Photonic crystal nanobeam microcavities based on slot and hollow-core waveguides were proposed and analyzed. Three-dimensional finite-difference time-domain simulations show that both an ultrasmall modal volume (V) and a high quality (Q) factor can be obtained simultaneously in hollow-core-nanobeam mirocavities (HCNMs) due to the strong confinement of fields in the two lateral directions. For a 6.5-μm-long HCNM, the Q factor and V are on the order of 10⁵ and 10⁻²(λ/2n)³, respectively. With compactness, lower fabrication requirements as well as ultrahigh Q/V, the proposed microcavities would be very promising in a variety of applications.

Abstract-A new nanocavity-based electrooptic modulator on silicon-on-insulator is proposed. This nanocavity is formed by a one-dimensional photonic crystal nanobeam cavity with a slot embedded. The slot lies in both the cavity and the distributed Bragg reflectors region. This results in an ultrasmall modal volume of 0.047 and a high -factor of . Another advantage of the structure is that polymers can be infiltrated into the slot. Accordingly, we rely on the fast and strong electrooptic effect of polymers, rather than on the free-carrier plasma dispersion effect of silicon. Analysis shows a modulator with a bandwidth of 86 GHz, a switching voltage of 0.2 V, and a length of only 14 m can be obtained.

We demonstrate a high-Q hybrid surface-plasmon-polariton-photonic crystal (SP3C) nanobeam cavity. The proposed cavities are analyzed numerically using the three-dimensional finite difference time domain (3D-FDTD) method. The results show that a Q-factor of 2076 and a modal volume V of 0.16(/2n) 3 can be achieved in a 50 nm silica-gap hybrid SP3C nanobeam cavity when it operates at telecommunications wavelengths and at room temperature. V can be further reduced to 0.02(/2n) 3 when the silica thickness decreases to 10 nm, which leads to a Q/V ratio that is 11 times that of the corresponding plasmonic-photonic nanobeam cavity (without silica). The ultrahigh Q/V ratio originates from the low-loss nature and deep sub-wavelength confinement of the hybrid plasmonic waveguide, as well as the mode gap effect used to reduce the radiation loss. The proposed structure is fully compatible with semiconductor fabrication techniques and could lead to a wide range of applications. Considerable efforts are currently focused on exploring ways to increase the quality factor Q and decrease the modal volume V of optical microcavities, because of their significant roles in many fundamental physics studies, including light-matter interactions and device physics [1]. A higher Q value indicates a very long photon lifetime, while a small V means that the cavity can localize the photon in a small spatial volume. Over the last decade, various types of optical microcavities were proposed and investigated, including whispering-gallery microcavities, Fabry-Perot microcavities, and photonic crystal microcavities [2]. Among these cavities, the nanobeam photonic crystal microcavity [3-11] provides not only a very high Q value, but also an ultrasmall V and an ultracompact footprint. Nanobeam photonic crystal microcavities with Q values even higher than 10 9 were demonstrated [12]. Although Q values have been raised considerably by sophisticated designs, the smallest achievable V in a conventional dielectric cavity is bound to the diffraction limit, i.e. it is of the order of several times (/2n) 3 [13]. However, in some cases, an ultrasmall V is more important than a higher Q. For example, when the cavity's resonance linewidth is smaller than the emission linewidth of the emitter, decreasing the effective modal volume is the only way to increase the spontaneous emission rate [14].New microcavity schemes were designed to further decrease V. For example, microcavities based on slot waveguides were proposed [14][15][16][17][18]. Microcavities using surface plasmon polaritons (SPPs) were also studied extensively [19,20]. SPPs are electron density waves excited at and propagating along metal-dielectric interfaces [21,22]. Because of their sub-wavelength confinement of optical fields, they are promising candidates for realization of ultrasmall-V nanocavities. However, the reported Q values of SPP cavities thus far are very small and high Q values were only predicted at cryogenic temperatures [23]. The intrinsic high loss caused by metallic absorption ...

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