1 / 16

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

hana
Download Presentation

Optical properties of materials

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related