IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 9, SEPTEMBER 1998 1205 First Demonstration of Highly Reflective and Highly Polarization Selective Diffraction Gratings (GIRO-Gratings) for Long-Wavelength VCSEL’s S. Goeman, S. Boons, B. Dhoedt, K. Vandeputte, K. Caekebeke, P. Van Daele, and R. Baets Abstract— We present experimental results on surface relief gratings in GaAs and InP with high reflectivity ( 85%) and po- larization selectivity to normal incidence from the semiconductor side. The potential for polarization stabilization with a reduced mirror complexity for long-wavelength VCSEL’s is discussed. Index Terms— Grating reflectors, semiconductor lasers, surface-emitting lasers. I. INTRODUCTION A LTHOUGH short wavelength VCSEL’s (850 and 980 nm) are commercially available, most of these devices still show unstable output polarization and transversal mul- timode behavior [1]. Several solutions have already been proposed to overcome these problems. Using AlAs oxidation in the top distributed Bragg reflector (DBR) mirror offers beside current confinement an improvement of transverse monomodal operation [2]. Concerning the polarization prob- lem, asymmetric etched post structures or growth on (311) substrates have been proposed [3]–[5]. This causes anisotropy of the gain, leading to pinning of the polarization. Also surface relief gratings have been proposed [6], [7]. In this letter, we propose a surface relief structure not only showing polarization selectivity but also a high reflectivity for TM polarization. This property is of specific interest to long-wavelength vertical-cavity surface-emitting lasers (VCSEL’s) in view of the low refractive index contrast in these devices leading to rather complex mirror structures (wafer fusion [8], metamorphic growth [9]) and associated electrical and thermal problems. Using the approach presented, it is possible to stabilise the polarization and to decrease the number of DBR pairs of the top VCSEL mirror, possibly leading to a smaller electrical and thermal resistance. In Section II, we will shortly discuss the grating design. Section III describes the fabrication and shows measurement results of GaAs and InP based GIant Reflectivity to Order (GIRO)-gratings. Section IV presents calculations showing that the fabricated GIRO-gratings on top of a reduced DBR stack will lead to complete polarization control. II. GRATING DESIGN In this section, we will only briefly discuss the the working principle and design rules of these “GIRO-gratings.” A more Manuscript received April 7, 1998; revised June 1, 1998. This work was supported in part by the European ACTS024- VERTICAL Project and in part by the Belgian DWTC-Project IUAP- 13. The work of S. Goeman and B. Dhoedt was supported by the Flemish IWT. The authors are with the Department of Information Technology (INTEC), Uninversity of Ghent—IMEC, B-9000 Gent, Belgium. Publisher Item Identifier S 1041-1135(98)06299-5. Fig. 1. TM reflectivity GaAs GIRO grating, filling factor 50%. Black region: TM reflectivity 90%, wavelength 1.55 m. detailed study will be explained elsewhere [10]. A GIRO- grating is a linear surface grating between semiconductor and air. A plane wave is incident normal to the grating plane, from the semiconductor side. By choosing the grating period, only the zeroth-order reflection exist in air and there are only two excited propagating optical modes in the grating region. The zeroth-order mode is highly concentrated in the grating ridges and the second-order mode is highly concentrated in the air gaps between the ridges. By choosing the proper grating depth, the average field at the grating air interface composed of these two modes is almost zero and, therefore, no optical power is coupled to the zeroth order in transmission. At the same time, the two optical modes interfere constructively at the grating semiconductor interface thereby coupling almost all power to the zeroth order in reflection and cancelling the higher diffraction orders (in practice, the 1,1 and 2,2 diffraction order). Ultimately, this leads to the following approximate design rules for such a grating: with and the effective indices of the zeroth- and the second-grating mode, the vacuum wavelength, the grating period, the grating depth, the semiconductor refractive index, the refractive index of air. We have found that these design rules only work for the TM polarization and not for the TE polarization. The reason for this is the different form of the optical modes for the two polarizations. In view of the approximations used to arrive at the design rules, we have designed GaAs and InP GIRO gratings for the wavelength of 1.55 m, using a diffraction model based on rigorous coupled wave analysis [11], using the design rules 1041–1135/98$10.00  1998 IEEE