Prof Michael Johnston

Abstract:

The spectral width of terahertz emission from ion-implanted terahertz emitters increases with ion damage, owing to ultrafast carrier capture. Carrier dynamics simulations reinforce these findings. Optical-pump, terahertz-probe experiments confirm the subpicosecond lifetimes of these materials. © 2006 Optical Society of America.

Abstract:

The terahertz radiation emitted from Fe+ ion-implanted InGaAs surface emitters and InP photoconductive switches was measured. We experimentally observe an increase in the spectral width of terahertz radiation at greater ion damage, which we attribute to the ultrafast capture of photoexcited carriers. Results from a three-dimensional carrier dynamics simulation support this explanation.

Abstract:

We have experimentally measured the terahertz radiation from a series of ion-implanted semiconductors, both from the bare semiconductor surface and from photoconductive switches fabricated on them. GaAs was implanted with As+ ions, and InGaAs and InP with Fe+ ions, and all samples were annealed post implantation. An increase in emission power is observed at high frequencies, which we attribute to the ultrafast trapping of carriers. We use a three-dimensional carrier dynamics simulation to model the emission process. The simulation accurately predicts the experimentally observed bandwidth increase, without resorting to any fitting parameters. Additionally, we discuss intervalley scattering, the influence of space-charge fields, and the relative performance of InP, GaAs and InAs based photoconductive emitters.

Abstract:

We have developed a detector which records the full polarization state of a terahertz (THz) pulse propagating in free space. The three-electrode photoconductive receiver simultaneously records the electric field of an electromagnetic pulse in two orthogonal directions as a function of time. A prototype device fabricated on Fe+ ion implanted InP exhibited a cross polarized extinction ratio better than 390:1. The design and optimization of this device are discussed along with its significance for the development of new forms of polarization sensitive time domain spectroscopy, including THz circular dichroism spectroscopy.

Abstract:

We have used differential terahertz spectroscopy to monitor performance degradation in state-of-the-art polymer field effect transistors (pFETs) based on poly[(9,9-dioetylfluorene-2,7-diyl)-co-(bithiophene)] (F8T2). After extended periods of operation holes are trapped in the polymer, increasing the device's threshold voltage. We monitor the trapped charge density using THz spectroscopy, and investigate the device's recovery as trapped holes are thermally removed. Illuminating the devices for a period with above-bandgap photons leads to a change in the terahertz transmission through the device, which is short lived after switching the light off.