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Register a new userGiant Tunable Faraday Effect in a Semiconductor Magneto-plasma for Broadband Terahertz Polarization Optics
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We report on a giant Faraday effect in an electron plasma in n-InSb probed via polarization-resolved terahertz (THz) time-domain spectroscopy. Polarization rotation angles and ellipticities reach as large as {pi}/2 and 1, respectively, over a wide frequency range (0.3-2.5 THz) at magnetic fields of a few Tesla. The experimental results together with theoretical simulations show its promising ability to construct broadband and tunable THz polarization optics, such as a circular polarizer, half-wave plate, and polarization modulators.
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The terahertz (THz) frequency range (0.1-10 THz) fills the gap between the microwave and optical parts of the electromagnetic spectrum. Recent progress in the generation and detection of the THz radiation has made it a powerful tool for fundamental research and resulted in a number of applications. However, some important components necessary to effectively manipulate THz radiation are still missing. In particular, active polarization and phase control over a broad THz band would have major applications in science and technology. It would, e.g., enable high-speed modulation for wireless communications and real-time chiral structure spectroscopy of proteins and DNA. In physics, this technology can be also used to precisely measure very weak Faraday and Kerr effects, as required, for instance, to probe the electrodynamics of topological insulators. Phase control of THz radiation has been demonstrated using various approaches. They depend either on the physical dimensions of the phase plate (and hence provide a fixed phase shift) or on a mechanically controlled time delay between optical pulses (and hence prevent fast modulation). Here, we present data that demonstrate the room temperature giant Faraday effect in HgTe can be electrically tuned over a wide frequency range (0.1-1 THz). The principle of operation is based on the field effect in a thin HgTe semimetal film. These findings together with the low scattering rate in HgTe open a new approach for high-speed amplitude and phase modulation in the THz frequency range.
We observe a giant spin-orbit splitting in bulk and surface states of the non-centrosymmetric semiconductor BiTeI. We show that the Fermi level can be placed in the valence or in the conduction band by controlling the surface termination. In both cases it intersects spin-polarized bands, in the corresponding surface depletion and accumulation layers. The momentum splitting of these bands is not affected by adsorbate-induced changes in the surface potential. These findings demonstrate that two properties crucial for enabling semiconductor-based spin electronics -- a large, robust spin splitting and ambipolar conduction -- are present in this material.
In this paper, a broadband tunable polarization converter based on graphene metasurfaces is proposed. This polarization converter works in the terahertz (THz) frequency region, using the advantage of graphene characteristics to have a tunable frequency response. The designed graphene-shaped periodic structure on top of the substrate is utilized to convert the incident wave polarization to the desired target in a flexible operational band in the THz frequencies. The polarization conversion ratio is more than 0.85 in a wide range of frequencies in the THz band from 4.86 to 8.42 THz (the fractional bandwidth is 54%). The proposed polarization converter is insensitive to the angle of the incident wave up to 40{deg}. Using graphene provides a tunable frequency response without changing the geometry of the designed structure.
Optical spectrometers are the central instruments for exploring the interaction between light and matter. The current pursuit of the field is to design a spectrometer without the need for wavelength multiplexing optics to effectively reduce the complexity and physical size of the hardware. Based on computational spectroscopic results and combining a broadband-responsive dynamic detector, we successfully demonstrate an optics-free single-detector spectrometer that maps the tunable quantum efficiency of a superconducting nanowire into an ill-conditioned matrix to build a solvable inverse mathematical equation. Such a spectrometer can realize a broadband spectral responsivity ranging from 660 to 1900 nm. The spectral resolution at the telecom is 6 nm, exceeding the energy resolving capacity of existing infrared single-photon detectors. Meanwhile, benefiting from the optics-free setup, precise time-of-flight measurements can be simultaneously achieved. We have demonstrated a spectral LiDAR with 8 spectral channels. This work provides a concise method for building multifunctional spectrometers and paves the way for applying superconducting nanowire detectors in spectroscopy.
Flexible manipulation of terahertz-wave polarization during the generation process is very important for terahertz applications, especially for the next-generation on-chip functional terahertz sources. However, current terahertz emitters could not satisfy such demand, hence calling for new mechanism and conceptually new terahertz source. Here we demonstrate a magnetic-field-controlled, highly-efficient, cost-effective, and broadband terahertz source with flexible switch of terahertz polarization states in ferromagnetic heterostructures driven by femtosecond laser pulses. We verify that the chirality, azimuthal angle, and ellipticity of the generated elliptical terahertz waves can be independently manipulated by delicately engineering of the external applied magnetic fields via effectively manipulating the photo-induced spin currents. Such an ultrafast photomagnetic interaction-based, magnetic-field-controlled, and broadband tunable solid-state terahertz source integrated with terahertz polarization tunability function not only has the capability to reveal physical mechanisms of femtosecond spin dynamics, but also demonstrates the feasibility to realize novel on-chip terahertz functional devices, boosting the potential applications for controlling elementary molecular rotations, phonon vibrations, spin precessions, high-speed terahertz communication, and accelerating the development of ultrafast terahertz opto-spintronics.
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