Optically-active solid-state defects in diamond and silicon carbide (SiC) are promising for the implementation of quantum technologies though use of their optically-addressable and -measurable electronic spin states. Nitrogen vacancy (NV) centres in diamond have a mature track record, including implementation of remote multi-node quantum communication networks [1], and form the basis of several commercial quantum computing platforms. Emitters in silicon carbide, including silicon vacancy (VSi) and divacancy (VV) centres, have arisen more recently and exhibit similar properties to NV centres, but in a more industrially-favourable material platform for quantum device fabrication.

 

Both materials possess high optical refractive indices, which strongly limit light harvested from quantum emitters due to reflections at the crystal interface. The use of solid immersion lenses (SILs) fabricated through focused ion beam milling to reduce these reflections has demonstrated enhanced collection efficiencies by over an order of magnitude [2], however the process is expensive and time-consuming. An alternative technique is resist-reflow lithography [3], where rounded lens shapes are formed through heating photoresist, which is scalable and cheap but allows extremely limited control over SIL profiles and aspect ratios.

 

Here, a double-mask technique based on grayscale lithography is presented for obtaining SILs with both large-array scalability and precise shape control [4]. A low-contrast photoresist is used as in conventional grayscale lithography to spatially control the mask height after development and yield arbitrarily-shaped rounded structures. These structures are transferred into a hard-mask (silica) layer, which are then etched into the SiC substrate to realise high-aspect-ratio SILs. We investigate the performance of hemispherical SILs fabricated through this method to enhance the emission of VSi centres, and observe enhancement of light collection efficiency from single defects by a factor of 4.4±1.0 [4]. This can be further improved through optimisation of the lens shape and registration of emitter positions to the centre of lens structures.

 

[1]   M. Pompili et al., NPJ Quantum Inf. 8, 1–10 (2022).

[2]   H. Bernien, Control, measurement and entanglement of remote quantum spin registers in diamond (TUDelft, 2014). 

[3]   F. Sardi et al., Appl. Phys. Lett. 117(2), 022105 (2020). 

[4]   C. Bekker et al., Appl. Phys. Lett. 122, 173507 (2023).