A coherent absorption dip in pump-probe experiment performed on a ten layer optically thin InGaAs∕GaAs quantum dot (QD) structure has been observed. Measurements performed for different wavelengths, polarizations, pulse widths, and temperatures allow assigning the dip to electromagnetically induced transparency (EIT). The EIT scheme is based on coupling of excitons with different spins in asymmetric QDs. Using spectrally narrow pulses, detrimental effect of the inhomogeneous broadening is reduced since only the QDs with transitions resonant with the pulse wavelength are addressed and participate in the EIT. The effect has been observed at temperatures up to 140K.

We present site-controlled low density InAs quantum dots grown by molecular beam epitaxy with a template based overgrowth technique allowing enlarged buffer layers upto 55 nm. Growing a seeding layer of InAs quantum dots in etched holes reduces closing of the holes, so that a second layer of InAs quantum dots can be aligned to the holes after a buffer layer overgrowth. Confocal microphotoluminescence measurements show a significant decrease of the low temperature photoluminescence linewidth of the quantum dots to an average value of ∼500 μeV and a minimum width of 460 μeV. This is to be compared to 2 to 4 meV of quantum dots grown on thin buffer layers. This improvement is due to the enlarged distance to residual defects at the overgrown surface.

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