The New Image of Optics
Classical optics is based on the laws of light reflection and refraction. Basic optical instruments are lenses, prisms, and mirrors — optical elements that reached the limit of perfection long ago. Further evolution of optics is associated with elements whose specific feature is the use of the phenomenon of light diffraction on micro and nanostructures...
I can’t comprehend my feelings
Charmed by a sudden attraction…
And I stand petrified, with my mind turned blind,
Till the moment I get it: DIFFRACTION!
(from Pastoral II by Igor Irten’ev)
Diffractive optics (which is sometimes called computer, binary, planar, or holographic optics) is the product of the era of information technologies, and it wouldn’t exist without such instruments as lasers and computers: creation of diffractive microstructures requires the use of special materials and new technologies for surface shaping. In the last 200 years, opticians had learned to make only one diffractive element — diffraction grating for spectral instruments, and it was for this purpose that highly accurate mechanical engraving machines were invented, whereas equipment for production of diffractive elements with an arbitrary surface topology has appeared only now.
The human eye accepts electromagnetic radiation with wavelengths from 400 nm (violet color) to 750 nm (red color). Interacting with the structure of the diffractive element, the light wave is deflected — the diffraction phenomenon arises. If the light radiation consists of several wavelengths (“white” light), it decomposes when it passes through the diffractive element or when it is reflected from the latter into a spectrum in the form of a rainbow.
Diffractive optical elements (DOEs) and holograms are getting into our everyday life gradually but irreversibly. Cashiers in stores read the bar-codes printed on packages using a laser device where the DOE performs several functions simultaneously: forms a laser beam, directs it to the bar-code, and collects the reflected image into a photodetector. The so-called “laser stylus” of CD players contains a diffractive lens, which helps to form a light beam whose size is smaller than fractions of a micron. Diffractive elements are widely used in measurement techniques, laser optics, and military technologies.
Holograms displaying all colors of a rainbow prevent falsifications and are placed on securities, banknotes, passport visas, and trade marks. There are special holographic postage stamps with three-dimensional images. Some books have illustrations where the colors are produced by white-light decomposition due to its reflection from a relief-bearing diffraction grating. The so-called “holographic keys” containing biometric data of the house owner or office keeper have become routine. And the number of applications of these new optical elements is growing every day…
Until recently, applications of diffractive elements have not been many due to the lack of technologies for microrelief creation. This microrelief consists of elements whose size is approximately half a micron and has a sophisticated three-dimensional shape (note that the overall size of the elements can be several meters in diameter). That is why the methods for DOE production are different from the methods for manufacturing microchips, for instance.
Diffraction is a delicate matter
It is convenient to consider the operation principles of diffractive optical elements and their difference from refractive elements by the example of a lens, which is the basic element in both classical and diffractive optics. The lens is designed for light focusing and constructing images of objects, i.e., for geometric and wave transformations of light beams. For example, the lens transforms an incoming parallel beam (planar wave) into an outgoing converging beam (spherical wave).
D. Gabor: ”The future cannot be foreseen, but it can be invented”Periodic gratings have always aroused much interest, because their capability of decomposing light into a spectrum made them a powerful analytical tool for physicists. The diffraction phenomenon had been discovered long ago, and the first diffraction grating was produced in 1785 by the American astronomer D. Rittenhouse who wound a hair between two screws with a very fine threading.
Almost 40 years later, it was “rediscovered” by the German physicist G. Fraunhofer. Being an excellent mechanic, he created the first mechanical machine for engraving periodic gratings with the help of a diamond cutter in a thin layer of gold applied onto the surface of a glass plate. His gratings were so good that he could measure absorption lines in the Solar spectrum (Fraunhofer lines). In those years, the technology of diffractive optics was developed mainly for the needs of spectroscopy, though J. Soret in 1875 created an annular diffraction grating, which allowed focusing up to 10 % of light energy.
At the end of the 19th century, the founding father of today’s diffraction gratings American scientist G. Rowland designed a set of engraving machines for engraving diffraction gratings with a period down to 1.5 mm and a size up to 18 cm. He also produced the first diffraction gratings on spherical surfaces, which simultaneously played the role of a grating and of a focusing lens.
In 1955, J. Harrison was the first to apply an interferometer (measuring device using the phenomenon of wave interference) for monitoring the diamond-cutter motion. The unit for measuring the cutter coordinate became the fraction of the wavelength of monochromatic light, which made it possible to compensate for vibrations and errors in the mechanical system of the machine. Such super-precise processing of optical surfaces is still widely used today.
In 1948, the future Nobel Prize winner, Hungarian D. Gabor proposed a method of optical holography — the method of recording, reproducing, and transforming wave fields which was based on interference and diffraction of light waves. The use of a laser for recording a hologram inspired the idea of using the latter as an optical element transforming laser radiation.
In the 1960s, the technology for fabricating diffraction gratings was developed based on generation of a periodic distribution of intensity in special photosensitive materials due to interference of laser radiation. Such holographic gratings of extremely high quality gained wide application. Optical holography, however, allowed recording holograms only of the objects that existed in reality. As the hologram recorded on a photographic film is nothing but an inhomogeneous blackening of photographic emulsion, it can be generated artificially, i. e., it is possible to synthesize a hologram by computing its structure. This idea was first put forward and implemented by the German physicist A. Loman in 1966.
Almost three years later, IBM specialists, using advanced (for that time) computers, created a focusing diffractive element with a continuous profile and called it a “kinoform.” A year after that, the first experiments on DOEs development were performed with the use of microelectronic technologies, which were experiencing a rapid evolution.
There is a substantial difference between the configurations of a diffractive and classical lens. In a conventional lens, the optical path from any point of an object to its image is constant (Fermat’s principle). The lens operates by means of refraction on its surface. The lens can be conventionally presented as a set of prisms with different angles which are increasing from the center toward the periphery; hence, the angles of refraction of light beams incident on each prism are different.
In a diffractive lens, the optical path at the zone boundaries experiences sudden changes equal to Nλ (where N is an integer). The lens operates by virtue of diffraction on a circular grating with the step decreasing toward the lens periphery. For N>>1, the diffractive structure transforms almost completely into a refractive structure, i.e., a DOE actually includes refractive elements.
Another important feature of the diffractive lens is its extremely small thickness.
Diffractive optical elements can also be used for creating the so-called hybrid lenses that combine diffractive and refractive elements. As the diffractive and refractive elements have the opposite signs of light dispersion, their optical hybridization allows constructing an optical element almost without any chromatism, i.e., capable of operating in white light. These properties are used in DVD players, night-vision equipment, and even intraocular lenses.
Siberian laser machine
Investigations in the field of diffractive optics were started at the Institute of Automation and Electrometry (IAE) of the Siberian Branch of the Russian Academy of Sciences in the early 1970s. At the first stage, the scientists tried to make a lens — basic diffractive element — by photographing an interference pattern with circular symmetry generated by a special Fabry-Perot interferometer. They managed to obtain a lens during one exposure, but each new element required a new interferometer…
The development of a universal method of DOE production was stimulated by the papers of A. Korpel, an American professor. At the first Soviet-American workshop on optical processing of information back in 1975, he demonstrated an add-on device to a TV set, which showed a color movie from a video disk 30 centimeters (cm) in diameter. Korpel thought that the most vital future technology will be direct laser-assisted recording onto a rotating disk.
After this workshop, Dr. Koronkevich suggested that IAE’s specialists should develop a laser-assisted recording system (LRS), which would be able to record digital information onto a special disk called a video disk. Soon it became clear that there was no sense in competing with large disk-producing companies, and it was decided to use this system to manufacture diffractive optical elements.
Because most optical systems are symmetrical relative to the optical axis, the device was developed in a polar coordinate system, thus allowing a much faster recording. The first LRS resembled a lathe where the focused beam of an argon laser worked as a cutter. In the 1980s this system made it possible to make diffractive elements with diameters up to 20 cm and a minimum size of the relief down to 1 mm.
An important difference between the diffractive lens and the classical lens is its thickness: the diffractive lens can be thinner by a factor of thousands than the conventional lens of the same focal power
It is known that laser radiation can be focused into a spot, smaller than the light wavelength, with a tremendous power density: the substance located in the focus can be almost instantaneously heated to a temperature of several thousand degrees. Such rapid variations of temperature alter the characteristics of many substances. Therefore, by controlling the motion of the laser spot and the power of laser radiation, one can modify the surface so that it will have the required properties and shape.
The first light-sensitive materials used at the IAE were thin films of chalcogenide glasses, which are vitreous semiconductors containing chemical elements of the VIth group of the Periodic Table (sulfur, selenium, etc.).
Then the possibility of DOE recording with the use of chromium film evaporation was examined, and the very first experiments suddenly revealed a thermochemical effect of formation of a latent image in these films. (Later it became known that a team of researchers from Leningrad had earlier discovered a similar technology.) The advantage of this technology is that it allows application of very uniform films of chromium onto surfaces of almost arbitrary size and, hence, formation of the diffractive structures exactly at the places that have been exposed to laser radiation.
Thus, the IAE’s researchers managed to obtain a relief with a minimum size of fractions of a micron. This method of recording high-quality diffractive amplitude elements, line and angular scales, encoded disks, and various photomasks immediately attracted attention of optical industry — note that none of the prior investigations had been funded.
In the 1980s—1990s, laser recording methods were constantly improving. Together with the Luch [Beam] company located in the town of Podolsk, the first master disks for magnetooptical memory were manufactured. The LRS of the second generation developed at the IAE became a prototype of the commercial version (CLWS-300С) designed together with the Technological Design Institute of Scientific Instrument Engineering of the Siberian Branch of the Russian Academy of Sciences. This system was purchased by research and production centers not only in Russia, but also in Germany, Italy and China.
New materials for optical recording (photoresists, amorphous silicon, LDW glasses, etc.) were used, which allowed creation of new classes of diffractive elements.
DOE applications: up in the Heavens...
Application of diffractive optical elements sometimes opens novel and unique prospects in various fields — from medicine to space research.
An illustrative example is telescopes, which still remain the most powerful instruments of studying the universe. The first telescope designed by Galileo in 1609 was 5 centimeters in diameter. In the past four centuries, the size of telescopic mirrors reached 10 meters, and scientists are planning to create giants with the mirror size up to 100 meters! Such telescopes will allow seeing an object the size of a ball on the Moon and distinguishing planets at the nearest stars.
It is insufficient, however, just to make a giant mirror: its surface has to be carefully checked; otherwise, tremendous labor and funding will be spent in vain. The parabolic surface of the telescope mirror should have the accuracy of hundredths of a micron. In other words, if an eight-meter mirror in enlarged to the size of the Black Sea, the ripples on its surface should be smaller that 1 millimeter (mm). Therefore, a reference instrument is needed for opticians to which the results of their efforts can be compared. It has turned out that diffractive optical elements are ideal candidates for that.
Recording techniques developed at the IAE made it possible to fabricate unique diffractive optical elements for inspecting the quality of 6.5- and 8.4-meter mirrors of the Magellan telescope and of the Large binocular telescope developed at the Steward Observatory of the Arizona University (Tusson, USA). Both telescopes have been put into operation.
The method of inspecting the quality of astronomic mirrors with the use of DOEs has been developed comparatively recently, and is now applied to creating all large mirrors. The impetus was the story with the Hubble space telescope, in which a 2.4-meter mirror was polished without DOE-assisted monitoring. As a result, the shape of the mirror surface differed from the prescribed template only by 0.5 µm, but this deteriorated the telescope’s resolution by a factor of 10! A special Shuttle mission was required to correct the situation.
A specific feature of DOEs designed for monitoring the quality of large astronomic mirrors is their relatively large size — up to 250 mm with the minimum size of the diffractive structure of approximately 0.5 µm, and the zones on the surface of such a DOE have to be applied with an error smaller than 50 nanometer.
Diffractive elements can act not only as assistants in creating telescopic mirrors but also as their competitors! Researchers from the Livermore Lab (USA) have recently proposed an unusual project where a long-focus diffractive lens applied onto a thin film is used as an objective lens. In contrast to a mirror, such a lens is much less sensitive to surface roughness, which is important for large-size objects. Also, this kind of lens is much lighter: a lens 25 meters in diameter weighs only about 100 kilograms (kg), whereas the mirror in the Hubble telescope which is one-tenth of the lens’s size weighs 800 kg.
Prototypes of diffractive lenses approximately 1 meter in diameter have been already created, and images of the Moon, some planets, and Solar spots have been obtained.
…and down to the Earth
The main optical characteristics of the crystalline lens of a healthy person are transparency and capability of absorbing ultraviolet radiation and of focusing of both near and far objects. Cataract of any origin is responsible for the loss of all these functions; hence, implantation of an artificial crystalline lens (or intraocular lens, IOL) after removing the damaged one is the most effective method in today’s eye surgery.
Several million intraocular lenses are implanted every year in the world. Normally, conventional single-focus refractive lenses are used, which allow recovery of two of the three basic functions of the natural eye lens. The problem of accommodation (i.e., clear vision at different distances) cannot be solved by lenses of this type.
Pseudo-accommodation becomes possible if we use hybrid (diffractive-refractive) lenses capable of forming simultaneous images of near and far objects. Clinical trials showed that the human brain, after certain training, can identify needed images of far or near objects. Therefore, implantation of hybrid IOLs allows one to live without spectacles.
The hybrid intraocular lens developed at the IAE consists of a traditional refractive lens, with one of the surfaces covered with a special diffractive microstructure in the form of a diffraction grating with a saw-tooth profile. The hybrid IOL forms two foci simultaneously by the entire surface, and bifocality is independent of the pupil diameter. To reduce the probability of biological deposits on the IOL microrelief, smooth transitions between the diffraction zones were formed.
The Akkord [Accord] lenses are now going through the first stage of clinical trials at the Novosibirsk Department of MNTK Mikrohirurgia Glaza (a medical company specializing in eye microsurgery). The lenses have been implanted to several dozens of patients. The post-operation inspection of vision functions showed that all patients can see far objects fairly well and can read a newspaper without glasses. Some patients have Akkord lenses implanted into both eyes to eliminate presbyopia (old-age far-sightedness). This application opens up fresh opportunities for correction of visual problems by surgery — and this technology will also work for elderly people.
In conclusion, it is worth noting that diffractive optical elements are not rivals of the traditional elements. It is obvious, nevertheless, that it is with the development and improvement of these optical elements that the future of optics is associated because their capabilities are really unlimited.
The easiest way to evaluate the level of investigations carried out by the Siberian researchers is to read the comments written by their colleagues who have visited the IAE.
The founder of digital holography Prof. A. Loman (Germany): “Your machine with its precise interferometric control and flexible computer control is excellent. The material chosen for recording is also good. We can create many useful planar optical elements.”
Professor G. Arseno (Canada): ”I am impressed with your machine for making kinoforms, not only because it is extremely useful, but also because light is used to create light-transforming elements. Treatment of materials by light brings spiritual satisfaction. Possibly a Michelangelo of light will appear in your lab…”