Οπτικές Ιδιότητες των Υλικών

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1 Οπτικές Ιδιότητες των Υλικών -Προέρχονται από την επίδραση της Ηλεκτρομαγνητικής Ακτινοβολίας με τα Υλικά -Οπτικά φαινόμενα είναι: Απορρόφηση, διάθλαση, Εκπομπή φωτός (φθορισμός φωσφορισμός), αλλαγή της ηλεκτρικής αγωγιμότητας των υλικών

φαινόμενα είναι: Απορρόφηση, διάθλαση, Εκπομπή φωτός

2 Απορρόφηση Η χαρακτηριστική απορρόφηση του ατόμου δεν επηρεάζεται από το σχηματισμό χημικών δεσμών καθώς προκύπτει από διέγερση εσωτερικών ηλεκτρονίων και όχι ηλεκτρονίων της εξωτερικής στοιβάδας Το φάσμα απορρόφησης μπορεί να είναι διακριτό ή και συνεχές Επίσης υπάρχει απορρόφηση λόγω ηλεκτρονικής διέγερσης, δόνησης ή περιστροφής των μορίων Η απορρόφηση ακτινοβολίας περιγράφεται από τη σχέση: Ι=Ι ο exp(-ax) ή ln[i/i o ]=-ax όπου Ι= ένταση προσπίπτουσας ακτινοβολίας, Ι ο = ένταση προσπίπτουσας ακτινοβολίας που διαπερνά το υλικό, χ = το πάχος του υλικού a=γραμμικός συντελεστής απορρόφησης της ακτινοβολίας από το υλικό ( ο οποίος εξαρτάται από την πυκνότητα του υλικού και τη φυσική συχνότητα δόνησης των μορίων του υλικού

ακτινοβολίας περιγράφεται από τη σχέση: Ι=Ι ο exp(-ax) ή ln[i/i o ]=-ax όπου Ι= ένταση προσπίπτουσας ακτινοβολίας, Ι ο = ένταση προσπίπτουσας ακτινοβολίας που διαπερνά το υλικό, χ =

3 Απορρόφηση των μετάλλων και των μονωτών Τα μέταλλα παρουσιάζουν ισχυρή απορρόφηση και ανάκλαση της προσπίπτουσας ακτινοβολίας από τις πολύ μικρές συχνότητες μέχρι τις μεγάλες στο υπεριώδες άρα είναι αδιαφανή σε μεγάλο εύρος συχνοτήτων ενώ ο Pb και το Cd απορροφούν και στις ψηλές συχνότητες. Τα μέταλλα επίσης λόγω της ηλεκτρονικής τους δομής παρουσιάζουν έντονη απορρόφηση σε ορισμένες συχνότητες (μήκη κύματος) π.χ Au απορροφά το πράσινο φως και ανακλά το κόκκινο. Αντίθετα οι μονωτές απορροφούν μικρό μέρος της προσπίπτουσας ακτινοβολίας

4 Electron energy, E n. E = KE 0 Continuum of energy. Electron is free Excited states n = -5 Ionization energy, E I Ground state n = 1-15 n The energy of the electron in the hydrogen atom (Z = 1).

40 5 4 3 2 Excited states n = -5 Ionization energy, E I

5 Energy 0 n = 0 = 1 = 2 = 3 5 5s 5p 5d 5f 4 4s 4p 4d 4f 3 3s 3p 3d 2 2s 2p Photon -13.6eV 1 1s An illustration of the allowed photon emission processes. Photon emission involves = ±1.

6 Energy of luminescent center in host E 2 Non-radiative decay h ex E 2 Excitation Luminescent emission, h em E 1 E 1 Non-radiative decay Photoluminescence: light absorption, excitation, non-radiative decay and light emission, and return to the ground state E1.(The energy levels have been displaced horizontally for clarity.)

absorption, excitation, non-radiative decay and light emission, and return to

7 Thermalization CB E c D R E t Trapping E g Luminescent center or activator h < E g A R h > E g Recombination VB E v a b c d Optical absorption generates an EHP. Both carriers thermalize. There are a number of recombination processes via a dopant that can result in a luminescent emission.

absorption generates an EHP. Both carriers thermalize.

8 Phosphor Emitted light Phosphor Emitted light Incident light Heat (a) Photoluminescence Incident light Phosphor Incident electrons Activators or luminescent centers (e.g. Cr 3+ ) Host matrix (e.g. Al 2 O 3 ) (c) A typical phosphor = host + activators Heat (b) Cathodoluminescence Photoluminescence, cathodoluminescence and a typical phosphor

10 Selected example of phosphors. Phosphor Activator Useful Emission Example excitation Comment or application Y 2 O 3 :Eu 3+ Eu 3+ Red UV Fluorescent lamp, color TV BaMgAl 10 O 17 :Eu 2+ Eu 2+ Blue UV Fluorescent lamp CeMgAl 11 O 19 :Tb 3+ Tb 3+ Green UV Fluorescent lamp Y 3 Al 5 O 12 :Ce 3+ Ce 3+ Yellow Blue, Violet White LED Sr 2 SiO 4 :Eu 3+ Eu 3+ Yellow Violet White LED (experimental) ZnS:Ag + Ag + Blue Electron beam Color TV blue phosphor Zn 0.68 Cd 0.32 S:Ag + Ag + Green Electron Beam Color TV green phosphor ZnS:Cu + Cu + Green Electron Beam Color TV green phosphor

17 :Eu 2+ Eu 2+ Blue UV Fluorescent lamp CeMgAl 11 O 19 :Tb 3+ Tb 3+ Green UV Fluorescent lamp Y 3 Al 5 O 12 :Ce 3+ Ce 3+ Yellow Blue, Violet

11 Phosphor (YAG): yellow emission InGaN chip: blue emission 1.0 Blue Yellow White LED (a) 0.5 Total white emission Blue chip emission Yellow phosphor emission Wavelength (nm) (b) (a) A typical white LED structure. (b) The spectral distribution of light emitted by a white LED. Blue luminescence is emitted by the GaInN chip and yellow phosphorescence or luminescence is produced by a phosphor. The combined spectrum looks white.

(a) A typical white LED structure. (b) The spectral distribution of light emitted by a white LED.

12 This flash light uses a white LED instead of an incandescent light bulb. The flash light can operate continuously for 200 hours and can project an intense spot over 30 feet.

13 Φωτοηλεκτρικό φαινόμενο Light CATHODE ANODE Electrons I Evacuated quartz tube A V The Photoelectric Effect.

14 Ανάκλαση - Διάθλαση του Φωτός y E y Direction of Propagation Velocity = c z x x B z The classical view of light as an electromagnetic wave. An electromagnetic wave is a travelling wave which has time varying electric and magnetic fields which are perpendicular to each other and to the direction of propagation.

An electromagnetic wave is a travelling wave which has time varying electric

15 Constructive interference Destructive interference P S 1 S 2 Photographic film showing Young's fringes Schematic illustration of Young's double slit experiment.

16 A t t k t Refracted Light B t y A t t n 2 O z A B n 1 i r i r A B k i k r A i B r Incident Light B i A r Reflected Light A light wave travelling in a medium with a greater refractive index (n 1 > n 2 ) suffers reflection and refraction at the boundary.

$light wave travelling in a medium with a greater refractive$

17 Transmitted (refracted) light t k t n 2 n 1 > n 2 k i i i k r c c i > c Incident light Reflected light TIR (a) (b) (c) Light wave travelling in a more dense medium strikes a less dense medium. Depending on the incidence angle with respect to qc, which is determined by the ratio of the refractive indices, the wave may be transmitted (refracted) or reflected. (a) i < c (b) i = c (c) i > c and total internal reflection (TIR).

$Depending on the incidence angle with respect to qc, which is determined by the ratio of the refractive$

18 Optical Fiber Information Digital signal t Emitter Input Output Photodetector Information TIR n 2 Fiber axis Light ray Core Cladding n 1 n 2 An optical fiber link for transmitting digital information in communications. The fiber core has a higher refractive index so that the light travels along the fiber inside the fiber core by total internal reflection at the core-cladding interface.

$The fiber core has a higher refractive index so that the light travels along the fiber inside$

19 A small hole is made in a plastic bottle full of water to generate a water jet. When the hole is illuminated with a laser beam (from a green laser pointer), the light is guided by total internal reflections along the jet to the tray. The light guiding by a water jet was demonstrated by John Tyndall in 1854 to the Royal Institution. (Water with air bubbles was used to increase the visibility of light. Air bubbles scatter light.)

internal reflections along the jet to the tray.

20 y y z E t, t E t, Transmitted wave k t Evanescent wave E t, t = 90o n 2 x into paper E k i, i E i i, r E r, i r n 1 > n 2 E r, kr E i, k r E i, Incident wave E r, (a) i < c then some of the wave is Reflected wave transmitted into the less dense medium. Some of the wave is reflected. Incident wave Reflected wave (b) i > c then the incident wave suffers total internal reflection. There is a decaying evanescent wave into medium 2 E r,

transmitted into the less dense medium. Some of the wave is reflected.

21 X-rays Detector 2 A d dsin dsin d Crystal B Atomic planes (c) (c) X-ray diffraction involves constructive interference of waves being "reflected" by various atomic planes in the crystal.

22 Εκπομπή ενισχυμένης ακτινοβολίας E 2 E 2 E 2 h h h IN h OUT h E 1 E 1 E 1 (a) Absorption (b) Spontaneous emission (c) Stimulated emission Absorption, spontaneous emission and stimulated emission

23 h 32 E 3 E 3 h 13 E 2 Metastable state E 2 E 1 E 1 (a) (b) The principle of the LASER. (a) Atoms in the ground state are pumped up to the energy level E 3 by incoming photons of energy h 13 = E 3 -E 1. (b) Atoms at E rapidly decay to the metastable state at energy level E 2 by emitting photons or emitting lettice vibrations. h 32 = E 3 -E 2.

24 E 3 E 3 E 2 h 21 E 2 OUT E 1 E 1 h 21 Coherent photons (c) (c) As the states at E are metastable, they quickly become populated and there is a population inversion between E and E. (d) A random photon of energy h = E -E can initiate stimulated emission. Photons from this stimulated emission can themselves further stimulate emissions leading to an avalanche of stimulated emissions and coherent photons being emtitted. (d)

25 Flat mirror (Reflectivity = 0.999) Very thin tube Concave mirror (Reflectivity = 0.985) He-Ne gas mixture Laser beam Current regulated HV power supply A schematic illustration of the He-Ne laser

28 He Ne ev (1s 1 2s 1 ) Collisions (2p 5 5s 1 ) ev nm Lasing emission (2p 5 3p 1 ) Electron impact (2p 5 3s 1 ) Collisions with the tube walls Fast spontaneous decay ~600 nm 0 (1s 2 ) Ground states (2p 6 ) The principle of operation of the He-Ne laser. He-Ne laser energy levels (for 632.8nm emission).

29 Emission Intensity Allowed Cavity Oscillations Relative intensity Doppler broadening n( /2) = L (a) (b) (c) (a) Doppler broadened emission vs. wavelength characteristics of the lasing medium. (b) Allowed oscillations and their wavelengths within the optical cavity. (c) The output spectrum is determined by satisfying (a) and (b) simultaneously.

30 Energy of the Er 3+ ion in the glass fiber 1.27 ev 980 nm 0 Pump E 3 Non-radiative decay 0.80 ev E nm 1550 nm In E 1 Out Energy diagram for the Er 3+ ion in the glass fiber medium and light amplification by stimulated emission from E 2 to E 1. Dashed arrows indicate radiationless transitions (energy emission by lattice vibrations)

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