Research Bits: November 21

MoS2 in-memory processor; graphene optical logic gate; mode-locked laser.


MoS2 in-memory processor

Researchers from École Polytechnique Fédérale de Lausanne (EPFL) developed a large-scale in-memory processor using the 2D semiconductor material, molybdenum disulfide (MoS2), for the channel material in the more than 1,000 transistors that comprise the processor. The MoS2-based in-memory processor is dedicated to vector-matrix multiplication, key for digital signal processing and the implementation of AI models.

Their processor combines 1,024 elements onto a one-by-one-centimeter chip. Each element is composed of a 2D MoS2 transistor as well as a floating gate, used to store a charge in its memory that controls the conductivity of each transistor. Coupling processing and memory in this way fundamentally changes how the processor carries out the calculation. “By setting the conductivity of each transistor, we can perform analog vector-matrix multiplication in a single step by applying voltages to our processor and measuring the output,” said Andras Kis, a professor at EPFL.

“The key advance in going from a single transistor to over 1,000 was the quality of the material that we can deposit. After a lot of process optimization, we can now produce entire wafers covered with a homogenous layer of uniform MoS2. This lets us adopt industry standard tools to design integrated circuits on a computer and translate these designs into physical circuits, opening the door to mass production,” added Kis. [1]

Graphene optical logic gate

Researchers from Guilin University of Electronic Technology, Jincheng Research Institute of Opto-Mechatronics Industry, and Texas A&M University proposed a graphene-based optical logic gate composed of Y-shaped graphene nanoribbons bonded on top of a layer of insulation. The team said the design is ideal for hosting plasmon waves: collective oscillations of electrons which arise at the interface between the graphene and the insulating medium. They can be triggered by the light waves in incoming optical signals, and can also generate outgoing signals themselves after the information is processed by the logic gate.

The device can be switched on and off using an external voltage, which manipulates the energy levels at which electrons in graphene are available for conveying electrical current. The team claims the device has a high ratio between the power level of the gate’s ‘on’ and ‘off’ states, alongside small size, low loss of information, and high stability. [2]

Nanophotonic mode-locked laser

Researchers from California Institute of Technology built a mode-locked laser, which can emit extremely short pulses, on a photonic chip. The nanophotonic laser is based on lithium niobate, a synthetic salt with unique optical and electrical properties that, in this case, allows the laser pulses to be controlled and shaped through the application of an external radio-frequency electrical signal. This approach is known as active mode-locking with intracavity phase modulation.

“Beyond its compact size, our laser also exhibits a range of intriguing properties. For example, we can precisely tune the repetition frequency of the output pulses in a wide range. We can leverage this to develop chip-scale stabilized frequency comb sources, which are vital for frequency metrology and precision sensing,” said Qiushi Guo, an assistant professor at the City University of New York, the first author of the paper, and a former postdoctoral scholar on the project. The team plans to improve the technology so it can operate at even shorter timescales and higher peak powers, with a goal of 50 femtoseconds. [3]


[1] Marega, G. M., Ji, H. G., Wang, Z., Pasquale, G., Tripathi, M., Radenovic, A., & Kis, A. (2023). Large-Scale Integrated Vector-Matrix Multiplication Processor Based on Monolayer MoS2. Nature Electronics.

[2] Zhu, A., Song, L., Cheng, L. et al. An ultra-compact and highly stable optical numerical comparator based on Y-shaped graphene nanoribbons. Eur. Phys. J. D 77, 169 (2023).

[3] Qiushi Guo et al., Ultrafast mode-locked laser in nanophotonic lithium niobate. Science 382, 708-713 (2023).

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