Recently, Dr. Guixin Li‘s group from the Department of Materials Science and Engineering from SUSTech published two papers in Nano Letters (impact factor: 12.712).

For instance, when illuminated by circularly polarized fundamental waves (FW) along the rotational axis, a meta-atom with m-fold rotational symmetry only allows harmonic generation orders of , where  is an arbitrary integer, and the sign corresponds to harmonic generations with the same and opposite spins as the FW, respectively. Similar to the definition of a geometric P-B phase element in the linear optics, the phase factors: and for harmonic generations can be defined as the nonlinear P-B phase, which depends only on the orientation angle of the meta-atoms. Therefore, second harmonic generation (SHG) from plasmonic meta-atoms with three-fold rotational symmetry has an opposite SAM as compared to that of FW and experiences a phase factor of 3σθ.

Research Topic I: Nonlinear Metasurface for Simultaneous Control of Spin and Orbital Angular Momentum in Second Harmonic Generation

Figure 1. Schematic of spin controlled generation of OAM of SHG by using nonlinear photonic metasurface.

The metasurface consists of C3 plasmonic meta-atoms with three-fold symmetry. For a fundamental wave (FW) with two spin states (LCP and RCP) normally incident onto the nonlinear photonic metasurface with C3 symmetry, generation of SHG waves with opposite handiness to that of FW are allowed. By encoding the phase singularity into the metasurfaces, the SHG vortex beams with spin controlled topological charges are generated.

The spin and orbital angular momentum (SAM and OAM) of light is providing a new gateway towards high capacity and robust optical communications. While the generation of light with angular momentum is well studied in linear optics, its further integration into nonlinear optical devices will open new avenues for increasing the capacity of optical communications through additional information channels at new frequencies. However, it has been challenging to manipulate both the SAM and OAM of nonlinear signals in harmonic generation processes with conventional nonlinear materials. Here, we report the generation of spin controlled OAM of light in harmonic generations by using ultrathin photonic metasurfaces. The spin manipulation of OAM mode of harmonic waves is experimentally verified by using second harmonic generation (SHG) from gold meta-atom with three-fold rotational symmetry. By introducing nonlinear phase singularity into the metasurface devices, we successfully generate and measure the topological charges of spin-controlled OAM mode of SHG through an on-chip metasurface interferometer. The nonlinear photonic metasurface proposed in this work not only opens new avenues for manipulating the OAM of nonlinear optical signals, but also benefits the understanding of the nonlinear spin-orbit interaction of light in nanoscale devices.

Collaborators: Prof. Yu Luo’s group from NTU, Singapore; Prof. Shuang Zhang’s group from University of Birmingham, UK. Dr. Wendi Li from HKU, Dr. Yanjun Liu and Dr. Siya Huang from SUSTech.

Research Topic II: Ultrathin Nonlinear Metasurface for Optical Image Encoding

Figure 2. Working principle of the nonlinear metasurface for different illumination conditions.

(a) If the metasurface is illuminated with incoherent and unpolarized visible light no image in the visible spectral range can be observed. (b) Under pumping of circularly polarized near-infrared laser at the regime of 1240 nm, an image with the character appears at the SHG wavelength (620 nm). (c) However, for a direct illumination with incoherent and unpolarized light at a wavelength of 620 nm, no information on the “META” characters is observable. The bottom row shows the corresponding measured real space microscopy images of the metasurface observed under different conditions (scale bar 20 μm).

Security of optical information is of great importance in modern society. Many cryptography techniques based on classical and quantum optics have been widely explored in the linear optical regime. Nonlinear optical encryption in which encoding and decoding involve nonlinear frequency conversions represents a new strategy for securing optical information. Here, we demonstrate that an ultrathin nonlinear photonic metasurface, consisting of meta-atoms with 3-fold rotational symmetry, can be used to hide optical images under illumination with a fundamental wave. However, the hidden image can be read out from second harmonic generation (SHG) waves. This is achieved by controlling the destructive and constructive interferences of SHG waves from two neighboring meta-atoms. In addition, we apply this concept to obtain gray scale SHG imaging. Nonlinear metasurfaces based on space variant optical interference open new avenues for multilevel image encryption, anticounterfeiting, and background free image reconstruction.

Collaborators: Prof. Thomas Zentgraf’s group from Department of Physics, University of Paderborn, Germany.

These works were supported by NSFC.

Links:

Li, G., et al. Nonlinear Metasurface for Simultaneous Control of Spin and Orbital Angular Momentum in Second Harmonic Generation. Nano Letters, 17 (12), 7974-7979, 2017.http://http://pubs.acs.org/doi/10.1021/acs.nanolett.7b04451

Felicitas W., et al. Ultrathin Nonlinear Metasurface for Optical Image Encoding. Nano Letters, 17 (5), 3171–3175, 2017.http://http://pubs.acs.org/doi/10.1021/acs.nanolett.7b00676

Dr. Guixin Li (PI),

Photonic Materials and Metamaterials Laboratory http:/en/people/detail/id/90