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BY-NC-ND 3.0 license Open Access Published by De Gruyter May 25, 2015

Bilayer graphene: physics and application outlook in photonics

  • Hugen Yan EMAIL logo
From the journal Nanophotonics

Abstract

Layered materials, such as graphene, transition metal dichacogenides and black phosphorus have attracted lots of attention recently. They are emerging novel materials in electronics and photonics, with tremendous potential in revolutionizing the traditional electronics and photonics industry. Marrying layered material to the nanophotonics is being proved fruitful. With the recent emphasis and development of metasurfaces in nanophotonics, atomically thin materials can find their unique position and strength in this field. In this article, I will focus on one specific two dimensional material: bilayer graphene. Basic physics will be reviewed, such as band-gap opening, electron-phonon interaction, phonon-plasmon interaction and Fano resonances in the optical response. Moreover, I will review the application of bilayer graphene as a sensitive and fast photodetector. An outlook will be given in the final part of the paper.

1 Introduction

The last decade has witnessed a huge amount of progress in graphene research, ranging from fundamental physics studies [1–3], to synthesis [4–6], electrical [7–10], mechanical [11] and optical characterizations [12–15] and exploration of potential applications in various fields [16–19]. The majority of the efforts are focused on single layer graphene, which is the thinnest material available, primarily due to the linear electronic band structure and the resulting Dirac fermions. Nevertheless, its two-layer counterpart, bilayer graphene, has attracted a significant amount of attention as well. The legendary “scotch tape” method [20] producing single layer graphene comes with interesting by-products: well-ordered bilayer and multilayer graphene [21]. For the bilayer graphene produced by this method, the stacking order is usually Bernal type (AB-stacking, Figure 1A). In this paper, we will exclusively focus on Bernal stacking bilayer graphene.

Figure 1 Bilayer graphene lattice and electronic band structure. (A) Lattice
						structure, γ1 is the interlayer coupling constant. (B)
						Illustration of the band structures for pristine and gated bilayer graphene.
						A band gap can exist for gated bilayer with an electrical field across the
						two layers (B reprinted with permission from [22], Copyright 2009 Nature
						Publishing Group).
Figure 1

Bilayer graphene lattice and electronic band structure. (A) Lattice structure, γ1 is the interlayer coupling constant. (B) Illustration of the band structures for pristine and gated bilayer graphene. A band gap can exist for gated bilayer with an electrical field across the two layers (B reprinted with permission from [22], Copyright 2009 Nature Publishing Group).

Bilayer graphene shares many merits with its single layer counterpart, such as high electrical mobility [7], high mechanical strength [23], flexibility and chemical stability. Notwithstanding, the structure difference between them gives rise to a plethora of different electronic and optical properties. Among the differences, the most notable ones are the capability to open an electronic band gap [24–26, 22, 27, 28] and the infrared activity of the Γ-point optical phonon in bilayer graphene [29, 30]. In this review paper, I will first discuss the band-gap opening in bilayer graphene and the studies of the band structure through infrared spectroscopy. Subsequently, the Fano resonance of the infrared phonon mode due to the interaction with the electronic transition quasi-continuum will be presented. After that, I will talk about the optical properties of bilayer graphene nanoribbons, focusing on the plasmonic effect and phonon-induced transparency. Before the conclusion of the paper with an outlook, dual-gated bilayer graphene as a hot electron bolometer will be reviewed.

2 Band structure and band-gap opening

Due to its unique linear electronic band structure and the existence of massless Dirac fermions, single layer graphene has attracted much attention and opened up a new research field in two dimensional materials [31]. Equally interestingly, bilayer graphene, with a different band structure, has massive chiral Dirac fermions and behaves differently in the quantum Hall effect [32]. Most importantly, bilayer graphene can open a band gap up to a few hundred meV with electrostatic gating or chemical doping [24, 22, 27], which makes bilayer graphene more advantageous in field effect transistors [28], infrared and terahertz light sources, photo-detectors [33] and light modulators.

Achieving a band gap in single layer graphene is particularly challenging, because it is not straightforward to break the A- and B-sublattice symmetry. While in bilayer graphene, the inversion symmetry of the lattice can be broken through the application of an electrical field across the two layers. The tight binding model of the bilayer graphene band structure can predict the band-gap opening effect [24, 26]. Experimentally, the band gap has been inferred through electrical transport measurements [25, 28, 34], probed by angle resolved photo-emission spectroscopy [35] and directly measured by infrared spectroscopy [22, 27, 36, 37]. The band gap derived from transport measurements is much smaller than the theoretical prediction, potentially due to the mid-gap states. The IR spectroscopy measurements give us consistent results with theory. In this paper, I will review the IR studies of bilayer graphene, given that this technique is also closely related to the photonic aspect of this intriguing material.

As shown in Figure1B, pristine bilayer graphene has no band gap, which is analogous to single layer graphene. Gated bilayer graphene with asymmetric charge doping possesses a band gap and the low energy band structure shows a Mexican hat shape, as illustrated in Figure1B. Dual-gate configuration for bilayer graphene enables independent tuning of both the electrical field across the two layers and the total charge carrier density. This configuration was utilized by Zhang et al. [22] (Figure 2A). With an equal amount of opposite charge carriers on both layers, the Fermi level of bilayer graphene is in the middle of the opened band gap. There are direct optical transitions across the band gap. Figure 2B shows the absorption spectra with different electrical displacement fields across the two layers. In the largest field achieved in the experiment, the absorption peak in Figure 2B can reach an energy as high as 250 meV. Figure 2C plots the measured band-gap energy as a function of the electrical displacement fields. It is in good agreement with the self-consistent tight-binding model [26].

Figure 2 Band-gap opening in dual gated bilayer graphene. (A) Sketch of the dual gated
						bilayer graphene device. (B) Infrared absorption spectra for different
						electrical displacement fields. The Fermi level is at charge neutral point.
						The spikes and dips at ∼200 meV are due to the optical phonon of the
						bilayer graphene. (C) Band gap as a function of the electrical displacement
						field. The black solid curve is from the self-consistent tight-binding
						calculation and the red curve is from DFT calculation. The dashed line is
						based on the unscreened tight-binding model (A–C reproduced with
						permission from [22], Copyright 2009 Nature Publishing Group).
Figure 2

Band-gap opening in dual gated bilayer graphene. (A) Sketch of the dual gated bilayer graphene device. (B) Infrared absorption spectra for different electrical displacement fields. The Fermi level is at charge neutral point. The spikes and dips at ∼200 meV are due to the optical phonon of the bilayer graphene. (C) Band gap as a function of the electrical displacement field. The black solid curve is from the self-consistent tight-binding calculation and the red curve is from DFT calculation. The dashed line is based on the unscreened tight-binding model (A–C reproduced with permission from [22], Copyright 2009 Nature Publishing Group).

With only one gate on bilayer graphene, the Fermi level can not stay in the gap. Consequently, it is not possible to observe the direct optical transition originated from the band gap [22]. Nevertheless, the existence of the band gap will affect other optical transitions, such as transitions 1 and 2 shown in Figure 3A. Using electrolyte as a top-gate, Mak et al. independently extracted the band gap in bilayer graphene [27]. Figure 3B plots the band gap as a function of the charge carrier density. With a carrier density n=1.5×1013 cm-2, the band gap approaches 200 meV.

Figure 3 Band-gap opening in top gated bilayer graphene. (A) Illustration of the band
						structure and the dominant two optical transitions which contribute to the
						absorption spectra. In this case, the optical transition direct across the
						opened band gap is not available. (B) Extracted band-gap energy as a
						function of the charge carrier density and top gate voltage. Solid curves
						are theoretical calculations. Red curve ΔEK is the
						calculated energy difference between transitions 1 and 2 shown in (A). Blue
						curve Eg is the computed band gap (A–B reprinted with
						permission from [27], Copyright 2009 American Physical Society).
Figure 3

Band-gap opening in top gated bilayer graphene. (A) Illustration of the band structure and the dominant two optical transitions which contribute to the absorption spectra. In this case, the optical transition direct across the opened band gap is not available. (B) Extracted band-gap energy as a function of the charge carrier density and top gate voltage. Solid curves are theoretical calculations. Red curve ΔEK is the calculated energy difference between transitions 1 and 2 shown in (A). Blue curve Eg is the computed band gap (A–B reprinted with permission from [27], Copyright 2009 American Physical Society).

In bilayer graphene field effect transistor devices, band-gap opening dramatically enhances the current on/off ratio [28], which makes bilayer graphene a potential candidate for logic devices. In terms of the implications in photonics, a band gap in the far and mid-infrared can enable the application of bilayer graphene in terahertz and infrared detection, light emission and modulation. Some of the topics will be revisited later on in the paper.

3 Phonons and Fano resonances

The Γ-point optical phonon with Eu symmetry (Figure 4A) in prinstine bilayer graphene already carries an electrical dipole moment [38]. Though the dipole is very small, it has infrared activity and in principle can be detected through infrared absorption measurements. While for single layer graphene, the corresponding phonon has no electrical dipole at all due to the equivalence of the A- and B-sublattice. It might be surprising that there is a dipole moment for the lattice vibration in bilayer graphene: a material composed of only one kind of atoms. In fact, a subtle in-equivalence of the A1 and B1 (or A2 and B2) atoms shown in Figure 4A implies a dipole moment in the Eu vibration mode. As shown in Figure 4A, the B1 atom is right underneath the A2 atom and the site is termed dimer site. While the A1 atom is underneath the center of a hexagonal of the above layer and the site is non-dimer site. Apparently, dimer and non-dimer sites are inequivalent. Any lattice vibrations with dimer site atoms displacing in one direction and non-dimer site atoms in the opposite direction can carry a macroscopic dipole moment and couple to light. This is the case for the Eu mode shown in Figure 4A. In pristine bilayer graphene, Eg mode (Figure 4A) has no dipole moment. However, with asymmetric doping in upper and lower graphene layers (inversion symmetry breaking), Eg mode can carry a dipole moment and becomes IR-active as well [38, 39].

Figure 4 Tunable Fano resonance in back-gated bilayer graphene. (A) Sketch of the
							Eu phonon and Eg phonon modes. (B) Extracted
						optical conductivity in the phonon region for different gate voltages. (C,
						D) The expanded view of the phonon spectra in the electron and hole doping
						regime (B–D reproduced with permission from [29], Copyright 2009
						American Physical Society).
Figure 4

Tunable Fano resonance in back-gated bilayer graphene. (A) Sketch of the Eu phonon and Eg phonon modes. (B) Extracted optical conductivity in the phonon region for different gate voltages. (C, D) The expanded view of the phonon spectra in the electron and hole doping regime (B–D reproduced with permission from [29], Copyright 2009 American Physical Society).

The infrared activity of the optical phonon in bilayer graphene makes infrared spectroscopy studies of phonons possible, in addition to the widely used Raman spectroscopy [40], which is the sole major spectroscopy technique for single layer graphene phonons. The bilayer graphene phonon infrared spectra depend strongly on the Fermi energy and electrical field across the two layers [29, 30, 38, 39]. Kuzmenko et al. studied the topic for back-gated bilayer graphene [29]. Figure 4B displays the extracted optical conductivity of bilayer graphene from the contribution of the phonon at different gate voltages. It shows both of the lineshape and the intensity of the peak have strong dependence on the gate voltage. At the charge neutral point (Vg=-30V), the phonon peak is barely visible. With increasing doping both in the electron and hole side, the phonon gains intensity and the spectral profile shows a typical asymmetric Fano lineshape [41]. It has been estimated that in pristine bilayer graphene, the Eu mode dipole moment is 3 orders of magnitude smaller than the inferred dipole moment from the typical spectra in Figure 4B. The huge enhancement of the dipole moment is explained by the charged phonon theory [38, 39]. The phonon is dressed by charge carriers in doped bilayer graphene and the dipole moment and infrared activity is amplified. In other words, the oscillator strength is transferred from the electronic transition to the phonon response. Meanwhile, the optical transition through phonon excitation can interfere with a much broader optical absorption from electronic transitions and form a Fano resonance system [41]. This can explain the observed Fano lineshape in Figure 4B.

Tang et al. performed similar measurements for dual-gated bilayer graphene, in which both the band gap and carrier density can be independently tuned [30]. In a particular case, the net carrier density was tuned to zero and the band gap was systematically varied. Figure 5A plots the absorption spectrum in the phonon frequency regime, the sharp phonon feature interferes with a broad spectrum from the band-gap excitonic absorption. As a consequence of the Fano interference, the spectrum is not a simple superposition of a phonon absorption peak and a excitonic peak, but rather a sharp transparency window within a broad excitonic absorption profile. Figure 5B shows the extracted phonon spectra for the cases with different band gaps, ranging from 130 meV to 245 meV. Both the phonon amplitude and lineshape change. Figure 5C plots the phonon amplitude as a function of the band gap. The amplitude reaches maximum when the band-gap energy coincides with the phonon energy, because in this case, the coupling between the phonon mode and the exciton mode reaches maximum and the phonon gains largest oscillator strength. Figure 5D shows the Fano parameter q as a function of the band gap. Tang et al. suggest that q is proportional to the ratio of the phonon dipole moment to the electronic transition dipole moment. When the band gap increases, the phonon dipole moment increases monotonically while the electronic transition dipole reaches maximum when the band-gap energy equals the phonon energy. As a consequence, ∣q∣ increases dramatically when the band gap is above the phonon energy [30].

Figure 5 Tunable Fano resonance in dual-gated bilayer graphene. (A) Absorption
						spectrum in the phonon region with zero doping and a band gap of 190 meV.
						The red dashed curve is the band-gap excitonic absorption profile. (B)
						Phonon spectra for various gapped bilayer graphene. The band gap ranges from
						130 meV to 245 meV. The net carrier density is kept at zero. The red curves
						are fits based on Fano model. (C) The phonon amplitude as a function of the
						band gap. (D) The Fano parameter q as a function of the
						band gap (A–D adapted with permission from [30], Copyright 2010
						Nature Publishing Group).
Figure 5

Tunable Fano resonance in dual-gated bilayer graphene. (A) Absorption spectrum in the phonon region with zero doping and a band gap of 190 meV. The red dashed curve is the band-gap excitonic absorption profile. (B) Phonon spectra for various gapped bilayer graphene. The band gap ranges from 130 meV to 245 meV. The net carrier density is kept at zero. The red curves are fits based on Fano model. (C) The phonon amplitude as a function of the band gap. (D) The Fano parameter q as a function of the band gap (A–D adapted with permission from [30], Copyright 2010 Nature Publishing Group).

An infrared active phonon in bilayer graphene makes it a particularly interesting material in mid-infrared photonics and optoelectronics. Tang et al. suggest possible applications in phonon lasers [30], which is an intriguing concept. With the bilayer band gap equal to the phonon energy, it might be possible to achieve phonon amplification through the population-inverted band-gap electrons.

4 Tunable phonon-induced transparency and slow light

Plasmons in graphene have attracted lots of attention due to their tunability, relatively low loss and strong energy confinement [17, 18, 42–64]. The plasmon frequency in graphene ranges from terahertz to the mid-infrared and covers a broad frequency range of technical importance. Multiple techniques are utilized to study the plasmons in graphene, such as electron energy loss spectroscopy [42, 43], near field nanoscopy [49] and imaging [50, 51], and infrared absorption spectroscopy of micro- and nano-structured graphene [17, 18, 47, 53, 54, 57, 61, 64]. While most of the efforts have been devoted to plasmons in single layer graphene, bilayer graphene is an equally (if not more) appealing material in this field.

As discussed in the last section, the infrared active phonon in bilayer graphene can interfere with the quasi-continuum electronic transitions and form Fano resonance systems. With the excitation of plasmons in nano-structured bilayer graphene, the phonon can strongly interfere with the plasmon and show phonon-induced transparency (PIT) in the absorption spectra when the plasmon frequency coincides with the phonon frequency. Moreover, since the plasmon frequency depends on many factors such as doping [18, 47, 56] and the size of structure [47, 56], the phonon-induced transparency is widely tunable.

PIT is a phenomenon in which a sharp transparency window within a broad absorption profile is created by phonons. This is a classical analogue of the renowned electromagnetically induced transparency (EIT) in quantum mechanical atomic systems [65]. Other classical analogues of EIT have been demonstrated in various systems, including opto-mechanical systems [66], coupled optical resonators [67] and plasmonic systems [68–70]. The latter is termed plasmon-induced transparency and has been widely studied in metal plasmonic systems. The plasmon-induced transparency typically involves two plasmon modes with contrasting lifetimes [68, 69]. While in the phonon-induced transparency demonstrated by Yan et al. in bilayer graphene, the interference originates from a phonon mode and a plasmon mode [56].

To demonstrate the tunable PIT effect, Yan et al. patterned bilayer graphene on SiO2/Si substrate into nanoribbon array (Figure 6B) and electrodes for back-gating were also fabricated (Figure 6A). With the nanoribbon width in the order of ∼100 nm, the plasmon frequency is in the mid-infrared. Figure 7A shows the absorption spectra of a bilayer graphene nanoribbon (width W=130 nm) array with light polarization parallel and perpendicular to the ribbons. Without plasmon excitation (parallel polarization), only a phonon absorption peak is observed in a broad featureless spectrum. However, with the excitation of the plasmon (perpendicular polarization), multiple peaks can be observed and the phonon feature amplitude is largely enhanced. The lineshape of the phonon shows strong asymmetry and can be fitted with a Fano resonance model (inset of Figure 7A). As seen from the spectrum for parallel polarization, the phonon peak has very little asymmetry without excitation of plasmons for those bilayer samples studied by Yan et al. The excitation of plasmon with a frequency nearby strongly affects the phonon behavior and the phonon-plasmon coupling dominates the phonon appearance. The two broader peaks(not the phonon peaks) shown in the spectrum of Figure 7A for the perpendicular polarization are the plasmon modes from bilayer graphene. More precisely, they are plasmon-phonon-polariton modes which are resulted from the hybridization of the graphene plasmon mode with the surface polar phonon modes of the SiO2 substrate [47, 52, 53, 61].

Figure 6 Experimental basics for the tunable phonon-induced transparency in bilayer
						graphene. (A) Illustration for the gated bilayer graphene nanoribbon device
						and the infrared measurement. (B) A typical SEM image for a nanoribbon array
						(A–B adapted with permission from [56], Copyright 2014 American
						Chemical Society).
Figure 6

Experimental basics for the tunable phonon-induced transparency in bilayer graphene. (A) Illustration for the gated bilayer graphene nanoribbon device and the infrared measurement. (B) A typical SEM image for a nanoribbon array (A–B adapted with permission from [56], Copyright 2014 American Chemical Society).

Figure 7 Phonon-plasmon Fano resonance system. (A) The infrared spectra of a
						nanoribbon array with ribbon width 130 nm for parallel and perpendicular
						polarizations. Inset shows the extracted phonon spectrum and a Fano fitting.
						(B) The typical spectrum for phonon-induced transparency (gray dots). The
						gray solid curve is a fitting based on the coupled oscillator model
						(A–B adapted with permission from [56], Copyright 2014 American
						Chemical Society).
Figure 7

Phonon-plasmon Fano resonance system. (A) The infrared spectra of a nanoribbon array with ribbon width 130 nm for parallel and perpendicular polarizations. Inset shows the extracted phonon spectrum and a Fano fitting. (B) The typical spectrum for phonon-induced transparency (gray dots). The gray solid curve is a fitting based on the coupled oscillator model (A–B adapted with permission from [56], Copyright 2014 American Chemical Society).

With a proper width of the bilayer graphene ribbon, the plasmon frequency can coincide or in the close vicinity of the phonon frequency and a PIT can be observed. Figure 7B shows the absorption spectra, with an emphasis on the phonon spectrum region. The spectrum for the perpendicular polarization shows a sharp transparency window in a broader absorption profile. This is the typical spectrum for the EIT and EIT-like phenomena. The spectrum can be fit (solid curve) by the coupled-oscillator model [71], which is a popular phenomenological model for describing EIT-like phenomena.

Compared to the plasmon-induced transparency, the most closely related EIT-like phenomenon, PIT has some advantages. First, the lifetime of phonons in bilayer graphene [72, 73] is at least one order of magnitude longer than that of the plasmon mode, which easily satisfies the requirement of a contrasting lifetime for the two modes. While for the typical plasmon-induced transparency, the dark mode lifetime is not so different from that of the bright mode [69], which makes the plasmon-induced transparency not as sharp. Second, bilayer graphene has the inherent phonon mode which naturally participates in the PIT effect. While in plasmon-induced transparency, one has to carefully engineer the metal structure to have both the dark mode and bright mode. Third, as will be shown subsequently, the PIT is widely tunable in bilayer graphene. The tunability in plasmon-induced transparency is usually lacking and yet very desirable.

There are multiple means to tune the PIT in bilayer graphene. The underlying principle is the same: tuning the plasmon resonance frequency to or away from the phonon frequency. Yan et al. have demonstrated three methods to tune PIT, including both passive and active tuning. Figure 8A shows the spectra for various ribbon widths. The plasmon frequency increases from below phonon frequency to above with decreasing ribbon width. The phonon absorption shows a typical Fano lineshape in the wide ribbons and gradually evolves into an absorption dip when the plasmon frequency coincides with the phonon frequency. When the plasmon frequency goes above the phonon frequency, the phonon feature again becomes very asymmetric but with an opposite skewing. Similar tuning can be achieved through chemical doping, as shown in Figure 8B. Most importantly, the PIT can be actively tuned through electrostatic gating, as demonstrated in Figure 8C. This opens an avenue for active bilayer graphene opto-electronic devices and reconfigurable metamaterials and metasurfaces.

Figure 8 Tunable phonon-induced transparency through (A) ribbon width control, (B)
						chemical doping, and (C) electrostatic gating (A–C adapted with
						permission from [56], Copyright 2014 American Chemical Society).
Figure 8

Tunable phonon-induced transparency through (A) ribbon width control, (B) chemical doping, and (C) electrostatic gating (A–C adapted with permission from [56], Copyright 2014 American Chemical Society).

The most stunning consequence of EIT is the slow light effect [74]. EIT can drastically modify the medium dispersion characteristics due to the fast-change of the index of refraction in a narrow frequency region. The group velocity of light in a dispersive media depends strongly on the dispersion of the index of refraction. The larger the dispersion, the smaller the group velocity. Based on the experimentally obtained parameters for the bilayer grpahene nanoribbons, Yan et al. calculated the group index as a function of the frequency and the plasmon wavevector q (directly related to the ribbon width). Figure 9 shows the computed result. A maximal group index as large as 500 can be reached. To observe the predicted slow-light effect, vertically stacked multiple bilayer graphene might be needed [17]. Alternatively, bilayer graphene on waveguide [75] has longer light-matter interaction length and provides a better chance to show the light delaying effect.

Figure 9 Slow light effect in bilayer graphene plasmonic devices. The figure shows the
						2-dimensional map of the light group index as a function of frequency and
						plasmon wavevector (adapted with permission from [56], Copyright 2014
						American Chemical Society).
Figure 9

Slow light effect in bilayer graphene plasmonic devices. The figure shows the 2-dimensional map of the light group index as a function of frequency and plasmon wavevector (adapted with permission from [56], Copyright 2014 American Chemical Society).

5 Application: hot electron bolometer

Single layer graphene has been exploited for photo-detection due to its fast speed and broad-band operation [76–83]. In addition to the merits of single layer graphene, bilayer graphene, with its ability to open up a band gap, has huge potential in infrared and terahertz radiation detection. The detection mechanism, as argued by Yan et al., is mostly a bolometric effect for the gapped bilayer graphene [33]. Two essential ingredients govern the bolometric response: the first one is the weak coupling of electrons and phonons which gives the opportunity of excited electrons to reach thermal equilibrium within the electronic system. The second ingredient is the electronic temperature dependent resistance of the gapped graphene. With the absorption of a photon by a bilayer graphene device, an energetic electron-hole pair will be generated. Because of strong electron-electron interaction and weak electron-phonon coupling, the hot electron-hole pair gives the excessive energy to other electrons and holes and quickly reaches thermal equilibrium among themselves [84], with a temperature much higher than the graphene lattice. These energetic carriers are usually termed hot electrons. The resistance of the device depends on the temperature of hot electrons. Therefore, the effect of the absorption of a photon will increase the resistance and the device can be named as hot electron bolometer. Figure 10A shows the device configuration used by Yan et al. The bolometric response is presented in Figure 10B. Both the photo-induced change in the voltage (ΔV) and the resistance (ΔR) as a function of the DC current are plotted. The decrease of ΔR with increasing DC current indicates that the heat dissipation due to the DC current can diminish the hot electron effect created by the photo-excitation.

Figure 10 Dual-gated bilayer graphene hot electron bolometer. (A) Sketch of the dual
						gated bilayer graphene hot electron bolometer device. (B) Photoresponse of
						the device. The device is at charge neutral point and the displacement field
						is 0.45 V/nm. Blue squares are the changes in the voltage and red dots are
						the photon-induced resistance change and x-axis is the DC
						current passing through the device (A–B reprinted with permission
						from [33], Copyright 2012 Nature Publishing Group).
Figure 10

Dual-gated bilayer graphene hot electron bolometer. (A) Sketch of the dual gated bilayer graphene hot electron bolometer device. (B) Photoresponse of the device. The device is at charge neutral point and the displacement field is 0.45 V/nm. Blue squares are the changes in the voltage and red dots are the photon-induced resistance change and x-axis is the DC current passing through the device (A–B reprinted with permission from [33], Copyright 2012 Nature Publishing Group).

From a technical point of view, bilayer graphene hot electron bolometer outperforms commercial silicon bolometers or even superconducting transition-edge sensors in two aspects. First, the noise equivalent power (33 fW Hz-1/2 at 5 K) is several times lower than the commercial counterparts. Second, the intrinsic speed (>1 GHz at 10 K) is 3–5 orders of magnitude larger than that of silicon bolometers or superconducting transition-edge sensors at similar temperatures. The responsivity of the bilayer graphene bolometer is good too. An estimation of 2×105 V/W is obtained. For comparison, silicon bolometer has responsivities in the range of 1×104 V/W to 1×107 V/W. These defining parameters for bilayer graphene bolometer indicate its competitiveness in the bolometer technologies.

6 Summary and Outlook

In this review article, I focus mainly on the Bernal stacking bilayer graphene, with topics including the band-gap opening, phonon Fano resonances, phonon-induced transparency and hot electron bolometer. Many other interesting topics are not covered, such as coherent phonons [85], Kohn anomalies in the optical phonons [86–88] and magneto-optics [89]. Moreover, I do not discuss the twisted bilayer graphene[90], which has many additional van Hove singularities in the optical joint density of states. Bilayer graphene is the simplest multilayer graphene. Many of the properties are shared with other multilayer graphene, such as band-gap opening [91], Fano resonances [92, 93] and coherent phonons [85]. More specifically, dual-gated ABC stacking trilayer graphene can have a band gap and it has tunable Fano resonance for the phonon mode in the infrared spectra [91, 93]. Generally speaking, ABC-stacking multilayers possess very similar electronic properties as AB stacking bilayer graphene, while ABA stacking multilayers do not [21, 94].

The effort to exploit bilayer graphene as a photo-detector has achieved some success. With a tunable band gap, bilayer graphene is potentially a very good candidate for tunable infrared and terahertz sources. The optical emission from the direct band-gap exciton recombination makes bilayer graphene a compact light emit diode. Nevertheless, there is no experimental demonstration yet and we believe this area will attract more attention in the near future.

The real application of bilayer graphene in photonics and optoelectronics requires large area high quality Bernal stacking bilayer graphene samples. Most of the experiments performed for bilayer graphene are for exfoliated samples obtained from bulk graphite. The size and the scale of the samples are very limited. Chemical vapor deposition method has successfully produced large area single layer graphene [16]. More recently, many groups have also demonstrated the capability to grow Bernal stacking bilayer graphene and band gap has been achieved in those CVD samples [95–97]. However, the stacking order seems not as good as those obtained from mechanical exfoliated samples. One evidence for that is the lacking of the IR active phonons in the IR spectrum. This will limit some of the unique applications of bilayer graphene. With a very dynamic progress in the CVD graphene research field, we believe that in the near future, Bernal stacking bilayer and multilayer graphene with quality as good as exfoliated counterparts will be ready and the exploration of the samples as a novel nano-photonics material will attract more players across various fields.


Corresponding author: Hugen Yan, IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, USA, e-mail:

Acknowledgments

The author is grateful to Drs. Phaedon Avouris, Tony Heinz and Fengnian Xia for their advice and insightful comments.

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Received: 2014-9-5
Accepted: 2014-10-20
Published Online: 2015-5-25

© 2015 Hugen Yan

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

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