How can lasers be used to cool integrated circuits?

Lasers are often used for heating materials, but they can also be harnessed for localized cooling of ICs. Laser cooling works by using the momentum of photons to slow down atoms. It’s used in cryogenic coolers for applications like quantum computing, and this principle is being adapted for cooling ICs.

Laser cooling is a two-step process. A laser is used to excite atoms within a material, which then emits photons in random directions, carrying away energy and cooling the material.

The laser directly interacts with the atoms to excite them away from their ground state and increases their motion in the direction of the photon’s momentum. Spontaneous emission of a photon occurs due to the interaction of atoms with the vacuum and takes place in approximately 30 ns (Figure 1).

Figure 1. Laser cooling is a two-step process where an atom absorbs a photon and then spontaneously emits a second photon. (Image: University of Alberta)

The expelled photon travels away in a random direction and pushes the atom in the opposite direction. The conservation of momentum dictates that the atom’s motion is slowed, and the atom is slightly cooled. When many of these absorption and emission events occur, the atoms, and therefore the material, can be significantly cooled, even down to cryogenic temperatures.

Doppler and Zeeman must be considered

For laser cooling to be effective, the frequency of the laser wavelength must be precisely matched to the energy differences between atomic states. That requires consideration of the Doppler effect.

The atoms are in motion relative to the laser light source, and the differences in the speed and direction of the motion of individual atoms result in slightly different wavelengths due to the Doppler effect.

The Zeeman effect refers to the splitting of a beam of light into multiple spectral components of slightly different wavelengths when the light is placed in a magnetic field. It can be used to create a Zeeman slower to take maximum advantage of the Doppler effect in laser cooling.

A basic Zeeman slower design utilizes a tapered solenoid that generates a large magnetic field at one end and a smaller field at the opposite end. Uncooled (hot) atoms enter the large field and are hit with a laser beam coming at them from the opposite end.

Traveling into the cooler, the atoms experience the Doppler frequency shift. The atoms absorb photons and are cooled by spontaneous emissions. As they move through the cooler, into increasingly lower field regions, the Zeeman effect is reduced, and so is the Doppler shift. The cooler is constructed so the atoms will remain in resonance as they move from one end to the other, greatly increasing the number of photons absorbed and the cooling effect.

Zeeman coolers form the basis for dilution refrigerators, which are used to create cryogenic environments. But there are other ways to harness laser cooling for ICs, potentially.

Photonic cold plates for cooling ICs

Instead of moving the atoms to be cooled through a Zeeman cooler, photonic cold plates bring the cooling laser light to precise heat sources like the semiconductor junctions on an IC. Called a photonic cold plate, this approach could replace liquid-based cold plates in some applications.

Photonic cold plates are designed with microscopic features measuring approximately 100 nm or smaller, which can channel and direct the cooling laser light. They are expected to provide higher performance cooling compared with water-cooled cold plates.

The photonic cold plate is fabricated using a gallium arsenide (GaAs) epitaxial layer less than a micron thick. A significant challenge is to produce an exceptionally pure GaAs layer. Any impurities would absorb the laser light and generate heat, thereby counteracting the cooling effect. Factication and patterning of ultra-pure GaAs epaxial structures are expected to be a key development for localized cooling of ICs (Figure 2).

Figure 2. Example of a micron-thick GaAs film for a photonic cold plate. (Image: Sandia National Labs)

Summary

Laser cooling is a well-established technique for creating cryogenic environments, which are essential for applications such as quantum computers. New techniques are being developed to enable the fabrication of tiny photonic cold plates that can be used for localized cooling of hot spots in high-performance ICs.