# A Review of Super-Resolution Imaging through Optical High-Order Interference [Invited]
## 8 Discussion and Perspectives  
The super-resolution imaging based on optical high-order interference could be beneficial in many applications, such as biomedical imaging [33; 37; 64; 65], laser radar [32], nano imaging [38] and astronomical imaging [9]. For comparison, a summary of the different super-resolution imaging schemes is listed in Table 1, showing the key difference between the imaging schemes, as well as the different conditions in different experiments. To improve the performance of the super-resolution imaging schemes, three main factors should be taken into consideration, i.e., the illuminating light field, the imaging element, and the detector. Besides the fundamental limit set by the SQL or HL, the imaging resolution is technically limited by the pixel size of the detector [33], as well as the spatial correlation length of the illuminating light field [42]. The image acquisition speed is limited by the speed of the detector, the properties of the illumination sources such as the scanning speed in the scanning-focused-beam illumination scheme, and the dynamic speed of the imaging element such as the refreshing rate of the spatial light modulator. Consequently, light sources of tailored properties, high-efficient and fast detectors, and fast spatial light modulators could be very helpful for prompting the application of the multi-photon super-resolution imaging. The optical high-order interference is not only useful for improving the imaging resolution of conventional lens-assisted imaging schemes, but also provides the possibility to design fundamentally new direct imaging schemes such as that realized with dynamic phase-controlled screen or scattering layer [40; 42]. This result indicates the possibility to explore novel type of multi-photon imaging schemes that can break the limitations existed in conventional lens-assisted imaging scheme. These super-resolution multi-photon imaging schemes are important additions to the library of multi-photon imaging [66; 67; 68; 69; 70; 71; 72]. Similar to the conventional lens-assisted imaging schemes, the multi-photon imaging schemes are in fact of limited resolution, despite that it can break the Rayleigh limit and reach the SQL or HL. Therefore, a future direction would be to break these limits to reach higher imaging resolution. One possible way is building hybrid super-resolution imaging schemes, i.e., combining several super-resolution imaging methods together to gain further resolution improvement. This is indeed feasible as revealed by a recent theoretical work in Ref. [44]. In the work, [PERSON] et al. developed a \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Light Source** & **Resolution Limit** & **Illumination** & **Experiment** & **Detectors** & **Imaging Element** & **Ref.** \\ \hline \multirow{3}{*}{Laser} & SQL & Scanning & & & SPAD-array & Lens & [31; 32] \\  & & focused & & & (32 \(\times\) 32) & & \\ \cline{2-6}  & HL & beam & & & CCD & Phase-controlled & [42; 43] \\ \hline \multirow{3}{*}{Fluctuating light} & SQL & Wide field & & & CCD, & & \\  & & & & EMCCD for & & & \\ \cline{2-6}  & SQL & Wide field & & & fluorescent imaging & & [33; 34; 35] \\ \cline{2-6}  & HL & Scanning & & & CCD & Phase-controlled & [42; 43] \\ \hline \multirow{3}{*}{Single-photon emitters} & \multirow{3}{*}{SQL} & Wide field & & & EMCCD & & \\ \cline{2-6}  & & Confocal & & & SPADs & & \\ \hline \multirow{3}{*}{Entangled two-photon light} & SQL & & Y & SPADs & Lens & [31; 39] \\ \cline{2-6}  & SQL & & & N & - & Phase-controlled & [42] \\ \cline{1-1} \cline{2-6}  & HL & Wide field & & & & & \\ \cline{1-1} \cline{2-6}  & & & Y & & (32 \(\times\) 32) & & \\ \cline{1-1} \cline{2-6}  & & & & N & - & Random & \\ \hline \hline \end{tabular}  
* Y indicates experimental demonstration has been performed, while N means the opposite.  
\end{table} 
Table 1: A summary of different super-resolution schemes based on optical high-order interference.