Why have terahertz frequencies been difficult to produce and detect?

Explanations for the ‘Terahertz Gap’ frequently refer to the gap between 'electronic' and 'photonic’ approaches or between classical physics and quantum or modern physics.  But what does this mean?

    
      The Electronic or Classical Physics Approach

Simplistically, radio and microwaves are generated and detected by moving electrons, i.e. using electrical currents.  Electromagnetic radiation from this part of the spectrum can be accurately described by classical physics (i.e. without needing the concepts of relativity and quantum mechanics developed in the early 20th Century) and can be managed by considering the radiation as a wave.


              
                The Photonic or Modern Physics Approach

Shorter wavelengths can be produced by causing electrons to move from higher energy levels to lower levels, releasing energy in the form of electromagnetic radiation. Short wavelength radiation is also generated when high velocity beans of particles such as electrons are forced to change direction and decelerate. These processes can only be described and harnessed with an understanding of relativity and quantum mechanics.  It is easier to consider the radiation as a particle, the photon.

There remains the question why neither of these approaches could be used to generate and detect terahertz radiation.  Both approaches encounter difficulties when they try to extend their range.  These difficulties reflect both fundamental physical limits and restraints caused by lack of appropriate technology.

             

                 Limitations to the Electronic Approach

In the case of the electronic approach a fundamental limit is reached because of the speed at which electrons move.  Achieving shorter wavelengths requires high frequency alternating currents, and there comes a point when the electrons do not travel far enough for a device to work before the polarity of the voltage changes and the electrons change direction (for example the electrons may not have time to cross a transistor channel).  In addition, high frequency alternating fields cause unwanted resistances and capacitances that reduce device power.

Technological limits have been battled with and overcome to a large extent, but can still provide barriers. This includes miniaturisation (the size of antenna for example is directly connected to the wavelength of the electromagnetic radiation that is being generated or detected, so decreasing wavelengths has required ever smaller devices) and the need to dissipate the heat that is generated by rapidly oscillating electrons.


                 Limitations to the Photonic Approach

From the photonic end a physical limitation to extending these techniques is due to the atomic structure of materials.  Electrons exist at discrete energy levels and the gap required between energy levels to produce terahertz emission is not commonly found in most materials. 

Additionally, the size of this energy gap is so small that it is dwarfed by the amount of energy that the electrons are getting from their environment – at room temperature the electrons are already moving between energy levels too vigorously to allow the control of the small discrete energy jumps needed to release photons with terahertz frequencies. Working at cooler temperatures helps this process but makes the equipment more expensive, bulkier and less practical to run, and lowering temperatures dramatically decreases the output power of the generated radiation. 


In the 21st Century, there have been significant efforts in science, technology and engineering to conquer the terahertz gap.  Novel techniques, often combining electronic and photonics approaches have been developed and technological barriers have been overcome.  The Further Reading and Resources resources page provides further information.