Todays summary Polarization Energy / Poyntings vector Reflection and refraction at a dielectric interface: –wave approach to derive Snells law –reflection

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Today’s summary

• Polarization

• Energy / Poynting’s vector

• Reflection and refraction at a dielectric interface:– wave approach to derive Snell’s law– reflection and transmission coefficients– total internal reflection (TIR) revisited

Polarization

Propagation and polarization In isotropic media(e.g. free space, amorphous glass, etc.)

More generally,

(reminder :Anisotropic in media, e.g. crystals, one could E not have parallel to D)

planar wavefront

electric field vector E

wave-vector k

Linear polarization (frozen time)

Linear polarization (fixed space)

Circular polarization (frozen time)

Circular polarization:linear components

Circular polarization (fixed space)

/4 plate

LinearLinear polarizationpolarization

birefringentl/4 plate

CircularCircular polarizationpolarization

λ/2 plate

Linear (90Linear (90oo-rotated)-rotated)

polarizationpolarization


birefringentλ/2 plate

Think about that

mirror

birefringentλ/4 plate


Relationship between E and B

Vectors k, E, B form aright-handed triad.

Note: free space or isotropic media only

Energy

The Poynting vector

S has units of W/m2so it represents

energy flux (energy per unit time & unit area)

Poynting vector and phasors (I)

For example, sinusoidal field propagating along z

Recall: for visible light, ω~1014-1015Hz

Poynting vector and phasors (II)Recall: for visible light, ω~1014-1015Hz

So any instrument will record theaverage average incident energy flux

where T is the period (T=λ/c)

is called the irradianceirradiance, aka intensityintensityof the optical field (units: W/m2)

Poynting vector and phasors (III)2

For example: sinusoidal electric field,

Then, at constant z:

Poynting vector and phasors (IV)

Recall phasor representation:

complex amplitude or " phasor":

Can we use phasors to compute intensity?

Poynting vector and phasors (V)Consider the superposition of two two fields of the same same frequency:

Now consider the two corresponding phasorsphasors:

Poynting vector and phasors (V)

Consider the superposition of two two fields of the same same frequency:

Now consider the two corresponding phasorsphasors:

and the quantity

Poynting vector and irradiance

Reflection/ RefractionFresnel coefficients

Reflection & transmission@ dielectric interface



I. Polarization normal to plane of incidenceI. Polarization normal to plane of incidence





Continuity of tangential electric fieldat the interface:

Since the exponents must be equalfor all x, we obtain



Continuity of tangential electric fieldat the interface:

law of reflection

Snell’s law of refraction

so wave description is equivalent to Fermat’s principle!! ☺



Incident electric field:

Reflected electric field:

Transmitted electric field:

Need to calculate the reflected and transmitted amplitudes E0r, E0t

i.e. need two two equations



Continuity of tangential electric fieldat the interface gives us one equation:

which after satisfying Snell’s lawbecomes



The second equation comes from continuity of tangential magnetic field

at the interface:



So continuity of tangential magnetic field Bx at the interface y=0 becomes:




II. Polarization parallel to plane of incidenceII. Polarization parallel to plane of incidence

Following a similar procedure ...


Reflection & transmission of energyenergy@ dielectric interface

Recall Poynting vector definition:

different on the two sides of the interface

Energy conservation

Reflection & transmission of energyenergy@ dielectric interface

Normal incidence

Note: Note: independent of polarization

Brewster angle

Recall Snell’s Law

This angle is known as Brewster’s angle. Under suchcircumstances, for an incoming unpolarized wave, only the component

polarized normal to the incident plane will be reflected.

Why does Brewster happen?

elemental

dipole radiator excited by the incident field




Turning the tables

Is there a relationship between r, t and r’, t’ ?

Relation between r, r’ and t, t’

Proof: algebraic from the Fresnel coefficientsor using the property of preservation of the preservation of the field properties upon time reversal

Proof using time reversal

Total Internal Reflection

Happens when

Substitute into Snell’s law

no energy transmitted

Total Internal ReflectionPropagating component

no energy transmitted

Total Internal Reflection

Pure exponential decayº º evanescent evanescent wave

It can be shown that:no energy transmitted

Phase delay upon reflection

Phase delay upon TIR

Phase delay upon TIR

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Todays summary Polarization Energy / Poyntings vector Reflection and refraction at a dielectric interface: –wave approach to derive Snells law –reflection