Tunable resonances in plasmonic crystals.

摘要

Plasmonic crystals---metallic grating arrays that resonantly couple free-space light into collective electron oscillations (surface plasmons)---are unique optical materials because they can confine and manipulate electromagnetic fields at subwavelength scales. Surface plasmon resonances can be tuned by changing factors such as grating geometry, surrounding dielectric environment, materials and excitation conditions. Most work has focused on the former parameters but not the latter, which are critical for controlling electromagnetic field confinement and plasmonic band structures. This dissertation describes several plasmonic crystals whose resonances can be tuned over a wide frequency range, from the optical to near infrared. First, by combining near-field and far-field characterization techniques, we provided direct evidence that surface plasmons mediated enhanced optical transmission of nanohole arrays. Near-field scanning optical microscopy images clearly revealed evanescent surface plasmon waves on gold films perforated with microscale arrays of nanoholes, and optical transmission spectra exhibited narrow resonances corresponding to high-order surface plasmon modes. Second, angle-resolved measurements of subwavelength hole arrays in gold and palladium were carried out to high angles and were converted to plasmonic band diagrams. Individual resonant modes were readily identified, and unusual photonic-plasmonic resonances from Rayleigh anomalies and surface plasmons polaritons were discovered. Finally, a new type of plasmonic crystal---two-dimensional arrays of nanopyramids---was used as a platform to screen a wide range of metals side-by-side and under different excitation conditions. These studies not only revealed unexpected plasmonic materials but also provided the first comprehensive understanding on how to tune surface plasmons for specific applications, such as biological and chemical sensing.