# Topics covered What is holography History of holography

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Topics covered: • What is holography? • History of holography – Timeline • Basic terms and concepts in holography – – – Interference Fresnel Zone Lens Holographic recording and reconstruction • Fundamental Imaging Techniques in Holography • Applications • Interesting articles to read on holography

What is holography? Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that it appears as if the object is in the same position relative to the recording medium as it was when recorded. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object was still present, thus making the recorded image (hologram) appear three dimensional. Holograms can also be made using other types of waves. The technique of holography can also be used to optically store, retrieve, and process information. While holography is commonly used to display static 3 -D pictures, it is not yet possible to generate arbitrary scenes by a holographic volumetric display.

History of Holography was invented in 1947 by Hungarian physicist Dennis Gabor (1900– 1979), work for which he received the Nobel Prize in Physics in 1971. Gabor's research focused on electron optics, which led him to the invention of holography. The basic idea was that for perfect optical imaging, the total of all the information has to be used; not only the amplitude, as in usual optical imaging, but also the phase. In this manner a complete holo-spatial picture can be obtained.

Basic terms and concepts in holography Interference: The superposition or interference of two light waves (with same frequency) will emerge from the points R and O. Taking an object wave and reference wave without restriction to generality: o = oe-iΦ r and o are the field amplitudes of the respective waves at the point of superposition P r = re-iΨ The phase Ψ = ΨR - 2π(r 1/λ) is determined by the starting phase of the wave at point R and the phase change at distance r 1. The same is valid for Φ = Φo- 2π(r 2/λ). At point P, the complex amplitudes add up: r + o The intensity I is the square of the sum of the complex amplitudes: I = |r + o|2 = r. r* + o. o* + o. r* + r. o* I = r 2 + o 2 + r. o. {e-i(Φ-Ψ) + ei(Φ-Ψ)} I = r 2 + o 2 + 2. r. o. cos (Φ-Ψ)

Questions: The photographic plate of hologram can only record the intensity of light. (1)True (2) False Which of the following statement is correct? (1)The hologram can directly record the phase and amplitude of the wave from object. (2)The hologram can only record the information of amplitude of object wave and reference wave. (3) The interference light intensity pattern from object wave and reference wave can be recorded by the hologram. (4)Reference wave is optional in the holographic setup.

Basic terms and concepts in holography Reconstruction: The reconstruction is performed by illuminating the hologram with the reference wave r. We will assume that the amplitude of transmission of the film material is proportional to I which is contrary to usual film processing. Therefore, the reconstruction yields the light amplitude u directly behind the hologram: u ~ r. I = r (|r|2 + |o|2) + rro* + |r|2 o = u 0 + u-1 + u+1. Governs the reference wave which is weakened by the darkening of the hologram by a factor of (|r|2 + |o|2) (zeroth diffraction order) Describes the conjugate complex object wave o*. Corresponds to the -1 st diffraction order. The object wave is itself reconstructed with amplitude of the reference wave |r|2 being constant over the whole hologram. This proves that the object wave o can be completely reconstructed. It represents the 1 st diffraction order.

Direct, object and conjugate waves Object wave Reference wave Real image Virtual image Conjugate wave z -z z=0 Direct wave

Hologram of a point source Construction of the hologram of a point source Any object can be represented as a collection of points Photographic plate Reference wave plane x z y Photosensitive plate 1. Records interference pattern (linear response) 2. The droplet has small grain structure ( ) Object wave - spherical

Basic terms and concepts in holography The distance between the neighborings is: Try deriving this from the previous equation

Basic terms and concepts in holography

In-line Hologram (Gabor) The technique of straightforward holography developed by Gabor places the light source and the object on the axis perpendicular to the holographic layer. Only transparent objects can be considered. If an axial point O is chosen as an object emitting a spherical wave the resulting hologram for a plane reference wave is a Fresnel zone lens. The disadvantage of in-line or straightforward holograms is obvious: during reconstruction the hologram is illuminated with a plane reference wave as shown in part (b) of the image. Since it represents a zone lens a virtual point appears at the same distance to the right of the hologram. During observation the two images lying on the same axis interfere which leads to image disturbances (shown in (b)). Moreover, the observer looks directly into the reconstruction wave. Because of these disadvantages this form of holography is only of historical interest.

In-line Hologram (Gabor)

Off-axis Hologram (Leith-Upatnieks) It turns out that it is more favorable to shift either the holographic layer or the object sideways. Laser beam, object, and hologram are not on the same axis anymore. The hologram represents the outer area of a fresnel zone lens. Again a virtual and a real image are formed during construction. The advantage of off-axis holography is that both images do not interfere during observation and image disturbances are avoided. By tilting the reference wave (or shifting the object) it is achieved that the three diffraction orders, namely the image, the conjugated image, and the illumination wave, are spatially separated. This has the advantage that also holograms of opaque objects can be produced since the reference wave is not obstructed by the object. In principle, a single beam or a multiple beam technique can be used.

Off-axis reconstruction: Direct, object and conjugate waves Use an off-axis system to record the hologram, ensuring separation of the three waves on reconstruction Reference wave Object wave Direct wave Virtual image Conjugate wave Real image

5. 1 Producing the hologram • Practical setup Light source: laser Object: solid, 3 D Photographic film: high resolution Hologram pattern: interference fringes Myriad of tiny domain —uniform gray —cannot be seen by naked eye containing a series of fringes of various lengths and spacing

Conventional vs. Holographic photography n Hologram: n n Freezes the intricate wavefront of light that carries all the visual information of the scene To view a hologram, the wavefront is reconstructed View what we would have seen if present at the original scene through the window defined by the hologram Provides depth perception and parallax

Reflection Hologram (Denisyuk) Until now holographs were presented at which the object and the reference wave impinge from the same side on the photographic layer. Holograms whose images are reconstructed in the reflection are of large importance especially in the field of graphics and art. In this case, the reference wave- and later the reconstruction wave- has to impinge from the observer’s side onto the hologram. The object wave in this type of recording impinges on the hologram from the opposite side. Of importance is the setup after Denisyuk in which the holographic layer is positioned across between the light source and the object. This results in the interference planes being almost parallel to the light sensitive layer.

Reflection Hologram (Denisyuk)

Hologram – n n Reflection vs. Transmission hologram: reference and object waves traverse the film from the same side Reflection hologram: reference and object waves traverse the emulsion from opposite sides View in Transmission View in reflection

5. 3 Application of Holography 3. Information Storage

Storing Data n n n We can convert binary data to an array of black-and-white pixels with a spatial light modulator (SLM). We can store multiple “pages” of data in our holographic crystal. We can then read back out our pages via the reference beam.

Surface storage n A commonality between recordable media is the fact that they store bits on a surface of the recording medium.

3 D Storage - holograms n n n A hologram is a recording of an interference pattern made by the interaction of two beams of light. Different image depending on the viewing angle Using the volume of the storage medium as opposed to only its surface

STM( scanning tunneling microscope ): surface information This x-ray hologram shows the positions of cobalt atoms to within 0. 1 Å

360 hologram Simple setup for making a 360 hologram

Interesting articles to read on holography: The Brightest, Sharpest, Fastest X-Ray Holograms Yet NTT Develops Stamp-Size 1 GB Hologram Memory Quantum holography system Holographic Storage Overview at CNET Laser Pointer Holograms How Holographic Storage Works