Chapter 14Chapter 14

Optical PropertiesOptical Properties

of Materialsof Materials

A. A. ELECTROMAGNETIC RADIATIONELECTROMAGNETIC RADIATION

In the classical sense, electromagnetic radiation is considered to be wave-like （ wave ） , consisting of electric and magnetic field components that are perpendicular to each other and also to the direction of propagation.All electromagnetic radiation traverses a vacuum at the same velocity, namely, 3×108m/s. This velocity, c, is related to the electric permittivity of vacuum Є0 and the magnetic permeability of a vacuum 0 through

00

1

c (21.1)

vc (21.2)

Frequency: hertz (Hz), and 1 Hz=1 cycle per second

F21.1 F21.2

From a quantum-mechanical perspective, rather than

consisting of waves, radiation composed of groups or

packets of energy, which are called photons （ particle ） .

Energy E of a photon.

hc

hvE (21.3)

h: Planck’s constant, 6.63×10-34 J-s

B. LIGHT LINTERACTIONS WITH SOLIDSB. LIGHT LINTERACTIONS WITH SOLIDS

When light proceeds from one medium into another ， the intensity I0 of the beam incident equal the sum of

the intensities of the transmitted absorbed, and

reflected beams, denoted as IT, I A and IR

I0 = IT + IA+ IR (21.4)

Radiation intensity: watts per square meter

T + A + R = 1 (21.5)

where T, A, and R represent, (IT/I0), (IA/I0), (IR/I0).

Transparent: transmitting light with relatively little absorption and reflection

Translucent: light is transmittd diffusely; light is scattered within the interior, to the degree that objects are not clearly distinguishable.

Opaque: impervious to the transmission of visible light

C. ATOMIC AND ELECTRONIC INTERACTIONSC. ATOMIC AND ELECTRONIC INTERACTIONS

C-1. ELECTRONIC POLARIZATIONC-1. ELECTRONIC POLARIZATION

A rapidly fluctuating electric field interacts to induce electronic poarization, or to shift the electron cloud relative to the nucleus of the atom. Two consequences: (1) some of the radiation energy may be absorbed, and (2) light waves are retarded in speed （ refraction ） .

bulk metals: opaque throughout the entire visible spectrum; all light radiation is either absorbed or reflected. Electrically insulating materials: can be made to be transparent. Semiconducting materials: transparent or opaque.

C-2. ELECTON TRANSITIONSC-2. ELECTON TRANSITIONS

For an isolated atom

hvE

F21.3

(21.6)

h: Planck’s constant • the energy states for the atom are discrete, only specific ΔE’s exist between the energy levels; only photons of frequencies corresponding to the possible ΔE’s for the atom can be absorbed by electron transitions. • all of a photon’s energy is absorbed in each excitation event. • stimulated electron cannot remain in an excited state indefinitely; after a short time, it falls or decays back into its ground state (or unexcited level)• for solid materials: electron band structure

C-3. OPTICAL PROPERTIES OF METALSC-3. OPTICAL PROPERTIES OF METALS

Metals are opaque because the incident radiation having frequencies within the visible range excites electrons into unoccupied energy states above the Fermi energy.incident radiation is absorbed

Within a very thin outer layer, usually less than 0.1m (only metallic films thinner than 0.1m are capable of transmitting visible light.).

Metals are opaque to all electromagnetic radiation, from radio waves, into about the middle of the ultraviolet radiation. (Metals are transparent to high frequency (x-and r-ray) radiation.)

F18.4 F21.4

Most of the absorbed radiation is reemitted from the surfac

e in the form of visible light of the same wavelength, which a

ppears as reflected light.

The reflectivity for most metals is between 0.90 and 0.95;

some small fraction of the energy from electron decay proces

ses is dissipated as heat.

The perceived color is determined by the wavelength distr

ibution of the radiation that is reflected. A bright silvery appe

arance when exposed to white light indicates that the metal i

s highly reflective over the entire range of the visible spectru

m,e.g., aluminum and silver. Copper and gold appear red-or

ange and yellow: some of the energy associated with light ph

otons having short wavelengths is not reemitted as visible lig

ht.

C-4. OPTICAL PROPERTIES OF NONMETALSC-4. OPTICAL PROPERTIES OF NONMETALS

Nonmetallic materials may be transparent to visible light ：reflection, absorption, refraction and transmission.

D. D. REFRACTIONREFRACTION

Index of refraction n: the ratio of the velocity in a vacuum c to the velocity in the medium v,

v

cn (21.7

)The magnitude of n depends on the wavelength of the light. (Thus, e.g., dispersion or separation of a beam of white light by a glass prism.)

1

v (21.8)

00

1

c (21.1)

F18.4

T21.1

Є and : permittivity and permeability of the particular substance.

rrv

cn

00

(21.9)

Io and IR : intensities of the incident and reflected beams, if the light is normal (or perpendicular) to the interface

Єr, r: dielectric constant and the relative magnetic permeabilety, since most substances are only slightly magnetic, r 1, rn (21.10)

The retardation of electromagnetic radiation in a medium results from electronic polarization: generally, the larger an atom or ion, the greater will be the electronic polarization, the slower the velocity, and the greater the index of refraction. Index of refraction for soda-lime glass:1.5; Highly leaded glasses containing 90 wt ％ PbO:2.1. crystalline ceramics that have cubic crystal structures, and for glasses, the index of refraction is independent of crystallographic direction (i.e., it is isotropic). Nocubic crystals, have an anisotropic n; the index is greatest along the directions that have the highest density of ions.

E. REFLECTIONE. REFLECTION

When light radiation passes from one medium into another having a different index of refraction, some of the light is scattered at the interface reflectivity R

0I

IR R (21.1

1)

Io and IR : intensities of the incident and reflected beams, if the light is normal (or perpendicular) to the interface

(21.12)

2

12

12

nn

nnR

2)1

1(

s

s

n

nR (21.1

3)

The index of refraction of air is very nearly unity. The higher the index of refraction of the solid, the greater is the reflectivity. The reflectivity vary with wavelength. Reflection losses for lenses and other optical instruments are minimized significantly by coating the reflecting surface with very thin layers of dielectric materials such as magnesium fluoride (MgF2).

n1 and n2 : indices of refraction of the media. If the incident light is not normal to the interface, R will depend on the angle of incidence. When light is transmitted from a vacuum or air into a solid s

F.ABSORPTIOF.ABSORPTIONN Nonmetallic materials may be opaque or

transparent to visible light; and, if transparent, they often appear colored . Light radiation is absorbed by two basic mechanisms:

•electronic polarization (Section 21.4): important only at light frequencies in the vicinity of the relaxation frequency of the constituent atoms.

•valence band-conduction band electron transitions: depends on the electron energy band structure.

absorption of a photon of light by the promotion or excitation of an electron from the nearly filled valence band, across the band gap, and into an empty state within the conduction band.(21.14

)(21.15)

F21.5

hν ＞ Eghcλ

＞ Eg

The maximum band gap energy Eg (max) for which absorption of visible light is possible

(min)

maxhc

Eg

eVm

smeV s

1.3104

)/103(1013.47

815

(21.16

a)

No visible light is absorbed by nonmetallic materials having band gap energies greater than about 3.1eV; these materials, if of high purity, will appear transparent and colorless.

The minimum band gap energy Eg (min) for which there is absorption of visible light is a

(min)

maxhc

Eg

eV

m

smeV s 8.1107

)/103(1013.47

815

F21.5

All visible light is absorbed for those semiconducting materials that have band gap energies less than about 1.8eV; these materials are opaque. Only a portion of the visible spectrum is absorbed by materials having band gap energies between 1.8 and 3.1 eV; consequently, these materials appear colored.

Every nonmetallic material becomes opaque at some wavelength, which depends on the magnitude of its Eg. For example, diamond, having a band gap of 5.6 eV, is opaque to radiation having wavelengths less than about 0.22m.

)( Eenergyholeelectron (21.17)

F21.5

If impurities or other electrically active defects are present,

electron levels within the band gap may be introduced (such as

the donor and acceptor levels (Section 18.11), except that they

lie closer to the center of the band gap), the electromagnetic

energy that was absorbed by electron excitation must be

dissipated, several mechanisms:

•direct electron and hole recombination

• multiple-step electron transitions:one possibility, (Figure 21.

6b) emission of two photons; alternatively, generation of a p

honon (Figure 21.6c) F21.6

The intensity of transmitted or nonabsorbed radiation I’T continuously decreases with distance x

XeT II 0'' (21.18)

I’0 :intensity of the nonreflected incident radiation. : absorption coefficient (in mm-1), Materials that have large values are considered to be highly absorptive.

G. TransmissionG. Transmission

lT eRII 2

0 )1( (21.19)

It is assumed that the same medium exists outside both front and back faces. R, A, and T depends on light wavelength

F21.7

H. COLORH. COLORTransparent materials appear colored, the color discerned is a result of the combination of wavelengths that are transmitted. If absorption is uniform for all visible wavelengths, the materials appears colorless; e.g., high-purity inorganic glasses and high-purity and single-crystal diamonds and sapphire.Fraction of the visible light having energies greater than Eg is selectively absorbed (by valence band conduction band electron transitions.) some of this absorbed radiation is reemitted as the excited electrons drop back into their original, lower-lying energy states. It is not necessary that this reemission occur at the same frequency as that of the absorption. As a result, for semiconducting materials the color depends on the frequency distribution of both transmitted and reemitted light beams.

F21.9F21.8

Fro example, cadmium sulfide (CdS) has a band gap of about 2.4eV; it absorbs blue and violet portions, some of this energy is reradiated as light having other wavelengths. Nonabsorbed visible light consists of photons having energies between about 1.8 and 2.4eV. Cadmium sulfide takes on a yellow-orange color

• Ceramics, specific impurities also introduce electron levels within the forbidden band gap, photons having energies less than the band gap may be emitted as a consequence of electron decay processes involving impurity atoms or ions. F3.17 F20.2

0

For example, high-purity and single-crystal aluminum oxide or sapphire is colorless. Ruby, which has a brilliant red color, is simply sapphire to which has been added 0.5 to 2% of chromium oxide (Cr2O3). Cr3+ ion substitutes for the Al 3+ ion in the Al2O3 crystal structure and introduces impurity levels within the wide energy band gap of the sapphire. Nonabsorbed or transmitted light mixed with reemitted light imparts to ruby its deep-red color.

Inorganic glasses are colored by incorporating transition or rare earth ions while the glass is still in the molten state: blue-green; Co 2+ , blue-violet; Cr3+, green; Mn2+, yellow; and Mn3+ , purple. These colored glasses are also used as glazes (釉料 ), decorative coatings (彩色鍍層 ) on ceramic ware (陶瓷器件 ).

I. OPACITY AND TRANSLUCENCY IN I. OPACITY AND TRANSLUCENCY IN INSULATORSINSULATORS

Many dielectric materials that are intrinsically transparent may be made translucent or even opaque because of interior reflection and refraction: multiple scattering events. Opacity results when the scattering is so extensive.

Internal scattering: •polycrystalline specimens in which the index of refraction is anisotropic normally appear translucent. Both reflection and refraction occur at grain boundaries.

• scattering of light also occurs in two-phase materials

• The greater difference in the refractive index difference, the

more efficient is the scattering

F21.10

Residual porosity in the form of finely dispersed pores. Also effectively scatter light radiation.

For intrinsic polymers (without additives and impurities), the degree of translucency is influenced primarily by the extent of crystallinity. Some scattering of visible light occurs at the boundaries between sperulites and between crystalline and amorphous regions, as a result of different indices of refraction. For highly crystalline specimens: translucency, even opacity. Highly amorphous polymers are completely transparent.

J. APPLICATIONS OF OPTICAL PROPERTIESJ. APPLICATIONS OF OPTICAL PROPERTIES

J-1. LUMINESCENCEJ-1. LUMINESCENCE

Some materials are capable of absorbing energy and then reemitting visible light in a phenomenon called luminescence. Energy is absorbed when an electron is promoted to an excited energy state; visible light is emitted when it falls back to a lower energy state if 1.8eV<hv<3.1eV. The absorbed energy may be supplied as higher-energy electromagnetic radiation (ultraviolet light) high energy electrons, or by heat, mechanical, or chemical energy. Luminescence is classified according to the delay time between absorption and reemission events.

If reemission occurs for times much less than one second: fl

uorescence; for longer times: phosphorescence; e.g., sulfid

es, oxides, tungstates, and a few organic materials. Ordina

rily, pure materials do not disply these phenomena, and to i

nduce them, impurities in controlled concentrations must be

added.

Luminescence applications:

• fluorescent lamps: glass housing, coated on the inside

with tungstates or silicates. Ultraviolet light is generated

from a mercury glow discharge, which causes the coating

to fluoresce and emit white light.

• television screen: inside of the screen is coated with a material that fluoresces as an electron beam inside the picture tube very rapidly traverses the screen

• Detection of x-rays and υ-rays

• light-emitting diodes(LEDs) : some p-n rectifying

junctions, (Section 18.15), may also be used to generate

visible light: electroluminescence. Such diodes that

luminesce visible light are the familiar light-emitting

diodes(LEDs), which are used for digital displays. The

characteristic color of an LED depends on the particular

semiconducting material that is used.

•White light LED Lighting:photoluminescence.

F18.19

#60 Fig.6

#2 #3 #25#24#5#4

Applications: photographic light meters. (cadmium sulfide is commonly utilized) sunlight be directly converted into electrical energy solar cells, empoly semiconductors. Reverse of that for the light-emitting diode. A p-n junction is used in which potoexcited electrons and holes are holes are drawn away from the junction, in opposite directions, and become part of an external current.Electron transitions heretofore discussed: spontaneous (without

any external provocation.) random times, incoherent.

J-2. PHOTOCONDUCTIVTTYJ-2. PHOTOCONDUCTIVTTY

The conductivity of semiconducting materials depends on: number of free electrons in the conduction band and number of holes in the valence. Photon-induced electron transitions: when a specimen of a photoconductive materials is illuminated, the conductivity increases: photoconductivity.

Laser: coherent light generated by electron transitions initiated by an external stimulus. “laser” light amplification by stimulated emission of radiation. solid-state ruby laser. Ruby single

crystal of Al2O3 (sapphire) added of 0.05% Cr 3+

ions. The ruby laser is in the form of a rod, the ends of which are flat, parallel, and highly polished. Both ends are silvered such that one is totally reflecting and the other partially transmitting.Photons of 0.56m from the xenon lamp excite electrons from the Cr3+ ions into higher energy states. Some fall back directly; other electrons decay into a metastable intermediate state, where they may reside for up to 3 ms 3ms is a relatively long time, a large number of these metatable states may become occupied.

350 400 450 500 550 600

Inte

nsity

(a.u

.)

Wavelength(nm)

500 550 600 650 700 750

Inte

nsity

(a.u

.)

Wavelength(nm)

Excitation spectra of CaS:Eu

Emission spectra of CaS:Eu

Phosphor powders

Blue LED chip

B

Ι. Introduction to Phosphors

A.Phosphors: luminescence conversion materials

material (powders) that emit visible light when irradiated with shorter wavelength light (UV or blue)

Beta-SiAlON:Eu Alpha-SiAlON:Eu CaSiAlN3:Eu

01

02

B. Applications(a)White light LED Lighting

(b)Back light source of flat panel displayLED For TV Backlight

C. Chemical Composition: ：Host Lattice activator(doping)

Oxides (conventional):

Nitrides and Oxynitrides (newly developed):

•Y2O3 ： Eu+3

•Y3Al5O12 ： Ce+3 (YAG)

•Tb3Al5O12 ： Ce+3

•Ca2Si5N8 ： Eu+2

•Ca2Si2N2O2 ： Eu+2

• Ca-α-SiAlON ： Eu+2

• Y-α-SiAlON ： Ce+3

D. Features of Nitride and Oxynitride Phosphors (compared to oxide phosphors)

Potential advantages• high thermal and chemical stability • high emission intensity (or high quantum yield)• long wavelength( orange-red) emission, improving color rendering index, which are usually difficult to be achieved by other phosphors.

More difficult to be synthesized ․more severe synthesis conditions, e.g., high reaction temperature, high N2 pressure, long reaction time or

use of moisture or oxygen sensitive starting materials.

Fig 6

Fundamentals of luminescence

Fig.2 The Jablonski diagram: (1) light absorption, (2) vibrational relaxation, (3) internal conversion (IC), (4) intersystem crossing (ISC), (5) radiative transition, and (6) nonradiative transition.

Typical photoluminescence excitation (ìmon ) 649 nm) and emission (ìex ) 460 nm)spectra of Ca0.98Eu0.02AlSiN3 (a) and the schematic picture of the influence of the environment of a Eu2+ on the positions of electronic states (b).