Metasurface-assisted phase-matching-free second harmonic generation in lithium niobate waveguides

• Published in: Nature communications Vol. 8; no. 1; pp. 2098 - 7
• Main Authors: Wang, Cheng, Li, Zhaoyi, Kim, Myoung-Hwan, Xiong, Xiao, Ren, Xi-Feng, Guo, Guang-Can, Yu, Nanfang, Lončar, Marko
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 13.12.2017; Nature Publishing Group; Nature Portfolio
• Subjects: 639/624/399/1015; 639/624/400/385; Antenna arrays; Antennas; Conversion; Crystals; ENGINEERING; Humanities and Social Sciences; Lithium; Lithium niobates; metamaterials; Metasurfaces; multidisciplinary; Narrowband; nonlinear optics; Phase matching; Photons; Science; science & technology; Science (multidisciplinary); Second harmonic generation; Thin films; Waveguides; Wavelength; Wavelengths; X-rays
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-017-02189-6

The phase-matching condition is a key aspect in nonlinear wavelength conversion processes, which requires the momenta of the photons involved in the processes to be conserved. Conventionally, nonlinear phase matching is achieved using either birefringent or periodically poled nonlinear crystals, which requires careful dispersion engineering and is usually narrowband. In recent years, metasurfaces consisting of densely packed arrays of optical antennas have been demonstrated to provide an effective optical momentum to bend light in arbitrary ways. Here, we demonstrate that gradient metasurface structures consisting of phased array antennas are able to circumvent the phase-matching requirement in on-chip nonlinear wavelength conversion. We experimentally demonstrate phase-matching-free second harmonic generation over many coherent lengths in thin film lithium niobate waveguides patterned with the gradient metasurfaces. Efficient second harmonic generation in the metasurface-based devices is observed over a wide range of pump wavelengths ( λ  = 1580–1650 nm). Phase matching is a crucial condition for nonlinear optical processes. Here, Wang et al. demonstrate that a gradient metasurface composed of phased array antennas allows phase-matching-free frequency conversion over a pump wavelength range of almost 100 nm.