Titanium:sapphire lasers are valued because they have the largest tunable wavelength range of any gain crystal and still are able to be precisely tuned. In the Nanoscale and Quantum Photonics Lab at Stanford University, led by Jelena Vučković, researchers are using Ti:sapphire lasers to excite artificial atoms in solid-state quantum optics experiments. The tabletop lasers, however, emit much more power than the researchers require. Like many researchers using Ti:sapphire lasers, the lab members attenuate the laser, often wasting some 99% of the output energy.
Unlike a commercial Ti:sapphire laser, the ideal Ti:sapphire laser for the lab’s solid-state quantum optics research would output only the power needed. It would also be easier to integrate with a variety of photonic structures. So Vučković’s group decided to create its own on-chip laser. Vučković is no stranger to creating miniature technology: Her lab had previously collaborated on the construction of a particle accelerator on a chip.
The lab’s primary development was learning to work around the properties of sapphire to allow for integrated photonics. Researchers can’t just grow a thin layer of high-quality crystalline sapphire on top of an insulator as they can with other materials. Instead, a bulk layer of titanium-doped sapphire needs to be ground down using a combination of chemical and mechanical processes. Sapphire is also chemically resistant, which means it is hard to etch the surface—a step needed for the miniaturization of the laser technology. The researchers developed a chromium mask that functions like a stencil over the Ti:sapphire and is sturdy enough to not erode after being subjected to the intense etching process.
The new laser has a footprint of only 0.15 mm2—the waveguide is etched onto the surface of a sapphire rod that makes up the lasing substrate—and can be incorporated directly into a laser-on-a-chip system. It operates as a continuous-wave laser, and the lower output power compared with the tabletop version is a better fit for the experiments in the Nanoscale and Quantum Photonics Lab. In addition to being much smaller than Ti:sapphire lasers currently in use, it requires only a basic green laser diode pump to operate. That lowers the cost of the pump laser by three orders of magnitude, and the researchers hope that price decrease will democratize the use of the laser.
Although the Vučković group researchers have already incorporated their new laser into their quantum optics work with artificial atoms, they still have plans for improvement. They are working to increase the coupling efficiency between the laser diode pump and the Ti:sapphire chip from the current 16% to reduce energy loss. They are also working on developing a pulsed laser. (J. Yang et al., Nature 630, 853, 2024.)
