Articles de revista
http://hdl.handle.net/2117/23799
2024-03-29T13:08:12ZElectron diffraction by vacuum fluctuations
http://hdl.handle.net/2117/359829
Electron diffraction by vacuum fluctuations
Giulio, Valerio Di; García de Abajo, Francisco Javier
Vacuum fluctuations are known to produce electron diffraction leading to decoherence and self-interference. These effects have so far been studied as either an extension of the Aharonov–Bohm effect in front of a planar perfect conductor or through path integral analysis. Here, we present a simpler, general, and rigorous derivation based on a direct solution of the quantum electrodynamic aloof interaction between the electron and a material structure in the temporal gauge. Our approach allows us to study dissipative media, for which we show examples of electron wave function shaping due to the interaction with real-metal surfaces. We further present a proof of the relation between the phase associated with vacuum fluctuations and the Aharonov–Bohm effect produced by the image self-interaction that is valid for arbitrary geometries. Besides their fundamental interest, our results could be useful for on-demand patterning of electron beams with potential application in nondestructive nanoscale imaging and spectroscopy.
2022-01-17T10:43:26ZGiulio, Valerio DiGarcía de Abajo, Francisco JavierVacuum fluctuations are known to produce electron diffraction leading to decoherence and self-interference. These effects have so far been studied as either an extension of the Aharonov–Bohm effect in front of a planar perfect conductor or through path integral analysis. Here, we present a simpler, general, and rigorous derivation based on a direct solution of the quantum electrodynamic aloof interaction between the electron and a material structure in the temporal gauge. Our approach allows us to study dissipative media, for which we show examples of electron wave function shaping due to the interaction with real-metal surfaces. We further present a proof of the relation between the phase associated with vacuum fluctuations and the Aharonov–Bohm effect produced by the image self-interaction that is valid for arbitrary geometries. Besides their fundamental interest, our results could be useful for on-demand patterning of electron beams with potential application in nondestructive nanoscale imaging and spectroscopy.Nonlinear plasmonic response in atomically thin metal films
http://hdl.handle.net/2117/358271
Nonlinear plasmonic response in atomically thin metal films
Rodríguez Echarri, Álvaro; Cox, Joel D.; Iyikanat, Fadil; García de Abajo, Francisco Javier
Nanoscale nonlinear optics is limited by the inherently weak nonlinear response of conventional materials and the small light–matter interaction volumes available in nanostructures. Plasmonic excitations can alleviate these limitations through subwavelength light focusing, boosting optical near fields that drive the nonlinear response, but also suffering from large inelastic losses that are further aggravated by fabrication imperfections. Here, we theoretically explore the enhanced nonlinear response arising from extremely confined plasmon polaritons in few-atom-thick crystalline noble metal films. Our results are based on quantum-mechanical simulations of the nonlinear optical response in atomically thin metal films that incorporate crucial electronic band structure features associated with vertical quantum confinement, electron spill-out, and surface states. We predict an overall enhancement in plasmon-mediated nonlinear optical phenomena with decreasing film thickness, underscoring the importance of surface and electronic structure in the response of ultrathin metal films.
2021-12-13T15:33:38ZRodríguez Echarri, ÁlvaroCox, Joel D.Iyikanat, FadilGarcía de Abajo, Francisco JavierNanoscale nonlinear optics is limited by the inherently weak nonlinear response of conventional materials and the small light–matter interaction volumes available in nanostructures. Plasmonic excitations can alleviate these limitations through subwavelength light focusing, boosting optical near fields that drive the nonlinear response, but also suffering from large inelastic losses that are further aggravated by fabrication imperfections. Here, we theoretically explore the enhanced nonlinear response arising from extremely confined plasmon polaritons in few-atom-thick crystalline noble metal films. Our results are based on quantum-mechanical simulations of the nonlinear optical response in atomically thin metal films that incorporate crucial electronic band structure features associated with vertical quantum confinement, electron spill-out, and surface states. We predict an overall enhancement in plasmon-mediated nonlinear optical phenomena with decreasing film thickness, underscoring the importance of surface and electronic structure in the response of ultrathin metal films.Visible Optical Resonances in Electrically Doped DNA
http://hdl.handle.net/2117/348866
Visible Optical Resonances in Electrically Doped DNA
Saavedra, J. R. M.; García de Abajo, Francisco Javier
Dexoyribonucleic acid (DNA) has recently been identified as a promising material for nanotechnology due to its unique mechanical, electrical, and optical properties. However, optical applications are severely hampered by the featureless response of neutral DNA at visible frequencies. Additionally, predictive simulations are computationally too demanding to cope with large DNA strands. Here, we develop a computationally efficient procedure to simulate the optical response of large DNA molecules and reveal the emergence of electrically tunable intense resonances in the visible spectral range. Our results support the potential of DNA for optoelectronics and biosensing applications.
2021-07-09T09:51:19ZSaavedra, J. R. M.García de Abajo, Francisco JavierDexoyribonucleic acid (DNA) has recently been identified as a promising material for nanotechnology due to its unique mechanical, electrical, and optical properties. However, optical applications are severely hampered by the featureless response of neutral DNA at visible frequencies. Additionally, predictive simulations are computationally too demanding to cope with large DNA strands. Here, we develop a computationally efficient procedure to simulate the optical response of large DNA molecules and reveal the emergence of electrically tunable intense resonances in the visible spectral range. Our results support the potential of DNA for optoelectronics and biosensing applications.Single-Plasmon Thermo-Optical Switching in Graphene
http://hdl.handle.net/2117/348864
Single-Plasmon Thermo-Optical Switching in Graphene
Cox, Joel D.; García de Abajo, Francisco Javier
While plasmons in noble metal nanostructures enable strong light-matter interactions on commensurate length scales, the overabundance of free electrons in these systems inhibits their tunability by weak external stimuli. Countering this limitation, the linear electronic dispersion in graphene endows the two-dimensional material with both an enhanced sensitivity to doping electron density, enabling active tunability of its highly confined plasmon resonances, and a very low electronic heat capacity that renders its thermo-optical response extraordinarily large. Here we show that these properties combined enables a substantial optical modulation in graphene nanostructures from the energy associated with just one of their supported plasmons. We base our analysis on realistic, complementary classical and quantum-mechanical simulations, which reveal that the energy of a single plasmon, absorbed in a small, moderately doped graphene nanoisland, can sufficiently modify its electronic temperature and chemical potential to produce unity-order modulation of the optical response within subpicosecond time scales, effectively shifting or damping the original plasmon absorption peak and thereby blockading subsequent excitation of a second plasmon. The proposed thermo-optical single-plasmon blockade consists in a viable ultralow power all-optical switching mechanism for doped graphene nanoislands, while their combination with quantum emitters could yield applications in biological sensing and quantum nano-optics.
2021-07-09T09:03:51ZCox, Joel D.García de Abajo, Francisco JavierWhile plasmons in noble metal nanostructures enable strong light-matter interactions on commensurate length scales, the overabundance of free electrons in these systems inhibits their tunability by weak external stimuli. Countering this limitation, the linear electronic dispersion in graphene endows the two-dimensional material with both an enhanced sensitivity to doping electron density, enabling active tunability of its highly confined plasmon resonances, and a very low electronic heat capacity that renders its thermo-optical response extraordinarily large. Here we show that these properties combined enables a substantial optical modulation in graphene nanostructures from the energy associated with just one of their supported plasmons. We base our analysis on realistic, complementary classical and quantum-mechanical simulations, which reveal that the energy of a single plasmon, absorbed in a small, moderately doped graphene nanoisland, can sufficiently modify its electronic temperature and chemical potential to produce unity-order modulation of the optical response within subpicosecond time scales, effectively shifting or damping the original plasmon absorption peak and thereby blockading subsequent excitation of a second plasmon. The proposed thermo-optical single-plasmon blockade consists in a viable ultralow power all-optical switching mechanism for doped graphene nanoislands, while their combination with quantum emitters could yield applications in biological sensing and quantum nano-optics.Nonlinear Graphene Nanoplasmonics
http://hdl.handle.net/2117/348861
Nonlinear Graphene Nanoplasmonics
Cox, Joel D.; García de Abajo, Francisco Javier
Nonlinear optics is limited by the weak nonlinear response of available materials, a problem that is generally circumvented by relying on macroscopic structures in which light propagates over many optical cycles, thus giving rise to accumulated unity-order nonlinear effects. While this strategy cannot be extended to subwavelength optics, such as in nanophotonic structures, one can alternatively use localized optical resonances with high quality factors to increase light–matter interaction times, although this approach is limited by inelastic losses partly associated with the nonlinear response. Plasmons—the collective oscillations of electrons in conducting media—offer the means to concentrate light into nanometric volumes, well below the light-wavelength-scale limit imposed by diffraction, amplifying the electromagnetic fields upon which nonlinear optical phenomena depend. Due to their abundant supply of free electrons, noble metals are the traditional material platform for plasmonics and have thus dominated research in nanophotonics over the past several decades, despite exhibiting large ohmic losses and inherent difficulties to actively modulate plasmon resonances, which are primarily determined by size, composition, and morphology.
Highly doped graphene has recently emerged as an appealing platform for plasmonics due to its unique optoelectronic properties, which give rise to relatively long-lived, highly confined, and actively tunable plasmon resonances that mainly appear in the infrared and terahertz frequency regimes. Efforts to extend graphene plasmonics to the near-infrared and visible ranges involve patterning of graphene into nanostructured elements, thus facilitating the optical excitation of localized resonances that can be blue-shifted through geometrical confinement while maintaining electrical tunability. Besides these appealing plasmonic attributes, the conical electronic dispersion relation of graphene renders its charge carrier motion in response to light intrinsically anharmonic, resulting in a comparatively intense nonlinear optical response. The combined synergy of extreme plasmonic field enhancement and large intrinsic optical nonlinearity are now motivating intensive research efforts in nonlinear graphene plasmonics, the recent progress of which we discuss in this Account. We start with a description of the appealing properties of plasmons in graphene nanostructures down to molecular sizes, followed by a discussion of the unprecedented level of intrinsic optical nonlinearity in graphene, its enhancement by resonant coupling to its highly confined plasmons to yield intense high harmonic generation and Kerr nonlinearities, the extraordinary thermo-optical capabilities of this material enabling large nonlinear optical switching down to the single-photon level, and its strong interaction with quantum emitters.
2021-07-09T08:44:35ZCox, Joel D.García de Abajo, Francisco JavierNonlinear optics is limited by the weak nonlinear response of available materials, a problem that is generally circumvented by relying on macroscopic structures in which light propagates over many optical cycles, thus giving rise to accumulated unity-order nonlinear effects. While this strategy cannot be extended to subwavelength optics, such as in nanophotonic structures, one can alternatively use localized optical resonances with high quality factors to increase light–matter interaction times, although this approach is limited by inelastic losses partly associated with the nonlinear response. Plasmons—the collective oscillations of electrons in conducting media—offer the means to concentrate light into nanometric volumes, well below the light-wavelength-scale limit imposed by diffraction, amplifying the electromagnetic fields upon which nonlinear optical phenomena depend. Due to their abundant supply of free electrons, noble metals are the traditional material platform for plasmonics and have thus dominated research in nanophotonics over the past several decades, despite exhibiting large ohmic losses and inherent difficulties to actively modulate plasmon resonances, which are primarily determined by size, composition, and morphology.
Highly doped graphene has recently emerged as an appealing platform for plasmonics due to its unique optoelectronic properties, which give rise to relatively long-lived, highly confined, and actively tunable plasmon resonances that mainly appear in the infrared and terahertz frequency regimes. Efforts to extend graphene plasmonics to the near-infrared and visible ranges involve patterning of graphene into nanostructured elements, thus facilitating the optical excitation of localized resonances that can be blue-shifted through geometrical confinement while maintaining electrical tunability. Besides these appealing plasmonic attributes, the conical electronic dispersion relation of graphene renders its charge carrier motion in response to light intrinsically anharmonic, resulting in a comparatively intense nonlinear optical response. The combined synergy of extreme plasmonic field enhancement and large intrinsic optical nonlinearity are now motivating intensive research efforts in nonlinear graphene plasmonics, the recent progress of which we discuss in this Account. We start with a description of the appealing properties of plasmons in graphene nanostructures down to molecular sizes, followed by a discussion of the unprecedented level of intrinsic optical nonlinearity in graphene, its enhancement by resonant coupling to its highly confined plasmons to yield intense high harmonic generation and Kerr nonlinearities, the extraordinary thermo-optical capabilities of this material enabling large nonlinear optical switching down to the single-photon level, and its strong interaction with quantum emitters.Nonlinear Interactions between Free Electrons and Nanographenes
http://hdl.handle.net/2117/348822
Nonlinear Interactions between Free Electrons and Nanographenes
Cox, Joel D.; García de Abajo, Francisco Javier
Free electrons act as a source of highly confined, spectrally broad optical fields that are widely used to map photonic modes with nanometer/millielectronvolt space/energy resolution through currently available electron energy-loss and cathodoluminescence spectroscopies. These techniques are understood as probes of the linear optical response, while nonlinear dynamics has escaped observation with similar degree of spatial detail, despite the strong enhancement of the electron evanescent field with decreasing electron energy. Here, we show that the field accompanying low-energy electrons can trigger anharmonic response in strongly nonlinear materials. Specifically, through realistic quantum-mechanical simulations, we find that the interaction between ≲100 eV electrons and plasmons in graphene nanostructures gives rise to substantial optical nonlinearities that are discernible as saturation and spectral shifts in the plasmonic features revealed in the cathodoluminescence emission and electron energy-loss spectra. Our results support the use of low-energy electron-beam spectroscopies for the exploration of nonlinear optical processes in nanostructures.
2021-07-08T15:24:23ZCox, Joel D.García de Abajo, Francisco JavierFree electrons act as a source of highly confined, spectrally broad optical fields that are widely used to map photonic modes with nanometer/millielectronvolt space/energy resolution through currently available electron energy-loss and cathodoluminescence spectroscopies. These techniques are understood as probes of the linear optical response, while nonlinear dynamics has escaped observation with similar degree of spatial detail, despite the strong enhancement of the electron evanescent field with decreasing electron energy. Here, we show that the field accompanying low-energy electrons can trigger anharmonic response in strongly nonlinear materials. Specifically, through realistic quantum-mechanical simulations, we find that the interaction between ≲100 eV electrons and plasmons in graphene nanostructures gives rise to substantial optical nonlinearities that are discernible as saturation and spectral shifts in the plasmonic features revealed in the cathodoluminescence emission and electron energy-loss spectra. Our results support the use of low-energy electron-beam spectroscopies for the exploration of nonlinear optical processes in nanostructures.Enhancement of nonlinear optical phenomena by localized resonances
http://hdl.handle.net/2117/121513
Enhancement of nonlinear optical phenomena by localized resonances
Rodríguez Echarri, A.; Cox, Joel D.; Yu, Renwen; García de Abajo, Francisco Javier
Nonlinear optics at the nanoscale is severely limited by the small departure of available materials from linear behavior. Despite intense efforts placed into overcoming this problem using multiple strategies for enhancing the near-field light intensity, all-optical active nanodevices remain a challenge. Here we introduce a material-independent scheme for quantifying the enhancement of the nonlinear response in nanostructures assisted by proximal metallic or dielectric nanoresonators. The proposed figures of merit, which we apply to configurations of current interest incorporating 2D materials and dielectric cavities, can be generally used to optimize nonlinear nanoscale elements.
2018-09-26T10:06:49ZRodríguez Echarri, A.Cox, Joel D.Yu, RenwenGarcía de Abajo, Francisco JavierNonlinear optics at the nanoscale is severely limited by the small departure of available materials from linear behavior. Despite intense efforts placed into overcoming this problem using multiple strategies for enhancing the near-field light intensity, all-optical active nanodevices remain a challenge. Here we introduce a material-independent scheme for quantifying the enhancement of the nonlinear response in nanostructures assisted by proximal metallic or dielectric nanoresonators. The proposed figures of merit, which we apply to configurations of current interest incorporating 2D materials and dielectric cavities, can be generally used to optimize nonlinear nanoscale elements.Enhanced graphene nonlinear response through geometrical plasmon focusing
http://hdl.handle.net/2117/116981
Enhanced graphene nonlinear response through geometrical plasmon focusing
Saavedra, J. R. M.; García de Abajo, Francisco Javier
We propose a simple approach to couple light into graphene plasmons and focus these excitations at
focal spots of a size determined by the plasmon wavelength, thus producing high optical field
enhancement that boosts the nonlinear response of the material. More precisely, we consider a
graphene structure in which incident light is coupled to its plasmons at the carbon edges and
subsequently focused on a spot of size comparable to the plasmon wavelength. We observe large
confinement of graphene plasmons, materializing in small, intense focal spots, in which the
extraordinary nonlinear response of this material leads to relatively intense harmonic generation.
This result shows the potential of plasmon focusing in suitably edged graphene structures to produce
large field confinement and nonlinear response without involving elaborated nanostructuring.
2018-05-07T13:28:26ZSaavedra, J. R. M.García de Abajo, Francisco JavierWe propose a simple approach to couple light into graphene plasmons and focus these excitations at
focal spots of a size determined by the plasmon wavelength, thus producing high optical field
enhancement that boosts the nonlinear response of the material. More precisely, we consider a
graphene structure in which incident light is coupled to its plasmons at the carbon edges and
subsequently focused on a spot of size comparable to the plasmon wavelength. We observe large
confinement of graphene plasmons, materializing in small, intense focal spots, in which the
extraordinary nonlinear response of this material leads to relatively intense harmonic generation.
This result shows the potential of plasmon focusing in suitably edged graphene structures to produce
large field confinement and nonlinear response without involving elaborated nanostructuring.Transient nonlinear plasmonics in nanostructured graphene
http://hdl.handle.net/2117/116964
Transient nonlinear plasmonics in nanostructured graphene
Cox, Joel D.; García de Abajo, Francisco Javier
Plasmons in highly doped graphene offer the means to dramatically enhance light absorption in the atomically thin
material. Ultimately the absorbed light energy induces an increase in electron temperature, accompanied by large
shifts in the chemical potential. This intrinsically incoherent effect leads to strong intensity-dependent modifications
of the optical response, complementing the remarkable coherent nonlinearities arising in graphene due to interband
transitions and anharmonic intraband electron motion. Through rigorous time-domain quantum-mechanical simulations
of graphene nanoribbons, we show that the incoherent mechanism dominates over the coherent response for
the high levels of intensity required to trigger nonperturbative optical phenomena such as saturable absorption. We
anticipate that these findings will elucidate the role of coherent and incoherent nonlinearities for future studies and
applications of plasmon-assisted nonlinear optics.
2018-05-07T08:55:39ZCox, Joel D.García de Abajo, Francisco JavierPlasmons in highly doped graphene offer the means to dramatically enhance light absorption in the atomically thin
material. Ultimately the absorbed light energy induces an increase in electron temperature, accompanied by large
shifts in the chemical potential. This intrinsically incoherent effect leads to strong intensity-dependent modifications
of the optical response, complementing the remarkable coherent nonlinearities arising in graphene due to interband
transitions and anharmonic intraband electron motion. Through rigorous time-domain quantum-mechanical simulations
of graphene nanoribbons, we show that the incoherent mechanism dominates over the coherent response for
the high levels of intensity required to trigger nonperturbative optical phenomena such as saturable absorption. We
anticipate that these findings will elucidate the role of coherent and incoherent nonlinearities for future studies and
applications of plasmon-assisted nonlinear optics.Electron refraction at lateral atomic interfaces
http://hdl.handle.net/2117/115222
Electron refraction at lateral atomic interfaces
Abd El-Fattah, Z. M.; Kher-Elden, M A.; Yassin, O.; El-Okr, M. M.; Ortega, J. E.; García de Abajo, Francisco Javier
We present theoretical simulations of electron refraction at the lateral atomic interface between a
“homogeneous” Cu(111) surface and the “nanostructured” one-monolayer (ML) Ag/Cu(111) dislocation
lattice. Calculations are performed for electron binding energies barely below the 1 ML Ag/
Cu(111) M-point gap (binding energy EB ¼53 meV, below the Fermi level) and slightly above its
C
-point energy (EB ¼160 meV), both characterized by isotropic/circular constant energy surfaces.
Using plane-wave-expansion and boundary-element methods, we show that electron refraction
occurs at the interface, the Snell law is obeyed, and a total internal reflection occurs beyond the
critical angle. Additionally, a weak negative refraction is observed for EB ¼53 meV electron
energy at beam incidence higher than the critical angle. Such an interesting observation stems from
the interface phase-matching and momentum conservation with the umklapp bands at the second
Brillouin zone of the dislocation lattice. The present analysis is not restricted to our Cu-Ag/Cu
model system but can be readily extended to technologically relevant interfaces with spinpolarized,
highly featured, and anisotropic constant energy contours, such as those characteristic
for Rashba systems and topological insulators. Published by AIP Publishing.
2018-03-15T14:58:26ZAbd El-Fattah, Z. M.Kher-Elden, M A.Yassin, O.El-Okr, M. M.Ortega, J. E.García de Abajo, Francisco JavierWe present theoretical simulations of electron refraction at the lateral atomic interface between a
“homogeneous” Cu(111) surface and the “nanostructured” one-monolayer (ML) Ag/Cu(111) dislocation
lattice. Calculations are performed for electron binding energies barely below the 1 ML Ag/
Cu(111) M-point gap (binding energy EB ¼53 meV, below the Fermi level) and slightly above its
C
-point energy (EB ¼160 meV), both characterized by isotropic/circular constant energy surfaces.
Using plane-wave-expansion and boundary-element methods, we show that electron refraction
occurs at the interface, the Snell law is obeyed, and a total internal reflection occurs beyond the
critical angle. Additionally, a weak negative refraction is observed for EB ¼53 meV electron
energy at beam incidence higher than the critical angle. Such an interesting observation stems from
the interface phase-matching and momentum conservation with the umklapp bands at the second
Brillouin zone of the dislocation lattice. The present analysis is not restricted to our Cu-Ag/Cu
model system but can be readily extended to technologically relevant interfaces with spinpolarized,
highly featured, and anisotropic constant energy contours, such as those characteristic
for Rashba systems and topological insulators. Published by AIP Publishing.