1. Semiconductors

2. 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 VB Probability of photon emission is very low Recombination is mediated by absorption or emission of a phonon Energy and momentum of the electron is transferred to lattice vibration.

3. 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 possible There is no photon emission GaAs - direct bandgap material – Used to make LEDs and LASER Wavelength of the emitted light depends upon the bandgap

4. Pure Semiconductor

5. n-type semiconductor p-type semiconductor

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

7. Energy band diagram of p-n junction at thermal equilibrium

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

9. 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

11. 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

12. 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 10 24 atoms/m 3, 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

13. 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

14. 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)

15. 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)

16. Calculation of the wavelength of emitted radiation Bandgap of GaAs = 1.44 eV

17. Drawbacks of homo-junction lasers 1.Threshold current density is very large (400 A/mm 2 ) 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

18. Peak emission wavelength of GaAsP diode laser is 1.55 μm What is its band gap?

19. 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

20. 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 plate The developed plate is called as Holograph

21. 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

22. 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

23. 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

24. 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

25. Reconstruction of a Hologram The virtual image is formed behind the hologram at the original site of the object The real image is formed in front of the hologram

26. 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

27. 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

28. 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