Roadmap on holography

From its inception holography has proven an extremely productive and attractive area of research. While specific technical applications give rise to 'hot topics', and three-dimensional (3D) visualisation comes in and out of fashion, the core principals involved continue to lead to exciting innovations in a wide range of areas. We humbly submit that it is impossible, in any journal document of this type, to fully reflect current and potential activity; however, our valiant contributors have produced a series of documents that go no small way to neatly capture progress across a wide range of core activities. As editors we have attempted to spread our net wide in order to illustrate the breadth of international activity. In relation to this we believe we have been at least partially successful. This Roadmap is organized under three headings: Materials, Applications and Concepts . We hope this structure is not misleading as in some cases the contributions do not fit neatly under any single heading and in several cases, there is significant overlap between articles listed in all the three sections. Significantly, some articles discuss work in areas not traditionally addressed side by side with others. We sincerely hope our approach will lead to an increase in awareness and a cross-fertilization in this wonderful field. Materials development and characterisation have always been of critically importance for the development of holography. The Roadmap begins with Wang et al (section 1) who briefly discuss novel materials for use in dynamic holographic displays. Tomita (section 2) then describes the development of nanocomposite photopolymers. Neipp and Frances (section 3) discuss the recording of holographic waveguides, while Gallego and Pascual (section 4) discuss shrinkage effects arising in photopolymers. Marovina et al (section 5) describe recent photorefractive materials and devices. Bruder et al (section 6) then discuss industrial-scale material production and volume holographic optical elements (vHOEs) fabrication. Under the heading of Applications, Kostuk (section 7) leads with the use of non-imaging HOES for solar energy conversion. Odinokov (section 8) discussed holographic data storage (HDS) and security issues. Matoba et al (section 9) reviews some advances in Multimodal 3D data acquisition using digital holography (DH). Wachulak (section 10) describes the performance of extreme ultraviolet (EUV) and soft x-ray (SXR) holography and tomography employing compact short wavelength sources. Gorelaya et al (section 11) then discuss holographic wavefront sensors (WFS) and next Chmelik (section 12), describes advance in incoherent holographic microscopy (HM). Ferrara and Coppola (section 13) describe the applications of digital polarized holography in the sciences. Márquez and Beléndez (section 14) discuss the use of liquid crystal on silicon (LCoS) microdisplays for use in HDS systems. Yang and Yuste (section 15) discuss the two-photon holographic imaging and manipulation of neural activity in vivo. Finally, in this section, Bianco and Zanutta (section 16) describe the use of holography in astronomical spectrographs. Under the heading Concepts, we begin with the description by Falldorf (section 17) of DH employing the spatial coherence function. Healy et al (section 18) describe the performance of autofocus in holographic imaging. Zhurminsky et al (section 19) discuss interference lithography for the fabrication of nanostructures. Situ and Wang (section 20) examine the applications of deep learning (DL) in DH. Abdurashitov and Tuchin (section 21) examine digital focusing in laser speckle contrast imaging (LSCI). Petrov (section 22) introduces ultrafast digital techniques and spatio-temporal metrology and Nomura (section 23) describes the evolution from the concepts of conventional DH to wide-sense DH. To conclude Morim and Saravanamuttu (section 24) describe the fabrication of functional 3D waveguide microstructures with nonlinear waves of light.