Whispering gallery modes with fractional optical angular momentum in photonic crystal micro-rings

Whispering gallery modes, a type of wave that can travel around concave surfaces, have shown promise for the development of new technologies, particularly in photonics. Due to geometrical limitations, in circularly symmetric optical microresonators (i.e., micrometer-scale structures that can confine light), these modes exhibit integer quantized angular momentum values. Although many effects take advantage of these modes, there are applications where a non-integer angular momentum may be desired.

Researchers from the National Institute of Standards and Technology and the University of Maryland recently developed a single photonic crystal micro-ring that enables whispering gallery modes with fractional optical angular momentum. This ring-shaped structure, introduced into a Physical examination letters (LRP), could open exciting possibilities for creating sensors, measurement tools, nonlinear optical devices, and other technologies.

“Our recent LRP paper builds on our previous work in Nature Photonicswhere we introduced the structure of a ‘microgear’ photonic crystal ring,” Xiyuan Lu, one of the researchers who conducted the study, told Phys.org. “In this new work, we demonstrate the moment half-integer orbital kinetic moment of light, compared to the even numbers achieved by the photonic crystal ring we studied earlier, which gives an integer orbital angular momentum of light similar to that of conventional micro-rings, called “gallery modes whispering”.

In their recent work, Lu and his colleagues set out to investigate the new capabilities of the photonic crystal micro-rings presented in their previous paper. The team also wanted to explore how introducing multiple defects to their resonator would affect its localization capabilities and spatial control of light.

“The meaning of ‘half-integer angular momentum’ is that the light has to make two round trips around the resonator to return to its initial phase (modulo 2*pi), which is different from conventional micro-rings, where it only only takes one round trip,” Kartik Srinivasan, another researcher involved in the study, told Phys.org. “This is why our resonator has sometimes been compared to a Mobius strip.”

To achieve half-integer angular momentum, the team simply designed their ring using an odd number of periodic cells in its circumference, instead of choosing an even number of cells. This allowed them to access the other half of the parameter space accessible by previously designed photonic crystal rings.

“Achieving the multiple faults of our device was also far from complicated, as we simply introduced multiple fault modulation on our ring,” Lu said. “Through this unique design, we could achieve two features, the half-integer angular momentum or the localization of multiple defects, in photonics on silicon, with a manufacturing method that can be upgraded.”

During the first tests, the ring-shaped micro-resonator developed by this research team obtained very promising results, exhibiting high Q and good coupling. Additionally, the device can be integrated into nonlinear photonics, quantum photonics, and biosensing applications as easily as whisper gallery modes in conventional micro-rings are integrated.

Designing high-Q photonic crystals, including geometries that incorporate defects to strongly localize light, is typically challenging and time-consuming, as it involves running multiple simulations and performing various optimization steps. In contrast, the micro-ring created by Lu and his colleagues has a very simple and straightforward design, which does not need to be perfected by simulations and optimizations.

“We saw that we could slow down light and localize it strongly in a small fraction of the ring (this was the subject of our previous article), and now we have shown that we can create half angular momentum states -integer and multiple strongly localized defect states in the same ring, and we can use these same defects to control the orientation of slow-light whisper gallery modes,” Srinivasan said.

“Although not all effects are available simultaneously, in some cases we can combine effects or apply an effect for one mode and another effect for another mode, and most importantly, we know how to design and control effects.”

The two recent papers by Lu, Srinivasan and colleagues show that introducing unique photonic crystal patterns into micro-ring resonators can profoundly alter the characteristics of electromagnetic fields. In the future, their resonator design and the resulting control of electromagnetic fields could help solve many research problems involving light-matter interactions, for example by allowing the realization of multiple quantum nodes in a micro-ring. or the nonlinear generation of light.

“An important point for me is that this work advances in a single step in how to implement multiple quantum nodes in a micro-ring, and in particular in raising an issue that weak defect coupling can be non-negligible and should be taken care of,” Lu said. “This weak coupling is a bit surprising to me.”

In their next studies, the researchers plan to test the interest of their design in controlling electromagnetic fields interacting with matter. More specifically, they plan to apply it to the development of nonlinear optical technologies and quantum optics consisting of atoms or quantum dots.

“We are interested in investigating the origin of the weak coupling, in order to better control it down the road,” Lu said. and quantum dots in these devices.”

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