SPECIAL SEMINAR: Optoelectronics investigations of electron dynamics in 2D-TMD semiconductor heterostructure photocells: from electron-hole pair multiplication to phonon assisted anti-stokes process

Two-dimensional heterostructures composed of atomically thin transition metal dichalcogenides provide the opportunity to design correlated electronic devices that allow us to study novel quantum phenomena that emerge in these low dimensional materials due to quantum confinement. Quantum optoelectronics providing a rich set of tools enable the study of these quantum electronic materials through the interaction of light with matter. Using advanced optoelectronic measurements, we discovered highly efficient multiplication of interlayer electron-hole pairs at the interface of a tungsten diselenide / molybdenum diselenide integrated into a field-effect heterojunction device. Electronic transport measurements of the interlayer current-voltage characteristics indicate that layer indirect electron-hole pairs are generated by hot electron impact excitation at temperatures near T=300 K. By exploiting this highly efficient interlayer e-h pair multiplication process, we demonstrated near-infrared optoelectronic devices that exhibit 350% enhancement of the optoelectronic responsivity at microwatt power levels. This efficient energy relaxation pathway makes 2D semiconductor heterostructures viable for a new class of hot-carrier energy harvesting devices that exploit layer-indirect electron-hole excitations. More importantly, it demonstrates the high level of electrostatic control over different parameters of the system. Results published in Nature Nanotechnology 12, 1134-1139 (2017). 

 

Establishing a high level of understanding on the fundamentals of this system through e-h pair multiplication process, we can now start looking for the answer to one of the most challenging questions of whether is it possible to cool a system in solid state via Raman process. Interestingly the concept of laser cooling of solids or optical refrigeration was first proposed by Pringsheim in 1929 nearly half a century before Doppler cooling of atoms was contemplated and even before the invention of laser. The potential of much lower temperatures, and the opportunity for direct integration into electronic and photonic devices has motivated research in this field from rare earth doped materials to bulk semiconductors. Here we propose laser cooling of 2D-TMD based atomic layer semiconductor heterostructures. To find the answer to the proposed question we need to be able to perform high precession experiments on these devices to precisely control the photon energy at the interface of these structures. So we designed and developed a scanning photocurrent spectroscopy microscope with a supercontinuum light source that enables excitations by tuning the wavelength within the ranges of VIS, near IR and IR. Through our novel spatially and spectrally resolved measurements on these high quality 2D-TMD devices, we’ve been able to isolate the interlayer exciton species through a phenomena called phonon assisted antistokes process and have identified an entirely new light-matter interaction within these states. We show that under optimized experimental conditions the phonon assisted antistokes processes become dominant near the exciton edge of these heterostructures. At low photon energies near 1eV, we observed a strong photocurrent peak with several low energy echoes spaced by 30meV below this fundamental absorption feature. These processes can be used to efficiently remove vibrational modes or phonons generated in the crystal structure by coupling them to an electron, reducing the temperature of the materials and introducing a very interesting concept of laser cooling based on TMD devices. This process, which we find to be highly efficient due to the alignment of the exciton dipole moment to the atomic displacement of the out-of-plane optical phonon modes, marks the first and most critical step toward laser cooling of atomic layer semiconductors, manuscript is in preparation.