Gao Yi, Lee Hyub, Xu Wen, Jiao Jiannan, Chen Peng, Kim Dong-Hwan, Kim Young-Jin
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore.
School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, Singapore, 637457, Singapore.
Sci Rep. 2019 Mar 25;9(1):5094. doi: 10.1038/s41598-019-41591-6.
Third harmonic generation (THG) is a nonlinear optical process attractive in high-resolution interfacial studies, sub-wavelength light manipulation, and bio-molecular detection due to its capability of converting low-energy quanta into a quantum of a higher energy. One of the limitations in utilizing THG is its low power conversion efficiency; thus, various THG enhancement methods have been researched by involving plasmonic coupling effects or utilizing electric band gap resonances at quantum dots or two-dimensional materials. Meanwhile, lanthanide ion-doped up-conversion nanoparticles (UCNPs) can be excited by a multi-photon process similar to THG, but its interaction or resonance with THG has not been studied to date. In this Communication, we demonstrate the first coherent amplification of third-order harmonic femtosecond pulses at multi-layered UCNP thin-film with an amplification factor of 7.8. This amplification is made by the resonance interaction of incident femtosecond laser field, generated third-order harmonics, and the electric band gaps of UCNPs. The power contribution of the third-order harmonic and the up-conversion luminescence (UCL) is strongly dependent on the sample geometry due to the reabsorption effect. For in-depth understanding of the emission characteristics, spectral-domain, time-domain, radio-frequency (RF) domain, and polarization-dependence analysis were addressed. This coherent amplification of third harmonic (TH) at UCNP thin-films enables us to attain higher power, shorter wavelength, and ultra-short femtosecond pulses generated from a simple thin-film structure near to the target samples, which will pave a way to an ultrafast short-wavelength laser platform for material characterization, sub-wavelength photonics, and biomolecular detection.
三次谐波产生（THG）是一种非线性光学过程，因其能够将低能量量子转换为更高能量的量子，在高分辨率界面研究、亚波长光操纵和生物分子检测方面具有吸引力。利用THG的局限性之一是其低功率转换效率；因此，人们通过涉及等离子体耦合效应或利用量子点或二维材料的电带隙共振来研究各种THG增强方法。同时，镧系离子掺杂的上转换纳米粒子（UCNPs）可以通过类似于THG的多光子过程被激发，但迄今为止尚未研究其与THG的相互作用或共振。在本通讯中，我们展示了在多层UCNP薄膜上对三阶谐波飞秒脉冲的首次相干放大，放大因子为7.8。这种放大是由入射飞秒激光场、产生的三阶谐波与UCNPs的电带隙的共振相互作用实现的。由于重吸收效应，三阶谐波和上转换发光（UCL）的功率贡献强烈依赖于样品几何形状。为了深入了解发射特性，我们进行了光谱域、时域、射频（RF）域和偏振依赖性分析。UCNP薄膜上三阶谐波（TH）的这种相干放大使我们能够从靠近目标样品的简单薄膜结构中获得更高功率、更短波长和超短飞秒脉冲，这将为用于材料表征、亚波长光子学和生物分子检测的超快短波长激光平台铺平道路。
