NIST improves chip-scale color conversion lasers to enable new quantum devices

29 Mar 2023

Efficiency and power output at two wavelengths boosted while still using the same input.

Researchers at the U.S. National Institute of Standards and Technology (NIST) say they have “greatly improved the efficiency and power output of a series of chip-scale devices that generate laser light at different wavelengths – while all using the same input laser source.”

Many quantum technologies, including miniature optical atomic clocks and future quantum computers, will require simultaneous access to multiple, widely varying laser wavelengths within a small region of space.

For example, up to six different laser wavelengths are needed for all the steps required for a leading atom-based design for quantum computation, including preparing the atoms, cooling them, reading out their energy states, and performing quantum logic operations.

To create multiple wavelengths on one chip, NIST researcher Kartik Srinivasan and his colleagues have spent the last few years studying nonlinear optical devices, such as those made of silicon nitride, which have a special property: The wavelength of laser light entering the device can differ from that which exits.

In their experiment, incoming light is converted into two different wavelengths. For instance, near-infrared laser light incident on the material is converted into shorter-wavelength visible laser light and longer-wavelength infrared laser light (at a lower frequency).

In previous work, the team demonstrated that this conversion process, known as optical parametric oscillation, can occur within a silicon nitride microresonator, a ring-shaped device small enough to be fabricated on a chip.

The light races around the ring some 5,000 times, building a high enough intensity for the silicon nitride to convert it into the two different frequencies. The two wavelengths are then coupled into a straight rectangular channel, also made of silicon nitride, that lies adjacent to the ring and acts as a transmission line, or waveguide, transporting the light where it is needed.

Wide range of wavelengths

The specific wavelengths generated are determined by the dimensions of the microresonator as well as the wavelength of the input laser light. Because many different microresonators with slightly different dimensions are created during the fabrication process, the technique provides access to a wide range of outputs on a single chip, all using the same input laser.

However, Srinivasan and his colleagues, which include researchers from the Joint Quantum Institute (JQI), a collaboration between NIST and the University of Maryland, found that the process was highly inefficient. Much less than 0.1 percent of the input laser light was converted into either of the two output wavelengths traveling in the waveguide. The team traced most of the inefficiency to poor coupling between the ring and the waveguide.

In the first study (published in Nature Communications), Srinivasan and his NIST/JQI collaborators, led by Jordan Stone, redesigned the straight waveguide so that it was U-shaped and wrapped around a portion of the ring. With this modification, the researchers were able to convert about 15 percent of the incoming light to the desired outputs, more than 150 times the amount in their earlier experiment.

In addition, the converted light possessed more than one milliwatt of power over a wide range of wavelengths, from the visible to the near-infrared. Generating a milliwatt of power is a milestone, said Srinivasan, because that amount is usually enough for several applications. For example, it can enable a tiny laser to excite electrons to jump, or transition, from one specific energy level to another inside an atom.

In addition, milliwatt power levels can be sufficient for laser stabilization. Some atoms have transition energies that are very stable and insensitive to environmental effects, and as a result, provide a good reference by which a laser frequency can be compared and corrected, ultimately improving its noise properties.

In the second study (published in APL Photonics), Srinivasan and his colleagues, led by Edgar Perez, improved the power output and efficiency of the technique even further. By increasing the coupling between the ring and the waveguide and suppressing effects that may interfere with the color conversion, the team increased the output laser power to as high as 20 milliwatts and converted as much as 29 percent of the incident laser light to the output wavelength.