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Optical properties of materials

Optical properties of materials. basic processes – reflection, refraction, absorption refractive index absorption coefficient photoconduction photoexcitation of charge carriers in semiconductors  ph /  dark ~ 10 5 - 10 6 CdS, PbS photoresistors, photocopying, photography

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Optical properties of materials

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  1. Optical properties of materials

  2. basic processes – reflection, refraction, absorption • refractive index • absorption coefficient • photoconduction • photoexcitation of charge carriers in semiconductors • ph/ dark ~ 105 - 106 • CdS, PbS • photoresistors, photocopying, photography • photoluminescence • excitons • spontaneous emission – fluorescence/phosphorescence

  3. electroluminescence • light emitting diode • Mn2+ doped ZnS, PPV [poly(p-phenylenevinylene] - Metal electrode Electric field EL material + Transparent electrode (ITO) Light

  4. e- LUMO LUMO h+ HOMO HOMO Light Cathode Anode Principle of EL

  5. lasers • population inversion • stimulated emission • coherent, monochromatic, intense • types: gas (CO2), solid state (Nd:YAG), diode (InGaN), … • laser printer, compact disc h Pump h h Atom in ground state Atom in ground state Atom in excited state Stimulation of emission Inverted population

  6. photovoltaic effect • p-n junction • solar cells - Metal electrode Electric field PV material + Transparent electrode (ITO) Light

  7. e- LUMO h+ HOMO Light Anode Cathode Principle of PV LUMO HOMO • Solar cell materials • silicon • TiO2 in Dye Sensitized Solar Cells • Conjugated polymer / fullerene in Bulk Heterojunction Solar Cells

  8. Nonlinear optical processes Linear optical processes Reflection, Refraction, Absorption No change in properties of medium or light Nonlinear optical processes Electro-optic effect (Pockel's effect) - (2) (-;,0) Electro-optic modulators Frequency doubling - (2) (-2;,) Harmonic generation Frequency tripling - (3) (-3;,,) Deep UV conversion

  9. First hyperpolarizability

  10. Refractive Index variation n = n0 + n2I Pockel’s effect (2nd order effect) Refractive index varies with applied dc field Kerr effect (3rd order effect) Refractive index varies with incident laser intensity

  11. Laser 1 No light Polariser Crossed Polariser (3) material Laser 2 Laser 1 Polariser Crossed Polariser Kerr gate

  12. Electro-optic modulation (Mach-Zehnder interferometer) • EO modulators in fibre optic applications • LiNbO3

  13. E  sin (t)  P(2)  sin2(t)  { 1 - cos(2t) 532 nm (green laser) 1064 nm (infra red laser) Second Harmonic Generation (SHG) • Harmonic generation crystals in lasers • KH2PO4(KDP) • -BaBO3 (BBO)

  14. Problem Set • Briefly describe the exciton states in the molecular crystal of anthracene. These are called Frenkelexcitons. What is the characteristic feature of such excitons ? • Lead sulfide is a photoresistor sensitive to far infra red radiation. Find out its band gap from some text book and discuss its photoresistor application. • Draw a schematic diagram to illustrate the band bendings when an n-type and a p-type semiconductor are used as the anode and cathode respectively in a photoelectrolysis cell. Indicate the flow of electrons and holes to the electrode surface. • Prove through logical arguments that a centrosymmetric system cannot exhibit even order nonlinear optical effects. • Using the cubic term in the expansion for the polarization in terms of electric field, show how the phenomenon of third harmonic generation arises. • List some popular organic crystals that show efficient second harmonic generation using 1064 nm light. What is their SHG capability ? • What is Pockel’s effect ? A more detailed presentation on NLO materials is also provided on the website

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