natureCOMMUNICATIONSARTICLECheck for updatesttps:/d6i.org/10.1038/s41467-020-19298-4OPENSlim-panel holographic video displayHoon Song,Chilsung Choi,Yunhee Kim',Juwon Seo,Alexander Morozov2,Hyunsik Park,Since its discovery almost 70 years ago,the hologram has been considered to reproduce themost realistic three dimensional images without visual side effects.Holographic video hasbeen extensively researched for commercialization,since Benton et al.at MIT Media Labdeveloped the first holographic video systems in 1990.However,commercially availableholographic video displays have not been introduced yet for several reasons:narrow viewingangle,bulky optics and heavy computing power.Here we present an interactive slim-panelholographic video display using a steering-backlight unit and a holographic video processor tosolve the above issues.The steering-backlight unit enables to expand the viewing angle by 30times and its diffractive waveguide architecture makes a slim display form-factor.Theholographic video processor computes high quality holograms in real-time on a single-chip.We suggest that the slim-panel holographic display can provide realistic three-dimensionalvideo in office and household environments.Dvintsev street,Moscow,Russia.3 University of Seoul 163 Seoulsiripdaero,Dongdaemun-gu,Seoul,South Korea.These authors contributed equally:Jungkwuen An Kanghee Won.5These authors jointly supervised this work Kichul Kim,Hong-Seok Lee.email:kkim@uos.ac.kr Ihs1210@samsung.comARTICLENATURE COMMUNICATIONS|https://doiorg/10.1038/s41467-020-19298-4holographic display uses light diffraction to create three-holographic media.It means that only a small size or a narrowdimensional (3D)images in space.When real objects andviewing angle dynamic hologram can be realised.Second,toholographic images are located in the same space,theygenerate a large coherent backlight,complicated optical com-can be perceived without inhomogeneity.Figure 1 shows how theponents and a considerable space is required for the manipula-holographic image can be shown with a real human hand.Bothtion of light.It is difficult to realise a holographic video display asthe fairy image and the hand are 0.3m away from the screen.slim as flat-panel displays commercialised nowadays.Last,theWith a holographic display,one can see the fairy image and thecalculation of hologram in real-time typically requires hugehand clearly focused without having any uncomfortable feelingcomputational cost,and the amount of computation increases asOn the contrary,conventional stereoscopic 3D images using onlythe SBP increases.Several studies have been carried out tobinocular parallax and vergence are recognised as images at theoptimise algorithms and to increase computation speed,but theyscreen2.3.One cannot clearly see the fairy image and the real handstill require clustered processors or high-performance parallelat the same time,and will experience visual fatigue caused by anprocessing systems to calculate high-quality hologram at videoaccommodation-vergence conflict2.Therefore,holographic dis-frame rateli-i6plays are essential parts of future video systems including inter-In this research,a real-time interactive slim-panel holographicactive 3D user interfaces.video display is demonstrated for the first time.It resolves all theBecause of its advantage in 3D image reproduction,the tech-above issues of low SBP,bulky optical system,and enormousnology for static holograms is quickly developed to a high stan-computational cost.To increase effective SBP (Supplementarydard by using hologram recording materials such as silver halideNote 1),a steering-backlight unit (S-BLU),which consists of acoherent-BLU(C-BLU)and a beam deflector (BD)is introducedare also used to reconstruct static holograms.However,thoseThe effective SBP is increased by 30 times compared to the ori-holographic media are non-updatable or have a limited updatingginal value by using the S-BLU.It is the highest SBP ever achievedfor a real-time holographic video system using an SLM.All thedynamic holograms.By using a spatial light modulator (SLM),optical components are designed and fabricated as a slim struc-which directly modulates the wavefront of light,it is possible toture.To generate 4-K (3840 x 2160)ultra-high-definition (UHD)update holograms at video rate.Many researches have demon-pixel holograms in real-time,a holographic video processor isstrated holographic video systems so far,including anisotropicimplemented on a single-chip field programmable gate arrayleaky modes and eye tracking.The recent study on a holographic(FPGA).display was reported,including a full-colour 3D computer-generated hologram (CGH)calculated in real-time,and a focus-adjustable reconstructed imagelo.However,it has been demon-Resultsstrated that vertical parallax only holograms using a bulky opticalBeyond SBP.In holographic display,SBP can be represented bysystem.It is expected to require quite an amount of time to reachthe equation,Wx =Ax N,where W is the image size,0 is thethe mobile holographic video for practical applicationsviewing angle,A is the wavelength,and N is the number of pixels.To build a mobile holographic video display,the followingFor a given number of controllable pixels,the maximum size ofbarriers need to be overcome.First of all,there is the limitationholographic image and the viewing angle are always trade-offof the space-bandwidth product (SBP),which determines bothrelation.For example,an SLM with full high-definition (FHD)the size of holographic image and the viewing angle.The staticresolution can provide the viewing angle of 0.25 in 10-in.displayholographic media can produce large holographic images with aor 30 in 0.1-in.display.For satisfying both large size and largelarge viewing angle,because the information of hologram isviewing angle,the SBP should be expanded by increasing therecorded in the sub-wavelength density and it can be recorded onnumber of pixels N.There are studies to increase the number ofthe large size film.However,SBP is limited by the pixel size andpixels Nby tiling many SLMs17 or by increasing the resolution ofthe number of pixels when an SLM is used as a dynamic holo-a single SLM18.To achieve the 30 viewing angle in 10-in.,221-Kgraphic medium.The SBP of the currently available SLM ishorizontal resolution is required,which is ~100 times highergenerally a few hundred times less than the SBP of the staticresolution than the FHD resolution.Apart from the difficulty infabricating such an SLM,the computation of the CGH in real-time may not be feasible,because the total number of pixelsincreases ~13,000 times.In this research,we expand the effective SBP withoutincreasing the number of pixels of the SLM.To overcome thesmall viewing angle of a large size SLM,we introduce S-BLU,which is able to tilt the angle of the backlight for reconstructingholograms as shown in Fig.2.Since the S-BLU steers thebacklight and can deliver the holographic image to a desireddirection,the viewing angle can be effectively expanded for anobserver by the amount of maximum steering angle of S-BLU.The BD,one of the key elements of S-BLU,plays the role ofsteering a beam.The BD consists of transmission phase arrays,which can provide the linear phase profile for tilting thetransmission angle.By using the BD,an extra enhancementFig.1 A photo of a holographic image taken with a real hand.ThePsLM/PBD).The enhancement factor proportionally increases withholographic image and the human hand are at the same distance from thethe ratio of pixel pitches between SLM and BD.In ourcamera.It provides natural depth perception and prompts a viewer to focusholographic video system,the pixel pitch of the BD,PBp,ison the object itself,not on the screen.The bulk-optic backlight unit is useddesigned as 2um while the pixel pitch of the SLM,PsLM is 58 um.to demonstrate the ultimate quality of the holographic image.The movieThen,the effective SBP is increased ~30 times without increasingdip is available in Supplementary Movie 1.the number of pixels in the SLM.Since the PBp can be reducedNATURE COMMUNICATIONS https://doi.org/10.1038/s41467-020-19298-4ARTICLEOriginal viewing angleSteering-backlight unitExpanded viewing angleSpatial light modulatorFig.2 The schematic diagram of expanding effective SBP by using steering-backlight unit.In addition to the original viewing angle of hologramsprovided by the spatial light modulator,the steering-backlight unit effectively enhances the overall viewing angle.Steering-backlight unitbaGlass substrateLiquid crystalGlass substrateBeam deflectorCGHEye tracking sensorOUTSpatial light modulatorCoherent-backlight unitGeometric phase lensMemoryRGB-DData nputdFFT Pocessor1Sngle-chip FPGAFig.3 Schematics of the optical architecture with key components and the holographic video processor.a Optical architecture consists of beamdeflectors,coherent-backlight units,a geometric phase lens and a spatial light modulator.b Principle of the beam deflector,which steers the transmittedlights optically like a prism:the vertical and horizontal phase arrays steer light up to-15 with the angular resolution of 0.02 at the wavelength of 520 nm.c Configuration of the coherent-backlight unit using waveguide:the first waveguide for both red and green light and the second waveguide for blue light arestacked together to increase the overall efficiency.d The holographic video processor is implemented on a single-chip FPGA.further independent to the SLM,using a BD is more scalable S-BLU architecture and holographic video processor.The S.mean to expand SBP than directly increasing the number ofpixels BLU is the key component for expanding the viewing angle ofof the SLM.holographic display.In Fig.3a,the S-BLU consists of the BD