Optical gating of photoluminescence from color centers in hexagonal boron nitride.

This is the final version. Available from the publisher via the DOI in this record.We report on multicolor excitation experiments with color centers in hexagonal boron nitride at cryogenic temperatures. We demonstrate controllable optical switching between bright and dark states of color centers emitting around 2eV. Resonant, or quasi-resonant excitation of photoluminescence also pumps the color center, via a two-photon process, into a dark state, where it becomes trapped. Repumping back into the bright state has a step-like spectrum with a defect dependent threshold between 2.25 and 2.6eV. This behavior is consistent with photoionization and charging between optically bright and dark states of the defect. Furthermore, a second zero phonon line, detuned by +0.4eV, is observed in absorption with orthogonal polarization to the emission, evidencing an additional energy level in the color center.Engineering and Physical Sciences Research Council (EPSRC

2D Photonic crystal thermo-optic switch based on AlGaAs/GaAs epitaxial structure

The realisation of a thermo-optically controlled symmetric Mach- Zehnder interferometer switch based on an AlGaAs/GaAs epitaxial waveguide structure operating at wavelengths in the region of λ = 1550 nm is reported. The device is based on a very compact two-dimensional photonic crystal channel waveguide structure. The measured and simulated transmission spectra for the devices are in good agreement. The π-phase shift switching power for the device is as low as 42 mW

Schooling and monetary rewards in Greece: an over-education false alarm?

The paper uses data from the 1993-94 household survey to investigate a number of issues stemming from the current educational policy debate in Greece. Private and social returns to investment in education are estimated and a test of the screening hypothesis is conducted. No evidence was found that higher education is being used as a screening device, and the returns to investment in education have been maintained to just below their level in the 1960s. The central finding of the paper is that the fear of further expansion of the university system is highly exaggerated, and that the economy could absorb even larger amounts of highly educated manpower. Of course, such expansion could not take place unless there is an end of the state monopoly on the financing and provision of tertiary education.