Semiconductors

Direct bandgap semiconductors (GaAs, InGaAs, InGaAsP)

The minimum of CB is directly above the maximum of VB

Electro-hole pair can recombine directly and transfer their energy to emit a photon

Indirect bandgap semiconductors (Ge, Si)

The minimum of the CB is not directly above maximum of the VBProbability of photon emission is very low Recombination is mediated by absorption or emission of a phononEnergy and momentum of the electron is transferred to lattice vibration.

Semiconductor LASER

Direct Bandgap Semiconductor

Large possibility for direct recombination of hole and electron emitting a photon

Indirect Bandgap Semiconductor

Direct recombination of hole and electron is not possibleThere is no photon emission

GaAs - direct bandgap material – Used to make LEDs and LASER

Wavelength of the emitted light depends upon the bandgap

Pure Semiconductor

n-type semiconductor p-type semiconductor

Energy band diagram of n-type and p-type semiconductors

Energy band diagram of p-n junction at thermal equilibrium

Energy band diagram of heavily doped p-n junction at thermal equilibrium

Semiconductor LASER (GaAs Diode LASER)

Active Medium

GaAs p-n junction diode

Doping Materials

p-type (Ge) & n-type (Te)

GaAs has high refractive index Reflectance at the material-air interface is large (External mirrors are not necessary)

Can give laser output with a wavelength 0.85 μm in pulsed mode

GaAs Laser - Working

Without any bias, there will be less number of electron-hole pairs in the junction region

1. Population inversion – Injection of electrons across the junction from n-doped region to the p-doped region by forward bias

2. Excess minority electrons in the p-region and excess minority holes in the n-region

3. Population inversion of minority carriers

4. When relatively large current is passed through the junction to provide excitation, the direction recombination process is efficient

5. Emitted photons increase the rate of recombination of injected electrons & holes – Thus more number of photons are produced

GaAs Laser - Working

6. Emitted photons from induced recombinations have same phase and frequency as that of the original inducing photons (We have stimulated emission of radiation along the p-n junction)

7. Wavelength of the emitted radiation depends upon concentration of donor and acceptor atoms in GaAs

8. When the donor and acceptor concentrations are about 1024 atoms/m3, the emitted wavelengths are 0.9020, 0.8425 and 0.8370 μm

9. Efficiency of laser emission increases when we cool the GaAs diode

10. In the reverse bias, no carrier injection takes place and no light emission

LASER action based on Energy Band structure

When n-region is heavily doped, the donor

levels and a portion of the conduction

band are occupied by electrons

Similarly when p-region is heavily doped,

the acceptor levels are unoccupied and

holes exist in the valence band

At thermal equilibrium, the Fermi level

should be uniform in the junction region

Fermi level in the n-side lies within the conduction band

Fermi level in the p-side lies within the valence band

LASER action based on Energy Band structure

When the forward bias is applied, the energy

levels shift and junction band diagram is

altered

Electrons and holes are injected across the

depletion region existing at the junction

The width of the depletion region decreases

and the minority carrier concentration in this

transition region increases exponentially

At lower threshold current, recombination of electrons and holes leads to

spontaneous emission

When the current increases, the transition region has high concentration of

electrons and holes (population inversion)

LASER action based on Energy Band structure

The transition region will be very narrow when

population inversion is achieved (Active region)

Now the spontaneously emitted photons will

start the stimulated emission

Rate of stimulated emission increases with

time due to more number of emitted photons

(Amplification of light)

Calculation of the wavelength of emitted radiation

Bandgap of GaAs = 1.44 eV

λ

chhνEg

J 10 1.6 eV 1

sJ 10 6.626h19-

-34

010

19-

834-

g

A8628m108628λ

J101.61.44

m/s)103() sJ10 (6.626λ

E

hcλ

(eV)E

1240λ(nm)

(eV)E

1.24λ(μm)

g

g

Drawbacks of homo-junction lasers

1. Threshold current density is very large (400 A/mm2)

2. Only pulsed mode output is obtained

3. Laser output has large beam divergence

4. Poor coherence and poor stability

5. Electromagnetic field confinement is poor

Peak emission wavelength of GaAsP diode laser is 1.55 μm

What is its band gap?

eV 0.8μm 1.55

1.24

λ(μm)

1.24(eV)E

(eV)E

1.24λ(μm)

g

g

HOLOGRAPHY

Holography is a lens-less photography

Need a laser source for producing and viewing the image

Image is in the form of interference pattern

Denis Gabor developed the phenomenon of holography

Principle of Holography

Holography is based on the principle of interference

Coherent light waves are needed (laser source)

Laser beam is split into two beams A and B using a beam splitter S

Beam A recognizes the object O and a part of light scattered by the object (object beam) falls on a photographic plate P

Reflected beam B (reference beam) also falls on the photographic plate

Superposition of reference and object beams produces an interference pattern and the pattern is recorded on the plateThe developed plate is called as Holograph

Principle of Holography

Conventional Photography

Negative is made first and a positive print is produced later using negative

Positive print is only a 2-D record of light intensity received from a 3-D object

It contains information about the square of the amplitude of the light wave that produced the image but information about the phase of the light is not recorded

Holography

Both the intensity and phase of the light waves are recorded and when viewed, the photograph shows a 3-D image of the object

Construction (Generation) of a Hologram

A laser light is split into two beams (reference and object beams)

Reference beam directly reaches the photographic plate

Object beam illuminates the object

Part of the light scattered by the object travels towards the plate

Scattered (object) beam and reference beam interferes and produces an interference pattern on the plate

Photographic plate with interference pattern is called a hologram

The hologram is developed, fixed and stored

Construction (Generation) of a Hologram

A hologram does not contain a distinct image of the object

It is only a record of the interference pattern formed by the superposition of two coherent light beams

Part of the light scattered by the object falls on the photographic plate after suffering reflections from various points of the object

Each and every point of the hologram receives from various points of the object

Thus, even if a hologram is broken into parts, each part is capable of reconstructing the whole object

Reconstruction of a Hologram

In the reconstruction process, the hologram is illuminated by laser beam

This beam is called reconstruction beam

This beam is identical to reference beam used in construction of hologram

The reconstruction beam illuminates the hologram at the same angle as the reference beam

The hologram acts as a diffraction grating and the reconstruction beam will undergo phenomenon of diffraction during passage through the hologram

The reconstruction beam after passing through the hologram produces a real as well as virtual image of the object

Reconstruction of a Hologram

The virtual image is formed behind the hologram at the original site of the objectThe real image is formed in front of the hologram

Reconstruction of a Hologram

An observer sees light waves diverging from the virtual image

The image is identical to the object

If the observer moves round the virtual image then other sides of the object which were not noticed earlier would be observed

Therefore, the virtual image exhibits all the true three dimensional characteristics

The real image can be recorded on a photographic plate

Applications of Holography

A hologram is a reliable medium for data storage

Several images can be stored on a hologram

The information on a hologram cam be decoded only by a coherent beam identical to that of the reference beam which can be chosen appropriately

Holographic non-destructive technique can be used to discover stresses in a pipe fitting, the stress points on a wheel

Applications of Holography

Holography plays an important role in optical signal processing

Can be used for character recognition and for identification of finger prints

Employed in the production of photographic masks used to produce microelectronic circuits

Holographic interferometry is the standard technique employed to assess the quality of aircraft tyres and

many other high performance aircrafts