Steering and Encoding the Polarization of the Second Harmonic in the Visible with a Monolithic LiNbO₃ Metasurface

Abstract

Nonlinear metasurfaces constitute a key asset in meta-optics, given their ability to scale down nonlinear optics to sub-micrometer thicknesses. To date, nonlinear metasurfaces have been mainly realized using narrow band gap semiconductors, with operation limited to the near-infrared range. Nonlinear meta-optics in the visible range can be realized using transparent materials with high refractive index, such as lithium niobate (LiNbO 3 ). Yet, efficient operation in this strategic spectral window has been so far prevented by the nanofabrication challenges associated with LiNbO 3 , which considerably limit the aspect ratio and minimum size of the nanostructures (i.e., meta-atoms). Here we demonstrate the first monolithic nonlinear periodic metasurface based on LiNbO 3 and operating in the visible range. Realized through ion beam milling, our metasurface features a second-harmonic (SH) conversion efficiency of 2.40 × 10 –8 at a pump intensity as low as 0.5 GW/cm 2 . By tuning the pump polarization, we demonstrate efficient steering and polarization encoding into narrow SH diffraction orders, opening novel opportunities for polarization-encoded nonlinear meta-optics.

Tunable unidirectional nonlinear emission from transition-metal-dichalcogenide metasurfaces

“… References Nonlinear media Refractive index ( n ) Extinction coefficient ( k ) SHG THG SH directionality Periodicity (nm) Fundamental wavelength (nm) (700–800 nm) Refs. 55 , 63 , 64 LiNbO 3 2.2 0 ✓ ✓ 55 ✗ 63 , 64 ✗ 590 900 63 590 64 815 1550 63 830 64 Refs. 16 , 17 Si 3.7–3.6 0.01 ✗ ✓ ✗ 750 14 1280 15 1350 14 2320 15 Refs.…”

Spontaneous Parametric Down-Conversion from Resonant Metasurfaces

“…Several recent works already demonstrated efficient SHG with LN nanoparticles 28 and metasurfaces. [29][30][31][32] LN is especially attractive for quantum nonlinear optics due to its broad transparency range and relatively low fluorescence compared to semiconductors like GaAs or GaP. In this work we take advantage of its largest nonlinear tensor component χ…”

Infrared full-Stokes polarimetry by parametric up-conversion

“…25,26 Thus it is applicable to vibrational spectroscopy for birefringent molecule identification, and thermal imaging for night vision detection. Furthermore, utilizing the current state-of-theart scheme of nonlinear enhancement by nanostructures such as metasurfaces [27][28][29][30] or intracavity up-conversion configuration, 31,32 the efficiency of the SFG process from the LN film could be furthermore effectively improved. We could foresee a direct impact of our results on a variety of IR polarimetric applications, such as optical crystallography, chemical sensing, disease diagnosis, and explosive detection, etc.…”