The junior research group „Silicon to Light“ focuses on developing efficient micro light sources which are based on or compatible with silicon. Efficiency limiting factors are analyzed and various concepts of nanostructuring and material modification in the nanoscopic range are employed in order to overcome them.
The eventual goal is an electrically pumped silicon based laser with tunable emission wavelength. Such a light source has the potential to revolutionize today's silicon based electronic chips by employing fast light pulses instead of the conventional and slow electrical pulses as the path of communication between different components of the chip. This would enable a new generation of ultrafast computer processors.
Several strategies are pursued in order to achieve efficient light emission and nonlinear optical frequency conversion in silicon based materials:
Enhanced NIR-Luminescence for Silicon photonics
When defects are deliberately introduced in photonic crystals, microresonators are formed, which lead to a confinement of light on a very small volume. Due to the Purcell-effect this causes the acceleration and enhancement of the luminescence of the material inside the resonator. Here a 2D-photonic crystal is shown, which consists of a hexagonal pore array with pore distances on the order of the wavelength of light. The non-existent pore in the middle represents the microresonator. The material contains Ge-quantum dots which emit in the NIR within a range of 1200-1600nm wavelength. In the photoluminescence spectrum the sharp resonance of the microresonator is observed, which leads to a ca. ten-fold enhancement of the luminescence.
A similar enhancement of the luminescence can be achieved by the periodic arrangement of metal nano particles. Neighbouring particles couple due to their scattering fields which leads to a mutual enhancement of the particle plasmon oscillation within a more narrow spectral range. Quantum dots, which are placed in the vicinity of these particles experience then an enhanced luminescence. Here the spectra of PbS-quantum dots are shown which were immersed in a square lattice of gold discs. The period of the disc lattice was 900nm, the diameters of the discs varied. The enhancement of the luminescence is especially visible for the larger gold discs.
Creation of second order optical nonlinearity in silicon
Silicon is a centrosymmetric material and therefore does normally not show a second order susceptibility (c(2)). However introducing an inhomogeneous strain into the silicon lattice a reduction of symmetry is achieved, so that a (c(2)) can be created. This is achieved by the deposition of a stressed cover layer. Left: Here a Si-waveguide is shown which contains a stressed SiO2-cover layer. Right: Due to the partial relaxation of the cover layer to the sides, the underlying Si-lattice is stretched and an overall V-shaped deformation of the lattice is created. In this range the centro-symmetry of the Si-crystal is broken.
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