A Supercontinuum? With Light’s Negative Counterpartby +Sophie Wrobel, +EuroTech; Germany
Matter is offset by anti-matter, and light, it seems, can be offset by ‘negative’ light – or, in more proper terminology, negative-frequency resonant radiation...
and with it, a new flurry of activity in supercontinuum research is set to take place.Background
You probably recall from grade school that any physical electric field is a real function. But: do positive and negative frequencies work the same way as in classical electromagnetism? Eleonora Rubino’s team, from the University of Insurbia in Como, Italy, has demonstrated that the answer to this question is No.
If you take light and pass it from one medium into another, it refracts, travelling at a different angle. But different wavelengths refract at different angles. That allows us to use some trickery to create a temporal displacement in light, creating ‘bubbles’ of intense light – called solitons. Normally, classical physics dictates that pulses are symmetrically phase-matched. But Rubino’s team claimed, and demonstrated, that it is possible to break that symmetry, resulting in two different resonant radiation frequencies instead of just one.
Under certain conditions, namely during phase-matching during the soliton production, the soliton emits a low-intensity, positive frequency resonant radiation. According to Eleonora Rubino’s calculations, this would require a corresponding negative
resonant radiation. Now, what does a negative frequency mean? It means that instead of resonating to the tune of just one frequency, we can cause resonance at two different wavelengths. And that means we have a new tool for creating consistent intensity across a broad spectrum, or a supercontinuum.How to make negative light
Rubino’s team took some photonic crystal fibers, which is particularly favorable for resonant radiation production, and launched short pulses of light across a broad input spectrum, designed to favor the energy transfer between a positive soliton and a negative resonant radiation, into them. Each pulse of light was 7 femtoseconds long. They observed the negative radiation at exactly the frequency they expected. They repeated the experiment in 2 cm of calcium fluoride, this time with 60 femtosecond Bessel pulses, and again succeeded at generating negative resonant radiation at their predicted wavelengths.What can you do with a supercontinuum?
Supercontinua are used in many fields – some notable example include coherence tomography, fluorescence microscopy, characterizing optical devices, generating multiple carrier waves in optical fiber communications, or measuring the carrier-envelope offset frequency of frequency combs. Rubino’s results mean that we can push supercontinua to even shorter wavelengths, leading to more precise optical instruments.Graphs:
Left, normal soliton; right, negative solitonTags: #ScienceEveryday _____________________________________________________
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