Generating Vectorial Optical Fields via Surface-Wave-Excited Complex-Amplitude Metasurfaces

On-chip photonic systems capable of efficiently generating pre-designed vectorial optical fields (VOFs) are highly desired in integrated photonics, but traditional devices are bulky and lack flexible control capabilities. Although ultra-compact metasurfaces (MSs) have exhibited powerful light-manipulation capabilities, they typically work under propagating-wave excitations and/or rely on only phase modulations to control light beams. Here, we propose a general strategy to design MSs that, under surface wave (SW) excitations, can independently control amplitudes and phases of locally scattered waves with two orthogonal polarizations, thus enabling efficient generation of pre-designed VOFs. As a benchmark test, we construct a complex amplitude MS and experimentally demonstrate that it can generate two directional beams exhibiting orthogonal polarizations and arbitrarily pre-designed intensities, under excitation of a terahertz (THz) SW at 0.4 THz. We next experimentally demonstrate another MS that can generate two focal points in the far field with distinct intensities, under the same THz SW excitation. Finally, based on a modified Gerchberg-Saxton (GS) algorithm incorporating both amplitude and phase modulations, we design and fabricate a series of THz SW-excited complex amplitude MSs and experimentally demonstrate that they can respectively generate pre-designed scalar and vectorial holographic images in the far field, exhibiting much improved qualities and flexibility than those generated by their phase-only counterparts. Our study establishes a novel on-chip platform to generate complex vectorial fields, paving the way for many applications in integrated optics such as encrypted holography, augmented reality, and so on.