| dc.description.abstract | This dissertation is concerned with the excitation of coherent optical phonons by femtosecond laser pulses. Investigation of coherent optical phonons can provide valuable understanding of the structure and phase transitions of a material. Studies have been performed upon two epilayers of cubic crystalline \mathrm{Ge_{2}Sb_{2}Te_{5}}
(GST) grown on InAs(111), and also on the reference InAs(111) sample. Time resolved pump-probe measurements of transient reflectivity and ellipticity response have been performed to excite and observe coherent optical phonons. The phonon dynamics is induced by an intense optical pump pulse and detected by a weak probe pulse with duration as short as 45fs.
Measurements of coherent optical phonons in GST/InAs (111) and InAs (111) samples have been performed as the pump polarisation, the sample orientation and pump fluence are varied. The transient reflectivity and ellipticity signals reveal coherent optical phonons with frequencies of 6.5 THz and \AC
3.4 THz in InAs(111) and the two epitaxial GST/InAs(111) samples respectively. Microscopic and macroscopic theories are presented, that consider the Raman tensor and phonon representation, and predict that both zinc blend and rock-salt structures possess a three dimensional T_{2}
optical phonon mode at the zone centre. This T_{2}
mode is Raman active in the zinc blend structure, while it is Raman inactive in rock-salt structure. A theory of transient stimulated Raman scattering (TSRS) is presented to explain how the amplitude of the coherent optical phonon depends upon the pump and probe polarisation when excitation occurs by a combination of impulsive stimulated Raman scattering (ISRS) and excitation of a surface space charge (SSC) distribution. Comparison of experiment with theory suggests that the 6.5 THz optical phonon observed in the ellipticity signal for InAs(111) has three dimensional T_{2}
character. A coherent optical phonon is observed between 2.9 and 3.4 THz in the ellipticity signal for both GST/InAs(111) samples, and in the reflectivity signal of one of the GST/InAs(111) samples, and is inferred to also possess three dimensional character. Cubic GST is believed to adopt a rock-salt like structure with approximately 20% vacancies of the Ge and Sb sites. The ordering of vacancies within GST, the displacement of ions from their positions in the rock salt structure, and a resulting lack of inversion symmetry results in the \AC
3.4 THz coherent optical phonon being Raman active with the underlying three dimensional T_{2}
–like character. Indeed the observation of the T_{2}
-like phonon mode confirms that the underlying crystallographic structure of GST is essentially cubic.
The TSRS theory predicts that, for a (111) surface of a cubic crystal, the amplitude of the oscillation generated by the phonon within the ellipticity signal should have a \sin(2(\theta-\phi))
dependence upon the orientation of the pump electric field, \theta
, and probe electric field, \phi
, within the plane of the sample. The pump beam is expected to excite a superposition of T_{2x},\,T_{2y}
, and T_{2z}
phonons. For the (111) surface of the zincblend and rocksalt structures, the experiment observations revealed that the ellipticity signals have \sin(2(\theta-\phi))
dependence on pump and probe polarization that is characteristic of the specular optical Kerr effect (SOKE). Hence, impulsive stimulated Raman scattering mechanism (ISRS) and the SOKE are seen to provide equivalent descriptions of the same phenomenom.
The (TSRS) theory predicts that, for a (111) surface of a cubic system, the T_{2x},\,T_{2y}
, and T_{2z}
modes can be observed in both the reflectivity and anisotropic reflectivity signals. However, the degeneracy of these three phonons may be lifted by structural distortion, so that dephasing of modes of similar phase but different frequency leads to a large apparent damping of the phonon oscillations.
The application of high pump fluence to the InAs(111) sample was found to lead to no change in the observed phonon frequency. However exposure of one of the GST/InAs(111) samples to high pump fluence led to the appearance of a new phonon mode, suggesting that a structural change had occurred. | en_GB |
