UNIT V
OPTOELECTRONICS
Optical Material
• A material which is transparent to light or to infrared, ultraviolet,or x-ray radiation is
known as optical material.
Ex:Glass ,polycrystalline materials.
• Optical materials are fabricated into optical elements such as lenses, mirrors, windows,
prisms, polarizers, detectors, and modulators. These materials serve to refract, reflect,
transmit, disperse, polarize, detect, and transform light. The term “light” refers here not
only to visible light but also to radiation in the adjoining ultraviolet and infrared spectral
regions. Classification of optical materials The optical materials are broadly classified
into the following categories based on the propagation nature of light.
Transparent material
• The materials which transmit light with negligible absorption and reflection are known as
transparent materials.These materials are see-through in nature. The dielectric insulating
materials are transparent.
Translucent material
• The material which scatter the incident light and hence, transmit the diffracted light on
the other side are known as translucent material. A clear and sharp image of the object is
not viewed through these material.
Opaque material
• The material which obstruct the passage of light through them are known as opaque
material. Generally for the whole range of visible spectrum, the bulk materials either
absorb or reflect the light rays and hence, become opaque.
,Metals
• When an electromagnetic wave is propagating through a metal, it gets attenuated to the
energy of the electron at a depth from the surface, known as skin depth δ given as,
δ = (2/νχσ)1/2
• Where is ν the frequency of the incident light, χ the magnetic susceptibility and the
conductivity of the metal.
• For example, the skin depth for copper will be 10-4 nm for an incident 1mm microwave.
In most of the metals, the δ will nearly 50nm.The energy will be reduced to the value of
the electron on the surface in about 10-13 for the incident radiation in the infrared regions.
The incident light energy will heat the metal, since some amount of energy is absorbed
within the skin depth region.
Insulators
• In case of insulator(dielectric) medium, the attenuation coefficient will be greater than
104 cm-1 for an incident wavelength of 1000nm in the near infrared regions. The time
taken for the reduction in the energy to the energy of the electron on the surface is 10-4 s,
which is less than that of a metal. This is due to the non-availability of conduction
electrons in the insulator at room temperature. As a result, the heating of the
electromagnetic wave due to the conduction current is negligible. On the other hand, the
electrons in the valence band absorb the light energy and are raised to either conduction
band or some impurity state ,and lie in the forbidden gap, with either radiation or non-
radiation process. The phonons are generated during the above process, which in turn
heat the material.
Semiconductors
• In case of semiconductors, the most important transition is from the valence band to the
conduction band. The above transition depends on the energy gap and wavelength ,i.e.
λ=hc/Eg for materials with wavelength λ. Semiconductor such as ZnSe(λ=0.48nm and
Eg=2.6eV) is transparent and its transmission is high in visible regions,while
, InSb(λ=6.21 nm and Eg=0.2 eV) is opaque to visible light and its transmission appears in
infrared regions.
Composite materials
• A composite is a composition of two or more materials in the first three categories, e.g.
metals, ceramics and polymers, that has properties from its constituents. Large number of
composite materials have been engineered.
• Few typical examples of composite materials are wood, clad metals, fibre glass,
reinforced plastics, cemented carbides, etc.
• Fibre glass is a most familiar composite material, in which glass fibres are embedded
within a polymeric material. A composite is designed to display a combination of the best
characteristics of each of the component materials. Fibre glass acquires strength from the
glass and the flexibility from the polymer.
Properties
1. Stiffness
2. Lightweight.
3. Fire Resistance.
4. Corrosion resistance
5. Chemical & Weathering Resistance
6. Specific strength-This is simply the rigidity or hardness of a material with regard to its
weight. For example, a number of composites such as fiberglass are having higher
strength comparable with steel and titanium.
Applications
The special properties of composite materials are very useful for the manufacturing of the
following materials in various fields.
i) Aerospace-wings, helicopter, blader landling gears seats, floors, fuel tanks.
ii) Automobiles-body planes, cabs, shafts, gears, bearing
iii) Chemical- pipes, tanks, pumps
iv) Electricals-panels, insulators, connectors.
OPTOELECTRONICS
Optical Material
• A material which is transparent to light or to infrared, ultraviolet,or x-ray radiation is
known as optical material.
Ex:Glass ,polycrystalline materials.
• Optical materials are fabricated into optical elements such as lenses, mirrors, windows,
prisms, polarizers, detectors, and modulators. These materials serve to refract, reflect,
transmit, disperse, polarize, detect, and transform light. The term “light” refers here not
only to visible light but also to radiation in the adjoining ultraviolet and infrared spectral
regions. Classification of optical materials The optical materials are broadly classified
into the following categories based on the propagation nature of light.
Transparent material
• The materials which transmit light with negligible absorption and reflection are known as
transparent materials.These materials are see-through in nature. The dielectric insulating
materials are transparent.
Translucent material
• The material which scatter the incident light and hence, transmit the diffracted light on
the other side are known as translucent material. A clear and sharp image of the object is
not viewed through these material.
Opaque material
• The material which obstruct the passage of light through them are known as opaque
material. Generally for the whole range of visible spectrum, the bulk materials either
absorb or reflect the light rays and hence, become opaque.
,Metals
• When an electromagnetic wave is propagating through a metal, it gets attenuated to the
energy of the electron at a depth from the surface, known as skin depth δ given as,
δ = (2/νχσ)1/2
• Where is ν the frequency of the incident light, χ the magnetic susceptibility and the
conductivity of the metal.
• For example, the skin depth for copper will be 10-4 nm for an incident 1mm microwave.
In most of the metals, the δ will nearly 50nm.The energy will be reduced to the value of
the electron on the surface in about 10-13 for the incident radiation in the infrared regions.
The incident light energy will heat the metal, since some amount of energy is absorbed
within the skin depth region.
Insulators
• In case of insulator(dielectric) medium, the attenuation coefficient will be greater than
104 cm-1 for an incident wavelength of 1000nm in the near infrared regions. The time
taken for the reduction in the energy to the energy of the electron on the surface is 10-4 s,
which is less than that of a metal. This is due to the non-availability of conduction
electrons in the insulator at room temperature. As a result, the heating of the
electromagnetic wave due to the conduction current is negligible. On the other hand, the
electrons in the valence band absorb the light energy and are raised to either conduction
band or some impurity state ,and lie in the forbidden gap, with either radiation or non-
radiation process. The phonons are generated during the above process, which in turn
heat the material.
Semiconductors
• In case of semiconductors, the most important transition is from the valence band to the
conduction band. The above transition depends on the energy gap and wavelength ,i.e.
λ=hc/Eg for materials with wavelength λ. Semiconductor such as ZnSe(λ=0.48nm and
Eg=2.6eV) is transparent and its transmission is high in visible regions,while
, InSb(λ=6.21 nm and Eg=0.2 eV) is opaque to visible light and its transmission appears in
infrared regions.
Composite materials
• A composite is a composition of two or more materials in the first three categories, e.g.
metals, ceramics and polymers, that has properties from its constituents. Large number of
composite materials have been engineered.
• Few typical examples of composite materials are wood, clad metals, fibre glass,
reinforced plastics, cemented carbides, etc.
• Fibre glass is a most familiar composite material, in which glass fibres are embedded
within a polymeric material. A composite is designed to display a combination of the best
characteristics of each of the component materials. Fibre glass acquires strength from the
glass and the flexibility from the polymer.
Properties
1. Stiffness
2. Lightweight.
3. Fire Resistance.
4. Corrosion resistance
5. Chemical & Weathering Resistance
6. Specific strength-This is simply the rigidity or hardness of a material with regard to its
weight. For example, a number of composites such as fiberglass are having higher
strength comparable with steel and titanium.
Applications
The special properties of composite materials are very useful for the manufacturing of the
following materials in various fields.
i) Aerospace-wings, helicopter, blader landling gears seats, floors, fuel tanks.
ii) Automobiles-body planes, cabs, shafts, gears, bearing
iii) Chemical- pipes, tanks, pumps
iv) Electricals-panels, insulators, connectors.
