Novel Materials Could Help Terahertz Chips Deliver Data at Terabits-Per-Second Rates

Photonic topological insulators and terahertz waves could together deliver data at ultra-fast speeds

Image: Nanyang Technological University/Nature Photonics

An artist's representation of the silicon chip. The orange wavy line represents terahertz rays, which travel topologically protected in the interface between the two different sets of triangular holes. On the right, data is encoded into transmitted terahertz rays. On the left, data is received from the terahertz rays in applications involving wireless communication.

By Charles Q. Choi

https://spectrum.ieee.org/nanoclast/computing/hardware/terahertz-chip

Novel materials known as photonic topological insulators could one day help terahertz waves send data across chips at unprecedented speeds of a trillion bits per second, a new study finds. 

Terahertz waves fall between optical waves and microwaves on the electromagnetic spectrum. Ranging in frequency from 0.1 to 10 terahertz, terahertz waves could be key to future 6G wireless networks. With those networks, engineers aim to transmit data at terabits (trillions of bits) per second.

Such data links could also greatly boost intra-chip and inter-chip communication to support artificial intelligence (AI) and cloud-based technologies, such as autonomous driving.

"Artificial intelligence and cloud-based applications require high volumes of data to be transmitted to a connected device with ultra-high-speed and low latency," says Ranjan Singh, a photonics researcher at Nanyang Technological University in Singapore and coauthor of the new work. "Take for example, an autonomous vehicle that uses AI to make decisions. In order to increase the efficiency of decision-making tasks, the AI-sensors need to receive data from neighboring vehicles at ultra-high speed to perform the actions in real time."

Conventional terahertz waveguides are vulnerable to fabrication defects and considerable signal loss at sharp bends. Now, researchers find the burgeoning field of topological photonics may help solve these problems.

Topology is the branch of mathematics that explores what features of shapes can survive deformation. For instance, an object shaped like a doughnut can get pushed and pulled into the shape of a mug, with the doughnut's hole forming the hole in the cup's handle, but it could not get deformed into a shape that lacked a hole without ripping the item apart.

Using insights from topology, researchers developed the first electronic topological insulators in 2007. Electrons traveling along the edges or surfaces of these materials strongly resist any disturbances that might hamper their flow, much as a doughnut might resist any change that would remove its hole.

Recently, scientists have designed photonic topological insulators in which photons of light are similarly "topologically protected." These materials possess regular variations within their structures that lead specific wavelengths of light to flow within them without scattering or losses, even around corners and imperfections.

Image: Nanyang Technological University/Nature Photonics

An optical image of the silicon chip. The white dashed line represents the interface between the two different sets of triangular holes.

Prior work on photonic topological insulators was largely focused on microwave and optical frequencies. Now researchers say they have for the first time experimentally achieved topological protection of terahertz waves.

Scientists fabricated a silicon chip that was 190 microns thick and measuring 8 millimeters by 26 millimeters. They perforated it with rows of triangular holes that alternated in size between 84.9 microns and 157.6 microns, with the smaller triangles pointing the opposite direction of the larger ones. These rows of holes were arranged in clusters where all the larger triangles either pointed up or down. Light entering this chip flowed topologically protected along the interface between the different sets of holes.

Photos: Nanyang Technological University/Nature Photonics

An experimental demonstration of uncompressed 4K high-definition video transmission using the new chip (right). The transmitted 4K high-definition video is shown on the monitor in the background. The terahertz-signal transmitter is on the left side; the receiver is on the right side.

In experiments, the researchers found terahertz waves could also travel smoothly with virtually no losses even when routed around 10 sharp corners, including five 120-degree turns and five 60-degree turns. They achieved data transfer rates of 11 gigabits per second at a frequency of 0.335 terahertz with a bit error rate of less than 1 in 100 billion. They also showed they could transmit uncompressed 4K high-definition video in real-time through their chip across those 10 sharp bends at a rate of 6 gigabits per second.

Previous research achieved data rates of 1.5 gigabits per second with terahertz waves and photonic crystals (structures possessing features smaller than the wavelengths of light they’re designed to deal with). Not only does the photonic topological insulator in the new work display higher data transfer rates, but traditional photonic crystals experience huge signal loss at bends, whereas such losses are negligible in the new material. "This is important when we consider miniaturization of devices in designing on-chip multiplexers and splitters, which normally require bending of waveguides," says Masayuki Fujita, a coauthor and photonics researcher at Osaka University in Japan.

The researchers note there are a number of ways to boost the data rates of their setup to achieve terabit-per-second speeds, though they haven’t yet demonstrated those rates in an experiment. These techniques include using higher frequencies, more bandwidth, and more complex data-encoding schemes.

The scientists detailed their findings on 13 April in the journal Nature Photonics.

Terahertz science discloses the ultrafast photocarrier dynamics in carbon nanotubes

Comparison of computed peak values of THz emission and photocurrent with experimental data.CREDIT @2020 American Chemical Society

https://www.eurekalert.org/pub_releases/2020-04/ou-tsd042020.php

OSAKA UNIVERSITY
A team of researchers from Osaka University, TU Wien, Nanyang Technological University, Rice University, University of Alberta and Southern Illinois University-Carbondale comes closer to unraveling the physics of quasiparticles in carbon nanotubes.

Carbon nanotubes (CNTs), a model one-dimensional (1D) material made up entirely of carbon atoms, have attracted considerable attention ever since their discovery because of the unique properties arising from quantum confinement effects. CNTs have been labeled as one of the materials for next-generation optoelectronic devices. Critical towards this advancement is understanding how quasiparticles - theoretical particles used to describe observable phenomena in solids - behave and interact with each other in a 1D system. This requires a fundamentally different model compared to a conventional 3D material like silicon as a consequence of the reduced dimensionality in CNTs.

"It was difficult to develop a terahertz radiation device with an external high electric field in a specific direction to CNT," says corresponding author Masayoshi Tonouchi.

By combining different experimental techniques, the team was able to directly probe the creation of free charge carriers in CNTs at different time scales after photoexcitation. Very complex interactions that involve different quasiparticles occur after the initial photoexcitation. These processes change over time, and being able to probe one of the quasiparticles makes it easier to understand the whole process.

Together with state-of-the-art simulations, the team was able to identify two key mechanisms that explain their data and helped them develop a detailed microscopic model describing quasiparticle interactions in a strong electric field in CNTs.

"We proposed a model in which electron-hole bound quasiparticles excited in the high energy E22 exciton band diverge to the low energy band and play a role in ultrafast electrical conduction. This model successfully explained the experimental facts and led to the clarification of the physical properties of CNTs."

Their results shed light on a number of long-standing issues in CNT ultrafast dynamics, moving us closer towards the realization of advanced optoelectronics based on CNTs and other low-dimensional materials.

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The article, "Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication," was published in ACS Nano Letters at DOI: https://doi.org/10.1021/acs.nanolett.9b05082.

TERAHERTZ SOURCES: Concentric-grating terahertz QCLs five times more powerful than ridge QCLs

01/30/2014

By Gail Overton
Senior Editor

http://www.laserfocusworld.com/articles/print/volume-50/issue-02/world-news/terahertz-sources-concentric-grating-terahertz-qcls-five-times-more-powerful-than-ridge-qcls.html

Quantum-cascade lasers (QCLs) operating in the terahertz region of the electromagnetic spectrum are in demand for numerous spectroscopy, communications, and imaging applications; however, ridge-waveguide-based terahertz QCLs with the commonly adopted metal-metal waveguide configuration suffer from a relatively low output power and a large beam divergence of approximately 180º as well as multimode operation. While smaller divergence angles on the order of 10º are possible using external or integrated optics, low output power and multimode operation persist.

An alternative terahertz QCL developed by researchers at Nanyang Technological University (NTU) and the Singapore Institute of Manufacturing Technology (both in Singapore), as well as the University of Leeds (Leeds, England), Hong Kong Polytechnic University (Kowloon, Hong Kong), and Shanghai Jiao Tong University (Shanghai, China) takes advantage of the circular symmetry of concentric circular gratings (CCGs) to develop vertical-emission QCLs with low divergence that are five times more powerful than their ridge-waveguide QCL counterparts.¹

Circular symmetry
Through numerical simulations using a commercial COMSOL (Burlington, MA) finite-element analysis tool, the research team developed an optimized concentric circular grating (CCG) using a two-dimensional (2D) partial-differential-equation mode in the COMSOL software. The modeling step takes the mode profile of vertical emission into account, producing a device optimized for terahertz operation at approximately 3.75 THz with a laser-gain medium sandwiched between the CCG at the top and a lower metal plate at the bottom. The optimal parameters of the CCG for maximum emission from theterahertz grating-based laser were an outer ring thickness of 16.2 μm and a total ring diameter of 356.7 μm with carefully engineered slots in each ring.

The concentric rings of the optimized metal-based CCG are electrically connected together via a three-spoke structure so that the entire gain medium below the CCG can be electrically pumped (see figure). The laser gain medium was fabricated using molecular beam epitaxy with a gallium arsenide/aluminum gallium arsenide (GaAs/Al_0.15Ga_0.85As) structure. The CCG structures were defined by standard optical lithography and liftoff, and the final active region was wet-etched into the circular form.

A scanning electron microscope (SEM) image shows the fabricated concentric circular grating (CCG) terahertz quantum-cascade laser (QCL); the yellow color highlights the gold layers and the concentric rings allow electrical pumping of the whole grating. The dual-lobed far-field emission is typical of a CCG terahertz QCL. (Courtesy of NTU)

Experimental analysis of the CCG terahertz QCLs showed operation up to 110 K compared to 130 K for conventional ridge-waveguide QCLs. However, the CCG QCLs exhibited stable single-mode operation (rather than multimode), at peak power levels of tens of milliwatts—around 5X higher than for comparably sized ridge-waveguide QCLs.

The far-field pattern of the CCG QCLs was a dual-lobed 13.5º × 7º shape due to boundary deformation of the active region occurring during anisotropic wet chemical etching for the circular fabrication portion of the process. Using an isotropic etching technique (like plasmon etching) could avoid this boundary-deformation problem.

“This approach opens up new opportunities in achieving functional beam control such as polarization control, beam steering, and special beams, while holding the single-mode operation simultaneously,” says NTU assistant professor Wang Qijie, lead author of this work. “The design is also simple by taking advantage of the circular symmetry, which is easily implemented.”

“In the near future, we aim to realize ring-shaped and radially polarized light emission by removing the laser boundary deformation,” says NTU Ph.D. student Liang Guozhen. “Such emission is highly desirable for efficient coupling when launching terahertz waves into a terahertz metal-wire waveguide or the metallic tip of a terahertz near-field imaging system. In addition, one can also achieve a narrower single-lobed far-field pattern by further optimizing the CCG structure.”

REFERENCE
1. G. Liang et al., Opt. Exp., 21, 26, 31872–31882 (2013).

Terahertz plasmonics

http://www.scopus.com/record/display.url?eid=2-s2.0-80053221143&origin=inward&txGid=GBKPi-RhYsF5wzzY9grdYof%3a2
Semiconductor microstructures can be used to tailor the dispersion properties of surface plasmon polaritons in the terahertz (THz) frequency range, and therefore can be used as important building blocks for terahertz optical devices. The physical principles of three structures are discussed: plasmonic second-order gratings, designer (spoof) surface plasmon polariton structures, and channel polariton structures. The effectiveness of these structures is demonstrated by utilising them to improve power throughput and to reduce the beam divergence of edge-emitting THz quantum cascade lasers. Plasmonics promises compact and low-loss solutions for manipulating light at THz wavelengths, and will have a large impact on applications such as imaging, light detection and ranging (LIDAR), and the heterodyne detection of chemicals. © 2010 The Institution of Engineering and Technology.