Introduction Light Interactions With Solids

IntroductionOptical PropertiesThe study of the optical properties of materials is a huge fieldand we will only be able to scratch the surface of this science. Light is an . wave: with a velocity given byc 1/ ( 0 0) 3 x 108 m/sOutline Introduction Basic Concepts 0 is the electric permittivity of avacuum 0 is the magnetic permeability of avacuum absorption reflection transmission refractionMagnetic fieldElectric field c wavelength*frequency ApplicationsIn view of this, it is not surprising that the electric fieldcomponent of the light waves interact with electrons. luminescence and fluorescence Laser and fiber opticsOptical properties: are the materials responses to exposureto electromagnetic radiation especially to visible light.Mech. Eng. Dept. - Concordia UniversityDr. M. MedrajMECH 221 lecture 24/1Dr. M. MedrajIntroduction 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 Frequency, Energyand wavelength aredifferent for eachradiation.From Quantummechanicalperspective,radiations arepackets of energycalled Alternatively T A R 1where T, A, and R arefractions of the amount ofincident light. T IT/I0, etc.The perceivedcolor is determinedby the wavelengthTransmitted: ITIncident: IoMECH 221 lecture 24/3transparent transparent: relatively little absorptionand reflection translucent: light scatters withinthe material opaque: relatively little transmissionopaquetranslucentFig. 21.10, Callister 8eGenerally, metals are opaque, electrical insulators can bemade transparent and some semiconductors are transparent.The spectrum of electromagnetic radiation, including wavelengthranges for the various colors in the visible spectrum.Mech. Eng. Dept. - Concordia UniversityAbsorbed: IAReflected: IR So materials are broadly classified asE h hc/ Dr. M. MedrajMECH 221 lecture 24/2Light Interactions with SolidsThe visible radiation isthe only radiation towhich the eye issensitiveh is Planck’s constant6.63x10-34 J-sMech. Eng. Dept. - Concordia UniversityDr. M. MedrajMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/4

OPTICAL PROPERTIES OF METALS: ABSORPTION Penetration depths for some materials are: Absorption of photons by electron transition:Energy of electron Metals have a finesuccession of energy states. This structure for metalsmeans that almost anyfrequency of light can beabsorbed.Iophntide gy h cInerenof Metal films thinner than thiswill transmit light - e.g. goldcoatings on space suithelmets E h required!filled statesfreq.ofincidentAdapted from Fig. 21.4(a), Callister 6e.lightPlanck’s constant(6.63 x 10-34 J/s) Since there is a very highconcentration of electrons,practically all the light isabsorbed within about 0.1µmof the surface.Dr. M. Medrajunfilled statesnot oDepending on the material and thewavelength, light can be absorbedby: nuclei – all materials electrons – metals andsmall band-gap materialsMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/5Optical Properties Of Metals: Reflection Electron transition emits a photon.Energy of electron Reflectivity IR/Io isbetween 0.90 and 0.95. Reflected light is samefrequency as incident. Metals appearreflective (shiny)!IROPTICAL PROPERTIES OF METALS: ABSORPTION water: 32 cmglass: 29 cmgraphite: 0.6 µmgold: 0.15µm So what happens to the excited atoms in the surfacelayers of metal atoms? they relax again, a photon The energy lost by the descending electron is the same asthe one originally incident So the metal reflects the light very well – about . formost metals metals are both opaque and reflective the remaining energy is usually lost as heatDr. M. MedrajMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/6OPTICAL PROPERTIES OF METALS: REFLECTION Reflection spectra for gold and aluminum are:unfilled states“conducting” electron Ere-emittedphoton frommaterialsurfacefilled statesaluminumspectrum isrelatively flatAdapted from Fig. 21.4(b), Callister 6e. The metal appears “silvery” since it acts as a perfect mirrorOK then, why are gold and copper not silvery? because the band structure of a real metal is not always as simple aswe have assumed; there can be some empty levels below EF and theenergy re-emitted from these absorptions is not in the visible spectrumMetals are more transparent to very high energy radiation (x-ray & ray).Dr. M. MedrajMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/7gold reflects lots ofred wavelengthsblueDr. M. MedrajMech. Eng. Dept. - Concordia UniversityredMECH 221 lecture 24/8

Selected Absorption: NonmetalsOptical Properties of Semiconductors Absorption by electron transition occurs if h Egap Cadmium Sulfide (CdS)Energy of electron- Egap 2.4eV,Semiconductors andinsulators behaveessentially the sameway, the only differencebeing in the size of the .unfilled statesblue light: h 3.1eV- absorbs higher energyvisible light (blue, violet),red light: h 1.7eVEgapincident photonenergy h GermaniumIofilled states Adapted from Fig. 21.5(a), Callister 6e. If Egap 1.8eV, full absorption; color is black (Si, GaAs) If Egap 3.1eV, no absorption; colorless (diamond) If Egap in between, partial absorption; material has a color.Dr. M. MedrajMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/9 Semiconductors can appear “metallic” if visible photons are all reflected(like Ge) but those with smaller Eg, such as CdS look coloured yellow for CdS which absorbs 540nm and aboveThis is applicable for pure materials but impurities can cause extraabsorption.Impurities divide up the band gap to allow transitions with energies lessthan EgMech. Eng. Dept. - Concordia UniversityDr. M. MedrajColor of NonmetalsMECH 221 lecture 24/10Transmitted Light: Refraction Transmitted light distorts electron clouds. Color determined by sum of frequencies ofnotransmittedlight- transmitted light,- re-emitted light from electron transitions. Example: Ruby Sapphire (Al2O3) (0.5 to 2) at% Cr2O3- Sapphire is colorless transmittedlight electronclouddistorts Result 1: Light is slower in a material vs vacuum.Index of refraction (n) speed of light in a vacuumspeed of light in a material(i.e., Egap 3.1eV)- adding Cr2O3 : alters the band gap blue light is absorbed yellow/green is absorbed red is transmitted Ruby has deep red color.CdS- Red/yellow/orange istransmitted and gives it color.- Adding large, heavy ions (e.g., leadcan decrease the speed of light.- Light can be"bent"Fig. 21.9, Callister 8e.MaterialLead glassSilica glassSoda-lime .491.49Selected values from Table 21.1, Result 2: Intensity of transmitted light decreasesCallister 6e.with distance traveled (thick pieces less transparent!)Dr. M. MedrajMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/11Dr. M. MedrajMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/12

TranslucencyApplication 1: Luminescence Even after the light has entered the material, itmight yet be reflected out again due to scatteringinside the material Even the transmitted light can lose information bybeing scattered internally Process:incidentradiationEgapfilled statesMECH 221 lecture 24/13 Energy of electronEnergy of electronsemiconductor:unfilled statesunfilled statesEgapIncidentradiation-A. No incident radiation:little current flowconductingelectron- Operation:- incident photon produces hole-elec. pair.- typically 0.5V potential.- current increases with light intensity.P-doped SiconductanceSielectronSi P SiEgapB. Incident radiation:increased current flowSilightn-type Sip-n junctionp-type SiBDr. M. Medrajcreation ofhole-electronpair-- SiSiB-doped SiThis phenomenon is utilized in photographic light meters. A photo-inducedcurrent is measured and its magnitude is a direct function of the intensity ofthe incident light radiation.MECH 221 lecture 24/15n-type Sip-n junctionp-type SiSihole Example: Photodetector (Cadmium sulfide)Mech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/14Application 3: Solar CellSiDr. M. MedrajUVradiationMech. Eng. Dept. - Concordia UniversityDr. M. Medrajfilled statesfilled states“ ” lightglass p-n junction: Adapted from Fig. 21.5(a), Callister 6e. Example: fluorescent lampsApplication 2: Photoconductivity Description:filled statesre-emissionoccursAdapted from Fig. 21.5(a), Callister 6e.coatinge.g., -aluminadopedw/EuropiumMech. Eng. Dept. - Concordia UniversityEgapemittedvisible lightelectrontransition occurs . in poly-crystalline materials fine pores in ceramics different phases of materialsDr. M. Medrajunfilled statesunfilled states so a beam of light will spread out or an image will becomeblurred In extreme cases, the material could becomeopaque due to excessive internal scattering Scattering can come from obvious causes:Energy of electronEnergy of electronCan be thought of as the reverseoperation of the light-emitting diode.Mech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/16

Application 4: LaserApplication 4: Laser LASER stands for Light Amplification by the Stimulated Emissionof Radiation. The key word here is “ ” All of the light emission we have mentioned so far is spontaneous. It happened just due to randomly occurring “natural” effects. The emitted light has the same energy and phase as theincident light ( ) Under normal circumstances, there are few excitedelectrons and many in the ground-state, so we get predominantly absorption If we could arrange for excited than non-excitedelectrons, then we would get mostly stimulated emission. Clearly, random spontaneous emission “wastes” electrontransitions by giving incoherent output.Dr. M. MedrajMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/17 The first is achieved by filling themetastable states with electronsgenerated by light from a xenon flashlamp. The second condition is achieved byconfining the photons to travel back andforth along the rod of ruby usingmirrored ends. Ruby is a common laser material, whichwe saw was Al2O3 (sapphire) with Cr3 impurities. When the electrons decay to themetastable state they may reside up to before stimulated emission longtime large number of thesemetastable states become occupied The energy levels of a laser material.avalanche of stimulated electrons.Dr. M. MedrajApplication 4: LaserMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/18Application 4: Laser In order to keep the coherentemission, we must ensure thatthe light which completes theround trip between the mirrorsreturns . with itself. Hence the distance between themirrors should obey 2L N where N is an integer, is the laser The rod’s endsare flat, paralleland highlypolished. Both ends aresilvered suchthat one istotally reflectingand the otherpartiallytransmitting.wavelength and L is the cavitylength. Semiconductor lasers work injust the same way except thatthey achieve the populationinversion electrically using acarefully designed bandstructure.Semiconductor laser (Callister Fig. 21.14)Dr. M. MedrajMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/19Dr. M. MedrajMech. Eng. Dept. - Concordia UniversityMECH 221 lecture 24/20