Carbon nanotube coating creates on-chip terahertz waveguides

There’s considerable interest in leveraging the bandwidth and other potential virtues of terahertz waves that occupy the spectrum between the conventional RF and optical worlds, generally considered to span 100 GHz (3 mm wavelength) to 10 THz (30 μm). However, managing electromagnetic energy at these wavelengths presents many challenges, as they are too short for most electronics, yet too long for all-optical components.

Nonetheless, there’s a significant amount of ongoing research in developing the materials and components needed, especially with many potential applications, including the emerging 6G standards being developed now.

At these frequencies and corresponding wavelengths, signal energy must be conveyed via waveguides—discrete wires won’t do, of course. But making the needed waveguide physical transitions is difficult when they are fabricated in silicon as part of a larger set of on-chip functions.

Addressing this issue, a team of researchers at The Skolkovo Institute of Science and Technology—or Skoltech, a private institute in Moscow—working with a team from KTH Royal Institute of Technology in Sweden, has developed a key technology that could support silicon-based terahertz waveguides and their on-chip transitions.

Their solution is based on carbon nanotubes, one of those amazing materials that keeps offering solutions to diverse problems. The single-wall carbon nanotube (SWCNT) was discovered in 1991 (see “A Brief Introduction of Carbon Nanotubes: History, Synthesis, and Properties“). Like fullerene and graphene, SWCNTs are one of the allotropes of carbon.

Allotropes present a different structural form of the same chemical element within the same physical state; because their atoms are bonded differently, allotropes have vastly different physical and chemical properties from each other—think diamond versus graphite.

A key challenge in building these complex terahertz arrangements is devising properly matched terminations. Without proper termination, reflections at device discontinuities can cascade, thus degrading performance and altering the intended operational profile. In addition, these terminations are necessary for characterization of multi-port devices such as directional couplers, where the unused ports must be terminated with matched loads.

The conventional solution is to use adiabatic or impedance-matched tapering of the waveguide cross-section to free space, gradually expanding the guided mode to induce radiation losses while operating as a dielectric rod antenna. However, the efficiency of these structures depends on the length of the tapering, therefore consuming valuable chip area; it can also radiate power in undesirable directions, thus complicating packaging, limiting integration density, and creating electromagnetic pollution.

Note that in the adiabatic-coupling approach, the optical mode is coupled from one waveguide to another by a slow change of a waveguide parameter (width, thickness, or both) such that the optical mode remains in the fundamental mode and does not couple to unwanted higher-order modes. As a result, the tapered waveguides need to be long enough to meet the requirements of the adiabatic conditions of slow change of waveguide parameter. However, at the same time, they need to meet the device compactness requirement. Therefore, there is a trade-off to be made

The research team devised and tested a carbon nanotube-based coating that blocks electromagnetic radiation, thereby creating waveguides compatible with terahertz wavelengths. The ultrathin single-walled carbon nanotube films that they synthesized are similar to those that they used previously to create small-scale components, such as lenses and antennas, but with a big difference, as this time it’s not for standalone components. Instead, they leveraged carbon-based material to control electromagnetic radiation in 2D-integrated optical circuits, eliminate interference, and enable additional functionality.

They demonstrated a compact, broadband termination by coating silicon dielectric rod waveguides (DRW) with ultrathin single-walled carbon nanotube films. Fabricated via a floating-catalyst (aerosol) chemical vapor-deposition process, the film thickness varies from 2 to 53 nm and was characterized in the 140-220 GHz range. A 53-nm thick film introduced up to 47 dB of attenuation while maintaining over 20 dB reflection loss, confirming nearly reflection-free absorption (Figure 1).

Figure 1 Reflection measurements of the SWCNT-loaded DRWs show ∣S₁₁∣ for the 6-mm long samples (a) and ∣S₁₁∣ for the 12-mm long samples (b). The light grey line is baseline reflection after calibration by measuring a thru-standard (flanges of the frequency extenders connected); dark grey is the reflection coefficient of an unloaded DRW. Source: Nature Communications

Shielding analysis shows absorption dominates over reflection, and they achieved a record specific shielding efficiency of 5.5 × 10⁹ dB cm²/g (Figure 2).

Figure 2 Shielding efficiency components for the SWCNT-coated dielectric waveguides: reflection component SE_R (a, b), absorption component SE_A. (c, d), and total shielding SE_T (e, f) for 6-mm (left column) and 12-mm (right column) samples over 140-220 GHz, with light grey as the equivalent shielding efficiency of an unloaded silicon waveguide provided for reference. Source: Nature Communications

This approach offers a footprint-efficient solution for high-density terahertz circuits without bulky, radiative terminations. The work is presented in their paper “Ultrathin Single-Walled Carbon Nanotube Surface Wave Absorbers for Terahertz Dielectric Waveguides” published in Nature Communications. It’s unfortunate that the paper does not have any microphotographs of the SWCNT waveguide and transitions in silicon, so you’ll just have to visualize those yourself.

Have you had any interaction with or uses for carbon nanotubes? If so, in what way? Do you see a role for them in any of your projects, whether terahertz or other?

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.

Related Content

• Nanotube sensors spark new use cases
• Carbon nanotubes boost image sensor sensitivity
• Metadevices may fill the terahertz component gap
• Optical combs yield extreme-accuracy gigahertz RF oscillator

The post Carbon nanotube coating creates on-chip terahertz waveguides appeared first on EDN.