Investigation Of Photoconductive Terahertz Antenna

Terahertz (THz) science and technology has great scientific value and can beemployed in a wide areas, in which the key technology is the generation of THz waves.Nowadays the photoconductive antenna is one of the most important ways to generateTHz waves. One of the major problems of the current photoconductive antennas is thatthe antenna efficiency is very low, thus it is difficult to obtain high power THz waves.To design terahertz (THz) photoconductive antennas with high output power and highoptical-THz conversion efficiency, the major works are as follows:1. Theory research on photoconductive antenna generating THz waveFirstly，a comparison of the major differences between conventional microwaveantennas and photoconductive antennas is given. The total efficiency(Optical-THzconversion efficiency), power and time variant resistance of a photoconductive antennais investigated. The total efficiency is the product of three efficiencies: laser-to-electrical,impedance matching and radiation efficiencies. The dominant and worst efficiencyseems to be the laser-to-electrical conversion efficiency and the impedance matchingefficiency is also low. The time variant resistance can aid to better comprehend the totaloptical-THz conversion efficiency.Secondly，a THz photoconductive antenna to the current instantaneous impulsemodel is established, according to Maxwell’s electromagnetic theory derived THznear-field and far field expressions. According to the mathematical model, numericalsimulation, respectively，and a detailed analysis of the pulse width, photoconductiveantenna material carrier lifetime, carrier relaxation time on the THz wave effect.2. An electromagnetic simulation software(CST Microwave Studio) is employed toaid the photoconductive antennas design and improvingFirstly，effects of substrate thickness on the radiation pattern and power ofphotoconductive terahertz antennas is investigated. The results show that most of theradiated power is radiated towards the substrate side rather than the free space, and athin substrate is preferred to maximize the radiated power. Secondly，a structure integrates a conical horn antenna or hyperhemisphericalSilicon lens to a photoconductive antenna is introduced, simulated results show that adirectivity of20.7dBi can be achieved by integrated a conical horn, a directivity ofmore than19dBi can be achieved by integrated hyperhemispherical substrate Siliconlens. So the two methods both can obtain high power THz waves.Finally, reviews and discussions on the main parameters of THz output power toimprove THz photoconductive antennas: several parameters which are related to opticalsources like laser intensity, pulse duration, excitation spot size, location and factorswhich are connected to photoconductive antennas such as bias field, antenna materialand geometry, dark resistivity of antenna and substrate martial have been discussed.The information presented in this paper should be very useful for design andanalysis of the performance of photoconductive antennas.