Abstract
A major challenge in optics is how to deliver and concentrate light from the micron-scale into the nano-scale. Light can not be guided, by conventional mechanisms, with optical beam sizes significantly smaller than its wavelength due to the diffraction limit. On the other hand, focusing of light into very small volumes beyond the diffraction limit can be achieved by exploiting the wavelength scalability of surface plasmon polaritons. By slowing down an optical wave and shrinking its wavelength during its propagation, optical energy can be compressed and concentrated down to nanometer scale, namely, nanofocusing. Here, we experimentally demonstrate and quantitatively measure the nanofocusing of surface plasmon polaritons in tapered metallic V-grooves down to the deep subwavelength scale - approximately lambda/40 at wavelength of 1.5 micron - with almost 50% power efficiency.
Highlights
On the other hand, focusing of light into very small volumes beyond the diffraction limit can be achieved by exploiting the wavelength scalability of surface plasmon polaritons
Such enhancements, localization and efficiency render nanofocusing an advantageous tool for a spectrum of nanooptical exploits, such as addressing nano-photonic circuits (or single-molecules and quantum dots), optical nanolithography, non-linear optical sensors, and more
Summary
Nanofocusing is characterized by nano-scale confinement, below the diffraction limit of light, accompanied by enhancement of the intensity or electric field strength. [1,2] Confinement below diffraction limit can be obtained by coupling light to plasma oscillations at a metallicdielectric surface forming surface plasmon polaritons (SPP). [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29] In particular, a plasmonic waveguide formed by a dielectric gap between two metal half spaces gives rise to a guided mode which does not experience mode cut-off at the nano-scale. [3,4] light can be concentrated into the nano-scale, by gradually decreasing the optical waveguide width, which is unattainable in conventional dielectric waveguides. [1,2] This confinement is accompanied by a gradual reduction of the surface-plasmon wavelength due to the unusual dispersion dependence on the thickness of the dielectric gap. Nanofocusing is characterized by nano-scale confinement, below the diffraction limit of light, accompanied by enhancement of the intensity or electric field strength. [3,4] light can be concentrated into the nano-scale, by gradually decreasing the optical waveguide width, which is unattainable in conventional dielectric waveguides. [9,13] The growing interest in nano-optics and the advances in nano-fabrication, have led to further theoretical investigations of nanofocusing on the wedge [10,11] and cone [12] as well as in several different plasmonic systems including V-grooves, [3,4,5,6,7,8] dielectric cones covered in metal film [15,16] and tapered rectangular metal strips. We measure the relative power emerging from V-grooves with different output widths, w, and find the output intensity is increased with decreased output width
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