Evanescent field of a sub-wavelength aperture enables sharper terahertz images

Ultrafast Laser Laboratory

Department of Electronic and Electrical Engineering

Spatial resolution in near-field scanning probe microscopy is determined by parameters of the probe; for sub-wavelength aperture probes, it is defined by aperture size. Transmission of electromagnetic waves through a sub-wavelength aperture is described by Bethe's theory, which predicts extremely low transmission for apertures smaller than 1/100 of the wavelength. According to the theory, the transmitted field amplitude follows a third power law (sixth power law for intensity). The power law limits the use of stand-alone apertures smaller than 20-30 microns for THz near-field imaging.

IMPROVING SPATIAL RESOLUTION OF SUB-WAVELENGTH APERTURE PROBES

Investigations of THz pulse transmission through single sub-wavelength apertures as small as 3 micron (1/100 of the wavelength at 1THz) within a close range (1-2 micron) from the aperture, show that the evanescent field is stronger than predicted by Bethe's theory in this close range.

By taking advantage of this effect, we developed high spatial resolution THz near-field microscopy probes incorporating 3micron apertures.

Impact of aperture size on near-field THz probe sensitivity:
Symbols show intensity of the transmitted THz wave for apertures a = 3, 5 and 10 micron detected at z = 1, 1.6 and 4 micron from the aperture (see inset). Placing the detector antenna at z = 1 micron improves resolution and the signal to noise ratio (SNR).

THz NEAR-FIELD PROBE INCORPORATING 3 MICRON APERTURE

In an article published recently in Applied Physics Letters , we demonstrate a spatial resolution of 3micron, the highest resolution achieved to date in THz time-domain imaging with a sub-wavelength aperture probe.

The developed probes open new horizons for the aperture-type THz near-field imaging.

Read more on applications of THz near-field microscopy for investigations of graphene, THz waveguides, and THz surface waves.

Integrated THz near-field probe characteristics:
(a) THz pulse transmitted through the 3 micron aperture; (b) scan of a metallic edge measured using the 3 micron aperture probe.

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