We synthesized BaTiO₃–epoxy nanocomposites (particle size < 100 nm) with volume fractions up to 25 vol. %, whose high-frequency complex permittivity was characterized from 8.2 to 12.5 GHz. The maximum dielectric constant approaches 9.499 with an acceptable loss tangent of 0.113. The dielectric loss gradually saturates when the particle concentration is higher than 15 vol. %. This special feature is an important key to realizing high-k and low-loss nanocomposites. By comparing the theoretical predictions and the experimental data, four applicable effective-medium models are suggested. The retrieved dielectric constant (loss tangent) of 100-nm BaTiO₃ nanopowder is in the range of 50–90 (0.1–0.15) at 8.2–12.5 GHz, exhibiting weak frequency dispersion. Two multilayer microwave devices—total reflection and antireflection coatings—are designed based on the fabricated nanocomposites. Both devices show good performance and allow broadband operation.

We report a design of low-loss THz Bragg fibers with a core size on the order of wavelength that operates near the cutoff frequency of its TE₀₁ mode. We also propose a broadband Y-type mode converter based on branched rectangular metallic waveguides to facilitate coupling between the TE₀₁ mode of the Bragg fiber and the TEM mode in free space with 60% efficiency. Our fiber holds strong promise to facilitate beam-wave interaction in gyrotron for high-efficiency THz generation.

This study examines the transverse magnetic (TM) waveguide modes, which have long been considered as the unsuitable ones for the operation of the electron cyclotron maser. The beam-wave coupling strength of the TM modes, as expected, is found to be relatively weak as compared with that of the transverse electric (TE) waveguide modes. Unlike TE modes, surprisingly, the linear behavior of the TM modes depends on the sign of the wave number k_z. The negative k_z has a much stronger linear efficiency than that of the positive k_z. The bunching mechanism analysis further exhibits that the azimuthal bunching and axial bunching do not compete but cooperate with each other for the backward-wave operation (negative k_z). The current findings are encouraging and imply that TM modes might be advantageous to the gyrotron backward-wave oscillators.

A strong and broadband negative group delay (NGD) is demonstrated in a circular waveguide with an asymmetric cross-shaped slotted structure in the millimeter-wave region. For an input of 45° linear polarization (relative to the orientation of the cross-shaped slots), the NGD fractional bandwidth is as high as 3.57%, in which the optimal group advancement obtained is nearly 24 times greater than the delay traveling with the speed of light in vacuum. This further indicates a very high fractional advancement of 12.10%. In addition, the NGD can be controlled by adjusting the polarization azimuthal angle of the incident pulse. The observed tunable NGD effect is attributed to the destructive cross interference between the two polarization eigenstates, which effectively reduces the released field energy at the observed polarization channel. A general theory is provided with excellent agreement in the experimental and simulation results.

Extraordinary group delays shorter than the transit time of light propagating at $c$ through an equal distance have been experimentally demonstrated in single-Fabry-Pérot (FP) waveguide systems and cascaded-FP structures under off-resonant conditions. These “superluminal” phenomena are well explained by the multiple-reflection destructive interference that reduces the intracavity stored energy when operating off resonances. Excellent agreement between theory and experiment is obtained when the dispersive effects of reflective boundaries are considered. These results provide further insight into the nature of apparent superluminality in regions of allowed propagation.