1. Optoelectronics

2. Optoelectronics
Optoelectronics is the study and application of electronic devices that source, detect and control light, usually considered a sub-field of photonics.
Optoelectronic devices are electrical-to-optical or optical-to-electrical transducers, or instruments that use such devices in their operation.
Electro-optics is often erroneously used as a synonym, but is in fact a wider branch of physics that deals with all interactions between light and electric fields, whether or not they form part of an electronic device.

3. What is Light?
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight.
Visible light has wavelength in a range from about 380 to about 740 nm, with a frequency range of about 405 THz to 790 THz.
Electromagnetic wave

4. EM Spectrum

5. Light interaction with solids
Optical classification:
Transparent
Transluscent
Opaque

6. Semiconductor
A semiconductor is a solid material that has electrical conductivity in between a conductor and that of an insulator.
Silicon (Si) is the most semiconductor material, but dozens of other materials are used as well.

7. Why semiconductor materials are so useful?
The main reason is that the behaviour of a semiconductor can be easily manipulated by the addition of impurities, known as doping.

8. Why semiconductor materials are so useful?
Semiconductor conductivity can be controlled by introduction of an electric field, by exposure to light, and even pressure and heat;
thus, they can make excellent sensors.
Current conduction in a semiconductor occurs via mobile or "free" electrons and holes, collectively known as charge carriers.

9. Diode Diode is a simplest semiconductor devices.
A diode has a low resistance in one direction and a high resistance to it in the reverse direction.
This property makes a diode useful as a rectifier, which can convert AC into DC.

10. Real diode (p-n junction)
A typical p-n junction diode characteristic curve

11. What is LED?
LEDs are semiconductor p-n junctions that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or IR spectrum regions. The quanta of light energy released is approximately proportional to the band gap of the semiconductor.

12. The pn Junction LED
Electron-hole recombination is the process that occurs in diodes.
In a regular diode: recombinations release energy thermal (heat) – nonradiative recombination.
In an LED: recombinations release the light – radiative recombination.
In reality, both types of recombination occur in a diode, when a majority of recombinations are radiative, we have an LED.

13. LEDs LEDs Red LED White LED LED for displays Blue LED
LED for traffic light

14. Photodiodes The photodiode is a p-n junction under reverse bias.
Exposing a semiconductor to light can generate electron-hole pairs, which increases the number of free carriers and its conductivity.
Only those that have correct wavelength can be absorbed by the semiconductor.
Separation of charge can be collected and measured as current or voltage.
If device is left open circuit  voltage detected  photovoltaic effect
If device is short-circuited (or under reverse bias)  photoconductive mode

15. Photodetectors
When a photon/light strikes a semiconductor, it can promote an electron from the valence band to the conduction band creating an electron-hole (e-h) pair.
The concentration of these e-h pairs is dependent on the amount of light striking the semiconductor, making the semiconductor suitable as an optical detector.
There are two ways to monitor the concentration of e-h pairs:
In photodiodes, a voltage bias is present and the concentration of light-induced e-h pairs determines the current through semiconductor.
Photovoltaic detectors contain a p-n junction, that causes the e-h pairs to separate to produce a voltage that can be measured.

16. Solar Cell/Photovoltaic Device
Photovoltaic devices or solar cells are semiconductor p-n junction that can convert solar radiation into electrical energy.
Diagram of a PV cell.
Photovoltaic cells, modules, panels and arrays.
Major photovoltaic system components.

17. Converting Sunlight to Electricity
A typical PV cell consists of semiconductor p-n junction.
Sunlight striking the cell raises the energy level of electrons and frees them from their atomic shells.
The electric field at the p-n junction drives the electrons into the n region while positive charges are driven to the p region.
A metal grid on the surface of the cell collects the electrons while a metal back-plate collects the positive charges.

18. Converting Sunlight to Electricity

19. Solar Cells

20. Laser
For atomic systems in thermal equilibrium, emission of light is the result of two main processes:
ABSORPTION of energy
SPONTANEOUS EMISSION of energy (a random photon is emitted)
A third mechanism is crucial to the formation of LASER action, which is
“STIMULATED EMISSION”.
Light Amplification of Stimulated Emission Radiation

21. Basic optical transitions
Laser
Basic optical transitions

22. Diode Laser
Diode lasers have been used for cutting, surgery, communication (optical fibre), CD writing and reading etc

23. The power-current curve of a laser diode
The power-current curve of a laser diode. Below threshold, the diode is an LED. Above threshold, the population is inverted and the light output increases rapidly

24. Boltzmann distribution vs Population inversion
How to create a population inversion?

25. Laser