Interference between a discrete resonance and a continuum background produces the characteristic asymmetric Fano profile σ ∝ (q+ε)²/(1+ε²), where q is the Fano parameter controlling asymmetry.
1.5
0.0
1.0
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Fano (1961): q→∞ gives Lorentzian; q=0 gives a dip (anti-resonance). Fano profiles appear in atomic photoionization, plasmonics, photonic crystals, and nanoelectronics.