Ne2E/m Polarization: P=Np=略-w(w+in) =0XeE, Dipole moment p=-er Dielectric susceptibility Ne2/mEo e(w)=1+Xe(w)=1+ w话-w(w+iy) f2 Ne2/meo 6 Oscillator strength Re(E,) Note that if the oscillator --lm(e,） strength is strong and the Re() (3)wl dissipation is small Im() f= 0.03w 2 pue enough,the real part of the permittivity can ⊙ 0 1=0.02500 become negative at 正 -2 frequencies just above the f=0.1娟 resonance frequency =4 0.7 0.8 0.9 1.01.11.21.3 Y2=0.010 00Polarization: Dipole moment Dielectric susceptibility Oscillator strength 1 0    0.025 2 0    0.01 Note that if the oscillator strength is strong and the dissipation is small enough, the real part of the permittivity can become negative at frequencies just above the resonance frequency