Terahertz photonics

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.

Abstract:

Research on polymer-based transistors is leading to the development of flexible, printable circuitry, which will be extremely cost effective to manufacture. However, the longterm performance of state-of-the-art polymer field effect transistors (pFETs) is limited by device degradation. We show that terahertz spectroscopy is an ideal tool to probe polymer device performance. Specifically we have monitored charge carrier trapping at the polymer-insulator boundary of a pFET. From these results we show that device degradation is primarily caused by a trapping of holes in the channel of the pFET, rather than by a change in hole mobility.

Abstract:

Ultrafast charge carrier dynamics in serniconductoring materials ultimately determine the performance of photoconductive terahertz (THz) emitters and receivers. Ion implantation of III-V semiconductors allows carrier dynamics to be tailored for a particular application, and thus the technique is increasingly being applied to the development of advanced materials for terahertz photonics. In this talk I will briefly introduce the technique of ion implantation and review some recent applications in THz photonics. I will then present time resolved conductivity studies of GaAs:As+, InGaAs:Fe+, InP:O+ and InP:Fe+ and relate these results to improved terahertz emitter and detector performance.