Metamaterials, artificially constructed structures that mimic lattices in natural materials, have made numerous contributions to the development of unconventional optical devices. With an increasing demand for more diverse functionalities, terahertz (THz) metamaterials are also expanding their domain, from the realm of mere passive devices to the broader area where functionalized active THz devices are particularly required. A brief review on THz metamaterials is given with a focus on research conducted in the authors' group. The first part is centered on enhanced THz optical responses from tightly coupled meta‐atom structures, such as high refractive index, enhanced optical activity, anomalous wavelength scaling, large phase retardation, and nondispersive polarization rotation. Next, electrically gated graphene metamaterials are reviewed with an emphasis on the functionalization of enhanced THz optical responses. Finally, the linear frequency conversion of THz waves in a rapidly time‐variant THz metamaterial is briefly discussed in the more general context of spatiotemporal control of light.

The spin Hall effect of light (SHEL) refers to a transverse and spin‐dependent shift of light in real space at an optical interface. Previous studies of enhancing the SHEL have involved extremely low efficiency, and achieving a large SHEL and high efficiency simultaneously has never been reported. Here, an approach using anisotropic impedance mismatching to attain a large SHEL with near‐unity efficiency in the microwave spectrum is proposed. A wire medium that has a near‐unity transmission for one polarization and low transmission for the other is used to achieve high efficiency. The spin‐dependent splitting is experimentally confirmed by measuring transmission coefficients and the spatial profile of Stokes parameters. The large SHEL with near‐unity efficiency will enable highly efficient devices with spin‐selective functionalities.

above, chiral metamaterials basically exhibit gigantic optical activity and circular dichroism, the potential of which can be clearly seen in the realization of compact polarization rotators, [23][24][25][26][27][28][29][30] and circular polarizers. [31][32][33][34] Chiral metamaterials mimic naturally existing chiral media, where the degeneracy between right-and left-circularly polarized waves (RCP and LCP waves) is lifted due to the magnetoelectric coupling in chiral molecules. [35] This results in the difference in refractive indices for the RCP and LCP waves, which can be described by a chirality parameter κ. The constitutive relations for a chiral medium are written as

Asymmetric total syntheses of (+)-pochonin D (1) and (+)-monocillin II (2), Hsp90 inhibitors with potent anticancer activity, have been accomplished where the macrolactone 3 was constructed through a chemo- and regioselective intramolecular nitrile oxide cycloaddition of diene 4.

Photonic crystals have revolutionized the field of optics with their unique dispersion and energy band gap engineering capabilities, such as the demonstration of extreme group and phase velocities, topologically protected photonic edge states, and control of spontaneous emission of photons. Time-variant media have also shown distinct functionalities, including nonreciprocal propagation, frequency conversion, and amplification of light. However, spatiotemporal modulation has mostly been studied as a simple harmonic wave function. Here, we analyze time-variant and spatially discrete photonic crystal structures, referred to as spatiotemporal crystals. The design of spatiotemporal crystals allows engineering of the momentum band gap within which parametric amplification can occur. As a potential platform for the construction of a parametric oscillator, a finite-sized spatiotemporal crystal is proposed and analyzed. Parametric oscillation is initiated by the energy and momentum conversion of an incident wave and the subsequent amplification by parametric gain within the momentum band gap. The oscillation process dominates over frequency mixing interactions above a transition threshold determined by the balance between gain and loss. Furthermore, the asymmetric formation of momentum band gaps can be realized by spatial phase control of the temporal modulation, which leads to directional radiation of oscillations at distinct frequencies. The proposed structure would enable simultaneous engineering of energy and momentum band gaps and provide a guideline for implementation of advanced dispersion-engineered parametric oscillators.