Photocurrent In Graphene Research Articles

Controlling photothermoelectric directional photocurrents in graphene with over 400 GHz bandwidth

Photodetection in the near- and mid-infrared spectrum requires a suitable absorbing material able to meet the respective targets while ideally being cost-effective. Graphene, with its extraordinary optoelectronic properties, could provide a material basis simultaneously serving both regimes. The zero-band gap offers almost wavelength independent absorption which lead to photodetectors operating in the infrared spectrum. However, to keep noise low, a detection mechanism with fast and zero bias operation would be needed. Here, we show a self-powered graphene photodetector with a > 400 GHz frequency response. The device combines a metamaterial perfect absorber architecture with graphene, where asymmetric resonators induce photothermoelectric directional photocurrents within the graphene channel. A quasi-instantaneous response linked to the photothermoelectric effect is found. Typical drift/diffusion times optimization are not needed for a high-speed response. Our results demonstrate that these photothermoelectric directional photocurrents have the potential to outperform the bandwidth of many other graphene photodetectors and most conventional technologies.

Optical control of ultrafast photocurrent in graphene

Optical control of ultrafast photocurrent in graphene

A hybrid graphene-PbS quantum dots photodetector with positive and negative photocurrents

Different graphene photodetectors may have different light response mechanisms, and in general, most graphene photodetectors tend to generate only positive or negative photocurrents. Here, we demonstrate a photodetector based on graphene grown by Chemical Vapor Deposition (CVD) and PbS quantum dots (QDs) with both positive and negative photocurrents. Under the irradiation of a 635 nm laser, the device generated positive photocurrents of 9 μA at a low laser power density (0.17 μW), the responsivity of which was up to 39.58 A/W due to the high gain mechanism. However, at high laser power density (9.59 μW), the device exhibits completely opposite characteristics due to thermal scattering. The negative photocurrents of 20 μA were generated and the responsivity of the device was 10.29 A/W. The device exhibits the coexistence of two response mechanisms. It is necessary to explore the mechanism of positive and negative photocurrent in graphene, which is helpful to investigate the regulation of graphene carriers, and can also provide a possible research direction for graphene-based memristor devices.

Plasmonic drag photocurrent in graphene at extreme nonlocality

It is commonly assumed that photocurrent in two-dimensional systems with centrosymmetric lattice is generated at structural inhomogenities, such as p-n junctions. Here, we study an alternative mechanism of photocurrent generation associated with inhomogenity of the driving electromagnetic field, termed as 'plasmonic drag'. It is associated with direct momentum transfer from field to conduction electrons, and can be characterized by a non-local non-linear conductivity $\sigma^{(2)}({\bf q},\omega)$. By constructing a classical kinetic model of non-linear conductivity with full account of non-locality, we show that it is resonantly enhanced for wave phase velocity coinciding with electron Fermi velocity. The enhancement is interpreted as phase locking between electrons and the wave. We discuss a possible experiment where non-uniform field is created by a propagating graphene plasmon, and find an upper limit of the current responsivity vs plasmon velocity. This limit is set by a competition between resonantly growing $\sigma^{(2)}({\bf q},\omega)$ and diverging kinetic energy of electrons as the wave velocity approaches Fermi velocity.

Nonlinear intensity dependence of edge photocurrents in graphene induced by terahertz radiation

We report on the observation of terahertz radiation induced edge photogalvanic currents in graphene, which are nonlinear in intensity. The increase of the radiation intensities up to MW/cm$^2$ results in a complex nonlinear intensity dependence of the photocurrent. The nonlinearity is controlled by the back gate voltage, temperature and radiation frequency. A microscopic theory of the nonlinear edge photocurrent is developed. Comparison of the experimental data and theory demonstrates that the nonlinearity of the photocurrent is caused by the interplay of two mechanisms, i.e. by direct inter-band optical transitions and Drude-like absorption. Both photocurrents saturate at high intensities, but have different intensity dependencies and saturation intensities. The total photocurrent shows a complex sign-alternating intensity dependence. The functional behaviour of the saturation intensities and amplitudes of both kinds of photogalvanic currents depending on gate voltages, temperature, radiation frequency and polarization is in a good agreement with the developed theory.

On the Origin of Photocurrents in Pristine Graphene

Recently Ma et al. (Nature Nanotech. 14, 145, 2019) reported an intrinsic photocurrent in graphene, which occurs as the authors believe “in a different parameter regime from all the previously observed photothermoelectric or photovoltaic photocurrents in graphene”. Here we present an alternative – obvious and transparent explanation of such experiments. We demonstrate that the photo effect occurs in the p–n junctions formed in graphene samples containing two regions of different widths with different particle concentrations. This difference in concentration for various sample widths is found to result from edge doping of samples.

Microscopic understanding of the photoconduction effect in graphene

We investigate the photoresponse of intrinsic graphene in an in-plane electric field. Toward that end, we employ a microscopic approach that allows us to determine the time- and momentum-resolved charge-carrier distributions as a result of the interplay between the field-induced acceleration of optically excited carriers and Coulomb- and phonon-driven carrier scattering. Calculating the generated photocurrent that is determined by the asymmetry of the carrier distribution, we reveal the microscopic foundation of the photoconduction effect in graphene. In particular, we discuss the possibility of tuning the photocurrent via externally accessible knobs, such as electric field, temperature, and substrate. Furthermore, we study the impact of Auger-induced carrier multiplication on the photocurrent in graphene.

Terahertz Electric Field Driven Electric Currents and Ratchet Effects in Graphene

AbstractTerahertz field induced photocurrents in graphene were studied experimentally and by microscopic modeling. Currents were generated by and pulsed laser radiation in large area as well as small‐size exfoliated graphene samples. We review general symmetry considerations leading to photocurrents depending on linear and circular polarized radiation and then present a number of situations where photocurrents were detected. Starting with the photon drag effect under oblique incidence, we proceed to the photogalvanic effect enhancement in the reststrahlen band of SiC and edge‐generated currents in graphene. Ratchet effects were considered for in‐plane magnetic fields and a structure inversion asymmetry as well as for graphene with non‐symmetric patterned top gates. Lastly, we demonstrate that graphene can be used as a fast, broadband detector of terahertz radiation.

Photoresponse enhancement in graphene/silicon infrared detector by controlling photocarrier collection

Graphene/silicon junction based photodetectors have attracted great interest due to their superior characteristics like large photosensitive area, fast photocarrier collection and low dark current. Currently, the weak optical absorption and short photocarrier lifetime of graphene remain major limitations for detection of infrared light with wavelengths above 1.2 μm. Here, we elucidate the mechanism of photocarrier transport in graphene/silicon junction based photodetector and propose a theoretical model to study the design and effect of finger-electrode structures on the photocurrent in graphene. We demonstrate that the top finger-like electrode in graphene/silicon photodetector can be designed to enhance the photocarrier collection efficiency in graphene by reducing the average transport distance of photocarriers. Therefore, the photoresponsivity of the graphene/silicon junction based photodetector can be increased. Our results have successfully demonstrated that by optimizing the design of finger electrodes, 4 times enhancement of photocurrents in graphene can be obtained at room temperature.

(Invited) Graphene Plasmons: Properties and Applications

After a short introduction to the single particle excitations of graphene, I will focus on the collective excitations (plasmons) of this material [1,2]. The properties of graphene plasmons, will be compared with those of the surface plasmons of conventional metals concentrating mainly on localized plasmons in lithographically patterned nano- and micro-structures (quantum dots and ribbons) [2-7]. I will discuss their optical behavior in the infrared and THz regions of the spectrum, size effects, doping effects, effects of external magnetic fields, and the hybridization of the graphene plasmons with substrate and adsorbed overlayer optical phonons and the plasmon damping mechanisms [3,6,7,11]. Applications of graphene plasmons in passive THz optical elements, the enhancement of photocurrents in graphene infrared photodetectors [8-10], and the enhancement of infrared absorption spectra of molecules and deposited thin dielectric films [11-13] will be discussed. [1] Grigorenko, A.N.; Polini, M; Novoselov, K.S., Nat. Photonics 6, 749 (2012). [2] Low, T.; Avouris, Ph. ACS Nano 8, 1086 (2014). [3] Yan, H.; et al. Nat. Nanotechnol. 7, 330 (2012). [3] Yan, H.; et al. Nat. Photonics 7, 394 (2013). [5] Freitag, M.; Low, T.; Xia, F.; Avouris, Ph. Nat. Photonics 7, 53 (2011). [6] Yan, H. et al. Nano Lett. 14, 4581 (2014). [7] Low, T. et al.; Phys. Rev. Lett. 112, 116801 (2014). [8] Freitag, M; et al. ACS Nano (2014), DOI: 10.1021/nn502822z [9] Freitag, M.; et al. Nat. Commun. 4, 1951 (2013). [10] Avouris, Ph., Freitag, M., IEEE J. Topics in Quantum Electronics, 20, 6000112 (2014). [11] Li, Y. et al.; Nano Lett. 14, 1573 (2014). [12] Freitag, M; et al. ACS Nano 8, 8350 (2014). [13] Wang, H.; et al. to be published.

Mid-infrared response of reduced graphene oxide and its high-temperature coefficient of resistance

Much effort has been made to study the formation mechanisms of photocurrents in graphene and reduced graphene oxide films under visible and near-infrared light irradiation. A built-in field and photo-thermal electrons have been applied to explain the experiments. However, much less attention has been paid to clarifying the mid-infrared response of reduced graphene oxide films at room temperature. Thus, mid-infrared photoresponse and annealing temperature-dependent resistance experiments were carried out on reduced graphene oxide films. A maximum photocurrent of 75 μA was observed at room temperature, which was dominated by the bolometer effect, where the resistance of the films decreased as the temperature increased after they had absorbed light. The electrons localized in the defect states and the residual oxygen groups were thermally excited into the conduction band, forming a photocurrent. In addition, a temperature increase of 2 °C for the films after light irradiation for 2 minutes was observed using absorption power calculations. This work details a way to use reduced graphene oxide films that contain appropriate defects and residual oxygen groups as bolometer-sensitive materials in the mid-infrared range.

Substrate-sensitive mid-infrared photoresponse in graphene.

We report mid-infrared photocurrent spectra of graphene nanoribbon arrays on SiO2 dielectrics showing dual signatures of the substrate interaction. First, hybrid polaritonic modes of graphene plasmons and dielectric surface polar phonons produce a thermal photocurrent in graphene with spectral features that are tunable by gate voltage, nanoribbon width, and light polarization. Second, phonon polaritons associated with the substrate are excited, which indirectly heat up the graphene, leading to a graphene photocurrent with fixed spectral features. Models for other commonly used substrates show that the responsivity of graphene infrared photodetectors can be tailored to specific mid-IR frequency bands by the choice of the substrate.

All-optical control of ultrafast photocurrents in unbiased graphene.

Graphene has recently become a unique playground for studying light-matter interaction effects in low-dimensional electronic systems. Being of strong fundamental importance, these effects also open a wide range of opportunities in photonics and optoelectronics. In particular, strong and broadband light absorption in graphene allows one to achieve high carrier densities essential for observation of nonlinear optical phenomena. Here, we make use of strong photon-drag effect to generate and optically manipulate ultrafast photocurrents in graphene at room temperature. In contrast to the recent reports on injection of photocurrents in graphene due to external or built-in electric field effects and by quantum interference, we force the massless charge carriers to move via direct transfer of linear momentum from photons of incident laser beam to excited electrons in unbiased sample. Direction and amplitude of the drag-current induced in graphene are determined by polarization, incidence angle and intensity of the obliquely incident laser beam. We also demonstrate that the irradiation of graphene with two laser beams of the same wavelength offers an opportunity to manipulate the photocurrents in time domain. The obtained all-optical control of the photocurrents opens new routes towards graphene based high-speed and broadband optoelectronic devices.

Photocurrent in graphene harnessed by tunable intrinsic plasmons

Graphene's optical properties in the infrared and terahertz can be tailored and enhanced by patterning graphene into periodic metamaterials with sub-wavelength feature sizes. Here we demonstrate polarization-sensitive and gate-tunable photodetection in graphene nanoribbon arrays. The long-lived hybrid plasmon-phonon modes utilized are coupled excitations of electron density oscillations and substrate (SiO2) surface polar phonons. Their excitation by s-polarization leads to an in-resonance photocurrent, an order of magnitude larger than the photocurrent observed for p-polarization, which excites electron-hole pairs. The plasmonic detectors exhibit photo-induced temperature increases up to four times as large as comparable two-dimensional graphene detectors. Moreover, the photocurrent sign becomes polarization sensitive in the narrowest nanoribbon arrays owing to differences in decay channels for photoexcited hybrid plasmon-phonons and electrons. Our work provides a path to light-sensitive and frequency-selective photodetectors based on graphene's plasmonic excitations.

Ultrafast hot-carrier-dominated photocurrent in graphene

The combination of its high electron mobility, broadband absorption and ultrafast luminescence make graphene attractive for optoelectronic and photonic applications, including transparent electrodes, mode-locked lasers and high-speed optical modulators. Photo-excited carriers that have not cooled to the temperature of the graphene lattice are known as hot carriers, and may limit device speed and energy efficiency. However, their roles in charge and energy transport are not fully understood. Here, we use time-resolved scanning photocurrent microscopy to demonstrate that hot carriers, rather than phonons, dominate energy transport across a tunable graphene p-n junction excited by ultrafast laser pulses. The photocurrent response time varies from 1.5ps at room temperature to 4ps at 20K, implying a fundamental bandwidth of ∼500GHz (refs12, 13, 21). Gate-dependent pump-probe measurements demonstrate that both thermoelectric and built-in electric field effects contribute to the photocurrent, with the contribution from each depending on the junction configuration. The photocurrent produced by a single pulsed laser also displays multiple polarity reversals as a function of carrier density, which is a possible signature of impact ionization.

Helicity-dependent photocurrents in graphene layers excited by midinfrared radiation of a CO2laser

We report the study of the helicity-driven photocurrents in graphene excited by midinfrared light of a CO${}_{2}$ laser. Illuminating an unbiased monolayer sheet of graphene with circularly polarized radiation generates--- under oblique incidence---an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. We show that the current is caused by the interplay of the circular ac Hall effect and the circular photogalvanic effect. By studying the frequency dependence of the current in graphene layers grown on the SiC substrate, we observe that the current exhibits a resonance at frequencies matching the longitudinal optical phonon in SiC.

Coherent control of photocurrents in graphene and carbon nanotubes

Coherent one photon ($2 \omega$) and two photon ($ \omega$) electronic excitations are studied for graphene sheets and for carbon nanotubes using a long wavelength theory for the low energy electronic states. For graphene sheets we find that coherent superposition of these excitations produces a polar asymmetry in the momentum space distribution of the excited carriers with an angular dependence which depends on the relative polarization and phases of the incident fields. For semiconducting nanotubes we find a similar effect which depends on the square of the semiconducting gap, and we calculate its frequency dependence. We find that the third order nonlinearity which controls the direction of the photocurrent is robust for semiconducting t ubes and vanishes in the continuum theory for conducting tubes. We calculate corrections to these results arising from higher order crystal field effects on the band structure and briefly discuss some applications of the theory.