When does the frequency of a source increase, rather than decrease, as it moves away from you? One place is in Bristol, UK, in a laboratory at BAE Systems, where Nigel Seddon and Trevor Bearpark have demonstrated this remarkable effect with radio waves. The idea is to have waves in a nonlinear medium that can exhibit anomalous dispersion. In this case - which as it turns out has been known since the 1940s - the effect occurs as the incoming waves strike the boundary between a region of anomalous dispersion that is moving away and a region of normal dispersion that has not yet been made anomalous. The backscattered component then comes back shifted not down, but up in frequency. This work offers a whole new way of thinking about how to generate frequencies in traditionally difficult regions of the spectrum.
Further reading
N Seddon and T Bearpark 2003 Science 302 1537.