A team of researchers led by Jelena Vučković, a professor of electrical engineering at Stanford University, has pioneered the integration of a titanium gemstone (Ti:sapphire) laser on a chip (which can be pumped with a green laser pointer). Compared to any other titanium gemstone laser currently available, this prototype is four orders of magnitude smaller (i.e., one ten-thousandth of the original) and three orders of magnitude lower in cost (i.e., one thousandth of the original).
Titanium gemstone lasers, due to their high gain bandwidth and ultrafast pulse output, are indispensable in fields such as cutting-edge quantum optics, spectroscopy and neuroscience. However, their large size and high price (hundreds of thousands of dollars each), as well as the need for high-powered devices (each selling for about $30,000) to pump them, have limited their widespread use.
"At Stanford's Laboratory for Nano and Quantum Photonics, we have conducted several quantum experiments based on solid-state spin quantum bits in materials such as diamond and silicon carbide. This experiment relies heavily on commercial titanium gemstone lasers." Joshua Yang, a PhD student in Vučković's team, explains.
In addition to being expensive, titanium gemstone lasers are complex and often require regular maintenance to keep them running well.Prof. Vučković's research team conducts a large number of experiments for which the titanium gemstone lasers do not have enough machine time, and therefore has to share the equipment and manage the experimental schedule. In addition, because the power required for the experiments is much lower than the output power of commercial titanium gemstone lasers, the laser output can only be attenuated by a few orders of magnitude, resulting in a large portion of the laser power being wasted.
Yang said, "Chip-scale titanium sapphire lasers, due to their low cost, compactness, and stability, can replace the commercial titanium gemstone laser systems currently employed for our precision experiments."
Figure 1: The chip-scale titanium gemstone laser developed by Prof. Jelena Vučković's research team. The laser rests diagonally against a titanium gemstone, both resting on a quarter.
Clever laser design
The chip-scale laser developed by the team consists of two main parts: a waveguide and a ring resonator.
A layer of titanium gemstones is placed on a silicon dioxide (SiO2) substrate, which is then placed on a sapphire crystal. The titanium gemstone layer is ground, etched and polished to a thickness of just a few hundred nanometers. It was then patterned with a waveguide, which acts as a vortex of tiny ridges that guide light as it travels through it.
A miniature heater is used to heat the waveguide, which changes the refractive index of the waveguide and the speed at which light travels through the waveguide, so that the output wavelength can be adjusted over a range of wavelengths, from red to infrared (currently adjustable to 60 nm).
"The spiral-shaped waveguide is equivalent to an amplifier for the laser, and the power increases as the laser passes through." Yang explains, "The ring resonator acts both as a filter to modulate the wavelength of the laser via microheaters, and as a resonant cavity for the laser - acting as a recirculation path for the laser transmission."
Figure 2: Optical image of a titanium gemstone waveguide amplifier measuring 0.5mm x 0.5mm.
Challenges for chip-scale titanium gemstone lasers
The biggest difficulty with titanium gemstone lasers is that they require high-intensity pumping to operate. By realizing titanium gemstone laser technology through a high-precision waveguide, the research team accomplished two important breakthroughs:
First, since pumping intensity is power divided by area, using titanium gemstone optical waveguides significantly reduces the pumping area. "This means that only less power (about 1000 times less) is needed to achieve a pumping intensity similar to that of commercial titanium gemstone systems." Yang explains, "So even a cheap green light semiconductor laser is powerful enough to pump this chip-scale laser."
Second, the titanium gemstone laser is integrated onto the chip. "The chip-scale sapphire laser (without more moving parts) has miniaturization, scalability and durability unmatched by commercial lasers for large-scale wafer-level semiconductor manufacturing." Yang added.
For Yang, the highlight of this work is the use of this chip-scale titanium gemstone laser for quantum experiments. He said, "It was a great surprise to see this tiny device replacing a bulky commercial laser system in a complex cavity quantum electrodynamics (QED) experiment. Because the chip-scale lasers we have developed are truly exceptional."
One of the challenges that Vučković's research team had to overcome to make the chip-scale titanium gemstone laser truly usable for quantum experiments was optimizing the coupling of the pump source. "For experiments, the laser is pumped through a free-space light path," Yang says, "but with photonic packaging technology, it is possible to integrate a green light semiconductor laser that serves as the pump source for the chip-scale titanium gemstone laser. By optimizing the coupling of the packaging system and the pump source, higher power laser output can be achieved, and the laser is both portable and durable."
Potential applications
Chip-scale titanium gemstone lasers have a wide range of applications, from quantum technologies such as quantum computing and atomic clocks to medical applications such as optical coherence tomography and two-photon microscopy.
"Hopefully this technology will mature and be used in these fields in the next few years," Yang said. After graduating this summer, he will work for Brightlight Photonics, a company that will work to facilitate the commercialization of chip-scale titanium gemstone lasers.
Currently, Jelena Vučković and her research team are working on a tunable mode-locked chip-scale titanium laser.
Pulsed lasers "will open up new opportunities for laser applications in quantum technology, classical information processing and biomedicine," Prof. Vučković said.
