Shohei Kosuga

We present a vertical-illumination InP/InGaAs photodiode (PD) with a monolithically integrated micro-lens having a 165-GHz bandwidth. Interference-based absorption enhancement resulted in a 0.53-A/W responsivity and a record 70-GHz external bandwidth-efficiency product, while maintaining a broader absorption spectral range compared with conventional resonant cavity enhanced PDs. The PD features an electric field confinement by an inverted p-down structure, which results in a low dark current of approximately 1 pA and low manufacturing variability. In addition, an InP microlens is monolithically integrated to increase the effective aperture by up to 20 μm, which improves assembly tolerance.

We fabricated an optically transparent monopole antenna using graphene film and investigated the feasibility of the film as an electrode material for antennas. A low sheet resistance (80 Ω/sq) was attained by stacking the graphene films and carrier doping with an ionic liquid. The optical transmittance of the carrier-doped three-layer stacked graphene film was greater than 90%, enabling it to be embedded in highly transparent objects without altering their landscape. Using the monopole antenna structure with a metal ground plane, we measured the reflection and radiation characteristics of the graphene monopole antenna, excluding the contribution from the power feeding components. The radiation efficiency of the graphene monopole antenna, which was measured by the Wheeler cap method, was determined to be 52.5% at 9.8 GHz. Through the measurements of the graphene monopole antenna, we demonstrated that the carrier-doped three-layer stacked graphene film can be used as an electrode material for optically transparent antennas.

The contact properties between metal and monolayer chemical vapor deposition (CVD) graphene were investigated. Coplanar waveguides (CPWs) composed of CVD graphene‐based signal lines and Au‐based ground lines were fabricated. The S‐parameters of the CPWs were experimentally measured from 1 to 15 GHz. The contact properties were analyzed using a lumped component‐based equivalent circuit model, which consists of the experimentally determined parallel contact resistance and the parallel contact capacitance. The calculated S‐parameters of the model roughly agreed with the measured ones, which indicated that the model was suitable for analyzing the contact properties between metal and graphene up to 15 GHz. Because the impedance of the contact capacitance (4.8 × 10⁻³ Ω) was four orders of magnitude lower than that of the contact resistance (50 Ω) at 15 GHz, the current flow was more capacitive and efficient than that in the DC band. The ratio of power consumption and power storage in the microwave band to the total power consumption in the DC band decreased with increasing frequency and increasing contact capacitance. Therefore, higher contact capacitance is preferable in designing microwave devices with a metal/graphene‐based feeding structure, such as antennas and transmission lines.

The radiation properties of a graphene‐based optically transparent half‐wavelength dipole antenna in microwave bands was investigated. The resonance frequencies of a reference Au antenna and a graphene antenna having the same antenna structures were 9.5 and 21.6 GHz, respectively, owing to the difference in conductivity. Despite the low conductivity of graphene, the gain of the graphene antenna was 1 dB higher than that of the Au antenna. We also showed the feasibility of using graphene‐based optically transparent antenna through the measurements of transmission characteristics between two graphene antenna.