The lack of low‐loss and high‐efficiency nonreciprocal isolators has become one of the limitations in the development of terahertz (THz) application systems. This work demonstrates that the longitudinally magnetized InSb can achieve one‐way transmission for one photonic spin state but not for linear polarization (LP) state due to the chiral mirror‐symmetry of the two spin states. To solve this issue, a silicon microstructure is fabricated on the InSb substrate to form a magnetoplasmon/dielectric metasurface, where both the time‐reversal and mirror‐reversal symmetric transmission of the two spin states can be broken. In this device, the forward LP state is efficiently transformed into one of the spin states and output with low loss, but the backward wave is forbidden, which achieves the one‐way transmission for the LP incidence with over 30 dB isolation and only 1.7 dB insertion loss. When a THz polarizer is added behind the device to obtain the LP output, the isolation can reach 40 dB, significantly better than the previous reports. This study is significant to understand the nonreciprocal transmission and manipulation mechanism of THz spin states in the magnetized semiconductor and promotes the development of high‐performance THz isolators under the weak magnetic field.
The transverse magneto-optical (MO) effect of InSb has been theoretically and experimentally investigated in the terahertz (THz) regime. The calculated photonic band structure and experimental measurements show that a unique circularly polarized magneto plasmon mode, and a linearly polarized transverse magnetic mode can be sensitively manipulated by a weak magnetic field. Moreover, these results indicate that transverse magnetized InSb can be used as a THz tunable high-pass filter and a MO modulator. The cutoff frequency of the filter can be broadly tuned from 0.4 to 0.8 THz when the magnetic field changes from 0 to 0.22 T, and the modulation depth of 20 dB can be obtained. This research has significance for the deep understanding to the THz MO effect of InSb and promotes the development of THz MO devices.
Active manipulation of photonic spin state and optical chirality leads to some key applications, such as in multichannel communication, polarization‐sensitive imaging, chiral spectroscopy, and chiral sensing. Magneto‐optical materials have unique advantages in the intrinsic transmission and magnetic control of photonic chiral spin states. Here, a scheme for dynamic terahertz (THz) anisotropy and chirality manipulations in the transversely magnetized InSb and its hybrid magneto‐optical metasurface structure is presented. A special transverse photonic spin state in the InSb and a transverse−longitudinal spin coupling effect in the hybrid magneto‐optical metasurface are revealed by the eigenmode analysis and numerical simulations. The strong magnetic birefringence effect induced by this spin mode is demonstrated in the experiment. Moreover, the symmetry‐breaking mechanism in this magneto‐optical structure leads to strong intrinsic chirality and polarization conversion. The experimental results confirm the magnetically active manipulation of spin states and their asymmetric transmission in this hybrid magneto‐optical metasurface, which achieve a polarization conversion rate of near 100% and an induced intrinsic chirality of over 15 dB. This work opens a new development for active THz polarization control and chiral manipulation in the magneto‐optical microstructure.
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