto-plasmonic crystal

Plasmonics is an active field that proposes to use the surface modes at the metal-dielectric interface in nanophotonics (sub-wavelength imaging and optics), spectroscopy (surface enhanced Raman spectroscopy), modifying the optical properties of materials among others. All these utilize the electron charge density wave or surface plasmon polaritons (SPP) at metal-dielectric interface.

Plasmonic crystals are specially designed metal-dielectric nanostructures that modify material properties due to plasmon mediation. One would require good theoretical, nanofabrication and characterization tools to predict and demonstrate novel phenomena. One such proposal is the enhancement of magneto-optical properties in magnetic materials so that the Kerr and Faraday effects are enhanced. Similarly, absorption and emission from semiconductors can be modified by plasmon mediation.

At TIFR, different types of plasmonic crystals are prepared. For example, Gold gratings of predetermined dimensions on dielectric thin films or dielectric gratings on metal-dielectric interfaces were made. We use electron beam lithography or interference lithography for patterning. By angle resolved, white light transmission measurements the SPP dispersion in this structure was measured and compared with numerical modeling.

Figure above shows magneto-plasmonic crystals made on Bi:YIG thin films. This crystal has been used to demonstrate the theoretically predicted enhancement in transverse magneto-optical Kerr Effect (TMOKE). As predicted two orders of magnitude enhacement in TMOKE has been demonstrated. This is the first demonstration of SPP mediated enhancement in TMOKE signal in transmission mode as well as TMOKE signal as a very efficient tool to identify the SPP resonances. These results demonstrate that plasmon mediation in designed plasmonic crystals can enhance the naturally weak optical properties to levels suitable for device realization.

References :

  1. A. S. Vengurlekar "Extraordinary optical transmission through metal films with subwavelength holes and slits",Current Science 98, 1020 (2010).
  2. V. I. Belotelov, I. A. Akimov, M. Pohl, V. A. Kotov, S. Kasture, A. S. Vengurlekar, Achanta Venu Gopal, D. R. Yakovlev, A. K. Zvezdin, and M. Bayer, "Enhanced magneto-optical effects in magnetoplasmonic crystals", Nature Nanotech. 6, 370 (2011).

Q & A

Pankaj Kumar Sharma : what is plasmonics
Venu Gopal Achanta : Solutions to Maxwell's equations for a metal-dielectric interface shows the existence of surface electromagnetic modes. Such surface modes are electron charge density waves or the surface plasmons. Those localized on a nanoparticle are the localized plasmons. Study involving exciting, controlling and making use of plasmons is Plasmonics. Some good books are, Surface Plasmons on Smooth and Rough Surfaces and on Gratings by H. Raether, Plasmonics Fundamentals and Applications by S. Maier, and Surface Plasmon Nanophotonics by M. I. Brongersma.

Tomin : Since plasmonic crystals are very small how do we measure its optical properties.
Venu Gopal Achanta : Though the period and the feature sizes are small, we can make plasmonic crystals that are big (over few mm^2 area). Thus, they can be probed.