The generation of terahertz electromagnetic pulses: generation properties and efficiency

• Main Authors: Shi-Hsiang Lu, 呂世香
• Other Authors: Sheng-fu Horng
• Format: Others
• Language: zh-TW
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/15523576152918784339

博士 === 國立清華大學 === 電機工程學系 === 89 === The generation and detection of THz radiation by a sub-picosecond (1 ps = 10e-12 sec) laser source has become a fast growing field since the 1980’s, due to its promising scientific and engineering applications. Yet many efforts are still required to clarify the underlying mechanism of THz radiation and to increase the emitting efficiency and bandwidth. In this dissertation, we investigate three commonly used THz emitters, i.e. photoconductive antennas, semiconductors under magnetic fields, and quantum well emitters. As to investigation of THz emitters, we also proposed a calculation scheme to analyze the THz radiation from quantum wells. Three types of GaAs-based materials, including semi-insulated GaAs (SI-GaAs), low-temperature GaAs deposited on SI-GaAs (LT-GaAs), low-temperature GaAs deposited on sapphire (LT-OS), which represent three crystallinty, are used to be the photoconductors of large-aperture photoconductive antennas (LAPAs). Among these three materials, because of its crystallinity, SI-GaAs has the largest carrier mobility, and hence the largest radiated field, which is ~2.5 times larger than that from LT-GaAs, and ~18 times larger than LT-OS. On the other hand, the lower carrier lifetimes of LT-GaAs and LT-OS induce slightly higher bandwidth in THz radiation. Therefore, there is a trade-off between the bandwidth and the intensity of THz radiating field. For these three materials, the radiating amplitudes are linearly proportional to the laser powers and externally biased electric fields. Moreover, the carrier mobility extracted from measured THz radiation is roughly consistent with the estimated value from photocurrent, time-resolved phototransmission, and electrical pulse measurements. This finding indicates that photoconduction is the dominant mechanism for THz radiation from LAPAs. In additional to conventional dipole antenna structure, we also measured THz radiation from a spiral antenna, and found highly dispersive THz waveforms, which indicate that the antenna structures strongly affect the waveforms of radiated field. The THz radiation from a spiral antenna exhibits obviously different waveforms in comparison with LAPA’s. In addition, by a novel nearly-filled-gap nonuniform illumination scheme, we found very short electrical pulses (FWHM∼190 fs) can be generated on a coplanar transmission line. The measured THz radiation from semiconductors under magnetic field consists of two components, one is from surface depletion field, and another is from carrier motion driven by magnetic field. The surface depletion field plays a role to “trigger” the carrier motion under magnetic field. Therefore, for a semiconductor under magnetic field, the amplitude of radiation field is proportional to surface depletion field. The other factors affecting the amplitude of radiation field include the effective mass and the applied magnetic field. At the same magnetic field, the measured amplitude from several semiconductors, including undoped InAs, n-doped InAs, SI-GaAs and InSb is proportional to their surface depletion field and inversely proportional to their effective mass. The measured THz beam profile indicates that the diverging angle of THz radiation beam from InAs under magnetic field is about 24°, and the radiation beam is not emitted along the specular direction of the pump beam. There is about 6° deviation between it. Therefore, the misalignment of THz beams must be carefully taken into account to utilize this type of emitters. Quantum-well THz emitters are relatively attractive for their adjustable radiating frequency. To design the optimal structure of a quantum well emitter, we propose a general k×p-type multiband transfer-matrix energy-band calculation method, which is efficient and can be generally employed in the calculation of the band structures of two-dimensional semiconductor systems. Strain, electric field, and magnetic field can all be incorporated in a unified manner. Our calculated results are consistent with those reported in the literature. With this multiband transfer-matrix method, the envelope functions as well as the in-plane dispersion relationship can be obtained, and all the remote bands can be included. Therefore, this approach is an appropriate tool for investigating the coupling effect between bands, which is important in the case of low-energy band gap or strong spin-orbit-coupling materials. Based on the calculated results of electronic states, the approach was employed to calculate THz radiation from quantum wells and the results are consistent with the results in the literature. Therefore, it is expected that the calculation approach is useful to analyze and to optimize THz radiation from quantum-well emitters. Moreover, we also measured and analyzed THz radiation from two-dimensional semiconductor structures. For the THz radiation from a p-i-n multiple quantum well, the coupling between wells are weak due to the thick barrier between them; therefore the charge oscillation disappears. In addition, due to the carrier motion interfered by barrier; the applied magnetic field parallel to the layer does almost not affect the THz radiation. The THz radiation from a modulation-doped heterostructure consists of two components; one is from surface depletion field, the other is from the applied magnetic field. This property is similar to some bulk semiconductors.