In contrast to the huge success story of electronics integration, photonic integration is still in its infancy. One of the most serious obstacles it faces is the need to use different materials to achieve different functions, unlike electronic integration. More complicated, many of the materials needed for photonic integration are not compatible with silicon integration technologies. So far, placing various functional nanowires in photonic circuits to achieve the desired function has shown that while it is entirely possible that nanowires are often too small to effectively limit the light. Although larger nanowires can increase light confinement and performance, they increase energy consumption and device size, both of which are considered "fatal" when designing integrated devices. In response to this question, a group of researchers at Japan's NTT Corp. proposed a method that includes combining subwavelength nanowires with a photonic crystal platform. They reported this week in the Journal of Applied Physics. The artificial structure of a photonic crystal with a periodically modulated index of refraction is at the heart of its work. "A small fraction of the refractive index modulated photonic crystal creates a strong optical confinement that creates an ultrahigh-quality optical nanocrystal resonator," said Masaya Notomi, a brilliant scientist at NTT's Basic Research Laboratory. "We took full advantage of this feature in the work we are doing." As early as 2014, a study by the same research group had shown that one subwavelength light can be strongly limited using nanowires of 100 nm diameter placed in silicon photonic crystals. At the time, "This is a preliminary demonstration of the constraint mechanism, but with our current work, we have successfully demonstrated the operation of subwavelength nanowire devices on silicon platforms, also through the use of this method," Notomi said. In other words: When a subwavelength nanowire can not be a resonator with a strong limit of light itself, it places the photonic crystal in the refractive index modulation conditions required to create a light confinement. "In our work, we have carefully prepared Group III-V semiconductor nanowires with large enough optical gain and placed them in a silicon photonic crystal with a groove structure. Using nanometer probe technology, we have realized an optical nano-resonance "Said Masato Takiguchi, lead author of the paper and working with other researchers at NTT's Basic Research Laboratory. "Through a series of careful portrayals, we have demonstrated that this sub-wavelength nanowire can achieve CW lasing and high-speed signal modulation at 10 Gbps." The use of nanowire lasers to achieve photon integration must meet three basic requirements. First of all, the nanowires should be as small as possible to achieve light constraints, ensuring a very small size and energy consumption, Takiguchi said. Second, nanowire lasers must be capable of producing high-speed signals. Third, the laser wavelength should be greater than 1.2 microns and should not be absorbed by the silicon, so it is important to create subwavelength nanowire lasers that can modulate high-speed signals in the optical communications bands of 1.3 and 1.55 microns. " In fact, the previously studied nanowire lasers all operate at pulsed lasers that are less than 0.9 microns in wavelength and can not be used with silicon photonic integrated circuits, except for the relatively thicker micron-line lasers that were at 1.55 microns, Notomi said. This is probably because the material gain is smaller at longer wavelengths, which makes it very difficult to achieve lasing on thin nanowires. In addition, "zero-speed demonstration of high-speed modulation of any type of nanowire has been achieved," he said. This is also due to the small gain volume. "In our current work, we solve these problems by combining nanowires and silicon photonic crystals," Notomi said. "Our result is the first implementation of CW lasing subwavelength nanowires and the first implementation of nanowire lasers for high speed signal modulation." The team was able to achieve 10 Gbps modulation, which is comparable to traditional, direct-modulated, high-speed lasers used for optical communications. "This proves that nanowire lasers have shown promise for information processing, especially for photonic integrated circuits," Notomi said. The most promising application of this group's current work is the nanowire-based photonic integrated circuits, which will use different nanowires for different functions such as lasers, photodetectors and switching in silicon photonic integrated circuits. "It is expected that on-chip photonic network processors will be implemented in about 15 years and nanowires based on photonic integration will be a viable solution," Notomi said. On the laser side, the team's next goal is to integrate nanowires into laser input / output waveguides. "While this integration is a daunting task for nanowire-based devices, we hope it will be easier to utilize the platform we studied, because the waveguide-connected photonic crystal platform is inherently superior," said Takiguchi. "Our goal is to drive current lasers at room temperature." The team also plans to use the same technology to create "photonic devices other than lasers, by choosing different nanowires," said Takiguchi. "We have to prove that we can integrate some photonic devices with different features on the same single chip."