The term flat optics is occasionally used for optical elements which are relatively thin and do not have curved surfaces like those of typical lenses. Examples are optical windows, filter plates, thin-film polarizers and diffractive optical elements. Fresnel lenses may also be considered as an approach for obtaining quasi-flat optics. Dielectric coatings are naturally flat and thin.
However, the term flat optics (or planar photonics) has relatively recently been introduced for a substantially more radical development [1, 2]. Here, extremely thin and flat optical elements are realized in the form of photonic metasurfaces (or metaoptics), containing nanoscale (sub-wavelength) structures (typically with high refractive index contrast) to obtain quite unusual optical properties. The nanoscale structuring is not done in three dimension, as for other photonic metamaterials, but only in a thin layer. For example, flat metalenses can be made, which can in certain application areas replace traditional lenses, e.g. for focusing or collimating light beams or for imaging. Such flat optics can be made for the visible spectral range as well as for the infrared – in principle also for the ultraviolet, but with stronger technological challenges. Transmissive as well as purely reflective devices can be made.
The essential differences in comparison with traditional lenses or other optical elements (mainly based on refractive optics) are the following:
- The optical function is obtained by a flat layer with an extremely small thickness of e.g. a few micrometers (although it is normally held by a substrate of substantially larger thickness, e.g. of hundreds of micrometers).
- A substantially wider range of optical functions is possible. For example, a suitable wavelength or polarization dependence can be obtained. Not only the functions of traditional optical elements like lenses, polarizers and beam splitters can be realized, but also functions with combinations of those, or functions which cannot be realized at all with traditional optical elements. For example, one may obtain negative refraction or very unusual polarization properties, also combinations of focusing and polarization control. Even tunable optical functions, implemented without mechanical movement of parts over substantial distances, are feasible.
- Optical aberrations of various kinds can be well controlled with a suitable metalens design – in effect similar to aspheric optics, but possibly going substantially further. One can design high-quality off-axis lenses, lenses with rather large numerical aperture etc. In many cases, one may integrate the complete functionality into a single optically relevant surface, rather than making a more complex setup containing multiple optical elements.
- The challenges of mounting multiple optical elements in a sufficiently precise way may be substantially reduced with a flat-optics technology, particularly if the required overall functionality can be realized with fewer elements, which can be simply mounted in a very close distance. This may also lead to extremely compact devices which can find new applications. However, substantial free-space sections may still often be needed, making the explained advantage less significant.
- The used fabrication techniques are entirely different. Frequently, one uses semiconductor chip processing technology (e.g. of CMOS type based on silicon) with certain kinds of high-resolution lithography based on short-wavelength radiation, e.g. extreme ultraviolet light; similar techniques can also be applied to various dielectric materials such as fused silica and titanium dioxide. They are wafer-based, highly automated and depend on complex and highly expensive fabrication machinery. In mass production, e.g. for smartphone cameras, the fabrication cost may be relatively low, at least for elements covering only small areas. This is because many optical components can then be fabricated together on a single wafer, which will subsequently be diced. In comparison, traditional optical elements usually need to be processed individually – with some exceptions in the area of micro-optics, however.
- The tasks of optical design are also very different for these profoundly different technological approaches. While traditional optical design, for example of imaging objectives with minimized optical aberrations, is often highly challenging, even with support by powerful simulation and design software, rather different and perhaps not less challenging tasks are associated with designing photonic metasurfaces which provide the required optical functions in a sufficiently broad spectral range. (Essentially achromatic performance is essential in some areas like imaging and adds substantial design challenges, but is not required in other application areas, for example involving lasers.) Of course, the challenges of design become relatively less important for industrial mass fabrication.
The Future of Flat Optics
Because of the interesting prospects for a radically new kind of technical approach, photonic metasurfaces can be seen as a disruptive technology . An important aspect is that it would be implemented by companies which have so far focused on microelectronics, but could now fabricate both the electronic and optical components for various kinds of devices, such as cameras, optical sensors and illuminators. For example, an imaging device may contain imaging optics, an image sensor and the required electronics, all fabricated with basically the same kind of technology at a single location, while traditional technology worked with very different components fabricated by different players at different locations. Traditional optics companies, having perfectionized optical fabrication technologies for glass molding, cutting, grinding, polishing etc., might lose substantial business in certain sectors if photonic metasurfaces will indeed prove to be the more practical and cost-effective approach in certain important application fields. New opportunities for flat optics may well arise if new types of optical devices are invented; these could utilize peculiar functions which become feasible only with metasurfaces.
The outlook for flat optics is currently hard to judge. While highly interesting operation principles and achieved optical functionalities have been demonstrated, it is not clear yet to which extent that new technology will be disruptive, replacing traditional optics. That will depend on many aspects, for example the following:
- While the basic idea of flexible wavefront shaping is already well developed, it will need to be implemented with different materials as required for different spectral regions and specific application areas. Semiconductors appear most suitable for the infrared, while dielectric materials will presumably be necessary for most applications involving shorter-wavelength light. The comprehensive development of suitable processes can be challenging and expensive, but may be pushed with substantial power if big technological players see sufficiently clear prospects for volume applications.
- It also needs to be further explored how to implement further functions of polarization control and particularly how to utilize them in devices with a potential for mass fabrication.
- Opportunities for applications also depend on developments in other fields of technology. For example, optical fiber communications is permanently pushed towards higher and higher transmission capacities, and new types of miniature optical devices may be helpful in that context. For example, that may apply to the use of space division multiplexing with multi-core fibers or multimode fibers.
- In order to fully realize advantages of extreme compactness and ease of accurate mounting, optical designs not requiring substantial free-space sections are required. However, such sections often play an essential role, for example in imaging. Ideas for new operation principles of devices might lead to breakthroughs in that direction.
- While optical surfaces are generally known to be quite sensitive, that is even more so for highly structured photonic metasurfaces. There function may be entirely spoiled e.g. if essential voids are filled by condensed water or affected by dirt. Structures containing nanopillars and the like may easily be destroyed even when being only lightly touched. The need to appropriately protect such surfaces in use, and not just a very careful handling during fabrication, might introduce limitations.
- Since flat optics technology can presumably play out its strengths only in volume applications, it is a problem that many optical devices are produced only in small volumes. To some extent, small volumes are caused by the substantial production cost, and in principle some types of optical devices may become much more used if they can be fabricated in much cheaper ways. However, for many feasible optical functions it is also not obvious what kind of volume applications based on them could satisfy real needs. It remains to be seen to which extent such new volume applications can be developed.
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