Scientists perfect miniaturized technique to generate precise wavelengths of visible laser light

In research, sometimes the bumpy road is the best path. By creating tiny periodic bumps in a miniature race track for light, researchers at the National Institute of Standards and Technology (NIST) and their colleagues at the Joint Quantum Institute (JQI), a research partnership between the University of Maryland and NIST, have converted near-infrared (NIR) laser light into specific desired wavelengths of visible light with high precision and efficiency.

The technique has potential applications in precision timekeeping and quantum information science, which require very specific wavelengths of visible laser light that cannot always be obtained with diode lasers (devices similar to laser lights). LED) to control atomic or semiconductor systems.

Ideally, the wavelengths should be generated in a compact device, such as a photonic chip, so that quantum sensors and optical atomic clocks can be deployed outside the laboratory, no longer tethered to optical equipment cumbersome.

In previous experiments, NIST researcher Kartik Srinivasan and his colleagues used perfectly smooth microresonators (ring-shaped devices with a diameter about a quarter of the thickness of a human hair) to transform a single wavelength of NIR light into two other wavelengths.

The resonator, small enough to fit on a microchip, can be designed so that one of the two output wavelengths lies in the visible light spectrum. The transformation occurs when NIR laser light, confined thousands of times around the ring-shaped resonator, reaches intensities high enough to interact strongly with the resonator material.

In theory, by choosing a particular radius, width and height of the resonator, which determine the properties of the light that can resonate in the ring, researchers can select one from a rainbow of possible colors with this technical. In practice, however, the method, known as optical parametric oscillation (OPO), is not always accurate. Even deviations as small as a few nanometers (billionths of a meter) from the specified dimensions of the microring produce visible light colors that differ significantly from the desired output wavelength.

As a result, the researchers had to make up to 100 silicon nitride microrings to be sure that at least some would have the right dimensions to generate the target wavelength. But even this painstaking measure does not guarantee success.

Now, Srinivasan and his collaborators, led by Jordan Stone of JQI, have demonstrated that by introducing imperfections (tiny periodic ripples or bumps) along the surface of a microresonator, they can select a specific output wavelength of visible light with an accuracy of 99.7. %. With improvements, Stone said, the technique should produce visible light wavelengths accurate to more than 99.9 percent of their target values, a necessary condition for powering optical atomic clocks and other high-precision devices.

The researchers describe their work in Natural photonics.

“In our previous experiments, we achieved the general range of an interesting wavelength, but for many applications this is not sufficient. You really need to define the wavelength with a high degree of precision.” , Stone said. “We now achieve this precision by incorporating a periodic arrangement of ripples on a microring resonator.”

The principle that governs the optical transformation of a single wavelength input into two outputs of different wavelengths is the law of conservation of energy: the energy carried by two of the input photons of the near infrared laser must be equal to the energy carried by the output. photons: one with a shorter wavelength (higher energy) and one with a longer wavelength (lower energy). In this case, the shortest wavelength is visible light.

Additionally, each of the input and output wavelengths must correspond to one of the resonance wavelengths allowed by the dimensions of the microring, just as the length of a tuning fork determines the specific note at which it resonates.

In their new study, the researchers designed a microring whose dimensions, without ripples, would not have allowed photons to resonate in the ring and produce new wavelengths because the process would not have retained energy.

However, when the team sculpted the ring with tiny periodic ripples, thereby changing its dimensions, they allowed the OPO to proceed, transforming the NIR laser light into one specific wavelength of visible light plus another much longer wavelength. These OPO-generated colors, unlike those previously created by smooth micro-rings, can be precisely controlled by the spacing and width of the bumps.

The ripples act like tiny mirrors that collectively reflect visible light flowing around the ring, but only for a particular wavelength. The reflections give rise to two identical waves traveling around the ring in opposite directions. Inside the ring, counter-propagating waves interfere with each other to create a pattern known as a standing wave, a waveform whose peaks remain fixed at a particular point in the ring. space when the wave vibrates, like a plucked guitar string.

This results in a shift towards a longer or shorter wavelength, depending on whether the standing wave interacts more with the peaks or troughs of the ripples. In both cases, the magnitude of the shift is determined by the height of the bump. Since the bumps only act as a mirror for a specific wavelength of light, the approach ensures that when OPO occurs, the generated signal wave has exactly the desired wavelength.

By slightly changing the wavelength of the infrared laser that drives the OPO process, any imperfections in the ripples can be compensated for, Stone said.