Search

Photoconductive materials are currently used to design antennas that radiate in the terahertz band. In this master thesis is proposed to study the design of different types of antennas and feeding mechanisms to increase the radiation efficiency., A model to simulate the dynamics of the electrons/holes pairs when they are generated in a semiconductor material by the application of an optical field and accelerated by a bias electrostatic field is presented. This model helps in the estimation of the photocurrent induced by the mobility of carriers to get a prediction of the THz power provided by a set of different prototypes of photoconductive antennas that have been designed and discussed.

Organic materials are suitable for light sensing devices showing unique features such as low cost, large area, and flexibility. Moreover, transparent photodetectors are interesting for smart interfaces, windows, and display-integrated electronics. The ease of depositing ultrathin organic films with simple techniques enables low light absorbing active layers, resulting in the realization of transparent devices. Here, we demonstrate a strategy to obtain high efficiency organic photodetectors and phototransistors based on transparent active layers with a visible transmittance higher than 90%. The photoactive layer is composed of two phases, each a few nanometers thick. First, an acceptor polymer, which is a good electron-transporting material, on top of which a small molecule donor material is deposited, forming noncontinuous domains. The small molecule phase acts as a trap for holes, thus inducing a high photoconductive gain, resulting in a high photoresponsivity. The organic transparent detectors proposed here can reach very high external quantum efficiency and responsivity values, which in the case of the phototransistors can be as high as âˆ1⁄474000% and 340 A W-1at 570 nm respectively, despite an absorber total thickness below 10 nm. Moreover, frequency dependent 2D photocurrent mapping allows discrimination between the contribution of a fast but inefficient and highly spatially localized photoinduced injection mechanism at the electrodes, and the onset of a slower and spatially extended photoconductive process, leading to high responsivity

Graphene, owing to its ability to support plasmon polariton waves in the terahertz frequency range, enables the miniaturization and electrical tunability of antennas to allow wireless communications among nanosystems. One of the main challenges in the characterization and demonstration of graphene antennas is finding suitable terahertz sources to feed the antenna. This paper characterizes the performance of a graphene RF plasmonic antenna fed with a photoconductive source. The terahertz source is modeled and, by means of a full-wave EM solver, the radiated power as well as the tunable resonant frequency of the device is estimated with respect to material, laser illumination, and antenna geometry parameters. The results show that with this setup the antenna radiates terahertz pulses with an average power up to 1 µW and shows promising electrical frequency tunability., Peer Reviewed, Postprint (author's final draft)

In this work we present a two-dimensional (2D) model for an organic thin film photo-conductive sensor containing a planar heterojunction and in-plane electrodes. The model simulates the flow of charge carriers based on the standard one-dimensional semiconductor transport and continuity equations, and combines this with a 2D model for the electric field. This procedure results in a hybrid differential/integral equation formulation. We present and analyse simulation results that resemble very well measured current-voltage characteristics of a real sensor under different illumination levels. We find that for currents below a critical value the sensor behaves like a resistor. Above this critical current the current increases much more slowly due to space charge accumulation close to the cathode. We explain the critical current as the maximum reverse current of the solar cell formed by the heterojunction covering the cathodic electrode. © 2014 IOP Publishing Ltd.

The phenomenon of image blurring on laser-printed or electro-photocopied paper has been discovered since the 1980s. In the 1990s, the problem was confirmed to be associated with the undesired surface conduction along the unique photoconductive polymer surface during the photoconduction process. Other than this, little progress has been made in investigating this phenomenon, due to the limited experimental techniques. In this thesis, the electrical properties of a commercially available photoconductor as a result of Corona charging were studied. Various techniques including vacuum deposition and step-function impedance spectroscopy were employed, to overcome the nature of the photoconductor that prevented the use of conventional techniques such as AC impedance spectroscopy. Negative dielectric constant (NDC) has been prevalently discovered at a broad range of frequencies (below 1Hz and up to 1 MHz) and it was questioned in the form of a physically-impossible inductor. This precipitous sign switch of dielectric constant is found in various areas ranging from physics, chemistry, biology to electronics. The magnitude of the NDC decreased drastically with the decrease of electric field frequency. The system obeyed the proposed free-carrier plasma model with a resonance frequency at MHz level. Commercially available polymeric photoconducting materials showing NDC at extremely low frequency are expected to provide unusual scattering to electromagnetic waves and therefore demonstrate profound implications with reduced cost. It has paved the way for many applications such as inductors in integrated chips without bulky coils and provides an insight into a possible revolution in electronics and photonics., Doctor of Philosophy (PhD)

We demonstrate subpicosecond photocarrier lifetimes at 1.55 mum in GaSb/ErSb nanoparticle superlattices grown by molecular beam epitaxy. Pump-probe measurements were made with a 1.55 mum mode-locked laser in transmission geometry to determine the photocarrier lifetime. The lifetime is found to be dependent on the size of the ErSb particles, amount of ErSb, and the distance between layers of particles. Through manipulation of these three parameters the photocarrier lifetime can be tuned down to less than 300 fs, the temporal limit of our experiment. (C) 2004 American Institute of Physics.

We demonstrate subpicosecond photocarrier lifetimes at 1.55 mum in GaSb/ErSb nanoparticle superlattices grown by molecular beam epitaxy. Pump-probe measurements were made with a 1.55 mum mode-locked laser in transmission geometry to determine the photocarrier lifetime. The lifetime is found to be dependent on the size of the ErSb particles, amount of ErSb, and the distance between layers of particles. Through manipulation of these three parameters the photocarrier lifetime can be tuned down to less than 300 fs, the temporal limit of our experiment. (C) 2004 American Institute of Physics.

The optical processor proposed here consists basically of a coherent light source and two liquid crystal light valves, of which the first converts incoherent input patterns into coherent ones; the second operates entirely on coherent signals. It is shown that, if the valve operating point is correctly chosen and its transfer characteristic linearized, negative feedback is implemented around the second valve. The valves perform as active system components, because they modulate the source power by their driving signals. The linear dynamic range, modulation transfer function, and time transient behaviour of the system are briefly studied