Near-field terahertz imaging using sub-wavelength apertures without cutoff

Area of Science:

• Optics and Photonics
• Electromagnetism
• Materials Science

Background:

• Conventional conically tapered waveguides exhibit strong transmission suppression below their cutoff frequency.
• This limitation restricts the operational frequency range for terahertz (THz) near-field imaging applications.
• Existing THz imaging probes often struggle with maintaining mode confinement and spatial resolution at lower frequencies.

Purpose of the Study:

• To demonstrate near-field imaging capabilities of a novel conical waveguide structure using broadband terahertz radiation.
• To investigate enhanced transmission below the cutoff frequency while maintaining mode confinement for imaging.
• To assess the imaging performance across multiple frequency regimes, including below cutoff.

Main Methods:

• Design and fabrication of a two-piece conical waveguide with an adjustable gap and thin sidewalls near the gap.
• Utilizing broadband terahertz radiation for excitation and near-field imaging.
• Simultaneous imaging at frequencies above, near, and below the waveguide's cutoff frequency.

Main Results:

• Significantly enhanced transmission was achieved at frequencies below the cutoff frequency.
• Mode confinement and spatial resolution were preserved, crucial for imaging applications.
• Near-field imaging demonstrated only mild degradation in image quality below the cutoff frequency.

Conclusions:

• The proposed conical waveguide design overcomes the cutoff frequency limitation of conventional structures.
• This advancement enables effective terahertz near-field imaging across a broader frequency spectrum.
• Further structural refinements hold promise for improved imaging performance well below the cutoff frequency.