Functional THz emitters based on Pancharatnam-Berry phase nonlinear metasurfaces

Recent advances in the science and technology of THz waves show promise for a wide variety of important applications in material inspection, imaging, and biomedical science amongst others. However, this promise is impeded by the lack of sufficiently functional THz emitters. Here, we introduce broadband THz emitters based on Pancharatnam-Berry phase nonlinear metasurfaces, which exhibit unique optical functionalities. Using these new emitters, we experimentally demonstrate tunable linear polarization of broadband single cycle THz pulses, the splitting of spin states and THz frequencies in the spatial domain, and the generation of few-cycle pulses with temporal polarization dispersion. Finally, we apply the ability of spin control of THz waves to demonstrate circular dichroism spectroscopy of amino acids. Altogether, we achieve nanoscale and all-optical control over the phase and polarization states of the emitted THz waves.

When the incident wave is linearly polarized, from a superposition of left and right circularly polarized waves, i.e., ( 1 ) = 1, ( 1 ) = , and ( 2 ) = 1, ( 2 ) = − , we get . After transformation back to the laboratory frame, the nonlinear dipole moment is given by Where 3 represents the nonlinear geometric P-B phase of the THz wave.

Supplementary Note 2 Linear and THz Responses of the Plasmonic Metasurfaces
The experimental setup for THz measurements is described in the Methods section of the manuscript. In Supplementary  The signal to noise ratio for the generated field from the C3 metasurface can be calculated as the average electric field strength divided by the standard deviation of the peak signal 1 , and is approximately 35. The dynamic range of the system is calculated to be 44, wherein the peak electric field strength is divided by the root mean squared value of the noise floor. In this case the signal is not optimized towards SNR. Significant improvement in the SNR could be achieved through increasing the time resolution of the scan, however it is not necessary for the scope of this study. The efficiency of the C3 uniform metasurface (shown in Fig. 1b) was examined by comparing the generated THz electric field to a 0.1mm thick ZnTe electro optic crystal, after pumping both sources with 1500 nm femtosecond pulses at a laser power of 25 mW.
Supplementary Figure 4 shows the two electric field traces, with peaks of 0.06 and 0.28 for the C3 and ZnTe metasurface, respectively, which is an approximate 5 times relative efficiency. It must be noted that at 1500 nm the pump and the THz waves in the ZnTe are not optimally phase matched, which may cause a reduction the generated THz signal. However, since the crystal that we used is 0.1mm thick and the coherence length in this case is larger than 0.2 mm the reduction in efficiency is not significant in this case.

Supplementary Note 3 Spatial Frequency Maps for the Circular and Composite THz Wavepackets
The spatial frequency map for the time domain electric fields of the LCP and RCP THz pulses between the simulation and experimental data. Frequencies above 1.5 THz are cut due to the limited NA of the optical collection system.

Supplementary Note 4 Effect of Diffraction on the Pulse Duration of THz Frequency Components
The effect of diffraction on the frequency components of the RCP pulse was examined from Ex field leading Ey, as expected from RCP light.

Supplementary Note 6 Time Domain Spectroscopy using Linearly Polarized THz Pulses
The spectra of Lactose and L-Cystine were also extracted using the metasurfaces shown in Fig. 1, which generate linearly polarized THz pulses. As expected the typical strong absorption peaks is noted at 0.71 THz for L-Cystine. A reference sample of Lactose is also shown, with peaks at 0.53 THz and 1.38 THz.