ENG2000 R I Hornsey Optic 1 ENG2000 Chapter 10 Optical Properties of Materials ENG2000 R I Hornsey Optic 2 Overview • The study of the optical properties of materials is a huge field and we will only[.]
ENG2000 Chapter 10 Optical Properties of Materials ENG2000: R.I Hornsey Optic: Overview • The study of the optical properties of materials is a huge field and we will only be able to touch on some of the most basic parts • So we will consider the essential properties such as absorption/reflection/transmission and refraction • Then we will look at other phenomena like luminescence and fluorescence • Finally we will mention applications, in particular optical fibres and lasers ENG2000: R.I Hornsey Optic: Nature of light • Light is an electromagnetic wave: § with a velocity given by c = 1/(0à0) = x 108 m/s ã In view of this, it is not surprising that the electric field component of the wave should interact with electrons electrostatically ENG2000: R.I Hornsey http://www.astronomynotes.com/light/emanim.gif Optic: • Many of the electronic properties of materials, information on the bonding, material composition etc was discovered using spectroscopy, the study of absorbed or emitted radiation § evidence for energy levels in atoms § evidence for energy bands and band-gaps § photoelectric effect ENG2000: R.I Hornsey Optic: General description of absorption • Because of conservation of energy, we can say that I0 = IT + IA + IR § Io is the intensity (W/m2) of incident light and subscripts refer to transmitted, absorbed or reflected • Alternatively T + A + R = where T, A, and R are fractions of the amount of incident light Đ T = IT/I0, etc ã So materials are broadly classed as § transparent:relatively little absorption and reflection § translucent:light scattered within the material (see right) § opaque:relatively little transmission http://www.tekano.pwp.blueyonder.co.uk/tekano/translucent.jpg ENG2000: R.I Hornsey Optic: • If the material is not perfectly transparent, the intensity decreases exponentially with distance • Consider a small thickness of material, x • The fall of intensity in x is I so I = - x.I § where α is the absorption coefficient (dimensions are m-1) • In the limit of x 0, we get dI =− I dx • The solution of which is I = I0 exp(– x) • Taking “ln” of both sides, we have: I x = − ln I0 § which is known as Lambert’s Law (he also has a unit of light intensity named for him) ENG2000: R.I Hornsey Optic: • Thus, if we can plot -ln(I) against x, we should find from the gradient • Depending on the material and the wavelength, light can be absorbed by § nuclei – all materials § electrons – metals and small band-gap materials ENG2000: R.I Hornsey Optic: ATOMIC ABSORPTION • How the solid absorbs the radiation depends on what it is! • Solids which bond ionically, show high absorption because ions of opposite charge move in opposite directions § in the same electric field § hence we get effectively twice the interaction between the light and the atoms • Generally, we would expect absorption mainly in the infrared § because these frequencies match the thermal vibrations of the atoms ENG2000: R.I Hornsey Optic: • If we think of our atom-on-springs model, there is a single resonance peak: absorption f f0 • But things are more complex when the atoms are connected – phonons § recall transverse and longitudinal optical phonons ENG2000: R.I Hornsey Optic: Electronic absorption • Absorption or emission due to excitation or relaxation of the electrons in the atoms ENG2000: R.I Hornsey http://www.nhn.ou.edu/~kieran/reuhome/vizqm/figs/hydrogen.gif Optic: 10