Demonstration of spin injection in a CW VECSEL at RT with a dynamic and accurate control of its polarization state (2017)

Thanks to the recent spintronics advancement, new applications should emerge, like fast communications with spin coding. For instance, data transmission by coding the polarization state of a spin-injected laser source [1] is enabling new modulation formats and wide frequency bandwidths. In comparison to semiconductor edge-emitting lasers where only one polarization is guided, vertical cavity surface emitting lasers (VCSELs) are more adapted to obtain a spin-laser. However, we recently showed, using an external cavity VCSEL (VECSEL), that even in such quasi-isotropic lasers, spin injection is inhibited by residual linear anisotropies due to the poor spin transfer efficiency [2]. These residual anisotropies set the laser polarization eigenstates to linear polarizations. Here, we propose to investigate theoretically and experimentally the impact of these anisotropies on the polarization eigenstates of a spin-VECSEL, so as to prepare the laser for spin injection. We finally highlight spin injection effects in a device where the linear birefringence and dichroism are reduced to the minimum. An original laser architecture is proposed to control dynamically the linear anisotropy of phase γ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lin</sub> and the linear dichroism of gain Δ G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lin</sub> so as to control the laser polarization state (Figure 1, a). The laser that we developed includes a ½-VCSEL structure emitting at 1.5 mm at room temperature (RT), provided by the Ecole Polytechnique Fédérale de Lausanne [3]. It is optically pumped with a continuous wave (CW) single mode laser diode at 980 nm. A 500 mm PLZT electro-optic ceramic is added into the cavity to control its linear birefringence [4]. A 100 mm YAG etalon is also inserted in order to ensure both single frequency oscillation and minimization of the linear dichroism of the cavity. Experimentally, when we tune the linear birefringence from high to low values, the VECSEL polarization ellipticity is increased from 4 to 23° (Fig. 1, b, red circles). By fitting these results with a vectorial model, which takes into account three parameters (γ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lin</sub> , Δ G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lin</sub> and the circular dichroism of gain Δ G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lin</sub> ), we can extract the values of Δ G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lin</sub> = 5.10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> and Δ G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lin</sub> = 2.10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> (Fig. 1, b, blue line). To highlight the effects of spin injection, we measure the laser polarization eigenstates modifications between a pumping with a linear polarization (unpolarized electrons) and a circular polarization (50% spin-polarized electrons) (Fig. 1, c). Compared to a linear pump polarization, an increase of the ellipticity of 22° is observed when spin down (right-handed circular pump polarization σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) is injected into the gain medium and a decrease of 20° when spin up is injected (left-handed circular pump polarization σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> ). The inset of the Figure 1, c) shows the switch of the laser polarization eigenstate when the VECSEL is optically injected with spin up and spin down. A threshold reduction of about 8 % is also obtained, as expected, witch spin injection. To conclude, spin injection in an optically pumped VECSEL operating at RT is demonstrated using a dynamic control of the laser residual linear anisotropies. We are now working in improving the spin injection efficiency so as to obtain a VECSEL polarization switch from right to left handed circular polarizations, that is, a switch of the VECSEL ellipticity from -45° to 45°.