Fine-tuning the color of light with a new optical device

Putting new capabilities in engineers’ hands.

Light is composite of many photons whose energy specifies a color of the rainbow—red, orange, yellow, green, blue, indigo, violet.

Now, using an optical device, Stanford scientists were able to change and fine-tune the frequencies of each photon in a stream of light to virtually any mixture of colors they want. This experiment established a new photonic architecture that could revolutionize various fields, including digital communications and artificial intelligence to cutting-edge quantum computing.

This new optical device puts a degree of control in the engineer’s hands not previously possible.

The structure of this new device includes a low-loss wire to carry a stream of photons. The photons then enter a series of rings, where each ring has a modulator that transforms the frequency of the passing photons. This frequency is what we see as color.

As scientists noted, there could be multiple rings, and engineers can finely control the modulators to dial in the desired frequency transformation.

Avik Dutt, a post-doctoral scholar in Fan’s lab and second author of the paper, said, “Our device is a significant departure from existing methods with a small footprint and offers tremendous new engineering flexibility.”

As mentioned above, the frequency of the photon indicates the color of the photon. Its frequency is also a factor of its wavelength. A red photon has a relatively slow frequency and a wavelength of about 650 nanometers, whereas a blue photon has a much faster frequency with a wavelength of about 450 nanometers.

The fine-tuning of color involves shifting a photon from a frequency of 500 nanometers to, say, 510 nanometers. The new device is powerful enough to perform these simple transformations and much more sophisticated ones with fine control.

Using this new device, an engineer could fine-tune that ratio to 73 percent at 500 nanometers and 27 percent at 510 nanometers, if so desired, all while preserving the total number of photons. This ability to set the ratio is what makes this device new and promising.

Siddharth Buddhiraju, who was a graduate student in Fan’s lab during the research and is the first author of the paper and who now works at Facebook Reality Labs, said, “We say this device allows for ‘arbitrary’ transformation, but that does not mean ‘random.’ Instead, we suggest that we can achieve any linear transformation that the engineer requires. There is a great amount of engineering control here.”

Shanhui Fan, a professor of electrical engineering at Stanford and senior author of the paper, said, “It’s very versatile. The engineer can control the frequencies and proportions very accurately, and a wide variety of transformations are possible. It puts new power in the engineer’s hands. How they will use it is up to them.”

Journal Reference:
  1. Buddhiraju, S., Dutt, A., Minkov, M. et al. Arbitrary linear transformations for photons in the frequency synthetic dimension. Nat Commun 12, 2401 (2021). DOI: 10.1038/s41467-021-22670-7

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