Abstract
Optical disk memory, optical fiber communication systems, and photolithography are examples of success in photonics. In the case of optical disk memory, storage and readout of the pit size smaller than λ/NA are not possible, where λ and NA are the wavelength of the incident light and a numerical aperture of the focusing lens, respectively. Shorter-wavelength lasers have been intensively developed in order to decrease the diffraction-limited pit size, i.e., the major efforts of increasing storage density have been how to use the shorter-wavelength light for storage and readout. However, the upper limit of the storage density to be realized by using visible light is several 10 Gb/in.2, while the value to be required in the year 2010 is 1 Tb/in.2.
Semiconductor lasers, optical waveguides, and optical switching devices have to confine the light in their bodies for effective operations. In the case of a semiconductor laser, its active layer has to be larger than the diffraction-limited volumes, i.e., λ3, for this confinement. In the case of an optical fiber, its core diameter has to be larger than λ. These examples mean that the sizes of photonic devices cannot be smaller than the wavelength of light, which is the diffraction-limited size of the photonic device. However, sizes of photonic switching devices for the optical fiber communication system in the year 2015 must become smaller than the diffraction-limited ones.
The most narrow linewidth of the pattern fabricated by photolithography is also limited by diffraction. Progress in decreasing the patterned size has been the result of the effort of using shorter-wavelength light for decreasing the diffraction-limited value. However, further shortening of the wavelength requires gigantic and expensive light sources, which can prohibit developing practical microfabrication systems. For the visible light sources, the 30–70-nm linewidth for 64–256 GbDRAMs is far beyond the diffraction limit.
To summarize, the society of the twenty-first century requires a novel optical technology in order to realize measurement, fabrication, control, and function with the size of several tens of nanometers. However, such miniaturization of optical technology is not possible as long as the conventional propagating light is used. This is the bottleneck imposed by light diffraction. One must go beyond the diffraction limit in order to open a new field of optical technology; this field is called as “nanophotonics.”
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