Electrons and holes in a semiconductor form hydrogen-atom-like bound states, called excitons. At high electron-hole densities the attractive Coulomb force becomes screened and excitons can no longer exist. BCS theory predicts that at such high densities cooperative many-body effects can at low temperatures induce a bound state, an electron-hole Cooper pair, comparable to an electron-electron Cooper pair in a superconductor. Here we report the observation of preformed electron-hole Cooper pairs in a semiconductor. By measuring stimulated emission from a dense electron-hole gas in ZnO, we have explored both the crossover from the electron-hole plasma to the preformed Cooper-pair regime, and the crossover from the exciton to the preformed Cooper-pair regime.

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We study non-linear absorption of intense monochromatic light through a dense natural rubidium (Rb) vapour. We measure transmission through a 10 cm long heated vapour cell for atom densities up to 3 × 1019 m−3 and saturation parameters up to 104, for linear and circular polarisation, close to resonance on the 87Rb D2 F = 1 → F′ = 0, 1, 2 transition. The strong absorption at low intensity is frustrated by an interplay of optical non-linearities (saturation and optical pumping) and non-linear effects due to the high atom density (collisional broadening and collisional depumping). To understand the results of the transmission measurements, we developed a model that incorporates these non-linear effects into the optical absorption. The model takes into account the absolute line strengths of all transitions from both hyperfine levels of the ground state of both isotopes of naturally abundant Rb. Doppler and collisional broadening are included in the Voigt profiles for the resonances. We show the effect of each of the non-linear processes on the calculation results of the model, and from comparison with experiment we conclude that all non-linear effects are necessary for a quantitative agreement.

The front cover shows a microscope image of a bridge waveguide, with tapered launchpads at each side. The launchpads contain the surface grating couplers designed in chapter 2 of this thesis. The freestanding, 200 nm wide and 100 µm long bridge was fabricated using e-beam lithography in a 220 nm thin silicon nitride membrane by Andries Lof at the Amolf Nanolab Amsterdam. The image is taken in the Fourier microscope setup shown in figure 2.5, with white light illumination using an LED. The back cover shows an image of the same bridge waveguide, which is mirrored to represent the bottom view of the sample. It is best viewed by holding the thesis face up and lifting it over your head to investigate the 'sample' from underneath. For this image, the bottom launchpad is illuminated with laserlight with a wavelength of 780 nm under an angle of 3 • from normal incidence. The light is coupled into the membrane and funneled into the waveguide bridge. A bright spot is visible at the waveguide bridge entrance due to enhanced scattering. The light then travels through the bridge and is coupled out at the top launchpad. The image demonstrates that light can be coupled into the nanophotonic waveguide bridge using surface grating couplers under near-normal incidence.

For future atom–photonic devices, the incoupling method is expected to become the main bottleneck in the scaling of the number of devices, as current devices use side-coupling, which is restricted to the sample edges. This paper describes how light from above is coupled directly from free space into a freestanding membrane of 220 nm thin silicon nitrite ( Si 3 N 4 ) with surface grating couplers. The design for these couplers is modeled, captured in an elegant design formula, and experimentally tested. A coupling efficiency of 12% makes experimental coupling to an array of atom–photonic devices possible. The elegant design formula makes grating design straightforward for all-dielectric materials and wavelengths. For this formula, determination of the correct effective index of the specific waveguide mode inside the grating layer is essential.