Study Of THz Emitters Fabricated With Integrated Diffraction Micro-structures

Ultrafast THz wave radiation is not only a forefront field of research but also ofgreat value in many applications, such as bio-medical diagnostics, homeland security,and outer space exploration. Therefore, enhancement of ultrafast THz output power isan urgent task for researchers. This thesis aims to design and use micro-structurediffraction devices fabricated on THz emitters, in order to improve THz output powergenerated by optical rectification. The main contents are described below:First, it is pointed out that THz emitters usually have a relatively high refractiveindex in the THz range, which leads to a large loss of THz power because of Fresnelreflection. In order to solve this problem, sub-wavelength antireflective (AR)structures with different parameters are designed and analyzed using effectivemedium theory and transfer matrix theory.Second, several AR structures of different parameters have been successfullymanufactured on the surface of gallium phosphide (GaP) crystal by fly cutting. Then,we use a typical THz time-domain system to detect both the THz emission andtransmission spectra, and use Golay-Cell to measure the actual THz output power. Itis demonstrated that the AR band becomes wider when the period of the AR structureis smaller, and the AR effect becomes more notable when the depth of the ARstructure becomes larger, both of which agree well with theoretical analysis.Finally, tilted pulse front pumping (TPFP) is one of the most efficient opticalmethods to generate THz wave. However, in current experiment systems, there mustbe an imaging system to focus the pump beam. As a result, distortion and aberrationlead to a lower optical-to-THz conversion efficiency. In this work, we study thefabrication of a transmission grating with a relatively high diffraction efficiency forthe negative first order on the front surface of the THz emitter. A new type of gratingwith a diffraction efficiency larger than90%—buried grating to decrease thereflection loss at the two boundaries between the grating and the incident medium aswell as between the grating and the substrate medium is designed based on a two-mode interference theory.