An All-Silicon Passive Optical Diode

Abstract

A passive optical diode effect would be useful for on-chip optical information processing but has been difficult to achieve. Using a method based on optical nonlinearity, we demonstrate a forwardbackward transmission ratio of up to 28 decibels within telecommunication wavelengths. Our device, which uses two silicon rings 5 micrometers in radius, is passive yet maintains optical nonreciprocity for a broad range of input power levels, and it performs equally well even if the backward input power is higher than the forward input. The silicon optical diode is ultracompact and is compatible with current complementary metal-oxide semiconductor processing.Nonreciprocal transmission is fundamental to information processing. Electrical nonreciprocity, or the diode effect, had been realized in integrated form with a semiconductor p-n junction. Optical nonreciprocity (ONR) is inherently difficult because of the time-reversal symmetry of light-matter interaction (1). Previously reported observations of ONR were based on the magneto-optic effect (2-4), optical nonlinearity (5-8), electroabsorption modulation (9), cholesteric liquid crystals (10), optomechanical cavities (11), indirect interband photonic transitions (12), and the opto-acoustic effect (13). However, complementary metal-oxide semiconductor (CMOS)-compatible passive optical diodes with a footprint and functionality analogous to those of p-n junctions have not been realized at the near-infrared wavelengths that are preferred for silicon (Si) photonics.Our optical diode (Fig. 1A) is based on strong optical nonlinearity in high-quality factor (Q) Si microrings (14-17). It consists of a high-Q all-pass notch filter (NF) operating near the critical coupling regime (17) (Fig. 1B) and an add-drop filter (ADF) (14,16,18) with asymmetric power coupling to the bus waveguides (Fig. 1C) NF is thermally tuned to match that of the ADF through the thermooptic effect of silicon (19).A microring accumulates optical energy at its resonant wavelength. The schematics in Fig. 1, E and F, show that light couples into the microring in the ADF through two different gaps, G 2 and G 3 . If we define forward and backward input power as P c,f and P in,b , respectively, the optical energy stored in the microring near its resonant wavelength, λ ADF , can be expressed as (1) and (2) where Q ADF is the ring's loaded quality factor, Q G 2 and Q G 3 are power coupling quality factors that are exponentially proportional to the gap sizes, and K(λ) represents all other terms that are independent of propagation direction for a linear system (14).The energy enhancement factor in the ring depends on the propagation direction because of our asymmetric design (Q G 2 ≈ 300,000,Q G 3 ≈ 192,000, and Q ADF ≈ 43,800, all through curve-fitting), and (U forward /U backward ) = (Q G 3 /Q G 2 ) = 0.64 for P c,f = P in,b . With high input power at λ 0 = λ ADF , the power density inside the ring will be amplified substantially because of its high Q factors and small radius; this induces optical nonlineari...