Unlocking the Terahertz spectrum

Visible light is a range of the electromagnetic (EM) spectrum which we can see using our eyes. Unsurprisingly, we use it to understand the world around us! But what about the other ranges of the EM spectrum? Turns out, we have been using them all along! We use X-rays for medical imaging, infrared for imaging and microwaves for communications.

Spectroscopy is a non-contact technique measuring interactions of the EM spectrum with different samples to characterise them. However, regions of the EM spectrum mentioned have their downsides when used to measure small devices. X-rays are ionising which may damage measured devices, and infrared and microwaves have trade-offs in terms of imaging resolution and penetration.

With the demand for computing ever increasing, electronic devices are becoming smaller and the use of non-silicon based and one-dimensional electronic systems are being extensively explored. Spectroscopy serves as a complementary technique to traditional contact-based characterisation techniques to characterise these systems. After all, it is physically difficult to see or touch them without significantly altering the properties of the measured system.

Terahertz (THz) spectroscopy emerges as an increasingly attractive technique. But what is THz? THz radiation is a range of EM wavelengths in between the microwave and infrared regions, from 0.1 to 10 THz. It is non-ionising and non-destructive, making it an appealing alternative to other regions of th EM spectrum for not only material and device characterisation, but biomedical imaging, airport security systems, industrial quality control and wireless communications too. It is especially relevant to electronic devices as the THz spectrum

enables us to measure the fundamental characteristics of materials and devices such as electrical conductivity, refractive index and charge carrier mobility.

However, the THz spectrum has been difficult to access via electronic or optical means, requiring more complex and novel ways for generation and detection. Turns out we can mix waves with different wavelengths to generate THz electric fields, like how we mix different colours, say blue and yellow to make the colour green. But how do we do this in practice?

Fortunately, there are materials that have non-linear optical properties that help us do that. The same materials can detect THz electric fields too, as the incident electric field will alter measurable properties of the crystal. On top of that, we can make nanostructure features to control the amplitude, phase and polarisation of electric fields, similar to how we control radio waves using an antenna.

Recently, scandium-containing III–nitride materials (ScAlN and ScGaN) have come to light as having a very high second-order non-linearity. My Midi project will investigate terahertz generation and detection within ScAlN and ScGaN thin films for the first time. My PhD project would be an extension to this, harnessing the benefits of nanostructured features to enable novel, ultracompact and affordable THz emitters and detectors. This will in turn enable more research into novel materials and devices and industrial quality control applications for a computing-centric future.

Aldric Goh

NanoDTC PhD Student, c2023