2, The University of Manchester, Manchester, , United Kingdom
Developing narrowband light sources in the mid- and far-infrared spectral regions has long been a major scientific and engineering challenge. Developments in quantum cascade laser (QCL) technology has been successful in providing single frequency sources in the mid and far infrared. However, these devices have limitations, most notably the lack of emission between 15-50µm, but additionally high-power consumption and limited tunability for far-infrared QCLs. Solving these challenges ultimately requires new material approaches to emitter design as semiconductor heterostructures do not currently provide a complete solution. Two dimensional (2D) materials have been the subject of intense scrutiny for their optoelectronic properties – such as polarized excitons, tuneable bandgap absorption and polariton behaviour. Indeed, there has been extensive research into making inter-band emitters and lasers from 2D materials, but realizing these at mid- and far-IR frequencies has proven a more significant challenge. This is mainly because achieving efficient emission requires careful design of both the optical mode and electronic states in the 2D material device. Here we will discuss an alternative approach; using polaritons within two dimensional materials to control an existing radiative process. This allows us to focus on efficient design of the optical modes in the structure, without the constraint of maintaining band-gap emission in the 2D material.
Specifically, we investigate graphene and hexagonal boron nitride (hBN) as polariton materials for controlling emission from quantum cascade structures and radiative emitters. Graphene has a carrier concentration that can be electrically controlled to create surface plasmon polaritons from the mid- to far-infrared. Here, we show how graphene can be coupled into a metal grating in the far-infrared to create an electrically tuneable optical response. By using such a structure in the waveguide of a QCL, we subsequently demonstrate spectral tuning of the terahertz laser emission. However, one of the limitations of this approach is that the device is still tied to the frequency of the QCL active region – ultimately limiting the spectral range of this approach. To solve this issue we turn to hexagonal boron nitride, which supports hyperbolic phonon polariton modes in each of its two Reststrahlen bands within the mid-wave and long-wave IR. By patterning resonators into hBN it has already been shown that multiple polariton modes be excited in such structures. By Kirchhoff’s law, an efficient absorber should also act as an efficient emitter for thermal radiation. Here, we demonstrate that hBN nanostructures can produce narrowband thermal emission by heating the device. Such emitters could be made compact by integrating local heat sources such as conductive elements such as graphite or graphene. We envision that a multiple material approach in 2D heterostructures will allow for better control of IR emission processes.