Intense Terahertz Radiation from GaAs Photoconductive Antenna Array

In this paper, we fabricated an intense photoconductive antenna array with eight antenna units. Each unit has separate electrodes and could be operated independently. The spacing between electrodes and the width of the electrodes are 150"m and 10"m, respectively. The substrate is commercial semi-insulating GaAs. The electrode pattern is made using a conven- tional photolithography technique. Ohmic contacts are obtained by using a standard mixture of Ni/Au-Ge/Au for the metallization and the thickness is 700nm. The antenna array is mounted in an integrated circuit shell with ceramic substrate. Those antennas are tested by TDS-Z2 system made in Zomega Terahertz Corporation. A flber laser (IMRA F-100) has been used in the system as the excitation source for the generation of THz radiation. The pump beam has a power of 90mW, wavelength of 800nm and the 1/e beam diameter on the emitter surface is about 20"m, and so only one antenna unit is illuminated in our experiment. The fleld waveform is measured using an electrooptic sampling method employing a 1-mm-thick ZnTe crystal. To understand the behavior of interdigital antenna array, THz transmission through the ceramic substrate is measured. Only 23% of THz wave transmitted through the substrate. If a hole is drilled through the ceramic substrate to make THz wave go through, the THz amplitude of antenna unit will increase more than 4 times, and the THz amplitude from the antenna unit is 4/3 times larger than the THz amplitude from the stripline antenna at the same electrical fleld. 1. INTRODUCTION The development of Terahertz (THz) source is of great importance for a large variety of scientiflc and technological application (1). Recently, various methods for generating terahertz electromagnetic radiation have attracted considerable attention. However, from 1984, Auston flrst demonstrated THz radiation can be generated by femtosecond laser illuminating a biased photoconductive an- tenna (2), this method is always a standard method until now. And photoconductive antenna is widely used as THz source in THz TDS system, which is used in many flelds, such as explosives and chemical detection (3), biology threat detection (4), quality control and non-destructive test- ing (5), material analysis and testing (6) etc. Various antenna geometries have been used, these include Hertzian dipole, resonant dipole antennas, spiral antennas, antenna array, and so on. The dipolar antennas and resonant dipole antennas are the most common structures. In this paper, we fabricated a photoconductive antenna array with interdigital electrodes on commercial SI-GaAs substrate and compared the performance of a antenna unit of the array with a conventional stripline antenna. And the absorption of ceramic substrate of antenna mount of interdigital antenna array is measured. The results indicate that the antenna unit can generate a higher THz wave and the interdigital antenna array is expected to emit THz wave with amplitude more than 10 times higher than that of the stripline antenna at same electrical fleld. 2. EXPERIMENT The photoconductive antenna array with interdigital electrodes is composed of eight photocon- ductive antenna units. The substrate of antenna is commercial high-resistivity (> 107›cm) liquid-encapsulated-Czochralsky-(LEC) grown, (100)-oriented, semi-insulating (SI) GaAs wafer. The schematic of the entire antenna array structure is shown in Fig. 1. Each unit has separate electrodes and could be operated independently. The spacing between electrodes and the width of the electrodes are 150"m and 10"m, respectively. The electrode pattern is made using a conven- tional photolithography technique. Ohmic contacts are obtained by using a standard mixture of Ni/Au-Ge/Au for the metallization and the thickness is 700nm. The antenna array is mounted in an integrated circuit shell with ceramic substrate, and the electrodes are connected with pins by golden wire. The performance of the emitter is compared with a conventional stripline antenna, which has a 100"m gap between electrodes, the substrate is also commercial SI-GaAs and the fabricate processing is same to the interdigital antenna array. Those antennas are tested by TDS-Z2 system made in Zomega Terahertz Corporation. A flber laser (IMRA F-100) has been used in the system as