Carrier-induced refractive index changes in InP-based circular microresonators for low-voltage high-speed modulation

Abstract

Abstract-Optical InP-based microresonator modulators which achieve low-voltage high-bandwidth modulation are presented, where resonant wavelength tuning of a circular resonator by free carrier injection is used as the modulation mechanism. Since thermal effects in small resonant cavities and switching speed limitations posed by minority carrier lifetime are the primary concerns in such types of devices, ion bombardment in microtoroidal structures is used to increase the speed of response. The modulation speed is enhanced by an order of magnitude. . In these devices, free carriers injected into the intrinsic region change the effective index of the whispering gallery modes (WGMs), and thus, blue-shift the resonant wavelengths in the spectral range of interest. Although other modulation mechanisms such as electroabsorption, depletion, and gain have been demonstrated [5], [6], we focus in this letter on the FCI mechanism as it is an attractive candidate to achieve low-voltage modulation in the InP-based material system. Index Terms-HighA current limitation of these devices is their limited high-frequency response which is controlled by the minority carrier lifetime. Also, these devices exhibit thermal tuning under forward bias that limits their performance. In this letter, we demonstrate greater than an order of magnitude improvement in the operation bandwidth of FCI modulators through the use of proton implantation to reduce the minority carrier lifetime in these devices [7]. Low voltage operation is maintained by instituting fabrication approaches that improve the sensitivity of the ion implanted devices. We will also present a model of the tuning mechanism including the effects of both FCI and thermal effects that quantifies the speed-voltage swing tradeoff in these devices. The principle of operation of the FCI modulator involves the resonant transfer of optical energy between two waveguides at the resonance of a microresonator that couples them. An example of such a coupling structure is shown in Fig. 1. Modulation of light in the through port is accomplished by rapid tuning of the index of refraction and, thus, the resonant frequency of the microresonator by FCI.The microresonator modulators in our study are fabricated with a combination of epitaxial growth and wafer bonding using a process that has been described in previous publications [1], [5], [8]. Some modifications have been introduced to maintain the small bias swing necessary to operate the modulator modified by the ion implantation process. Device fabrication starts with the growth of the epi-structure that contains the disk core with a sacrificial InGaAsP layer on top that will be removed after the ion implantation. This structure is bombarded with protons to achieve lifetime reduction. Protons with energy of 40 keV were chosen so as to cause maximum damage at the center of the disk waveguide core layer. After the ion implantation, the sacrificial layer is removed to provide a relatively defect free growth interface for the rest of the epitaxial st...

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