Ultrasmall Mode Volumes in Dielectric Optical Microcavities

We theoretically demonstrate a mechanism for reduction of mode volume in high index contrast optical microcavities to below a cubic half wavelength. We show that by using dielectric discontinuities with subwavelength dimensions as a means of local field enhancement, the effective mode volume ($V_{\mathrm{eff}}$) becomes wavelength independent. Cavities with $V_{\mathrm{eff}}$ on the order of ${10}^{-2}(\lambda /2n{)}^{-3}$ can be achieved using such discontinuities, with a corresponding increase in the Purcell factor of nearly 2 orders of magnitude relative to previously demonstrated high index photonic crystal cavities.

Figure 1

(a)–(c) The index profile for the slab waveguide with embedded low index slot regions of various slot widths ($w_s$). (d)–(f) The field distribution of the fundamental mode in the slab waveguide for various values of $w_s$. The electric field is polarized normal to the interface. $E_0$ is the maximum value of the electric field for the slab with no slot and $\Delta n$ is the ratio $n_H/n_L$.

Figure 2

The ratio of the effective mode volume of a slot waveguide compared to a slab waveguide for $\Delta n=1.5$ (circles), $\Delta n=2.5$ (triangles), and $\Delta n=3.5$ (squares), where $\Delta n$ is the ratio of high to low refractive indices. The slab thickness is $\lambda /n_H$.

Figure 3

(a) $|E{|}^2$ field spatial distribution from 3D FDTD in the a cavity based a on buried waveguide with an embedded low index slot at its resonant wavelength of 1431.3 nm. (b) $|E{|}^2$ field spatial distribution from 3D FDTD in a quasi-1D microcavity based on a buried waveguide without a slot for the resonant wavelength of 1556.4 nm.