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Tunable light tightly bound to a single sheet of carbon atoms:
graphene as a novel platform for nano-optoelectronics

Final Report Summary - CARBONLIGHT (Tunable light tightly bound to a single sheet of carbon atoms:graphene as a novel platform for nano-optoelectronics)

1- Graphene nano-photonics and strong light-matter interactions

Graphene plasmons were predicted to possess simultaneous ultrastrong field confinement and very low damping, enabling new classes of devices for deep-subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light–matter interactions and nano-optoelectronic switches. In this project, we have exploited near-field microscopy to image propagating plasmons in high- quality graphene encapsulated between two films of hexagonal boron nitride (h-BN). We find unprecedentedly low plasmon damping, with lifetimes up to 0.5 ps, combined with strong field confinement and confirm the high uniformity of this plasmonic medium.
Graphene is also an excellent medium for strong light-matter interactions due to the high Purcell factors. We have demonstrated strong interactions between graphene and single photon emitters. Because graphene is gapless with tunable carrier density, it can effectively behave as a semiconductor, a dielectric, or a metal. We exploit this to electrically control of optical emitter relaxation pathways and optical emission energy. In addition, we have studied the energy transfer between molecules and graphene and find a universal scaling-law of the energy transfer rate, which contains no material parameters. This represents itself as a universal distance ruler for nanometer distances.

2- Plasmons in Van der Waals heterostructures

The ultimate limit of control of light at the nanoscale is the atomic scale. By stacking multiple layers of graphene on hexagonal boron nitride (h-BN), heterostructures with unique nanophotonic properties can be constructed, where the distance between plasmonic materials can be controlled with atom-scale precision. We have shown show how various types of atomically-thick tunable heterostructures can be used as building block for quantum plasmonics:

• Ultra-compact phase modulators based on graphene plasmons
Modulating the amplitude and phase of light is a key ingredient for many of applications such as wavefront shaping, transformation optics, phased arrays, modulators and sensors. Performing this task with high efficiency and small footprint is a major challenge for the development of optoelectronic devices. We have developed a phase modulator based on graphene capable of tuning the light phase between 0 and 2π in situ. To achieve this, the unique wavelength tunability of graphene plasmons was exploited.

• Electrical detection of graphene plasmons and h-BN hyperbolic phonons
Plasmons are key players when trying to squeeze light into tiny circuits and control its flow electrically. We have been able to fabricate an all-graphene mid-infrared plasmon detector operating at room temperature, where a single graphene sheet serves simultaneously as the plasmonic generation medium and detector. We used a new experimental design in which they employed two local gates—a split metal sheet located underneath the graphene—to fully tune the thermoelectric and plasmonic behaviour of the graphene. Graphene is not only the plasmonic material but also a thermoelectric detector for the plasmons. The resulting device converts the conveniently available natural decay product of the plasmon, electronic heat, directly into a voltage through the thermoelectric effect. The results paper opens a new pathway to graphene "plasmo-electronics", which would allow to perform mid-infrared opto-electronics at very small length scales.

• Plasmons in charge-neutral quantum tunnelling devices
So far, the smallest device that can control electrons at that scale is the quantum tunnelling transistor, of a few atoms big, where quantum mechanics dictates the motion of electrons. We have added light to these quantum tunnelling devices, and found that quantum mechanics also dictates the motion of plasmons, which is the synchronous motion of light and electrons inside the transistor. The transistor was built from two-dimensional materials using a bottom-up technique: two layers of graphene separated by one nanometer (three monolayers) of insulating two-dimensional hexagonal Boron Nitride material. Then, a voltage was applied between the graphene layers, which generated the quantum tunnelling of electrons, meanwhile electrons were also removed from one graphene layer (leaving holes behind) and inserted in the other graphene layer. Interestingly enough, plasmons showed to propagate in these devices, while the net charge of the device remained zero. With near-field microscopy, the researchers visualized the plasmons and found that the plasmons exhibit ultra-strong optical field confinement, down to the nanometer scale.
The results of this study show that quantum tunnelling transistors can be used to control electrons and light simultaneously. Such discovery could pave the way to the development of nano-optoelectronic devices, incorporating light into future computer chips. Future research will aim for all-electrical generation and detection of plasmons in the quantum transistors.

• Optical probing of quantum plasmonic effects at the nanoscale
We have shown how light at the nanoscale can be used to “see” the quantum nature of an electronic material. We managed to do that by capturing light in graphene and slowing down light it down so that it moves almost as slow as the electrons in the graphene. Then something special happens: electrons and light start to move in concert, unveiling their quantum nature at such large scale that it could observed with a special type of microscope. The experiments were performed with ultra-high quality graphene. With a near-field nanoscope, we observed graphene plasmons propagate more than 300 times slower than light, and dramatically different from what is expected from classical physics laws.
This technique paves now the way for exploring many new types quantum materials, including superconductors where electricity can flow without energy consumption, or topological materials that allow for quantum information processing with topological qubits. This work could just be the beginning of a new era of near field nanoscopy.