Optical frequency conversion is important nonlinear process for generating coherent radiation in spectral regions where there are no convenient laser sources. For example, nonlinear processes are... Show moreOptical frequency conversion is important nonlinear process for generating coherent radiation in spectral regions where there are no convenient laser sources. For example, nonlinear processes are used to generate ultraviolet radiation for biomedical applications or mid-infrared radiation in the wavelength range of 3–12 μm for remote sensing of atmosphere. Besides these practical applications, frequency conversion can be used to create entangled photon pairs in quantum optics experiments to test the fundamental laws of quantum mechanics. This thesis describes an experimental investigation of second harmonic generation in III-V semiconductor photonic structures. These nanostructures have both wavelength and subwavelength dimensions. In particular, ensembles of aligned gallium phosphide nanowires and two-dimensional aluminum gallium arsenide photonic crystal slabs with a square lattice of holes are studied. In both cases, the III-V semiconductor material provides a large second-order nonlinearity, while the special arrangement of dielectric material of the nanostructure introduces additional dispersion. This extra dispersion can be used to phase match the nonlinear process in order to make frequency conversion efficient. We demonstrate that it is possible to resonantly couple to leaky modes of a photonic crystal slab at both the fundamental and second harmonic frequency and enhance the second harmonic signal ~10,000 times. Show less
This thesis describes coupling of light to periodic structures. A material is patterned with a regular pattern on a length scale comparable to the wavelength of light. With these structures, light... Show moreThis thesis describes coupling of light to periodic structures. A material is patterned with a regular pattern on a length scale comparable to the wavelength of light. With these structures, light can be manipulated very precisely. The structures find applications in semiconductor lasers, light emitting diodes (LEDs), photovoltaic cells, and detectors of light. A periodic array of holes in a layer of semiconductor or in a thin metal film causes a coupling between the incident light and light that is trapped inside the layer. This coupling can be studied by measuring the reflection and transmission. The environment has an important role here; e.g. placing glass antennas in the holes can increase the coupling between light and plasmons. A thin, superconducting wire can be used as a detector of light. To increase the surface area, the wire is folded into a meander. The optical properties of this detector are very dependent on the polarization, due to the regular periodic structure of the meander. Moreover, we found that the absorption of a very thin absorbing layer can be almost 100%, when it is illuminated under the right angle, from the substrate. This can be used to increase the efficiency of the detectors. Show less
In this thesis we consider several effects of a Dirac spectrum in photonic crystals on the scattering and propagation of light. We calculate the effect of a Dirac point (a conical singularity in... Show moreIn this thesis we consider several effects of a Dirac spectrum in photonic crystals on the scattering and propagation of light. We calculate the effect of a Dirac point (a conical singularity in the band structure) on the transmission of radiation through a photonic crystal. We find that the transmission at the Dirac point is inversely proportional to the longitudinal dimension of the crystal. Further we propose a method to detect the pseudospin-1/2 Berry phase produced by the Dirac-type band structure of a triangular-lattice photonic crystal. In addition we show that the half-integer spin and the associated Berry phase remain observable in the presence of disorder in the crystal: the destructive interference caused by the Berry phase suppresses the reflected intensity at an angle which is related to the angle of incidence by time-reversal symmetry causing extinction of coherent backscattering. We test all our predictions numerically by solving the full Maxwell equations. We also demonstrate that the Goos-Hanchen effect, which has been observed for the first time in optics and has applications in particular in photonic crystals, is of high importance for graphene. We show that the Goos-Hanchen effect in an n-doped channel with p-doped boundaries doubles the degeneracy of the lowest propagating mode, introducing a two-fold degeneracy on top of the usual spin and valley degeneracies. This can be observed as a stepwise increase by 8e^2/h of the conductance with increasing channel width. Show less
