Terahertz (THz) waves have attracted much attention due to their interesting characteristics, such as low photon energy that can measure intraband transitions of various materials, high penetrability in non-conducting materials without ionizing and a response associated with the vibration energy of many molecules.
In order to utilize THz waves, it is necessary to develop THz components and devices including a generator, a detector, a quarter wave plate, a polarizer for this frequency range. Among them, the THz detector is an essential device for measuring generated THz waves. THz photoconductive antennas (PCAs) are one of the first THz pulse detectors. Although the THz PCAs have been commonly used in many THz research fields, theoretical and experimental studies on different configuration of THz PCAs have not been fully performed for broad spectral ranges.
In the present work, we calculated photocurrents of different PCA shapes with the Drude-Lorentz model and the equivalent time-varying lumped element circuit model and simulated spectra of the detectable THz pulses by a finite integration technique (FIT) via an electromagnetic (EM)-simulator. A silicon lens (Si-lens) was used with the THz PCAs to decrease the significant difference of the refractive index between the air and substrate and the loss of THz pulses. The Si-lens is optimized by adjusting the radius of curvature and the shape through THz ray tracing. Based on the simulated results, we fabricated the THz PCAs consisting of an antenna chip and antenna body. The antenna chip is comprised of metal lines printed by photo-lithography and e-beam evaporation on low-temperature gallium arsenide (LT-GaAs) and attached to the antenna body with wires for connecting an external electric device.
To investigate the characteristics of the designed THz PCAs experimentally, a THz time-domain spectroscopy system based on a mode-locked Ti:Sapphire laser with a pulse duration of 65 fs and a repetition rate of 100 MHz, operating at a wavelength of 800 nm, is used. The measured THz spectra of the different THz PCAs with H-, I- and Bowtie-shapes agreed well with the simulated results. The I- and Bowtie-shaped PCAs could detect low THz frequencies below 0.6 THz with a high sensitivity. On the other hand, the detectable range of the H-shaped PCA was over 3 THz, due to the short dipole length. In addition, we confirmed that the different sizes of Si-lens affect the THz detection efficiency substantially. Therefore, the reliability of the simulation process is confirmed by comparing the simulation and experimental results.
