Competing Ultrafast Energy Relaxation Pathways in Photoexcited Graphene

For most optoelectronic applications of graphene a thorough understanding of the processes that govern energy relaxation of photoexcited carriers is essential. The ultrafast energy relaxation in graphene occurs through two competing pathways: carrier-carrier scattering -- creating an elevated carrier temperature -- and optical phonon emission. At present, it is not clear what determines the dominating relaxation pathway. Here we reach a unifying picture of the ultrafast energy relaxation by investigating the terahertz photoconductivity, while varying the Fermi energy, photon energy, and fluence over a wide range. We find that sufficiently low fluence ($\lesssim$ 4 $\mu$J/cm$^2$) in conjunction with sufficiently high Fermi energy ($\gtrsim$ 0.1 eV) gives rise to energy relaxation that is dominated by carrier-carrier scattering, which leads to efficient carrier heating. Upon increasing the fluence or decreasing the Fermi energy, the carrier heating efficiency decreases, presumably due to energy relaxation that becomes increasingly dominated by phonon emission. Carrier heating through carrier-carrier scattering accounts for the negative photoconductivity for doped graphene observed at terahertz frequencies. We present a simple model that reproduces the data for a wide range of Fermi levels and excitation energies, and allows us to qualitatively assess how the branching ratio between the two distinct relaxation pathways depends on excitation fluence and Fermi energy.Comment: Nano Letters 201

Out-of-plane heat transfer in van der Waals stacks through electron-hyperbolic phonon coupling.

Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities and superior electrical and optoelectronic properties 1-7 . Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting 8-16 require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements, we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons 17-19 in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism using distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures

Perspective on terahertz spectroscopy of graphene

Graphene has been an intensely studied material, owing to its unique band structure and concomitant outstanding electronic properties. In the past decades, ultrafast terahertz (THz) spectroscopy has developed into a powerful tool to characterize ultrafast charge carrier dynamics in a wide range of materials and material structures. In this Perspective we review recent experimental work exploring the ultrafast electron dynamics in graphene in the THz spectral range, and present a simple thermodynamic picture describing the THz linear, nonlinear, and photo-induced conductivity of this remarkable material

Large area conductive nanoaperture arrays with strong optical resonances and spectrally flat terahertz transmission

Krewer KL, Jiang K, Bley K, et al. Large area conductive nanoaperture arrays with strong optical resonances and spectrally flat terahertz transmission. Applied Physics Letters. 2017;111(2).Using simple and inexpensive nanosphere lithography, we produce large, centimeter-squared sized thin golden films patterned with a hexagonal array of nanoapertures with controllable dimensions on the order of 100–300 nm, spaced by a 350–375 nm pitch distance. The optical transmission spectra of our samples are dominated by the resonant plasmonic features in the spectral range 500–700 nm, caused by the nanostructure in the film. At the same time, the transmission at terahertz (THz) radiation is as high as ∼10% and is spectrally flat. Our measurements are in agreement with finite difference time domain simulations. Such thin metal hole array films allow for very efficient injection of optical energy, while at the same time maintaining reasonably high transparency of THz waves. These structures can be used in any application combining strong optical sensitivity and THz transparency, in optical biomolecular sensing, or as optically transparent electrodes

Control of Energy Relaxation Pathways in Graphene: Carrier-Carrier Scattering vs Phonon Emission

We investigate the relaxation pathways of photoexcited carriers in graphene. These carriers relax their energy through either carrier-carrier scattering or phonon emission, depending on photoexcitation conditions and the Fermi level

Direct observation of mode-specific phonon-band gap coupling in methylammonium lead halide perovskites

Methylammonium lead iodide perovskite, a promising material for efficient photovoltaics, shows a unique temperature dependence of its optical properties. Kim et al. quantify the coupling between the optical gap and a lattice phonon at 1 THz, which favorably contributes to the thermal variation of the gap