Experimental verification of surface plasmon amplification on a metallic transmission grating

We report on a near-field amplification in a transmission metallic grating, whereby the spatially and spectrally resolved near-field intensity reaches {approx}20 times the incident intensity at the surface plasmon polariton resonance. The amplified value is maintained up to {approx}2 {mu}m away from the surface. Our experiments show that the near-field amplification in the transmission grating, which is strongly implied in a recent superlens design, indeed occurs at the surface plasmon polariton resonance. Theoretical calculation shows good agreement with experiment and also reveals that the horizontal magnetic field is predominantly amplified. Our results suggest that a grating-assisted superlens should have its optimal functional wavelength right around the surface plasmon resonance.