Tunable second harmonic generation by an all-dielectric diffractive metasurface embedded in liquid crystals

Abstract

We experimentally demonstrate the possibility to modulate the second harmonic power emitted by nonlinear AlGaAs metasurfaces embedded in a liquid crystal (LC) matrix. This result is obtained by changing the relative in-plane orientation between the LC director and the linear polarization of the light at the excitation wavelength. According to numerical simulations, second-harmonic is efficiently radiated by the metasurfaces thanks to the sizeable second-order susceptibility of the material and the resonant excitation of either electric or magnetic dipole field distributions inside each meta-atom at the illuminating fundamental wavelength. This resonant behavior strongly depends on the geometric parameters, the crystallographic orientation, and the anisotropy of the metasurface, which can be optimized to modulate the emitted second harmonic power by about one order of magnitude. The devised hybrid platforms are therefore appealing in view of enabling the electrical control of flat nonlinear optical devices.

Manipulating Light with Tunable Nanoantennas and Metasurfaces

“…Let us consider the metasurface depicted in Figure 4(a) [69]. The structure is a periodic array of nanodiscs of height equal to 200 nm.…”

“…The latter is a crucial aspect for obtaining a metasurface that acts as a SH modulator. This promising consideration motivated the fabrication of such a device, as described in [69]. Briefly, the structure is fabricated by molecular-beam epitaxy on a non-intentionally doped GaAs wafer, with 200 nm layer of Al 0.18 Ga 0.82 As on the top of an aluminum-rich substrate, which is later oxidized.…”

“…Metasurfaces with varying cylinder radius in the range 260-315 nm and with period fixed at 910 nm have been obtained, as already reported in Figure 4. The metasurfaces are then embedded in a 12 μm-thick matrix of E7 LC following the procedure reported in [69].…”

“…Importantly, for a metasurface with radius r = 285 nm the modulation of the SH power reaches the high value of about one order of magnitude, as highlighted by the dashed black line in Figure 7. In [69] the authors reported another structure where the LC director is always parallel to the (110) crystallographic axis. However, for the latter metasurface no significant SH modulation is expected and we will not further discuss this case here.…”

Tailoring the resonant modes in liquid crystal based all-dielectric metasurfaces

“…A variety of tuning mechanisms are being employed to tailor the responses of dielectric metasurfaces such as phase transitions in phase change materials (PCM's), opto-electronic and thermal tuning in liquid crystals, mechanical tuning using MEMS and microfluidic devices, etc [13][14][15][16][17][18][19][20] . Liquid crystals is a versatile medium that can support tunable, multi-functional and programmable electromagnetic response via thermal, optical, electrical and magnetic tuning 13,16,[21][22][23][24][25][26] . Liquid crystal (LC) is a versatile medium with high birefringence and high dielectric anisotropy.…”