INTRODUCTION TO OPTICAL MATERIALS FOR FIBER

INTRODUCTION TO OPTICAL MATERIALS

1. Introduction to Optical Materials Optical materials are substances that can manipulate the flow of light by interacting with electromagnetic radiation. Their properties determine how they interact with different wavelengths of the electromagnetic spectrum, including visible light, infrared (IR), and ultraviolet (UV). LEDs Optical Fiber Solar Cell LASER

2. Classification of Optical Materials

3. Electromagnetic Spectrum Radiation is energy that travels through space as waves of oscillating electric and magnetic fields, moving at the speed of light. This radiation forms the electromagnetic Spectrum a broad range of waves classified by their frequency and wavelength, including radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays. EM radiation can originate from natural sources like the sun and stars, or from man-made devices. 1. Radio Waves : Low energy, used for communication and various devices. Microwaves : Used in microwave ovens and communication. Infrared : Perceived as heat. Electromagnetic Spectrum Visible Light : The portion of the spectrum visible to the human eye. Ultraviolet (UV) Radiation : Higher energy than visible light. X-rays : Used in medical imaging. Gamma Rays : Highest energy and shortest wavelength

4. Light Interaction with Materials Light interacts with materials through reflection, absorption, transmission, and scattering, along with refraction, diffraction, and emission.

5. Fundamental Interactions Absorption: When light's energy is transferred to the material, increasing its temperature or transforming it into other forms of energy, such as chemical energy in plants. Reflection: Light bounces off the surface of a material. The type of reflection (specular or diffuse) depends on the material's surface. Transmission: Light passes through a material. Transparent objects transmit all light, while translucent objects transmit some light. Scattering: Light waves are redirected in many directions as they interact with small particles or structures within the material, creating phenomena like the color of clouds. Refraction: Light bends as it passes from one medium (like air) to another (like water), causing the pencil-in-water illusion. Diffraction: Light waves bend and spread out as they encounter an obstacle or an opening, such as light passing through a small hole. Emission: Materials can also emit light, which is the opposite of absorption Diffraction Emission Scattering

6. Carrier Generation, and recombination processes and lifetime Types of carrier generations Photogeneration In photogeneration, light of frequency v falls on a semiconductor. Let hv be the energy of light photon greater than the bandgap of the semiconductor. Figure (a) shows the absorption of light energy  hv   (> E g ) . By absorption of light photon, one electron jumps from valence band to conduction band generating an electron-hole pair. For different wavelengths of light with different energies ( hv 2 , hv 3 ) it can take an electron in higher conduction band states. Phonon generation Phonon generation occurs when a semiconductor is under thermal excitation. With increase in temperature of the semiconductor, lattice vibrations increase which give rise to more phonons. Due to more lattice vibrations, covalent bonds in the semiconductor break down and electron-hole pairs are generated.As shown in Figure (b). Impact Ionization In this process, one energetic charge carrier will create another charge carrier. When a semiconductor is under an electric field, electrons gain energy from the applied electric field and hit other Si-atoms. In this process, a bond breaks out generating more carriers. very high electric field, it results in a avalanche breakdown. (Fig. (c)) (c) (a) (b) Carrier generation Carrier generation is the creation of mobile electrons and holes in a semiconductor material, typically through thermal energy or photon absorption Carrier recombination Recombination is simply a process of converting free electrons into valence electrons. Consider an energy band structure for the semiconductors. Carrier lifetime The time for which, on an average, a charge carriers will exist before recombination with a carrier of opposite charge is called carrier life time.

7. Absorption, emission, and scattering of light in metals, insulators, and semiconductors Metals Absorption : Light with photon energy greater than the energy difference between filled and partially filled bands (the plasma frequency) is absorbed, exciting electrons to higher energy levels. Emission: Excited electrons can quickly relax, releasing the absorbed energy as photons in the form of visible light, explaining high reflectivity and emission from heated metals. Scattering: Free electrons in metals can scatter light. This interaction can lead to the reflection of light or the modification of the light's polarization and direction. Insulators Absorption: Light absorption is negligible for photons with energies less than the band gap energy, as there are no available energy levels for electrons to move into from the filled valence band. Emission: When a photon with energy greater than the band gap interacts with an insulator, an electron is promoted to the conduction band. When this electron returns to the valence band, a photon is emitted, which is the basis of light emission in some materials. Scattering: Light can be scattered from the surface or internal structure of insulators, but this is primarily a physical interaction rather than an electronic transition based on the band gap. Semiconductors Absorption: Photons with energy exceeding the band gap can excite an electron from the valence band to the conduction band, creating an electron-hole pair. Emission: When a relaxed electron in the conduction band recombines with a hole in the valence band, it emits a photon with energy roughly equal to the band gap energy, enabling devices like LEDs and lasers. Scattering: In semiconductors, scattering can also occur due to surface defects or variations in the crystal lattice, affecting how light propagates through the material.

8. Carrier Generation, and recombination processes and lifetime