SR-PA

Researchers are pushing the boundary of spatial resolution. Typically for optical imaging the spatial resolution is limited by the diffraction limit of wavelength/2. There are several approaches to break this diffraction limit and achieve super-resolution. Here we are working towards achieving super-resolution with the use of optical nanojets. The concept has been used in other microscopy field to achieve resolution in the order of <100 nm. In ORPAM (optical resolution photoacoustic microscopy), the axial resolution is limited by the detection bandwidth. However, the transverse (lateral) resolution is determined by the light focus. By using photonic nanojet (PNJ) we showed that (through simulation) it is possible to achieve super-resolution in photoacoustic imaging. This will provide us another way of doing photoacoustic nanoscopy. More details can be found in Upputuri et al., Journal of Biomedical Optics 19 (2014).

Single round microsphere can generate PNJ with sub-wavelength waist, but its short axial length limits its applications to surface imaging only. We investigated different sphere designs to achieve ultra-long nanojets which will make the nanojet more viable in far-field applications such as photoacoustic imaging. The PNJ properties including effective length, waist size, working distance, and peak intensity can be tuned and controlled by changing the sphere design and its refractive index. A new truncated multi-layer microsphere (TMLM) design could generate an ultra-elongated PNJ while retaining a large working distance. Through simulation study we observed more than ten fold enhancement in lateral resolution. More details can be found in Upputuri et al., Journal of Biomedical Optics 22 (2017).