A surface-emitting laser incorporating a high-index-contrast subwavelength grating

Semiconductor diode lasers can be used in a variety of applications including telecommunications, displays, solid-state lighting, sensing and printing. Among them, vertical-cavity surface-emitting lasers (VCSELs) are particularly promising. Because they emit light normal to the constituent wafer surface, it is possible to extract light more efficiently and to fabricate two-dimensional device arrays. A VCSEL contains two distributed Bragg reflector (DBR) mirrors for optical feedback, separated by a very short active gain region. Typically, the reflectivity of the DBRs must exceed 99.5% in order for the VCSEL to lase. However, the realization of practical VCSELs that can be used over a broad spectrum of wavelengths has been hindered by the poor optical and thermal properties of candidate DBR materials. In this Letter, we present surface-emitting lasers that incorporate a single-layer highindex-contrast subwavelength grating7,8 (HCG). The HCG provides both efficient optical feedback and control of the wavelength and polarization of the emitted light. Such integration reduces the required VCSEL mirror epitaxial thickness by a factor of two and increases fabrication tolerance. This work will directly influence the future designs of VCSELs, photovoltaic cells and light-emitting diodes at blue –green, 1.3 –1.55 mm and midto far-infrared wavelengths. Broadband mirrors with high reflectivity are essential for constructing a VCSEL cavity with a high quality factor. They are typically composed of semiconductor DBRs, structures formed from multiple layers of alternating dielectric materials with periodic variation of refractive indices. Their reflectivity and bandwidth depend on the refractive-index contrast of the constituent materials and the precision of thickness control within each layer. Because of epitaxial growth constraints for matching the material atomic lattice, typical combinations of DBR materials often have small refractive-index differences. Thus, it is often necessary to have a rather large number of DBR pairs (25–40) to attain a high enough reflectivity in addition to the resulting small mirror bandwidth (Dl/l 3–9% where l is the wavelength of the light). This has been one of the major difficulties in current VCSEL fabrication, especially for blue–green and long-infrared wavelengths. The problem becomes more challenging when making wavelength-tunable VCSELs, where the requirements on mirror bandwidth and reflectivity are even more stringent. To overcome the limitation of DBRs in surface-emitting optoelectronic devices, we propose and demonstrate the use of a single-layer, high-index-contrast subwavelength grating in place of a conventional DBR. For example, 35–40 pairs of GaAs–AlGaAs DBRs approximately 5 mm in thickness can be replaced by a 0.235 mm thick AlGaAs-based HCG with an equivalent reflectivity. Thus, by using an HCG, we can potentially reduce the required VCSEL mirror epitaxial thickness and simplify material growth requirements for achieving thickness and composition accuracy. The HCG differs from a second-order grating both in terms of structural design and in performance, in that coupling efficiency and reflectivity are both significantly lower. Furthermore, the HCG reflectivity bandwidth is 10 times wider than that of a conventional DBR and indeed 100 times wider than that of a second-order grating. In earlier work we have reported theoretical calculations and experimental reflectivity measurements of a single HCG mirror. However, it was difficult to determine the absolute value of the reflectivity, the phase delay and the dependence on beam divergence, which are all critical for VCSEL design. It was therefore uncertain whether an HCG was suitable for integration with a VCSEL structure to be used in the near-field regime. In this paper we demonstrate, for the first time, the realization of a VCSEL with an HCG mirror (HCG-VCSEL). The schematic of the device is shown in Fig. 1a. The device consists of a conventional semiconductor-based bottom n-DBR mirror, a l-cavity layer containing the active region and an HCGbased top mirror. The top mirror is composed of two parts: a four-pair p-doped DBR and a freely suspended HCG. The p-DBR is mainly used to provide current spreading while protecting the active region during fabrication. Although the p-DBR does increase the overall reflectivity of the top mirror, our simulation shows that the number of p-DBR pairs can be reduced, because a single-layer HCG, as the top mirror, is capable of providing sufficient reflectivity (R . 99.9%). Electric-current injection is carried out through the top contact (through the p-doped HCG layer) and the bottom contact (through the n-DBR). An aluminium oxide aperture, formed from the thermal oxidation of an AlGaAs layer in the p-DBR section immediately above the cavity layer, provides efficient current and optical confinement. Figure 1b shows the scanning electron microscope (SEM) image of the fabricated HCG-integrated VCSEL, where the HCG is defined in the centre of the VCSEL mesa, aligned with the oxide aperture. Figure 1c and d shows close-up views of the freely suspended HCG structure. A stress-relief trench surrounding the grating was necessary to eliminate buckling of the suspended LETTERS