arXiv subjects

Joel D. Cox

Publications and source records attributed to Joel D. Cox.

36 records · Page 2Linked to original sources

arXiv2022

Optoelectronic control of atomic bistability with graphene

We explore the emergence and active control of optical bistability in a two-level atom near a graphene sheet. Our theory incorporates self-interaction of the optically-driven atom and its coupling to electromagnetic vacuum modes, both of which are sensitive to the electrically-tunable interband transition threshold in graphene. We show that electro-optical bistability and hysteresis can manifest in the intensity, spectrum, and quantum statistics of the light emitted by the atom, which undergoes critical slow-down to steady-state. The optically-driven atom-graphene interaction constitutes a platform for active control of driven atomic systems in quantum coherent control and atomic physics.

quant-ph↗

arXiv2022

Giant enhancement of third-harmonic generation in graphene-metal heterostructures

Nonlinear nanophotonics leverages engineered nanostructures to funnel light into small volumes and intensify nonlinear optical processes with spectral and spatial control. Due to its intrinsically large and electrically tunable nonlinear optical response, graphene is an especially promising nanomaterial for nonlinear optoelectronic applications. Here we report on exceptionally strong optical nonlinearities in graphene-insulator-metal heterostructures, demonstrating an enhancement by three orders of magnitude in the third-harmonic signal compared to bare graphene. Furthermore, by increasing the graphene Fermi energy through an external gate voltage, we find that graphene plasmons mediate the optical nonlinearity and modify the third-harmonic signal. Our findings show that graphene-insulator-metal is a promising heterostructure for optically-controlled and electrically-tunable nano-optoelectronic components.

cond-mat.mes-hall↗

arXiv2021

Direct generation of entangled photon pairs in nonlinear optical waveguides

Entangled photons are pivotal elements in emerging quantum information technologies. While several schemes are available for the production of entangled photons, they typically require the assistance of cumbersome optical elements to couple them to other components involved in logic operations. Here, we introduce a scheme by which entangled photon pairs are directly generated as guided mode states in optical waveguides. The scheme relies on the intrinsic nonlinearity of the waveguide material, circumventing the use of bulky optical components. Specifically, we consider an optical fiber under normal illumination, so that photon down-conversion can take place to waveguide states emitted with opposite momentum into a spectral region populated by only two accessible modes. By additionally configuring the external illumination to interfere different incident directions, we can produce maximally entangled photon-pair states, directly generated as waveguide modes with conversion efficiencies that are competitive with respect to existing macroscopic schemes. These results should find application in the design of more efficient and compact quantum optics devices.

quant-ph↗

arXiv2021

Polaritons in two-dimensional parabolic waveguides

The suite of highly confined polaritons supported by two-dimensional (2D) materials constitutes a versatile platform for nano-optics, offering the means to channel light on deep-subwavelength scales. Graphene, in particular, has attracted considerable interest due to its ability to support long-lived plasmons that can be actively tuned via electrical gating. While the excellent optoelectronic properties of graphene are widely exploited in plasmonics, its mechanical flexibility remains relatively underexplored in the same context. Here, we present a semi-analytical formalism to describe plasmons and other polaritons supported in waveguides formed by bending a 2D material into a parabolic shape. Specifically, for graphene parabolas, our theory reveals that the already large field confinement associated with graphene plasmons can be substantially increased by bending an otherwise flat graphene sheet into a parabola shape, thereby forming a plasmonic waveguide without introducing potentially lossy edge terminations via patterning. Further, we show that the high field confinement associated with such channel polaritons in 2D parabolic waveguides can enhance the spontaneous emission rate of a quantum emitter near the parabola vertex. Our findings apply generally to 2D polaritons in atomically thin materials deposited onto grooves or wedges prepared on a substrate or freely suspended in a quasi-parabolic (catenary) shape. We envision that both the optoelectronic and mechanical flexibility of 2D materials can be harnessed in tandem to produce 2D channel polaritons with versatile properties that can be applied to a wide range of nano-optics functionalities, including subwavelength polaritonic circuitry and bright single-photon sources.

cond-mat.mes-hall↗

arXiv2021

Surface-response functions obtained from equilibrium electron-density profiles

Surface-response functions are one of the most promising routes for bridging the gap between fully quantum-mechanical calculations and phenomenological models in quantum nanoplasmonics. Within all the currently available recipes for obtaining such response functions, \emph{ab initio} calculations remain one of the most predominant, wherein the surface-response function are retrieved via the metal's non-equilibrium response to an external perturbation. Here, we present a complementary approach where one of the most appealing surface-response functions, namely the Feibelman $d$-parameters, yield a finite contribution even in the case where they are calculated directly from the equilibrium properties described under the local-response approximation (LRA), but with a spatially varying equilibrium electron density. Using model calculations that mimic both spill-in and spill-out of the equilibrium electron density, we show that the obtained $d$-parameters are in qualitative agreement with more elaborate, but also more computationally demanding, \emph{ab initio} methods. The analytical work presented here illustrates how microscopic surface-response functions can emerge out of entirely local electrodynamic considerations.

cond-mat.mes-hall↗

arXiv2020

Anisotropic Second Harmonic Generation From Monocrystalline Gold Flakes

Noble metals with well-defined crystallographic orientation constitute an appealing class of materials for controlling light-matter interactions on the nanoscale. Nonlinear optical processes, being particularly sensitive to anisotropy, are a natural and versatile probe of crystallinity in nano-optical devices. Here we study the nonlinear optical response of monocrystalline gold flakes, revealing a polarization dependence in second-harmonic generation from the {111} surface that is markedly absent in polycrystalline films. Apart from suggesting an approach for directional enhancement of nonlinear response in plasmonic systems, we anticipate that our findings can be used as a rapid and non-destructive method for characterization of crystal quality and orientation that may be of significant importance in future applications.

physics.optics↗

arXiv2020

Optical response of noble metal nanostructures: Quantum surface effects in crystallographic facets

Noble metal nanostructures are ubiquitous elements in nano-optics, supporting plasmon modes that can focus light down to length scales commensurate with nonlocal effects associated with quantum confinement and spatial dispersion in the underlying electron gas. Nonlocal effects are naturally more prominent for crystalline noble metals, which potentially offer lower intrinsic loss than their amorphous counterparts, and with particular crystal facets giving rise to distinct electronic surface states. Here, we employ a quantum-mechanical model to describe nonclassical effects impacting the optical response of crystalline noble-metal films and demonstrate that these can be well-captured using a set of surface-response functions known as Feibelman $d$-parameters. In particular, we characterize the $d$-parameters associated with the (111) and (100) crystal facets of gold, silver, and copper, emphasizing the importance of surface effects arising due to electron wave function spill-out and the surface-projected band gap emerging from atomic-layer corrugation. We then show that the extracted $d$-parameters can be straightforwardly applied to describe the optical response of various nanoscale metal morphologies of interest, including metallic ultra-thin films, graphene-metal heterostructures hosting extremely confined acoustic graphene plasmons, and crystallographic faceted metallic nanoparticles supporting localized surface plasmons. The tabulated $d$-parameters reported here can circumvent computationally expensive first-principles atomistic simulations to describe microscopic nonlocal effects in the optical response of mesoscopic crystalline metal surfaces, which are becoming widely available with increasing control over morphology down to atomic length scales for state-of-the-art experiments in nano-optics.

cond-mat.mes-hall↗

arXiv2019

Strong-field-driven dynamics and high-harmonic generation in interacting 1D systems

We explore the roles of electronic band structure and Coulomb interactions in solid-state HHG by studying the optical response of linear atomic chains and carbon nanotubes to intense ultrashort pulses. Specifically, we simulate electron dynamics by solving the single-particle density matrix equation of motion in the presence of intense ultrafast optical fields, incorporating tight-binding electronic states and a self-consistent electron-electron interaction. While linear atomic chains constitute an idealized system, our realistic 1D model readily provides insight on the temporal evolution of electronic states in reciprocal space, both in the absence or presence of electron interactions, which we demonstrate to play an important role in the HHG yield. This model further predicts that doped semiconductors generate high harmonics more efficiently than their metallic and undoped counterparts. To complement this idealized system we also show results for HHG in more realistic quasi-1D structures such as carbon nanotubes, the behavior of which is found to be in good qualitative agreement with the atomic chains. Our findings apply directly to extreme nonlinear optical phenomena in atoms on surfaces, carbon-based structures, linear arrays of dopant atoms in semiconductors, and linear molecules, such as polycyclic aromatic hydrocarbon chains, and can be straightforwardly extended to optimize existing platforms for HHG or identify new solid-state alternatives in the context of nonlinear plasmonics.

cond-mat.mes-hall↗

arXiv2019

Nonlinear atom-plasmon interactions enabled by nanostructured graphene

Electrically tunable graphene plasmons are anticipated to enable strong light-matter interactions with resonant quantum emitters. However, plasmon resonances in graphene are typically limited to infrared frequencies, below those of optical excitations in robust quantum light sources and many biologically interesting molecules. Here we propose to utilize near fields generated by the plasmon-assisted nonlinear optical response of nanostructured graphene to resonantly couple with proximal quantum emitters operating in the near-infrared. We show that the nonlinear near-field produced by a graphene nanodisk can strongly excite and coherently control quantum states in two- and three-level atomic systems when the third harmonic of its plasmon resonance is tuned to a particular electronic transition. In the present scheme, emitter and plasmon resonances are nondegenerate, circumventing strong enhancement of spontaneous emission. We envision potential applications for the proposed nonlinear plasmonic coupling scheme in sensing and temporal quantum control.

cond-mat.mes-hall↗

arXiv2019

Quantum Effects in the Acoustic Plasmons of Atomically-Thin Heterostructures

Recent advances in nanofabrication technology now enable unprecedented control over 2D heterostructures, in which single- or few-atom thick materials with synergetic opto-electronic properties can be combined to develop next-generation nanophotonic devices. Precise control of light can be achieved at the interface between 2D metal and dielectric layers, where surface plasmon polaritons strongly confine electromagnetic energy. Here we reveal quantum and finite-size effects in hybrid systems consisting of graphene and few-atomic-layer noble metals, based on a quantum description that captures the electronic band structure of these materials. These phenomena are found to play an important role in the metal screening of the plasmonic fields, determining the extent to which they propagate in the graphene layer. In particular, we find that a monoatomic metal layer is capable of pushing graphene plasmons toward the intraband transition region, rendering them acoustic, while the addition of more metal layers only produces minor changes in the dispersion but strongly affects the lifetime. We further find that a quantum approach is required to correctly account for the sizable Landau damping associated with single-particle excitations in the metal. We anticipate that these results will aid in the design of future platforms for extreme light-matter interaction on the nanoscale.

cond-mat.mtrl-sci↗

arXiv2018

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 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 behavior, thus facilitating their use for the design of actual devices.

cond-mat.mes-hall↗

arXiv2017

How To Identify Plasmons from the Optical Response of Nanostructures

A promising trend in plasmonics involves shrinking the size of plasmon-supporting structures down to a few nanometers, thus enabling control over light-matter interaction at extreme-subwavelength scales. In this limit, quantum mechanical effects, such as nonlocal screening and size quantization, strongly affect the plasmonic response, rendering it substantially different from classical predictions. For very small clusters and molecules, collective plasmonic modes are hard to distinguish from other excitations such as single-electron transitions. Using rigorous quantum mechanical computational techniques for a wide variety of physical systems, we describe how an optical resonance of a nanostructure can be classified as either plasmonic or nonplasmonic. More precisely, we define a universal metric for such classification, the generalized plasmonicity index (GPI), which can be straightforwardly implemented in any computational electronic-structure method or classical electromagnetic approach to discriminate plasmons from single-particle excitations and photonic modes. Using the GPI, we investigate the plasmonicity of optical resonances in a wide range of systems including: the emergence of plasmonic behavior in small jellium spheres as the size and the number of electrons increase; atomic-scale metallic clusters as a function of the number of atoms; and nanostructured graphene as a function of size and doping down to the molecular plasmons in polycyclic aromatic hydrocarbons. Our study provides a rigorous foundation for the further development of ultrasmall nanostructures based on molecular plasmonics

cond-mat.mes-hall↗

arXiv2017

Quantum Effects in the Nonlinear Response of Graphene Plasmons

The ability of graphene to support long-lived, electrically tunable plasmons that interact strongly with light, combined with its highly nonlinear optical response, has generated great expectations for application of the atomically-thin material to nanophotonic devices. These expectations are mainly reinforced by classical analyses performed using the response derived from extended graphene, neglecting finite-size and nonlocal effects that become important when the carbon layer is structured on the nanometer scale in actual device designs. Here we show that finite-size effects produce large contributions that increase the nonlinear response of nanostructured graphene to significantly higher levels than those predicted by classical theories. We base our analysis on a quantum-mechanical description of graphene using tight-binding electronic states combined with the random-phase approximation. While classical and quantum descriptions agree well for the linear response when either the plasmon energy is below the Fermi energy or the size of the structure exceeds a few tens of nanometers, this is not always the case for the nonlinear response, and in particular, third-order Kerr-type nonlinearities are generally underestimated by the classical theory. Our results reveal the complex quantum nature of the optical response in nanostructured graphene, while further supporting the exceptional potential of this material for nonlinear nanophotonic devices.

cond-mat.mes-hall↗

arXiv2017

Petahertz optical response in graphene

The temporal dynamics of charge carriers determines the speed with which electronics can be realized in condensed matter, and their direct manipulation with optical fields promises electronic processing at unprecedented petahertz frequencies, consisting in a million-fold increase from state of the art technology. Graphene is of particular interest for the implementation of petahertz optoelectronics due to its unique transport properties, such as high carrier mobility with near-ballistic transport and exceptionally strong coupling to optical fields. The back action of carriers in response to an optical field is therefore of key importance towards applications. Here we investigate the instantaneous response of graphene to petahertz optical fields and elucidate the role of hot carriers on a sub-100 fs timescale. Measurements of the nonlinear response and its dependence on interaction time and field polarization allow us to identify the back action of hot carriers over timescales that are commensurate with the optical field. An intuitive picture is given for the carrier trajectories in response to the optical-field polarization state. We note that the peculiar interplay between optical fields and charge carriers in graphene may also apply to surface states in topological insulators with similar Dirac cone dispersion relations.

cond-mat.mes-hall↗

arXiv2016

Plasmon-assisted high-harmonic generation in graphene

High-harmonic generation (HHG) in condensed-matter systems is both a source of fundamental insight into quantum electron motion and a promising candidate to realize compact ultraviolet and ultrafast light sources. Here we argue that the large light intensity required for this phenomenon to occur can be reached by exploiting localized plasmons in conducting nanostructures. In particular, we demonstrate that doped graphene nanostructures combine a strong plasmonic near-field enhancement and a pronounced intrinsic nonlinearity that result in efficient broadband HHG within a single material platform. We extract this conclusion from time-domain simulations using two complementary nonperturbative approaches based on atomistic one-electron density-matrix and massless Dirac-fermion Bloch-equation pictures. High harmonics are predicted to be emitted with unprecedentedly large intensity by tuning the incident light to the localized plasmons of ribbons and finite islands. In contrast to atomic systems, we observe no cutoff in harmonic order. Our results support the strong potential of nanostructured graphene as a robust, electrically tunable platform for HHG.

cond-mat.mes-hall↗

arXiv2015

Plasmon-enhanced nonlinear wave mixing in nanostructured graphene

Localized plasmons in metallic nanostructures have been widely used to enhance nonlinear optical effects due to their ability to concentrate and enhance light down to extreme-subwavelength scales. As alternatives to noble metal nanoparticles, graphene nanostructures can host long-lived plasmons that efficiently couple to light and are actively tunable via electrical doping. Here we show that doped graphene nanoislands present unique opportunities for enhancing nonlinear optical wave-mixing processes between two externally applied optical fields at the nanoscale. These small islands can support pronounced plasmons at multiple frequencies, resulting in extraordinarily high wave-mixing susceptibilities when one or more of the input or output frequencies coincide with a plasmon resonance. By varying the doping charge density in a nanoisland with a fixed geometry, enhanced wave mixing can be realized over a wide spectral range in the visible and near infrared. We concentrate in particular on second- and third-order processes, including sum and difference frequency generation, as well as on four-wave mixing. Our calculations for armchair graphene triangles composed of up to several hundred carbon atoms display large wave mixing polarizabilities compared with metal nanoparticles of similar lateral size, thus supporting nanographene as an excellent material for tunable nonlinear optical nanodevices.

cond-mat.mes-hall↗

arXiv2014

Extraordinary nonlinear plasmonics in graphene nanoislands

Nonlinear optical processes rely on the intrinsically weak interactions between photons enabled by their coupling with matter. Unfortunately, many applications in nonlinear optics are severely hindered by the small response of conventional materials. Metallic nanostructures partially alleviate this situation, as the large light enhancement associated with their localized plasmons amplifies their nonlinear response to record high levels. Graphene hosts long-lived, electrically tunable plasmons that also interact strongly with light. Here we show that the nonlinear polarizabilities of graphene nanoislands can be electrically tuned to surpass by several orders of magnitude those of metal nanoparticles of similar size. This extraordinary behavior extends over the visible and near-infrared for islands consisting of hundreds of carbon atoms doped with moderate carrier densities. Our quantum-mechanical simulations of the plasmon-enhanced optical response of nanographene reveal this material as an ideal platform for the development of electrically tunable nonlinear optical nanodevices.

cond-mat.mtrl-sci↗

arXiv2012

Dipole-dipole interaction between a quantum dot and graphene nanodisk

We study theoretically the dipole-dipole interaction and energy transfer in a hybrid system consisting of a quantum dot and graphene nanodisk embedded in a nonlinear photonic crystal. In our model a probe laser field is applied to measure the energy transfer between the quantum dot and graphene nanodisk while a control field manipulates the energy transfer process. These fields create excitons in the quantum dot and surface plasmon polaritons in the graphene nanodisk which interact via the dipole-dipole interaction. Here the nonlinear photonic crystal acts as a tunable photonic reservoir for the quantum dot, and is used to control the energy transfer. We have found that the spectrum of power absorption in the quantum dot has two peaks due to the creation of two dressed excitons in the presence of the dipole-dipole interaction. The energy transfer rate spectrum of the graphene nanodisk also has two peaks due to the absorption of these two dressed excitons. Additionally, energy transfer between the quantum dot and the graphene nanodisk can be switched on and off by applying a pump laser to the photonic crystal or by adjusting the strength of the dipole-dipole interaction. We show that the intensity and frequencies of the peaks in the energy transfer rate spectra can be modified by changing the number of graphene monolayers in the nanodisk or the separation between the quantum dot and graphene. Our results agree with existing experiments on a qualitative basis. The principle of our system can be employed to fabricate nano-biosensors, optical nano-switches, and energy transfer devices.

cond-mat.mes-hall↗