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High power, narrow-bandwidth radiation at microwave frequencies is currently only available from conventional vacuum tubes up to the edge of the terahertz frequency range. Higher frequencies, extending into the extreme ultraviolet and X-ray spectral range, are usually only available, at high powers, from free-electron-lasers (FELs). However, the large size and cost of FELs is driving a search for alternatives methods.
Study by the University of Strathclyde has found that the normal direct correspondence between the bandwidths of the current source and emitted radiation can be broken which challenges a widely-held understanding of electromagnetic radiation.
The finding is said to have produced narrowband light sources where electromagnetic radiation would not normally be possible.
Professor Dino Jaroszynski said: “Our study has shown that some common media with interesting optical properties can be taken advantage of if we imbed, or bury, an oscillating current source in them. Media such as plasma, semiconductors and photonic structures have a ‘cut-off’, where propagation of electromagnetic radiation with frequencies lower than the ‘cut-off’ frequency is not possible.
“We noticed that the radiation impedance is increased at the cut-off. One consequence of this is that, for a broadband current source immersed in this type of dispersive medium, the cut-off frequency ‘mode’ is selectively enhanced, resulting in narrow bandwidth emission.
“In our research, we uncovered a hidden face of the cut-off and realised a new paradigm of narrowband light sources in media that would not usually allow electromagnetic radiation to propagate.”
When applied to THz systems, say the researchers, it is a way of obtaining strong, coherent monochromatic THz pulses in compact systems.