Infrared Nanophotonics Based on Graphene Plasmonics - ACS

Review pubs.acs.org/journal/apchd5

Infrared Nanophotonics Based on Graphene Plasmonics Qiushi Guo,† Cheng Li,† Bingchen Deng,† Shaofan Yuan,† Francisco Guinea,‡,§ and Fengnian Xia*,† †

Department of Electrical Engineering, Yale University, New Haven, Connecticut 06511, United States IMDEA Nanociencia, Calle de Faraday 9, E-28049 Madrid, Spain § Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom ‡

ABSTRACT: Graphene plasmonics has recently attracted remarkable attention, with reports of numerous intriguing properties and novel device demonstrations. As a twodimensional (2-D) semimetal with ultrahigh carrier mobility, graphene can host plasmon waves that exhibit extremely tight spatial confinement, exceptionally long plasmon lifetime, and an electrostatically tunable response in the mid-infrared (midIR) and terahertz (THz). These properties render graphene a viable plasmonic material for achieving novel functionalities in various mid-IR to THz photonic systems. From the device perspective, we review the key distinguishing features of graphene plasmons and highlight the latest developments, such as midIR and THz tunable infrared sources, modulators, and photodetectors. Finally, we discuss future challenges and new opportunities for graphene plasmonics in infrared photonic systems. KEYWORDS: graphene, plasmons, mid-infrared photonics, terahertz photonics, modulators, photodetectors, spectroscopy

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supporting the highly confined surface plasmon waves (as illustrated in Figure 1b). However, at lower frequencies, such as mid-IR and THz, metals behave more like good conductors, so that the electric field penetration and high spatial confinement vanishes.17,20−22 Recall that for any 3- or 2-D conductor, the electromagnetic perturbation of the equilibrium electrons in a conductor results in a Coulomb restoring force, which in turn drives charge and field oscillations simultaneously. The key factor that accounts for fundamental properties of plasmonic waves is the kinetic energy of the collective oscillating electrons.23 Interestingly, the reduction of a conductor’s thickness from 3-D to 2-D will drastically increase the kinetic energy. This is because when the metal film thickness is smaller than the penetration depth, the electric fields generated by plasmon waves at the top and bottom surfaces are coupled17,23,24 (as illustrated in Figure 1c). Consequently, the plasmon waves in a 2-D conductor (e.g., graphene) have a dispersion relation that departs further from the light line compared to their 3-D counterpart; this is depicted by the red solid line in Figure 1a. Remarkably, these properties enable 2-D plasmons to achieve a significantly lower group velocity at midIR to GHz frequencies and, thus, a much greater subwavelength confinement than 3-D surface plasmons in this wavelength range. As an atomically thin 2-D conductor, doped graphene is known to host extremely confined plasmons (∼106 smaller than the diffraction limit25), thus, enabling stronger light− matter interactions at mid-IR and THz frequencies.26−28 Its

lectromagnetic waves in the mid-infrared and terahertz wavelength ranges are of great utility for numerous applications, including chemical bond spectroscopy,1 imaging,2 free space communications,3 trace gas sensing,4 medical diagnostics,5 and homeland security,6 to name a few. Despite applications being well-known for decades, developments in mid-IR technologies have been largely hindered by the limited availability of mid-IR active materials and the challenges associated with the weak interactions between the long wavelength electromagnetic wave and matter. Recently, the research in mid-IR photonics and systems has been spurred by the advent of the quantum cascade lasers (QCLs),7,8 which are compact, wavelength flexible, and now commercially available. Consequently, there is now growing interest in miniaturizing and improving bulky and costly optical components such as detectors, spectrometers, and modulators in mid-IR photonics.9−16 To this end, researchers are actively seeking novel materials, enabling physical mechanisms and device architectures that can achieve enhanced light−matter interaction in mid-IR and THz. One approach to enhancing the light−matter interaction is by exciting the surface plasmons in metals, which can give rise to very strong optical near fields concentrated at a subwavelength dimension.17−19 In 3-D noble metals such as gold or silver, such high field confinement can occur at frequencies (usually visible and near-IR) close to the bulk plasma frequency of the metal. The dispersion relation of the surface plasmon at the interface between the dielectric and a typical 3-D metal such as gold or silver is illustrated by a gray solid line in Figure 1a. At these frequencies, the electric field can penetrate into the metal such that the nonzero component of the electric field parallel to the surface can establish an oscillating spatial charge distribution, which is essential for © XXXX American Chemical Society

Special Issue: 2D Materials for Nanophotonics Received: May 30, 2017 Published: August 8, 2017 A

DOI: 10.1021/acsphotonics.7b00547 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. (a) Sketch of dispersion curves for light wave, surface plasmonic wave at the interface between 3-D noble metal and air and graphene plasmonic wave. In the figure, meff is the effective mass of electron in 3-D metal, ne is the volume density of electrons in 3-D metal, ε0 is the vacuum permittivity, εr is the average dielectric constant of the surrounding medium, |EF| is the Fermi level of graphene, ℏ is the reduced Planck constant, and c is the speed of light. Illustration of the charge, electric field (arrows) associated with (b) surface plasmonic wave on 3-D bulk metal and (c) with 2D plasmonic wave in graphene.

Figure 2. (a) Top: Schematic of graphene nanodisk arrays with electrolyte gating. The electrolyte (ion gel) was spin-coated on top of the graphene nanostructures and a gold top gate contact was subsequently introduced. Bottom: measured transmission and absorption spectra for graphene nanodisk arrays when Fermi level is tuned from 0.2 to 0.8 eV. (b) Top: Schematic of microdisk array fabricated on graphene/insulator stack structure. Bottom: Transmission extinction spectra of stacked graphene plasmonic devices with different layer numbers. Inset: illustration of dipole− dipole coupling in two closely spaced graphene disks. (c) Top: Schematic of graphene plasmonic resonators integrated with a λ/4 cavity. The inset illustrates the device supports a standing wave at the cavity resonance frequency. The electrical field maximum is designed to overlap with the graphene. The dielectric spacer has a thickness d ∼ λ/4n, where n is the permittivity of the dielectric. Bottom: Absorption spectra of graphene resonators with various widths. The dashed curve represents the simulated parallel electric field at the dielectric surface without graphene. Panel a is reprinted with permission from ref 42. Copyright 2013 American Chemical Society. Panel b is reprinted with permission from ref 43. Copyright 2012 Nature Publishing. Panel c is reprinted with permission from ref 44. Copyright 2014 American Physical Society.

high mobility translates to a small plasmon damping rate, which leads to a longer plasmon propagation length and plasmon lifetime (∼500 fs for exfoliated graphene on hexagonal boron nitride (hBN) substrate29), even at room temperature. Unlike

3-D bulk metals in which the high density of free electrons can almost completely screen the external electric field, graphene is a semimetal with a small density of states. Free electrons or holes can be induced through chemical doping or electrical B


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gating effectively due to its 2-D nature. Consequently, the optical response of graphene plasmonic devices can be dynamically controlled.26,30,31 This Review is devoted to the emerging, but rapidly developing, field of graphene plasmonics, with a focus on its applications in IR photonic devices and systems.

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In addition to highly doping SLG, distributing carriers into multiple graphene layers can effectively enhance the plasmonic resonance frequency and magnitude due to larger cumulative optical conductivity. In 2012, Yan et al. reported graphene plasmonic extinction ∼50% at THz frequencies in a stacked device with five graphene layers,43 as shown in Figure 2b. THz filters and polarizers have also been demonstrated using such graphene stacks. Deng et al. and Rodrig et al. recently showed that double layer graphene stacking structures exhibit superior extinction in mid-IR (∼10%) compared with SLG.50,51 Moreover, electrostatic spectral tuning of plasmonic resonances in double layer graphene was found to be more efficient than SLG, primarily due to redistribution of carriers over different layers.50 It is interesting to explore if such wider spectral tuning range still exists in stacks with more than two graphene layers, as the free carrier screening of the external electric field can be more pronounced. Besides the stronger plasmonic resonances, significant upshift of the plasmonic resonance frequency has also been observed in graphene stacks.43,50,51 This is a direct consequence of strong dipole−dipole interactions among graphene layers, which lead to a stronger restoring force and higher plasmonic resonance frequency. As an important implication for a specific frequency range of interest, one can fabricate graphene plasmonic structures with much larger feature sizes with graphene stacks. This is particularly important in increasing the plasmon lifetime since the edge scattering can be mitigated in larger structures.39 With further increased feature size of graphene structures and stacked layer thickness, the graphene plasmons can enter the retardation regime where the quasi-electrostatic treatment of the light−matter interaction fails.52,53 In this regime, ultralow damping and strong radiative coupling of graphene plasmons can be achieved.54,55 One can also achieve stronger graphene−light interaction by integrating graphene plasmonic structures with other photonic structures, such as a Fabry−Perot cavity comprised of an opaque reflector and a lossless dielectric spacer.44,56−59 The underlying mechanism is to overcome the inefficient coupling between free-space photons and highly confined graphene plasmons through multiple reflections. In 2014, Jang et al. demonstrated 24.5% total SLG plasmonic absorption of incoming mid-IR radiation by placing graphene a quarter wavelength away from a gold back reflector (Figure 2c).44 Specifically, at the cavity resonance frequencies, the gold reflector creates a standing wave between the incident and reflected light that maximizes the electric field overlapping with graphene (Figure 2c, inset). Once the graphene plasmonic frequency is tuned to coincide with such frequencies, a drastic increase of graphene plasmonic absorption can occur. Such an effect can be further augmented using aforementioned graphene stacks as plasmonic resonators or focusing IR radiation to the graphene plasmonic resonators using subwavelength metallic slit arrays. Using the latter approach, Kim et al. recently achieved near unity (96.9%) plasmonics absorption in graphene at 1389 cm−1 (∼7.2 μm). With the strong light−graphene interaction, such a device also allows for exceptional modulation efficiency of 95.9% in reflection.60

ENHANCING GRAPHENE PLASMONIC RESPONSE

Despite the exciting and attractive features of graphene as an active plasmonic material for mid-IR and THz photonics, a major hurdle in utilizing graphene plasmonics for real device applications is the relatively weak plasmonic response due to graphene’s atomically thin nature, which results in unsatisfactory device efficiency. In the past decade, tremendous advances have already been made in synthesizing large-scale graphene on various substrates through chemical vapor deposition (CVD),32 thus, paving the way for the scalable fabrication of graphene plasmonic devices. However, the stateof-the-art CVD single-layer graphene (SLG) still exhibits relatively weak plasmonic resonances. There are probably three reasons for this. First, induced impurities, defects, and the grain boundaries during the material growth and transfer result in much lower carrier mobility in large scale polycrystalline CVD graphene (typically ranging from 100 cm2 V−1 s−1 to 4000 cm2 V−1 s−1)32,33 compared to mechanically exfoliated graphene whose mobility can be mainly limited by electron− phonon scattering.34−38 Therefore, the plasmon lifetime in CVD graphene is usually short (∼tens of femtoseconds).39 Second, a high and stable chemical doping of graphene without sacrificing graphene’s mobility remains challenging. Third, the extreme optical confinement comes at a price, as it makes it challenging to couple the external light to the plasmonic waves due to the large momentum mismatch. As a result, typical lightto-plasmon coupling cross sections can barely exceed the projected geometrical size of the features involved.40,41 In pursuit of stronger graphene plasmonic response and light− matter interactions, common experimental schemes involve increasing graphene’s doping concentration, stacking graphene multilayers, and utilizing external photonic structures, which are summarized in Figure 2. Traditional electrostatic gating of graphene can only induce a modest carrier density change on the order of 1012 cm−2, which is generally limited by the small capacitance as well as the breakdown voltage of the dielectric. In contrast to dielectricbased gating, electrolyte gating can concentrate excess charges directly on the graphene surface. Typically, a high capacitance of C = 3.2 μF cm−2 (250× higher than that of 300 nm thick silicon dioxide) and carrier density change up to Δns = 1014 cm−2 can be reached upon application of a small gate voltage.45−49 Using the electrolyte gating, Fang et al. experimentally realized the significantly higher graphene (hole) doping up to 0.8 eV shift in Fermi-level from Dirac point,42,47 corresponding to a carrier density ns ∼ 5 × 1013 cm−2. With such a high doping level, the nanostructured graphene exhibits absorbance as high as 28% for photon energies around 0.17 eV (wavelength ∼7.2 μm), as shown in Figure 2a. Because of the slow ion migrations driven by the electric field, the electrolyte gating usually suffers from a slow response speed, making it not viable for IR modulation applications, which require high operational speed. However, it may find applications in which fast response is not critical, for instance, optical filtering or beam-steering.

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ACTIVE GRAPHENE PLASMONICS ENABLED INFRARED LIGHT SOURCES The extraordinary field confinement and the dynamically tunable optical response of graphene plasmonics render it a powerful tool for extending the functionalities of existing infrared sources such as QCL, whose spectral control is known C


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Figure 3. (a) Left: Schematic of the THz QCL integrated with metallic ADFB structures and a graphene sheet overlaid on top. A polymer electrolyte covers the device, thus, enabling the electrolyte gating of graphene. Right: Measured QCL emission spectra for ungated (low ns) and gated (high ns) graphene. Insets: Simulated electric field profiles of a single metallic slit with low doped (top) and highly doped (bottom) graphene at 2.8 THz. (b) Left: Schematic of tunable graphene plasmonic thermal emitter and experimental apparatus. The graphene nanoribbon arrays are fabricated on a 1 μm thick SiNx membrane with an Au back reflector. Voltage is applied across the SiNx in order to control the doping of graphene. Right: Measured IR emissivity for 40 nm wide graphene nanoresonators at various carrier densities. The device is heated at 250 °C. (c) Top: Illustration of graphene nanoribbons functioning as both plasmonic resonators and joule heaters upon application of bias voltages. The thermally excited SiNx phonons can interact with graphene plasmons. Bottom: evolution of IR extinction spectra of 80 nm graphene nanoribbon arrays under various voltage bias (Vb), which indicates that the plasmon-phonon interaction can be electro-thermally controlled. Panel a is reprinted with permission from ref 61. Copyright 2016 The American Association for the Advancement of Science. Panel b is reprinted with permission from ref 58. Copyright 2014 Nature Publishing. Panel c is reprinted with permission from ref 73. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

governing principle for such devices is Kirchoff’s law, which states that for any material the blackbody emissivity must equal its absorptivity at thermal equilibrium.62 Based on the same principles that underlie enhanced and spectrally tunable graphene plasmonic absorption, the dynamic modulation of mid-IR emission can also be accomplished. As shown by Brar et al. (illustrated by Figure 3b), the graphene plasmonic resonators can serve as antennae for blackbody radiation, enabling narrow spectral emission peaks in the mid-infrared region. The frequency and intensity of these spectral responses can be modulated via electrostatic gating. Owing to the extremely small mode volumes, a large Purcell factor in graphene plasmonic resonators has been demonstrated, approaching 107.63,64 Therefore, the thermal emission modulation rate can be extremely fast.65 This makes graphene plasmonic tuning advantageous as compared to other tuning mechanisms of thermal emitters, such as using phase change materials.66 Furthermore, electrically controllable thermal emitters can be realized by utilizing graphene nanostructures functioning as both plasmonic resonators and Joule heaters, where the local temperature can be controlled by the external voltage bias (as shown in Figure 3c). Such an approach effectively circumvents the need for external heaters and further reduces the device footprint. On the other hand, thermally or electrothermally induced surface phonon polariton (SPP) modes in the underlying substrate can in turn interact with collective plasmon modes in graphene via the Coulomb interaction,38 which results in novel hybrid plasmon-phonon modes that

to be limited. One example of particular note was demonstrated by Chakraborty et al.,61 who demonstrated the reversible tuning of the emission from a terahertz QCL (as illustrated in Figure 3a). First, customized noble metal slits known as aperiodic distributed feedback (ADFB) structures were fabricated on top of the laser active waveguiding region to control the output spectral properties of THz lasers. Such structures only allow emissions with wavelengths on the order of the slit width. Second, the metal slit structures can also function as subwavelength scatters which facilitate the excitation of graphene plasmons. Polymer electrolyte was then deposited over the device to control the carrier density (ns) of the graphene. At low ns (EF ∼ 50 meV), graphene plasmons have wavelength (λp) on the order of the slit width. In this scenario, it can be viewed as if the graphene plasmon wave “fixes” the patterned waveguide, yielding a laser emission spectrum that is no longer affected by the metal slits. By contrast, when ns is large (EF ∼ 300 meV), λp becomes much larger than the slit width. In this case, propagating THz waves are efficiently scattered by the subwavelength slits, which indicates that the laser emission can be controlled by the slits. This work opens new opportunities for realizing THz or mid-IR semiconductor lasers where gate-tunable spectral control is made possible by graphene. Further studies on the coherence statistics of these optical modes and how graphene plasmons affect the coherent properties will be of substantial interest. Thermal emitters with lower spatial and time coherence also emerge as novel mid-IR sources when various constraints (cost, power, or fabrication difficulty) are imposed upon QCLs. The D


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Figure 4. (a) Transmitted light IR modulator enabled by graphene plasmonic nanoribbons coupled to subwavelength Au slit array. As shown in the dashed circle, graphene nanoribbons are embedded in the metal slits. Incoming mid-IR radiation with transverse magnetic (TM) polarization excites surface plasmons on the top metal−air interface (1). The plasmons can tunnel through the subwavelength metallic slits (2) and, subsequently, excite surface plasmons on the bottom metal−dielectric interface (3). The graphene plasmons inside the Au slits can be tuned to interfere with Au’s surface plasmons, thus, blocking or unblocking the electromagnetic coupling between the surface plasmons on the top and bottom surface. (b) False colored SEM image of the graphene nanoribbons inside a subwavelength Au slit. (c) Simulated device transmission spectra as a function of graphene’s Fermi level (EF). (d) Comparison of measured modulation efficiency at 1397 cm−1 as a function of EF between bare graphene plasmonic modulator and graphene plasmon enabled EOT modulator. Panels a−d are reprinted with permission from ref 79. Copyright 2016 Nature Publishing.

large device footprint, require high voltage operation, and have limited modulation bandwidth.75 A modulating mechanism such as quantum confined Stark shift of intersubband transitions in semiconductor quantum wells is not well-suited for phase-only modulation, primarily due to the high absorption losses associated with the intersubband transitions.76,77 In recent years, researchers have been actively pursuing the use of the graphene’s electrostatically tunable optical conductivity or plasmonic resonances for building mid-IR modulatiors. Crucially, since mid-IR or far-IR modulators are mostly applied to free space optics rather than guided optics, a large modulation depth requires the proper design of relevant photonic structures to further enhance the short interaction length between long wavelength light and graphene. In 2013, Yao et al. demonstrated a reflective mode mid-IR modulator by incorporating graphene in the gap of the coupled metallic plasmonic antennae, which maximized the field overlap with the active graphene layer.78 Moreover, a 3 dB cutoff frequency of 30 MHz has been shown. With the similar device geometry and operating principle, Sherrott et al. recently demonstrated voltage modulated reflected light phase from 0° to 206° around 8.70 μm, which can be translated to a beam-steering efficiency of 23% for reflected light angles up to 30°.78 For some applications, such as modulators or beam shapers directly integrated on mid-IR light sources, devices that do not possess a back-reflector and that allow for efficient modulation of transmitted light are strongly desired. Kim et al. recently reported a graphene plasmonic device capable of modulating

exhibit significantly longer lifetime and smaller mode volume.67−69 The interaction is sensitive to the local temperature change (phonon oscillation strength) and thus can be controllable. It is known that nanostructured crystalline polar dielectrics (e.g., SiC and hBN) can support SPP modes and are capable of emitting narrowband mid-IR radiation,70−72 although the properties (e.g., frequency, amplitude, and phase) can be hardly manipulated once the structures are fabricated. Hence, the marriage of graphene plasmonics with SPP modes can add a suite of powerful functionalities and flexibilities to current SPP-based thermal emitters.

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GRAPHENE PLASMONIC INFRARED MODULATION Modulation of the light intensity, phase, and polarization is critical in various optical imaging and communication systems. In contrast to modulators operating at telecommunication wavelengths that are predominantly used for fiber communications, mid- or far-infrared spatial light modulators (SLMs) are uniquely targeted to environment/health monitoring, navigation, and surveillance through scattering media (e.g., foggy or dusty environments). Efficient and fast modulation of the phase front of mid-IR radiations and reconfigurable beam-steering devices are of special significance in developing remote-sensing and light detection and ranging (LIDAR) systems.74 High resolution tunable spectral filters can also serve as an enabling component in chip-scale mid-IR spectrometers. However, novel solutions toward realizing such devices with good performance are still needed. Mid-IR acousto-optical modulators possess a E


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Figure 5. (a) Schematic view of the device for thermoelectric detection of graphene plasmons. A 10.6 μm (28 THz) laser illuminates at a movable metallized AFM tip, launching plasmons in the hBN−graphene−hBN heterostructure. Two metal electrodes (split gates) underneath the hBN dielectric are used to spatially modulate the charge carrier density and polarity across the graphene layer creating a graphene homojunction. (b) Illustration of thermoelectric detection mechanism. The electron−electron interactions during the plasmon decaying process result in elevated carrier temperature, subsequently drive an outward majority carrier (holes in the figure) diffusion. A gate voltage induced homojunction breaks the symmetry of the graphene channel, resulting in a nonzero net DC current. (c) Spatially resolved plasmon-induced photocurrent as the AFM tip is scanned across the graphene junction. (d) False colored SEM image of the metamaterial-enhanced graphene plasmonic mid-IR detector. Two metals with different work functions are used as contacts to imbalance the carrier diffusion along different directions. (e) Frequency dependent photovoltage of the detector. The inset shows the detector responsivity as a function of incident light frequency. (f) Frequency dependent photovoltage of the detector with ∼20 nm thick coating of PMMA. In (e) and (f), the black (red) line shows the photovoltage with incident light polarized perpendicular (parallel) to the graphene ribbons. Panels a−c are reprinted with permission from ref 90. Copyright 2017 Nature Publishing. Panels d and e are reprinted with permission from ref 93. Copyright 2016 American Chemical Society.

transmitted light of 28.6% efficiency at 1397 cm−1 (7.16 μm). Their device is comprised of periodic subwavelength metal slits that support spectrally tunable extraordinary optical transmission80 (EOT) in mid-IR. The EOT originates from the photon tunneling of surface plasmons from the top metal layer to the bottom metal−dielectric interface81 via the metallic slits. Subsequently, the surface plasmons on the bottom metal− dielectric interface are scattered into the free space by the periodic metallic structure. (Figure 4a). By embedding graphene plasmonic resonators in the metal slits (Figure 4b), plasmonic resonances in graphene nanoresonators can alter the local electromagnetic field (or the damping) experienced by EOT modes when the graphene plasmonic resonant frequency is close to the EOT frequency. As a result, the carrier density modulated graphene plasmons can block or enable the coupling between top and bottom surface plasmons, thus, suppressing or enabling the EOT resonance (Figure 4c). In this design, a dielectric Fabry−Perot resonator was also employed to increase the graphene plasmonic resonance amplitude, which in turn boosted the modulation efficiency. One can expect that

superior modulation efficiency can be achieved if the graphene’s mobility (or graphene plasmon lifetime) can be increased, allowing strong coupling between the graphene plasmonic mode and the EOT mode.

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GRAPHENE PLASMONICS FOR INFRARED PHOTODETECTION AND SENSING Another exciting frontier that graphene plasmonics offers lies in harvesting low energy photons at room temperature, which can potentially overcome the limitations of conventional semiconductor devices. Traditional detectors based on interband transitions (e.g., mercury cadmium telluride) require cryogenic cooling to suppress dark-current noise, especially when operating at long wavelength infrared wavelengths (LWIR). In addition, device miniaturization and their heterogeneous integration with silicon or germanium platforms remain challenging.82 Uncooled thermal detectors such as thermopiles or bolometers usually possess large device footprints, low responsivities and slow speeds due to thermal time constants being in the millisecond range.83 These drawbacks have F


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junction, for such a device design, challenges still remain, particularly in terms of lowering the NEP. In fact, the Johnson− Nyquist voltage noise will increase with larger device resistance as a result of connecting multiple junctions. On a different front, infrared spectroscopy has long played a pivotal role in identifying chemical or biomolecules through their vibrational spectrum. A major drawback is that the long wavelength of infrared light interacts very weakly with nanometer size molecules, resulting in the relatively low sensitivity of such technique. At mid-IR frequencies, graphene plasmons have light confinement down to tens of nanometers and enable unprecedentedly high overlap with nanoscale biomolecules.94−96 Additionally, graphene also facilitates the physical adsorption of various molecules via noncovalent π−π stacking.97,98 Recently, superior sensitivity over noble metals for surface-enhanced IR absorption (SEIRA) has been demonstrated in graphene.99 As an important step forward, the abovementioned graphene mid-IR detector can also be devised as a monolithic mid-IR spectrometer to uniquely identify molecules [e.g., poly(methyl methacrylate) (PMMA)], which has characteristic vibrational modes (e.g., CO and O−CH3 bonds), as shown in Figure 5f, with the molecule’s induced plasmonic signal perturbations transformed into photovoltage changes. This may greatly simplify the current SEIRA-based sensors, which are now heavily reliant on bulky and expensive IR spectrometers.

significantly impeded their applications in thermal imaging and free space communications. Similar to noble metals, the nonradiative decay process of graphene plasmons can trigger physical processes with multiple outcomes. Due to strong electron−electron interactions,84 the damping of graphene plasmons gives rise to an effective electron temperature Tel higher than that of the lattice (Tl). Hot carriers created by the graphene plasmons can remain at Tel for many picoseconds85−87 because of weak electron−phonon coupling in graphene.88 Therefore, hot carriers in graphene may be extracted before the thermalization (equilibration of Tel and Tl via the slower scattering between hot carriers and acoustic phonons). Unlike bulk metals, the semimetallic graphene allows one to extract the hot carriers by inhomogeneously doping the graphene channel, which in turn shapes the direction of flow of hot electrons/holes via thermal diffusion.85,89 Electrical readouts (voltage or current) can then be produced. Lundeberg et al. reported the electrical readout and imaging of propagating graphene plasmons by utilizing a single p−n junction made of high quality mechanically exfoliated graphene on hBN dielectric (Figure 5a,b).90 Two metal electrodes (split gates) underneath the hBN dielectric were used to spatially modulate the charge carrier density across the graphene. The doping of the p and n regions are high enough to sustain graphene plasmon propagation. The mid-IR radiation at 10.6 μm (28 THz) was incident on a movable metallic AFM tip. Here, the AFM tip has multiple functionalities including launching the propagating plasmons, boosting the local incident power density and importantly, and enabling the highresolution real-space photocurrent mapping (Figure 5c). The photogenerated hot carriers can produce a thermoelectric current IPTE = (SR − SL)ΔTel/R due to the photothermoelectric (PTE) effect (Seebeck effect), where SL,R (in μV K−1) is the Seebeck coefficient in the left and right graphene regions with different Fermi levels, ΔTel is the junction-average rise in electronic temperature relative to ambient and R is the resistance of the device. As a mid-IR photodetector, a high responsivity of ∼20 mA/W and a Johnson−Nyquist noise limited noise equivalent power (NEP) of 400 pW/ Hz was reported. It ought to be noted that, since the thermal diffusion of hot carriers, rather than lattice heating,91,92 is responsible for generating the current, PTE graphene detectors can achieve fast frequency response. The PTE approach is also generalizable to THz frequencies, where carriers in graphene have an even stronger response. In order to leverage the above-mentioned device physics to detect real infrared radiation, one needs to account for the large beam spot size and the exceedingly low power density on each graphene junction. One possible solution is to employ the localized graphene plasmonic resonances in large scale CVD graphene instead of the propagating plasmons in mechanically exfoliated graphene. Additionally, connecting each graphene junction in series can add up the voltage response of each junction element. Luxmoore et al. reported a metamaterial cavity enhanced graphene-plasmonic PTE detector (shown in Figure 5d), which offered a voltage responsivity of 120 mV/W at 1500 cm−1 and responsivity that exceeds 10 mV/W at other frequencies93 (Figure 5e). This was made possible by the judicious design of metal-based metamaterial cavities, which serve to enhance the graphene’s plasmonic absorption, imbalance the hot carrier diffusion along different directions, and more importantly, wire each junction element together. Despite the improved voltage responsivity compared to the unit

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OUTLOOK AND PERSPECTIVES Graphene plasmonics represents an emerging frontier which brings together multidisciplinary fields of material science, optics and electronics. It has uncovered exciting prospects in infrared photonics research, where novel technologies and solutions are still being developed. Graphene offers a combination of distinctive features in plasmonics, particularly the dynamically tunable response, extremely high degree of spatial confinement, and long plasmon lifetime. Beyond these unique optical properties, the excellent electronic properties of graphene, such as high carrier mobility and controllable doping make it possible to actively control or perform electronic readouts of the excitations and perturbations of plasmons. This is, of course, in addition to conventional optical probing methods. From a device and system viewpoint, graphene is a layered material with an intrinsically passivated surface. It is thus amenable to monolithic integration with conventional silicon-based platforms100−103 as well as other important IR materials.104 This makes graphene remarkably advantageous over materials with bulk lattices whose heterogeneous integration with other materials usually induce defects105 and deep level trapping centers106 at the interface. Graphene plasmonics also has some demanding challenges that must be overcome before making an impact in terms of industry-standard IR device applications. Among many others, the high quality synthesis of large-scale graphene is of paramount importance and will have the highest positive impact on this field. Specifically, the improvements in graphene’s quality (i.e., carrier mobility) can significantly prolong the lifetime of graphene plasmons, which is strongly correlated with various device performance metrics, such as the light−matter interaction strength, absorbance, phase shifts, sensitivity, spectral resolution, modulation efficiency, detectivity, and so forth. Additionally, a wide variety of novel device physics and concepts call for a significantly prolonged plasmon lifetime as compared to the values attained in the state-of-theG


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art CVD graphene.107−112 Going beyond high quality SLG, advancements in synthesizing large-scale Bernal-stacked bilayer graphene or trilayer graphene will also enable the observation of a host of intriguing plasmonic phenomena113−117 in IR frequencies. Another open challenge concerns how to efficiently harvest the hot carriers generated by graphene plasmons using electrical methods. Advancements in this area will not only lead to exciting opportunities, including efficient IR detection or energy harvesting,118,119 but also may transform some of the widely used tools in IR, such as infrared cameras, spectrometers, and highly sensitive bio/chemical sensors, into compact chip-scale platforms. An efficient plasmon-electron conversion method can also be used as a thermometer to probe the near-field heat transfer between graphene plasmonics structures, which was predicted to be highly efficient.120,121 It is known that the excitation of graphene plasmons results in electron oscillations with a collective mass.122 The electronic readout of graphene plasmons can also offer a different perspective to look at the physics associated with the plasmon excitations.123 Solutions based on the thermoelectric effect90,93 and bolometric effect124 are so far still of a modest plasmonelectron conversion efficiency and are difficult to scale up for sensitive detecting IR radiation of low power density. Moreover, there remains a pressing need for detailed and fundamental physical understandings of the hot carrier dynamics and transports in graphene since they are indispensable for rationally designing high performance graphene-based hot electron devices. In summary, the various properties of graphene plasmonics make it well suited for integrated, multifunctional and compact photonic devices and systems, yet its potential is not fully reached. We believe that continued work in graphene plasmonics, together with the recent of enrichment of mid-IR and THz devices and systems would finally lead to proliferation in both fundamental investigations and various applications including imaging, communications, environmental monitoring, medical diagnostics, and spectroscopic studies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fengnian Xia: 0000-0001-5176-368X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation CAREER Award (ECCS 1552461) and the Office of Naval Research (N000141410565) for the support of this work. F.G. was supported by funding from the European Union through the ERC Advanced Grant NOVGRAPHENE through Grant Agreement Nr. 290846, and from the European Commission under the Graphene Flagship, Contract CNECTICT-604391. We thank Kyle LaBrosse for proofreading the manuscript.

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Infrared Nanophotonics Based on Graphene Plasmonics - ACS

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