Geometry phase for generating multiple focal points with different polarization states.

Conventional lenses are always large and bulky to achieve desired wave-manipulating functions, hindering the development of integrated and miniaturized optical systems. Metasurfaces, two-dimensional counterparts of metamaterials, can accurately tailor the wavefront of electromagnetic waves at subwavelength scale, providing a flexible platform for designing ultra-compact and ultra-flat lenses, namely as metalenses. However, the previous geometry-phase-based metalenses usually generate focal point(s) with only one special polarization state, i.e., either linearly-polarized (LP) state or circularly-polarized (CP) state, which inevitably degrades further applications. Here, we propose and experimentally demonstrate an approach for designing terahertz (THz) metalenses based on geometry phase that can generate multiple focal points with different polarization states. Under the illumination of LP THz waves, three focal points with left-hand CP (LCP), right-hand CP (RCP) and LP states are observed. Furthermore, the position of each focal point can be flexibly manipulated in free space. Geometry metasurfaces consisting of micro-rods with the same shape but different in-plane orientations are fabricated to demonstrate these properties. This unique approach may enable an unprecedented capability in designing multifunctional THz devices with potential applications in imaging, detecting and communications.

Multiband and broadband active controllable terahertz absorption in dual-side grating-gate graphene field-effect transistors.

We show that the even order ungated modes can be excited under normal incidence while the odd order ungated modes cannot in traditional single-side grating-gate graphene field-effect transistors. The odd order ungated modes will suppress the excitation efficiency of the gated modes. In order to realize multiband detection by effectively exciting the higher order gated modes, the frequency of the 1st order ungated mode should be tuned up, which can be realized by shortening the length of the ungated region. We propose to use the dual-side grating-gate structure to shorten the length of the ungated region. Gated mode up to 21st order can be realized in complementary dual-side grating-gate structure. The ultra-multiband absorption can be actively controlled to cover 1.06-10 THz when the graphene Fermi energy is tuned from 0.2 eV to 0.6 eV. Even order gated modes will be excited by gradually overlapping the two grating layers because of the break of symmetry. Broadband detection from 0.1-8.2 THz can be realized by the effective excitation and overlap of the odd and even order gated modes.

"Terhune-like" transformation of the terahertz polarization ellipse "mutually induced" by three-wave joint propagation in liquid.

In this Letter, we show experimentally for the first time, to the best of our knowledge, the possibility to observe the effect of polarization mutual action of three elliptically polarized waves, with one of them at terahertz frequency, when they propagate in the isotropic nonlinear medium. When three light pulses are propagated at frequencies ω, 2ω, and ωTHz through liquid nitrogen, we observed the rotation of the ellipse main axis and the ellipticity change. We have shown that this effect is very well described theoretically in the framework of a physical approach analogous to the self-rotation of the polarization ellipse first described in 1964 by Maker et al., but expanded for the case of multi-frequency interaction.

Tunable Transmissive Terahertz Linear Polarizer for Arbitrary Linear Incidence Based on Low-Dimensional Metamaterials.

In this work, we propose a structure consisting of three metamaterial layers and a metallic grating layer to rotate the polarization of arbitrary linearly polarized incidence to the y-direction with high transmissivity by electrically tuning these metamaterials. The transfer matrix method together with a harmonic oscillator model is adopted to theoretically study the proposed structure. Numerical simulation based on the finite difference time-domain method is performed assuming that the metamaterial layers are constituted by graphene ribbon arrays. The calculation and simulation results show that the Drude absorption is responsible for the polarization rotation. Fermi level and scattering rate of graphene are important for the transmissivity. For a polarization rotation of around 90°, the thickness of either the upper or lower dielectric separations influences the transmission window. For a polarization rotation of around 45° and 135°, the lower dielectric separations decide the frequency of the transmission window, while the upper dielectric separations just slightly influence the transmissivity.

Terahertz Broadband Polarization Conversion for Transmitted Waves Based on Graphene Plasmon Resonances.

We applied the harmonic oscillator model combined with the transfer matrix method to study the polarization conversion for transmitted waves in metallic grating/plasmon-excitation layer/metallic grating structure in the terahertz (THz) region. By comparing the calculated spectra and the simulated (by the finite-difference-time-domain method) ones, we found that they correspond well with each other. Both methods show that the Drude background absorption and the excited plasmon resonances are responsible for polarization conversion. The transmission is close to 0 when the distance between the top/bottom metallic gratings and gated graphene is an integer multiple of half the wavelength of the incident wave (in the dielectrics), at which points the plasmon resonances are greatly suppressed by the destructive interference between the backward/forward electromagnetic waves and that reflected by the top/bottom metallic gratings. Away from these points, the transmission can be higher than 80%. The electron density and the excitation efficiency of the plasmon-excitation layer were found to be important for the bandwidth of the polarization conversion window, while the scattering rate was found to influence mainly the polarization conversion rate. Multi-broadband polarization conversion is realized by exciting plasmon modes between the 0 transmission points in the THz region.