Improved chip - level color conversion laser can realize the next generation of quantum devices

In two new studies, researchers at the National Institute of Standards and Technology (NIST) have dramatically improved the efficiency and power output of a range of chip-level devices that produce lasers of different colors while using the same input laser source.

Four nano-photon resonators, each with a slightly different geometry, produce a different color of visible light from the same near-infrared pumped laser. Source: NIST

Many quantum technologies, including tiny optical atomic clocks and future quantum computers, will require simultaneous acquisition of a wide range of laser colors in a small region of space. For example, all the steps required for leading quantum computing designs based on atoms require up to six different laser colors, including preparing the atoms, cooling the atoms, reading out their energy states, and performing quantum logic operations.

To create multiple laser colors on a single chip, NIST researcher Kartik Srinivasan and his colleagues have spent the past few years studying nonlinear optical devices, such as those made of silicon nitride, that have a special property: the color of the laser that enters the device can be different from the color that comes out. In their experiment, the incoming light was converted into two different colors -- they corresponded to two different frequencies. For example, a near-infrared laser incident on a material is converted into a visible laser with a shorter wavelength (higher frequency than the light source) and an infrared laser with a longer wavelength (lower frequency).

In previous work, the team demonstrated that this conversion process, known as optical parametric oscillation, can occur in silicon nitride micro-resonators, annular devices small enough to be manufactured on a chip. The light rotated around the ring about 5,000 times, creating a high enough intensity that the silicon nitride was able to convert it to two different frequencies. The two colours are then coupled into a straight rectangular channel, also made of silicon nitride, which sits next to the ring and acts as a transmission line, or waveguide, to carry the light where it is needed.

The specific colors produced are determined by the size of the microresonator and the color of the input laser. The technology offers a wide range of output colors on a single chip, all using the same input laser, because many different sizes of microresonators are created during the manufacturing process.

However, Srinivasan and his colleagues, who include researchers from the Joint Quantum Institute (JQI), a collaboration between the National Institute of Technology and the University of Maryland, found that the process is very inefficient. Less than 0.1% of the input laser is converted to one of the two output colors propagating through the waveguide. The team attributed much of the inefficiency to poor coupling between the ring and the waveguide.

In the first study, Srinivasan and his NIST/JQI collaborators, led by Jordan Stone, redesigned the straight waveguide so that it was U-shaped and wrapped around part of the ring. With this modification, the researchers were able to convert about 15 percent of the incident light into the desired output color, more than 150 times more than in their earlier experiments. In addition, the converted light has more than one milliwatt of power over a wide range of wavelengths, from visible to near infrared.

Generating one milliwatt of power is a milestone, Srinivasan said, because that amount is usually enough for several applications. For example, it can cause a tiny laser to excite electrons to jump from one particular energy level within an atom, or to transition to another. Exciting these transitions is part of a common protocol for producing quantum states of light, such as single-photon states, from single atoms or atom-like systems, such as quantum dots.

In addition, milliwatt power levels can meet the laser stabilization needs. The transition energy of some atoms is very stable and insensitive to environmental effects, thus providing a good reference by which to compare and correct laser frequencies and ultimately improve their noise characteristics.

The researchers report their results in the December 2, 2022 issue of the journal APL Photonics.

In a second study, Srinivasan and his colleagues, led by Edgar Perez, further improved the technology's power output and efficiency. By increasing the coupling between the ring and the waveguide and suppressing effects that could interfere with the color conversion, the team increased the output laser power to 20 milliwatts and converted up to 29 percent of the incident laser to the output color. Although the colors in this study were limited to the near-infrared, the team plans to extend their work to visible wavelengths.

The researchers report their findings in the January 16, 2023, issue of Nature Communications.

Source: Laser Manufacturing Network