Nanophotonics theory
http://hdl.handle.net/2117/23798
2019-08-17T13:28:01ZEnhancement 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, F. 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, F. 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, F. 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, F. 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, F. 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, F. 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, F. J.
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, F. J.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.Ultrafast nonlinear optical response of Dirac fermions in graphene
http://hdl.handle.net/2117/115206
Ultrafast nonlinear optical response of Dirac fermions in graphene
Baudisch, Matthias; Marini, Andrea; Cox, Joel D.; Zhu, Tony; Silva, Francisco; Teichmann, Stephan; Massicotte, Mathieu; Koppens, Frank; Levitov, Leonid S.; García de Abajo, F. Javier; Biegert, Jens
The speed of solid-state electronic devices, determined by the temporal dynamics of charge
carriers, could potentially reach unprecedented petahertz frequencies through direct
manipulation by optical fields, consisting in a million-fold increase from state-of-the-art
technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong
coupling to optical fields, from which stems an important back-action of photoexcited carriers.
Here we investigate the instantaneous response of graphene to ultrafast optical fields,
elucidating the role of hot carriers on sub-100 fs timescales. The measured nonlinear
response and its dependence on interaction time and field polarization reveal the back-action
of hot carriers over timescales 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.
2018-03-15T09:57:25ZBaudisch, MatthiasMarini, AndreaCox, Joel D.Zhu, TonySilva, FranciscoTeichmann, StephanMassicotte, MathieuKoppens, FrankLevitov, Leonid S.García de Abajo, F. JavierBiegert, JensThe speed of solid-state electronic devices, determined by the temporal dynamics of charge
carriers, could potentially reach unprecedented petahertz frequencies through direct
manipulation by optical fields, consisting in a million-fold increase from state-of-the-art
technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong
coupling to optical fields, from which stems an important back-action of photoexcited carriers.
Here we investigate the instantaneous response of graphene to ultrafast optical fields,
elucidating the role of hot carriers on sub-100 fs timescales. The measured nonlinear
response and its dependence on interaction time and field polarization reveal the back-action
of hot carriers over timescales 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.Optimization of Nanoparticle-Based SERS Substrates through Large-Scale Realistic Simulations
http://hdl.handle.net/2117/103861
Optimization of Nanoparticle-Based SERS Substrates through Large-Scale Realistic Simulations
Solís, Diego M.; Taboada, José M,; Obelleiro, Fernando; Liz-Marzán, Luis M.; García de Abajo, Francisco Javier
Surface-enhanced Raman scattering (SERS) has
become a widely used spectroscopic technique for chemical
identification, providing unbeaten sensitivity down to the singlemolecule
level. The amplification of the optical near field
produced by collective electron excitations plasmons in
nanostructured metal surfaces gives rise to a dramatic increase
by many orders of magnitude in the Raman scattering intensities
from neighboring molecules. This effect strongly depends on
the detailed geometry and composition of the plasmonsupporting
metallic structures. However, the search for
optimized SERS substrates has largely relied on empirical
data, due in part to the complexity of the structures, whose
simulation becomes prohibitively demanding. In this work, we
use state-of-the-art electromagnetic computation techniques to
produce predictive simulations for a wide range of nanoparticle-based SERS substrates, including realistic configurations
consisting of random arrangements of hundreds of nanoparticles with various morphologies. This allows us to derive rules of
thumb for the influence of particle anisotropy and substrate coverage on the obtained SERS enhancement and optimum spectral
ranges of operation. Our results provide a solid background to understand and design optimized SERS substrates.
2017-04-28T14:45:56ZSolís, Diego M.Taboada, José M,Obelleiro, FernandoLiz-Marzán, Luis M.García de Abajo, Francisco JavierSurface-enhanced Raman scattering (SERS) has
become a widely used spectroscopic technique for chemical
identification, providing unbeaten sensitivity down to the singlemolecule
level. The amplification of the optical near field
produced by collective electron excitations plasmons in
nanostructured metal surfaces gives rise to a dramatic increase
by many orders of magnitude in the Raman scattering intensities
from neighboring molecules. This effect strongly depends on
the detailed geometry and composition of the plasmonsupporting
metallic structures. However, the search for
optimized SERS substrates has largely relied on empirical
data, due in part to the complexity of the structures, whose
simulation becomes prohibitively demanding. In this work, we
use state-of-the-art electromagnetic computation techniques to
produce predictive simulations for a wide range of nanoparticle-based SERS substrates, including realistic configurations
consisting of random arrangements of hundreds of nanoparticles with various morphologies. This allows us to derive rules of
thumb for the influence of particle anisotropy and substrate coverage on the obtained SERS enhancement and optimum spectral
ranges of operation. Our results provide a solid background to understand and design optimized SERS substrates.Nonlinear Plasmonic Sensing with Nanographene
http://hdl.handle.net/2117/101765
Nonlinear Plasmonic Sensing with Nanographene
Yu, Renwen; Cox, Joel D.; García de Abajo, Francisco Javier
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, 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.
2017-03-01T09:46:12ZYu, RenwenCox, Joel D.García de Abajo, Francisco JavierPlasmons 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, 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.Plasmons in doped finite carbon nanotubes and their interactions with fast electrons and quantum emitters
http://hdl.handle.net/2117/90950
Plasmons in doped finite carbon nanotubes and their interactions with fast electrons and quantum emitters
Vega, Sandra de; Cox, Joel D.; García de Abajo, Javier
We study the potential of highly doped finite carbon nanotubes to serve as plasmonic elements that mediate the interaction between quantum emitters. Similar to graphene, nanotubes support intense plasmons that can be modulated by varying their level of electrical doping. These excitations exhibit large interaction with light and electron beams, as revealed upon examination of the corresponding light extinction cross-section and electron energy-loss spectra. We show that quantum emitters experience record-high Purcell factors, while they undergo strong mutual interaction mediated by their coupling to the tube plasmons. Our results show the potential of doped finite nanotubes as tunable plasmonic materials for quantum optics applications
2016-10-21T11:28:58ZVega, Sandra deCox, Joel D.García de Abajo, JavierWe study the potential of highly doped finite carbon nanotubes to serve as plasmonic elements that mediate the interaction between quantum emitters. Similar to graphene, nanotubes support intense plasmons that can be modulated by varying their level of electrical doping. These excitations exhibit large interaction with light and electron beams, as revealed upon examination of the corresponding light extinction cross-section and electron energy-loss spectra. We show that quantum emitters experience record-high Purcell factors, while they undergo strong mutual interaction mediated by their coupling to the tube plasmons. Our results show the potential of doped finite nanotubes as tunable plasmonic materials for quantum optics applicationsSmith-Purcell radiation emission in aperiodic arrays
http://hdl.handle.net/2117/90932
Smith-Purcell radiation emission in aperiodic arrays
Saavedra, J. R. M.; Castells-Graells, D.; García de Abajo, F. Javier
We study the Smith-Purcell light emission produced by electrons moving parallel to linear aperiodic particle
arrays. This constitutes a generalization of this type of phenomenon from periodic to aperiodic structures. As in the
periodic case, the emission is found to exhibit intense features in its angular and frequency distributions, associated
with the condition of constructive interference between the contributions arising from different particles in the
array. This condition can also be expressed in terms of momentum conservation involving reciprocal wave-vector
transfers from the array. We consider two examples of quasiperiodic and hyperuniform aperiodic arrays that
allow us to illustrate this idea. Our study provides insight into the interaction of fast electrons with aperiodic
arrays characterized by strong features in reciprocal space, which dominate the electron-array coupling.
2016-10-20T15:26:06ZSaavedra, J. R. M.Castells-Graells, D.García de Abajo, F. JavierWe study the Smith-Purcell light emission produced by electrons moving parallel to linear aperiodic particle
arrays. This constitutes a generalization of this type of phenomenon from periodic to aperiodic structures. As in the
periodic case, the emission is found to exhibit intense features in its angular and frequency distributions, associated
with the condition of constructive interference between the contributions arising from different particles in the
array. This condition can also be expressed in terms of momentum conservation involving reciprocal wave-vector
transfers from the array. We consider two examples of quasiperiodic and hyperuniform aperiodic arrays that
allow us to illustrate this idea. Our study provides insight into the interaction of fast electrons with aperiodic
arrays characterized by strong features in reciprocal space, which dominate the electron-array coupling.Propagation and localization of quantum dot emission along a gap-plasmonic transmission line
http://hdl.handle.net/2117/79294
Propagation and localization of quantum dot emission along a gap-plasmonic transmission line
Castro-Lopez, M.; Manjavacas, A.; García de Abajo, Francisco Javier; Hulst, Niek F. van
Plasmonic transmission lines have great potential to serve
as direct interconnects between nanoscale light spots. The guiding of gap
plasmons in the slot between adjacent nanowire pairs provides improved
propagation of surface plasmon polaritons while keeping strong light con-
finement. Yet propagation is fundamentally limited by losses in the metal.
Here we show a workaround operation of the gap-plasmon transmission
line, exploiting both gap and external modes present in the structure.
Interference between these modes allows us to take advantage of the
larger propagation distance of the external mode while preserving the high
confinement of the gap mode, resulting in nanoscale confinement of the
optical field over a longer distance. The performance of the gap-plasmon
transmission line is probed experimentally by recording the propagation of
quantum dots luminescence over distances of more than 4 µm. We observe
a 35% increase in the effective propagation length of this multimode system
compared to the theoretical limit for a pure gap mode. The applicability of
this simple method to nanofabricated structures is theoretically confirmed
and offers a realistic way to combine longer propagation distances with
lateral plasmon confinement for far field nanoscale interconnects.
2015-11-16T11:23:15ZCastro-Lopez, M.Manjavacas, A.García de Abajo, Francisco JavierHulst, Niek F. vanPlasmonic transmission lines have great potential to serve
as direct interconnects between nanoscale light spots. The guiding of gap
plasmons in the slot between adjacent nanowire pairs provides improved
propagation of surface plasmon polaritons while keeping strong light con-
finement. Yet propagation is fundamentally limited by losses in the metal.
Here we show a workaround operation of the gap-plasmon transmission
line, exploiting both gap and external modes present in the structure.
Interference between these modes allows us to take advantage of the
larger propagation distance of the external mode while preserving the high
confinement of the gap mode, resulting in nanoscale confinement of the
optical field over a longer distance. The performance of the gap-plasmon
transmission line is probed experimentally by recording the propagation of
quantum dots luminescence over distances of more than 4 µm. We observe
a 35% increase in the effective propagation length of this multimode system
compared to the theoretical limit for a pure gap mode. The applicability of
this simple method to nanofabricated structures is theoretically confirmed
and offers a realistic way to combine longer propagation distances with
lateral plasmon confinement for far field nanoscale interconnects.