Sensitive room-temperature terahertz detection via the photothermoelectric effect in graphene

Abstract

Terahertz (THz) radiation has uses from security to medicine [1]; however, sensitive roomtemperature detection of THz is notoriously difficult [2]. The hot-electron photothermoelectric effect in graphene is a promising detection mechanism: photoexcited carriers rapidly thermalize due to strong electron-electron interactions [3,4], but lose energy to the lattice more slowly [3,5]. The electron temperature gradient drives electron diffusion, and asymmetry due to local gating [6,7] or dissimilar contact metals[8] produces a net current via the thermoelectric effect. Here we demonstrate a graphene thermoelectric THz photodetector with sensitivity exceeding 10 V/W (700 V/W) at room temperature and noise equivalent power less than 1100 pW/Hz 1/2 (20 pW/Hz 1/2 ), referenced to the incident (absorbed) power. This implies a performance which is competitive with the best room-temperature THz detectors [9] for an optimally coupled device, while time-resolved measurements indicate that our graphene detector is eight to nine orders of magnitude faster than those [7,10]. A simple model of the response, including contact asymmetries (resistance, work function and Fermi-energy pinning) reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.Graphene has unique advantages for hot-electron photothermoelectric detection. Gapless graphene has strong interband absorption at all frequencies. The electronic heat capacity of single-layer graphene is much lower than in bulk materials, resulting in a larger change in temperature for the same absorbed energy. The photothermoelectric effect has a picosecond response time, set by the electronphonon relaxation rate. [10,11]. Hot electron effects have been exploited in graphene for sensitive bolometry in THz and millimeter-wave at cryogenic temperatures, by using temperature-dependent resistance in gapped bilayer graphene [12], which is sizable only at low temperature, or noise thermometry [13], which requires complex RF electronics. In contrast, our photothermoelectric approach is temperature insensitive and produces an observable dc signal even under room temperature conditions.To realize our graphene hot electron thermoelectric photodetector we generate an asymmetry by contacting graphene with dissimilar metals using a standard double-angle evaporation technique as shown in Figs. 1a-e (also see Methods). Fig. 1f shows optical and atomic-force micrographs of our monolayer graphene device. Two metal electrodes, each consisting of partially overlapping Cr and Au regions, contact the monolayer graphene flake. The 3 µm × 3 µm graphene channel is selected to be shorter than the estimated electron diffusion length [14]. Fig. 1g shows the schematic of our detector in cross section. Figs. 1h-k illustrate the principle of operation: Electrons in graphene are heated by the incident light and the contacts serve as a heat sink, resulting in a non-uniform electron temperature T(x)as a function of position x within the device (Fig. 1h)...

Antenna Enhanced Graphene THz Emitter and Detector

“…For the detector, we achieve four orders of magnitude sensitivity improvements, which, in conjunction with its high speed 19,20 , makes the GFET a strong competitor to other contemporary THz sensors.…”

“…The antenna and the silicon lens are seen to improve the response to 1.1V/W at zero gate voltage, and V G tuning in Fig.4f further increases it to 4.9V/W, representing a 4 orders of magnitude improvement over Ref. [19].To assess the ultimate performance of the GFET thermoelectric detector, we perform a DC (direct current) rectification measurement to find the electric heating responsivity of the same detector D3. A DC bias voltage is used to heat the sample and the thermoelectric current is detected by comparing the magnitude of the electric currents for the two opposite directions of bias voltage (see Methods).…”

Hot-carrier photocurrent effects at graphene–metal interfaces

“…The photocurrent generated at the graphene-metal interface has been studied since 2008 [3][4][5][6] and has been shown to give rise to an ultrafast photoresponse with picosecond switching dynamics [5,7,8]. The response is furthermore extremely broadband, covering the visible, infrared and far-infrared (THz) wavelength ranges [2,9,10]. The photocurrent can moreover be enhanced by plasmonic effects [11] and by suspending the graphene sheet [12].…”

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

“…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.…”