Renwen Yu

Resonant visible light modulation with graphene

Fast modulation and switching of light at visible and near-infrared (vis-NIR) frequencies is of utmost importance for optical signal processing and sensing technologies. No fundamental limit appears to prevent us from designing wavelength-sized devices capable of controlling the light phase and intensity at terahertz speeds in those spectral ranges. However, this problem remains largely unsolved, despite recent advances in the use of quantum wells and phase-change materials for that purpose. Here, we explore an alternative solution based upon the remarkable electro-optical properties of graphene. In particular, we predict unity-order changes in the transmission and absorption of vis-NIR light produced upon electrical doping of graphene sheets coupled to realistically engineered optical cavities. The light intensity is enhanced at the graphene plane, and so is its absorption, which can be switched and modulated via Pauli blocking through varying the level of doping. Specifically, we explore dielectric planar cavities operated under either tunneling or Fabry-Perot resonant transmission conditions, as well as Mie modes in silicon nanospheres and lattice resonances in metal particle arrays. Our simulations reveal absolute variations in transmission exceeding 90% using feasible material parameters, thus supporting the application of graphene in fast electro-optics at vis-NIR frequencies.

Manipulating the interaction between localized and delocalized surface plasmon-polaritons in graphene

The excitation of localized or delocalized surface plasmon polaritons in nanostructured or extended graphene has attracted a steadily increasing attention due to their promising applications in sensors, switches, and filters. These single resonances may couple and intriguing spectral signatures can be achieved by exploiting the entailing hybridization. Whereas thus far only the coupling between localized or delocalized surface plasmon polaritons has been studied in graphene nanostructures, we consider here the interaction between a localized and a delocalized surface plasmon polariton. This interaction can be achieved by two different schemes that reside on either evanescent near- field coupling or far-field interference. All observable phenomena are corroborated by analytical considerations, providing insight into the physics and paving the way for compact and tunable optical components at infrared and terahertz frequencies.

Ultrafast radiative heat transfer

Light absorption in conducting materials produces heating of their conduction electrons, followed by relaxation into phonons within picoseconds, and subsequent diffusion into the surrounding media over longer timescales. This conventional picture of optical heating is supplemented by radiative cooling, which typically takes place at an even lower pace, only becoming relevant for structures held in vacuum or under extreme thermal isolation. Here, we reveal an ultrafast radiative cooling regime between neighboring plasmon-supporting graphene nanostructures in which noncontact heat transfer becomes a dominant channel. We predict that more than 50% of the electronic heat energy deposited on a graphene disk can be transferred to a neighboring nanoisland within a femtosecond timescale. This phenomenon is facilitated by the combination of low electronic heat capacity and large plasmonic field concentration in doped graphene. Similar effects should occur in other van der Waals materials, thus opening an unexplored avenue toward efficient heat management.Electron relaxation, which is the dominant release channel of electronic heat in nanostructures, occurs with characteristic times of several picoseconds. Here, the authors predict that an ultrafast (femtosecond) radiative cooling regime takes place in plasmonically active neighboring graphene nanodisks prior to electron relaxation.

Electrical detection of single graphene plasmons

Plasmons-the collective oscillations of electrons in conducting materials-play a pivotal role in nanophotonics because of their ability to couple electronic and photonic degrees of freedom. In particular, plasmons in graphene-the atomically thin carbon material-offer strong spatial confinement and long lifetimes, accompanied by extraordinary optoelectronic properties derived from its peculiar electronic band structure. Understandably, this material has generated great expectations for its application to enhanced integrated devices. However, an efficient scheme for detecting graphene plasmons remains a challenge.

Analytical Modeling of Graphene Plasmons

The two-dimensionality of graphene and other layered materials can be exploited to simplify the theoretical description of their plasmonic and polaritonic modes. We present an analytical theory that allows us to simulate these excitations in laterally patterned structures in terms of plasmon wave functions (PWFs). Closed-form expressions are offered for their associated extinction spectra, involving only two real parameters for each plasmon mode and graphene morphology, which we calculate and tabulate once and for all. Classical and quantum-mechanical formulations of this PWF formalism are introduced, in excellent mutual agreement for armchaired islands with >10 nm characteristic size. Examples of application are presented to predict both plasmon-induced transparency in interacting nanoribbons and excellent sensing capabilities through the response to the dielectric environment. We argue that the PWF formalism has general applicability and allows us to analytically describe a wide range of 2D polaritonic ...

Plasmonic Nano-Oven by Concatenation of Multishell Photothermal Enhancement

Metallodielectric multishell nanoparticles are capable of hosting collective plasmon oscillations distributed among different metallic layers, which result in large near-field enhancement at specific regions of the structure, where light absorption is maximized. We exploit this capability of multishell nanoparticles, combined with thermal boundary resistances and spatial tailoring of the optical near fields, to design plasmonic nano-ovens capable of achieving high temperatures at the core region using moderate illumination intensities. We find a large optical intensity enhancement of ∼104 over a relatively broad core region with a simple design consisting of three metal layers. This provides an unusual thermal environment, which together with the high pressures of ∼105 atm produced by concatenated curved layers holds great potential for exploring physical and chemical processes under extreme optical/thermal/pressure conditions in confined nanoscale spaces, while the outer surface of the nano-oven is close to ambient conditions.

Efficient electrical detection of mid-infrared graphene plasmons at room temperature

Optical excitation and subsequent decay of graphene plasmons can produce a significant increase in charge-carrier temperature. An efficient method to convert this temperature elevation into electrical signals can enable important mid-infrared applications. However, the modest thermoelectric coefficient and weak temperature dependence of carrier transport in graphene hinder this goal. Here, we demonstrate mid-infrared graphene detectors consisting of arrays of plasmonic resonators interconnected by quasi-one-dimensional nanoribbons. Localized barriers associated with disorder in the nanoribbons produce a dramatic temperature dependence of carrier transport, thus enabling the electrical detection of plasmon decay in the nearby graphene resonators. Our device has a subwavelength footprint of 5 × 5 μm2 and operates at 12.2 μm with an external responsivity of 16 mA W–1 and a low noise-equivalent power of 1.3 nW Hz–1/2 at room temperature. It is fabricated using large-scale graphene and possesses a simple two-terminal geometry, representing an essential step towards the realization of an on-chip graphene mid-infrared detector array.A system of patterned graphene nanoresonators/nanoribbons can be used as an efficient mid-infrared detector, based on plasmonic resonant absorption and subsequent carrier thermalization.

Analytical description of nonlinear plasmonic phenomena in nanostructured graphene

The remarkably-high intrinsic optical nonlinearity of graphene can be pushed even further when the optical frequency is tuned to plasmon resonances hosted by the material when it is doped [1-4]. Atomistic simulations provide an accurate description of these phenomena, although their computational cost is prohibitive for large graphene nanostructures [3, 4]. An alternative formalism consists in relying on classical electromagnetism, using the local nonlinear conductivities extracted from models of extended graphene. Here we present an analytical, classical electromagnetic description of the nonlinear optical response associated with tunable plasmons in graphene nanostructures, in excellent agreement with atomistic simulations of sufficiently large structures (10s of nm in lateral size) when describing second- and third-harmonic generation, as well as the Kerr nonlinearity. We base our analytical approach on an eigenmode decomposition of the optical field associated with the plasmon-driven resonant response of graphene ribbons and finite islands. The analytical description constitutes a valuable asset to explore nonlinear optical phenomena in the context of graphene plasmonics, and can also be applied to model nonlinearities in other planar plasmonic materials, such as thin metal layers and black phosphorous.

Nonlinear plasmonic sensing with nanographene

Plasmons provide excellent sensitivity to detect analyte molecules through their strong interaction with the dielectric environment. Plasmonic sensors based on noble metals are, however, limited by the spectral broadening of these excitations. Here we identify a new mechanism that reveals the presence of individual molecules through the radical changes that they produce in the plasmons of graphene nanoislands. An elementary charge or a weak permanent dipole carried by the molecule are shown to be sufficient to trigger observable modifications in the linear absorption spectra and the nonlinear response of the nanoislands. In particular, a strong second-harmonic signal [1], forbidden by symmetry in the unexposed graphene nanostructure, emerges due to a redistribution of conduction electrons produced by interaction with the molecule. These results pave the way toward ultrasensitive nonlinear detection of dipolar molecules and molecular radicals that is made possible by the extraordinary optoelectronic properties of graphene [2].

Active modulation of visible light with graphene-loaded ultrathin metal plasmonic antennas

Electro-optical modulation of visible and near-infrared light is important for a wide variety of applications, ranging from communications to sensing and smart windows. However, currently available approaches result in rather bulky devices, suffer from low integrability [1], and can hardly operate at the low power consumption and fast switching rates of microelectronic drivers. Here we show that planar nanostructures patterned in ultrathin metal-graphene hybrid films sustain highly tunable plasmons in the visible and near-infrared spectral regions. Strong variations in the reflection and absorption of incident light take place when the plasmons are tuned on- and off-resonance with respect to externally incident light. As a result, a remarkable modulation depth exceeding 90% in transmission and even more dramatic in reflection (>600%) is predicted for graphene-loaded silver film of 1–5 nm thickness and currently attainable lateral dimensions [2], These new structures hold great potential for fast low-power electro-optical modulation.