Fig.1

Schematic demonstration of whispering gallery modes excited in a closed cavity"

Fig.2

Geometry of an EM Helmholtz cavity with radius r and angular width of the opening θ, θi is the angle of incidence and the center line of the opening is the x axis "

Fig.3

The absolute value of the first five Mie scattering coefficients, $|{B_m}|$ , as a function of wavelength for a Helmholtz cavity with r = 60 mm and $\theta = 14^\circ $ , the angle of incidence is ${\theta _i} = {0^\circ }$ "

Fig.4

The absolute value of the Mie scattering coefficients a) and b) versus incident wavelength for Helmholtz cavities with the same radius r = 60 mm but different angular widths of the opening; The quality factor, Q, of the dipole WGM c) and the hexapole WGM d) versus incident wavelength for Helmholtz cavities with the same radius r = 60 mm but different angular widths of the opening. The angle of incidence is ${\theta _i} = {0^\circ}$ "

Fig.5

The E field distribution when an EM wave with a) $\lambda = 53.8\;{\rm{mm}}$ and b) $\lambda = 59.2\;{\rm{mm}}$ strikes a Helmholtz cavity with radius r = 60 mm and angle width of the opening $\theta = {20^\circ}$ ; c) The E field distribution and e) the magnetic field vector distribution when an EM wave with $\lambda = 53.8\;{\rm{mm}}$ strikes a Helmholtz cavity with radius r = 60 mm and angle width of the opening $\theta = {8^\circ}$ ; d) The E field distribution f) and the magnetic field vector distribution when an EM wave with $\lambda = 59.2\;{\rm{mm}}$ strikes a Helmholtz cavity with radius r = 60 mm and angle width of the opening $\theta = 10^\circ $ . The angle of incidence is ${\theta _i} = {0^\circ}$ "

Fig.6

a) The absolute value of the Mie scattering coefficients, $|{B_1}|$ , versus incident wavelength for Helmholtz cavities with radius r = 60 mm and angular width of the opening $\theta = {8^\circ}$ at different incident angle; b) The absolute value of the Mie scattering coefficients, $|{B_3}|$ , versus incident wavelength for Helmholtz cavities with radius r = 60 mm and angular width of the opening $\theta = {10^\circ}$ at different incident angle; The E field distribution when an EM wave with $\lambda = 59.2\;{\rm{mm}}$ strikes a Helmholtz cavity with radius r = 60 mm and angle width of the opening $\theta = 10^\circ $ at an angle of incidence c) ${\theta _i} = {15^\circ}$ and d) ${\theta _i} = {25^\circ}$ "

Fig.7

a) The E field distribution for the Helmholtz cavity with radius r = 60 mm, wall thickness $t = 0.2$ mm and angular width of the opening $\theta = {\rm{8}}^\circ $ at $\lambda = 53.8\;{\rm{mm}}$ ; b) The E field distribution for the Helmholtz cavity with radius r = 60 mm, wall thickness $t = 0.2$ mm and angular width of the opening $\theta = 10^\circ $ at $\lambda = 59.2\;{\rm{mm}}$ . The angle of incidence is ${\theta _i} = {0^\circ}$ "

Fig.8

The H field distribution for the Helmholtz cavity with radius r = 60 mm and angular width of the opening $\theta = {\rm{8}}^\circ $ for the transverse-magnetic polarized wave with $\lambda = 53.5\;{\rm{mm}}$ ; b) The H field distribution for the Helmholtz cavity with radius r = 60 mm and angular width of the opening $\theta = 10^\circ $ at $\lambda = 58.8\;{\rm{mm}}$ . The angle of incidence is ${\theta _i} = {0^\circ}$ "