The discovery of a single layer of carbon atoms, known as graphene1, led to great interest in 2D materials. Whereas graphene is highly transparent to visible light2, 2D materials that are highly reflective could be used as lightweight mirrors in optical or optoelectronic systems. The existence of such materials has been questioned, but, writing in Physical Review Letters, Back et al.3 and Scuri et al.4 report that single layers of molybdenum diselenide can have high levels of reflectance.
The importance of the authors’ work can be understood by considering the reflection of light from a homogeneous, free-standing thin film of material. When a wave of light of a particular colour — or, equivalently, frequency — hits the film, the oscillating electric field that is associated with the light wiggles the charged particles in the material. This drives the oscillation of electric dipoles (separations between positively and negatively charged particles) at the same frequency as that of the incident light (Fig. 1a).
The oscillating dipoles re-radiate light waves in both the forward and backward directions with respect to the direction of the incident wave. Whereas the latter occurrence gives rise to reflection, the former destructively interferes with the incident wave, producing transmitted light that has a lower intensity than that of the incident light. The material’s response to an oscillating electric field is, in general, not uniform with respect to incident waves from across the electromagnetic spectrum. At a particular frequency, the dipoles have a large oscillation amplitude — a phenomenon known as resonance — which results in more reflection and less transmission of light than at any other frequency.
Like all oscillators in real physical systems, the oscillations of the dipoles are damped, which means that they die out if the event that drives them is stopped. There are two ways in which the energy that is stored in the dipoles can be lost: it can be re-radiated (as discussed previously) or it can be absorbed by the material and converted into heat. These processes are known as radiative and non-radiative damping, respectively. In most materials, both mechanisms of damping operate. The incident light is therefore partly reflected, partly absorbed and partly transmitted.
However, in a material in which radiative damping dominates, the absorption losses would be negligible, and all of the incident electromagnetic energy would be re-radiated. Furthermore, the re-radiation in the forward direction would perfectly cancel out the incident light, through destructive interference. Owing to conservation of energy, the incident light would be reflected entirely, and the material would act as a perfect mirror (Fig. 1b). This holds true even when the material comprises a single layer of atoms, provided that the oscillating dipoles are being driven at their resonance frequency.
Although theoretical studies have suggested that such conditions could be realized in a 2D array of ultracold atoms5,6, the authors demonstrate near-perfect mirrors in a solid-state system. They use a single layer of molybdenum diselenide, which is a semiconductor and belongs to a family of materials known as the transition-metal dichalcogenides. In such materials, the oscillating dipoles generated by the incident light are excitons7 — bound pairs of an electron and a hole (the absence of an electron). The more tightly bound the excitons are, the larger the radiative damping will be, and the more perfectly the mirror will behave. Previous experimental work has shown that the exciton binding in single-layer transition-metal dichalcogenides is extremely strong8,9,10, which results in a rate of radiative damping that is much greater than that of conventional semiconductors.
Back et al. and Scuri et al. fabricated high-quality samples of single-layered molybdenum diselenide by encapsulating the material in atomically thin, inert films of hexagonal boron nitride, and then carried out their experiments at a low temperature (4 kelvin). Under these conditions, the authors show that radiative damping of the excitons is the dominant process. They demonstrate mirrors that can reflect a considerable proportion of the incident light — up to 85% in Scuri and colleagues’ study — at the exciton resonance frequency of the material.
Although the authors’ near-perfect mirrors work only in light from a narrow range of the electromagnetic spectrum (in the vicinity of the resonance frequency), the two studies open up intriguing possibilities for the fields of nanophotonics and quantum optics. For instance, quantum nonlinear optics requires strong interactions between photons at the single-photon level, which is difficult to achieve in conventional materials. The authors’ work shows that quantum nonlinear optics could be realized in single-layer transition-metal dichalcogenides because of the extremely strong light–matter interactions that can be achieved11.
The authors also demonstrate that the application of a voltage causes the mirrors to switch from being highly reflective to highly transparent. Such mirrors could therefore be used as light modulators, or as other reconfigurable components, in optical and optoelectronic systems. Moreover, the excitons in single-layer transition-metal dichalcogenides have a feature known as the internal-valley degree of freedom7, which might enable the mirrors’ reflectance to be controlled purely by varying the polarization of the incident light.
About a decade ago, during the early stages of research on 2D materials, many scientists were asking whether a single layer of atoms could be highly reflective. Thanks to Back et al. and Scuri et al., we now know that the answer is yes.
Nature 556, 177-178 (2018)