Chip-scale laser beams light nearly straight up from lithium niobate microdisk

A research team in China has demonstrated an on-chip light source that emits coherent light almost vertically out of the surface of a photonic chip, reaching a wavelength near 817 nanometers that conventional semiconductor emitters struggle to cover. The work, published in the peer-reviewed journal Opto-Electronic Technology, combines a thin-film lithium niobate microdisk resonator with a carefully engineered nonlinear optical process, offering a new route to integrated light sources whose emission colors are not dictated by the gain bandwidth of the underlying semiconductor material. The achievement is significant because vertically emitting sources are among the most practical building blocks for photonic integrated circuits, yet their spectral reach has long been constrained by material limitations.

Vertically emitted integrated light sources radiate from the chip surface at near-normal angles, and that geometry carries real engineering advantages. Because the light leaves the chip perpendicular to its plane, such devices can be packed into dense two-dimensional arrays, their output can be collected with simple free-space optics, and they can serve as convenient bridges between on-chip platforms and external optical systems. These attributes make them attractive for optical communications, optical displays, precision metrology, integrated quantum light sources, and terahertz photonics. Conventional devices of this type, such as vertical-cavity surface-emitting lasers, however, are tied to the gain bandwidths of their constituent materials, which limits the wavelengths they can produce.

The spectral gap that motivated the new work sits near 810 nanometers, a region where vertically emitted integrated light sources remain scarce despite strong demand. Precision atomic, molecular, and optical physics experiments, the quantum technologies that build on them, and laser surgery all rely on light in this part of the spectrum. Filling such gaps is where nonlinear optical frequency converters come in: by mixing existing laser light in a nonlinear crystal, new wavelengths can be generated that no single semiconductor gain medium could reach. The difficulty has been that integrating both a frequency converter and a vertically emitted source on the same chip poses substantial challenges, since the nonlinear process must be phase matched while the output must still escape the chip surface.

To overcome this, the team, led by Professor Ya Cheng of East China Normal University in collaboration with Professor Jintian Lin of the Shanghai Institute of Optics and Fine Mechanics and Associate Professor Jielei Ni of Shenzhen University, 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 inside the resonator between the pump light and a counterpropagating stimulated Raman lasing signal excited by that same pump, the researchers realized a coherent nonlinear light source with near-vertical emission at 817 nanometers.

The device at the heart of the experiment is an X-cut thin-film lithium niobate microdisk resonator with a diameter of approximately 66 micrometers. Lithium niobate is prized in integrated photonics for its strong second-order nonlinearity and wide transparency window, making it a natural choice for frequency conversion on chip. To shape the resonator’s mode structure, the researchers introduced a weak perturbation by placing an optical tapered fiber in contact with the periphery of the microdisk. This contact broke the rotational symmetry of the original whispering-gallery modes, which normally circulate around the disk rim, and induced the coherent recombination of a quasi-degenerate whispering-gallery-mode sequence into high-Q nearly-degenerate square modes.

Those square modes do two jobs at once. First, they significantly enhance the Rayleigh backscattering of the forward-propagating pump light, which is essential for building up a counterpropagating wave inside the resonator. Second, they provide favorable spatial overlap between the optical fields involved in the nonlinear interaction, strengthening the sum-frequency generation process. The high quality factors of the modes, meaning the photons circulate many times before leaking out, establish the strong intracavity field enhancement on which efficient nonlinear conversion depends. This combination of mode engineering and counterpropagating geometry forms the technical foundation of the demonstration.

The experimental sequence began with continuous-wave pump light at 1555.42 nanometers coupled into the microdisk. Once the pump power exceeded the threshold for stimulated Raman scattering, pronounced stimulated Raman lasing signals centered at 1721.26 nanometers appeared in both the forward and backward propagation directions. The threshold powers were remarkably low, measured at 0.439 milliwatts in the forward direction and 0.427 milliwatts in the backward direction, with slope efficiencies of 32.3 percent and 21.1 percent respectively. The two split resonances of the pump mode exhibited loaded quality factors of 4.48 million and 4.39 million, values that enable the substantial intracavity field enhancement exploited in the subsequent conversion step.

As the pump power was increased further, several nonlinear optical signals emerged in the short-wavelength region of the spectrum. Among them, the signal at 817.06 nanometers was generated through sum-frequency generation between the pump light and the counterpropagating stimulated Raman laser. Crucially, this signal was emitted from the top surface of the microdisk at nearly normal angles, with a conversion efficiency of 0.272 percent per milliwatt of pump power. The emission angle deviated from the normal direction by only approximately plus or minus 6.7 degrees, confirming that the transverse phase matching scheme in the counterpropagating configuration delivers on its promise of near-vertical output from the chip surface.

The significance of the result lies in how it decouples emission wavelength from material gain. Instead of searching for a semiconductor that lases at 810 nanometers, the researchers used a single telecom-band pump laser and the intrinsic nonlinearity of lithium niobate to synthesize the desired wavelength on the same chip. Because the frequency conversion process is governed by phase matching and mode design rather than by a gain bandwidth, the same architecture could in principle be retuned to other target wavelengths by adjusting the resonator geometry and pump frequency. The authors describe the work as establishing a new technical route for extending the spectral coverage of vertically emitted integrated light sources, and as providing a pathway toward integrated coherent light sources, quantum light sources, and free-space optical interfaces.

The demonstration also underscores the growing maturity of thin-film lithium niobate photonics as a platform where high-Q resonators, stimulated Raman scattering, and second-order nonlinear mixing coexist on a single chip smaller than the width of a human hair. For applications in atomic and molecular physics, where lasers near 810 nanometers are workhorses, a chip-scale source that emits nearly vertically could simplify experimental setups and enable dense arrays of independently addressable emitters. For quantum technologies, on-chip nonlinear sources with free-space output offer a direct interface between flying photonic qubits and integrated circuits. While the reported conversion efficiency and output power remain modest compared with bulk laboratory systems, the low Raman thresholds, quality factors in the millions, and near-normal emission angle together mark a convincing proof of principle that nonlinear frequency conversion and vertical emission can be unified on a photonic chip.

Subject of Research: On-chip vertically emitted nonlinear light sources in thin-film lithium niobate microdisk resonators

Article Title: Researchers develop on-chip light source that emits light nearly vertically

Article References: Researchers develop on-chip light source that emits light nearly vertically. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: thin-film lithium niobate, microdisk resonator, stimulated Raman scattering, sum-frequency generation, phase matching, nonlinear optics, integrated photonics, vertical emission, whispering-gallery modes, on-chip light source, quantum photonics, near-infrared laser