Focusing light down to subwavelength scales to enhance the light-matter interaction has been highly sought after, which has promoted significant researches and applications in nanophotonics. Plasmonic nanoantennae are a significant tool to achieve this goal since they can confine light into ultra-small volumes far below the diffraction limit. Second-order nonlinearity on the other hand shows significant importance in photonics. The second-order nonlinearity in small volumes is also particularly desirable. Metal is centrosymmetric; thus, the second-order nonlinearity must vanish in bulk and only exists at surfaces due to broken spatial inversion symmetry. Other ways to achieve SHG of metals is to break the symmetry in plasmonic nanostructures. By leveraging the strong field enhancement, SHG in nanostructures features stability, tunability, coherence, fast response, and polarization sensitivity, which is highly powerful in frequency conversion, sensing, and quantum optics. In despite, the second order nonlinear conversion efficiency is still weak. Boosting SHG yield or nonlinear conversion efficiency on the subwavelength scale is still challenging yet a hot topic for nano-nonlinear optics.

Here, we design plasmonic bowtie nanoantennae on thin-film lithium niobate (TFLN) for deep-subwavelength light confinement to boost the second-harmonic generation (SHG) in TFLN via the plasmonic hotspot enhancement. About 20 times enhancement of SHG intensity is realized under resonant fs laser excitation and the SHG enhancement in metal hotspots is much larger than 20-fold.

Figure 5. Experimental result of SHG enhancement. (a) Spectrum of the FH femtosecond laser. (b) SH spectra from the z-oriented, y-oriented bowtie nanoantenna arrays with P=950 nm and unpatterned TFLN. (c) Quadratic relationship between SH and FH power with different G’s.  P=950 nm. (d) SHG enhancement factor varying with P and G.

In addition, we also studied the effect of the orientation of the plasmon hotspot antenna on the SHG enhancement. About 6 times SHG enhancement is experimentally realized for the y-orient bowtie nanoantenna array under z-polarized FH pump as compared with unpatterned TFLN. The experimental results show that the hotspot enhanced SHG has strong polarization sensitivity due to the polarization dependence of plasmon resonance and second-order nonlinearity.

It is worth noticing that the simulated mode volume is only ~6×10^(-6) (λ/n)^3 and n is the refractive index of LN The achieved nonlinear conversion efficiency (η=P_SHG/P_FF) reaches 1.3×10^(-7). This work proposes a route for subwavelength nonlinear optics on the TFLN platform.

The paper was published in “Zengya Li, Xiaona Ye, Zhuoran Hu, Hao Li, Shijie Liu, Yuanlin Zheng, and Xianfeng Chen, "Plasmonic hotspot arrays boost second harmonic generation in thin-film lithium niobate," Opt. Express 32, 13140-13155 (2024)”.

Link: https://opg.optica.org/oe/fulltext.cfm?uri=oe-32-8-13140&id=548343