A new tuneable terahertz laser has been made by incorporating graphene. This is an important breakthrough as lasers emit light at specific frequencies that are usually fixed once the device has been fabricated. Researchers at The University of Manchester have combined an atomically thin graphene sheet with a quantum cascade laser to make a terahertz laser that can be tuned simply by adjusting the density of current carriers of the graphene layer. The device is based on manipulating the plasmons (oscillations of the current carriers) in the layer. These have a frequency that depends on the number of current carriers - and in graphene this is easily adjusted using an electric field.
In two-dimensional materials such as graphene, the plasmons often have low frequencies in the terahertz or infrared part of the electromagnetic spectrum, and can be made to couple to light. This enabled the researchers excite the plasmons in a graphene sheet overlying a metallic waveguide. An irregular pattern of sub-wavelength slits was introduced into the waveguide (which would normally control the emission spectrum of the laser). However, with graphene in place, the researchers found that they could switch the laser output between the spectrum created by the slits, and one expected from an unpatterned waveguide, just by changing the plasmon frequency.
This research shows that graphene could be used to reversibly control the frequency of terahertz lasers. This could lead to vastly improved scanning systems that dramatically improve the efficiency and accuracy of materials analysis in the pharmaceutical, security and agricultural industries. For example, highly sensitive scanners for detecting concealed weapons at airports could become more accurate and much safer for frequent use using the new technology.
- Terahertz radiation occupies the part of the electromagnetic spectrum between infrared and microwave frequencies - so it is very low energy, long wavelength radiation. Terahertz radiation is invisible to the naked eye and is non-destructive and non-invasive - much safer than X-rays, which are currently often used in materials analysis.
- A quantum cascade laser is a semiconductor laser that emits radiation in the infrared to terahertz part of the electromagnetic spectrum. It gets its name because the light emission is caused as excited electrons 'cascade' through a set of energy levels with small separations in a repeated stack of semiconductor quantum wells.
- A waveguide is a structure that guides waves, such as electromagnetic waves. They enable a signal to propagate with minimal loss of energy. Here a metal waveguide is used constrain the waves to propagate along the long axis of the laser device.