Electro-optic phase modulators are critical components in modern communication, microwave photonic, and quantum photonic systems. Important for these applications is to achieve modulators with low half-wave voltage at high frequencies. Here we demonstrate an integrated phase modulator, based on a thin-film lithium niobite platform, that simultaneously features small on-chip loss (~1 dB) and low half-wave voltage over a large spectral range (3.5 - 4.5V at 5 - 40 GHz). By driving the modulator with a strong 30-GHz microwave signal corresponding to around four half-wave voltages, we generate an optical frequency comb consisting of over 40 sidebands spanning 10 nm in the telecom L-band. The high electro-optic performance combined with the high RF power-handling ability (3.1 W) of our integrated phase modulator is crucial for future photonics and microwave systems.

A novel waveguide-to-microstrip inline transition was proposed using a wedge-waveguide iris to form an ultra-compact U-bend waveguide. The wedge-waveguide iris not only functioned as a wide wall of the height-reduced waveguide for the $E$ -plane probe, but also played a critical role in impedance matching and parasitic resonance suppression. The working principle was investigated through theoretical analysis and full-wave simulation, and the relevant design processes for the transition were discussed. Moreover, two back-to-back transition prototypes, one working at the $W$ -band and the other working at the WR4.3 band, were designed, fabricated, and measured. The back-to-back prototypes obtained an insertion loss of less than 0.8 and 1.2 dB, with a return loss better than 15 dB in the $W$ -band and WR4.3 band, respectively. The measurement results were consistent with simulation results. A miniaturized 110-GHz tripler incorporating this transition for inline output was fabricated. A conversion efficiency of 2.5%–3.6% was achieved in 90–120 GHz at 100-mW driving power.

High quality ( $Q)$ -factor metamaterial absorbers are an emerging technology, which can be used in refractive index sensing for dielectric coating and in radar cross section (RCS) reduction for concealment of an object. Nevertheless, angular-sensitive or the required complex structure hampered its application. Here, a metamaterial absorber based on a simple inverted-cross-star (ICS) structure with high $Q$ -factor operating within microwave frequencies is demonstrated. The metamaterial absorber is composed of ICS metallic pattern and a metal ground plane on the sides of a Rogers RT5880 dielectric slab with a thickness of 0.127 mm. The excellent angular insensitivity for both TE and TM polarizations of the proposed metamaterial absorber is demonstrated by Finite Integeral Technology (FIT) numerical simulation, i.e., the absorptivity is about 85% for TE polarization and more than 99% for TM polarization under the incident angle as wide as 70 $^{\circ}$ . The RCS of a manufactured prototype is measured in an anechoic chamber. In addition, the potential application of the proposed metamaterial absorber in refractive index sensing is also studied. This proposed prototype, with the advantages of high $Q$ -factor, structural simplicity, small thickness, angular-and polarization-insensitiveness, and frequency flexibility, has potential application in the field of antennas with RCS reduction and metamaterial sensing.

We proposed a new variety of substrate integrated plasmonic filters (SIPFs) with multiple passband characteristics based on the concept of spoof surface plasmon polaritons (SSPPs). By etching a periodic interdigital structure array on the top metal layer of substrate integrated waveguide (SIW), we can achieve high-efficiency microwave SSPP transmission with an insertion loss of 1 dB in the passband of 7.4-12.5 GHz, and a high rejection level of 27 dB in the rejection bands. By introducing antisymmetric C-ring resonators (ASCRs) in the filter structure, the passband number and bandwidth of the filter can be flexibly manipulated. To validate the proposed design, four SSPP filters prototypes are fabricated and tested, showing good filtering performances with a high transmission coefficient ( $\text{S}21\mathbf {>}-$ 1 dB) and low reflection coefficient ( $\text{S}11\,\,\mathbf { < } - $ 12 dB) in their passbands, and a high rejection level over 15 dB between them. By changing the geometric parameters of ASCR, the number of passbands can be increased from 1 to 3, and the passband center frequency and bandwidth can be engineered accordingly. The proposed SSPP filters with good multiple passband characteristics may have great potential applications in microwave wireless integrated plasmonic circuits and communication systems.

The microwave spectrum has become more and more crowded, motivating the investigation of new techniques to increase the communication capacity. Vortex beams carrying orbital angular momentum (OAM) have the capability for providing an extra degree of freedom to increase the channel and capacity of the wireless communication system. In this Letter, we demonstrate an effective route to achieving efficient bidirectional multi-mode microwave circularly polarized vortex beam generation enabled by spoof surface plasmon polariton (spoof SPPs, SSPPs). According to the dispersion relation of the SSPP waveguide and the radiation principle for a leaky-wave antenna, multi-OAM-mode vortex beams with different topological charges can be generated by changing the operating frequency without changing the structure. Interestingly, the emitter simultaneously radiates the right-hand circularly polarized and the left-hand circularly polarized vortex beams with opposite topological charge values upward and downward, respectively. Simulation and experimental results show that this planar SSPP vortex emitter has the advantages of single-fed, bidirectional, multi-mode, and integration compatibility.

A high-efficiency W-band 12-way power combiner system based on the TM ₀₁ mode in a circular waveguide has been presented in this study. It is a radial structure composed of three elements: a circular waveguide TM ₀₁ -mode transducer, an isolated 12-way radial power combiner, and amplifier modules. The measured passive combining efficiency is observed to be greater than 89.5% over the range of 85–100 GHz. The isolation between the inputs is better than 10 dB, with a maximum back-to-back return loss of 13 dB. By using 12 GaN monolithic microwave integrated circuit (MMIC) 2-W power amplifiers, the active combiner system produces a maximum power output of up to 22 W with an average combining efficiency of 81.7% within the band of 92–97 GHz. The power-added efficiency within the operating band has been measured to be up to 20.8%.