Graphene Terahertz Research Articles

Acoustic terahertz graphene plasmons revealed by photocurrent nanoscopy.

Terahertz (THz) fields are widely used for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, λp, compared with the photon wavelength, λ0, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far-only GPs with nearly free-space wavelengths have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (λp ≈ λ0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.

Graphene terahertz devices for communications applications

The extraordinary electronic, thermal and optical properties of graphene, together with its two dimensional nature, and with the possibility of facile integration, have enabled its application into a new generation of high-performance devices capable of extending the performance of existing terahertz communications technologies. Although promising for wireless communications applications, the terahertz region of the spectrum, i.e. the frequency range between 300 GHz and 10 THz, is in fact still characterized by a lack of efficient, compact, solid state components capable of operating at room temperature. In this regard, graphene-based terahertz components have shown very promising results in terms of modulation, detection, as well as generation of terahertz waves. This paper will review and discuss recent progress on graphene based devices for modulation, detection and generation of terahertz waves, which are among the key components for future terahertz band communications systems.

Polarization-insensitive tunable multiple electromagnetically induced transparencies analogue in terahertz graphene metamaterial

A graphene-based metamaterial structure composed of multilayer graphene/dielectric stacking configuration is proposed, which achieves multiple analogue of electromagnetically induced transparencies (EIT) effect at terahertz frequencies. Using the phase-coupling scheme, a theoretical model is established to study the EIT-like effect of the proposed structure, and the theoretical calculations coincide well with the numerical simulated results. By varying the Fermi energy level of the graphene, the EIT-like windows can be dynamically tuned in a wide range of terahertz spectra. Particularly, since the symmetry of the structure, the EIT-like effect is polarization-insensitive and can be performed very well in a large incident angles, nearly 80° for both transverse electric and transverse magnetic waves. The proposed structure has potential applications in tunable terahertz chip-integrated optical devices, especially for dynamic multi-band filters, sensors, modulators and nonlinear devices.

Time-dependent simulation of particle and displacement currents in THz graphene transistors

Although time-independent models provide very useful dynamical information with a reduced computational burden, going beyond the quasi-static approximation provides enriched information when dealing with terahertz (THz) frequencies. In this work, the THz noise of dual-gate graphene transistors with DC polarization is analyzed from a careful simulation of the time-dependent particle and displacement currents. From such currents, the power spectral density (PSD) of the total current fluctuations are computed at the source, drain and gate contacts. The role of the lateral dimensions of the transistors, the Klein tunneling and the positive–negative energy injection on the PSD are analyzed. Through the comparison of the PSD with and without band-to-band tunneling and graphene injection, it is shown that the unavoidable Klein tunneling and positive–negative energy injection in graphene structures imply an increment of noise without similar increment on the current, degrading the (either low or high frequency) signal-to-noise ratio. Finally, it is shown that the shorter the vertical height (in comparison with the length of the active region in the transport direction), the larger the maximum frequency of the PSD. As a byproduct of this result, an alternative strategy (without length scaling) to optimize the intrinsic cut-off frequency of graphene transistors is envisioned.

Flexible terahertz modulator based on coplanar-gate graphene field-effect transistor structure.

The terahertz (THz) modulators, as an essential component of the THz system, have been developed by many efforts until now. However, the development of flexible THz modulators is hindered due to the lack of flexible THz modulating materials. Herein, for the first time to the best of our knowledge, we demonstrated the feasibility of flexible THz modulators based on the coplanar-gate field-effect transistor (FET) structure of ion-gel/graphene/polyethylene terephthalate. The THz transmittance through this THz graphene modulator can be well controlled with a modulation depth up to 22% by tuning the carrier concentration of graphene via electrical gating. Furthermore, because of the integration of high flexibilities of graphene, ion-gel, and polyethylene terephthalate (PET), the proposed THz graphene modulator shows superior flexible performance, where the modulation properties can be maintained almost unchanged, not only under bending deformations, but also before and after bending 1000 times. In addition, due to the unique structure of ion-gel/graphene/PET, the flexible THz graphene modulator has a low insertion loss (1.2 dB). Therefore, this Letter is expected to be beneficial for the potential applications, ranging from the traditional compact THz system to a new flexible THz technology.

グラフェンのテラヘルツオプトエレクトロニクス

Graphene has attracted considerable attention due to its extraordinary carrier transport, optoelectronic, and plasmonic properties originated from its gapless and linear energy spectra enabling various functionalities with extremely high quantum efficiencies that could never be obtained in any existing materials. This paper reviews recent advances in graphene optoelectronics particularly focused on the physics and device functionalities in the terahertz (THz) electromagnetic spectral range. Optical response of graphene is characterized by its optical conductivity and nonequilibrium carrier energy relaxation dynamics, enabling amplification of THz radiation when it is optically or electrically pumped. Current-injection THz lasing has been realized very recently. Graphene plasmon polaritons can greatly enhance the THz light and graphene matter interaction, enabling giant enhancement in detector responsivity as well as amplifier/laser gain. Graphene-based van der Waals heterostructures could give more interesting and energy-efficient functionalities.

Theoretical Limits on the Efficiency of Reconfigurable and Nonreciprocal Graphene Antennas

Theoretical upper limits on the radiation efficiency of reconfigurable antennas based on graphene are presented in closed form and discussed. The theory provides useful information for the design of these structures independently of the type and geometry of the antenna. The case of beam steering, which is very relevant for recently proposed graphene terahertz reflectarrays, is analyzed in detail. Finally, the theory is extended to the nonreciprocal case demonstrating that magnetostatically biased graphene allows the creation of nonreciprocal antennas, but that also in this case there are precise theoretical limits on the antenna efficiency.

Terahertz wave reflection impedance matching properties of graphene layers at oblique incidence

With the development of terahertz (THz) anti-radar technology, the demands for new antireflection coating technology and materials based on impedance matching mechanism have increased significantly. Herein, the reflection impedance matching properties of p-polarized THz wave at a randomly stacked multi-layer graphene interface is studied using both experiment and theory at oblique incidence. The experiment is performed on a THz-time domain spectroscopy system with variable reflection angle configuration. The reflected pulses from the quartz/graphene/air interface are observed to change with the incident angle and the layer number of graphene. Remarkable pulse shape reversal and impedance matching governed amplitude suppression with broadband response are achieved. Theoretical analysis based on graphene's THz Drude conductivity and a variable angle THz wave propagation model has been proved to agree well with the experimental results. This work makes it clear that the impedance matching mechanisms for the potential THz antireflection devices based on graphene in oblique incidence situation.

Integrated Terahertz Graphene Modulator with 100% Modulation Depth

Terahertz (THz) frequency technology has many potential applications in nondestructive imaging, spectroscopic sensing, and high-bit-rate free-space communications, with an optical modulator being a key component. However, it has proved challenging to achieve high-speed modulation with a high modulation depth across a broad bandwidth of THz frequencies. Here, we demonstrate that a monolithically integrated graphene modulator can efficiently modulate the light intensity of the THz radiation from a THz quantum cascade laser with a 100% modulation depth for certain region of the pumping current, as a result of the strongly enhanced interaction between the laser field and the graphene enabled by this integration scheme. Moreover, the small area of the resulting device in comparison to existing THz modulators enables a faster modulation speed, greater than 100 MHz, which can be further improved through optimized designs of the laser cavity and modulator architectures. Furthermore, as the graphene absorption spe...

Antenna Enhanced Graphene THz Emitter and Detector.

Recent intense electrical and optical studies of graphene have pushed the material to the forefront of optoelectronic research. Of particular interest is the few terahertz (THz) frequency regime where efficient light sources and highly sensitive detectors are very challenging to make. Here we present THz sources and detectors made with graphene field effect transistors (GFETs) enhanced by a double-patch antenna and an on-chip silicon lens. We report the first experimental observation of 1-3 THz radiation from graphene, as well as more than 3 orders of magnitude performance improvements in a half-edge-contacted GFET thermoelectric detector operating at ∼2 THz. The quantitative analysis of the emitting power and its unusual charge density dependence indicate significant nonthermal noise contribution from the GFET. The polarization resolved detection measurements with different illumination geometries allow for detailed and quantitative analysis of various factors that contribute to the overall detector performance. Our experimental results represent a significant advance toward practically useful graphene THz devices.

Terahertz-wave generation using graphene: Toward new types of terahertz lasers

This paper reviews recent advances in terahertz-wave generation in graphene toward the creation of new types of terahertz lasers. First, fundamental basis of the optoelectronic properties of graphene is introduced. Second, nonequilibrium carrier relaxation and recombination dynamics in optically or electrically pumped graphene are described to introduce a possibility of negative dynamic conductivity in a wide terahertz range. Third, recent theoretical advances toward the creation of current-injection graphene terahertz lasers are described. Fourth, unique terahertz dynamics of the 2-D plasmons in graphene are described. Finally, the advantages of graphene materials and devices for terahertz-wave generation are summarized.

Room temperature performance analysis of bilayer graphene terahertz photodetector

In this paper, the room temperature performance of bilayer graphene terahertz photodetector is studied by calculating of its dark current and dark current limited detectivity at room temperature. In the first step the spectral dependences of the photodetector responsivity as function of the device structural parameters is calculated. It is demonstrated that at the radiation frequency close to the resonant frequencies of plasma oscillations, the responsivity can exhibit sharp and very high maxima corresponding to fairly large responsivity values. In the second step the dark current and dark current limited detectivity for different temperatures, bias voltage, and widths of graphene layer is calculated and analyzed. The obtained results show that for the bilayer graphene photodetector with W=5nm, L=20 μm, Vb=2V, Vg=2V, value of dark current limited detectivity D*, at room temperature T=300°K and also T=200°K are 9×106 cmHz/W and ∼6×109 cmHz/W, respectively.

Graphene-Based Terahertz Waveguide Modulators

We report two proof-of-principle designs of silicon terahertz (THz) modulators based on guiding structures wherein graphene is introduced to actively control the properties of the waveguides, thus tuning their transmissivity. Using well-established models to characterize the electromagnetic parameters of electrically-doped graphene, we estimate the theoretical performance of the proposed structures, in particular the required switching voltage and the achievable modulation depth. These results predict considerably improved performance with respect to the state-of-the-art of graphene THz modulators, together with reduced size and larger THz bandwidth.

Graphene-based optical modulators.

Optical modulators (OMs) are a key device in modern optical systems. Due to its unique optical properties, graphene has been recently utilized in the fabrication of optical modulators, which promise high performance such as broadband response, high modulation speed, and high modulation depth. In this paper, the latest experimental and theoretical demonstrations of graphene optical modulators (GOMs) with different structures and functions are reviewed. Particularly, the principles of electro-optical and all-optical modulators are illustrated. Additionally, the limitation of GOMs and possible methods to improve performance and practicability are discussed. At last, graphene terahertz modulators (GTMs) are introduced.

Large area graphene and graphene oxide patterning and nanographene fabrication by one-step lithography

A cheap and green technology based on electro-erosion, performed at room conditions and scalable to large area (cm2) is demonstrated for structuring graphene and graphene oxide films on any substrate. This one-step technique based on electrical discharges produced by a direct current voltage source competes favorably with laser patterning. The threshold voltage for complete graphene elimination as determined by Raman mapping is around 20V. At low relative humidity conditions (30%) the transformation to graphene oxide is also detected for operating voltages above 40V, the oxidation being probably mediated by the residual adsorbed water at the surface. We also show the close correlation of atomic force microscopy (AFM) phase images with the modified graphene characteristics while AFM topographic images are dominated by extrinsic aspects. The use of a spring probe relaxes the requirement of precision for the tip-sample distance and sample flatness to around 10μm which is very convenient for large scale applications. Furthermore, this technique allows the formation of high quality nanographene, with size around 18nm, and graphene micro-ribbon lattices in a very fast way and very well defined edges, with dimensions down to 1μm width and mm length, very promising for terahertz graphene plasmonic applications.

Graphene terahertz modulators by ionic liquid gating.

Graphene based THz modulators are promising due to the conical band structure and high carrier mobility of graphene. Here, we tune the Fermi level of graphene via electrical gating with the help of ionic liquid to control the THz transmittance. It is found that, in the THz range, both the absorbance and reflectance of the device increase proportionately to the available density of states due to intraband transitions. Compact, stable, and repeatable THz transmittance modulation up to 93% (or 99%) for a single (or stacked) device has been demonstrated in a broad frequency range from 0.1 to 2.5 THz, with an applied voltage of only 3 V at room temperature.

Progress of terahertz devices based on graphene

Graphene has unique electronic properties stemming from a linear gapless carrier energy spectrum, and has dominant advantages in the research of devices such as lasers, detectors and modulators in terahertz region due to its tunable energy gap and extremely high carrier mobility. In this review, we summarize its latest progress in applications of terahertz devices such as lasers, detectors and modulators. Terahertz lasers based on graphene can reach a gain as high as 104 cm-1, and terahertz detectors with different structures such as a bilayer graphene field-effect transistor with top gate and buried gate can achieve NEP (noise equivalent power) ～ m nW/Hz. Graphene terahertz modulators, which are equipped with transmission configuration and reflection configuration, can have a very high modulation depth. These results may be helpful for developing the high-efficiency graphene terahertz devices.

FDTD Formulation for Graphene Modeling Based on Piecewise Linear Recursive Convolution and Thin Material Sheets Techniques

A finite-difference time-domain formulation based on piecewise linear recursive convolution method and on thin material sheets technique is developed for modeling terahertz graphene antennas and some other photonic components. The graphene sheets are modeled by specific recursive equations obtained for tangential electric field components, allowing one to easily apply voltage or current sources between the sheets. The effective conductivity of graphene sheets in Yee's three-dimensional lattice is calculated and used in simulations. A bow-tie-like geometry is investigated, aiming at resonance tuning. The developed numerical formulation is validated by comparison of results to data published in literature.

A Graphene-Based Hybrid Plasmonic Waveguide With Ultra-Deep Subwavelength Confinement

Reduction of propagation loss of terahertz graphene plasmon can be made by increasing the chemical potential of graphene layer, but at the cost of significantly increased modal area, which fundamentally limits the packing density on a chip. By utilizing the strong coupling between the dielectric waveguide and plasmonic modes, we propose hybrid plasmonic terahertz waveguides that not only significantly suppress the mode field confinement, but also maintain a compact modal size. A typical propagation length is 127 μm, and optical field is confined into an ultra-small area of approximately 32.6 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 3 THz. This structure also exhibits ultra-low crosstalk, which shows great promise for constructing various functional devices in future terahertz integrated circuits.

Challenges to create graphene terahertz lasers

The gapless, linear energy spectrum of graphene can lead to population inversion and negative dynamic conductivity in the terahertz range when graphene is optically or electrically pumped, which makes it possible to create a new type of graphene terahertz lasers. This paper reviews recent advances in scientific challenges to create graphene-based terahertz lasers.