Metasurface plus photoelectric quantum effect yields sensitive THz detector

I’m always interested in how researchers, scientists, engineers, and manufacturing specialists leverage apparently unrelated advances to their own advantage for devising innovative techniques and advances. This phenomenon is not new, of course; it’s been one of the driving forces behind technological advances for hundreds of years in situations ranging from fairly modest to some that are impressively esoteric.

This seems to be the case especially for sophisticated sensors. The latest one I have seen addresses the challenge of detecting terahertz energy. It’s not news to this audience that the terahertz region of the electromagnetic spectrum, generally defined as 100 GHz to 10 THz, has many barriers when developing complete viable systems. It’s informally called the “THz gap” where the frequencies are too high and the wavelengths too short for most electronic components; yet too low and too long, respectively, for using optical ones.

A deep-physics concept

Addressing this issue, a joint team at the University of Cambridge and Swansea University (both in the U.K.) took advantage of some “new” physics. Their sensor merges a metasurface with a recently discovered quantum physics effect that is a spinoff of the well-known photoelectric effect.

In the photoelectric effect, there is the emission of electrons (current) from a material (usually a metal) when it’s struck by electromagnetic radiation such as light with high-enough energy. In contrast, their terahertz detector makes use of the in-plane photoelectric effect (IPPE), where incoming terahertz photons transfer energy to electrons that are confined within a two-dimensional electron gas. Those energized electrons cross a carefully designed potential step, generating an electrical current that can be measured.

If you are not familiar with the photoelectric effect, it has a major place in the history of modern quantum physics. The effect had been observed since the latter half of the 1880s with solid experimental data, but non-quantum physics could not explain the data and in fact was at odds with the data. Albert Einstein proposed a radically new explanation in his seminal 1905 paper “On a Heuristic Viewpoint Concerning the Emission and Transformation of Light”—one of four truly significant papers he had published in that “miraculous year”, which resulted in the Nobel Prize award in 1921.

That photoelectric effect, however, is different in the in-plane version, a phenomenon that was observed only recently, in 2022 (“An in-plane photoelectric effect in two-dimensional electron systems for terahertz detection”). In the IPPE quantum process, incoming terahertz photons transfer energy to electrons confined within a two-dimensional electron gas. Those energized electrons cross a carefully designed potential step, generating an electrical current that can be measured.

Unlike conventional photoelectric detectors, this mechanism does not require photons to exceed a minimum energy threshold. Because the process occurs entirely within the plane of the material, it also avoids several efficiency limitations that affect traditional detector designs.

While earlier detectors using this concept showed promising performance, they suffered from one major drawback. They captured only a small portion of the incoming radiation because they relied on single-antenna structures.

A new approach

To overcome that limitation, the team devised a metasurface with a patterned structure that concentrates electromagnetic energy into regions much smaller than the wavelength of the incoming radiation. In the new design, a repeating “brickwork” pattern gathers terahertz waves and channels them into narrow gaps where detection takes place (Figure 1).

Figure 1 Schematic diagram of the MetaPETS detector with the brickwork metamaterial (a). Zoom-in view showing a unit cell indicated by a black dashed rectangle. The unit cell sizes in the 𝑥- and 𝑦-directions, 𝑑𝑥 and 𝑑𝑦, are indicated (b). Zoom-in view highlighting the capacitive gap, illustrating the width 𝑤 of the capacitor in the lateral (𝑥)-direction, and the gap size 𝑔 of the capacitor. The 2DEG is depleted along the edge of the mesa, as indicated, causing the area of the conducting 2DEG to be offset inwards from the lithographically defined mesa edge (c). Zoom-in view highlighting the summation of photocurrents generated by the PETS detection elements at the lower right corner. The electron flow occurs within the 2DEG, and its direction is shown by the red arrows. Without loss of generality, we show an example with gate 1 positively biased and gate 2 negatively biased (d). Photocurrent detection circuit (e). Source: SPIE Digital Library

Each of these tiny gaps functions as an individual detector. By distributing many of them across the metasurface and connecting them electronically, the researchers were able to combine their outputs into a stronger overall signal.

But that was only the first step. Individual photoelectric tunable-step (PETS) detector elements were then integrated into the capacitive gaps, as they experience the strongest electromagnetic fields. This ensures optimal coupling of the metasurface to the detection elements and significantly boosts the detection sensitivity compared to simply connecting the elements in parallel.

Fabrication relied on a semiconductor structure containing a high-mobility electron gas. The manufacturing process closely resembles techniques already used to produce field-effect transistors, making future integration with electronic circuits more feasible. Since the metasurface itself concentrates the incoming radiation, the detector does not require external focusing components such as silicon lenses and their precise alignment, which simplifies assembly, reduces cost, and could make large-scale manufacturing easier.

In tests, the detector was cooled to 10 K and exposed to radiation near 1.9 THz. It generated a clear electrical signal that matched the on and off pattern of the incoming radiation. Measurements showed a responsivity of 2.7 amperes per watt. The proof-of-concept device also achieved an external quantum efficiency of 2.1% at 1.9 THz.

Figure 2 Time-averaged photocurrent in response to the 1.881 THz quantum cascade laser (QCL) radiation, measured in the source-drain circuit while mechanically blocking and unblocking the THz waveguide (WG). The two gates are biased at +0.76 V and −0.095 V, respectively (a). Real-time photocurrent measurement using an oscilloscope, showing the response to a pulse emitted by the QCL during its on time with 100 sweeps averaged. The two gates are biased at +0.76 V and −0.095 V, respectively (b). 2D maps are shown as a function of the two gate voltages: photocurrent (c) and four-wire conductance of the device (d). Source: SPIE Digital Library

According to the researchers, this represents roughly a 20-fold improvement over previously demonstrated PETS detectors. Much of that gain comes from the metasurface’s ability to capture more incoming radiation and direct it into the active detection regions.

The work is described in both theory and practical facets in their readable paper “Quantum metasurface-based photoelectric tunable-step terahertz detector” published in Advanced Photonics. Of course, they note that there’s more work to be done. The researchers believe the technology could eventually operate at temperatures higher than those required by many competing detector designs. Similar PETS devices have already demonstrated performance at temperatures reachable using compact cryocoolers rather than liquid helium.

Have you used a highly sophisticated sensor that blends advanced technologies, or had to innovate and implement such a sensor yourself? What were the unexpected challenges you encountered?

Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors, signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing.

Source link