1. 1 Au-shell cavity mode - Mie calculations R core = 228 nm R total = 266 nm t Au = 38 nm medium = silica cavity mode 700 nm cavity mode 880 nm 880 nm = 340 THz

2. 2 Finite Difference Time Domain method Step 1: Excitation kyky ExEx Step 2: Relaxation Step 3: Fourier Transform Plane wave excitation on and off resonance stores some energy in particle Particle oscillates, reemitting at its resonance frequency Fast Fourier transform of the relaxation E(t) to generate frequency spectrum

3. 3 Snapshots - Au shell (R=266 nm, t Au = 38 nm) in silica box (1.5x1.5 μm 2 ) excitation off-resonance at 150 THz (2 μm) -2 0 +1 +2 335 THz (895 nm) ExEx Fast Fourier Transform E-field monitor in center  p =1.3x10 16 rad/s  =1.25 x10 14 rad/s  d =9.54

4. 4 Snapshots - Au shell (R=266 nm, t Au = 38 nm) in silica box (1.5x1.5 μm 2 ) excitation off-resonance at 150 THz (2 μm) excitation on-resonance at 335 THz (895 nm) cavity mode! Electric field intensity max= 6.5 at center 0 5 -2 0 +1 +2 ExEx

5. Cavity parameters Quality factor Q=35 The maximum field enhancement within the core amounts to a factor of 6.5 The mode volume V=0.2 ( /n) 3 … 10 2 -10 3 smaller than than that in micordisc/microtoroid WGM cavities A characteristic Purcell factor – assuming homogeneous field distribution in the cavity core and =895 nm

6. Cavity optimization A characteristic Purcell factor assuming homogeneous field distribution in the cavity core, =895 nm and Q=150:

7. 7 Cavity mode is tunable - T-matrix vs FDTD calculations Penninkhof et al, JAP 103, 123105 (2008)

8. 8 Cavity mode is tunable by shape - oblate Au shell spheroid aspect ratio =2.5 L / 410 THz T / 240 THz