Design of sub-wavelength size indium tin oxide clad optical disk resonators with quality factors>10⁴ in visible wavelengths

Abstract

The Q-factors of nano-scale disk optical resonators incorporating planar indium tin oxide (ITO) electrodes are significantly improved by optimizing the cavity geometry. Calculated Q-factors exceed 10 4 for optical cavities of sub-wavelength size in all dimensions.Optical resonators incorporating metallic materials are considered as a possible solution for shrinking the resonator size below free space resonance wavelength (λ 0 ) in all dimensions [1,2]. The metallic material would also satisfy the electrode requirement for current injection and electric field application in electrically-driven compact nano-and micro-cavity photonic devices such as current-injection nano-lasers. However, sub-wavelength (sub-λ) size metallic resonators suffer from low Qfactors in the order of a few hundreds since conventional metallic materials such as gold and silver are extremely lossy in the visible and near-infrared wavelengths. On the other hand, compact optical cavities clad by low-loss transparent conductive oxide (TCO) electrodes are promising for room temperature (RT), continuous-wave (CW), current injection low-threshold micro-lasers due to significantly high Q-factors [3].In this work, we show that both the radiation and the absorption losses are significantly reduced in simple nano-scale disk optical resonators incorporating planar indium tin oxide (ITO) electrodes by optimizing the cavity geometry. The disk aspect ratio-disk radius(r) divided by disk height (h)-is considered as the geometric parameter for optimization of the resonator Qfactors. FDTD analysis show that calculated Q-factors exceed 10 4 for optical cavities of sub-λ size in all dimensions while ITO electrode are directly placed on top of the disk. Fig. 1. (a) Schematic representation of the disk optical cavity with ITO top electrode, AlOx bottom cladding layer and AlAs current path. (b) Q-factor, and (c) disk diameter versus disk aspect ratio (r/h) for TM m11 , TE m11 and TE m12 modes with m=6 and 7.Analyzed ITO-containing disk resonators are shown in Fig.1 (a). An active disk medium is sandwiched between an ITO layer and AlO x layer. The bottom AlAs cylinder provides a current-injection path. ITO is chosen as the electrode material and as well as a cladding medium, considering its superior optical and electrical properties suitable for high-Q, ohmic-electrodecontaining sub-λ scale optical resonators that are essential for current-injection nano-lasers. The real part of the refractive index of ITO is smaller than that of semiconductors (typically ~ 3.4) and the absorption is negligible in visible wavelength spectra [4]. Even at infrared wavelengths, optical absorption is still small compared to gold and silver. Although the real part of the refractive index increases in the visible wavelengths, total internal reflection is well maintained for high-Q whispering gallery modes (WGMs). In this study, wavelength of 670 nm is chosen as the resonance wavelength.We analyzed cavity mode properties-mode frequencies, Q-factors, effective mode volumes (V eff ) and con...