University of California
From Electron-Hole Pair Multiplication Towards Solid State Optical Cooling in Van der Waals Heterostructures
Using advanced optoelectronic measurements on a TMD-based heterostructure, we discovered highly efficient multiplication of interlayer electron-hole pairs at the interface of WSe2 / MoSe2 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 which makes them viable for a new class of hot-carrier energy harvesting device, 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 proposed laser cooling of 2D-TMD based atomic layer semiconductor heterostructures. To precisely control the photon energy at the interface of these structures a scanning photocurrent spectroscopy microscope is developed with a supercontinuum light source that enables excitations by tuning the wavelength within the ranges of VIS, near IR and IR. Through novel spatially and spectrally resolved measurements on these high quality 2D-TMD devices, we’ve been able to isolate the interlayer excitons through a phenomenon called phonon assisted antistokes process near the exciton band 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.