74 research outputs found
Theory of the in-plane photoelectric effect in a two-dimensional electron system
A new photoelectric phenomenon, the in-plane photoelectric (IPPE) effect, has
been recently discovered at terahertz (THz) frequencies in a
GaAs/AlGaAs heterostructure with a two-dimensional (2D) electron
gas (W. Michailow et al., Science Advances, DOI: 10.1126/sciadv.abi8398). In
contrast to the conventional PE phenomena, the IPPE effect is observed at
normal incidence of radiation, the height of the in-plane potential step, which
electrons overcome after absorption of a THz photon, is electrically tunable by
gate voltages, and the effect is maximal at a negative electron "work
function", when the Fermi energy lies above the potential barrier. Based on the
discovered phenomenon, efficient detection of THz radiation has been
demonstrated. In this work we present a detailed theory of the IPPE effect
providing analytical results for the THz wave generated photocurrent, the
quantum efficiency, and the internal responsivity of the detector, in
dependence on the frequency, the gate voltages, and the geometrical parameters
of the detector. The calculations are performed for macroscopically wide
samples at zero temperature. Results of the theory are applicable to any
semiconductor systems with 2D electron gases, including III-V structures,
silicon-based field effect transistors, and the novel 2D layered,
graphene-related materials.Comment: 21 pages, 15 figures, substantially revised improved versio
Combined electrical transport and capacitance spectroscopy of a field effect transistor
We have measured both the current-voltage (-)
and capacitance-voltage (-) characteristics of a
field effect transistor. From the measured capacitance
we calculate the electron surface density and show that its gate voltage
dependence follows the theoretical prediction resulting from the
two-dimensional free electron model. This model allows us to fit the measured
- characteristics over the \emph{entire range} of
. Combining this experimental result with the measured
current-voltage characteristics, we determine the field effect mobility as a
function of gate voltage. We show that for our device this improved combined
approach yields significantly smaller values (more than a factor of 4) of the
electron mobility than the conventional analysis of the current-voltage
characteristics only.Comment: to appear in Applied Physics Letter
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Active metamaterial polarization modulators for the Terahertz frequency range
Abstract
Active control of chirality in the terahertz frequency range is of great importance in many scientific areas, which include research into fundamental optical phenomena, investigation of novel materials, spectroscopy, imaging, wireless communications and chemistry. The lack of efficient, integrated and fast-reconfigurable polarization modulators has hindered, so far, the full exploitation of applications in all the aforementioned fields. Metamaterials are artificial resonant elements possessing unique remarkable properties such as high efficiency and miniaturization capability. The interplay of metallic metamaterial arrays with electrostatically tunable monolayer graphene has been demonstrated to be a valid approach for the realization of a novel class of THz devices. In this work, the realization of active chiral graphene/metamaterial modulator is presented. The versatility of this experimental approach allowed the device integration with broadband sources such as terahertz time domain spectrometers as well as with quantum cascade lasers. A continuous rotation of the polarization plane > 30° has been reported with a reconfiguration speed > 5 MHz. These results pave the way to the integration of fast terahertz polarization modulators in all the applications where these devices are in great demand.</jats:p
Terahertz aperture SNOM mapping of metamaterial coupled resonators
Metamaterials have emerged as the basis of a novel optoelectronic platform operating in the terahertz (THz) range, due to their versatility and strong light-matter interaction. The necessary design of efficient modulators and detectors requires a detailed investigation of metamaterial resonances and their interplay with an active medium, e.g. graphene. An aperture-SNOM (a-SNOM) system based on picosecond THz pulses was used to investigate the spectral characteristics of a set of lithographically tuned metamaterial coupled resonators. This approach allowed the mapping of the supported E-field of each resonator a few microns from the device plane, yielding bonding and antibonding modes reminiscent of electromagnetic induced transparency
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An in-plane photoelectric effect in two-dimensional electron systems for terahertz detection
Many mid- and far-infrared semiconductor photodetectors rely on a photonic response, when the photon energy is large enough to excite and extract electrons due to optical transitions. Toward the terahertz range with photon energies of a few milli–electron volts, classical mechanisms are used instead. This is the case in two-dimensional electron systems, where terahertz detection is dominated by plasmonic mixing and by scattering-based thermal phenomena. Here, we report on the observation of a quantum, collision-free phenomenon that yields a giant photoresponse at terahertz frequencies (1.9 THz), more than 10-fold as large as expected from plasmonic mixing. We artificially create an electrically tunable potential step within a degenerate two-dimensional electron gas. When exposed to terahertz radiation, electrons absorb photons and generate a large photocurrent under zero source-drain bias. The observed phenomenon, which we call the “in-plane photoelectric effect,” provides an opportunity for efficient direct detection across the entire terahertz range.George and Lilian Schiff Studentship, Schiff Foundation, University of Cambridge
Honorary Vice-Chancellor’s Award, Cambridge Trust, University of Cambridg
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Active metamaterial polarization modulators for the Terahertz frequency range
Abstract
Active control of chirality in the terahertz frequency range is of great importance in many scientific areas, which include research into fundamental optical phenomena, investigation of novel materials, spectroscopy, imaging, wireless communications and chemistry. The lack of efficient, integrated and fast-reconfigurable polarization modulators has hindered, so far, the full exploitation of applications in all the aforementioned fields. Metamaterials are artificial resonant elements possessing unique remarkable properties such as high efficiency and miniaturization capability. The interplay of metallic metamaterial arrays with electrostatically tunable monolayer graphene has been demonstrated to be a valid approach for the realization of a novel class of THz devices. In this work, the realization of active chiral graphene/metamaterial modulator is presented. The versatility of this experimental approach allowed the device integration with broadband sources such as terahertz time domain spectrometers as well as with quantum cascade lasers. A continuous rotation of the polarization plane > 30° has been reported with a reconfiguration speed > 5 MHz. These results pave the way to the integration of fast terahertz polarization modulators in all the applications where these devices are in great demand.</jats:p
Enhanced delivery and detection of terahertz frequency radiation from a quantum cascade laser within dilution refrigerator
We report on significant enhancements to the integration of terahertz (THz) quantum cascade lasers (QCL) and THz detection with a two-dimensional electron gas (2DEG) within a dilution refrigerator obtained by the inclusion of a multi-mesh 6 THz low-pass filter to block IR radiation, a Winston cone to focus light output, and gating the 2DEG for optimised sensitivity. We show that these improvements allow us to obtain a > 2.5 times reduced sample electron temperature (160 mK compared with 430 mK previously), during cyclotron resonance (CR) measurements of a 2DEG under QCL illumination. This opens up a route to performing sub-100 mK experiments using excitation by THz QCLs
Directed delivery of terahertz frequency radiation from quantum cascade lasers within a dry 3He dilution refrigerator
Metamaterial/graphene active terahertz modulators
Within the last years there has been a tremendous thrust into research and technology in the THz spectral region (broadly defined as 0.1-10 THz) mainly driven by the unique potential where this radiation finds applications in, such as imaging, spectroscopy and communication. In all these fields a fast, integrated and versatile platform for modulating light is required. Metamaterial/graphene devices fulfill all these requirements as their subwavelength nature lends itself naturally to strong light-matter interaction, and therefore highly efficient and miniaturized devices. Graphene's unique properties, e.g. the large carrier concentration modulation, provide a large degree of compatibility with several architectures which can be exploited in a range of modulation or detection schemes. Finally, metamaterial/graphene devices realize a fast, versatile platform, which can be easily scaled to other frequencies, and adapted into amplitude, frequency, polarization and phase modulators, as well as integrated detectors, for the next generation of wireless-communication
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