Prof Michael Johnston

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.

Abstract:

A semi-classical Monte Carlo model for studying three-dimensional carrier dynamics in photoconductive switches is presented. The model was used to simulate the process of photoexcitation in GaAs-based photoconductive antennas illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analyzed the power and frequency bandwidth of THz radiation emitted from these devices as a function of bias voltage, pump pulse duration and pump pulse location. We show that the mechanisms limiting the THz power emitted from photoconductive switches fall into two regimes: when illuminated with short duration (<40 fs) laser pulses the energy distribution of the Gaussian pulses constrains the emitted power, while for long (>40 fs) pulses, screening is the primary power-limiting mechanism. A discussion of the dynamics of bias field screening in the gap region is presented. The emitted terahertz power was found to be enhanced when the exciting laser pulse was in close proximity to the anode of the photoconductive emitter, in agreement with experimental results. We show that this enhancement arises from the electric field distribution within the emitter combined with a difference in the mobilities of electrons and holes. © 2005 The American Physical Society.