Graphene's high mobility and Fermi velocity, combined with its constant light absorption in the visible to far-infrared range, make it an ideal material to fabricate high-speed and ultrabroadband photodetectors. However, the precise mechanism of photodetection is still debated. Here, we report wavelength and polarization-dependent measurements of metal−graphene−metal photodetectors. This allows us to quantify and control the relative contributions of both photothermo-and photoelectric effects, both adding to the overall photoresponse. This paves the way for a more efficient photodetector design for ultrafast operating speeds.KEYWORDS: Graphene, photodetectors, Raman spectroscopy, photoresponse, optoelectronics T he unique optical and electronic properties of graphene make it ideal for photonics and optoelectronics. 1 A variety of prototype devices have already been demonstrated, such as transparent electrodes in displays 2 and photovoltaic modules, 3 optical modulators, 4 plasmonic devices, 4−9 microcavities, 10,11 and ultrafast lasers. 12 Among these, a significant effort is being devoted to photodetectors (PDs). 6,10,11,13−25 Various photodetection schemes and architectures have been proposed to date. The simplest configuration is the metal− graphene−metal (MGM) PD, in which graphene is contacted with metal electrodes as the source and drain. 13−18 These PDs can be combined with metal nanostructures enabling local surface plasmons and increased absorption, realizing an enhancement in responsivity (i.e., the ratio of the lightgenerated electrical current to the incident light power). 6,26 Microcavity based PDs were also used, with increased light absorption at the cavity resonance frequency, again achieving an increase in responsivity. 10,11 Another scheme is the integration of graphene with a waveguide to increase the effective interaction length with light. 25,27 Hybrid approaches employ semiconducting nanodots as light-absorbing media. 22 In this case, light excites electron−hole (e−h) pairs in the nanodots; the electrons are trapped in the nanodot, while the holes are transferred to graphene, thus effectively gating it. 22 Under applied drain−source bias, this results in a shift in the Dirac point, thus a modulation of the drain−source current. 22 Due to the long trapping time of the electrons within the dot, the transferred holes can cycle many times through the phototransistor before relaxation and e−h recombination. This gives a photoconductive gain; i.e., one absorbed photon effectively results in an electrical current of several electrons. Responsivities >10 7 A/W were reported, 22 but with a millisecond time scale, not suitable for, e.g., high-speed optical communications. Devices were also fabricated for detection of THz light. 28,29 In this low energy range, Pauli blocking forbids the direct excitation of e−h pairs due to finite doping. Instead, an antenna coupled to source and gate of the device excites plasma waves within the channel. These are rectified, leading to a detectable dc out...

Graphene, a two-dimensional material with a high mobility and a tunable conductivity, is uniquely suited for plasmonics. The frequency dispersion of plasmons in bulk graphene has been studied both theoretically and experimentally, whereas no theoretical models have been reported and tested against experiments for confined plasmon modes in graphene microstructures. In this paper, we present measurements as well as analytical and computational models for such confined modes. We show that plsmon modes can be described by an eigenvalue equation. We compare the experiments with the theory for plasmon modes in arrays of graphene strips and demonstrate a good agreement. This comparison reveals the important role played by interaction among the plasmon modes of neighboring graphene structures.

The ultimate limitations on carrier mobilities in metal dichalcogenides, and the dynamics associated with carrier relaxation, are unclear. We present measurements of the frequency-dependent conductivity of multilayer dichalcogenide MoS2 by optical-pump terahertz-probe spectroscopy. We find mobilities in this material approaching 4200 cm 2 V −1 s −1 at low temperatures. The temperature dependence of scattering indicates that the mobility, an order of magnitude larger than previously reported for MoS2, is intrinsically limited by acoustic phonon scattering at THz frequencies. Our measurements of carrier relaxation reveal picosecond cooling times followed by recombination lasting tens of nanoseconds and dominated by Auger scattering into defects. Our results provide a useful context in which to understand and evaluate the performance of MoS2-based electronic and optoelectronic devices.Layered two-dimensional transition metal dichalcogenides have recently enjoyed a resurgence of interest from the scientific community both from a new science perspective and also for novel applications [1][2][3][4][5][6][7][8][9][10]. In contrast to graphene, metal dichalcogenides have non-zero bandgaps and are also efficient light emitters, making them attractive for electronics and optoelectronics [1][2][3][4][5][6][7][8][9][10]. The ability to synthesize atomically thin semiconducting crystals and their heterostructures and transfer them to arbitrary substrates has opened the possibility of transparent, flexible electronics and optoelectronics based on these material systems [10][11][12][13][14][15][16][17]. The best reported carrier mobilities in metal dichalcogenides, typically in the few hundred cm 2 V −1 s −1 range for MoS 2 [18,19], are not as large as in graphene. But the reported mobilities are large in comparison to those of organic materials often used for flexible electronics [1]. Physical mechanisms limiting the mobility of monolayer and multilayer MoS 2 field effect transistors have been the subject of several experimental [18][19][20][21][22][23][24] and theoretical [25,26] investigations. Charged impurity scattering, electron-phonon interaction, and screening by the surrounding dielectric environment are all believed to affect the mobility. Due to the challenge of isolating these effects, the intrinsic mobility of MoS 2 , and the ultimate performance of electronic devices, remains unclear. Similarly, carrier intraband and interband scattering and relaxation rates, which determine the performance of almost all proposed and demonstrated electronic and optoelectronic metal dichalcogenide devices, remain poorly understood.In this report, we present optical-pump THz-probe measurements of the time-and frequency-dependent conductivity of multilayer MoS 2 . Previously reported electrical measurements [18,19,21,22] have probed the MoS 2 DC carrier transport, and all-optical measurements [24,27] have probed the exciton temporal dynamics. In contrast, optical-pump THz-probe spectroscopy can measure the time development of the co...

A variety of different graphene plasmonic structures and devices have been proposed and demonstrated experimentally. Plasmon modes in graphene microstructures interact strongly via the depolarization fields. An accurate quantitative description of the coupling between plasmon modes is required for designing and understanding complex plasmonic devices. Drawing inspiration from microphotonics, we present a coupled-mode theory for graphene plasmonics in which the plasmon eigenmodes of a coupled system are expressed in terms of the plasmon eigenmodes of its uncoupled sub-systems. The coupled-mode theory enables accurate computation of the coupling between the plasmon modes and of the resulting dynamics. We compare theory with experiments performed on the plasmon modes in coupled arrays of graphene strips. In experiments, we tune the coupling by changing the spacing between the graphene strips in the array. Our results show that the coupling parameters obtained from the coupled-mode theory and the plasmon frequency changes resulting from this coupling agree very well with experiments. The work presented here provides a framework for designing and understanding coupled graphene plasmonic structures.Graphene, a single layer of carbon atoms arranged in honeycomb lattice, has emerged as an important material for plasmonics [1][2][3][4][5][6][7][8]. The high carrier mobility and the widely tunable conductivity in graphene together with the ability to fabricate graphene microstructures of different sizes implies that plasmons in graphene structures can have high quality factors and frequencies tunable from a few THz to more than 100 THz. Graphene plasmonic structures have the potential to form building blocks for novel THz/IR devices such as detectors, emitters, oscillators, switches, filters, and sensors. Several theoretical works have explored techniques to compute the modes in individual as well as in arrays of graphene plasmon resonators [9][10][11][12][13][14]. In order to realize the full potential of graphene plasmonics, and develop the ability to combine several graphene plasmonic resonators and engineer complex device structures, suitable techniques are needed model the interactions between plasmonic resonators in simple, yet effective and accurate, ways. In the field of microphotonics, the equivalent role is played by coupled-mode theories [15,16]. In coupled-mode theories, the field of a coupled system is expanded in terms of the fields of the eigenmodes of its uncoupled subsystems [15,16]. Accurate computation of the coupling parameters and the response of the coupled system without detailed first-principles electromagnetic simulations are few of the main benefits of coupled-mode theories. Coupled-mode theories have proven to be extremely effective tools in designing and understanding complex optical integrated structures [17]. In the field of graphene plasmonics, complex devices incorporating several coupled plasmonic resonators have been proposed and demonstrated for various applications [1,2,[18][19][20][21]...

No abstract

No abstract