Efficient coupling to W 1 Photonic crystal waveguide on InP membrane through thinly supported access guides

Thinly supported access ridges have been designed in order to improve the coupling efficiency into W1 Photonic crystal –PhCwaveguides on InP membrane. This integration allows to control the termination of the W1 PhC guide. Low propagation losses have been measured for these thinly supported guides. Introduction InP-based Photonic Crystal –PhC– membranes are attractive because they can combine the advantage of operating PhC guides (mainly W1) below the air light-line [1], so theoretically in a lossless regime, with the possibility of including emitters such as quantum boxes emitting at 1. 55μm. The theoretical lossless regime can be reached in the case of a perfect system. Part of the imperfections comes from e-beam lithography [2] and part comes from holes etching. We have investigated here an ICP-RIE process leading to very smooth etched surfaces. Previous work had implemented a Cl2/Arbased processes for deep etching. But, we were not able to suppress some residual roughness [3]. In the case of a membrane, deep etching is no more an issue, since the membrane thickness does not exceed 350-400nm (λ/4) in order to operate in a monomode regime. But the etched surfaces have to be very smooth, because any additional roughness will produce scattering, thus increasing propagation losses. We have developed a process including N2 in the gas mixture, as already proposed by P.Strasser et al [4], and discussed in [5]. Injecting light in a strongly confined PhC guide is still difficult due to the strong modal mismatch. Also, the role of the termination of the W1 guide has been evidenced to play a crucial role [6]. We propose here a new type of access guide, thinly supported access guides that are realized during the same technological steps than the holes. They allow to control the final termination of the PhC guide. relative position of the injected power. We fabricated W1 PhC guides on a 340nm thick InP membrane. A 770nm sacrificial GaInAs layer is selectively etched away to produce the membrane. Different lengths of W1 are fabricated in order to obtain the propagation losses through the Fabry-Perot oscillation method. The PhC triangular matrix has a 420nm period. The air-filling factor is close to 30%. These PhC parameters allow to cross the air light-line in a wavelength domain close to 1500nm. Due to the strong effect of termination [6], the reasonably low propagation losses found here are a first estimate, and have to be confirmed. PhC technology PhC holes are defined by e-beam lithography in a PMMA resist layer, and then transferred with a CHF3-based RIE process in a SiO2 250nm-thick underlying layer deposited on the semiconductor material. After removing the PMMA resist, this SiO2 layer is used as a mask for ICP etching. ICP etching is performed in a Sentech SI500 system. We first investigated different Cl2/N2 ratio , while keeping a low Cl2 fraction in order to avoid a too large chemical contribution. This two-gas mixture does not allow the obtainment a smooth and simultaneously vertical surface: the surface passivation is always too large, and lead to closed holes as visible on the SEM picture (cross section) of fig.1-a. The nonverticality of the holes is detrimental for propagation because it produces additional losses. In order to keep the advantage of surface passivation through N2, and FA2