Download Lecture Section 80

Field energy in a dispersive
medium
Section 80
Energy flux density
This formula has been shown to hold in
• vacuum (volume 2.)
• in metals at zero frequency (v. 8 section 30)
• and in non-dispersive dielectrics (v. 8 section 75).
Does the same Poynting Vector give energy flux density for dispersive media?
The component of S normal to a surface involves only Et and Ht,
which are both continuous across a boundary.
Sn is continuous across boundary
Energy flowing from vacuum across boundary must be conserved, so
remains valid for energy flux density in a dispersive dielectric.
Four cases to consider:
1. Non-dispersive bodies: Internal energy
2. Monochromatic light in dispersive bodies: rate of dissipation
3. Non-monochromatic wave packet in dispersive bodies: net dissipation
4. Non-monochromatic wave packet in transparent dispersive body: Build up of
internal energy
Case 1: non-dispersive bodies
(75.15)
Take the real parts of the fields before combining them in this quadratic function
Real constants, no w dependence
where
= electromagnetic energy density
= internal energy difference for body
with and without fields, holding entropy
and density constant.
=total work done by turning on the fields
No energy is lost and no heat is generated!
Cases 2 and 3: Dispersive media.
Dispersive media dissipate energy
Dispersive media are absorbing.
Mean evolved heat density per unit time Q = <-divS>t
Electromagnetic energy U in the medium is not constant unless there is a
net inflow of electromagnetic energy.
Under conditions of constant field amplitude, there is a constant inflow of
energy <dU/dt>t = <-divS>t
which is converted to heat at a rate Q
Case 2: Dispersive medium, monochromatic fields.
The flow of electromagnetic energy into unit volume is
dU = (E.dD +H.dB)/4p. Some flows out the other side.
Some remains as heat = <dU>t
E = E0e-iwt
(We can’t write U itself
as a definite quantity
because of dissipation.)
The rate of inflow of electromagnetic energy density is
dU/dt =
Use real fields in non-linear functions
Now take time average to find rate of heat generation Q per unit volume.
Products E.E and E*.E* contain oscillating factors
Oscillations disappear from products E.E*
Dissipation of field energy per unit time is determined by imaginary parts e” and m”
Complex fields
used here.
Real fields used here
e” and m” are positive:
Second law of thermodynamics
dQ = TdS > 0
Recipe: For monochromatic light, e.g. CO2 laser light, dissipation is determined by
the value of e” at the corresponding frequency.
60.2
But what are you going to do if the light is not monochromatic?
Rate of heat generation is not
constant since amplitude of nonmonochromatic field is time
dependent.
What value of e” would you
use? The average?
LWIR
Conversion of broad band infrared radiation
to heat is an important practical problem
Proc. SPIE 9819 – 50 (2016).
Which is true?
1.
2.
3.
Real and imaginary parts of permittivity are always positive.
Real part of permittivity can be negative, but the imaginary part is
always positive.
Both parts of the permittivity can be positive or negative.
Case 3: non-monochromatic fields in dispersive medium
We consider a pulse or wavepacket that goes to zero sufficiently rapidly as t 
All light emission processes are finite duration.
Non-monochromaticity
Instead of dissipation per unit time, consider time-integrated net dissipation.
We have to do this because dissipative effects depend on the entire history of the fields
An expression for instantaneous rate of energy dissipation isn’t possible in this case.
Any time dependent field can be written as a sum of monochromatic fields
Fourier expansion
Required to ensure that the field is real
Electric part of net dissipation
is a triple integral
First integrate over w’. Then the delta function changes w’ to -w
This looks just like an integral over frequencies of the monochromatic case.
(Ew has units of electric field times time).
After the pulse has come and gone, energy is left behind in the medium as heat.
Since the integrand is even
Electric and magnetic losses are determined by
By second law and
may be positive or negative
Transparency ranges
e” and m” are never zero except at w = 0.
However, they may be very small e”<<|e’|
Then, neglect absorption.
Now we can talk about definite internal energy as in the static
case, except now it is not constant.
In static case, these were real constants independent of w
For electromagnetic waves in a transparent dispersive medium,
we don’t simply replace the constants e and m by the functions
e(w) and m(w).
It’s more complicated.
Energy density U doesn’t change in transparent media for purely monochromatic
fields, because their amplitudes are unchanged since the dawn until the end of time.
Consider nearly monochromatic wavepacket
with a narrow range of frequencies
These amplitudes vary slowly compared with the “carrier”
Long smooth pulse
Substitute the real parts of the fields into
Next, average over the short period
The electric part is
Constant
over this
period.
Products
Leaving
Average to zero
The electric induction is
Integral operator
In the expression for dU/dt, we need the time derivative of D and D*.
This is an integraldifferential operator
If
= constant.
This is the zeroth approximation, where nearly
monochromatic wave is assumed perfectly monochromatic.
for monochromatic fields
Next approximation: Smoothly varying nearly monochromatic wave packet.
Expand the slowly varying amplitude as a Fourier series.
Only small frequencies a << w0 appear in the
expansion of the amplitude, since its variations are
slow compared to the carrier w0.
We will consider just one Fourier
component for now
Considering only the ath Fourier
component in the expansion of E0(t)
The operator is acting on a
monochromatic wave
Taylor expand f(a + w0) about a = 0.
Now sum the Fourier components for the amplitude.
Drop subscript on w0
Substitute into
neglect e” in transparency regions, so e is real. Two of the four terms cancel.
Rate of change of
electromagnetic
energy density
Where averaged internal
energy has definite value
Averaged over fast oscillations
Magnetic part added
Mean value of the electromagnetic part of the internal energy density in a transparent
medium
Compare to result for static case and for dispersion-less medium (dynamic case 1)
If there is no dispersion
Usual static result
In terms of the real fields
Mean value of the electromagnetic part of the
internal energy density of transparent medium
If you cut off the external source of electromagnetic energy,
energy in medium must all eventually convert to heat.
Second law
Then
The formula
Was found by expanding dD/dt to first order in the small frequencies a,
which are only small if the amplitude E0(t) varies slowly.
The formula applies to slowly and smoothly varying wave trains, and not
to short pulses.