Generation of photovoltage in graphene on a femtosecond timescale through efficient carrier heating

Abstract

*Graphene is a promising material for ultrafast and broadband photodetection. Earlier studies have addressed the general operation of graphene-based photothermoelectric devices and the switching speed, which is limited by the charge carrier cooling time, on the order of picoseconds. However, the generation of the photovoltage could occur at a much faster timescale, as it is associated with the carrier heating time. Here, we measure the photovoltage generation time and find it to be faster than 50 fs. As a proof-of-principle application of this ultrafast photodetector, we use graphene to directly measure, electrically, the pulse duration of a sub-50 fs laser pulse. The observation that carrier heating is ultrafast suggests that energy from absorbed photons can be efficiently transferred to carrier heat. To study this, we examine the spectral response and find a constant spectral responsivity of between 500 and 1,500 nm. This is consistent with efficient electron heating. These results are promising for ultrafast femtosecond and broadband photodetector applications. Photovoltage generation through the photothermoelectric (PTE) effect occurs when light is focused at the interface of monolayer and bilayer graphene, or at the interface between regions of graphene with different Fermi energies E F (refs 1-6). In such graphene PTE devices-which operate over a large spectral range 7,8 that extends even into the far-infrared 9 -local heating of electrons by absorbed light, in combination with a difference in Seebeck coefficients between the two regions, gives rise to a PTE voltage V PTE = (S 2 − S 1 )(T el − T 0 ). Here, S 1 and S 2 are the Seebeck coefficients of regions 1 and 2, respectively, T el is the hot electron temperature after photoexcitation and electron heating, and T 0 is the temperature of the electrode heat sinks. The performance of PTE graphene devices is intimately connected to the dynamics of the photoexcited electrons and holes, which have mainly been studied in graphene samples through ultrafast optical pumpprobe measurements [10][11][12][13][14][15][16][17] . As shown in Fig. 1a, the dynamics start with (i) photoexcitation and electron-hole pair generation, followed by (ii) electron heating through carrier-carrier scattering, in competition with lattice heating, both of which take place on a sub-100 fs timescale, and finally (iii) electron cooling by thermal equilibration with the lattice, which takes place on a picosecond timescale. The effect of the picosecond cooling step (iii) on the switching speed of graphene devices has been studied using timeresolved photovoltage scanning experiments with ∼200 fs time resolution [18][19][20] . These studies showed that the picosecond electron cooling time limits the intrinsic photo-switching rate of these devices to a few hundred gigahertz, because faster switching would reduce the switching contrast, as the system does not have time to return to the ground state. Indeed, gigahertz switching speeds have been demonstrated in graphene-based devices [21][22][23]...

Hot-carrier photocurrent effects at graphene–metal interfaces

“…The electron heating and cooling dynamics in bulk graphene have been studied using pump-probe measurements, such as optical pumpprobe [22,31,33,34], femtosecond time-resolved angleresolved photo-electron spectroscopy (ARPES) [30,32], and time-resolved optical pump-terahertz (THz) probe spectroscopy [26,[35][36][37][38][39][40][41]. The photoexcited carrier dynamics have also been studied in graphene-based devices through time-resolved photocurrent scanning microscopy [7,8,17,29]. These studies indicate that light absorption leads to the following dynamics (see [28] and references therein for a more detailed treatment): absorbed light induces electronhole pair excitation, assuming that the photon energy E exc is more than twice as large as the Fermi energy E F .…”

“…These electron temperature dynamics have been studied in quite some detail at pn-junctions [8,17,29]. To establish a better understanding of the mechanism and dynamics of the photoresponse near contacts, we compare the photovoltage dynamics for the two regions (at the contact and at the pnjunction).…”

“…We apply ultrafast time-resolved photocurrent scanning microscopy measurements to our dual-gated device and compare the dynamics at the pn-junction with the dynamics at the graphene-metal interface. The setup is very similar as the ones described in [7,8,17,29] and uses pulse pair excitation with two ultrashort pulses (with a wavelength of ∼800 nm) and a variable time delay between the two pulses. Due to an intrinsic nonlinearity (the electron heat capacity of graphene depends on electron temperature [48]), a lower photocurrent is generated when the two pulses overlap in time, than when they contribute to photocurrent independently, i.e.…”

“…We refer to the lowering of the photocurrent at short time delays as the photocurrent dip. The dynamics of the photocurrent dip directly reflect the temperature dynamics of the photoexcited electrons in graphene [7,8,17,29].…”

Giant photoeffect in proton transport through graphene membranes

“…Metal nanoparticles are known to dope graphene underneath, which leads to an in-plane electric field in the graphene areas surrounding them 18,20 . It is also known that illumination of such built-in junctions creates long-lived (>1 ps) hot electrons in graphene 24,25 , and these electrons generate a photovoltage 5,26,27 , similar to illumination of semiconducting p-n junctions. The photovoltage V ph in graphene is proportional to its electron temperature, T e .…”