Brightness Enhancement in Non-Hermitian VCSELs

Abstract

Vertical Cavity Surface Emitting lasers (VCSELs) are compact and efficient light sources useful for a variety of applications. However, due to lack of a transverse mode control mechanism, such lasers suffer from poor spatial beam quality, intrinsic spatiotemporal instabilities and nonlinear destabilizing effects such as filamentation and spatial hole burning [1]. Therefore, there is a need for new strategies to manipulate the light wave dynamics to enhance the stability of VCSELs. Recently, non-Hermitian media have become a flexible platform for new functionalities such as asymmetric coupling, unidirectional invisibility, single mode lasing [2–3]. In this presentation, we propose a novel stabilization mechanism for VCSLEs to obtain bright and narrow beams. The mechanism relies on non-Hermitian configuration of the laser potential, achieved by simultaneous spatial modulation of the refractive index and gain-loss profiles. In particular, we consider axisymmetric non-Hermitian potentials expressed as: n(r)=nR cos(qr)-in I cos(qr-ϕ) where n R and n I are the amplitude of the refractive index and gain-loss modulations, and ϕ is the relative phase shift between them. Such potentials may confine the emitted light around the central part of VCSELs, through unidirectional-inward radial coupling among the transverse modes [4]. The interplay of the relative strength and relative phase of the index and gain-loss modulations manipulate the wave dynamics of such lasers to emit powerful and narrow beams of high brightness. We use the mean-field paraxial model to study the spatiotemporal dynamics of such VCSELs with non-Hermitian potentials. The output emission of conventional VCSEL and modified VCSEL with concentric non-Hermitian configuration is shown in Fig. 1(a,b), illustrating irregular and stable localized pattern, respectively. We assess the performance through the central intensity enhancement [see Fig. 1(c)] and field concertation [see Fig. 1(d)]. The spatial dynamics for a representative point is provided in Fig. 1(e). The stationary intensity profile and its corresponding cross-section show that intensity is strongly concentrated at r=0 due to the favoured radial coupling of inward propagating waves, as depicted in the transverse field flow on the inset [see Fig. 1(e)].