Compared with other regions of the electromagnetic spectrum terahertz (THz), the frequency range between the infrared and the microwave, has been relatively neglected. A group from the Chinese University of Hong Kong and Warwick University have recently shown that broadband, large and fast modulation of THz beams is in fact possible, and can even be achieved with one very neat device.

Much effort has been spent designing cameras and spectrometers that operate at THz frequencies. They have already proved useful in airport security scanners, and for identifying underlying layers of old paintings.

One important component of these pieces of equipment are modulators, which control the amplitude or phase of a THz beam. These must operate quickly, consume little energy, give consistent modulation over a large frequency range, and produce large changes in the intensity or phase of a THz beam. Approaches so far include metamaterials, semiconductors and liquid crystal devices, none of which meet all the necessary requirements.

In comes Mr. Brewster…

In 1815 David Brewster published a paper describing the angle of incidence required to achieve zero reflection from a transparent body. Now over two hundred years later a team of scientists led by Jianbin Xu and Emma Pickwell-MacPherson have applied this knowledge, along with some more recent technological advances, to create a record-breaking THz modulator.

Pickwell-MacPherson commented, “Our first step was to demonstrate that broadband THz modulation can be achieved with a much lower change in the conductivity by employing  total internal reflection (TIR) geometry rather than transmission geometry (read more about it in Advanced Optical Materials). This has blossomed into the realization of several new device designs, with this latest one exploiting the Brewster angle.”

The device consists of a single stack of graphene, aluminium oxide (Al2O3) and titanium oxide (TiO­­x) on a quartz substrate. A p-polarized THz beam is reflected from the stack, and when Brewster’s angle is reached the reflection goes to zero. The addition of a layer of graphene here allows for an extra element of tunability. When a voltage is applied across the graphene between two gold contacts, the conductivity changes. This alters the Brewster angle for the stack, so for a given angle of incidence the reflected THz may be “switched on or off” by controlling the voltage.

Choose your mode of operation

Shining the p-polarized THz beam onto the device at an angle of 65°, and altering the voltage across the graphene from -12V to +14V, you can modulate the amplitude of the THz by between 99.3% and 99.9% across the entire frequency range of 0.5–1.6 THz. This range is limited by the experimental constraints; in theory even larger bandwidths could be achieved.

But that’s not the only option they have. The researchers took advantage of the fact that at angles greater than the Brewster angle, the reflected beam undergoes a 180° phase change. A THz beam incident at an angle of 68° will experience a phase change of at least 140° across the same frequency range when the voltage is changed from -12V to +16V. Across this range of voltages the Brewster angle varies between 72° and 64°.

The need for speed

The rise time of the modulation is around 1ms, so modulation frequencies of 1 kHz are easily achieved. However, if the modulation depth can be compromised, frequencies of up to 10 kHz can also be reached. Although other solid-state THz modulators operate at significantly higher frequencies of around 2.4 MHz, all is not lost as some small tweaks can improve the modulation frequency of this device. Currently it is limited by the resistance and capacitance of the layers between the gold contacts. By reducing the size to around 1 mm and replacing the TiOx with another layer of graphene, the modulation can reach speeds comparable to other devices.

Xu, who is Director of Materials Science and Technology Research Centre, the Chinese University of Hong Kong, explained that, “the additional benefit of this device is that it can be retrofitted into existing commercially available THz spectrometers.” This graphene-controlled Brewster angle THz modulator truly propels us into the future of THz technologies in real-life applications.

Read more about the work in the paper published in Nature Communications.