TABLE OF CONTENTS Total internal reflection occurs when light comes from an optically denser material to another material at incidence angles greater than the critical angle determined by Snell's law. As discussed in Chap. 8, the amplitude of the reflection coefficient becomes unity at incidence angles greater than the critical angle. Although no energy is transferred from medium 1 to medium 2, there exists an electromagnetic field in the second medium near the surface. This electromagnetic field can store as well as exchange energy with medium 1 at any instant of time. The time-averaged energy flux must be zero across the interface. Total internal reflection has important applications in optical fibers and waveguides. When medium 2 is not infinitely extended but a very thin layer sandwiched between the first medium and the third medium (which may be made of the same material as that of medium 1), photons can tunnel through the second medium into the third, even though the angle of incidence is greater than the critical angle. This phenomenon is called photon tunneling, radiation tunneling, or frustrated total internal reflection, and has been studied for over 300 years since Newton's time. Detailed descriptions of the original experiments and analyses by Isaac Newton can be found from his classical book, Opticks (reprinted by Dover Publications in 1952). The enhanced energy transfer by photon tunneling may have applications in thermophotovoltaic energy conversion devices as well as nanothermal manufacturing using heated AFM cantilever tips.
