Near-field measurements reveal large near-to-far-field spectral shifts of THz resonances

A Dutch research group led by Prof. Jaime Gómez Rivas at AMOLF, Amsterdam, has quantified how far the near-field and far-field spectral responses of resonant structures can differ. The results were published in Physical Review B.

Far field is not the whole story

Spectroscopy of resonant structures is usually done with a source and a detector in the so-called far field, at distances much larger than the wavelength. A far-field measurement reveals, for example, the frequencies of maximum absorption. But this need not be the frequency at which the resonator responds with the strongest field at its immediate near-field surface.

For resonant sensing elements, that difference matters — the wrong frequency gives misleading results.

Measuring the near field directly

AMOLF solved the problem by measuring the near-field response directly at THz frequencies with a photoconductive microprobe, the TeraSpike TD-800-X-HRS from Protemics. The two main challenges of near-field measurements are

• picking up the field information at a sufficiently small, sub-wavelength distance from the structure under test, and
• leaving the resonant behaviour uninfluenced by the measurement itself.

Both were accomplished thanks to the low-invasive, miniaturised design of the microprobe.