Light-Matter Interactions

What happens when EMR waves encounter matter?

Classically, esp if matter is dielectric (non-conducting) it depends on field intensity

!P = !

0"1( )!E +!

0"

2( )!E

!E +!

0"

3( )!E

!E

!E + ...

linear spectroscopy nonlinear spectroscopy

sources detectors

E z,t( ) = E0 exp i !t " nreal! z c( )#$ %&exp "nimag!z c( )

dispersion absorption

Components

polarizers

lenses

filters*

gratings*

Spectroscopy

linear

absorbance

fluorescence

scattering

nonlinear

Light-Matter InteractionsWhat happens when photons encounter matter?

absorption

(excitation)

spontaneous

emission

En

erg

y

!E=h"

stimulated

emission

QM: If photon E (h") is equal to !E between states -

EPR Spectroscopy

NMR Spectroscopy

rotations

flip nuclei

in B

µ &

radio

waves

IR Absorbance

Spec.

Raman

Spectroscopy

vibrate

atoms

IR

Visible Absorbance

Fluorescence Spec.

valence

shell e-

Visible

UV Absorbance

Fluorescence Spec.

valence

shell e-

UV

X-ray Photoelectron

X-ray Diffraction

inner shell

e-

X-rays

Ei,#i

Ej,#j

Rij = !i

"# µ! jd$

in atoms:

% =8& 3

NRij2'

max

6909hcgi

Non-linear Processes

!P = !

0"1( )!E +!

0"

2( )!E

!E +!

0"

3( )!E

!E

!E + ...

Intense beams induced higher order interactions in anisotropic materials. SHG

requires crystals such as KH2PO4 (KDP), NH4H2PO4 (ADP), BBO

E2= Emax

2exp

2i!(t " nreal

cz)

#$%

&'( = Emax

2exp i2!(t " nreal

cz)

#$%

&'(

Linear spectroscopy:

1 h" + material

k1 k1

k2

2nd order spectroscopy:

2 h" + material

k1 k2

2nd harmonic

Generation (SHG)

3nd order spectroscopy:

3 h" + material

k1k4

k3

k2 Four wave mixing

k2!k1

k2=2k1

k4=k1+k2-k3

Reflection & Refraction

!i

!t

"i "r

"t

v = !µ"1

c = !0µ0

"1

n = 1

n =c

v=

!µ

!0µ0

Light-Matter Interactions3a. Absorption

The electric field distorts the charge distribution in the dielectric by generating

dipoles that transiently increase the total E field in the medium, polarizing the medium.

In this polarized medium the motion of the EMR wave is distorted:

E z,t( ) = Emax exp i! (t "nreal

cz)

#$%

&'(exp "!

nimag

cz

#$%

&'(

1. oscillates at $

2. travels more slowly &

is phase shifted in space

3. Intensity decays with

propagation (due to abs)

E!z" = c2!0 Emax2exp i2" (t #

nreal

cz)

$%&

'()exp #2"

nimag

cz

$%&

'()t* dt =

c2!0Emax

2

2exp #2"

nimag

cz

$%&

'()

E = E 0 exp #2"nimag

cz

$%&

'()

The irradiance of the beam depends on nimag & how far it has traveled in the medium:

Light-Matter Interactions

n1

n2

Polarization

n1

n2

Birefringent Crystals

no

ne no

001

none

Polarizers

d

x

y

Modulators