META 2019 23-26/07/2019 Lisbon, Portugal Invited talk
Ventsislav K. Valev is a Professor of Physics and Research Fellow of the Royal Society, 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.
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.