Abstract
We describe an improved optical beam control technique combined with a waveguide probe for imaging optical near-field using multiple wavelengths. The beam control technique varies the incident angle of pseudocollimated beams whose spot diameter is ∼10 μm such that it is equal to an arbitrary angle with ∼0.03° precision and the beam position to ∼0.1 μm precision. This helps resonantly excite surface plasmons using visible 660 nm and near-infrared 850 nm on an ∼10 μm width three-dimensional microwaveguide. We demonstrate the operation of this system by imaging the optical near-field and capturing the interference of surface plasmons in a gold microstructure. This system achieved dual-color imaging of the optical near-field of different modulation frequencies with submicrometer lateral spatial resolution.
Highlights
We describe an improved optical beam control technique combined with a waveguide probe for imaging optical near-field using multiple wavelengths
In the advanced technique of scattering-type Near-field scanning optical microscopy (NSOM), strong electric fields on sharpened probes enhance optical signals,6 enable nanometer-scale imaging of the dielectric constant of microstructures,7,8 and provide Raman spectra from microscopic materials
We present an ideal optical setup, which controls the position of the optical spot and incident angle arbitrary, for the plasmon resonance waveguide probe
Summary
Near-field scanning optical microscopy (NSOM) and related techniques are versatile methods for nondestructive and noninvasive optical imaging at nanometer-scale spatial resolutions. Numerous technologies have been developed, with classical aperturetype probes realizing thermal imaging, dichroism mapping, time-resolved Kerr microscopy, and dual-probe imaging. In the advanced technique of scattering-type NSOM, strong electric fields on sharpened probes enhance optical signals, enable nanometer-scale imaging of the dielectric constant of microstructures, and provide Raman spectra from microscopic materials. The scattering-type NSOM is known to be sensitive to subsurface structures.. We present an ideal optical setup, which controls the position of the optical spot and incident angle arbitrary, for the plasmon resonance waveguide probe. This configuration precisely adjusts the optical excitation conditions of multiple wavelengths. The excitation light usage rate, which is determined by the intrinsic absorption of the silicon and the Fresnel reflection at the silicon surface, is calculated from the three different thicknesses of the waveguide [Fig. 1(e)] In this calculation, the incident angle is set to the resonant angle, and the Fresnel loss is ∼1%. Our method controls the input position and incident angle on the input pupil of the microscope objective, which focuses the excitation beam on the waveguide
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