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Dr.
C. Vijayan Professor Dept.
of Physics IIT Madras |
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Links
to other introductory articles: nanostructures , photonic materials,
photoacoustics 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. Links to other introductory articles:
nanostructures , photonic materials, photoacoustics |