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

Spatial dispersion effect of aligned carbon nanotubes (CNTs) in the terahertz (THz) region has significance for both theoretical and applied consideration due to the unique intrinsically anisotropic physical properties of CNTs. Herein, we report the angular dependent reflection of p-polarized THz wave from vertically aligned multi-walled CNT arrays in both experiment and theory. The spectra indicate that the reflection depends on the film thickness of vertically aligned CNTs, the incident angle, and the frequency. The calculation model is based on the spatial dispersion effect of aligned CNTs and performed with effective impedance method and the Maxwell-Garnett approximation. The results fit well with the experiment when the thickness of CNT film is thin, which reveals a coherent superposition mechanism of the CNT surface reflection and CNTs/Si interface reflection. For thick CNT films, the CNTs/Si interface response determines the reflection at small incident angles, while the CNTs surface effect dominates at large incident angles. This work investigates the spatial dispersion effect of vertically aligned CNT arrays in the THz region, and paves a way for potential anisotropic THz applications based on CNTs with oblique incidence requirements.

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“…A custom-designed angle-dependent THz-TDS39 is used to perform the reflection measurement. THz experimental geometry is shown in Fig.…”

Section: Methodsmentioning

confidence: 99%

Angular dependent anisotropic terahertz response of vertically aligned multi-walled carbon nanotube arrays with spatial dispersion

Zhou

Yiwen

et al. 2016

Sci Rep

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“…[ 25 ] Such combinations have opened new avenues for fundamental studies and the designs of solid‐state device with varying functionalities. [ 26–28 ] Besides, a potential THz antireflection device based on graphene was achieved by stacking different layers based on the impedance matching effect, [ 38 ] the results suggested the reflectance of THz wave is sensitive to the sheet conductivity of interface and broadband zero reflection could be obtained at a particular incident angle for graphene with different layers. The VO 2 film can provide a conductivity switching of 3–5 orders of magnitude by thermally induced phase transition, and highly reflect THz wave with broad operation bandwidth in metallic phase.…”

Section: Figurementioning

confidence: 99%

“…[ 31,32,42 ] As shown in Figure 2c, the impedance matching state (with VO 2 /3l graphene) could realize a near zero reflection amplitude at the graphene/VO 2 interface. This phenomenon can be elucidated using the Fresnel equations, the relative amplitude reflection of THz wave from the graphene/VO 2 interface can be simplified as [ 38–40 ]

\begin{matrix} r_{gra / {vo}_{2} - ref} = \frac{\begin{matrix} {(\cos θ_{in} n_{Sub})}^{2} - {(\cos θ_{r} n_{Air})}^{2} - \\ (\cos θ_{in} n_{Sub} + \cos θ_{r} n_{Air}) \cos θ_{in} \cos θ_{r} Z_{0} σ \end{matrix}}{\begin{matrix} {(\cos θ_{in} n_{Sub})}^{2} - {(\cos θ_{r} n_{Air})}^{2} + \\ (\cos θ_{in} n_{Sub} - \cos θ_{r} n_{Air}) \cos θ_{in} \cos θ_{r} Z_{0} σ \end{matrix}} \end{matrix}

…”

Section: Figurementioning

confidence: 99%

A Phase Transition Oxide/Graphene Interface for Incident‐Angle‐Agile, Ultrabroadband, and Deep THz Modulation

Zhu

et al. 2020

Adv Materials Inter

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Although plenty of functional materials have been explored to dynamically control the terahertz (THz) transmission, the limits in modulation depth, operation bandwidth, or angle‐agile characteristics hinder their applications in highly active THz devices. Here, a 2D/3D (graphene/phase transition oxide VO2) interface is demonstrated for unprecedented high‐efficiency modulation of THz waves with incident‐angle‐agile and ultrabroadband. An impedance matching state can be met at the graphene/VO2 interface by adjusting the stacked graphene layers, which leads to a minimal terahertz reflection amplitude. Then the interfacial impedance can be changed continuously via the semiconductor–metal phase transition of VO2 and achieve a THz switching. Based on this mechanism, this heterostructure demonstrates giant THz average amplitude modulation of 93%, 94%, and 96% in 0.3–1.5 THz at the incident angle of 35°, 48°, and 53°, respectively. Moreover, the corresponding insertion loss is only −7.4, −4.1, and −6.5, which is much lower than most of the present THz modulation. These coupling phenomena and functions arising from the integration of 2D material and 3D phase transition oxide film broaden the previous materials system for dynamic controlling of THz wave, and would promote advanced THz smart devices for practical applications.

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“…However, in order to Δ get high bolometric sensitivities, the very weak R( T e )/ T e of intrinsic graphene had to be Δ Δ artificially increased by introducing disorder [17] , patterning nanostructures [18] , or opening a band gap in bilayer graphene [14] . The fundamental challenge in this approach is set by the enormous impedance mismatch between these high impedance devices, with both -free space [19,20] and the readout circuit, which consequently results in very low light coupling efficiencies and readout bandwidths, accordingly. To overcome this issue and achieve all at the same time -high bolometric sensitivity, high readout bandwidth and efficient light absorption, it is necessary to introduce a T e readout scheme that works for high-quality, low impedance graphene and to couple the bolometer to resonant light structures.…”

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confidence: 99%

High-speed bolometry based on Johnson noise detection of hot electrons in cavity-coupled graphene (Conference Presentation)

Efetov

Englund

Hone

et al. 2019

2D Photonic Materials and Devices II

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Since the invention of the bolometer, its main design principles relied on efficient light absorption into a low-heat-capacity material and its exceptional thermal isolation from the environment. While the reduced thermal coupling to its surroundings allows for an enhanced thermal response, it in turn strongly reduces the thermal time constant and dramatically lowers the detector's bandwidth. With its unique combination of a record small electronic heat capacity and a weak electron-phonon coupling, graphene has emerged as an extreme bolometric medium that allows for both, high sensitivity and high bandwidths. Here, we introduce a hot-electron bolometer based on a novel Johnson noise readout of the electron gas in graphene, which is critically coupled to incident radiation through a photonic nanocavity. This proof-of-concept operates in the telecom spectrum, achieves an enhanced bolometric response at charge neutrality with a noise equivalent power NEP < 5pW/ √Hz, a thermal relaxation time of < 34ps, an improved light absorption by a factor~3, and an operation temperature τ up to T=300K. High sensitivity and fast response are the most important metrics for infrared sensing and imaging and together form the primary tradeoff space in bolometry [1]. To simultaneously improve both characteristics requires a paradigm shift on the thermal properties of bolometric materials. Due to a vanishingly small density of states (DOS) at the charge neutrality point (CNP) [2,3] , graphene has a record-low electronic heat capacity which can reach values approaching one Boltzmann constant C e ~k b [4-6]. In addition, its small Fermi surface and the high energy of its phonons result in an extremely weak electron-phonon (e-ph) heat exchange [7-9] ., The combination will allow a strong thermal isolation of the electrons in graphene for higher sensitivity without sacrificing the detector response time. These unique thermal properties and its broadband photon absorption [10] , make graphene a promising platform for ultrasensitive and ultra-fast hot electron bolometers, calorimeters and single photon detectors for low energy light [11-13]. While there already have been device concepts demonstrating bolometric response of graphene [14-16] , many challenges remain on the way to practical applications, like the very low light absorption of only 2.3% [10] and low measurement bandwidths. Previous work

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

Cited by 51 publications

References 34 publications

Angular dependent anisotropic terahertz response of vertically aligned multi-walled carbon nanotube arrays with spatial dispersion

Angular dependent anisotropic terahertz response of vertically aligned multi-walled carbon nanotube arrays with spatial dispersion

A Phase Transition Oxide/Graphene Interface for Incident‐Angle‐Agile, Ultrabroadband, and Deep THz Modulation

High-speed bolometry based on Johnson noise detection of hot electrons in cavity-coupled graphene (Conference Presentation)

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