When the topic of two-dimensional (2D) materials is presented, more often than not, graphene will be involved in this conversation. Due to its high carrier mobility, impressive strength, thermal and electrical conductivity and overall lightweight nature, this single layer of carbon atoms has become an ideal component of a growing number of applications for a variety of industrial purposes. While graphene is the thinnest and strongest material known to man, its lack of a natural bandgap prevents it from being used for important transistor and optoelectronic devices.
With an atom, electrons are placed in an array of states, which represents their different energy level, momentum and spin. These states then form regions known as bands, and the bandgap therefore describes the regions located between those bands. The importance that the presence of a bandgap plays is evident in understanding the role of semiconductors for optoelectronic devices.
An example of the role of the bandgap in a semiconductor material is in silicon, which is the material of choice for numerous optoelectronic devices, particularly solar cells, whose bandgap is wide enough to allow for the electrons within the material to easily cross the bandgap, following the introduction of photons from visible light into the material. While graphene does not inherently contain this bandgap, there are several ways to engineer a bandgap into this wonder material, however, such measures often reduce the material’s ability to conduct electricity.
In an effort to look towards other 2D materials that closely resemble the properties, structure and fabrication process of graphene, Researchers have found that transition metal dichalcogenides (TMDs), particularly monolayered TMDs, exhibit distinctive electronic and optical properties that have enhanced a number of devices including photodetectors, photovoltaics, thin film transistors and many more. The semiconducting properties of TMDs are largely attributed to their inherent and direct bandgaps of around 1-2eV for monolayer TMDs. Additionally, monolayer TMDs exhibit strong excitonic transitions that allow this material to provide an efficient optical-gain for certain applications, such as nanolasers.
Within a traditional laser, a gain medium is known as the source of the laser’s optical gain that is a result of stimulated emission that is provided from the pumping of energy into the cavity from an external source. The development of efficient nanolasers has been a project of interest for numerous Researchers, however, there remains a clear lack in developing an efficient gain medium that does not compromise the energy efficiency and overall size of the device in which it is placed.
With prospective applications of nanolasers including a potential role as a light source for future photonic systems, detection schemes and integration for highly flexible device displays, nanolasers have previously only been capable of functioning at cryogenic temperatures. The determining factor of this low-temperature operation of nanolasers is associated with the low cavity Q factor of nanolasers, which describes the amount of energy that can be retained within the cavity.
A group of Researchers from the Arizona State University and Tsinghua University in Beijing, China, have recently developed a single-layered nanolaser that is capable of functioning at room temperature. By utilizing a monolayer of TMD material molybdenum ditelluride (MoTe2), a silicon nanobeam cavity enclosed the material to allow for this optimal operation temperature. Within a laser, the two most important components are the gain medium that both produces and amplifies the photons that are providing energy to the material, as well as the cavity that will then trap the acquired photons.
By avoiding the need to cool down the nanolaser material during operation, there is an increase in the amount of energy present within the emitted laser that is no longer lost during this process. Additionally, the excitons present within the MoTe2 material were shown to emit a wavelength that is transparent to the silicon, thereby allowing this newly discovered cavity material to allow for the integration of this well-known semiconductor material into a variety of different electronic and photonic devices.
The Researchers in this study are hopeful that the both thermal and mechanical stress-resistant MoTe2 nanobeam structure will find its useful addition in electrical injection 2D TMD-based lasers, strong-cavity-TMD monolayer coupling physics and valley-spin-polarized lasers.
- “Room-temperature continuous-wave lasing from monolayer molybdenum ditelluride integrated with a silicon nanobeam cavity” Y. Li, J. Zhang, et al. Nature Nanotechnology. (2017). DOI: 10.1038/nnano.2017.128.