Page 20 - Photonics Essentials an introduction with experiments

Electrons and Photons

14 Introductory Concepts

Since photons always travel at the speed of light, it is natural to
think about the flow of energy or power in a light beam. Power is
measured in watts:
Watts = power that comes out of the light bulb = energy/sec
Watts = number of photons of frequency f/sec
× energy, summed over all f
Power = n f · E f
f
So the total power is made up of the sum of all these little packets of
E = hf
It is sometimes more convenient in many applications to use angu-
lar frequency instead of regular frequency:
= 2 f
To make everything work out right you have to divide Planck’s con-
stant by 2 :
h/2
E =
In photonics, you will use and E almost always. Rarely will you
calculate f. The most important reason for this is experimental in ori-
gin. There are no instruments that measure frequency of photons di-
rectly.

2.4 Properties of Electrons
Electrons are the ONICS of photONICS. Electrons can interact with
photons one at a time (mostly) through the medium of a semiconduc-
tor crystal. When a semiconductor absorbs a photon, the energy of the
photon can be transferred to an electron as potential energy. When
the electron loses potential energy, the semiconductor can account for
the energy difference by emitting a photon.

Exercise 2.4
A photon with energy 1.5 eV strikes GaAs. The energy is absorbed by
breaking one bond, promoting one electron from a bonding state (va-
lence band) to an antibonding state (conduction band), and leaving a
vacant state (hole) in the valence band. Some time later, the electron
recombines with the hole, completing the bond and releasing a photon
of 1.42 eV, the bonding energy of GaAs at room temperature.

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