We investigate theoretically the influence of type and density of background carriers in the active region on the quantum efficiency of InGaN-based light emitters using an extension of the ABC rate model. A method to determine experimentally whether a certain type of Auger recombination is relevant in InGaN quantum wells is derived from these considerations. Using this approach, we show that the physical process which is the dominant cause for the efficiency droop is superlinear in the electron density and can thus be assigned to nnp-Auger recombination.

Photoalignment refers to a technique for the definition of the orientation of liquid crystals using light. The approach is based on the use of materials that either absorb or trigger a chemical reaction with an anisotropy that depends on the polarisation of light. Azobenzenes are a wellestablished for aligning liquid crystals in research applications. Despite their popularity, only few details of the process are publicly available. We consider here the optimisation of the photoalignment process using the dichroic dye Brilliant Yellow. Its solubility in various solvents; the alignment quality as a function of exposure time and intensity; the influence of humidity at various stages; and the stability of the process in the presence of environmental light are investigated. Additionally, the dichroic dyes Congo Red and Chrysophenine are tested to increase the portfolio of available photoalignment materials.

We present a compact tunable Fabry–Pérot filter operating at mid-infrared wavelengths (3–5 µm) with novel actuators based on liquid crystal elastomers (LCEs). These are a new class of shape-memory materials with potential for achieving large reversible actuation based on different external stimuli. For actuating the Fabry–Pérot filter, microheaters and thermo-actuated LCE layers were fabricated as spacers between two Bragg mirrors allowing a controlled change of the air-filled resonator. The ability to integrate LCEs with standard microfabrication techniques was thus demonstrated. The full-width at half maximum of the filter transmission peak is 143 nm at 4.58 µm. A tuning range of 260 nm in the second interference order is realized by applying a maximum voltage of 7.3 V.

Lasing and self-frequency doubling are achieved in a millimeter-sized laser-active whispering-gallery resonator made of neodymium-doped lithium niobate. A low-cost 808-nm laser diode without external frequency stabilization is sufficient to pump the neodymium ions. Laser oscillation around 1.08 μm drives a frequency-doubling process within the same cavity providing green light. The electrical-optical efficiency of the system reaches up to 2×10. To the best of our knowledge, this is the first demonstration of combining lasing and χ frequency conversion in a single high-Q whispering-gallery resonator. This approach is general and can be applied to other materials and other nonlinear optical processes.

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