A Light-Trapping, Color-Converting Crystal

An illustration of the researchers’ design. The holes in this microscopic slab structure are arranged and resized in order to control and hold two wavelengths of light. (Source: M. Minkov)

Five years ago, Stanford postdoctoral scholar Momchil Minkov encountered a puzzle that he was impatient to solve. At the heart of his field of nonlinear optics are devices that change light from one color to another – a process important for many tech­nologies within tele­communications, computing and laser-based equipment and science. But Minkov wanted a device that also traps both colors of light, a complex feat that could vastly improve the effi­ciency of this light-changing process and he wanted it to be micro­scopic. In order to prove the near-impossible was still possible, Minkov and Shanhui Fan, professor of electrical engineering at Stanford, developed guide­lines for creating a crystal structure with an uncon­ventional two-part form. Now, the team is beginning to build its theorized structure for experimental testing.

Anyone who’s encountered a green laser pointer has seen nonlinear optics in action. Inside that laser pointer, a crystal structure converts laser light from infrared to green. This research aims to enact a similar wave­length-halving conversion but in a much smaller space, which could lead to a large improve­ment in energy efficiency due to complex inter­actions between the light beams. The team’s goal was to force the coexistence of the two laser beams using a photonic crystal cavity, which can focus light in a microscopic volume. However, existing photonic crystal cavities usually only confine one wavelength of light and their structures are highly customized to accom­modate that one wavelength.

So instead of making one uniform structure to do it all, these researchers devised a structure that combines two different ways to confine light, one to hold onto the infrared light and another to hold the green, all still contained within one tiny crystal. “Having different methods for containing each light turned out to be easier than using one mechanism for both fre­quencies and, in some sense, it’s completely different from what people thought they needed to do in order to accomplish this feat,” Fan said.

After ironing out the details of their two-part structure, the researchers produced a list of four conditions, which should guide colleagues in building a photonic crystal cavity capable of holding two very different wave­lengths of light. Their result reads more like a recipe than a schematic because light-mani­pulating structures are useful for so many tasks and technologies that designs for them have to be flexible. “We have a general recipe that says, ‘Tell me what your material is and I’ll tell you the rules you need to follow to get a photonic crystal cavity that’s pretty small and confines light at both frequencies,’” Minkov said.

If telecommu­nications channels were a highway, flipping between different wavelengths of light would equal a quick lane change to avoid a slowdown – and one structure that holds multiple channels means a faster flip. Nonlinear optics is also important for quantum computers because calcu­lations in these computers rely on the creation of entangled particles, which can be formed through the opposite process that occurs in the Fan lab crystal – creating twinned red particles of light from one green particle of light.

Envisioning possible appli­cations of their work helps these researchers choose what they’ll study. But they are also motivated by their desire for a good challenge and the intricate strangeness of their science. “Basically, we work with a slab structure with holes and by arranging these holes, we can control and hold light,” Fan said. “We move and resize these little holes by billionths of a meter and that marks the difference between success and failure. It’s very strange and endlessly fas­cinating.” These researchers will soon be facing off with these intri­cacies in the lab, as they are beginning to build their photonic crystal cavity for experi­mental testing. (Source: Stanford U.)

Reference: M. Minkov et al.: Doubly resonant χ(2) nonlinear photonic crystal cavity based on a bound state in the continuum, Optica 6, 1039 (2019); DOI: 10.1364/OPTICA.6.001039

Link: Fan Lab, Dept. of Electrical Engineering, Stanford University, Stanford, USA

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