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Metasurfaces hold promise to miniaturize many bulk (~mm) optical elements and concurrently enable the implementation of multiple optical functionalities in individual integrated optical devices. Nonlinear optics is a fast-rising area of application, albeit the sub-wavelength thickness of metasurfaces means that their nonlinear conversion efficiency is often very low. In this framework, lithium niobate (LN) is a ferroelectric material widely employed for optoelectronics and nonlinear optics thanks to its high refractive index, broad transparency window, and large electro-optical response and second-order nonlinearity. Over the last few years, much effort has thus been directed towards the realization of LN nanoparticles and nanostructures [1,2].
Here, we report highly directional second-harmonic (SH) generation of visible light by a monolithic LN metasurface [3]. The metasurface was milled on a commercially available z-cut LiNbO3 substrate via dual-beam focused ion beam lithography. Using an 80 MHz train of 140 fs laser pulses with 500 MW/cm2 peak intensity as a pump, we measured SH conversion efficiencies (SH power /exciting power) up to 2.5E-8. The numerically-assisted design of the metasurface [4] exploits the resonant modes of the single pillar: a magnetic dipole at the exciting wavelength of 820 nm, along with an electric dipole and a magnetic quadrupole at the SH wavelength. The interplay between the single-pillar emission and the grating mode of the metasurface brings about an efficient (~20 dB) rerouting of the SH emission from the zeroth into the first diffraction orders, which exhibit a unconventional polarization behavior. The studied system exemplifies a design concept of NL diffractive metasurface and is suitable for applications such as NL holography with polarization encoding.
[1] A. Fedotova, et al., Nano Letters 20, 8608–8614 (2020)
[2] T. Santiago-Cruz, et al. arXiv 2103.08524 (2021)
[3] L. Carletti, et al. ACS Photonics 8, 731–737 (2021)
[4] L. Carletti, et al. Optics Express 27, 33391–33398 (2019)
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