Ultra-sensitive mid-infrared evanescent field sensors combining thin-film strip waveguides with quantum cascade lasers.

We demonstrate ultra-sensitive chemical sensing in the mid-infrared spectral regime with a combination of quantum cascade lasers (QCLs) with GaAs/Al(0.2)Ga(0.8)As strip waveguides fabricated via metal-organic vapor-phase epitaxy (MOVPE) and reactive ion etching (RIE) using evanescent field absorption spectroscopy. These strip waveguides have been designed with a width of 200 µm, thereby facilitating 2-D confinement and mode-matched propagation of mid-infrared radiation emitted from a distributed feedback (DFB) QCL at a wavelength of 10.3 µm. Acetic anhydride was detected with a limit of detection (LOD) of 18 pL (19.4 ng) deposited at the waveguide surface by overlapping of the vibrational absorption of the methyl group with the emission frequency of the QCL. The obtained results indicate a remarkable enhancement in sensitivity by three orders of magnitude compared to previously reported multimode planar silver halide waveguides. Further reduction of the waveguide strip width to 50 µm resulted in an additional sensitivity enhancement yielding a calculated LOD of 0.05 pL for the exemplary analyte acetic anhydride, which is among the most sensitive evanescent field absorption measurements with a miniaturized mid-infrared sensor system reported to date.

Optical properties of red emitting self-assembled InP/(Al0.20Ga0.80)0.51In0.49P quantum dot based micropillars.

Using focused ion beam etching techniques, micropillar cavities were fabricated from a high reflective AlAs/AlGaAs distributed Bragg reflector planar cavity containing self-assembled InP quantum dots in (Al(0.20)Ga(0.80))(0.51)In(0.49)P barrier layers. The mode spectra of pillars with different diameters were investigated using micro-photoluminescence, showing excellent agreement with theory. Quality factors of the pillar cavities up to 3650 were observed. Furthermore, for a microcavity pillar with 1.26 mum diameter, single-photon emission is demonstrated by performing photon correlation measurements under pulsed excitation.

Low density MOVPE grown InGaAs QDs exhibiting ultra-narrow single exciton linewidths.

Low density (approximately 10(7) cm(-2)), small sized InGaAs quantum dots were grown on a GaAs substrate by metal-organic vapor-phase epitaxy and a special annealing technique. The structural quantum dot properties and the influence of the annealing technique was investigated by atomic force microscope measurements. High-resolution micro-photoluminescence spectra reveal narrow photoluminescence lines, with linewidths down to 11 microeV and fine structure splittings of 25 microeV. High signal to noise ratios (approximately 140) and a nearly background free autocorrelation measurement indicate an excellent optical quality and single photon emission behavior. Furthermore, time resolved measurements reveal excitonic decay times typically in the range between 800 and 2300 ps and biexcitonic decay times around 300 ps.