Researchers Demystify Contradiction of Supermode Microcavity Lasers

Stimulated scattering in supermode microcavities, such as Raman or Brillouin lasers, has proven useful for precision measurements by exploiting the beat note in their lasing spectra. The existence of the beat note (which corresponds to the energy splitting of supermodes) presents a question that researchers have wrestled with for decades: Are supermode microcavity lasers single- or dual-mode?

In pursuit of an answer, a team from Peking University has revealed the lasing dynamics of a stimulated scattering laser in a supermode microcavity. Experimentally, the team demonstrated the supermode microcavity laser’s single-mode nature.

Diagram showing generation and manipulation of supermode Raman laser in a microcavity. Courtesy of Peking University.

The dilemma to this point stems from a contradiction between the laser theory and experimental observations. In theory, because of the homogenous gain, the pump field should always be clamped after the laser emits, meaning the laser is single-mode. However, the widely observed beat notes suggest that the lasers are dual-mode.

“This beating phenomenon really contradicts what we have learned from the textbook, but it does exist,” said postdoctoral researcher Cao Qitao of Peking University.

The researchers used surface scattering to construct a pair of supermodes in a whispering-gallery microcavity and generated an ultralow-threshold Raman laser. They then adopted an add-drop coupling structure that enabled them to directly acquire the intracavity pump power, by which the clamping effect of the pump field was observed for the first time. With the help of a heterodyne method (combination of multiple frequencies to produce a single, unique frequency), the researchers observed the side-mode suppression ratio (SMSR) to be over 30 dB, so that the single-mode characteristic of the Raman laser in supermode microcavity is unambiguously demonstrated.

“To reveal the underlying physics of the previously observed beating phenomenon, we utilized the self-injection method to modulate the mode losses of the two supermodes,” said Zhang Peiji, a Ph.D. student at Peking University.

The team introduced self-injection by applying a weak reflectance on the output laser. Part of the output laser is then injected back into the cavity to interfere with the intracavity laser field. With that method in play, the previously observed periodic beating phenomenon (which suggests dual-mode) emerged in the time domain. Further theoretical analysis revealed that the widely reported beat note arises from the transient interference during the switching process between the supermode lasers, rather than the simultaneous lasing of the two supermodes.

In terms of applications, the self-injection method could contribute to the selective generation of near-degenerate lasers and the improvement of their SMSRs.

In addition to revealing insights on the lasing spectrum paradox, the researchers said, the work supports improvements to microlaser-based precision measurements and information that could be beneficial to the development of reconfigurable light sources and optical memory devices.

The research was published in PNAS (www.doi.org/10.1073/pnas.2101605118).