Ultrabroadband Nonlinear Optics in Nanophotonic Periodically Poled Lithium Niobate Waveguides (1909.08806v1)

Abstract: Quasi-phasematched interactions in waveguides with quadratic nonlinearities enable highly efficient nonlinear frequency conversion. In this article, we demonstrate the first generation of devices that combine the dispersion-engineering available in nanophotonic waveguides with quasi-phasematched nonlinear interactions available in periodically poled lithium niobate (PPLN). This combination enables quasi-static interactions of femtosecond pulses, reducing the pulse energy requirements by several orders of magnitude, from picojoules to femtojoules. We experimentally demonstrate two effects associated with second harmonic generation. First, we observe efficient quasi-phasematched second harmonic generation with <100 fJ of pulse energy. Second, in the limit of strong phase-mismatch, we observe spectral broadening of both harmonics with as little as 2-pJ of pulse energy. These results lay a foundation for a new class of nonlinear devices, in which co-engineering of dispersion with quasi-phasematching enables efficient nonlinear optics at the femtojoule level.

Ultrabroadband Nonlinear Optics in Nanophotonic Periodically Poled Lithium Niobate Waveguides

This paper presents a detailed examination of ultrabroadband nonlinear optics facilitated by nanophotonic periodically poled lithium niobate (PPLN) waveguides. The authors delineate the combination of dispersion engineering inherent to nanophotonic waveguides with quasi-phasematched nonlinear interactions characteristic of PPLN. This amalgamation remarkably reduces pulse energy requirements from picojoules to femtojoules and opens the door to a new class of highly efficient nonlinear photonic devices.

Summary of Key Results

The paper reports two main experimental observations concerning second harmonic generation (SHG). First, the authors achieved efficient quasi-phasematched SHG using pulse energies as low as 100 femtojoules. Secondly, under strong phase mismatch conditions, they noted spectral broadening effects in both harmonics with pulse energies as minimal as 2 picojoules. These findings suggest the feasibility of ultrabroadband quasi-phasematched interactions in dispersion-engineered waveguides.

The SHG bandwidth achieved (>250 nm) represents a substantial enhancement over traditional methods available for waveguides of the same length, highlighting the effective coupling of quasi-phasematching and pulse shaping through dispersion management. The conversion efficiency surpassed 50% with a pulse energy of only 60 femtojoules.

Moreover, the paper illuminates the process of supercontinuum generation (SCG) in phase-mismatched PPLN waveguides. It demonstrates that these devices, when driven with pulse energies surpassing 1 picojoule, can produce multi-octave supercontinua. The enhanced effective nonlinearity, estimated at n<sub\>2</sub> = 4.8 x 10^-17 m<sup\>2</sup>/W, is found to outperform the conventional χ⁽³⁾ response, offering profound implications for future developments in nonlinear optical applications.

Implications and Future Developments

The implications of this research are manifold, both from a theoretical and practical perspective. The introduction of dispersion-engineered quasi-phasematched χ⁽²⁾ interactions in nanophotonic waveguides allows for the realization of highly efficient nonlinear optical processes at substantially reduced power levels. This capability is paramount for the advancement of integrated photonic circuits and compact, energy-efficient optical devices.

The experimental results presented underline the potential for miniaturizing nonlinear optical components while improving performance metrics dramatically in terms of bandwidth and energy efficiency. Such enhancements cater not only to conventional nonlinear optics but also to future quantum optics applications, where managing pulse energy and coherence provide significant practical benefits.

Future studies could explore the coherence properties of the supercontinuum generated, particularly exploring its suitability for frequency comb stabilization and precise spectroscopic applications. Additionally, there is scope for further investigating the fabrication techniques and geometrical parameter optimizations to extend the applicability of nanophotonic PPLN waveguides to various optical wavelengths and regimes, potentially paving the way for new breakthroughs in ultrafast optics and photonic systems design.