Plasmonic metamaterials


The term plasmonics comes from concept of surface plasmon polariton, commonly referred to as plasmon, which is a kind of surface wave propagating at the interface between a metal and a dielectric due to the collective oscillation of the electrons to incident electromagnetic radiation. This kind of excitation is the main cause of the phenomenon of extraordinary transmission (EOT) across thin metallic hole arrays, which allows their use as wavelength-selective elements. Plasmons are also behind the response at optical wavelengths of the meta-atoms forming metamaterials. However, its application goes beyond, and also includes the design of certain optical metamaterials as their electromagnetic behavior is linked to the plasmon excitation.

Multiple applications venture into the field of plasmonics due to the physical characteristics that plasmons present. Thus, their reduced wavelength allows their use in high resolution technologies such as lithography and microscopy. Moreover, its ability to confine the light in very small dimensions together with their sensitivity to material properties over they propagate, postulate them as promising candidates in ultrasensitive sensors. Our research has yielded ultra-compact filtering structures based on the phenomenon of the EOT. We have also designed and characterized plasmonic nanostructures for biosensing applications. Finally, we have been able to show a way to unidirectional launching of surface plasmons by using circularly polarized light.


Driving plasmonic nanostructures by placing them in a silicon waveguide gap

Excitation and measurement of isolated subwavelength metallic nanostructures supporting plasmonic resonances becomes extremely challenging because of the diffraction limit. The situation gets much worse if trying to address simultaneously multiple nanostructures in parallel. A possible solution is the use of high-index silicon waveguides to excite the nanostructure and collect part of the light it scatters. Previous approaches placed the metallic nanostructure on top of a dielectric waveguide. Since the plasmonic resonance was excited via the evanescent field of the waveguide, the interaction was quite poor, resulting in low contrast ratios at the output. In this work, we have used a new approach: a subwavelength gap is created on a silicon waveguide and the plasmonic nanostructure (a gold nanostrip) is placed in the middle of it in order to achieve a full delivery of the guided power to the nanostructure. We show experimentally contrast ratios over 10 dB, although values > 40 dB are predicted in simulations, which can be extremely useful in application such as sensing or optical switching. Moreover, our scheme can be easily implemented in a multiplex approach, enabling multiple driving of plasmonic nanostructures (such as nanoantennas). Out results have been published in Optics Express [1].



[1] "Experimental measurement of plasmonic nanostructures embedded in silicon waveguide gaps", A. Espinosa-Soria; A. Griol; Alejandro Martínez; Optics Express, Vol. 24, Issue 9, 9592-9601, May 2016.

Controlling plasmon directionality with near field sources

We have studied, in collaboration with King’s College London, the excitation of surface plasmons by near field sources and found experimentally that the use of circularly polarized illumination gives rise to a rather unintuitive phenomenon of unidirectionality in plasmon excitation. This phenomenon can be explained using the novel concept of near field interference applied to a circularly polarized dipole: dipoles are extensively used to model near field sources such as metallic tips, structures, or holes. When the polarization of the dipole is circular, the near-field evanescent components interfere such that only those propagating on one direction exist. The concept is so fundamental that we found it was of wide applicability to every other electromagnetic waveguide, vastly increasing the potential applications. The results are published in Science [1].



[1] "Near-Field Interference for the Unidirectional Excitation of Electromagnetic Guided Modes", F. J. Rodríguez-Fortuño; G. Marino; P. Ginzburg; D. O’Connor; A. Martínez; G. A. Wurtz; A. V. Zayats, SCIENCE, Vol. 340, n. 6130, pp. 328 - 330, April 2013.

Non-hermitian resonant plasmonic nanoparticles for polarization conversion

Researchers at NTC and King’s College London have proposed the use of locally induced losses in the environment of plasmonic nanoparticles to achieve polarization conversion. The active control of losses could allow real time selection of the polarization conversion behavior, and turned around, the scheme could be used for the sensing of the losses of analytes. The theoretical study is published in Nano Letters [1].


[1] "Analogue of the Quantum Hanle Effect and Polarization Conversion in Non-Hermitian Plasmonic Metamaterials", P. Ginzburg; F. J. Rodríguez-Fortuño; A. Martínez; A. V. Zayats, NANO LETTERS, Vol. 12, n. 12, pp. 6309 – 6314, December 2012.

Localized Surface Plasmon Resonance Sensors

Researchers at NTC have fabricated and measured high-performance chemical/biological sensors based on localized surface plasmon resonances (LSPRs) in arrays of functionalized metallic nanoparticles. The high electric field confinement achieved by LSPRs allows very high sensitivity to the analytes being sensed. The functionalization of either the substrate [1] or the metallic surface [2] of the nanoparticles allows the binding of the analyte being sensed into the regions of high electric field, achieving high resonance shifts.


[1] "Demonstration of near infrared gas sensing using gold nanodisks on functionalized silicon", P. J. Rodríguez; M. Martínez-Marco; F. J. Rodríguez-Fortuño; B. Tomás-Navarro; R. Ortuño; S. Peransi-Llopis; A. Martínez, OPTICS EXPRESS, Vol. 19, n. 8, pp. 7664-7672, April 2011.

[2] "Highly-sensitive chemical detection in the infrared regime using plasmonic gold nanocrosses", F. J. Rodríguez-Fortuño; M. Martínez-Marco; B. Tomás-Navarro; R. Ortuño; J. Martí; A. Martínez; P. J. Rodríguez, APPLIED PHYSICS LETTERS, Vol. 98, n. 13, pp. 133118(1)-133118(3), March 2011.

Relationship between EOT and negative index with surface plasmons

We have obtained analytically the dispersion relation of SPPs in a double-layer metallic structure patterned with subwavelength hole arrays and shown by numerical analysis that light coupling to the external and internal SPPs originates EOT.
In addition, internal SPPs show certain unique properties different from the external SPPs: at the internal-SPP resonant frequencies a negative effective permeability is achieved as a VCL is formed between the metallic layers which can be used to design negative-index metamaterials.


[1] "Role of surface plasmon polaritons on optical transmission through double layer metallic hole arrays", R. Ortuño, C. García-Meca, F.J. Rodríguez-Fortuño, J. Martí, A. Martínez, Phys. Rev. B, vol.79, pp. 75425, 2009.

[2] "Negative Refraction index metamaterials aided by extraordinary optical transmission", C. García-Meca, R. Ortuño, F. J. Rodríguez-Fortuño, J. Martí, and A. Martínez, Opt. Express 17, 6026-6031 (2009).