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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.

 

 

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Strong Rotational Anisotropies Affect Nonlinear Chiral Metamaterials

D. C. Hooper, A. G. Mark, C. Kuppe, J. T. Collins, P. Fischer, V. K. Valev
Adv. Mater. 29, 1605110 (2017)

This work was the subject of a press release from the University of Bath.

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.

The paper was highlighted by Phys.Org. ScienceDaily, NanoWerk, AzoNano.

Editorial feature at AzoNano.


Chiral Nanomaterials and Chiral Light

V. K. Valev
Optics & Photnics News 27, 35-41 (2016) July/August Issue


Advances in nanofabrication are expanding opportunities to exploit and customize the "handedness" of materials and of light itself.

A pdf copy of the paper can be download here.


Light-induced actuating nanotransducers: ANTs

T. Ding, V. K. Valev, A. R. Salmon, C. J. Forman, S. K. Smoukov, O. A. Scherman, D. Frenkel, J. J. Baumberg
PNAS 113, 5503-5507 (2016). [Open Access]

This work was the subject of a press release from the University of Bath and the University of Cambridge.  

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

S. Chen, F. Zeuner, M. Weismann, B. Reineke, G. Li, V. K. Valev, K. W. Cheah, N. C. Panoiu, T. Zentgraf, S. Zhang
Adv. Mater. 28, 2992–2999 ( 2016) [Open Access]


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.

 

This work was highlighted by the University of Cambridge and the press release was shared and re-posted over 2000 times. Also highlighted by Newsweek

 

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]

 

This work was highlighted by the University of Cambridge, Phys.org Materials Views.
 

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

V. K. Valev, J. J. Baumberg, C. Sibilia and T. Verbiest
Adv. Mater. 25, 2517-2534 (2013)
.

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]

V. K. Valev
Langmuir 28, 15454-15471 (2012)
.

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 employed.


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)
.

This work has been highlighted in ScienceDaily (July 18, 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 illumination intensities.

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 sub-wavelength laser-ablation.

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.


Plasmon-enhanced sub-wavelength laser ablation: plasmonic nanojets

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).

This work has been highlighted in Nature Photonics, ScienceDaily (January 13, 2012) and Knack.

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 plasmons.

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 nanoscale

V. K. 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 resonances.

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 circuit.

The 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).

This work has been highlighted in Laser Focus World, ScienceDaily (June 6, 2011), also diffused by Nanotech-Now, NanoWerk, Photonics.com, AzoOptics, AzoNano, and AlphaGalileo.

Quoting: "Imaging of Surface Plasmons May Be a Lot Easier Than Previously Thought"

"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 techniques.

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."


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