Dr. C. Vijayan
Dept. of Physics
Welcome to Photonics
A Technology for Tomorrow
The different stages of technological evolution are characterized in terms of the dominant type of tools used in each of them. We have passed trough the `stone age', the `copper age' and the `iron age' and are probably now in the `electron age'. After having done a wonderful job, today it looks as if electronic technologies have started experiencing their limits. For example, today's `information explosion' and communication problems demand very large bandwidths without cross talk, larger than that electronics can provide. What could be the technology that can tackle the growing needs of tomorrow?
Photonics is being studied today as a possible alternate technology for the future. Investigations are on to harness the capability of the photon to carry information and energy. Fortunately, success has already been achieved in the area of communications. Optical fibers, not copper cables, are already being used to carry huge amounts of information across the great oceans. Even the internet cables around us are now optical and not electronic. However, communication is only one of the three `C' s, that has been taken care of, the other two being computing and control. Photonic switches and optical computers are still in the laboratories and not yet in the marketplace. The major bottleneck in this area is not in solving technological problems or in perfecting theoretical understanding, but in developing suitable materials. That is where contributions from Materials Scientists are called for.
The Nature of Optical Nonlinearity
Optical processes of materials have always attracted the attention of Materials Scientists. Spectroscopic characterization and analysis of materials using the techniques of optical absorption, luminescence, Raman scattering are standard techniques in research. These studies help in structure analysis and in understanding the electronic processes and energy levels in systems. Several interesting photochemical processes and reactions such as photosynthesis have attracted the attentions of Materials Scientists and Biologists. The advent of lasers has revolutionized optical technology including spectroscopic instrumentation.
The high intensity radiation from lasers is also capable of causing new processes to occur in materials. In such cases, most of the materials can have a `nonlinear interaction' with the electric field. The nonlinear interaction results in several novel processes, which have the potential for communication, control and computing applications.
Essentially, the proportionality of the induced electrical polarization P in the medium to the electric field E breaks down and the resulting polarization can be considered to be made up of several terms consisting of products of higher order susceptibility c(n) and the magnitude of the electric field E. This mathematical formalism helps us to classify optical nonlinearity of materials and to present several important aspects of it in a convenient way.
P and E are vector quantities and c(n) is a tensor of rank (n+1), with 3(n+1) individual components. These tensor components describe the directional dependence of optical properties of crystals. The second and subsequent terms inside the brackets for the expression for susceptibility are progressively much smaller than the first term. This means that nonlinear optical effects would vanish in the low optical intensity regime as intensity is proportional to the square of the amplitude E of the electromagnetic wave. A material can be expected to exhibit n th order optical nonlinearity when either of the quantities c(n) or E is large enough. E depends on the intensity of the laser used and c(n) is a property of the material. Thus the amount of nonlinearity induced will depend on both the nature of the material as well as the intensity of the laser used.
From Optics to Photonics
The nth term in the above expression for c(n) describes an `(n+1) wave-mixing' phenomenon. According to this formalism, the first order nonlinearity (n=1) actually corresponds to linear optics and hence c(1) governs most of the ordinary optical phenomena such as reflection, refraction, diffraction, interference, polarization etc. In linear optics, we have the principle superposition of waves according to which light waves passing through a medium do not exchange energy with one another. This is not the case for n>1, and for n=2, we have wave mixing phenomena such as second harmonic generation (SHG), second order sum-and-difference frequency generation etc. SHG refers to the generation of light at frequency 2w when light of frequency w interacts with a medium. For example, infrared light of wavelength 1060 nm from an Nd:Yag laser could be converted to the green light of wavelength 532 nm. In the case of frequency mixing, light at frequencies w1 and w2 can generate light at w1 ±w2 in a second order medium, in addition to light at 2w1 and 2w2. A smart combination of such processes in a proper medium can ultimately provide several laser wavelengths by process known as optical parametric mixing. This is the technology being widely used by the latest models of high power tunable lasers in the market today.
Second order phenomena are NOT exhibited by materials which possessing a center of inversion. The existence of inversion symmetry forbids c(2) -related processes in materials. This restricts second order materials to certain classes of crystals. This is not the case with third order phenomena, which could be exhibited by all materials. Apart from higher order wave mixing phenomena, several new processes are also possible here. The refractive index of the material n becomes a function of the intensity of light I according to the relation n(I) = n0 + n2 (I) where n0 is the linear refractive index and n2I is the nonlinear contribution. This in fact makes the medium act as a lens when a strong beam of light with a Gaussian cross section passes through it. This is because the central part of the beam sees a larger effective refractive index and consequently travels slower, when compared to the peripheral part. Some materials self-focus the beam whereas some others self-defocus, depending on whether n2 is positive or negative. The change in refractive index with intensity is the basic principle based on which several photonic devices such as optical switches, transistors, modulators couplers, limiters etc.
Several methods have been designed to measure the nonlinear optical susceptibilities in materials. These are based on the optical processes involved. For example, third order nonlinear susceptibility can be measured conveniently using the self-focusing effect. In an experiment known as z-scan, a laser beam with Gaussian cross section is passed through a convex lens and the far field beam pattern is studied as the medium is moved along the beam direction, across the focal point of the lens. Obviously, the beam profile will change if the medium also starts acting as another lens. Thus a plot of the intensity (at any point) in the far field cross section of the beam as a function of the position of the sample enables us to calculate the beam distortion and hence the third order nonlinear susceptibility.
The dreams of Photonics Technology would be realized only if proper materials with large nonlinear susceptibility and fast response apart from ease of preparation and handling could be developed. Most of the natural materials such as inorganic crystals, organic liquids, semiconductors etc. have only either one of these conditions satisfied and not both simultaneously. This is a frontier area of intense research in the area of developing new Photonic Materials and improving the nonlinear optical response of known materials.
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