Sandestin, Florida, USA Invited Talk
Ventsislav K. Valev is a Research Fellow of the Royal Society and Reader in the Physics Department of the University of Bath, where he heads the MultiPhoton NanoPhotonics group. Prior to taking up this post, he was a Research Fellow in the Cavendish Laboratory, at the University of Cambridge.
The MultiPhoton NanoPhotonics research group focuses on the interaction between powerful laser light and nanostructured materials. In particular, we explore the application of chiral plasmonic nano/meta-materials to achieve enhanced chiroptical effects with potential benefits for the pharmaceutical industry. Powerful lasers constitute highly sensitive probes for material properties at the nanoscale, especially through nonlinear optical effect, such as Second Harmonic Generation. But just as light can be used to study nanomaterials, it can also be used to build them. We have thus demonstrated the world's smallest nanojets and have shown how light could be employed as a tiny needle threading gold strings through chains of nanoparticles.
In teaching students, our projects often take a distinct science fiction aspect, as we build laser-powered nano-photonic steam engines (Steampunk Science) or program a humanoid robot to become a lab assistant.
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 emer- gence 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 nanopar- ticles 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 prop- erties 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.
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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, 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.
Decorations map plasmonic patterns with the resolution of scanning probe
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).
Quoting: "Imaging of Surface Plasmons May Be a Lot Easier Than Previously
"An unusual observation turned into a scientific breakthrough when
K.U.Leuven researchers investigating the optical properties of nanomaterials
discovered that so-called surface plasmons leave imprints on the surface of
the nanostructures. This led to a new type of high resolution microscopy for
imaging the electric fields of nanostructures.
Surface plasmon hotspots can be imprinted on metallic nanostructures for
subsequent high resolution imaging with standard surface probe
Nanomaterials, consisting of extremely small
particles or thin layers, tend to acquire unexpected properties.
Optical nanomaterials are a class of materials that have emerged
over the last ten years and that have quickly become a hot topic
in material science due to their counterintuitive optical
behavior and revolutionary potential applications. Optical
nanomaterials are mainly based on surface plasmon resonances -
the property whereby, in metallic nanostructures, light can
collectively excite surface electron waves. These electron waves
have the same frequency as light, but much shorter wavelengths,
which allow their manipulation at the nanoscale. In other words,
with the help of plasmons, light can be captured, modified and
even stored in nanostructures. This emerging technology finds
applications in surprising areas, ranging from cancer treatment
(by targeting cancer cells with nanoparticles that will produce
heat when excited) to invisibility (by causing light to follow a
trail of nanoparticles, that acts as an invisibility cloak to
whatever is underneath them).
The imaging of surface plasmons provides a
direct way to map and understand the local electric fields that
are responsible for the unusual electromagnetic properties of
optical nanomaterials. However, the imaging of surface plasmons
is quite challenging. While there are methods to image plasmons
with high resolution, they come at a considerable increase in
both cost and complexity. But now, Ventsislav K. Valev and his
colleagues have demonstrated a powerful and user friendly method
for imaging plasmonic patterns in nanostructures.
"We were performing routine characterization of
freshly grown samples, when I asked Yogesh, one of our Ph.D. students,
to look at a sample that had already been studied. There was absolutely
no reason to do this; I just had a hunch," sais Ventsislav Valev.
"Surprisingly, this sample appeared to be decorated and I immediately
recognized the pattern. Somehow, the optical properties have been
imprinted on the surface of the nanostructures."
The scientists indeed found out that upon
illuminating nanostructures made of nickel or palladium, the resulting
surface plasmon pattern is imprinted on the structures themselves. This
imprinting is done through displacing material from the nanostructure to
the regions where the plasmon enhancements are the largest. In this
manner, the plasmons are effectively decorated, allowing for subsequent
imaging with standard surface probe techniques, such as scanning
electron microscopy or atomic force microscopy. The imprinting method is
quite unique, combining aspects of both imaging and writing techniques."