Figure 9.

Radiation enhancement G_R of the Raman signal from the aggregate of molecules of RBD protein coated with a spherical silver envelope with a radius of R = 90 nm as a function of silver layer thickness d. (a,b) The electric field distribution in the dipole and quadrupole resonances excited by the molecular dipole, which is indicated with an open arrow; the dipole oscillates with a frequency of 2πc/(λ = 834 nm), which corresponds to the Trp Stokes shift (754 cm⁻¹) from the fundamental laser frequency of 2πc/(λ = 785 nm); (a) the dipole resonance is excited in the silver envelop with a thickness of d = 8 nm; (b) the quadrupole resonance is excited at d = 4 nm; (c) radiation enhancement G_R (d); molecular dipoles are distributed over the internal surface of the silver envelope with a radius of R = 90 nm; the thickness of the dipole layer is 8 nm; the dipoles radiate at wavelengths of λ = 834, 878, and 895 nm (the brown, red, and green curves), which correspond to the main Stokes shifts (754, 1344, and 1565 cm⁻¹) in the Raman signal from the RBD protein (cf. Figure 6 and Figure 7); the Raman signal from the plasmon resonator is normalized to the Raman signal generated by the bare molecular layer.