Computer-aided design of grating VCSELs emitting circularly-polarized light

Vertical-cavity surface-emitting lasers (VCSELs) are widely used in automotive systems, sensors, short and long-range communications. However, emerging applications like polarization LIDARs, atomic clocks and atomic magnetometers require stable circular polarization in the laser beam. This thesis explores potential designs for circularly-polarized (CP) VCSELs and identifies key parameters that influence CP performance. CP VCSELs have been analysed by means of our in-house VCSEL ELectroMagnetic Suite (VELMS), serving as optical mode solver of Maxwell's equations, based on the mathematical framework of coupled mode theory. In particular, this thesis is focused on engineering optical anisotropies, investigating the impact of their orientation on the polarization features of the emitted light. The electro-optic and elasto-optic effects, single and double sub-wavelength gratings have been considered either independently or in combination. Anisotropies have been incorporated into the model by extending the 1D version of VELMS, now accounting for the vectorial components of the optical field and able to extract polarization features. The interplay between misaligned anisotropies is pivotal for CP; in this view, tilted sub-wavelength gratings play a key role. Such tilted gratings have been analysed with different models, either by treating them as a homogeneous anisotropic medium described in terms of polarization-dependent effective indices, or by rigorous coupled wave analysis (RCWA). The output polarization is described by the Stokes parameters, with particular emphasis on S3. Indeed, S3 +/-1 represent fully CP light, either clockwise or counterclockwise. The first structure analyzed is a single-grating VCSEL, with electro and elasto-optic effects oriented along the VCSEL's crystalline axis and with the grating at the outcoupling facet rotated with respect to them. When optimized in terms of technological parameters, specifically grating thickness and rotation angle, this configuration can produce CP. However, this solution could be strongly affected by unwanted technological factors, such as unwanted strain, impacting on the elasto-optic effect, eventually degrading the CP output to elliptically polarized light. Aiming at a more robust implementation, a more complex design has been proposed, where decoupled anisotropies are induced by multiple gratings. Specifically, a double grating structure with a spacer in the middle has been investigated. In this design, both gratings are placed at the VCSEL outcoupling facet. The bottom grating, is aligned with the crystalline axes, while the upper grating, exposed to air, is tilted. By carefully employing for simulations the parameters of reasonable materials, obtained by processes compatible with standard VCSEL technology, promising results have been obtained. Employing a sol-gel of TiO2 for both the spacer and the upper grating, and considering reasonable thermal curing, it is possible to obtain a refractive index of 2. CP has been achieved for a structure where the upper grating has a thickness of 70 nm, with the spacer thickness of 100 nm, yet featuring a periodic behavior. A critical condition for this structure is that the period of the bottom grating must be smaller than that of the upper one. The upper grating exhibits optimal performance when rotated at 45° with a period of [200, 400] nm and a thickness of [300, 400] nm. The proposed structures are extremely promising for future implementations in the view of more compact devices.