Announcing a new publication from Opto-Electronic Technology; DOI 10.29026/oet.2026.260011 .
On-chip light source are essential building blocks for photonic integrated circuits. Among them, vertically emitted integrated light sources which radiate from the chip surface at near-normal angles offer distinct advantages, including high-density array integration, ease of signal collection, and convenient interfacing between free-space optical systems and on-chip platforms. These attributes make them highly attractive for applications in optical communications, optical displays, precision metrology, integrated quantum light sources, and terahertz photonics. However, the emission wavelengths of conventional semiconductor light sources, such as vertical-cavity surface-emitting lasers, are typically constrained by the gain bandwidths of their constituent materials, leading to limited spectral coverage. In particular, vertically emitted integrated light sources remain scarce near 810 nm, despite strong demand in precision atomic, molecular, and optical (AMO) physics, related quantum technologies, and laser surgery.
Nonlinear optical frequency converters offer a promising pathway to extend the spectral coverage of vertically emitted light sources. Nevertheless, integrating both a frequency converter and a vertically emitted light source on the same chip poses substantial challenges. To address this issue, a research team introduced a transverse phase matching nonlinear interaction in a counterpropagating configuration. Under continuous-wave excitation from a single pump laser, nearly-degenerate square modes were excited in a thin-film lithium niobate microdisk resonator. Through sum-frequency generation in the resonator between the pump light and the counterpropagating stimulated Raman lasing signal excited by the pump light, a coherent nonlinear light source with near-vertical emission at 817 nm was successfully realized. This work establishes a new technical route for extending the spectral coverage of vertically emitted integrated light sources.
Professor Ya Cheng's group at East China Normal University, in collaboration with Professor Jintian Lin from Shanghai Institute of Optics and Fine Mechanics, and Associate Professor Jielei Ni at Shenzhen University, published a research article entitled "Nearly vertically emitted nonlinear light sources with high efficiencies enabled by high-Q nearly-degenerate polygon modes" in Opto-Electronic Technology.
Motivated by the growing demand for on-chip, nearly vertically emitted nonlinear light sources, the research team employed a transverse phase matching mechanism in a counterpropagating configuration and exploited high-Q nearly-degenerate square modes to enhance the spatial overlap between the optical fields involved in the nonlinear interactions. This strategy ultimately enabled the realization of an on-chip, nearly vertically emitted coherent nonlinear light source in the near-infrared region. The researchers fabricated an X-cut thin-film lithium niobate microdisk resonator with a diameter of approximately 66 μm. Weak perturbation was introduced into the resonator by placing an optical tapered fiber in contact with the periphery of the microdisk, which broke the rotational symmetry of the original whispering-gallery modes and induced the coherent recombination of a quasi-degenerate whispering-gallery-mode sequence, thereby forming high-Q nearly-degenerate square modes. These modes not only significantly enhanced the Rayleigh backscattering of the forward-propagating pump light, but also provided favorable spatial overlap between the interacting optical fields, thereby strengthening the nonlinear interactions and establishing the foundation for efficient sum-frequency generation (SFG) between the pump light and the counterpropagating stimulated Raman laser.
When continuous-wave pump light at 1555.42 nm was coupled into the microdisk and the pump power exceeded the stimulated Raman scattering threshold, pronounced stimulated Raman lasing (SRL) signals centered at 1721.26 nm were observed in both the forward and backward propagation directions. The corresponding threshold powers were as low as 0.439 mW and 0.427 mW, respectively, while the slope efficiencies reached 32.3% and 21.1% (Figure 1). The two split resonances of the pump mode exhibited loaded Q factors of 4.48×106 and 4.39×106 , respectively, enabling substantial intracavity field enhancement. As the pump power was further increased, several nonlinear optical signals emerged in the short-wavelength region (Figure 2). Among them, the signal at 817.06 nm was generated through SFG between the pump light and the counterpropagating stimulated Raman laser. This signal was emitted from the top surface of the microdisk at nearly normal angles, with a conversion efficiency of 0.272%/mW. The emission angle was approximately ±6.7° deviated from the normal direction. These results demonstrate that the combination of high-Q nearly-degenerate square modes and a counterpropagating transverse phase matching configuration provides an effective approach for realizing on-chip, nearly vertically emitted nonlinear light sources. This work provides a new pathway toward integrated coherent light sources, quantum light sources, and free-space optical interfaces.
Keywords: thin-film lithium niobate, stimulated Raman scattering, sum-frequency generation, phase matching, optical microresonator
