CPM Semniar

Ultrafast optoelectronics in 2D materials and their heterostructures

Mathieu Massicotte

Two-dimensional (2D) layered materials, such as graphene and transition metal dichalcogenides (TMDs), have recently emerged as prime candidates for optoelectronic applications. This new class of one-atom-thick materials has sparked huge interest due to their exceptional electrical and optical properties, which can be very different from those of their bulk counterpart. Since the first isolation of graphene in 2004, the library of 2D materials has grown considerably and now comprises many other crystals covering a wide range of complementary properties. Assembling these 2D building blocks into vertical heterostructures opens up exciting possibilities for designing artificial materials with atomic-layer precision. The resulting van der Waals heterostructures (vdWH), in addition to combining the properties of their constituent layers, provide a rich playground for studying photophysical phenomena and implementing novel photodetection schemes.

In this talk, I will present our efforts to understand the optoelectronic response of devices based on 2D materials and vdWHs in order to facilitate the design of high-performance photodetectors. Studying how these light-matter interactions induce an electric signal in actual devices poses many experimental challenges. Besides the fabrication of high-quality devices, one of the main difficulties is to assess the ultrafast electronic processes involved in the photocurrent generation. To this end, we employ a versatile time-resolved photocurrent measurement technique which combines electronic detection with subpicosecond optical excitation.

From the broad library of 2D materials, we focus our attention on the two that have attracted most interest: graphene and TMDs. First, we investigate the physics of excitons and free carriers in monolayer TMDs and identified the tunnel ionization of excitons and the drift of free carriers as the main processes limiting the photocurrent generation. Next, we show that these limitations are overcome in heterostructures made of graphene and TMDs, yielding fast and efficient photodetectors. We also demonstrate that their spectral range can be extended to the infrared via a new photodetection mechanism called photo-thermionic emission. As a whole, these results clearly demonstrate the potential of 2D materials for ultrafast photodetection applications, such as on-chip high-speed optical communications.