Terahertz Ultrafast Pulse Shaping

Research Area: Ultrafast Spectroscopy
Status: In progress
Project leaders: Othonos, Andreas
Members: Demetra Tsokkou
George Georgiou
Proposed start date: 2005-01-01
Proposed end date: 2007-12-30
Funded by: Levendis Research Program
University of Cyprus
Funding: 164330.00 EUR

We propose to design and develop a femtosecond pulse shaper system for generating ultrafast terahertz pulses that will be used for time resolved experiments in semiconductors. The project will be carried out in three phases. In the first phase of the project, a programmable control liquid crystal system pulse shaper will be developed. This will allow control of amplitude and phase of the outgoing ultrafast laser pulse. In the second phase of the project we will use the pulse shaper apparatus to develop and study terahertz generation based on photoconductive switches. The objective here is to develop and control THz radiation with the best efficiency and highest temporal resolution possible. Using the complete electric field as feedback, arbitrary laser pulse shapes can be optimally generated in two different ways. First, a local convergence algorithm can be used to apply reliable and accurate spectral chirps. Second, an evolutionary algorithm can be employed to reach specific temporal profiles. This pulse shaper will serve as the source for terahertz generator based on photoconductive switch using low temperature grown GaAs. After the generation of ultrafast terahertz pulses, we will investigate in details the pulse shaping effect on the terahertz pulses. Then, we will use this novel system to study and investigate semiconductors in the terahertz range.

Description:

Since the development of the laser, before four decades, there has been a great deal of interest in the quest to generate extreme short laser pulse in the picosecond and femtosecond range (10-15 sec). Over the last several years the progress in this direction has been spectacular. Pulses as short as few fs, that is only a few optical cycles in duration are now available from mode-locked titanium sapphire lasers. The widespread access to femtosecond laser systems has accelerated the already considerable interest in this technology for studies of ultrafast phenomena in solid state materials, generation and investigation of high density plasmas, for fundamental studies of extremely high intensity laser-matter interaction as well as for characterization of high-speed electronic and optoelectronic devices and systems, optical communications, chemical and biological materials, and other applications.

Femtosecond lasers constitute the world’s best pulse generators. However, many applications require not only optical pulse generators, but also ultrafast optical waveforms generators, in analogy with electronic function generators. Typically an input pulse from a femtosecond laser is directed to a pulse-shaping set-up, which reshapes the input pulse into the desired waveform. The sharpest features in the reshaped waveform may have duration comparable to the input pulse (fourier limited transform) duration. One may think of the new waveform as consisting of a multiplicity of such features pasted together, with each feature independently controllable in phase as well as amplitude.

The project will be carried out in three phases as described in the next few paragraphs. In the first phase of the project we will develop a programmable pulse shaper system. In the second and most elaborate part of the project we will develop a Terahertz-radiator using a Hertzian dipole antenna embedded in a charged coplanar line. The Terahertz radiation will be controlled using the programmable pulse shaper and in the final phase of the project we will develop a pump-probe system for transient measurements-characterization with sub-picosecond resolution.

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