A research team from The Chinese University of Hong Kong (CUHK) and the University of Warwick has reached a crucial milestone towards developing single-pixel terahertz radiation (T-ray) imaging technology. Their single-pixel T-ray camera reached 100 times faster acquisition than the previous state-of-the-art without adding any significant costs to the entire system or sacrificing the sub-picosecond temporal resolution needed for the most sought-after applications, potentially opening the opportunity for them to be used in non-invasive security and medical screening. The breakthrough has been published in the journal Nature Communications.

The potentials and problems of Terahertz radiation

Terahertz (THz) radiation, or T-rays, sit in-between infrared and Wi-Fi on the electromagnetic spectrum. T-rays have different properties from other electromagnetic waves, most notably they can see through many common materials such as plastics, ceramics and clothes, making them potentially useful in non-invasive inspections. Another quality is that the low-energy photons of T-rays are non-ionizing, making them very safe in biological settings including security and medical screening. They are also highly sensitive to water and can observe minute changes to the hydration state of biological matter. This means that diseases perturbing the water content of biological matter, such as skin cancer, can potentially be detected using T-rays in vivo without any histological markers.

Efficient detection and generation of T-rays has been possible in laboratory settings for the last 25 years. However, THz technology is still not widely used in commercial settings as the cost, robustness and/or ease of use is still lagging behind for commercial adoption in industrial settings.

The advantages of single-pixel cameras

Professor Emma Pickwell-Macpherson from the Department of Electronic Engineering at CUHK and the Department of Physics at University of Warwick, said: “We use what is called ‘a single-pixel camera’ to obtain our images. In short, we spatially modulate the THz beam and shine this light onto an object. Then, using a single-element detector, we record the light that is transmitted (or reflected) through the object we want to image. We keep doing this for many different spatial patterns until we can mathematically reconstruct an image of our object.”

The researchers have to keep changing the shape of the THz beam many times which means this method is usually slower compared to multi-pixel detector arrays. However, multi-pixel arrays for the terahertz regime usually lack sub-picosecond temporal resolution, require cryogenic temperatures to operate or incur large equipment costs (>US$ 350,000). The setup developed by the CUHK Engineering and Warwick team, which is based on a single-element detector, is reasonably priced (~US$20,000), robust, has sub-picosecond temporal resolution (needed for accurate diagnosis) and operates at room temperature.

Professor Pickwell-Macpherson adds: “Our latest work improves upon the acquisition rate of single-pixel terahertz cameras by a factor of 100 from the previous state-of-the-art, acquiring a 32x32 video at 6 frames-per-second. We do this by firstly determining the optimal modulation geometry, secondly by modelizing the temporal response of our imaging system for improvement in signal-to-noise, and thirdly by reducing the total number of measurements with compressed sensing techniques. In fact, part of our work shows that we can reach a five times faster acquisition rate if we have sufficient signal-to-noise ratio.”

The research team led by Professor Pickwell-Macpherson has previously developed several THz devices including THz modulators that make use of the total internal reflection geometry to achieve high modulation depths across a broadband frequency range and a new approach for amplitude and phase modulation exploiting the Brewster angle. They are also working to improve the resolution of single pixel THz imaging through signal processing approaches. Future work will focus on improving the signal-to-noise and optimizing the software needed for accurate medical diagnosis, with the ultimate goal being to use single-pixel THz imaging for in vivo cancer diagnosis.

The research paper has been published in Nature Communications: https://doi.org/10.1038/s41467-020-16370-x

Read more about the research: http://bme.ee.cuhk.edu.hk/thzgroup/index.php and here: go.warwick.ac.uk/ultrafast

The ever-increasing growth in data traffic requires more powerful transmission networks.  To respond to such demand, a group of researchers led by Prof. Xiankai Sun and Prof. Hon Ki Tsang in the Department of Electronic Engineering, The Chinese University of Hong Kong (CUHK) has recently revealed a way to use light to convey large rates of data in advanced optical chips. Their findings and demonstrations shed new light on increasing the data capacity with low insertion loss and crosstalk. These research results have been recently published in the prestigious scientific journal Nature Communications.

The conventional optical communication is based on total internal reflection, creating a high-refractive-index channel, for the light wave to propagate. Bound states in the continuum (BICs) refer to a type of wave that can coexist with continuous waves without any radiation loss.  Applying BICs in photonic integrated circuits enables low-loss light guidance in low-refractive-index channels on high-refractive-index substrates, lowering the cost and the complexity of processing.  The research team applied them on an etchless lithium niobate integrated photonic platform and has successfully confined light without adopting the high-refractive-index channel.

Prof. Xiankai Sun said, “The BIC concept makes it unnecessary to invent new high-refractive-index polymers to form waveguide channels on the high-refractive-index substrate or etch the substrate in order to guide light in channels in the substrate.”

Optical interconnections with ultrahigh data capacity are needed in high-performance computers and data centres.  To further increase the data transmission capacity, optical multiplexing technologies are used to transmit multiple channels of data in parallel. By making use of carefully engineered high-order BICs on a planar lithium niobate substrate, the team demonstrated the viability of the BIC concept for use in high-capacity optical communication links by using different spatial modes for mode-division multiplexing.

Prof. Sun added, “With this new technology, we can obtain an aggregate data rate of 160 Gb/s per wavelength light carrier on the lithium niobate platform which, with the additional advantages of high thermal stability and high linearity, will bring optical communication to a new level.”

Department of Electronic Engineering, CUHK

The Department of Electronic Engineering was established in 1970 by the 2009 Nobel Laureate in Physics, Prof. Charles Kao. The department aims at educating students to enhance their potential to become leaders in electronic engineering and instill in them the desire to pursue knowledge and advance the state of the art in electronic engineering, which includes hardware, software, and design aspects of electronics as the core, ranging from materials, devices, circuits to systems and their applications. The department has 21 professors and teaching staff complemented by 56 research and technical support staff members, serving 227 undergraduate students, 128 research postgraduate students pursuing PhD and MPhil degrees, and 39 postgraduate students pursuing MSc degrees.