Lithium niobate microdisk emits near-vertical light at 817 nm

Researchers at East China Normal University, Shanghai Institute of Optics and Fine Mechanics, and Shenzhen University demonstrated an on-chip nonlinear light source that emits nearly vertically at 817.06 nm. The result could help extend integrated photonics into wavelengths needed for atomic physics, quantum technologies, and other applications where vertically emitted sources remain scarce.

Why it matters: - Vertically emitted on-chip light sources make it easier to collect signals and connect chip-based photonics with free-space optics. - Near-810 nm emission is especially important for precision atomic, molecular and optical physics, quantum technologies, and laser surgery. - Conventional vertical-cavity surface-emitting lasers are usually limited by material gain bandwidths, which narrows their wavelength coverage. - The new approach offers a way to expand vertically emitted integrated light sources into spectral regions that have been difficult to reach.

What happened: - A research team led by Professor Ya Cheng, with collaborators at Shanghai Institute of Optics and Fine Mechanics and Shenzhen University, published the study in Opto-Electronic Technology. - The team built an X-cut thin-film lithium niobate microdisk resonator with a diameter of about 66 μm. - The device produced a coherent nonlinear light source that emitted near-vertically at 817.06 nm. - The light came from sum-frequency generation between the pump field and a counterpropagating stimulated Raman laser signal. - The source was driven under continuous-wave excitation from a single pump laser.

The details: - The researchers used a transverse phase-matching mechanism in a counterpropagating configuration. - An optical tapered fiber in contact with the microdisk edge introduced a weak perturbation that broke the original rotational symmetry of the whispering-gallery modes. - That perturbation created coherent recombination of a quasi-degenerate whispering-gallery-mode sequence and formed high-Q nearly-degenerate square modes. - The nearly-degenerate square modes strengthened Rayleigh backscattering of the forward-propagating pump light. - The same modes improved spatial overlap among the interacting optical fields, which boosted the nonlinear interaction. - Continuous-wave pump light at 1555.42 nm was coupled into the microdisk. - When the pump power exceeded the stimulated Raman scattering threshold, stimulated Raman lasing appeared at 1721.26 nm in both forward and backward directions. - The stimulated Raman thresholds were 0.439 mW and 0.427 mW. - The corresponding slope efficiencies were 32.3% and 21.1%. - The two split pump resonances had loaded Q factors of 4.48×10^6 and 4.39×10^6. - As pump power increased, multiple short-wavelength nonlinear signals appeared. - The 817.06 nm signal was emitted from the top surface of the microdisk at nearly normal angles. - The measured emission angle was about ±6.7° from normal. - The conversion efficiency was 0.272%/mW. - The work points to high-Q nearly-degenerate square modes plus counterpropagating phase matching as a route to efficient on-chip nonlinear light sources.

Between the lines: - The key advance is not just the wavelength, but the combination of near-vertical emission and efficient nonlinear conversion on a chip. - Using a single pump laser to generate both stimulated Raman lasing and sum-frequency output simplifies the architecture. - The design suggests a practical path for integrated coherent light sources and free-space optical interfaces, rather than only conventional in-plane photonic circuits.

What's next: - The approach could be adapted to other wavelength targets where vertically emitted integrated sources are scarce. - The same platform may support future integrated coherent light sources and quantum light sources. - The design may also be useful for chip-to-free-space coupling in photonic systems that need compact, high-density emitters.

The bottom line: - A thin-film lithium niobate microdisk has demonstrated efficient, nearly vertical nonlinear emission at 817.06 nm, opening a new route for chip-based light sources beyond the limits of standard vertical emitters.

More information: the journal website