Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu Engineering 45 Optical Properties Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu

Learning Goals – Optical Props Learn How Light and Solid Materials Interact Why materials have characteristic colors Why some materials transparent and others not Optical applications: Luminescence Photoconductivity Solar Cell Optical Fiber Communications

Properties of Solid Materials Mechanical: Characteristics of materials displayed when forces are applied to them. Physical: Characteristics of materials that relate to the interaction of materials with various forms of energy. Chemical: Material characteristics that relate to the structure of a material. Dimensional: Size, shape, and finish

Material Properties Chemical Physical Mechanical Dimensional Composition Melting Point Tensile properties Standard Shapes Microstructure Thermal Toughness Standard Sizes Phases Magnetic Ductility Surface Texture Grain Size Electrical Fatigue Stability Corrosion Optical Hardness Mfg. Tolerances Crystallinity Acoustic Creep Molecular Weight Gravimetric Compression Flammability

ElectroMagnetic Radiation Energy associated with Light, Radio Signals, X-rays and Others is Transmitted as ElectroMagnetic (EM) Radiation (EMR) Electromagnetic radiation Transmits energy in the form of a Sinusoidal wave Which Contains ELECTRICAL & MAGNETIC Field-Components The EM waves Travel in Tandem, and are perpendicular to Each Other The Direction Of Propagation

The EM Spectrum EM Waves Cover a Wide Range of WAVELENGTHS, , and FREQUENCIES,  : miles→femtometers “Light” is generally divided into Three Segments UltraViolet: 0.001→0.35 µm NOT Visible, High in Energy Visible: 0.35→0.7 µm A VERY Small Slice of the EM spectrum InfraRed: 0.7-1000 µm Not Visible; carries “sensible” energy (heat)

EM Radiation Quantified All EM Waves Travel at the Speed of Light, c c is a Universal Constant with a value of 300 Mm/s (186 000 miles/sec) c is related to the Electric & Magnetic Universal Constants Where (Recalling From Previous Lectures) 0  ELECTRIC Permittivity of Free Space (a vacuum) µ0  MAGNETIC Permeability of Free Space (a vacuum)

EM Radiation Quantified The Wavelength and Frequency of EM waves are related thru c EM radiation has a Wave↔Particle Duality The Energy, E, of a Light Particle Where   WaveLength in meters per cycle   Frequency in Hertz (cycles/sec) Where h  Planck’s Constant (6.63x10-34 J-s) h is the PHOTON Energy Solar Photon-Flux (photons/sq-m-s is important in solar cell design calculations

EM-Solid Interaction Consider EM Radiation with Intensity I0 (in W/m2) Impinging on a Solid The EM-Solid interaction Alters the incident Beam by 3 possible Phenomena The EM Beam can be Reflected Absorbed Transmitted Solar Photon-Flux (photons/sq-m-s is important in solar cell design calculations

EM-Solid Interaction cont Mathematically Where all the IK are Intensities in W/sq-m Now Divide E-Balance Eqn by I0 An Energy Balance on the Solid: E-in = E-reflected + E-absorbed + E-transmitted Solar Photon-Flux (photons/sq-m-s is important in solar cell design calculations

EM-Solid Interaction cont.2 Where: R  REFLECTANCE (IR/I0) A  ABSORBANCE (IA/I0) T  TRANSMITTANCE (IT/I0) Using R, A, T, Classify EM-Solid Behavior Opaque → T = 0 Transparent → T >> A+R Light Not Scattered Translucent→ T > A+R Light Scattered Solar Photon-Flux (photons/sq-m-s is important in solar cell design calculations

Metals – Optical Absorption Energy of electron Incident photon filled states unfilled states D E = h required I o of Energy h Metals – Optical Absorption Metals Interact with Light Thru QUANTIZED Photon Absorption by Electrons Metals have Very Closely Spaced e- Energy Levels Thus Almost ALL incident Photons are ABSORBED within about 100 nm of the surface

Metals – Optical Reflection re-emitted photon from material surface Energy of electron filled states unfilled states D E IR “conducting” electron Metals – Optical Reflection The Absorbed Energy is ReEmitted by e- “falling” back to Lower Energy states Since Metals have Very Closely Spaced e- Energy Levels The Light is emitted at many ’s Thus Outgoing Light Looks About the Same as Incoming Light → High Reflectance

Metals - Colors Metals also ABSORB Some Photons Dissipated as heat Metals that Absorb few, or in broad-spectrum, reflect “WHITE” Light and Appear Silvery Some Metals absorb Preferentially, and the Reflected Light is Colored due the absence of the Absorbed light e.g., Cu Absorbs in the Violet-Blue; leaving Reflected light rich in Orange-Red Cu Bar Sn-Plated Cu Bar

NonMetals – Selective Absorb. incident photon energy hn Energy of electron filled states unfilled states E gap I o blue light: h 3.3 ev red light: h 1.8 ev NonMetals – Selective Absorb. In The Case of Materials with “Forbidden” Gaps in the Band Structure, Absorption Occurs only if h>Egap For These Materials there is Very little ReEmission The Material Color Depends on the Width of the BandGap

Color Cases – BandGap Matls Egap < 1.8 eV ALL Visible Light Absorbed; Solid Appears Gray or Black in Color e.g., Si with Egap = 1.1 eV Egap > 3.3 eV NO Visible Light Absorbed; Solid Appears Clear and Transmissive e.g., Diamond Egap = 5.45 eV, SiO2 Egap = 8-9 eV 1.8 eV < Egap < 3.3 eV Some Light is absorbed and Material has a color

NonMetal Colors Color determined by sum of frequencies transmitted light re-emitted light from electron transitions e.g., Cadmium Sulfide (CdS) Egap = 2.4eV Absorbs higher energy visible light (blue, violet), Red/yellow/orange is transmitted and gives it this color CdS

NonMetal Colors cont. Ex: Ruby = Sapphire (Al2O3) + 0.5-2 at% Cr2O3 Sapphire is colorless (i.e., Egap > 3.1eV) adding Cr2O3 alters the band gap blue light is absorbed yellow/green is absorbed red is transmitted Result: Ruby is deep Red in color

Wavelength vs. Band Gap Example: What is the maximum wavelength absorbed by Ge? Find Ge BandGap: Eg = 0.67 eV Thus Need Ephoton = hc/λmax ≥ Eg Use the Photon Energy Eqn:

Light Refraction When Light Encounters a Matter-Containing Environment, it SLOWS DOWN Due to Interaction with Electrons transmitted light + electron cloud distorts + no transmitted light Define the INDEX of REFRACTION, n

Light Refraction cont The slowing of light in a Non-Vacuum Medium Results in Refraction, or Bending of the light Path Light Refracts per Snell’s Law :

Refraction Physics Recall Thus n Now the relations for v and c Most Matls are NOT magnetic → µr  1 So Where ε & µ are respectively the Permittivity & Permeability of the Material Now Recall e.g. Germanium n = 3.97 → n2 = 15.76 r = 16.0 (very close)

Application  Luminescence emitted light h1+ h2+... Energy of electron filled states unfilled states E gap Re-emission Occurs Incident Radiation h0 Electron Excitation Energy of electron filled states unfilled states E gap UV radiation coating e.g.; -alumina, doped w/ Europium “white” light glass Application  Luminescence Based on EM Induced e− excitation, and then Relaxation with Broad-Spectrum h Emission e.g. fluorescent lamps

Application  PhotoConduction Incident radiation Energy of electron filled states unfilled states E gap Conducting e- + - B. Incident radiation: Increased current flow Application  PhotoConduction h Absorption by NO-Junction SemiConductors results in the Elevation of an e- to the Conduction Band Where it Can Carry an E-Field Driven Current semi conductor: Energy of electron filled states unfilled states E gap + - A. No incident radiation: little current flow e.g. Cadmium Sulfide

Application  Si Solar Cell Recall The PN Junction Operation for Si Cell: An incident PHOTON produces HOLE-ELECTRON pair. Typically 0.5-0.7 V potential Theoretical Max = 1.1 V (Egap). Current INCREASES with INCREASED Light INTENSITY Need to Minimize Reflectance n-type Si p -type Si p-n junction B-doped Si B hole P conduction electron -doped n p + E - hv generates Minority carriers, e.g. h+ in n-type. The h+ that make it to the jcn are then “collected” (swept downhill) by E-field and contribute to current flow n-type => POS ion cores P-type => NEGATIVE Ion Cores

Application – Heat Mirror Natural SunLight is Very Pleasant However, In Sunny Climes Windows that Admit Visible Light ALSO transmit InfraRed EM radiation that Heats the Building; increasing AirConditioning costs Soln → “Heat” Mirror Window

Application – Heat Mirror cont A Perfect Heat Mirror Would Transmit 100% of EM radiation (light) in the visible 350-700 nm Wavelength range Reflect 100% of EMR over 700 nm Heat Mirror Windows are Constructed from thin-film coated “window glass” HM Film Stack → dielectric / metal / dielectric (D/M/D) e.g., 300Å TiO2 / 130Å Ag / 300Å TiO2 http://www.cerac.com/pubs/cmn/cmn6_4.htm

The Solar Spectrum All Done for Today http://faculty.evansville.edu/ck6/bstud/euler.html ".. indeed, far and away the most prolific writer in the history of the subject" writes Howard Eves in An Introduction to the History of Mathematics. Euler's contribution to mathematics is represented here by a few of the notations conventionalized by him or in his honor. Around the world, these are read, written, and spoken thousands of times every day: e for the base of the natural logarithm (a.k.a. "the calculus number") a, b, c for the sidelengths of a triangle ABC f(x) for functional value R and r for the circumradius and inradius of a triangle sin x and cos x for values of the sine and cosine functions i for the imaginary unit, the "square root of -1" capital sigma for summation. capital delta for finite difference. Euler grew up near Basel, Switzerland, and studied at an early age under Johann Bernoulli. He finished studies at the University of Basel when only 15 years old. From 1727 to 1741, Euler worked in St. Perersburg, Russia, and then moved to the Akademie in Berlin. In 1766 he returned to St. Petersburg, where he remained.

WhiteBoard Work Derive Eqns 21.18 21.19 Thick, Strongly Absorbing Medium of thickness d 21.19 Weakly Absorbing (transparent) medium with Reflection, R, and thickness d

Hot Miror (Heat Reflecting) Heat Mirror Hot Miror (Heat Reflecting) What - These "hot mirror" filters transmit the visible spectrum and reflect the infrared. At any specified angle of incidence, the average transmission is more than 93% from 425 to 675 nm. The average reflectance of our standard Hot Mirror is more than 95% from 750 to 1150 nm. Extended Hot Mirror: The average reflectance is more than 90% from 750 to 1600 nm. Long IR Hot Mirror The average reflectance is more than 90% from 1700 to 3000 nm Cold Mirror (Heat Transmitting) These "cold mirror" filters reflect the visible spectrum and transmit heat (infrared). At any specified angle of incidence, average reflectance is more that 95% from 450 to 675 nm. Transmission is more than 85% from 800 to 1200 nm.