Researchers at the Joint Quantum Institute (JQI) have designed and tested photonic chips that convert one color of light into a rainbow of additional colors — a long-coveted breakthrough that could hold the key to building quantum computers and enabling precision measurements of frequency or time. Additionally, the developed chips all work without any active inputs or painstaking optimization — a major improvement over previous methods, according to the researchers. In addition to holding broad implications for integrated photonics-enabled applications such as nonlinear optical computing, metrology, and frequency conversion, the researchers’ framework avoids the hassle of active tuning or precise engineering to satisfy frequency-phase matching conditions. The achievement builds on the group’s previous advancements in nonlinear photonics. Ordinarily, the interactions between light and a photonic device are linear, which means the light can be bent or absorbed but its frequency won’t change, as in a prism. By contrast, nonlinear interactions occur when light is concentrated so intensely that it alters the behavior of the device, which in turn alters the light. This feedback can generate an array of different frequencies, which can be collected from the output of the chip and used for measurement, synchronization, or a variety of other tasks. Researchers at JQI have designed and tested new chips that reliably convert one color of light (represented by the orange pulse, lower left) into many colors. Courtesy of JQI/Mahmoud Jalali Mehrabad. Unfortunately, nonlinear interactions are usually very weak. Today, multiple methods to strengthen these interactions exist; one widely used approach involves meticulously engineered chips tailored with photonic resonators. Still, tradeoffs remain in when trying to produce a particular set of new frequencies using a single resonator. “If you want to simultaneously have second harmonic generation, third harmonic generation, fourth harmonic—it gets harder and harder,” says Mahmoud Jalali Mehrabad, the lead author of the current paper. “You usually compensate, or you sacrifice one of them to get good third harmonic generation, but cannot get second harmonic generation, or vice versa.” In an effort to avoid some of these tradeoffs, members of the research team previously pioneered ways of boosting nonlinear effects by using a hoard of tiny resonators that all work in concert. They showed in earlier work, for example, how a chip with hundreds of microscopic rings arranged into an array of resonators can amplify nonlinear effects and guide light around its edge. And last year, they showed that a chip patterned with such a grid could transmute a pulsed laser into a nested frequency comb. However, it took many iterations to design chips with the right shape to generate the precise frequency comb they were after, and only some of their chips actually worked. Now, the researchers and their colleagues discovered that the array of resonators used in previous work already increases the chances of satisfying so-called “frequency-phase matching conditions” in a passive way — that is, without the use of any active compensation or numerous rounds of design. Instead of trying to engineer the precise frequencies they wanted to create and iterating the design of the chip in hopes of getting one that worked, they considered whether the array of resonators produced any stable nonlinear effects across all the chips. When they checked, they were surprised to find that their chips would generate second, third, and even fourth harmonics for incoming light with a frequency of around 190 THz. This is a standard frequency used in telecommunications and fiber optic communications. The researchers ultimately realized that the reason all their chips worked was related to the structure of their resonator array. Light circulated quickly around the small rings in the array, which set a fast timescale. But there was also a “super-ring” formed by all the smaller rings, and light circulated around it more slowly. Having these two timescales in the chip had an important effect on the frequency-phase matching conditions that the researchers hadn’t appreciated before. Instead of having to rely on meticulous design and active compensation to arrange for a particular frequency-phase matching condition, the two timescales provide researchers with multiple shots at nurturing the necessary interactions. In other words, the two timescales essentially provide the frequency-phase matching for free. The researchers tested six chips manufactured on the same wafer by sending in laser light with the standard 190-THz frequency, imaging a chip from above and analyzing the frequencies leaving an output port. They found that each chip was indeed generating the second, third and fourth harmonics, which for their input laser happened to be red, green, and blue light. They also tested three single-ring devices. Even with the inclusion of embedded heaters to provide active compensation, they only saw second harmonic generation from one device over a narrow range of heater temperature and input frequency. By contrast, the two-timescale resonator arrays had no active compensation and worked over a relatively broad range of input frequencies. The researchers even showed that as they dialed up the intensity of their input light, the chips started to produce more frequencies around each of the harmonics, reminiscent of the nested frequency comb created in an earlier result. The research was funded by the Air Force Office of Scientific Research, the Army Research Office, the National Science Foundation and the Office of Naval Research. The research was published in Science (www.doi.org/10.1126/science.adu6368).
