Achromatic terahertz quarter waveplate based on silicon grating

Terahertz Metagrating Emitters with Beam Steering and Full Linear Polarization Control

We report the realization of broadband THz plasmonic metagrating emitters for simultaneous beam steering and all-optical linear polarization control. Two types of metagratings are designed and experimentally demonstrated. First, the plasmonic meta-atoms are arranged in a metagrating with a binary phase modulation which results in the nonlinear generation of THz waves to the ±1 diffraction orders, with complete suppression of the zeroth order. Complete tunability of the diffracted THz linear polarization direction is demonstrated through simple rotation of the pump polarization. Then, the concept of lateral phase shift is introduced into the design of the metagratings using interlaced phase gradients. By controlling the spatial shift of the submetagrating, we are able to continuously control the linear polarization states of the generated THz waves. This method results in a higher nonlinear diffraction efficiency relative to binary phase modulation. These functional THz metagratings show exciting promise to meet the challenges associated with the current diverse array of applications utilizing THz technology.

3D-printed THz wave- and phaseplates

Three-dimensional printing based on fused deposition modeling has been shown to provide a cost-efficient and time-saving tool for fabricating a variety of THz optics for a frequency range of <0.2 THz. By using a broadband THz source, with a useful spectral range from 0.08 THz to 1.5 THz, we show that 3D-printed waveplates operate well up to 0.6 THz and have bandwidths similar to commercial products. Specifically, we investigate quarter- and half-waveplates, q-plates, and spiral phaseplates. We demonstrate a route to achieve broadband performance, so that 3D-printed waveplates can also be used with broadband, few-cycle THz pulses, for instance, in nonlinear THz spectroscopy or other THz high field applications.

Broadband and switchable terahertz polarization converter based on graphene metasurfaces

In this work, we propose broadband and switchable terahertz (THz) polarization converters based on either graphene patch metasurface (GPMS) or its complementary structure (graphene hole metasurface, GHMS). The patch and hole are simply cross-shaped, composed of two orthogonal arms, along which plasmonic resonances mediated by Fabry-Perot cavity play a key role in polarization conversion (PC). An incidence of linear polarization will be converted to its cross-polarization (LTL) or circular polarization (LTC), as the reflected wave in the direction of two arms owning the same amplitude and π phase difference (LTL), or ±π/2 phase difference (LTC). Such requirements can be met by optimizing the width and length of two arms, thickness of dielectric layer, and Fermi level E_F of graphene. By using GPMS, LTL PC of polarization conversion ratio (PCR) over 90% is achieved in the frequency range of 2.92 THz to 6.26 THz, and by using GHMS, LTC PC of ellipticity χ ≤ -0.9 at the frequencies from 4.45 THz to 6.47 THz. By varying the Fermi level, the operating frequency can be actively tuned, and the functionality can be switched without structural modulation; for instance, GPMS supports LTL PC as E_F = 0.6 eV and LTC PC of χ ≥ 0.9 as E_F = 1.0 eV, in the frequency range of 2.69 THz to 4.19 THz. Moreover, GHMS can be optimized to sustain LTL PC and LTC PC of |χ| ≥ 0.9, in the frequency range of 4.96 THz to 6.52 THz, which indicates that the handedness of circular polarization can be further specified. The proposed polarization converters of broad bandwidth, active tunability, and switchable functionality will essentially make a significant progress in THz technology and device applications, and can be widely utilized in THz communications, sensing and spectroscopy.

Liquid-crystal-based magnetically tunable terahertz achromatic quarter-wave plate

Development of the wideband and tunable quasi-optic terahertz (THz) components is in high demand. In this work, we demonstrate a tunable achromatic quarter-wave plate (AQWP) for the THz frequency range. The phase retardation of this device can be set at 90° ± 9° from 0.20 to 0.50 THz. The operation range from 0.20 to 0.50 THz can be tuned to from 0.30 to 0.70 THz by introducing three nematic liquid crystals phase retarders, of which the birefringence can be magnetically tuned. The frequency-dependent phase retardation is in good agreement with theoretical predictions.

High quality terahertz glass wave plates

In this paper we present four λ/2-wave plates made out of fused silica glass for operation in the terahertz frequency range. The design of the wave plates is based on form birefringence. They were fabricated by selective laser-induced etching resulting in a series of glass bars separated by air grooves. Wave plates operating at single, two and several frequencies were designed, fabricated and characterized.

Terahertz wire-grid circular polarizer tuned by lock-in detection method

We report the design, construction, and testing of a broadband-tunable terahertz circular polarizer, which we developed for our magneto-optical measurements using the far-infrared/THz laser source in the range of 0.25-7.5 THz. We present a thorough analysis of the lock-in amplifier signal generated by using a rotating analyzer with regard to setting the desired state of polarization. The phase-sensitive detection method is applied to a combination of a wire-grid polarizer and a parallel translation mirror providing a tunable retardance. The proposed technique is appropriate not only to free-standing grids but also to those deposited on transparent substrates which may introduce additional non-linear effects. The method is preferred when the distance between the mirror and the grid cannot be exactly determined, but the relative displacements are measured. The device enables switching between left- and right-handed polarization states on a time scale of a few seconds. Practical use of the circular polarizer is demonstrated by directly probing the far infra-red magneto-optical properties of the two-dimensional electron gas on the 458 μm laser line.

Artificial high birefringence in all-dielectric gradient grating for broadband terahertz waves

Subwavelength dielectric gratings are widely applied in the phase and polarization manipulation of light. However, the dispersion of the normal dielectric gratings is not flat while their birefringences are not enough in the THz regime. In this paper, we have fabricated two all-dielectric gratings with gradient grids in the THz regime, of which artificial birefringence is much larger than that of the equal-grid dielectric grating demonstrated by both experiments and simulations. The transmission and dispersion characteristics are also improved since the gradient grids break the periodicity of grating lattices as a chirp feature. From 0.6–1.4 THz, a broadband birefringence reaches 0.35 with a low dispersion and good linearity of phase shift, and the maximum phase shift is 1.4π. Furthermore, these gradient gratings are applied as half-wave plates and realize a linear polarization conversion with a conversion rate over 99%, also much higher than the equal-grid gratings. These gradient gratings show great advantages compared to the periodic gratings and provide a new way in the designing of artificial birefringence material.