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Optics and photonics

Terahertz radiation is created using semiconductor surface states

19 Aug 2021
New terahertz source
Surface feature: schematic showing the semiconductor lattice in contact with a nanoantenna array. (Courtesy: Deniz Turan/UCLA)

A highly efficient way to convert optical photons into terahertz radiation has been developed by researchers in the US and Germany. The team, led by Mona Jarrahi at the University of California, Los Angeles, showed how the electric fields associated with semiconductor “surface states” can be used to create electrons that emit radiation in the  terahertz range.

Terahertz radiation falls between infrared light and microwaves in the electromagnetic spectrum and has a range of potential applications including security scanning and medical imaging. However, creating practical sources of terahertz radiation is challenging.

One option is to convert light into terahertz radiation using a wavelength conversion system that relies on nonlinear optical effects. The downside of this approach, however, is that it can require bulky and complex optical setups, severely limiting their potential applications.

Now, Jarrahi’s team has come up with a new conversion technique that avoids these problems by using semiconductor surface states.

Incomplete bonds

At the surface of a semiconductor there is break in the periodicity of the crystal lattice and a region of incomplete chemical bonds. The result is the existence of localized electronic surface states, with energies that can lie within the semiconductor’s bandgap – where no electron states can exist within the bulk material. As many semiconductor applications rely on the bandgap, surface states can diminish the semiconductor’s performance, and engineers have found ways to minimize their effects.

Jarrahi and colleagues’ technique uses the large electric field at the surface of a semiconductor that is created by the incomplete bonds. When light strikes the surface, electrons are excited to higher energy levels and can move around the surface, where they are accelerated by the large electric field. These electrons will radiate away their extra energy at the surface, creating terahertz radiation. To enhance this emission, the team placed an array of nanoantennas on the surface, which concentrates the incoming light into the surface region.

The wavelength of the emitted terahertz radiation is a beat frequency of the incident photons and the team produced terahertz radiation with wavelengths ranging from 0.1–1 mm. Using low-energy optical infrared pulses, the system produced terahertz radiation with an efficiency some four orders of magnitude greater than existing nonlinear optical methods.

The team also showed how the technology could be incorporated into an endoscopy probe – which involves coupling to thin, highly efficient optical fibres. Such an instrument could be used to carry out detailed imaging and spectroscopy in opaque environments where optical techniques do not work. The researchers also believe that their technique could be extended to extended work in other regions of the electromagnetic spectrum.

The research is described in Nature Communications.

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