Quasi-Phase-Matched Up- and Down-Conversion in Periodically Poled Layered Semiconductors

Nonlinear optics lies at the heart of classical and quantum light generation. The invention of periodic poling revolutionized nonlinear optics and its commercial applications by enabling robust quasi-phase-matching in crystals such as lithium niobate[1]. However, reaching useful frequency conversion efficiencies requires macroscopic dimensions, effectively limiting on-chip integration with ultracompact footprints.<br/><br/>Here we realize <b>periodically poled transition metal dichalcogenides (PPTMDs)</b>, using 3R-MoS₂. Due to its large nonlinearity, we achieve macroscopic frequency conversion efficiency (0.01%-0.1%) over a microscopic thickness of only 3μm (just three poling periods), <i>i.e.</i>, 10⁸ times more efficient than a monolayer TMD. For comparison, at least 10-100× longer path lengths are needed to achieve similar conversion efficiencies in existing crystals.<br/><br/>Further, we report the generation of <b>entangled photon pairs at telecom wavelengths</b> via quasi-phase-matched spontaneous parametric down-conversion (SPDC) at the key 1550 nm telecom wavelength, reaching a maximum coincidence-to-accidental-ratio (CAR) of 354 ± 32, outperforming any existing van der Waals-based SPDC source.<br/><br/>This work opens the new and unexplored field of <b>phase-matched nonlinear optics with microscopic van der Waals crystals</b>. As on-chip integrable, programmable, microscopic, entangled photon sources, PPTMDs hold potential for new applications that require simple, ultracompact technologies for integrated quantum circuitry and sensing.<br/><br/>[1] Fejer, M. M. et al. “Quasi-phase-matched second harmonic generation: tuning and tolerances”, IEEE Journal of Quantum Electronics, 28, 11 (1992)<br/>[2] &lt;!--[endif]--&gt;C. Trovatello et al. “Quasi-phase-matched up- and down-conversion in periodically poled layered semiconductors” https://arxiv.org/abs/2312.05444 (2023)