On-Chip Laser Light Generates Spectrum of Colors

National Institute of Standards and Technology (NIST) and University of Maryland researchers demonstrated the ability to convert invisible near-infrared laser light into multiple visible colors, using newly introduced microchip technology. The approach in generating red, yellow, and green laser light, among other colors, on integrated microchips supports the implementation of technology necessary to ensure precision timekeeping and conduct quantum information science.

These areas of science and technology often rely on atomic and/or solid-state systems powered by visible laser light delivered at precise and specific wavelengths.

A team led by NIST’s Xiyuan Lu used a process called third-order parametric oscillation (OPO) to convert incident light in the near infrared into two different frequencies. OPO requires a nonlinear optical material to perform the light conversion; one of the resulting frequencies is higher than that of the incident light, meaning it exists in the visible range, and the other is lower in frequency, extending deeper into the infrared.

Though OPO effectively produces particular, different colors of light in large instruments, the NIST-led research successfully applied the effects to produce visible light wavelengths on a microchip, which is a device capable of mass production and application.

The researchers miniaturized their OPO method by directing near-infrared light into a microresonator that was fabricated on a silicon chip. Once inside the resonator, the light circulated 5000 times before dissipating, generating an intensity high enough to access the nonlinear range, in which the light is converted into two different output frequencies.

In testing the approach, the team fabricated dozens of nanophotonic microresonators, each with unique geometries, onto each microchip. The strategy, which intended to generate a wide range of colors, ultimately enabled a single near-infrared laser to itself generate a wide range of specific visible light and infrared colors.

The input pump laser used in testing varied in wavelength, though only by a small amount, the researchers said. Though the laser operated over a narrow range of infrared wavelengths, the visible light colors ranged from 560 to 760 nm (green to red). The infrared wavelengths ranged from 800 to 1200 nm.

Series of nanophotonic resonators, each slightly different in geometry, generates different colors of visible light from the same near-infrared pump laser. Courtesy of NIST.

By adjusting the dimensions of the microresonators, the method is capable of accessing any of the wavelengths. In addition to its small-scale construction, as opposed to a system requiring tabletop lasers, the new method additionally circumvents the need for different semiconductor materials.

Beyond quantum communication and computing applications, as the work showed the possible integration of lasers with optical circuits, the research has value in medicine and defense-specific cases, as well as in industrial manufacturing. Work is currently underway on developing highly compact atomic optical systems technologies that are operational at low power, giving them functionality outside laboratory settings. Where compact, high-performance lasers in the near infrared are used in telecommunications, realizing similar or equivalent performance at visible wavelengths has challenged scientists.

While an approach to overcome that barrier has used multiple semiconductor materials to make lasers, the NIST and University of Maryland team focused on silicon nitride. The material responds nonlinearly to light, making it amenable to the research team’s deployment of OPO. If the intensity of incoming light is high enough, the color of light already in the system will not necessarily match the color of the entering light, due to the interacting electrons re-radiating that light in colors that differ from the incident light.

The effect contrasts light bouncing off a mirror and refracted through a lens; the color of the light remains the same in such an instance.

“Though a first demonstration, we are excited at the possibility of combining this nonlinear optics technique with well-established near-infrared laser technology to create new types of on-chip light sources that can be used in a variety of applications,” Lu said.

The work is part of NIST’s “NIST on a Chip” program, aimed at miniaturizing NIST’s existing measurement technology and encouraging its direct distribution to users in industry, medicine, defense, and academia.

The research was published in Optica (www.doi.org/10.1364/OPTICA.393810).