In our 2021 Science paper entitled "Topological insulator vertical-cavity surface-emitting laser (VCSEL) array", Lustig et al., Science 373, 1514-1517 (2021) we demonstrated the basic concept described in the proposal. We have shown that we can use a topological arrangement of the VCSEL elements to force injection-locking of the individual VCSEls and made them act as a single coherent source with the power of the individual lasers combined coherently.
However, that was just an initial proof-of-concept, as those VCSELs were not suitable for operation as an actual product. Far from that. Namely, these VCSEL were based on quantum dots as the gain medium powering the laser. For high-power lasing, the gain medium should be quantum wells, not quantum dots. Second, the VCSELs were pumped optically, whereas for a product the lasers should be pumped electrically. Third, the VCSEL array was at low temperatures (up to 200oK), whereas it is desirable to have the lasers at room temperatures and higher. As part of the PoC grant, we intended to overcome those obstacles and bring this nice proof-of-concept into a real prototype.
The first major mission was to repeat the experiments described in our Science paper with VCSELs based on quantum wells. This is where we have faced the crucial challenge.
Normally, the VCSEL elements are normally "defined" on the wafer by the position of their electrodes, which define the region of gain, and outside that region the wafer has large loss for light at the frequency range of the laser operation. This is the standard technology. When two such VCSELs are brought to close proximity, they tend to lock in phase and they operate in two possible modes: the in-phase mode or the -out-of-phase mode. The latter has a lower threshold for laser operation, as the out-of-phase operation forces zero intensity in the region between the VCSELs, which is the lossy region. As such, this mode is less lossy than the in-phase mode which has non-zero intensity in the region between the emitters, and thus suffers greater loss. This means that the less-lossy mode has a lower threshold for lasing, and will always be the dominant mode. Scaling this up to many emitters, the logic is the same: the highest mode has phase difference between adjacent emitters, and has the lowest threshold, and this is the dominant lasing mode in the array. The problem is that this kind of -out-of-phase mode is NOT topological. In our 2021 Science paper, we were able to avoid this problem by defining the individual VCSELs through chemical etching. Namely, we literally carved pillars surrounded by a medium with a lower refractive index, and under pumping each pillar became a VCSEL. We were able to do that because we could etch quite deeply into the wafer, as the quantum dot medium (which provides the gain for lasing) was mostly in the center of each VCSEL, and etching around it did not damage the quantum dot. It turns out that transferring this idea to quantum wells (instead of quantum dots) is problematic because the quantum wells extend as a uniform layer throughout the wafer, and etching inevitably damages the quantum wells. The solution is to do partial etching, such that the chemical etching is deep enough to define the VCSELs to be "index guided" (rather than "gain-guided") but not too deep to damage the quantum well material.
It took us many iterations to achieve this stage. We achieved this only very recently, towards the end of the PoC. We are now at a stage where we see the signature of the topological arrangement of VCSELs with quantum wells as the gain medium.
