Upcoming:

PIERS 2019
17-20/06/2019
Rome, Italy
Invited talk

CLEO/EUROPE 2019
23-27/06/2019
Munich, Germany
Program Committee

Chirality 2019
14-17/07/2019
Bordeaux, France

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.

 
 

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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)

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

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.


Measuring optical activity in the far-field from a racemic nanomaterial: diffraction spectroscopy from plasmonic nanogratings

C. Kuppe, X. Zheng, C. Williams, A. W. A. Murphy, J. T. Collins, S. N. Gordeev, G. A. E. Vandenboschb, V. K. Valev
Nanoscale Horiz. (2019) DOI: 10.1039/C9NH00067D

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.


“Hot Edges” in Inverse Opal Structure Enable Efficient CO2 Electrochemical Reduction and Sensitive in-situ Raman Characterization

Y. Yanga,, L. Ohnoutek, S. Ajmala, X. Zhenga, Y. Fenga, K. Lia, T. Wanga, Y. Denga, Y. Liua, X. Donga, V. K. Valev, L. Zhang
J. Mat. Chem. A 7, 11836-11846 (2019)

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

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

Z. Liu, A. W. A. Murphy, C. Kuppe, D. C. Hooper, V. K. Valev, A. Ilie
ACS Nano 13, 3896–3909 (2019)

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

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)

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

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

D. C. Hooper, C. Kuppe, D. Wang, W. Wang, J. Guan, T. W. Odom, V. K. Valev
Nano Lett. 19, 165-172 (2019)

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

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)

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

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

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

J. T. Collins, D. C. Hooper, A. G. Mark, C. Kuppe, V. K. Valevv
ACS Nano 12, 5445–5451 (2018)

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

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

C. Kuppe, C. Williams, J. You, J. T. Collins, S. N. Gordeev, T. D. Wilkinson, N. C. Panoiu, V. K. Valev
Adv. Opt. Mater. 6, 1800098 (2018)

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

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

V. K. Valev, C. Kuppe, D. Slavov

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

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. 2018-07-12-Valev-VC-Award-PE.mp4

BBC Radio Bristol 09/10/2017, at 16:05.


Chirality and Chiroptical Effects in Metal Nanostructures: Fundamentals and Current Trends

J. T. Collins, C. Kuppe, D. C. Hooper, C. Sibilia, M. Centini, V. K. Valev
Adv. Opt. Mater. 5, 1700182 (2017)

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.


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

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.

 

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.


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