Analysis of structurally sensitive loss in GaN-based VCSEL cavities and its effect on modal discrimination

Abstract

Lateral loss causes optical energy to leave the laser cavity in the transverse, lateral, direction, and is sometimes neglected to simplify the numerical simulations. However, in contrast to outcoupling and absorption losses, we show that the lateral loss can change drastically with only nanometer-sized changes of the cavity structure, from being virtually zero to becoming the major source of cavity loss, since the cavity becomes antiguiding. This can be explained as the opening of a channel of efficient resonant lateral leakage of optical power at a certain oblique propagation angle. A number of different realizations of current apertures and top mirror designs in GaN-based VCSEL cavities, which have been suggested for realization of microcavity lasers emitting in the blue wavelength range, are simulated. Many of these are shown to lead to unintentional antiguiding, which can more than double the threshold gain for lasing. Notably, for strong enough antiguiding the resonant lateral leakage decreases so that the threshold gain values might again be tolerable. This regime has been suggested for robust single-mode operation since earlier predictions, building on analogies with slab waveguides, hinted at a very strong suppression of higher order modes. However, our simulations indicate that for the VCSEL cavities the derived formulas grossly overestimate the modal discrimination.

Progress and challenges in electrically pumped GaN-based VCSELs

“…Buried tunnel junctions have also been demonstrated to laterally confine the current 130 , and if the introduced structural step from the overgrown TJ transfers into the top DBR, it might also provide simultaneous optical guiding 132,133 .…”

“…However, to achieve an efficient device with low threshold it is important to ensure a good lateral overlap between the optical mode and the carriers in the QWs. In fact, it has been shown that most apertures used for current confinement result in optically anti-guided structures with associated high losses 132,133 , for more details see Section 4. Switching to index-guided structures instead, will contribute to lowering threshold currents and can be an effective way to reduce filamentation, something often seen in GaN-VCSELs 27,28,30,32,39,40,46 , and also seen in early proton-implanted gain-guided GaAs-VCSELs 135,136 .…”

“…The structure and material data used are the same as in Ref. 132 , except for the bottom DBR, which in Fig. 4 consist of 11 mirror pairs of TiO2/SiO2 and in Fig.…”

“…An anti-guided structure is known to lead to a higher lateral radiation loss and thereby higher threshold currents and lower output powers, if lasing is at all possible. To investigate this further, a three-dimensional (3D) Beam Propagation Method (BPM) was deployed to calculate the threshold gain, accounting for the lateral radiation loss mechanisms such as diffraction and lateral leakage of light 132 for a number of different aperture schemes. The calculated threshold material gain as a function of the depression parameter δ is plotted in Fig.…”

Electrically Pumped III-N Microcavity Light Emitters Incorporating an Oxide Confinement Aperture

“…So far, electrically pumped polariton emitters and conventional VCSELs have been demonstrated in III-nitride based MCs at room temperature (RT). However, the pside designs including indium-tin-oxide (ITO) transparent conducting layer (TCL) and dielectric materials based current aperture in the reported devices induce a negative guiding effect and lateral optical loss [13][14][15]. To provide a sufficient positive guiding effect, Cheng et al utilized an intra-cavity low-index AlN layer to serve as a lateral optical/current aperture and achieved a high-quality-factor (Q) emission; however, no lasing action was observed in their MC light emitter due to the lack of TCL [16].…”