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, a general strategy is proposed to design MSs that, under surface wave (SW) excitations, can independently control the amplitudes and phases of locally scattered waves with two orthogonal polarizations, thus enabling efficient generation of pre-designed VOFs. As a benchmark test, a complex amplitude MS was constructed to experimentally demonstrate the generation of two directional beams exhibiting orthogonal polarizations and arbitrarily pre-designed intensities, under excitation of SW at 0.4 THz. Next, another MS was experimentally demonstrated to 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, a series of THz SW-excited complex amplitude MSs were designed and fabricated, and were experimentally demonstrated to 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. The present work establishes a novel on-chip platform to generate complex vectorial fields, paving the way for many applications in integrated optics such as encrypted holography, and augmented reality.