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. They achieved broadband second-harmonic generation with a 3-dB bandwidth of 13 nm in a micro-racetrack resonator.
Study: Chip-scale nonlinear bandwidth enhancement via birefringent mode hybridization. Image Credit: narong sutinkham/Shutterstock.com
The same approach worked for ultrashort pulse frequency conversion in a bent waveguide. This research is expected to benefit on-chip tunable frequency conversion and quantum light source generation on photonic platforms.
Related Work
Past work has 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. Lithium niobate, particularly thin-film lithium niobate (TFLN), has been recognized for enhancing second-harmonic generation (SHG) by tightly confining optical fields and enabling group velocity tailoring. While periodically poled lithium niobate (PPLN) has achieved broadband SHG, quasi-group-velocity matching (QGVM) has also been explored for broadband nonlinear frequency conversion. However, chip-scale integration remains a challenge.
Enhanced Nonlinear Frequency Conversion
The proposed structure in this work features a microracetrack resonator on X-cut TFLN for SHG. The straight section of the waveguide aligns with the Y axis, and the SHG process is influenced by the azimuthal angle of propagation, where the effective nonlinear coefficient oscillates between values depending on the wave vector direction.
Phase mismatch between the fundamental wave (FW) and SH in different resonator sections is calculated, and the second-harmonic intensity grows continuously in the racetrack resonator under certain phase-matching conditions, like quasi-phase matching (QPM) techniques.
The work introduces a relaxed phase-matching condition called spontaneous quasi-phase matching (SQPM), where slight shifts in phase mismatch can be compensated across different sections of the racetrack resonator. This condition allows for broadband nonlinear frequency conversion but with lower conversion efficiency than conventional QPM. The conversion efficiency in SQPM largely depends on the structure size, offering more flexibility in designing racetrack resonators for on-chip integration.
The structure's broadband SQPM condition is derived from the geometry of the racetrack resonator and its dispersion properties. A key challenge is achieving the necessary phase-matching conditions in an isotropic platform, where the group-velocity and phase-velocity mismatches are difficult to manipulate. However, the racetrack resonator's birefringent nature introduces mode hybridization in the half-circle waveguide, providing additional control over modal dispersion, which is crucial for efficient SHG.
Simulations show that in the half-circle waveguide, the transverse electric mode 0 (TE0) mode of the FW transitions into higher-order modes, such as transverse magnetic mode 1 (TM1) and TE1, as it propagates, significantly altering the dispersion characteristics. The variation in refractive indices between the straight and curved waveguides causes a shift in dispersion, which can be used to compensate for temporal walk-off between the FW and second harmonic (SH) pulses. It allows the pulses to realign as they enter the next straight waveguide section, ensuring efficient overlap for SHG.
Broadband SQPM SHG Demonstration
The team designed a dual-resonant racetrack resonator to validate the broadband SQPM SHG experimentally. This resonator featured a straight waveguide and a half-circle radius, which resulted in specific phase mismatches at the FW wavelength. The fabrication process primarily determined the geometric parameters, which could be enhanced through improved machining or deep etching techniques to reduce bending loss. The dispersion properties of the structure were analyzed, revealing that the phase mismatch varied with the FW wavelength.
The analysts calculated the summation of phase mismatch, discovering that it reached an extremum while satisfying the QPM condition at a particular wavelength. They highlighted that slight adjustments in temperature or geometric parameters could simultaneously realize the SQPM and quasi-group velocity mismatch (QGVM) conditions. The perfect SQPM condition yielded a theoretical maximum SH intensity. In contrast, the broadband SQPM condition reduced overall SH intensity without significantly affecting conversion efficiency across the wavelength range.
The experimental setup involved fabricating and characterizing the perfect and phase-compensated SQPM racetrack resonators. A pulley-type bus waveguide facilitated on-chip coupling characterized by specific dimensions. The setup included grating couplers to input and output light from the chip.
The transmission spectrum of the FW light was recorded, showing a typical mode with a high intrinsic Q factor. The measured SH intensities from both resonators revealed a substantial SHG spectrum with a significant bandwidth under phase-compensated SQPM, greatly 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 high SQPM order used, with the potential for improved performance by exploring lower-order SQPM designs.
Conclusion
To sum up, the researchers demonstrated a novel approach for achieving QGVM SHG in a racetrack resonator and bent waveguide on X-cut TFLN. They leveraged birefringence-induced mode transitions of the SH light, allowing the group velocity mismatch to change its sign during propagation flexibly.
The study significantly enhanced SHG bandwidths, applicable to various parametric processes, including sum-frequency generation and optical parametric oscillation. This work promised substantial advancements in chip-scale nonlinear frequency conversion, particularly between ultrashort optical pulses and quantum states, contingent on minimizing transmission loss through improved 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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