Electrically controlled silicon photonic devices use carbon nanotube heaters to modulate and filter terahertz waves on chip with stable performance, enabling compact tunable components for high frequency applications.
(Nanowerk Spotlight) Light in the terahertz range is drawing interest for communications and sensing. It can support data rates higher than today’s wireless systems and detect features in materials at fine scales. Engineers want devices that send, receive, and shape terahertz waves on a single chip.
Conventional electronic circuits struggle at these frequencies because currents encounter high resistance and lose energy. Optical circuits can guide terahertz waves but often lose power at sharp bends or from surface defects. These problems limit the ability to build compact terahertz systems.
One strategy to avoid this loss uses principles from topology, which is a branch of mathematics focused on properties that stay the same when shapes are stretched or bent. In photonics, topology can be used to create channels that trap light along an interface between two materials. Light in such a channel can move around sharp turns or surface roughness without scattering away. This property makes topological waveguides well suited for routing signals in small spaces.
A special kind of topological structure, called a valley Hall photonic crystal, is made by drilling patterns of holes in a material like silicon. The design breaks certain symmetries inside the crystal, so light traveling in one direction becomes trapped along the boundary with another, mirrored version of the crystal. These guided waves stay confined even when the boundary turns sharply.
At the same time, thin films of carbon nanotubes have emerged as useful components in optoelectronic devices. The tubes can be processed into large area sheets that are light, flexible, and conduct electricity well. When a voltage is applied to such a sheet, it produces heat. In a nearby structure this heat can change the local optical properties, making the carbon nanotube film an efficient heater.
By combining the guiding properties of topological crystals with the thermal response from carbon nanotube films, researchers can build terahertz components that are both stable and tunable. The study published in Advanced Functional Materials (“Electrically Tunable On‐Chip Topological Photonics with Integrated Carbon Nanotubes”) demonstrates this combination in two devices: a broadband modulator and a narrowband filter.
Both are made from silicon valley Hall photonic crystals with integrated carbon nanotube heaters. Each device operates near three hundred thirty gigahertz and is controlled through a low voltage electrical bias.
Carbon nanotube (CNT)-integrated electrically tunable topological devices. a) Schematic of CNT-integrated electrically tunable topological chips. CNT sheets are precisely positioned: 10-unit cells away from the bottom edge of the omega-shaped topological waveguide and 7-unit cells away from the edge of the cavity. A bias ranging from 0 V to 26 V results in a decrease in broadband transmission in the waveguide and a redshift in resonance frequency for the coupled waveguide-cavity system. The upper inset illustrates the VPC unit cell having a periodicity of a = 242.5 μm and a height of h = 215 μm. Each unit cell features two triangular air holes with side lengths l1 and l2. Δl = l2−l1 represents the difference between the two inverted triangular air holes. Negative and positive Δl values correspond to Type A and Type B VPC unit cells, respectively, as depicted in the bottom inset. b) Band diagram of VPC unit cells with (Δl = 0) and without (Δl ≠ 0) inversion symmetry. The blue area represents the region above the light line. c) Dispersion of topological edge states supported by the AB and BA type zigzag interfaces. The orange‒yellow region highlights the projected bulk bands. The gray dashed line shows the light line. d) Schematic of the modulation mechanism of CNT: Applying a bias from 0 to 26 V heats the CNT sheets (individual CNTs depicted as examples). Owing to their high thermal conductivity, CNT sheets transfer heat to the silicon-based VPC chip, thereby altering its refractive index and modulating the THz wave propagating inside the topological chip. (Image: Reprinted from DOI:10.1002/adfm.202514656, CC BY) (click on image to enlarge)
The foundation of the platform is a silicon slab patterned with pairs of triangular air holes. Each pair forms a “unit cell,” which is the smallest repeating pattern in a crystal. The triangles differ slightly in size, which breaks a geometric property called inversion symmetry. Inversion symmetry means the structure would look the same if flipped upside down and rotated.
Breaking this symmetry opens a “bandgap,” which is a range of frequencies that cannot move through the bulk crystal. However, at the interface between two mirrored versions of this patterned silicon, special modes known as “topological edge states” appear in the bandgap. They guide terahertz waves along the boundary even when the path includes sharp turns or defects.
To create the modulator, the researchers built a waveguide shaped like the Greek letter omega. It includes four one 120-degree bends and covers an area of 11.5 by 11.5 millimeters. A thin carbon nanotube film was placed beside the waveguide and connected to two electrodes. When voltage is applied, the film warms through electrical resistance. This heating shifts the refractive index of the silicon, which is a measure of how much light slows down inside the material. A small shift in refractive index changes the frequencies that can pass through the waveguide, allowing the device to act as a controllable modulator.
With no voltage applied, the waveguide passes signals between 315.41 and 348.06 gigahertz and shows almost no loss at 331.74 gigahertz. This indicates that the topological structure moves terahertz light efficiently, even around tight bends. When 26 volts are applied, the average signal level across that same range is reduced by 71 percent. At one frequency, the reduction reaches 90 percent. The change is strongest near the upper edge of the passband because heating shifts the allowed range downward. Frequencies near the upper edge move into the bandgap and lose transmission.
The second device in the study is a tunable filter. It combines a straight topological waveguide with a cavity. A cavity is a small region that traps waves at certain frequencies. When the frequency of light in the waveguide matches a cavity resonance, power drops in the waveguide because it tunnels into the cavity.
Two such resonances appear in this device, one at 330.30 gigahertz and one at 334.48 gigahertz. Each has a “quality factor,” or Q, which is the center frequency divided by the bandwidth of the resonance. A higher Q means the filter is more selective.
As with the modulator, a carbon nanotube film is used to warm the silicon near the cavity. Increasing the bias voltage shifts both resonances downward by about 0.52 to 0.54 gigahertz. Importantly, the Q factors stay nearly constant during this shift, which preserves frequency selectivity.
The depths of the two resonance dips change in opposite directions under heating due to changes in the balance between “intrinsic loss” and “coupling loss.” Intrinsic loss refers to energy absorbed or scattered in the cavity. Coupling loss refers to energy passed in or out of the waveguide. When these losses balance, the device is “critically coupled” and transmission at the resonance drops to its lowest point.
To isolate heating as the cause of the tuning, the researchers monitored temperature changes with an infrared camera. On the modulator, temperature rose from 23 to 131 degrees Celsius as the voltage increased to 26 volts. On the filter, temperature rose from 21 to 50 degrees because the heated area was smaller.
Electrical current increased proportionally with voltage, which confirms that the film’s electrical resistance stayed constant and that heating power was driven by the square of the voltage. Measurements at a fixed frequency showed that transmission fell by about 0.007 for every degree Celsius the device warmed. Resonance frequency in the filter shifted by about 17 to 18 megahertz per degree.
Those measurements were used to estimate the “thermo optic coefficient” of the silicon in the device. This number tells how much the refractive index changes with temperature. The measured value, about 1.89 times ten to the minus four per degree Celsius, matches other reported values for porous or patterned silicon at similar frequencies.
Switching times were also examined. The modulator took about 29 seconds to reach a new steady state when voltage was applied and about 3.9 seconds to cool when voltage was removed. The filter responded in about 4.7 to 4.9 seconds in both directions. These differences reflect the volume of heated material. The larger modulator structure retains heat longer, which slows response.
The researchers suggest that speed can be improved by lowering the thermal mass or placing the heater closer to the waveguide, as long as added optical loss is avoided.
This study shows that it is possible to integrate electrical tuning into terahertz topological photonic structures while keeping low loss and compact size. The modulator offers wideband amplitude control useful for switching and variable attenuation. The cavity filter enables frequency selection with small, precise shifts that preserve filtering quality. Carbon nanotube films offer a flexible way to add this control because they can be placed and wired with minimal changes to the photonic crystal design.
These devices point toward on chip terahertz systems for data links, sensing, and imaging. They combine reliable guiding around bends with electrical control through simple drivers. The method can be adapted to other structures in the same platform, creating a toolkit for building tunable terahertz circuits in silicon.
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