The MultiPhoton NanoPhotonics research group focuses on the interaction between powerful laser light and nanostructured materials. In January 2019, we "made history" by demonstrating a new physical effect, that had eluded scientists for 40 years. The effect had been referred to as an impossible theory.
Our main expertise is in building laser experiments for studying novel materials, such as plasmonic nanostructures, metamaterials, 2D materials and quantum optical materials. We aim to discover new properties and to test theoretical predictions. Our focus is on the physics of photons, electrons and magnetism confined to tiny volumes of space – nanoparticles or 2D sheets. We seek out new and useful intersections between classical electromagnetism and quantum mechanics. Our investigations are both fundamental and applied, with potential benefits for the pharmaceutical, food, perfume, and agrochemical industries.
In both research and teaching projects, our work often takes a distinct science fiction aspect, as we build laser-powered nano-photonic steam engines (Steampunk Science) or program humanoid robots to assist us.
Abstract: Chiroptical spectroscopies provide structural analyses of molecules and nanoparticles but they require sample volumes that are incompatible with generating large chemical libraries. New optical tools are needed to characterize chirality for the ultrasmall (<1 µl) volumes required in the high-throughput synthetic and analytical stations for chiral compounds. Here we show experimentally a novel photonic effect that enables such capabilities—third-harmonic Mie scattering optical activity—observed from suspensions of CdTe nanostructured helices in volumes <<1 µl. Third-harmonic Mie scattering was recorded on illuminating CdTe helices with 1,065, 1,095 and 1,125 nm laser beams and the intensity was around ten-times higher in the forward direction than sideways. The third-harmonic ellipticity was as high as 3° and we attribute this effect to the interference of chiral and achiral effective nonlinear susceptibility tensor components. Third-harmonic Mie scattering on semiconductor helices opens a path for rapid high-throughput chiroptical characterization of sample volumes as small as 10-5 µl.
Optical activity in third-harmonic Rayleigh scattering: a new route for measuring chirality
Abstract: In 3D isotropic liquids, optical third-harmonic generation is forbidden, with circularly polarized light (CPL). Yet the associated nonlinear susceptibility directly influences the optical properties at the fundamental frequency by intensity dependence (Kerr effect). Here, we reveal the hidden third-harmonic optical properties upon CPL illumination by demonstrating a new effect, in hyper-Rayleigh scattering. This effect is succinctly enunciated: the intensity of light scattered at the third-harmonic frequency of the CPL incident light depends on the chirality of the scatterers. It is referred to as third-harmonic (hyper) Rayleigh scattering optical activity (THRS OA) and we first observed it from Ag nanohelices randomly dispersed in water. We also provide the first analytical theory model for the new effect in nanohelices, highlighting the role of localized transition dipoles along the helical length. Whereas nonlinear optics is usually experimentally challenging, THRS OA is remarkably user-friendly. It offers access to intricate optical properties (hyperpolarizabilities) that have so far been more easily accessible by computation and that are essential for our understanding of light-matter interactions. The new effect could find applications in hyper-sensitive characterization of the chirality in molecules and in nanostructures; this chirality plays a fundamental role in the function of bio/nano-machinery, with promising applications in next generation technologies.
Doubly-resonant enhancement of second harmonic generation from a WS2 nanomesh polymorph with a modified energy landscape
Abstract: Although transition metal dichalcogenides (TMDs, such as WS2, WSe2, MoS2, MoSe2) have emerged as highly promising two-dimensional (2D) materials for nonlinear optical applications, they are limited by intrinsically small light-matter interaction length and (typically) flat-lying geometries. Here, we present the first hyperspectral multiphoton analysis of a tridimensional webbed network of densely-packed, randomly distributed stacks (1-5 layers in the stack) containing twisted and/or fused 2D nanosheets of WS2 – referred to as “nanomesh”. We map the optical second harmonic generation (SHG) across the three characteristic spectral features (A, B and C) and we establish the two-photon luminescence and third harmonic generation signatures. We reveal that the nanomesh presents very different and enhanced multiphoton spectral signatures from those of flat-lying WS2 multilayers. We pinpoint the origin of these differences to hotspots whose location changes depending on the wavelength of illumination. We attribute the main SHG enhancements to double resonances due to a modified energy landscape by the presence of defects (such as vacancies and their passivated variants, or grain boundaries) that induce intra-bandgap energy levels. The SHG hotspots determine the overall SHG intensity and its spectral bandwidth, both of which are crucial parameters for applications. As a result, the nanomesh is a highly efficient (≈ 9.05×103 times the SHG from standard z-cut quartz at 875 nm), broadband (ranging from 850 to 1100 nm) nanomaterial that is robust against degradation, scalable and ultra-thin (compared to traditional SHG materials). This nanomesh is a prime candidate for integration into quantum optical technologies, such as devices on a chip, thereby enabling unprecedented miniaturization for these technologies.
Single Nanoparticle Chiroptics in a Liquid: Optical Activity in Hyper-Rayleigh Scattering from Au Helicoids
Abstract: Linear optical methods of determining the chirality of organic and inorganic materials have relied on weak chiral optical (chiroptical) effects. Nonlinear chiroptical characterization holds the potential of much greater sensitivity and smaller interaction volumes. However, suitable materials on which to perform measurements have been lacking for decades. Here, we present the first nonlinear chiroptical characterization of crystallographic chirality in gold helicoids (≈150 nm size) and core/shell helicoids with the newly discovered hyper-Rayleigh scattering optical activity (HRS OA) technique. The observed chiroptical signal is, on average, originating from between ≈0.05 and ≈0.13 helicoids, i.e., less than a single nanoparticle. The measured HRS OA ellipticities reach ≈3°, for a concentration ≈109 times smaller than that of chiral molecules with similar nonlinear chiroptical response. These huge values indicate that the helicoids are excellent candidates for future nonlinear chiroptical materials and applications.
'Hot' in plasmonics: temperature-related concepts and applications of metal nanostructures
Abstract: This review covers recent advances in nonlinear optics, hot electrons for renewable energy (e.g. solar cells and water-splitting), acousto-optics, nano-metalworking, nanorobotics, steam generation and photothermal cancer therapy. In all these areas, the key enabling property is the ability of metallic nanoparticles to harvest and control light at the sub-wavelength scale, by supporting coherent electronic oscillations, called localized surface plasmon resonances (LSPR). Various physical properties and potential areas of application emerge depending on the decay mechanism of the LSPR and, especially, depending on the considered timescale. The field of plasmonics has mainly been associated with manipulating electromagnetic near-fields at the nanoscale, where absorption is an obstacle. However, plasmonic absorption leads to a stream of temperature-related phenomena that have only recently attracted significant attention. The goal of this review is to highlight exciting new areas of research (such as nanorobotics, nano-metalworking or acousto-optical techniques) and to survey the most recent progress in more established areas (such as hot electrons, photothermal therapy and plasmonic steam generation). To set each research area in context, the text is organized around the thermal cycle of the nanoparticles.
Measuring optical activity in the far-field from a racemic nanomaterial: diffraction spectroscopy from plasmonic nanogratings
Abstract: Recent progress in nanofabrication has redrawn the boundaries of the applicability of chiroptical (chiral optical) effects. Chirality, often expressed as a twist in biomolecules, is crucial for pharmaceuticals, where it can result in extremely different chemical properties. Because chiroptical effects are typically very weak in molecules, plasmonic nanomaterials are often proposed as a promising platform to significantly enhance these effects. Unfortunately, the ideal plasmonic nanomaterial has conflicting requirements: its chirality should enhance that of the chiral molecules and yet it should have no chiroptical response on its own. Here, we propose a unique reconciliation to satisfy the requirements: a racemic plasmonic nanomaterial, consisting of equal amounts of opposite chiral unit cells. We show how diffraction spectroscopy can be used to unveil the presence of chirality in such racemic nanogratings in the far-field. Our experiments are supported by numerical simulations and yield a circular intensity difference of up to 15%. The physical origin is demonstrated by full wave simulations in combination with a Green's function – group-theory-based analysis. Contributions from Circular Dichroism in the Angular Distribution of Photoelectrons (CDAD) and pseudo/extrinsic chirality are ruled out. Our findings enable the far-field measurement and tuning of racemic nanomaterials, which is crucial for hyper-sensitive chiral molecular characterization.
Atomic dispensers for thermoplasmonic control of alkali vapor pressure in quantum optical applications
K. R. Rusimova, D. Slavov, F. Pradaux-Caggiano, J. T. Collins, S. N. Gordeev, D. R. Carbery, W. J. Wadsworth, P. J. Mosley, V. K. Valev Nat. Commun. 10, 2328 (2019)
Abstract: Alkali metal vapors enable access to single electron systems, suitable for demonstrating fundamental light-matter interactions and promising for quantum logic operations, storage and sensing. However, progress is hampered by the need for robust and repeatable control over the atomic vapor density and over the associated optical depth. Until now, a moderate improvement of the optical depth was attainable through bulk heating or laser desorption – both time-consuming techniques. Here, we use plasmonic nanoparticles to convert light into localized thermal energy and to achieve optical depths in warm vapors, corresponding to a ~16 times increase in vapor pressure in less than 20 ms, with possible reload times much shorter than an hour. Our results enable robust and compact light-matter devices, such as efficient quantum memories and photon-photon logic gates, in which strong optical non-linearities are crucial.
“Hot Edges” in Inverse Opal Structure Enable Efficient CO2 Electrochemical Reduction and Sensitive in-situ Raman Characterization
Abstract: Conversion of CO2 into fuels and chemicals via electroreduction has attracted significant interest. Via mesostructure design to tune the electric field distribution in the electrode, it is demonstrated that Cu-In alloy with inverse opal (CI-1-IO) structure provides efficient electrochemical CO2 reduction and allows for sensitive detection of the CO2 reduction intermediates via surface-enhanced Raman scattering. The significant enhancement of Raman signal of the intermediates on CI-1-IO surface can be attributed to electric field enhancement on the “hot edges” of the inverse opal structure. Additionally, a highest CO2 reduction Faradaic efficiency (FE) of 92% (sum of formate and CO) is achieved at -0.6 V vs. RHE on CI-1-IO electrode. The DRIFTS results show that the Cu-In alloy with inverse opal have a faster adsorption kinetic and higher adsorption capacity of CO2. The “hot edges” of the bowl-like structure concentrate electric fields, due to the high curvature and also concentrate K+ on the active sites, which can lower the energy barrier of the CO2 reduction reaction. This research provides new insight into the design of materials for efficient CO2 conversion and the intermediates detection during CO2 reduction processes.
WS2 Nanotubes, 2D Nanomeshes, and 2D In-Plane Films through One Single Chemical Vapor Deposition Route
Abstract: We demonstrate a versatile, catalyst free chemical vapor deposition process on insulating substrates capable of producing in one single stream one-dimensional (1D) WO3–x suboxides leading to a wide range of substrate-supported 2H-WS2 polymorphs: a tunable class of out-of-plane (of the substrate) nanophases, with 1D nanotubes and a pure WS2, two-dimensional (2D) nanomesh (defined as a network of webbed, micron-size, few-layer 2D sheets) at its extremes; and in-plane (parallel to the substrate) mono- and few-layer 2D domains. This entails a two-stage approach in which the 2WO3 + 7S → 2WS2 + 3SO2 reaction is intentionally decoupled. First, various morphologies of nanowires or nanorods of high stoichiometry, WO2.92/WO2.9 suboxides (belonging to the class of Magnéli phases) were formed, followed by their sulfurization to undergo reduction to the aforementioned WS2 polymorphs. The continuous transition of WS2 from nanotubes to the out-of-plane 2D nanomesh, via intermediary, mixed 1D-2D phases, delivers tunable functional properties, for example, linear and nonlinear optical properties, such as reflectivity (linked to optical excitations in the material), and second harmonic generation (SHG) and onset of saturable absorption. The SHG effect is very strong across the entire tunable class of WS2 nanomaterials, weakest in nanotubes, and strongest in the 2D nanomesh. Furthermore, a mechanism via suboxide (WO3–x) intermediate as a possible path to 2D domain growth is demonstrated. 2D, in-plane WS2 domains grow via “self-seeding and feeding” where short WO2.92/WO2.9 nanorods provide both the nucleation sites and the precursor feedstock. Understanding the reaction path (here, in the W–O–S space) is an emerging approach toward controlling the nucleation, growth, and morphology of 2D domains and films of transition-metal dichalcogenides.
First Observation of Optical Activity in Hyper-Rayleigh Scattering
J.T. Collins, K.R. Rusimova, D.C. Hooper, H.-H. Jeong, L. Ohnoutek, F. Pradaux-Caggiano, T. Verbiest, D. R. Carbery, P. Fischer, and V. K. Valev Phys Rev. X 9, 011024 (2019)
Abstract: Chiral nano- or metamaterials and surfaces enable striking photonic properties, such as negative refractive index and superchiral light, driving promising applications in novel optical components, nanorobotics, and enhanced chiral molecular interactions with light. In characterizing chirality, although nonlinear chiroptical techniques are typically much more sensitive than their linear optical counterparts, separating true chirality from anisotropy is a major challenge. Here, we report the first observation of optical activity in second-harmonic hyper-Rayleigh scattering (HRS). We demonstrate the effect in a 3D isotropic suspension of Ag nanohelices in water. The effect is 5 orders of magnitude stronger than linear optical activity and is well pronounced above the multiphoton luminescence background. Because of its sensitivity, isotropic environment, and straightforward experimental geometry, HRS optical activity constitutes a fundamental experimental breakthrough in chiral photonics for media including nanomaterials, metamaterials, and chemical molecules.
BBC Radio Norfolk 06/02/2019, at 17:45.
Second harmonic spectroscopy of surface lattice resonances
Abstract: Because of their large figures of merit, surface lattice resonances (SLRs) in metal nanoparticle arrays are very promising for chemical and biomolecular sensing, in both liquid and gas media. SLRs are sensitive to refractive index changes both near the surface of the nanoparticles (surface sensitivity) and in the volume between them (bulk sensitivity). Due to its intrinsic surface-sensitivity and a power-law dependence on electric fields, second harmonic generation (SHG) spectroscopy can improve upon both the surface and volume sensitivities of SLRs. In this report on SHG spectroscopy of plasmonic nanoparticles, we show that the SHG signal is greatly increased (up to 450 times) by the SLRs. We also demonstrate very narrow resonances in SHG intensity (~5 nm FWHM). We illustrate how the SHG resonances are highly sensitive to SLRs by varying the fundamental wavelength, angle of incidence, nanoparticle material and lattice constant of the arrays. Finally, we identify an SHG resonance (10 nm FWHM) that is electric dipole forbidden and can be attributed to higher-order multipoles, enhanced by the strong near-fields of SLRs. Our results open up new and very promising avenues for chemical and biomolecular sensing, based on SHG spectroscopy of SLRs.
Enantiomorphing Chiral Plasmonic Nanostructures: A Counterintuitive Sign Reversal of the Nonlinear Circular Dichroism
J. T. Collins, X. Zheng, N. V. S. Braz, E. Slenders, S. Zu, G. A. E. Vandenbosch, V. V. Moshchalkov, Z. Fang, M. Ameloot, P. A. Warburton, V. K. Valev Adv. Opt. Mater. 6, 1800153 (2018)
Abstract: Plasmonic nanostructures have demonstrated a remarkable ability to control light in ways never observed in nature, as the optical response is closely linked to their flexible geometric design. Due to lack of mirror symmetry, chiral nanostructures allow twisted electric field "hotspots" to form at the material surface. These hotspots depend strongly on the optical wavelength and nanostructure geometry. Understanding the properties of these chiral hotspots is crucial for their applications; for instance, in enhancing the optical interactions with chiral molecules. Here, the results of an elegant experiment are presented: by designing 35 intermediate geometries, the structure is "enantiomorphed" from one handedness to the other, passing through an achiral geometry. Nonlinear multiphoton microscopy is used to demonstrate a new kind of double-bisignate circular dichroism due to enantiomorphing, rather than wavelength change. From group theory, a fundamental origin of this plasmonic chiroptical response is proposed. The analysis allows the optimization of plasmonic chiroptical materials.
Prince Edward presented Dr Valev with the Vice-Chancellor's Award for Public Engagement with Research
This Award is made in recognition of Dr Ventsislav Valev's long standing commitment to meaningfully involving a wide range of non-academic partners with his research, and encouraging postgraduate researchers within his department to participate in public engagement.
Ventsi's research focuses the interaction of powerful lasers with tiny bits of gold, known as nanoparticles. His team is developing new optical materials that can find applications from miniaturised telecommunication components to manufacturing healthier and safer pharmaceuticals.
Since 2009, Ventsi has developed a strong public engagement with research portfolio including: coverage across radio, television and print media; publishing of a science fiction novel and a dozen such short stories; collaborations and exhibitions in conjunction with a sculptor; and an extensive series of science workshops on light for primary schools near Bath. To date, this series has engaged over 900 children through 31 workshops and resulted in significant numbers of children believing they could become scientists.
As a member of the Department of Physics' Public Engagement Working Group, Ventsi is integral to promoting and coordinating the department's public engagement activity. He has also engaged his PhD students in delivering the school workshops, developing their confidence and presentation skills in so doing.
Beyond the University, Ventsi's public engagement work has been recognised by the Royal Society and the Science & Technology Facilities Council through the award of several grants. Ventsi is also a member of the Royal Society Public Engagement Committee whose role it is to oversee all related activities of the Society.
Second-Harmonic Generation Optical Rotation Solely Attributable to Chirality in Plasmonic Metasurfaces
Abstract: Chiral plasmonic nanostructures, those lacking mirror symmetry, can be designed to manipulate the polarization of incident light resulting in chiroptical (chiral optical) effects such as circular dichroism (CD) and optical rotation (OR). Due to high symmetry sensitivity, corresponding effects in second-harmonic generation (SHG-CD and SHG-OR) are typically much stronger in comparison. These nonlinear effects have long been used for chiral molecular analysis and characterization; however both linear and nonlinear optical rotation can occur even in achiral structures, if the structure is birefringent due to anisotropy. Crucially, chiroptical effects resulting from anisotropy typically exhibit a strong dependence on structural orientation. Here we report a large second-harmonic generation optical rotation of ±45°, due to intrinsic chirality in a highly anisotropic helical metamaterial. The SHG intensity is found to strongly relate to the structural anisotropy; however, the angle of SHG-OR is invariant under sample rotation. We show that by tuning the geometry of anisotropic nanostructures, the interaction between anisotropy, chirality, and experimental geometry can allow even greater control over the chiroptical properties of plasmonic metamaterials.
Circular dichroism in higher-order diffraction beams from chiral quasi-planar nanostructures
Abstract: Miniaturization down to the nanoscale has enabled a new paradigm of ultrathin optical devices, capable of manipulating the direction, polarization, and frequency of light. Great interest is drawn by the promising prospects of deep subwavelength material dimensions. However, interesting properties and opportunities offered by structures with sizes comparable to the wavelength of light appear to have been overlooked. Here, quasiplanar chiral arrays made of gold are considered and show that higher order diffracted beams can yield extremely large chiroptical responses for optical frequencies. The chosen sample geometry demonstrates spectrally tunable polarization conversion and extremely large circular dichroism. Experimental and numerical data are in good agreement, for both sample chiral forms, and for the complementary geometries under Babinet's principle. Specifically, the experimental results show that the fractional circular dichroism (CD) can be as high as 20%, in the third order diffraction beam. Based on the numerical results, a great potential for improvement is anticipated, which makes higher order diffraction CD a very promising candidate for ultrathin optical applications.
Response to Government call for evidence: laser pointers
Our main concern is with laser pointers that seem safe to the user but are in fact very powerful. Such lasers can lull people into a false sense of security and lead to damage or injury, without malicious intent. We have developed home-made laser modules that allow careful control of the electrical current and the temperature of laser diodes. Such laser diodes are at the heart of every laser pointer. As laser pointers come in different colours (wavelengths), we tested laser diodes in a range of wavelengths (including violet, blue, green, red and infrared).
Our results show that the "frequency doubled" laser pointers (usually green, some blue and violet pointers) can be particularly dangerous even if they seem safe to the user. The reason is that such laser pointers are constructed in a different manner from other pointers.
BBC Radio Bristol 09/10/2017, at 16:05.
Chirality and Chiroptical Effects in Metal Nanostructures: Fundamentals and Current Trends
Throughout the 19th and 20th century, chirality has mostly been associated with chemistry. However, while chirality can be very useful for understanding molecules, molecules are not well suited for understanding chirality. Indeed, the size of atoms, the length of molecular bonds and the orientations of orbitals cannot be varied at will. It is therefore difficult to study the emergence and evolution of chirality in molecules, as a function of geometrical parameters. By contrast, chiral metal nanostructures offer an unprecedented flexibility of design. Modern nanofabrication allows chiral metal nanoparticles to tune the geometric and optical chirality parameters, which are key for properties such as negative refractive index and superchiral light. Chiral meta/nano-materials are promising for numerous technological applications, such as chiral molecular sensing, separation and synthesis, super-resolution imaging, nanorobotics, and ultra-thin broadband optical components for chiral light. This review covers some of the fundamentals and highlights recent trends. We begin by discussing linear chiroptical effects. We then survey the design of modern chiral materials. Next, the emergence and use of chirality parameters are summarized. In the following part, we cover the properties of nonlinear chiroptical materials. Finally, in the conclusion section, we point out current limitations and future directions of development.
We investigated a chiral metamaterial with substantially sub-wavelength dimensions (<lambda/10), made of nanohelices (Au80%-Cu20%). As the archetypical chiral geometry, the helical design is particularly suitable because it is pronouncedly three-dimensional, it gives directly rise to superchiral field configurations along the center of the helix and its structural chirality parameter is straightforward to estimate as a function of varying dimensions. Within this metamaterial, we clearly identify three different rotational anisotropies and demonstrate how they can mask the true chiral effect, rendering the measured nonlinear chiroptical signals unreliable. Our experimental results highlight the need for a general method to extract the true chiral contributions to the SHG signal. Such a method would be hugely valuable in the present context of increasingly complex chiral meta/nanomaterials.
Scientists have dreamt of nanomachines that can navigate in water, sense their environment, communicate, and respond. Various power sources and propulsion systems have been proposed but they lack speed, strength, and control. We introduce here a previously undefined paradigm for nanoactuation which is incredibly simple, but solves many problems. It is optically powered (although other modes are also possible), and potentially offers unusually large force/mass. This looks to be widely generalizable, because the actuating nanotransducers can be selectively bound to designated active sites. The concept can underpin a plethora of future designs and already we produce a dramatic optical response over large areas at high speed.
BBC Radio Somerset
BBC Radio Bristol
Giant Nonlinear Optical Activity of Achiral Origin in Planar Metasurfaces with Quadratic and Cubic Nonlinearities
3D chirality is shown to be unnecessary for introducing strong circular dichroism for harmonic generations. Specifically, near-unity circular dichroism for both second-harmonic generation and third-harmonic generations is demonstrated on suitably designed ultrathin plasmonic metasurfaces with only 2D planar chirality. The study opens up new routes for designing chip-type biosensing platform, which may allow for highly sensitive detection of bio- and chemical molecules with weak chirality.
Threading plasmonic nanoparticle strings with light
L. O. Herrmann, V. K. Valev*, C. Tserkezis, J. S. Barnard, S. Kasera, O.
A. Scherman, J. Aizpurua, J. J. Baumberg* Nat. Commun.
5, 4568 (2014).[Open Access] * Corresponding authors.
New nanomaterials find increasing application in communications,
renewable energies, electronics and sensing. Because of its
unsurpassed speed and highly tuneable interaction with matter,
using light to guide the self-assembly of nanomaterials could
open up novel technological frontiers. However large-scale
light-induced assembly remains challenging. Here we demonstrate
an efficient route to nano-assembly through plasmon-induced
laser-threading of gold nanoparticle strings, producing
conducting threads 12 ± 2 nm wide. This precision is achieved
because the nanoparticles are first chemically-assembled into
chains with rigidly-controlled separations of 0.9 nm primed for
re-sculpting. Laser-induced threading occurs on a large scale in
water, tracked via a previously-unknown optical resonance in the
near-IR corresponding to a hybrid chain/rod-like charge transfer
plasmon. The nano-thread width depends on the chain mode
resonances, the nanoparticle size, the chain length, and the
peak laser power, enabling nm-scale tuning of the optical and
conducting properties of such nanomaterials.
Nonlinear superchiral meta-surfaces: tuning chirality
and disentangling non-reciprocity at the nanoscale
V. K. Valev, J. J. Baumberg, B. De Clercq, N. Braz, X. Zheng, E. J. Osley, S. Vandendriessche, M. Hojeij, C. Blejean, J. Mertens, C. G. Biris, V. Volskiy, M. Ameloot, Y. Ekinci, G. A. E. Vandenbosch, P. A. Warburton, V. V. Moshchalkov, N. C. Panoiu, T. Verbiest Adv. Mater.
26, 4074-4081 (2014).[Open Access]
Due to the favorable power-law scaling of near-field
enhancements, the nonlinear optical properties of chiral plasmonic nano- and metamaterials are of prime fundamental and
practical interest. However, these optical properties remain
largely unexplored. Here we demonstrate that nonlinear
chiroptical effects are sensitive to superchiral light
enhancements and can therefore be used to guide the design of
superchiral devices for enhanced chiroptical sensing and
asymmetric molecular synthesis or catalysis. While maximal
response in linear chiral metamaterials is achieved for deep sub-wavelength
dimensions, we show that the chiral coupling in the nonlinear
case has a local maximum for a distance of half the second
harmonic wavelength. Fundamentally, whereas conservation under
space and time reversal causes chiral linear metamaterials to be
reciprocal, we demonstrate that the nonlinear ones are non-reciprocal.
These results provide a framework for exploiting the benefits of
chiral nonlinear meta-surfaces.
Chirality and Chiroptical Effects in Plasmonic
Nanostructures: Fundamentals, Recent Progress, and Outlook
This work has been rated as a Very Important Paper [VIP] by Advanced Materials.
Strong chiroptical effects recently reported result from the
interaction of light with chiral plasmonic nanostructures. Such
nanostructures can be used to enhance the chiroptical response
of chiral molecules and could also significantly increase the
enantiomeric excess of direct asymmetric synthesis and catalysis.
Moreover, in optical metamaterials, chirality leads to negative
refractive index and all the promising applications thereof. In
this Progress Report, we highlight four different strategies
which have been used to achieve giant chiroptical effects in
chiral nanostructures. These strategies consecutively highlight
the importance of chirality in the nanostructures (for linear
and nonlinear chiroptical effects), in the experimental setup
and in the light itself. Because, in the future, manipulating
chirality will play an important role, we present two examples
of chiral switches. Whereas in the first one, switching the
chirality of incoming light causes a reversal of the handedness
in the nanostructures, in the second one, switching the
handedness of the nanostructures causes a reversal in the
chirality of outgoing light.
Characterization of Nanostructured Plasmonic Surfaces with Second Harmonic
Generation [Invited Feature Article]
Because of its high surface and interface sensitivity, the
nonlinear optical technique of second harmonic generation (SHG)
appears as a designated method for investigating nanostructured
metal surfaces. Indeed, the latter present a high
surface-to-volume ratio, but, even more importantly, they can
exhibit strong near-field enhancements, or "hotspots". Hotspots
often appear as a result of geometric features at the nanoscale
or of surface plasmon resonances, which are collective electron
oscillations at the surface that, on the nanoscale, can readily
be excited by light. In the last ten years, near-field hotspots
have been responsible for a dramatic development in the field of
nano-optics. In this Feature Article, the influence of hotspots
on the SHG response of nanostructured metal surfaces is
discussed at both the microscopic and the macroscopic level. At
the microscopic level, the nanostructured metal surfaces were
characterized by scanning SHG microscopy, complemented by
rigorous numerical simulations of the near-field and of the
local electric currents at the fundamental frequency. At the
macroscopic level, the SHG - Circular Dichroism and the
Magnetization-induced SHG characterization techniques were
Distributing the optical near-field for efficient field-enhancements in nanostructures
V. K. Valev
V. K. Valev, B. De Clercq, C. G. Biris, X. Zheng, S. Vandendriessche, M.
Hojeij, D. Denkova, Y. Jeyaram, N. C. Panoiu, Y. Ekinci, A. V. Silhanek, V.
Volskiy, G. A. E. Vandenbosch, M.
Ameloot, V. V. Moshchalkov, and T. Verbiest, Adv. Mater.
24, OP208-OP215, (2012).
At present, the research field of plasmonics
is rapidly growing and local field enhancements (hotspots) are
becoming increasingly important for chemical- and bio-sensing.
However, by definition, hotspots are highly localized and, for
intense illumination, they can become too hot, causing damage.
Here we present a nanoengineered sample pattern that, when
illuminated with circularly polarized light, can distribute the
optical near-field over the entire sample surface, thereby
increasing the useful area and allowing the use of higher
The results we show are quite
counter-intuitive. Indeed, one might expect randomly oriented
linearly polarized light to also distribute the optical
near-field over the entire surface of the nanostructures. We
show in our manuscript that this is not the case because the
expectation fails to take into account the optical properties of
this material: while for linearly polarized light the electron
density is mainly subject to strong coupling between the
nanostructures, for circularly polarized light the electron
density distribution is mainly confined within them. Our
findings are supported by two sets of independent theoretical
simulations and by two experimental techniques - second harmonic
generation scanning microscopy and plasmon-induced
The type of ring-shaped nanostructured samples we present can
find a broad range of applications in chemical transformations,
photochemical reactions, catalytic reactions and SERS;
essentially, everywhere where the interaction between molecules
and local field enhancements plays an important role.
V. K. Valev, D. Denkova, X. Zheng, A. I. Kuznetsov, C. Reinhardt, B. N. Chichkov, G.
Tsutsumanova, E.J. Osley, V. Petkov, B. De Clercq, A. V. Silhanek, Y.
Jeyaram, V. Volskiy, P. A. Warburton, G. A. E. Vandenbosch,
S. Russev, O. A. Aktsipetrov, M. Ameloot, V. V. Moshchalkov, T. Verbiest, Adv. Mater. 24, OP29-OP35 (2012).
When a pebble drops on the surface of water, it is often
observed that a water column, or "back-jet", surges upwards.
Counter-intuitive though it might be, a similar phenomenon can
occur when light shines on a metal film surface. Indeed, tightly
focused femtosecond laser pulses carry sufficient energy to
locally melt the surface of a gold film and the impact from
these laser pulses produces a back-jet of molten gold with
nanoscale dimensions - a nanojet.
As the name suggests, nanojets on the surface of a homogeneous
gold film are quite small, their size being determined by the
distribution of energy in the light pulse. This distribution of
energy is in turn dependent on the wavelength of light.
Consequently, although these nanojets are quite small, they
cannot be much smaller than the wavelength of light. Well, we
have shown that they actually can, with the help of surface
Surface plasmons are coherent oscillations of the electron
density in metal nanostructures that can readily be excited by
light. Essentially, in response to the incident light's electric field, the electron
density oscillates in the plasmonic hotspots producing an
electric current. Associated Ohmic losses raise the temperature
of the nanomaterial within the plasmonic hotspot above the
melting point. A nanojet and nonosphere ejection can then be
observed precisely from the plasmonic hotspots.
U-Shaped switches for optical information processing at the
Valev, A. V. Silhanek, B. De Clercq, W. Gillijns, Y. Jeyaram, X. Zheng,
V. Volskiy, O. A. Aktsipetrov, G. A. E. Vandenbosch, M. Ameloot, V. V.
Moshchalkov, T. Verbiest, Small 7, 2573-2576 (2011).
Fully light based circuits are becoming a realistic possibility,
due to the recent advances in metamaterials. The possibility
arises from the fact that light waves can couple to collective
excitations of electrons at the surfaces of metallic
nanostructures, a property referred to as: surface plasmon
We report on a novel
way to transmit information from a beam of light to the
plasmonic outputs of U-shaped nanostructures: four distinct
logical states can be transmitted depending on the polarization
of the incoming light. Upon coupling the output extremities of
the U-shaped switches to plasmonic metamaterial waveguides, we
believe that information can be channeled through an all-optical
figure to the left representes a schematic diagram of the
plasmonic switch for optical information processing at the
nanometer scale. Depending on the polarization state of the
incoming light (at 800 nm wavelength), the two branches (outputs A and B) of a golden U-shaped
nanostructure, give rise to localized second harmonic sources (at 400
nm wavelength), or hotspots, that are due to local field enhancements. The
nanostructure is 600 nm long, 400 nm wide, 25 nm thick. A and B
are both 200 nm wide.
Hotspot Decorations map plasmonic patterns with the resolution of scanning probe techniques
V. K. Valev, A. V. Silhanek, Y. Jeyaram, D. Denkova, B. De Clercq, V. Petkov, X. Zheng, V. Volskiy, W. Gillijns, G. A. E. Vandenbosch, O. A. Aktsipetrov, M. Ameloot, V. V. Moshchalkov, and T. Verbiest Phys. Rev. Lett. 106, 226803 (2011).
Abstract: In high definition mapping of the plasmonic patterns on the surfaces of nanostructures, the diffraction limit of light remains an important obstacle. Here we demonstrate that this diffraction limit can be completely circumvented. We show that upon illuminating nanostructures made of nickel and palladium, the resulting surface-plasmon pattern is imprinted on the structures themselves; the hotspots (regions of local field enhancement) are decorated with overgrowths, allowing for their subsequent imaging with scanning-probe techniques. The resulting resolution of plasmon pattern imaging is correspondingly improved.