Optimization of the generation efficiency of EM-radiation in the THz frequency range. (Added 2019)
Using a novel material called BNA, it is possible to directly generate THz radiation by optical rectification of ultrashort pulses with a centre wavelength of 800-850 nm.
The project involves building a set-up for generation of THz radiation. By modifying the laser beam parameters and centre wavelength it will be possible to optimize the efficiency. The work allows you to carry out practical applications of THz generation
Optimizing the generation efficiency for THz generation in BNA
Kontaktperson: Jörgen Larsson
Electron diffraction studies of lattice dynamics in light elements. (Added 2019)
Ultrafast electron diffraction on the 100 fs timescale is a powerful method of probing rapidly evolving structures. Of particular interest is electron-phonon and phonon-phonon scattering which can be studied below the damage threshold of the material
The work will involve probing lattice dynamics of light elements which can give insights into phonon dynamics in these materials. The experiments will be implemented using the pump-probe technique. In this instance a laser with wavelength in the infrared region will transfer energy to the sample (pump) and ultrashort electron burst will probe the material. The set-up has been developed, so the project will include mounting the samples, carry out experiments and subsequently analyse the results. (Fall 2019- we are about to study phonon-phonon interaction in diamond).
Observing structural changes in light elements following laser excitation
Contact Jörgen Larsson
Lattice dynamics of KDP near the ferroelectric phase transition temperature
The core activity of the group is the investigation of structural phase transition dynamics in real time by means of X-ray diffraction. A material of particular interest is KDP (potassium dihydrogen phosphate). KDP undergoes a ferroelectric phase transition at TC=123 K.Going below the Curie temperature TC the crystal structure changes in a way that each unit cell develops a permanent net dipole moment.Analogous to the magnetic dipoles in a ferromagnet these electrical dipoles align preferably parallel to each other building domains of equal orientation. This type of phase transition is driven by so-called soft phonons, which are phonons in a mode whose frequency converges to zero if the temperature drops and approaches the Curie temperature.The atomic motions associated with the soft phonon correspond to the displacements that accompany the ferroelectric phase transition. So far nobody has “seen” this in real time with atomic resolution.It is our intention to change this.
Your task would be to implement a dynamical model of KDP within the lattice dynamical program package UNISOFT. This basically means defining atomic positions and inter-atomic potentials which can be found in the literature and making sure that this reproduces measured phonon dispersion relations. In a second step you would model the effects of phonons - in particular of those in the soft mode - onto the diffracted X-ray signal using COINS a code developed within the group.The outcome of these simulations basically decides about the setup of upcoming experiments and tells us for what we have to look for.
Contact Jörgen Larsson