In a paper published in the journal Advanced Photonics, researchers demonstrated broadband nonlinear frequency conversion using mode-hybridization on X-cut thin-film lithium niobate. This technique enabled broadband second-harmonic generation (SHG) with a 3-dB bandwidth of 13 nm within a micro-racetrack resonator.
Study: Chip-scale nonlinear bandwidth enhancement via birefringent mode hybridization. Image Credit: narong sutinkham/Shutterstock.com
The same approach also proved effective for ultrashort pulse frequency conversion in a bent waveguide, with potential applications in on-chip tunable frequency conversion and quantum light source generation on photonic platforms.
Related Research
Past efforts have focused on achieving efficient second-order nonlinear processes with tunable pump bandwidth for applications like wavelength division multiplexing, ultrashort pulse nonlinearity, and quantum key distribution.
Thin-film lithium niobate (TFLN) has gained recognition for enhancing SHG by tightly confining optical fields and enabling precise control of group velocity. While periodically poled lithium niobate (PPLN) has shown promise for broadband SHG, quasi-group-velocity matching (QGVM) is also being explored to advance broadband nonlinear frequency conversion. However, integrating these processes at the chip scale remains a challenge.
Enhanced Nonlinear Frequency Conversion
In this study, a microracetrack resonator on X-cut thin-film lithium niobate (TFLN) was investigated for its potential in second-harmonic generation (SHG). The waveguide's straight section was aligned along the Y-axis, and the SHG process was found to be influenced by the azimuthal angle of propagation, causing oscillations in the effective nonlinear coefficient based on the wave vector's direction.
Phase mismatches between the fundamental wave (FW) and the second harmonic (SH) were calculated across different sections of the resonator. The study demonstrated that under certain phase-matching conditions, such as quasi-phase matching (QPM), second-harmonic intensity could increase continuously within the racetrack resonator.
A relaxed phase-matching condition, referred to as spontaneous quasi-phase matching (SQPM), was introduced in this work. This approach allowed minor phase mismatches to be compensated across different parts of the racetrack, enabling broadband nonlinear frequency conversion, though at a lower efficiency compared to conventional QPM. The conversion efficiency in SQPM was found to depend largely on the structure's size, offering increased flexibility for designing racetrack resonators suitable for on-chip integration.
The study attributed the broadband SQPM condition to the geometry and dispersion properties of the racetrack resonator. One of the key challenges identified was achieving the necessary phase-matching conditions in an isotropic platform, where group-velocity and phase-velocity mismatches are difficult to manipulate. However, the racetrack resonator’s birefringent nature introduced mode hybridization in the curved sections, which provided additional control over modal dispersion—an important factor for efficient SHG.
Simulations revealed that within the half-circle waveguide, the fundamental wave’s transverse electric mode 0 (TE0) transitioned into higher-order modes, such as transverse magnetic mode 1 (TM1) and TE1, altering the dispersion characteristics. The variation in refractive indices between straight and curved waveguides caused a shift in dispersion, which compensated for the temporal walk-off between FW and SH pulses. This enabled the pulses to realign as they entered the next straight section, ensuring efficient pulse overlap and enhancing SHG performance.
Broadband SQPM SHG Demonstration
The research team designed a dual-resonant racetrack resonator to experimentally validate broadband SQPM for SHG. This resonator featured both a straight waveguide and a half-circle radius, leading to specific phase mismatches at the FW wavelength. The fabrication process determined the geometric parameters, which could potentially be optimized through improved machining or deep etching techniques to minimize bending loss. The dispersion properties of the structure were analyzed, revealing that the phase mismatch varied with the FW wavelength.
The analysis of phase mismatch summation revealed that it reached an extremum when the QPM condition was met at a particular wavelength. The team noted that slight adjustments in temperature or geometric parameters could simultaneously satisfy both the SQPM and quasi-group velocity mismatch (QGVM) conditions. The perfect SQPM condition was shown to yield a theoretical maximum SH intensity, while the broadband SQPM condition reduced the overall SH intensity without significantly compromising conversion efficiency across the wavelength range.
The experimental setup involved fabricating and characterizing both perfect and phase-compensated SQPM racetrack resonators. A pulley-type bus waveguide facilitated on-chip coupling, with specific dimensions tailored to the setup. Grating couplers were used to input and output light from the chip.
The transmission spectrum of the FW light was recorded, revealing a characteristic mode with a high intrinsic Q factor. Measurements of SH intensities from both resonators demonstrated a substantial SHG spectrum with a broad bandwidth under phase-compensated SQPM, which was significantly enhanced compared to the perfect SQPM resonator.
The highest on-chip SH power was achieved at a specific FW wavelength under a defined on-chip pump power. The lower conversion efficiency was attributed to the use of a high SQPM order, suggesting that exploring lower-order SQPM designs could further improve performance.
Conclusion
In summary, the researchers demonstrated a novel approach for achieving QGVM in SHG using a racetrack resonator and bent waveguide on X-cut TFLN. By leveraging birefringence-induced mode transitions of the second-harmonic light, they enabled the group velocity mismatch to flexibly change its sign during propagation.
This study significantly broadened the SHG bandwidths, with applications extending to various parametric processes such as sum-frequency generation and optical parametric oscillation. The findings hold promise for advancing chip-scale nonlinear frequency conversion, especially for interactions between ultrashort optical pulses and quantum states. Future progress in this area depends on minimizing transmission loss through enhanced fabrication technologies.
Journal Reference
Yuan, T., et al. (2024). Chip-scale nonlinear bandwidth enhancement via birefringent mode hybridization. Advanced Photonics, 6:5. DOI: 10.1117/1. ap.6.5.056012, http://dx.doi.org/10.1117/1.AP.6.5.056012
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