Optical excitations of deformed AB-stacked graphenes are studied through the gradient approximation. The interlayer atomic interactions induce prominent peaks, shoulder structures, and transition gaps in the low-energy absorption spectra. The uniaxial stress changes the energy spacing at the band-edge states and the Fermi momenta, which reflects on the spectrum peak frequencies and the transition gaps, respectively. These optical characteristics are also influenced by the layer number. Besides, deformation shows some similar and different effects in comparison with electric and magnetic fields. These predicted optical properties can be verified by optical measurements.

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Relativistic electromagnetic ion cyclotron instabilities driven by fusion-produced fast ions in magnetized plasmas can have two peaks in their growth rate spectrum. The wave numbers of these two peaks are close to the first and second peaks, respectively, of the Bessel function that is in the resonance driving term. The driving of the second Bessel and growth rate peak occurring at a higher wave number is weaker than that of the first peak. Surprisingly, as in contrast to conventional wisdom, the second peak can dominate near the instability threshold. For the higher energy of fusion-produced fast ion such as 14.7MeV, the slow ion temperature is required to be higher for overcoming the threshold to drive a cubic instability, which is determined by an Alfvénic condition. This cubic instability is due to the coupling of the first-order slow ion resonance and second-order fast ion resonance. This finite temperature effect is on the slow ion resonance and increases with wave number and thus the threshold is first satisfied near the second peak. Therefore, the second peak appears earlier in the instability spectrum and dominates near the threshold. The cubic instability has a much larger frequency mismatch than a coupled quadratic instability; a larger frequency mismatch indicates more fast ion energy to loss before the nonlinear saturation of the instability. When the slow ion temperature or density is about twice that of the threshold, the second peak has transited from the cubic to the coupled quadratic instability while the first peak remains as the cubic instability, in contrast to the previous 3.02MeV proton case.