A key requirement in achieving effective interaction between the cavity mode and the QD exciton is a high degree of spatial overlap: the QD position needs to be within 100 nm of the anti-node of the cavity electric field. To date this has proven particularly challenging and work has relied on chance to achieve alignment between randomly positioned QDs in high-density samples and cavity modes. Such a probabilistic approach results in poor yields (<1%) and is thus unsuitable for commercial implementation. Furthermore, for true single-photon emission it is important to excite an optical cavity without emission from nearby QDs, necessitating low QD densities. The ability to register the position of a single QD is therefore important for achieving accurate QD alignment with a cavity mode.

In order to address this we have implemented a novel photolithographic technique that enables us to register the position of a particular QD before the photonic crystal cavity is written around it. The registration process involves the fabrication of alignment markers at cryogenic temperatures by using two-photon absorption laser photolithography in a photoresist (SU-8). Advantages of this technique are that it does not rely on chance (i.e. it is suitable for low-density QD samples) and it allows the selection of QDs with the desired properties (e.g. wavelength and lifetime). Also, unlike other approaches this technique can be performed with the QD in situ without the need for any active positioning (e.g. using stacked QD growth). By using piezoelectric transducers to control our objective we have been able to register the position of a QD to within 50 nm. Work is in progress to improve the Q of photonic crystal cavities written around these QDs.