Polarization conveys valuable information for electromagnetic signal processing exhibiting tremendous potential in developing application driven photonic devices. Manipulation of polarization state of an electromagnetic wave has drawn a lot of research interests in many different fields, especially in the terahertz regime. Here, we propose a unique approach to efficiently rotate the linear polarization of terahertz wave in a broadband configuration with tri-layer metasurfaces. We experimentally observe a nearly perfect orthogonal polarization conversion with an ultrahigh efficiency, demonstrating a ultrathin terahetz rotator. The Fabry-Perot cavity effect in the tri-layer metasurfaces is attributed to the underlying mechanism of high transmittance and polarization rotation.

A perfect metamaterial polarization rotator

Metamaterials offer exciting opportunities that enable precise control of light propagation, its intensity and phase by designing an artificial medium of choice. Inducing birefringence via engineered metamolecules presents a fascinating mechanism to manipulate the phase of electromagnetic waves and facilitates the design of polarimetric devices. In this paper, a high-efficiency, broadband, tunable and flexible quarter-wave plate based on a multilayer metamaterial is presented. Excellent achromatic π/2 phase retardance with high transmission is observed upon terahertz propagation through the quarter-wave plate. The calculated Stokes parameter represents the output polarization state numerically, indicating an excellent broadband conversion of linearly polarized light into circularly polarized light. The metamaterial-based quarter-wave plate demonstrated in this work could be an important step forward in the development of functional terahertz polarization conversion devices for practical applications.

Highly flexible broadband terahertz metamaterial quarter-wave plate

We demonstrate the first electronically switchable Bragg structure for THz frequencies. The structure works as stop-band filter and as mirror. It exhibits the 60 GHz broad stop-band around 300 GHz which can be removed by reorienting liquid crystal molecules in an external electric field. Our first proof-of-principle experiments agree very well with transfer matrix calculations.

Liquid crystal based electrically switchable Bragg structure for THz waves

With growing interest in switchable devices for the THz frequency range, there is a strong demand for liquid crystals (LC) exhibiting both a high birefringence and a low absorption. We present the refractive index and absorption coefficient of the liquid crystal mixtures 1852 and 1825 in the frequency range between 0.2 and 2.5 THz. Both mixtures are designed specifically for high birefringence Δn of 0.32 and 0.38, respectively, in the THz region. In addition, they show low absorption coefficients for both ordinary and extraordinary polarization. This low absorbance in combination with the high birefringence makes these LCs particularly well suited for applications in switchable devices for THz optics.

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Highly birefringent, low-loss liquid crystals for terahertz applications

A terahertz modulator based on the insulator-metal transition (IMT) in a photonic crystal waveguide (PCW) coated by vanadium dioxide (VO2) film is proposed. The numerical simulations show that a dielectric state and a metallic state with quite different photonic band structures and transmission properties in the proposed PCW are reciprocally converted because of the IMT of VO2, and the pass-bands of this PCW are greatly shifted from 0.68 to 0.8 and 1.02 to 1.25 THz to 0.8-1.45 THz. This PCW significantly enhances the modulation depth and sensitivity compared with bare VO2 film. Extensive investigation demonstrates that the thickness of VO2 film greatly affects the IMT process in the PCW, and limits the ultimate modulation depth of the device. The proposed modulation scheme will be of great significance for potential THz applications.

Terahertz modulator based on insulator–metal transition in photonic crystal waveguide

The effects of magnetic birefringence of liquid crystal E7 on the photonic bandgaps and the transmitting properties in two-dimensional photonic crystal waveguides are investigated by using the plane wave expansion and finite-difference time domain methods. Detailed calculations on the shifts and variations in the frequency width of bandgaps reveal that under the control of an external magnetic field the two-dimensional liquid-crystal-filled photonic crystal waveguide can serve as a switch and continuously tunable bandpass filter at the terahertz wave band.

Liquid-crystal-filled photonic crystal for terahertz switch and filter

Supercontinuum generation in chloroform-filled photonic crystal fibers

The THz spectral region, lying between the microwave and the far infrared, is currently attracting widespread
interest in relation to potential applications that span many areas of pure and applied science and technology such as in
security, human health and communications. The development of practical source of THz radiation is crucial to the
realization of these applications. Using optical parametric oscillator with intersecting pump results in compact, low
threshold THz radiation source. Lithium niobate is one of the most suitable materials for generating THz waves
efficiently because of its large nonlinear coefficient and its transparency over a wide wavelength range. In this
communication we theoretically analyze generation of tunable terahertz (THz) radiation which is based on optical
parametric process, in particular using a non-collinear quasi-phase matched geometry in the nonlinear crystal
periodically poled lithium niobate (PPLN), and where wide and continuous tuning is obtained by changing the poled
period and the angle between the resonated idler wave and the pump wave. Also the THz wave should fulfill the energy
and momentum conservation laws. Furthermore, the bandwidth of the wavelength and the frequency of THz wave are
analyzed. The optimal THz wave parametric oscillator for efficient coupling output power can be designed according to
our theoretical results

Jinghe Yuan

Papers

Liquid-crystal-filled photonic crystal for terahertz switch and filter

Supercontinuum generation in chloroform-filled photonic crystal fibers

Theoretical analysis of terahertz wave parametric source based on intersecting cavity