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The concept of photonic crystals was born in the late 1980's when two important letters were published that showed the possibility to control light propagation by a periodic structure. A photonic crystals consists of two or more materials with different dielectric functions periodically arranged on the length scale of light. If the conditions are favorable, a gap will open in the dispersion relation, often called photonic band structure, and electromagnetic waves with frequency in the gap range cannot propagate through the photonic crystal. In this thesis, mainly two types of structures and their properties have been numerically investigated: two-dimensional structures that are either square or triangular. In the calculations, both dielectric and polaritonic materials have been used. Polaritonic materials have an interval of high reflectance in the IR range, due to strong lattice resonances. Within such an interval, the real part of the dielectric function is negative, which causes a metal-like behavior. A polaritonic material, BeO has been introduced in photonic crystals to study the coexistence of structure and polaritonic gaps. Band structures and for some cases transmission spectra have been calculated to study the existence of complete gaps, i.e. energy intervals in which an incoming electromagnetic wave is totally reflected regardless of polarization and angle of incidence. A brief discussion on signature management and thermal emission, and calculations for low-emittance coatings is included. It is shown that a 50-60µm layer of a 3D photonic crystal can be sufficient to achieve a thermal emittance of 20%.

Calculations of the electronic properties of AIN, GaN, InN and their alloys are presented. Initial calculations are performed using the first principles pseudopotenial method to obtain accurate lattice constants. Further calculations then investigate bonding in the nitrides through population analysis and density of state calculations, the empirical pseudopotential method is also used in this work. Pseudopotentials 'or each of the nitrides are constructed using a functional form that allows strained material and alloys to be studied. The conventional k,p valence band parameters for both zincblende and wurtzite are obtained from the empirical band structure using two different methods. A Monte-Carlo fitting of the k.p band structure to the pseudopotential data (or an effective mass method for the zincblende structure) is used to produce one set. Another set is obtained directly from the momentum matrix elements and energy eigenvalues at the centre of the Brillouin zone. Both methods of calculating k.p parameters produce band structure in excellent agreement with the original empirical band calculations near the centre of the Brillouin zone. The advantage of the direct method is that it produces consistent sets of parameters, and can be used in studies involving a series of alloy compositions. Further empirical pseudopotential method calculations are then performed for alloys of the nitrides. In particular, the variation of the band gap with alloy composition is investigated, and good agreement with theory and experiment is found. The direct method is used to obtain k.p parameters for the alloys, and is contrasted with the fitting approach. The behaviour of the nitrides under strain is also studied. In particular, valence band offsets for nitride heterojunctions are calculated, and a strong forward-backward asymmetry in the band offset is found, in good agreement with other results in the literature.

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Using time-resolved photo-induced reflectivity, we reported for the first time a systematic work on the ultrafast response of Bi₂Sr₂CaCu₂O_8+δ (BSCCO-2212) and Tl₂Ba₂CuO_6+δ (TBCO-2201), measurements of detwinned YBa₂Cu₃O_7-δ (YBCO-123) single crystal with the electric field ̲E parallel to the a and b-axis and high-resolution measurements of the rising edge dynamics of YBCO-123 thin films. We identified similar photo-induced responses for BSCCO, TBCO and for YBCO with ̲E ⊥ ̲b, which indicates that we observed a universal response of HTSC coming from the CuO2 superconducting planes. This latter dynamics is composed of three different components corresponding to the superconducting, pseudogap and normal state. A bi-molecular model has been put forward to explain the linear temperature dependence of the decay rate; the model implies that the re-formation of the condensate is limited by the rate at which quasiparticle interact. Moreover, we observed superconducting fluctuations up to 13K above T_c and a divergence of the long-lived component magnitude at very low temperature, which is explained by a cw heating model. In the pseudogap state, we have several indications that the negative peak observed between T_c and T* has a different origin from that of the superconducting signal below T_c. We argued that the probe mechanism of the pseudogap signal is electronic excitations of the pseudogap correlations. In the normal state, the observed dynamics is similar to that of simple metals. In the second part of this thesis, the rising edge dynamics of YBCO has been resolved in time. The model developed to interpret the results implies that the hot quasiparticles relaxation time down to the Fermi energy is 55fs. In this context, we proved that the Mazin model cannot explain both the oscillatory and the non-oscillatory part of the dynamics in YBCO. Finally, in YBCO-123, a new response has been observed with ̲E ∥ ̲b. We argued that the origin of this component is intraband transitions. This dynamics is solely responsive to the pseudogap, coming from the difference in scattering rate between pre-formed pairs and quasiparticles in the Drude reflectivity. The response with ̲E ⊥ ̲b exhibits a strong a-b plane anisotropy in its long-lived component, which can be interpreted as a d-wave gap symmetry using the thermally-activated model.

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CdSe thin films were deposited by thermal evaporation and e-beam evaporation techniques on to well cleaned glass substrates. Low dose of boron have been implanted on a group of samples. EDAX and X-ray patterns revealed that almost stoichiometric polycrystalline films have been deposited in (002) preferred orientation. An analysis of optical measurements revealed a sharp increase in absorption coefficient below 700 nm and existence of a direct allowed transition. The calculated band gap was around 1.7 eV. The room temperature conductivity values of the samples were found to be between 9.4 and 7.5x10-4 (&amp / #937 / -cm)-1 and 1.6x10-6 and 5.7x10-7 (&amp / #937 / -cm)-1for the thermally evaporated and e-beam evaporated samples respectively. After B implantation conductivity of these films increased 5 and 8 times respectively. Hall mobility measurements could be performed only on the thermally evaporated and B-implanted e-beam evaporated samples and found to be between 8.8 and 86.8 (cm2/V.s). The dominant conduction mechanism were determined to be thermionic emission above 250 K for all samples. Tunneling and v variable range hopping mechanisms have been observed between 150-240 K and 80- 140 K respectively. Photoconductivity &amp / #8211 / illumination intensity plots indicated two recombination centers dominating at the low and high regions of studied temperature range of 80-400 K. Photoresponse measurements have corrected optical band gap measurements by giving peak value at 1.72 eV.

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