Session D6: Focus Session: Graphene Devices - Bolometers and Optical Response

Probing Dirac electron transport in graphene by far-infrared spectroscopy

The transport properties of Dirac fermions in graphene are a subject of intense interest. While various scattering mechanisms including impurities, graphene phonons and substrate phonons have been examined by dc transport measurements,\footnote{Das Sarma, S., Adam, S., Hwang, E. H. \& Rossi, E. Rev. Mod. Phys. 83, 407-470, (2011).} direct determination of the scattering rates under different experimental conditions remains challenging.\footnote{Horng, J. et al. Phys. Rev. B 83, 165113, (2011).} In this paper we report on the far-infrared optical conductivity spectrum of monolayer graphene samples obtained by Fourier transform infrared spectroscopy. From the frequency dependence of the optical conductivity we determine both the Drude weight and the carrier scattering rate. The dependence of these transport parameters on temperature and electrostatic doping will be presented, and the importance of many-body Coulomb interactions between Dirac electrons will be discussed.   

Ultrathin dual-gated graphene p-n junction photodetectors

Optoelectronic devices composed of atomically thin graphene and boron nitride membranes yield great promise for next-generation photonics and optoelectronic research, yet numerous fabrication challenges remain. We use chemical vapor deposited (CVD) graphene to produce atomically thin, local bottom-gates for high-quality exfoliated graphene optoelectronic devices. By incorporating CVD graphene instead of the more conventional silicon bottom-gate electrodes, we create very low-profile dual-gated field effect p-n junction devices with hexagonal boron nitride as the insulating gate dielectric layer. Combining electron-beam and photolithography techniques, we can shape the bottom-gates to locally modulate the carrier density in the active graphene layer. In addition to avoiding optical transmission through thick top-gate electrodes, our approach allows us to perform temperature dependent photoresponse measurements over various device length scales and with direct control of local electronic carrier densities.   

Graphene-based Optical Modulator

Data communications have been growing at a speed even faster than Moore's Law, with a 44-fold increase expected within the next 10 years. Data Transfer on such scale would have to recruit optical communication technology and inspire new designs of light sources, modulators, and photodetectors. The past decade has seen the flourish of researches in silicon-based optical modulators. However, their performance is limited by the weak refractive index changes in silicon, and consequently large footprint and stringent fabrication tolerance are required. Here we raise a totally new mechanism for optical modulation. Instead of changing the refractive index in silicon, we use graphene as an active layer and change its absorption coefficient by turning on/off the interband transitions. This turning is realized through shifting the Fermi level by simply a back gate. In this way, we can operate the optical modulator at a relatively high speed (1.2 GHz) over a broad range (1.3 to 1.6 $\mu$m), while keep the smallest footprint ($\sim $25 $\mu$m$^2$). More details of the device will be discussed in the talk.   

Tunable optical properties of graphene

Graphene, a single layer of carbon atoms, exhibits novel two-dimensional electronic behavior. Optical spectroscopy provides a powerful toolkit study graphene physics. In this talk, I will show how we can use infrared spectroscopy to probe gate-dependent interband transitions as well as intraband transitions. I will also discuss how we can use electrical gating to control inelastic light scattering processes in graphene.   

Photoconductivity of electro-oxidized epitaxial graphene

We report the enhanced photosensitivity of epitaxial graphene (EG) after electrochemical oxidation in nitric acid. The onset of photoconductivity appears at a photon energy of $\sim $ eV1.7 while the responsivity reaches 200 A/W in the UV spectral range (3.5 eV, 350 nm). The observed photoresponse is attributed to the formation of deep traps at the electro-oxidized EG interface, which release charge carriers under illumination and a significantly prolonged life time of photoexcitations due to the effect of the traps on the recombination dynamics. The enhanced photosensitivity and high selectivity in the UV spectral range make electro-oxidized EG an interesting alternative to the less spectrally selective Si for UV detection, although further optimization of the chemistry is required to shorten the photoresponse time while preserving the high sensitivity.   

Photo-controlling electrical properties of graphene-based field-effect transistor with photo-reactive copolymer

Here, we presented a photo-controlling graphene-based FET device. The tri-stable current of graphene device can be achieved in room temperature using the photo-assisted poling (PAP) and photo bleaching (PB) on photo-reactive copolymer PMMA-DR1. PMMA-DR1 was used as a dielectric film between graphene and top gate, where the top gate was the ITO film for applied voltage and excited laser passing. PAP operation created a polarization of PMMA-DR1 film, while the current of graphene FET device was varied due to Fermi energy of graphene was directly influenced by the polarization. The strength of polarization was associated with the gate voltage and laser power during the process of PAP operation. In contrast to the PAP operation, the PB operation was able to destroy the polarization of PMMA-DR1 film, while the current of graphene FET device returned to the initial value. Combing with PAP and PB operations, three current states identified as 1, 0 and -1 states can be achieved in room temperature. The current change ratios were 150{\%} and 50{\%} for 1 state and -1 state, comparing with initial state 0.   

Imaging of Polarization-dependent Photocurrent in Graphene Photodevices

Recently, a metal-graphene-metal photodetector for high-speed optical communications was reported. In addition, a graphene-based photodetector was reported to be able to absorb broadband light owing to the unique band structure of graphene [Mueller et al., Nature Photonics 4, 297 (2010)]. We investigated the polarization dependence of the photocurrent generated in metal-graphene-metal junctions. The graphene photodevice was fabricated by depositing Pd/Au and Ti/Au electrodes on single-layer graphene samples. When the polarization of incident laser beam is rotated with respect to the metal-graphene-metal junction, the photocurrent is significantly modulated. In addition, we measured the exact positions where the photocurrent is generated by measuring the photocurrent and Raman images of the graphene photodevices simultaneously.   

Supercollisions and Electron-Lattice Cooling in Graphene

Hot carrier proliferation is dependent on slow cooling between electron and lattice systems. In graphene, these cooling rates can be slow due to a small Fermi surface size and the momentum-conserving character of electron-phonon scattering. This in concert with the small ratio between the sound velocity, $s$, and the Fermi velocity, $v_F$, such that $s/v_F= 1/100$ produces a phase space constrained by the Fermi wavevector, $k_F$. In each of these first order scattering events, the energy exchanged between electronic and lattice systems is of the order $T_{\rm BG} = s\hbar k_F$ which at typical doping densities can be many times smaller than $k_BT$; non-thermal phonons are the dominant contributors to scattering dramatically suppressing cooling power. This constraint can be lifted by considering ``supercollisions" that utilize the full thermal distribution of phonons. While the frequency of supercollisions may be lower than the first order process, energy transfer for supercollisions is now on the order of $k_BT$ which is many times larger than the first order process. We we will show that this large exchange of energy allows supercollisions to give a dramatic boost to the cooling rate dominating over the first order process.   

Hot Carrier-Assisted Intrinsic Photoresponse in Graphene

Graphene is considered an excellent candidate for photodetection and energy harvesting applications due to its broadband optical response and high internal quantum efficiency, yet measurements have not clearly determined the photocurrent generation mechanism. Here, we report on the intrinsic photoresponse of dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation of wavelength 850 nm at the p-n interface leads to striking six-fold photovoltage patterns as a function of bottom- and top-gate voltages. These patterns, together with the measured spatial and density dependence of the photoresponse, provide strong evidence that non-local hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene [1,2] The hot carrier regime manifests as a strong photo-thermoelectric effect in which the photogenerated carrier population remains hot while the lattice stays cool. [1] Science v. 334, p. 648-652 (2011). [2] Nano Lett. ASAP 10.1021/nl202318u (2011).   

Graphene Terahertz Photodetector

A graphene photodetctor device is fabricated using mechanically exfoliated single layer graphene on SiO2/Si substrate contacted by two dissimilar metal electrodes (chromium and gold) using standard electron beam lithography. The graphene is etched into a strip shape with specific width and coupled to a bow tie antenna structure to improve coupling to long-wavelength radiation and enhance the electric field in the center of the device. We have observed the response of the graphene photodetector to optical (632.8nm) and infrared laser (118um) radiation as a function of gate voltage and device width. Experimental results and comparison to a model of graphene plasmon-enhanced photodetction will be discussed.   

Bolometric photo response of dual-gated bilayer graphene

We study the photo response of dual-gated bilayer graphene devices under infrared radiation. By comparison to Joule heating measurements using a second harmonic transport technique, we determine that the photo response is bolometric instead of photoconductive. The measured large electron-phonon heat resistance of our device is in good agreement with theoretical estimates in magnitude and temperature dependence, and enables our graphene bolometer operating at a temperature of 5 K to have a low noise equivalent power (33 fW/Hz$^{1/2}$) and fast response time (sub nanosecond).   

GHz response of bilayer graphene hot electron bolometer

An intrinsic GHz speed of a hot-electron bolometer (HEB) based on dual-gated bilayer graphene (BLG) was recently reported (J. Yan, arXiv:1111.1202). The thermal response time is governed by the weak electron acoustic phonon scattering which also results in a high thermal resistance for the lattice cooling of the hot electrons. The time response of BLG HEB was measured as a function of temperature, bias current, and laser power using two time delayed pulses from a 1.03 $\mu$m pulsed laser. In addition, we probed the energy gap dependence of the time response revealing information about the electron density in gapped BLG. We report the temperature dependence of the heat capacity and thermal resistance obtained from these measurements. This work is supported by IARPA grant \#W911NF1010443   

Non-linear plasma transport in graphene channels and application to the detection of terahertz signals

The non-linear electron plasma response to electromagnetic signal applied to a gated graphene conduction channel can be used to make a graphene based Dyakonov-Shur terahertz detector. The hydrodynamic model predicts a resonance response to electromagnetic radiation at the plasma oscillation frequency. With less damping and higher mobility, the graphene conduction channels may provide higher quality plasma response than possible with semiconductor channels. Our analysis of plasma oscillations in a graphene channel is based on the hydrodynamic equations which we derive from the Boltzmann equation accounting for both electrons and holes, and including the effects of viscosity and finite mobility.