Effects of various laser focusing conditions on the efficiency of photoconductive antenna arrays
We report on the fabrication and testing of photoconductive antenna (PCA) arrays. We studied the effects of laser focusing conditions using tightly and loosely focused laser beams. In the tightly focused condition, an aspheric lens was used to generate a spherical beam spot that excites only a single dipole in the PCA array. Meanwhile, a cylindrical lens was used to generate a loosely focused laser line in order to excite multiple dipoles in the PCA array. The loose focusing of the PCA arrays lead to 9 orders of magnitude higher efficiency than the tightly focused laser condition. It was observed that increasing the number of illuminated dipoles in the PCA array results to increased THz emission. The 50 micron PCA was observed to have comparable THz emission and power regardless of focusing condition. However, under loosely focused laser conditions, the 50 micron PCA array was more efficient than the 200 microns PCA array.