The 1971 Nobel Prize in physics was awarded to the Hungarian-British electrical engineer Dennis Gabor for inventing the field of holography. It wasn’t until six years later, however, that the discipline would gain widespread recognition. In 1977, the movie “Star Wars” arrived in U.S. theaters, and the robot R2-D2 projected Princess Leia Organa saying the memorable words, “Help me, Obi-Wan Kenobi. You’re my only hope.” While Leia’s projection technically was not a hologram but a volumetric light-field display (a technique used to bring back to the stage deceased entertainers such as Elvis Presley and Tupac Shakur), three-dimensional holograms have been inextricably linked with a vision of the future. Current applications in holography are poised to usher it in. UCLA’s Aydogan Ozcan developed a portable microscope that uses holography to detect environmental pathogens and pollutants. Courtesy of Aydogan Ozcan/University of California, Los Angeles. Marvelous phones “We’ve already lost control of the term ‘hologram,’ and we have George Lucas to thank!” said Dan Novy, a postdoctoral associate in the Object-Based Media group at the MIT Media Lab. Before joining MIT (first as a graduate student), Novy spent nearly two decades working in the visual effects industry, where he won awards, including an Emmy, for his work on popular television shows and films such as “Red Planet,” “Deep Blue Sea,” and “Blood Diamond.” The Object-Based Media group is working on holographic waveguides that may turn another invention of science fiction into a reality — the smartphone of Tony Stark, aka Iron Man, from the popular Marvel films. Stark’s phone, which appears to be an ultrathin version of a modern smartphone but made entirely from transparent glass or plastic, both displays and projects interactive, 3D images and movies. “[We] wanted to replace the big, bulky, black monolithic [smartphones] with something more elegant,” Novy said. “A hologram is the only medium by which you can create an object that is a perfect replica of an object you’re interested in — a true simulacrum.” The MIT group’s waveguide uses surface acoustic waves to alter the index of refraction. The researchers were able to create full-motion, 30-fps, three-color RGB holograms when they pulsed the waveguide with coherent light and then modulated that light with an acoustic chirp1. “We can actually do color mixing in the waveguide,” Novy said. Smartphones with the waveguides could be made nearly one-third thinner than traditional devices, much like Stark’s phone, because they would not require red, green, and blue phosphors. The scientists and engineers experimented with manufacturing techniques as well. Novy said they used traditional proton-exchange methods but also femtosecond lasers to create the waveguides from a lithium niobate substrate2. “Femtosecond laser manufacturing is almost literally like sending a job to the printer from your computer as far as the process, but it is extremely slow right now,” he said. Novy foresees holograms appearing on nearly any transparent surface to provide entertainment, information, and assistance. “Holographic head-up displays on car windshields could be overlaid onto streets to help drivers navigate, for example,” he said. Augmented and virtual reality As Novy points out, holographic techniques have the potential to transform nearly any display. This could be a boon for augmented and virtual reality (AR/VR) devices, which have had trouble gaining widespread adoption because of a longstanding problem: Even after relatively short periods, users often experience headaches, eye strain, and nausea when wearing displays. These issues are caused by the vergence-accommodation mismatch, in which a user’s brain must reconcile the disparity between perceiving virtual objects in three dimensions while focusing on the flat screen of the display just centimeters in front of the eyes3. Aydogan Ozcan, a UCLA professor, has combined holographic techniques with other recent trends -- artificial intelligence and deep learning -- to create more powerful imaging methods. True holograms, Novy said, would allow the eyes to relax. “The wavefront of the virtual, perceived object would just rest directly on your retina.” Large technology companies have already understood the value of holography for AR/VR. In 2018, Apple Inc. acquired Colorado-based Akonia Holographics LLC, which developed both holographic storage solutions as well as optical technology for holography. A ‘Star Trek’ future One device from science fiction that has yet to be replicated is the “Star Trek” tricorder — a hand-held, multifunction biological data sensor and imager used by Dr. Leonard “Bones” McCoy in the original series. While there have been attempts to re-create the device and even an XPRIZE inducement, researchers have fallen short. Holographic techniques may enable this fictional invention to become a reality. Aydogan Ozcan, UCLA professor of electrical and computer engineering, is particularly enthusiastic about holography’s use as a 3D reconstruction mechanism in microscopic imaging (Figure 1). “It is one of the most important elements,” he said. Ozcan has developed various portable, hand-held microscopes and sensors that use holography in place of traditional optics for volumetric imaging4. Figure 1. With a holographic flow cytometer, researchers investigated and differentiated types of algae in blooms without having to take ocean water samples back to a lab. Some types of algae can be toxic to fish and devastate fish farms. Courtesy of Aydogan Ozcan/University of California, Los Angeles. “Holographic techniques are great for looking into weaker-scattering, hard-to-see objects — cells, for example — in liquid culture that nearly match the refractive index of the medium,” he said. Phase contrast, he added, is the key. Traditional, bright-field microscopes use incoherent light (like that from a lightbulb), which is focused by a lens, while holographic microscopy typically uses a coherent light source (such as a laser), focused through a pinhole. The issue, however, is that lasers are often delicate and can generate noise in images because of too much coherence. Instead, Ozcan’s group used a partially coherent light source, an LED, and focused it using larger pinholes of up to 100 μm. The sample of interest rests directly on top of a CCD or CMOS imager chip. “The trick,” said Ozcan, “was to change the geometry of the imaging system to make it on-chip; it was a win-win in many ways.” Ozcan’s lensless microscope, which fits in the palm and weighs less than a baseball, has a very large field of view (FOV) — at least two orders of magnitude larger than traditional holographic microscopes. It can image objects over an area of 20 to 30 sq mm via conventional CMOS imagers such as those used in mobile phones. The FOV can be further increased to 20 sq cm using higher-end CCDs. The UCLA professor has combined holographic techniques with other recent trends — artificial intelligence and deep learning — to create more powerful imaging methods (Figure 2). Figure 2. AI combined with holographic transformation and lensless microscopy could usher in devices from science fiction. This artist’s depiction illustrates a biological sample of interest placed directly on an imager and transformed into data. Courtesy of Aydogan Ozcan/University of California, Los Angeles. “Deep learning changes the game,” Ozcan said. “You can train a neural network on holographic and bright-field microscopic images and get the best of both worlds — the 3D advantage of holography and the contrast and resolution of bright-field microscopy.” He added that data science and AI will transform imaging at large. “Holography is certainly one of the first fields in microscopy to substantially benefit from this revolution.” The combination of holographic techniques and AI has enabled Ozcan’s devices to function like the Star Trek tricorder — with high throughput and rapid detection of toxic algae in drinking water, or airborne allergens such as pollen and mold spores — by merely imaging the samples holographically without first having to label them5,6. “In the next five years, we’ll see a renaissance in computational imaging, with powerful and unique bridges being built between different microscopy modalities,” he said. “There will be a revolution in field measurement devices at large, whether it’s for point-of-care diagnostics, sensing, or looking for natural or foreign pollutants.” Acknowledgments The author would like to thank Dan Novy, the Massachusetts Institute of Technology, and Aydogan Ozcan, the University of California, Los Angeles. References 1. S. Jolly et al. (2017). Near-to-eye electroholography via guided-wave acousto-optics for augmented reality. Proc SPIE, Vol. 10127, Practical Holography XXXI: Materials and Applications, 101270J. 2. N. Savidis et al. (2016). Fabrication of waveguide spatial light modulators via femtosecond laser micromachining. Proc SPIE, Vol. 9759, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics IX, 97590R. 3. S. Jolly S et al. (2016). Progress in off-plane computer-generated waveguide holography for near-to-eye 3D display. Proc SPIE, Vol. 9771, Practical Holography XXX: Materials and Applications, 97710L. 4. Y. Zhang et al. (2018). Motility-based label-free detection of parasites in bodily fluids using holographic speckle analysis and deep learning. Light: Sci Appl, Vol. 7, Article No. 108. 5. Z. Göröcs et al. (2018). A deep learning-enabled portable imaging ?ow cytometer for cost-effective, high throughput, and label-free analysis of natural water samples. Light: Sci Appl, Vol. 7, Article No. 66. 6. T. Liu et al. (2019). Deep learning-based color holographic microscopy. J Biophotonics, Vol. 12, No. 11.