An effect which is principally known but hard to measure so far has now been quantified by a Dutch research group led by Prof. Jaime Gomez Rivas (AMOLF, Amsterdam): Resonant structures exposed to light may oscillate at a much slower speed than implied by standard spectroscopic measurements.

These are typically done using a light source and detector in the so-called “far-field” distance, which is much larger than the applied wavelength. A spectroscopic far-field measurement reveals for example the frequencies of maximum absorption of the investigated resonant structures. However, this frequency may be not the one where the resonator responds with the strongest excitation intensity at its immediate “near-field” surface. However, in order to use resonant structures as sensing elements this information is very important to avoid any misleading results. The solution applied at AMOLF to solve this problem is to measure the near-field response directly with an advanced photoconductive microprobe (TeraSpike TD-800-X-HRS, Protemics) at Terahertz frequencies. The main challenges in doing near-field measurements are picking-up the field information from the structure under test in sufficiently close sub-wavelength distance and to keep its resonant behavior uninfluenced from the measurement itself. Both tasks were accomplished successfully thanks to the low-invasive miniaturized design of the applied microprobes. 

The work has been recently published in Phys Rev. B under the following reference:

A. Bhattacharya, G. Georgiou, S. Sawallich, C. Matheisen, M. Nagel, and J. Gómez Rivas, Large near-to-far field spectral shifts for terahertz resonances, Phys. Rev. B 93, 035438 (2016). [HTML]