Lasers made from titanium sapphire (Ti:sapphire) are considered “unparalleled” in performance and are essential in many fields, including cutting-edge quantum optics, spectroscopy, and neuroscience. But that performance comes at a high price: Ti:sapphire lasers are large, several cubic feet in volume, and expensive, costing hundreds of thousands of dollars each. They also require high-power lasers, costing as much as $30,000 each, to provide enough energy to function.
As a result, Ti:sapphire lasers have not yet seen the widespread practical application they could have hoped for. But now, Stanford researchers have made a breakthrough in scale, efficiency, and cost, building a Ti:sapphire laser on a chip. Their prototype is four orders of magnitude smaller (10,000 times smaller) and three orders of magnitude cheaper (1,000 times cheaper) than any Ti:sapphire laser built to date.
“This is a complete departure from the old model.” Jelena VucicJensen Huang, the Global Leadership Professor and professor of electrical engineering, is senior author of the paper introducing the chip-scale titanium sapphire laser. Published In the journal Nature“Instead of one big, expensive laser, any lab might soon have hundreds of these precious lasers on a single chip, and they could all be powered by a green laser pointer.”
Big Benefits
“Breaking out from tabletop size and making something that can be produced on a chip at low cost opens up the possibility of using these powerful lasers for a variety of important applications,” said Joshua Yang, a doctoral student in the Vukovich lab and co-first author of the study with researchers in the Vukovich lab. Nanoscale and Quantum Photonics Laboratory Colleague, Research Engineer Caspar Van Gasse doctor student Daniil M. Lukin.
Technically speaking, titanium sapphire lasers are so valuable because they have the largest “gain bandwidth” of any laser crystal, Yang explains. In simple terms, gain bandwidth means the range of colors the laser can produce is larger compared to other lasers. They're also superfast, Yang says: A pulse of light is fired every quadrillionth of a second.
But Ti:sapphire lasers are also hard to come by. Even Vuković's lab, where he runs cutting-edge quantum-optics experiments, has only a few of the precious lasers. The new Ti:sapphire laser fits on a chip measuring just a square millimeter. If the researchers could mass-produce it on wafers, they might be able to pack thousands, or even tens of thousands, of Ti:sapphire lasers onto a disk that fits in the palm of a human hand.
“The chip is lightweight, portable, cheap and efficient. It has no moving parts and can be mass-produced,” Yang said. “What's not to like? This democratizes the Ti:sapphire laser.”
manner
To make their new laser, the researchers started by placing a bulk layer of titanium sapphire on a platform of silicon dioxide (SiO2), and then topping it with an all-natural sapphire crystal. They then ground, etched, and polished the Ti:sapphire to create an incredibly thin layer just a few hundred nanometers thick. They then created a swirl of tiny bumps in that thin layer. These bumps are like fiber optic cables that swirl light around, increasing its intensity. In effect, this pattern is known as a waveguide.
Optical image of a Ti:sapphire waveguide amplifier, which is 0.5 mm square. | Yang et al., Nature
“Mathematically speaking, intensity is power divided by area, so if you keep the same power as a larger laser, but reduce the area that the laser is focused on, the intensity skyrockets,” Yang says. “Because our laser is small, it actually becomes more efficient.”
The remaining piece of the puzzle is a microscale heater that warms the light passing through the waveguide, allowing the Vučković team to change the wavelength of the emitted light, tuning the color of the light between 700 and 1,000 nanometers, from red to infrared.
Focus on Applications
Vučković, Yang and colleagues are most excited by the range of fields such lasers could impact. In quantum physics, the new laser offers a cheap, practical solution that could significantly shrink state-of-the-art quantum computers. In neuroscience, the researchers foresee immediate applications in optogenetics, a field that allows scientists to control neurons by directing light with relatively bulky optical fibers in the brain. Small-scale lasers could be integrated into more compact probes, opening up new avenues of experimentation, they say. In ophthalmology, it could find new uses for chirped-pulse amplification in Nobel Prize-winning laser surgery or provide a cheaper, more compact optical coherence tomography technique used to assess retinal health.
The team is next working to perfect chip-scale Ti:sapphire lasers and develop ways to mass-produce them on wafers, thousands at a time. Yang will complete his PhD based on this work this summer and is working to bring the technology to market.
“You can put thousands of lasers on a single four-inch wafer,” Yang says, “and the cost per laser goes to almost zero. That's very exciting.”
