Browsing by Author "Wang, Xiangfeng"
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Item Creating a near-perfect circularly polarized terahertz beam through the nonreciprocity of a magnetoplasma(Optica Publishing Group, 2023) Ju, Xuewei; Hu, Zhiqiang; Zhu, Guofeng; Huang, Feng; Chen, Yanqing; Guo, Cuixia; Belyanin, Alexey; Kono, Junichiro; Wang, XiangfengCompared to other parts of the electromagnetic spectrum, the terahertz frequency range lacks efficient polarization manipulation techniques, which is impeding the proliferation of terahertz technology. In this work, we demonstrate a tunable and broadband linear-to-circular polarization converter based on an InSb plate containing a free-carrier magnetoplasma. In a wide spectral region (∼ 0.45 THz), the magnetoplasma selectively absorbs one circularly polarized mode due to electron cyclotron resonance and also reflects it at the edges of the absorption band. Both effects are nonreciprocal and contribute to form a near-zero transmission band with a high isolation of –36 dB, resulting in the output of a near-perfect circularly polarized terahertz wave for an incident linearly polarized beam. The near-zero transmission band is tunable with magnetic field to cover a wide frequency range from 0.3 to 4.8 THz.Item Development of a coherent THz magneto-spectroscopy system(2006) Wang, Xiangfeng; Kono, JunichiroWe have developed a coherent time-domain THz magneto-spectroscopy system. It can be used for measuring the refractive index, complex conductivities, and cyclotron resonance of different semiconductor structures as well as for investigating fundamental physical phenomena. As a first application, we have carried out time-domain cyclotron resonance studies of an ultrahigh-mobility two-dimensional electron gas at low temperatures. We observed coherent cyclotron resonance oscillations that persist as long as ∼ 50 ps. We show that the basic physics of these oscillations can be described as the free induction decay of a coherent superposition between the lowest unfilled Landau level and the highest filled Landau level prepared by an incident THz pulse. Using the 0 Tesla data as a reference, we successfully extracted the real and imaginary conductivities for different magnetic fields in the frequency domain. Finally, I will discuss a few future experiments that will be performed with this novel system.Item Giant tunable Faraday effect in a semiconductor magneto-plasma for broadband terahertz polarization optics(Optical Society of America, 2012) Arikawa, Takashi; Wang, Xiangfeng; Belyanin, Alexey A.; Kono, JunichiroWe 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 π/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.Item Time-domain terahertz magneto-spectroscopy of semiconductors(2009) Wang, Xiangfeng; Kono, JunichiroThe terahertz frequency range, 0.1-10 THz, is one of the richest frequency ranges in condensed matter spectroscopy. Many important excitations and dynamical phenomena occur in this range, including superconducting gaps, protein conformational modes, phonons, and plasmons, just to name a few. Spectroscopic studies in this region provide valuable insights into the quantum states and dynamics of confined, driven, or interacting electrons in solids. In this dissertation research I have developed a time-domain THz magneto-spectroscopy system to investigate various THz magnetic excitations in semiconductors, including a high-mobility two-dimensional electron gas (2DEG) in a GaAs quantum well and lightly-doped InSb. In the 2DEG, I have observed very long-lived (up to ∼ 50 ps) coherent THz oscillations, which correspond to a time-domain observation of cyclotron resonance. From the data both the real and imaginary parts of the conductivity can be simultaneously determined because of the phase-sensitive-detection nature of this technique. Magnetic field and temperature dependent results provide some important information on electron scattering in this system. In InSb, I have found that the THz transmittance of the sample sensitively changes with the temperature and magnetic field, showing a number of non-intuitive spectral features. In particular, I observed a sudden appearance and disappearance of transparency with increasing temperature, which resulted in a transparency window of a narrow temperature region (160-190 K), over a frequency range of 0.1-0.8 THz. Detailed theoretical simulations based on a cold magneto-plasma model demonstrate that this novel phenomenon is a manifestation of coherent interference of the cyclotron-resonance-active and cyclotron-resonance-inactive modes co-propagating through the magneto-plasma along the magnetic field direction. Finally, I have obtained some experimental results on the 1s--2p-- impurity transition at 1.6 K that provide insight on the nature of the magnetic-field-induced metal-to-insulator transition that is known to occur in this system. The materials studied in this research are highly tunable with external fields and doping and thus promising for future THz devices such as tunable THz detectors, filters, and Faraday rotators.Item Tunable ultrasharp terahertz plasma edge in a lightly doped narrow-gap semiconductor(Optical Society of America, 2021) Ju, Xuewei; Hu, Zhiqiang; Huang, Feng; Wu, Haibin; Belyanin, Alexey; Kono, Junichiro; Wang, XiangfengPlasma edges in metals typically occur in the visible range, producing characteristic colors of metals. In a lightly doped semiconductor, the plasma edge can occur in the terahertz (THz) frequency range. Due to low scattering rates and variable electron densities in semiconductors, such THz plasma edges can be extremely sharp and greatly tunable. Here, we show that an ultrasharp THz plasma edge exists in a lightly n-doped InSb crystal with a record-high transmittance slope of 80 dB/THz. The frequency at which this sharp edge happens can be readily tuned by changing the temperature, electron density, scattering rate, and sample thickness. The edge frequency exhibited a surprising increase with decreasing temperature below 15 K, which we explain as a result of a weak-to-strong transition in the scattering rate, going from ωτ ≫ 1 to ωτ ∼ 1. These results indicate that doped narrow-gap semiconductors provide a versatile platform for manipulating THz waves in a controllable manner, especially as a high-pass filter with an unprecedented on/off ratio.