Nonlinear materials had a fascinating effect on the technology development. For example, in the 1950/60’s the nonlinearity of ferrites (associated with spinwaves) was exploited for microwave and millimetre wave parametric amplifiers, and then in the 1960/70’s the nonlinearity of veractor diodes were used for a similar purpose. In the 1980’s, the nonlinearity of electro-optic crystals was exploited for quantum optics research in the area of quantum entanglement. So what happen to this microwave and millimetre wave parametric amplifier technology and could it be used to develop quantum technology in this band, as potentially they offer a window on the vacuum photons?
One significant challenge of millimeter-wave or micro-wave for applications similar to quantum optics is the IR background noise in that bands. At room temperature (300K) the IR background has its maximum around 20-25 meV, which is somewhere between 10-100 µm or a few THz. Certainly, the black body spectrum extends with a long tail to even lower frequencies. For any qunatum optics, you want your detectors and experiments only to click, when there is a signal, which is in very many cases a single photon. If you have thermal noise, you will get many additional, uncorrelated clicks, making the experiments harder and making you loose on statistics. Spectral filtering will only get you so far, even inside your waveguides or experimental setup (or whatever structures you are using) thermal radiation will always be present. But if one operates in the optical or near IR bands, your photons have energies around 1eV and more. For practical most purposes there is no thermal radiation present. If something clicks, it was your photon. I think that is the significant reason to move to optical frequencies.
That is atleast one physical reason, why mm-wave etc. was abonded for that purpose. I can imagine, that many early research was first directed into those bands, as mm and micro-wave technology was available (just think about radar and the military implications, which was the driving factor for microwave technology), while (integrated) optics was still under development in the 60s and 70s. Much like the MASER was demonstrated before the LASER or early experiments on 3D "photonic crystals" have been done in microwaves first due to easier fabrication.
That is atleast the reason I can imagine, why it was abondoned.
Michael
Yes, certainly i can see that this would be a problem, many thanks for the response.
Neil
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