John Erland

Designing finite-height two-dimensional photonic crystal waveguides

Guidelines for designing planar waveguides based on introducing line-defects in two-dimensional photonic-crystal slabs are obtained by comparing calculations on two-dimensional structures with dispersion relations for the media above and below the slab.

Observation of propagation of surface plasmon polaritons along line defects in a periodically corrugated metal surface.

Propagation of surface plasmon polaritons (SPPs) excited in the wavelength range 720-830 nm at a corrugated gold-film surface with areas of 150-nm-wide and 45-nm-high scatterers arranged in a 380-nm-period triangular lattice containing line defects is investigated by use of near-field optical microscopy. We demonstrate that the SPP at 740-750 nm propagates along 2.2-microm -wide and 16-microm -long line defects with ~50% loss, whereas its propagation along narrower line defects is strongly damped and in periodically corrugated areas is inhibited. We observe significant deterioration of these effects for both longer and shorter wavelengths and conclude that the SPP guiding occurs as a result of the SPP bandgap effect in the structures.

Second-harmonic imaging of semiconductor quantum dots

Resonant second-harmonic generation is observed at room temperature in reflection from self-assembled InAlGaAs quantum dots grown on a GaAs (001) substrate. The detected second-harmonic signal peaks at a pump wavelength of ∼885 nm corresponding to the quantum-dot photoluminescence maximum. In addition, the second-harmonic spectrum exhibits another smaller but well-pronounced peak at 765 nm not found in the linear experiments. We attribute this peak to the generation of second-harmonic radiation in the AlGaAs spacer layer enhanced by the local symmetry at the quantum-dot interface. We further observe that second-harmonic images of the quantum-dot surface structure show wavelength-dependent spatial variations. Imaging at different wavelength is used to demonstrate second-harmonic generation from the semiconductor quantum dots.

Second harmonic spectroscopy of semiconductor nanostructures

Summary form only given. Semiconductor nanostructures and their application to optoelectronic devices have attracted much attention recently. Lower-dimensional structures, and in particular quantum dots, are highly anisotropic resulting in broken symmetry as compared to their bulk counterparts. This is not only reflected in highly anisotropic linear polarization properties, as studied recently in pyramide-shaped self-assembled InGaAs quantum dots, but also in second harmonic generation (SHG), which can be greatly enhanced allowing for detailed studies of such structures. SHG has contributed considerably as a technique to investigate solid state systems where the local inversion symmetry is broken by e.g. a surface or an interface, defect states or simply by structures so small that the bulk symmetry properties no longer are valid. Our idea is to use SHG in the configurations, where the bulk and surface contributions are forbidden for a homogeneous sample, so that the only source of SHG is associated with nanostructures embedded in the host material.

Designing finite height photonic crystal waveguides; confinement of light and dispersion relations

Guidelines are obtained for characteristic design parameters of finite-height photonic crystal waveguides using diagrams of photonic bandgaps for infinite-height photonic crystals. This is achieved by requiring photonic crystal designs with bandgaps well below a fundamental upper frequency limit for leakage-free guidance of light related to the properties of the media above/below the finite-height photonic crystal waveguide. The approach has the advantage that it can be applied to a large number of diagrams for infinite-height crystals that are already available in the literature, and furthermore the approach is not computer intensive compared to more rigorous numerical approaches to three-dimensional structures. We consider optical waveguide designs based on introducing a line defect in photonic crystals with air holes arranged on a triangular lattice in a silicon slab. For the media above/below the slab we consider the choices of silica and air. Dispersion relations are calculated for various waveguide designs. The analysis reveals a complex distribution of bands related to guided modes and provides information on how the guidance properties are modified as the waveguide width is changed. Furthermore, the analysis reveals the existence of bandgaps that are almost omni-directional. For a specific choice of photonic crystal waveguide placed on a silica substrate these bandgaps that have previously been overlooked gives the only possibility of leakage-free bandgap guidance of TM-polarized light.

Ultrafast nonlinear optics in GaAs/AlGaAs quantum wells

By degenerate four-wave mixing experiments in a two-beam geometry, we have investigated the initial coherence and dephasing of quasi two-dimensional excitions and biexcitons in GaAs multiple quantum wells. The dephasing has contributions from phonon scattering, interface-roughness scattering and exciton-exciton scattering. Inhomogeneous broadening and generation of coherent wavepackets play a significant role in the coherent exciton dynamics. The incoherent exciton dynamics, diffusion and recombination, is studied by three-beam transient grating experiments. A significant difference in the interface roughness scattering of coherent and incoherent (thermalized) excitons is observed.

Nonlinear quantum beat spectroscopy of bound biexcitons in II–VI semiconductors

Abstract With the technique of nonlinear quantum beat spectroscopy (NQBS), based on time-integrated, spectrally resolved four-wave mixing, the nonlinearities of biexcitons localized at neutral acceptor sites in CdSe are investigated. The NQBS offers the possibility to distinguish between quantum beats from a three-level system and polarization interference from independent two-level systems. The localized biexciton states are discussed in analogy with excited states of holes in neutral donor complexes.

Exciton dynamics in CdSe

Abstract The time evolution of the adsorption due to exciton-biexciton transitions and the subsequent relaxation, diffusion and recombination of excitons and exciton complexes are investigated in CdSe by differential transmission experiments and by laser-induced grating experiments. Transitions from free excitons and impurity bound excitons as well as direct two-photon absorption (TPA) to the biexciton state are identified. For a quasi-equilibrium situation, a few hundred picoseconds after the excitation, a diffusion coefficient D x = 3.6 cm 2 /s and a recombination lifetime T x = 1500 ps is determined for free excitons at a lattice temperature of 30 K.

Picosecond spectroscopy of exciton-biexciton transitions in CdSe

Abstract We have observed the direct two-photon absorption (TPA) to the excitonic molecule state in CdSe for the first time by a differential transmission technique and estimated the binding energy E b xm = 4.1 meV of the molecule. Excitons bound to impurities strongly enhance the TPA and as well induce two-step transitions to the biexciton state. Out of the time resolved measurements of the induced absorption a lifetime T 1 = 600 ps of the bound exciton is determined.

Seeding of polariton stimulation in a homogeneously broadened microcavity

In time-resolved light emission from a high-quality semiconductor microcavity after pulsed excitation suitable for angle-resonant polariton–polariton scattering on the lower-polariton branch, we find strong evidence for final-state stimulation of this process. The self-stimulated emission, following single-pulse excitation, appears on a fast time scale of only a few tens of ps with a maximum at 15 ps. This is in striking contrast to the photoluminescence decay time of 110 ps observed in the low-density limit. By injection of polaritons into the final state by a seeding pulse, the dynamics and the intensity of this emission can be controlled. The time-resolved data and the density dependences are in agreement with a rate equation model neglecting polarization mixing effects. This model gives a coupling coefficient of bLP,k=0 = 2.4 × 10—9 cm4 s—1 for the stimulated angle-resonant polariton–polariton scattering process.