BE-II SEMESTER ADVANCED PHYSICS

1. BE-II SEMESTER ADVANCED PHYSICS UNIT-IV OPTICAL FIBRE AND NANOTECHNOLOGY DEPARTMENT OF APPLIED PHYSICS

2. SYLLABUS • Optical fibers: Propagation by total internal reflection, • structure and classification • Modes of propagation in fiber, Acceptance angle, • Numerical aperture, Attenuation and dispersion. • Light sources and Detectors. Applications: I ) As a • Sensors - i) Temperature Sensor ii) Pollution / Smoke • detector iii) Liquid level sensor. • II) As a Detectors- i) PIN detector ii) Avalanche • Detector.

3. SYLLABUS • Introduction to nanoscience and nanotechnology, • Classification of nano materials, • Synthesis of Nanomaterials, General idea about • physical and chemical methods. • Comparison of properties of nanomaterials with bulk • materials, • Some special nanomaterials:1) Zeolites, 2) Graphine, • Application of nanomaterials in engineering, • Impact of Nanoscience and nanotechnology

4. LEARNING OBJECTIVES • Understand what total internal reflection is. • Understand how to work out critical angle • Describe various types of optical fibers • Explain fiber dispersion, attenuation and other • properties • Develop basic knowledge of nanotechnology. • Study properties, synthesis and applications.

5. Fibre Optics

6. Fibre optics • A thin transparent conduit as thin as human hair made up of glass or clear plastic which is designed to guide light waves along its length.

7. Main Section of Optical Fibre glass or plastic cladding fiber core • Core – Inner most light guiding region • Cladding – coaxial middle region having RI less than core • Buffer coating(Sheath) - Plastic coating that protects the fiber from damage and moisture. Plastic jacket

8. Structure of optical fibre Sheath Cladding Core 250 -900µm ≈ 8-60µm 125 µm

9. Total Internal Reflection Normal 1 2 3 µ2 4 5 µ1 • F.O. works on the principle of Total Internal • Reflection. • TIR takes place when a ray of • light strikes a medium boundary • (core-cladding ) at an angle • larger than the critical angle • with respect to the normal • to the surface. θ2 θ1

10. Critical Angle Normal 1 2 3 µ2 4 5 µ1 • The angle of incidence for which the angle of • refraction is 900 is known as critical angle θc. • Acc. Snell’s law, θ2 When θ2 = 90o, θ1

11. Propagation of light through a Optical fibre / Acceptance Angle Normal B n1 θi θc Launching medium air θr totally internally reflected ray C A θi Core Incident ray n2 Cladding

12. Derivation • Applying Snell’s law to the launching face of the fibre, ------------ (1) • Applying Snell’s law, ------------------ (2) • Using equation (1) & (2) • For air medium, n0= 1.The angle θ0 is called the acceptance angle of the fibre. -

13. Propagation of light through a cladded fibre: • Acceptance Angle: The maximum angle that a light ray can have relative to the axis of the fibre and propagate down the fibre. θ0 = sin-1[ n12- n22 ]1/2 • It depends on core diameter and the material. • Acceptance cone:-Itis twice of the acceptance angle i.e. 2θ0.

14. Numerical Aperture • Numerical Aperture is defined as the sine of the acceptance angle. Fractional Refractive Index Change • Fractional refractive index change is the fractional difference between the refractive indices of the core and the cladding.

15. Modes of propagation • Modes are the possible number of paths of light that propogates through optical fibre. • The waves travel in number of directions and not all waves are trapped within the optical fibre. • The light rays traveling through a fibre are classified as 1. Axial rays 2. Zigzag rays.

16. Classification of Optical Fibre Optical Fibre On the basis of materials On the basis of refractive index profile On the basis of light propagation Step index fibre Graded index fibre Glass fibre Plastic fibre PCS (polymer clad silica) Single mode fibre (SMF) Multimode fibre (MMF)

17. Index of refraction Single mode fibre • Single mode fiber has a core diameter of 8 to 9 µm. • Allows only single beam of light to travel along the axis of optical fibre. • ∆, NA and Acceptance angle are very small.

18. Multimode Step-Index Fiber • Multimode fiber has a core diameter of 50 or 62.5 µm (sometimes even larger) • Allows several light paths or modes • This causes modal dispersion – some modes takes longer time to pass through the fiber than others because they travel a longer distance Index of refraction

19. Index of refraction Multimode Graded-Index Fibre • The index of refraction gradually changes across the core • Modes that travel further also move faster. • This reduces modal dispersion so the bandwidth is greatly increased.

20. Classification on the basis of material used • Glass fibres: 1) SiO2 core – B2O3.SiO2 cladding 2) GeO2.SiO2 core – SiO2 cladding • Plastic fibres: 1) Polystyrene core 2) Polymethyl methacrylate core • PCS fibres: The core is made from high purity quartz.

21. V-number (Normalized frequency) • The mathematical description of the V number is: • V = 2 Π* NA * a / λ • For V < 2.405, the fibre can support only one mode. • For V > 2.405, the fibre can support many modes simultaneously. • For V=2.405, the wavelength is called cut-off wavelength.

22. Advantages of optical Fibre • Cheaper • Smaller in size, lighter in weight, flexible yet strong • Not hazardous • Immune to EMI(electromagnetic interference) and RFI(radiofrequency interference) • No cross talk • Wider bandwidth • Low loss per unit length

23. Losses • ATTENUATION: 1) Absorption by material 2) Rayleigh Scattering 3) Geometric effects • DISPERSION: 1) Intramodal Dispersion 2) Intermodal Dispersion 3) Material Dispersion change in shape of signal .

24. ATTENUATION • The attenuation of optical signal is defined as the ratio of the optical input power from a fibre of length L to the optical output power. • It is expressed in decibel per kilometer (dB/Km). • Pi is the power of optical signal launched at one • end of the fibre • P0 is the power of the optical signal emerging from • the other end of the fibre.

25. 2. Macrobend losses Mechanisms of Attenuation • Intrinsic losses • B) Extrinsic losses 1. Microbend losses

26. Application of fibre optics1) Force Sensor/Pressure Sensor • Based on variations of light intensity. • When an optical fibre is pressed, a small change • occurs in light propagation direction due to • microbending of the fibre. • The change in intensity of the transmitted light is • proportional to the force applied on the optical fibre.

27. 2) Temperature sensors • Temperature is measured by the modulation of • intensity of the reflected light from a target, a silicon • layer. • The change in intensity of the reflected light is • proportional to the change in temperature.

28. 3) Smoke / Pollution Detector • A smoke and pollution detector can be built by • using optical fibres. • A beam of light radiating from one end of a fibre • can be collected by another optical fibre. • If foreign particles are present in the region • between the two fibres, they scatter light. • The variation in intensity of the light collected by • the second optical fibre reveals the presence of • foreign particles.

29. 4) Liquid Level Detector Principle: The liquid level detector described here is based on the principle of total internal reflection.

30. P I n Hole Electron  PIN DETECTOR • The device structure • consists of P and N regions • separated by a very lightly • n-doped intrinsic (i) region. • A semiconductor photodiode • is a reverse biased P-N • Junctions. Photons are • absorbed in the semi- • conductor and create • electron-hole pairs. BIAS VOLTAGE Photodiode Photon

31. Energy band diagram for a pin detector Photogenerated Electron BandgapEg p  ί Conduction Band Photon  hEgPhotogenerated n Hole Depletion Valence Band Region

32. p+ p n+ Avalanche Photo-Diode (APD) • An electron-hole pair is created by the absorption • of a photon. • The electron and hole accelerate rapidly in • opposite directions creating pairs. • Reverse bias voltage results in an effective • amplification of the photodiode output current. e-h Creation Multiplication Region V

33. NANOSCIENCE AND NANOTECHNOLOGY

34. Introduction • Nanoscience is concerned with the study of • phenomena and manipulation of materials at atomic, • molecular and macromolecular scales. • Nanotechnology is the design, characterization, • production and application of structures, devices • and systems by controlling shape and size at the • nanometer scale.

35. Introduction Cont… • Nanomaterials is a field that takes a material • science-based approach to nanotechnology. • It studies the materials with morphological • features on the nanoscale. • Nanoscale is usually defined as smaller than a • one tenth of a micrometer in at least one • dimension.

36. Classification of nano materials • Nanomaterials could be organized into four forms: •   Zero Dimensions •    One Dimensions •   Two Dimensions •      Three Dimensions

37. Zero Dimensions(Nano scale in zero dimension) • Ex. Nano particles, Precipitates, Colloids, Quantum • Dots, Fullerenes, Dendrimers etc. • Nano particles: particles of less than 100 nm diameter. • Quantum Dots: Nano particles of semiconductor. • Fullerenes: A molecule composed of carbon. • Spherical fullerene are called bucky balls. • Cylindrical are called carbon nano tubes or bucky • tubes • Dendrimers: They are spherical polymeric molecule.

38. Dendrimers: • These are nanosized • polymers built from • branched units. • The surface of a dendrimers • has numerous chain ends. • which can be tailored to • perform specific chemical functions. • This property could also be useful for catalysis. One Dimensions(Nano scale in one dimension) • Ex. Thin film, Surface coating • Used in silicon integrated circuit industry as many • devices rely on thin films for their operation.

39. Two Dimensions(Nano scale in two dimension) • Ex. Nano wires and nano tubes, biopolymers etc. • When two lengths of three dimensional • nanostructure is of nano dimension then the • structure is known as nanowire. Three Dimensions • Equiaxed nanometer sized grains (bulk structure)

40. BOTTOM UP Nanoparticle Synthesis TOP-DOWN Via attrition(erosion) and milling Involves mechanical thermal cycles Yields - broad size distribution (10-100 nm) - varied particle shape or geometry - impurities Application: - Nano-composites and Nano-grained, bulk materials. Via - Pyrolysis - Inert gas condensation - Solvothermal Reaction - Sol-gel Fabrication

41. Physical Vapour Deposition(Bottom – Up Synthesis): • In this method thin film is deposited onto various • surfaces by condensation of a vaporized form of • material under high vacuum condition. • The shape, size and chemical composition of a nano • structured material is controlled. • Various methods of Physical Vapour Deposition • involves – Evaporation, Sputtering, glow discharge, • RF Sputtering etc.

42. Process of Execution: • (i) precursor vaporization (typically involves a catalyst) • (ii) nucleation, and • (iii) growth stage • Effectiveness demands: • - simple process • low cost • continuous operation Aerosol Spray Methods • - High yield (e.g., Spray Pyrolysis)

43. Sol-gel Process • Formation of stable sol solution • Gelation via a polycondensation or polyesterification reaction • Gel aging into a solid mass causes contraction of the gel network, also (i) phase transformations and (ii) Ostwald ripening

44. Drying of the gel to remove liquid phases can lead to • fundamental changes in the structure of the gel. • Dehydration at temperatures as high as 800oC, • used to remove M-OH groups for stabilizing the gel, • i.e., to protect it from rehydration. • Densification and decomposition of the gels at high • temperatures (T > 800oC), i.e., to collapse the pores • in the gel network and to drive out remaining organic • contaminants.

45. Sol-gel Process

46. Comparison of properties of nano-materials with bulk materials 1. Physical Properties 2. Magnetic Properties 3. Mechanical Properties 4. Optical Properties 5. Electrical Properties

47. 1. Physical Properties: • When size of bulk material reduced to nano-size, • surface area to volume ratio increases. • Surface pressure changes and results in change in • inter-particle spacing. • Inter atomic spacing decreases with size. • Variation in surface free energy changes the • chemical potential which directly affects the melting • point of the material.

48. 2. Magnetic properties • Magnetic behavior of nano particle varies mostly due to surface area effect, including symmetry breaking charge transfer and magnetic interaction. • As the size reduced to nano-size, it results into increase in coercivity and retaintivity (Residual magnetism) of the material. • In case of Fe, Co, and Ni magnetic moment increases with decrease in co-ordination number. • Thus small particles are more magnetic than the bulk material.

49. 3. Mechanical Properties • Mechanical strength enhances due to reduction of particle size to nano-scale. • Because of nano size, many mechanical properties such as hardness, elastic modulus, fracture, toughness, scratch resistivity, fatigue strength can be modified. • Reduction in grain size lowers the transition temperature in steel from ductile to brittle. • The nano-phase materials are super plastic materials as it shows extensive tensile deformation without cracking.

50. Some special nanomaterials Zeolites (Crystalline microporous material) • Zeolites are crystallin aluminosilicates generating network of pores and cavities • Molecular dimension are less than 100nm There are 34 naturally and 100 synthetic types of zeolite. • Molecular sieving, high thermal stability, acidity, adsorption capacities, shape selectivity, ion exchange & physicochemical properties.