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Helicity preserving nanostructures and metasurfaces have been recently proposed as suitable candidates to enhance spectroscopic features of chiral molecules. With this in mind, we highlight that losses in the constituent nonmagnetic materials dramatically affect the possibility of constructing structures which conserve helicity. We first present a general procedure that permits the evaluation of the normalized helicity expectation value, hLiN, i.e. the observable that allows for the identification of helicity preserving scatterers. We then apply this procedure to the case of a chiral sphere, which in an orientation averaged picture can capture the optical response of chiral inorganic nanostructures, obtaining a widely applicable analytical expression of Image hLiN. Finally, we numerically show that optical losses impose an upper bound to the helicity expectation value on nonmagnetic core–shells and chiral spheres., J. L. A., J. O. T. and A. G. E. acknowledge support from the Spanish Ministerio de Ciencia e Innovación (PID2019-109905GA-C2) and from Eusko Jaurlaritza (IT1164-19 and KK-2021/00082). A. G. E. acknowledges funding from Programa Red Guipuzcoana de Ciencia, Tecnología e Innovación 2021 (Grant Nr. 2021-CIEN-000070-01. Gipuzkoa Next). A. G. E. and G. M. T. acknowledge funding from the Basque Government's IKUR initiative on Quantum technologies (Department of Education).

Resumen del trabajo presentado a la Conferencia Española de Nanofotónica (CEN), celebrada en Vigo del 20 al 22 de septiembre de 2021.

9 pags., 4 figs., Increasing the sensitivity of chiral spectroscopic techniques such as circular dichroism (CD) spectroscopy is a current aspiration in the research field of nanophotonics. Enhancing CD spectroscopy depends on two complementary requirements: the enhancement of the electromagnetic fields perceived by the molecules under study and the conservation of the helicity of those fields, guaranteed by duality symmetry. In this work, we introduce a systematic method to design nanostructured dual periodic photonic systems capable of enhancing molecular CD spectroscopy resonantly. As an illustration, we engineer a dual 1D silicon nanoparticle array and show that its collective optical modes can be efficiently employed to resonantly enhance by 2 orders of magnitude the local density of optical chirality and, thus, the CD signal obtained from a given molecular sample on its vicinity., J.L.A., J.J.S., and A.G.E. acknowledge the PID2019-109905GAC22 Project of the Spanish Ministerio de Ciencia, Innovacion y Universidades (MICIU). D.R.A. acknowledges partial financial support from Grant No. MELODIA PGC2018- 095777-B-C21 of MICIU and from the Ministerio de Educación, Cultura y Deporte through the FPU Ph.D. Fellowship FPU15/03566. A.N. acknowledges finacial support from the MICIU through the FPI Ph.D. Fellowship BES-2017- 080073. G.M.T. acknowledges the FIS2017-87363-P Project of the Spanish MINECO. A.G.E. and J.A. acknowledge the FIS2016-80174-P Project of the Spanish MINECO. A.G.E. received funding from the Gipuzkoako Foru Aldundia OF23/ 2019 (ES) Project, “Fellows Gipuzkoa” Fellowship through FEDER “Una Manera de hacer Europa”, and by Eusko Jaurlaritza Grant Numbers KK-2017/00089, IT1164-19, PI2016-41, and KK-2019/00101.

This article is dedicated to the memory of Juan José Sáenz., In this work we study the spatio-temporal correlations of photons produced by spontaneous parametric down conversion. In particular, we study how the waists of the detection and pump beams impact on the spectral bandwidth of the photons. Our results indicate that this parameter is greatly affected by the spatial properties of the detection beam, while not as much by the pump beam. This allows for a simple experimental implementation to control the bandwidth of the biphoton spectra, which only entails modifying the optical configuration to collect the photons. Moreover, we have performed Hong–Ou–Mandel interferometry measurements that also provide the phase of the biphoton wavefunction, and thereby its temporal shape. We explain all these results with a toy model derived under certain approximations, which accurately recovers most of the interesting experimental details., JJMV, JLA, MMF and GMT acknowledge the FIS2017-87363-P project of the Spanish Ministerio de Educación, Cultura y Deporte. NT acknowledges support by the Griffith University, Australia Postdoctoral Fellowship Scheme by the Australian Research Council (ARC) Centre of Excellence CE17010001, and by the Alexander von Humboldt Foundation, Germany .

Lens-axicon doublets have been used to produce Bessel-Gaussian beams, a narrow non-diffracting beam of relatively constant width. One problem of using Bessel-Gaussian beams is that there is a compromise between achieving a long effective focal length with a small central core radius and distributing the beam intensity between the central core and the off-axis rings. Here, we explore the advantage of tuning the lens-axicon separation, which allows us to have an additional degree of freedom to tailor the beam profile. Moreover, the separation between the lens and the axicon reduces the spherical aberrations in the beam profile, which can then be modeled within the paraxial regime. We study the detrimental effects of the spherical aberrations and provide several options to minimize them. We examine both sharp and shallow axicons used in combination with different converging lenses. We perform a series of detailed experiments to image the structure of the beam through the Bessel region. The spatial light distribution of the lens-axicon system is analyzed by using high dynamic range imaging and complemented with consistent theoretical calculations within the paraxial regime., The Australian Research Council Discovery Project (DP160103332)

The conservation of the photon total angular momentum in the incident direction in an axially symmetric scattering process is a very well known fact. Nonetheless, the redistribution of this conserved magnitude into its spin and orbital components, an effect known as the spin-orbit interaction (SOI) of light, is still a matter of active research. Here, we discuss the effect of the absorption on the SOI in the scattering of a subwavelength silicon particle. Describing the scattering process of an electric and a magnetic dipole, we show via the asymmetry parameter that the SOI of light in the scattering of high refractive index nanoparticles endures in the presence of optical losses. This effect results in optical mirages whose maximum values surpass those of an electric dipolar scatterer., This research was supported by the Spanish Ministerio de Economía y Competitividad (MICINN) and European Regional Development Fund (ERDF) Projects (FIS2015- 69295-C3, FIS2016-80174-P, FIS2017-82804-P, FIS2017- 91413-EXP) and FPU PhD Fellowship (FPU15/03566), and by the Basque Government Projects (PI-2016-1- 0041, KK- 2017/00089) and PhD Fellowship (PRE2018-2-0252). AGE received funding from the Fellows Gipuzkoa fellowship of the Gipuzkoako Foru Aldundia through FEDER “Una Manera de hacer Europa”.

The strength of optical trapping of a nanodiamond can be increased by cooperative effects between its numerous colour centres — or artificial atoms: an observation that brings together ideas from atom and nanoparticle trapping.

Resumen del trabajo presentado a la Spanish Conference on Nanophotonics (Conferencia Española de Nanofotónica-CEN), celebrada en Donostia-San Sebastián (España) del 3 al 5 de octubre de 2018., Spin-orbit interaction of light can lead to the so-called optical mirages, i.e. a perceived displacement in the position of a particle due to the spiraling structure of the scattered light. In electric dipoles, the maximum displacement is subwavelength and does not depend on the optical properties of the scatterer. Here we will show that the optical mirage in high refractive index dielectric nanoparticles depends strongly on the ratio between electric and magnetic dipolar responses. When the dual symmetry is satisfied (at the first Kerker condition), there is a considerable enhancement (far above the wavelength) of the spin-orbit optical mirage which can be related to the emergence of an optical vortex in the backscattering direction.

In this work, we experimentally show that quantum entanglement can be symmetry protected in the interaction with a single subwavelength plasmonic nanoaperture, with a total volume of V∼0.2λ3. In particular, we experimentally demonstrate that two-photon entanglement can be either completely preserved or completely lost after the interaction with the nanoaperture, solely depending on the relative phase between the quantum states. We achieve this effect by using specially engineered two-photon states to match the properties of the nanoaperture. In this way we can access a symmetry protected state, i.e., a state constrained by the geometry of the interaction to retain its entanglement. In spite of the small volume of interaction, we show that the symmetry protected entangled state retains its main properties. This connection between nanophotonics and quantum optics probes the fundamental limits of the phenomenon of quantum interference., L. M. and F. S. acknowledge financial support from the European Union Horizon 2020 program, within the European Research Council (ERC) Grant No. 694683, PHOSPhOR. G. M. T. acknowledges financial support from the Australian Research Council through the Future fellowship program FT110100924 and the Spanish Ministry of Science, Innovation and Universities (MICIN) Project No. FIS2017-87363-P.

A fundamental problem of using photonic states as carriers of quantum information is that they interact weakly with matter and that the interaction volume is typically limited by the wavelength of light. The use of metallic structures in quantum plasmonics has the potential to alleviate these problems. Here, we present the first results showing that a single subwavelength plasmonic nanoaperture can controllably modify the quantum state of light. We achieve this effect by using a specially engineered two photon state to match the properties of the nanoaperture.

A technique for multiwavelength phase measurements with quantum light is proposed and applied to the study of optical activity., The phenomenon of molecular optical activity manifests itself as the rotation of the plane of linear polarization when light passes through chiral media. Measurements of optical activity and its wavelength dependence, that is, optical rotatory dispersion, can reveal information about intricate properties of molecules, such as the three-dimensional arrangement of atoms comprising a molecule. Given a limited probe power, quantum metrology offers the possibility of outperforming classical measurements. This has particular appeal when samples may be damaged by high power, which is a potential concern for chiroptical studies. We present the first experiment in which multiwavelength polarization-entangled photon pairs are used to measure the optical activity and optical rotatory dispersion exhibited by a solution of chiral molecules. Our work paves the way for quantum-enhanced measurements of chirality, with potential applications in chemistry, biology, materials science, and the pharmaceutical industry. The scheme that we use for probing wavelength dependence not only allows one to surpass the information extracted per photon in a classical measurement but also can be used for more general differential measurements.

We report on a new regime for optical trapping of diamond nanoparticles in liquid. While holding a nanodiamond (~150 nm) containing many NV centres (~10^3) at the focus of classical optical tweezers, we add a second laser slightly detuned from the dipole transition of the NVs. We measure a change in trap stiffness of ~10%. Remarkably, we show that our results must include collective effects – ‘superradiance’ – between colour centres, which has never been reported before for solid-state systems at room temperature. We discuss how these resonant forces depend on (and can be enhanced by) the type, number, dipole strength and spectral linewidth of the centres.

In this work we present a simple method to reconstruct the complex spectral wavefunction of a biphoton, and hence gain complete information about the spectral and temporal properties of a photon pair. The technique, which relies on quantum interference, is applicable to biphoton states produced with a monochromatic pump when a shift of the pump frequency produces a shift in the relative frequencies contributing to the biphoton. We demonstrate an example of such a situation in type-II parametric down-conversion (SPDC) allowing arbitrary paraxial spatial pump and detection modes. Moreover, our test cases demonstrate the possibility to shape the spectral wavefunction. This is achieved by choosing the spatial mode of the pump and of the detection modes, and takes advantage of spatiotemporal correlations., Supplementary information also available. Comments and feedback appreciated. Compared to the previous version, here we have made the following changes: -corrected a typo in the text between Eq. (11) and (12) -corrected a typo in the references -added references

We propose a method to study and characterize the spatial and temporal properties of degenerate photon pairs emitted in SPDC, using a filtering system combined with temperature variation of the nonlinear crystal. The photons can be distinguished. We relate these to the measured Hong-Ou-Mandel interference dip of the photons, measured in a parallel experiment. The theoretical plots match very well with the experimental results.

We theoretically show that the dipole force of an ensemble of quantum emitters embedded in a dielectric nanosphere can be exploited to achieve near-field optical levitation. The key ingredient is that the polarizability from the ensemble of embedded quantum emitters can be larger than the bulk polarizability of the sphere, thereby enabling the use of repulsive optical potentials and consequently the levitation using optical near-fields. In levitated cavity quantum optomechanics, this could be used to boost the single-photon coupling by combining larger polarizability to mass ratio, larger field gradients, and smaller cavity volumes while remaining in the resolved sideband regime and at room temperature. A case study is done with a nanodiamond containing a high-density of silicon-vacancy color centers that is optically levitated in the evanescent field of a tappered nano-fiber and coupled to a high-finesse microsphere cavity., Comment: 11 pages (including appendix), 6 figures

In this paper, we propose a new type of rotational Doppler shift measurement based on the OAM of light which is capable of measuring the rotation of a point source in the plane orthogonal to the observer line of sight. By analysing the correlations between OAM states of light emitted by rotating sources, the rotational Doppler shift, and hence the rate of rotation, can be measured. We demonstrate that an OAM interferometer capable of extracting the rotational Doppler shift from OAM correlations can be constructed from a standard OAM modesorter combined with a phase filter.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., The polarization of the light scattered by an optically dense and random solution of dielectric nanoparticles shows peculiar properties when the scatterers exhibit strong electric and magnetic polarizabilities. While the distribution of the scattering intensity in these systems shows the typical irregular speckle patterns, the helicity of the incident light can be fully conserved when the electric and magnetic polarizabilities of the scatterers are equal. We show that the multiple scattering of helical beams by a random dispersion of >dual> dipolar nanospheres leads to a speckle pattern exhibiting a perfect isotropic constant polarization, a situation that could be useful in coherent control of light as well as in lasing in random media., M. K. S. and J. A. acknowledge funding from Project No. FIS2013-41184-P of the Spanish Ministry of Economy and Competitiveness, the Project No. ETORTEK IE14-393 NANOGUNE’14 of the Department of Industry of the Government of the Basque Country, Project No. IT756-13 of the Department of Education and Culture of the Basque Country, and Grant No. AP-2012-4204 from the Spanish Ministry of Education, Culture, and Sport. J. J. S. acknowledges financial support by the Spanish MINECO (Grant No. FIS2012-36113) and by IKERBASQUE. G. M.-T. is supported by the Australian Research Council (Grant No. FT110100924).

We will show how to extract information from the Mie coefficients to properly design dual systems combined with chiral elements for having optically active structures. Such optically active elements will scatter the light omnidirectionally, where the amount of rotation of light is fixed in any given direction independently of the incident polarization. The key elements are the preservation of helicity by equaling the electric and magnetic responses from the material, and by the proper manipulation of the angular momentum.

We show the relation between optical activity and helicity for the scattering of chiral particles. The results show how to design chiral structures for an omnidirectional optically active response independent of the incident polarization.

We propose a novel detection method based on the symmetry breaking induced by the bio-molecule to be detected. This optical method provides substantial advantages over current approaches for the conception of biosensors.

In recent years, there have been a series of proposals to exploit the orbital angular momentum (OAM) of light for astronomical applications. The OAM of light potentially represents a new way in which to probe the universe. The study of this property of light entails the development of new instrumentation and problems which must be addressed. One of the key issues is whether we can overcome the loss of the information carried by OAM due to atmospheric turbulence. We experimentally analyze the effect of atmospheric turbulence on the OAM content of a signal over a range of realistic turbulence strengths typical for astronomical observations. With an adaptive optics system we are able to recover up to 89% power in an initial non-zero OAM mode (ℓ = 1) at low turbulence strengths (0.30" FWHM seeing). However, for poorer seeing conditions (1.1" FWHM seeing), the amount of power recovered is significantly lower (5%), showing that for the terrestrial detection of astronomical OAM, a careful design of the adaptive optics system is needed.

We present a method to enhance the ripple structure of the scattered electromagnetic field in the visible range through the use of Laguerre-Gaussian beams. The position of these enhanced ripples as well as their linewidths can be controlled using different optical beams and sizes of the spheres., 10 pages, 4 figures, 1 table, http://dx.doi.org/10.1016/j.jqsrt.2012.10.010

We study the spiral spectra scattered off transparent dielectric spheres when probed by different Laguerre–Gaussian light beams, carrying nested topological wavefront dislocations. We show that such scattering data may be employed to determine geometrical properties of the spheres, such as their position. The technique is a generalization of standard Mie scattering, and it can be extended to study and to characterize nanospheres. © 2012 Optical Society of America OCIS codes: 080.4865, 290.4020. Light beams carry angular momentum, which can be divided in two components, namely, orbital angular momentum (OAM) and spin angular momentum (SAM). In the case of paraxial fields, the two contributions can be separated and thus, independently prepared and measured [1]. The SAM, i.e., the polarization of light, is routinely used to characterize materials in ellipsometry and in several other applications in metrology. It is not always properly appreciated that the OAM may also be used to probe geometrical properties of objects [2,3] using the unique properties of the topological wavefront dislocations, or optical vortices, associated to light beams carrying OAM. The technique is based on the elucidation and interpretation of the relevant parts of the so-called spiral spectra of the light beams scattered off objects when illuminated by different probing beams. In this Letter we study several spiral spectral modes scattered by paraxial transparent dielectric spheres when probed by Laguerre– Gaussian (LG) modes. By and large, the spiral spectrum may be used to elucidate different geometrical properties of spheroids, such us their size, position, and ellipticity, as well as the refractive index contrast between the particle and the surrounding medium. Here we focus on how the spiral spectra vary with the position and size of spherical polystyrene beads. Our setup may be viewed as a generalization of standard Mie scattering [4], including its application to com

We propose a novel deterministic protocol, based on continuous light flows, that enables us to control the concentration of light in generic plasmonic nanostructures. Based on an exact inversion of the response tensor of the nanosystem, the so-called Deterministic Optical Inversion (DOPTI) protocol provides a physical solution for the incident field leading to a desired near field pattern, expressed in the form of a coherent superposition of high order beams. We demonstrate the high degree of control achieved on complex plasmonic architectures and quantify its efficiency and accuracy.

We report on a novel approach, based on sub-wavelength spatial phase variations at the focus of high-order beams, to reconfigure the optical near field distribution near plasmonic nanostructures. We first show how the introduction of phase jumps in the incident field driving a gap nanoantenna strongly affects its near field response. Beyond, we demonstrate the feasibility of exploiting this approach to selectively switch on and off hot-spots sites within a complex antenna architecture.

paraxial beam we can easily control one component of the angular momentum i.e., the component parallel to the propagation direction but can we also control the total angular momentum of the beam? The answer to this question is found by expressing the paraxial beam in the basis of eigenstates of the total angular momentum, i.e., the multipolar states.

The generation of paired photons entangled in orbital angular momentum (OAM), provides a new degree of freedom that is increasingly being used as a source of quantum states. Elucidation of the OAM spectrum of OAM spectrum of the generated photons is of paramount importance, since quantum information applications require the ability to generate arbitrary entangled states with the appropriate OAM correlations. Here we discuss the OAM spectrum of the photon generated via SPDC in two complementary scenarios. On one hand, we should consider the whole geometry of the nonlinear process, taking into account azimuthal variations of the nonlinear coefficient or the phase matching conditions. On the other hand, all relevant experiments reported to date detect only a small section of the full down-conversion cone. In this scenario, the measured OAM correlations depend of the emission angle of the photons and the strength of the Poynting vector walk-off. We will present experiments in SPDC where this dependence is clearly revealed.

We demonstrate experimentally the tunable control of the joint spectrum, i.e. waveform and degree of frequency correlations, of paired photons generated in spontaneous parametric downconversion. This control is mediated by the spatial shape of the pump beam in a type-I noncollinear configuration. We discuss the applicability of this technique to other sources of frequency entangled photons, such as electromagnetically induced Raman transitions., 5 Pages, 4 Figures

We show that the orbital angular momentum (OAM) correlations, and the amount of spatial entanglement, of paired photons generated in spontaneous parametric downconver- sion are strongly affected by the use of noncollinear configurations and by the presence of Poynting vector walk-off. We discuss how our results are related to the question of the conservation of OAM in a nonlinear optics process, and we derive selection rules that allow to determine the sought-after OAM correlations, especially for those configurations used in current quantum information experiments.

In spontaneous parametric down-conversion photons are known to be created coherently and with equal probability over the entire length of the crystal. Then, there is no particular position in the crystal where a photon pair is created. We make the seemingly contradictory observation that we can control the time delay with the crystal position along the propagation direction. We resolve this contradiction by showing that the spatio-temporal correlations critically affect the temporal properties of the pair of photons, when using a finite detector size. We expect this to have important implications for experiments that require indistinguishable photons.

The OAM of light provides a new resource to explore quantum physics in a d-dimensional Hilbert space, beyond the two dimensional Hilbert space generated by the polarization state of the photons. The implementation of such a ddimensional quantum channel requires the generation of arbitrary engineered entangled states, thus controlling the OAM of the entangled photons is of paramount importance for many applications. Here we address the orbital angular momentum, i.e. the spatial shape, of photons generated in SPDC in non collinear geometries, when the interacting waves can exhibit Poynting vector walk off. The spatial shape depends on the interplay between the state ellipticity caused by the nonlinear geometry, and the Poynting vector walk off. The importance of both effects is dictated by the relationship between three characteristics lengths: the length of the nonlinear crystal, the walk off length and the non collinear length. The effects described here are relevant to current experiments, especially for the implementation of quantum information protocols based on spatially encoded information. Finally, the consideration of new geometries for SPDC, more specifically, highly non collinear configurations, will lead us to the discussion of the relationship between the OAM of the classical beam that pumps the nonlinear crystal, and the quantum OAM of the down converted photons. Regarding experimental measurements related to this issue, it is of great importance to make a clear distinction between the measurement of locally paraxial light beams in a suitable transverse frame, and the description of the global down conversion process, which is not necessarily paraxial.

This paper shows theoretically and experimentally that locally, paraxial measurements of the orbital angular momentum (OAM) conducted with entangled photons generated in noncollinear geometries, or nearly collinear geometries with Poynting vector walk off, they do not comply, in general, with the known selection rules for the spiral index of the pump, signal and idler mode functions. Conditions under which this selection rule can apply are also discussed.

We show experimentally how the spatial shape of the pump beam controls the ellipticity of the spatial mode function in noncollinear spontaneous parametric down-conversion. The degree of ellipticity depends on the pump beam width, especially for highly focused beams. We introduce an effective length, the so-called noncollinear length, that determines the importance of the ellipticity of the spatial mode function. We also discuss the ellipticity induced by the spectrum of the pump beam.

Twisted light, or light with orbital angular momentum (OAM), plays an emerging role in both classical and quantum science, with important applications in areas as diverse as biophotonics, micromachines, spintronics, or quantum information. It offers fascinating opportunities for exploring new fundamental ideas in physics, as well as for being used as a tool for practical applications. One important point is to determine how to generate single photons, and two-photon states, with an appropriate OAM content. Here we describe the paraxial orbital angular momentum of entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) in different non-collinear geometries. These geometries introduce a variety of new features. In particular, we find the OAM of entangled pairs generated in purely transverse-emitting configurations, where the entangled photons counter-propagate perpendicularly to the direction of propagation of the pump beam. The spatial walk-off of all interacting waves in the parametric process also determines the OAM content of the down-converted photons, and here its influence is also revealed.

We elucidate the paraxial orbital angular momentum of entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) in different non-collinear geometries. To date, most investigations addressed SPDC in nearly collinear phase-matching geometries, where the pump, the signal and idler photons propagate coaxially almost along the same direction. However, non-collinear geometries introduce a variety of new features. The OAM of the entangled photons strongly depend on the propagation direction of the photons. Here we show that locally paraxial measurements of the OAM conducted with entangled photons generated in non collinear geometries, they do not comply with the known selection rules for the spiral index of the pump, signal and idler mode functions (Mair et al., Nature 412, 313 (2001)). In particular, we find the orbital angular momentum of entangled pairs generated in purely transverse-emitting configurations, where the entangled photons counter-propagate perpendicularly to the direction of propagation of the pump beam. In transverse emitting configurations, the spatial shape of the down converted in one transverse dimensions strongly depends on the corresponding spatial shape of the input pump beam, while in the other transverse dimension, the shape is tailored by the longitudinal phase matching. The spatial walk-off of all interacting waves in the parametric process also determines the OAM content of the down-converted photons, and here its influence is also revealed.

We review a method which allows the simultaneous frequency-conversion and shaping of a light beam into a variety of patterns. The method is based on the controllable dynamics of vortices nested in the light beams. We outline the theoretical idea behind the technique and discuss experimental observations conducted in SH generation in lithium triborate which demonstrate its feasibility. The technique might find applications in specialized optical tweezers aimed at the wavelength-optimized assembly, fabrication and manipulation of complex microstructures and nanostructures.

Conservation of the orbital angular momentum (OAM) of light in photon downconversion has been observed experimentally in nearly collinear phase-matching geometries, where the pump, signal and idler photons propagate along almost the same direction [Nature 412 (2001) 313]. However, here we predict that such paraxial measurements conducted with entangled photons in noncollinear geometries are not expected to comply with OAM conservation in the above sense. Under proper conditions, the strength of such apparent anomaly approaches 100%. We discuss the physical origin of the effect and suggest experimental schemes where it can be verified.

We put forward an experimentally feasible technique to generate engineered entangled states in d-dimensional Hilbert spaces in parametric down-conversion of photons. The scheme is based on the orbital angular momentum of light and translates the classical, topological information imprinted in the light beam that pumps the two-photon source into quantum information contained in the weights and phases of the quantum entangled two-photon states.

We report what is believed to be the first experimental observation of spontaneous nucleation of arrays of optical vortices in the process of second-harmonic generation in a crystal with moderate Poynting vector walk-off, pumped with focused beams containing screw phase dislocations. Our experiments were conducted in lithium triborate under conditions of small-efficiency second-harmonic generation pumped by nanosecond pulses at 1064 nm.

It is now nearly 20 years since Allen et al published their seminal paper at Leiden: `Orbital angular momentum of light and the transformation of Laguerre–Gaussian laser modes' (L Allen, M W Beijersbergen, R J C Spreeuw and J P Woerdman 1992 Phys. Rev. A 45 8185). Although it had long been recognized that light could carry an angular momentum in addition to that associated with photon spin, this had previously only been associated with high-order and rare atomic/molecular transitions. The breakthrough of Allen et al was their recognition that laser beams carrying orbital angular momentum could be produced within a standard optics laboratory. The natural basis set for such beams were the Laguerre–Gaussian laser modes whose perfectly helical phasefronts give an azimuthal component to the Poynting vector and hence energy and momentum flow around the beam axis. In this original work from Leiden, they used a clever combination of cylindrical lenses to transform between Laguerre–Gaussian and Hermite–Gaussian laser modes. However, in parallel with these studies had been the use of diffractive optics (holograms) to introduce helical phasefronts to a normal laser beam. Although at the time the link to orbital angular momentum went unnoticed, this holographic technique is now the method of choice for both generation and measurement of light beams containing orbital angular momentum. In 1995, Rubinsztein-Dunlop and co-workers showed that the orbital angular momentum of a focused light beam could be transferred within optical tweezers to set small particles into rotation. However, these optical spanners were just the beginning of what orbital angular momentum could do. The paper of Allen et al in 1992 undoubtedly created a new field of optics research, producing many papers which themselves have impacted various fields ranging from optical manipulation, non-linear and quantum optics to imaging. Beyond optical wavelengths, orbital angular momentum is becoming an important concept in electron, x-ray and radio frequency beams. Whatever the future holds, the story is not yet over.

It is generally assumed that a light beam with orbital angular momentum (OAM) per photon of lh, is transformed, when traversing a Dove prism, into a light beam with OAM per photon of -lh. In this paper, we show theoretically and experimentally that this OAM transformation rule does not apply for highly focused light beams. This result should be taken into account when designing classical and quantum algorithms that make use of Dove prims to manipulate the OAM of light.

We present the complete radiation diagram of the field scattered by a nanohole in a metallic film under Gaussian illumination. A modal expansion of the Green's Tensor for a planar multilayered structure allows us to semi-analytically obtain the radiation diagram of the field scattered by an arbitrary dielectric perturbation present in the structure. Given that the electrical field induced in the perturbation by the excitation input is available, the expansion coefficients of the scattered field in the eigenfunctions of the structure's vector wave equation are derived. In the two semi-infinite bottom and top media, those eigenfunctions coincide with the planewave basis vectors, including both propagating and evanescent components.

The scattering of circularly polarised Gaussian beams impinging on a cylindrical nanohole in a metallic thin film contains phase singularities. These experimental observations are due to changes in the helicity of the scattered photons.

We report on the observation of resonant forces in optically trapped diamond nanoparticles containing NV centres, in a liquid environment. We use a laser beam slightly detuned from the dipole transition of target colour NV centres to exert resonant (attractive and repulsive) forces onto them and, in turn, to the host diamond nanoparticles. When tuning the wavelength in the vicinity of the zero-phonon line of the NV centres, we observe a significant effect (~10%) on the overall trapping strength of conventional optical tweezers due to the resonant forces. We discuss using full optical resonant trapping as a method to manipulate, in liquid, nanodiamonds as small as ~tens of nanometers containing suitable colour centres (e.g. SiV centres) for biological applications.

In this talk we will give an overview of the latest advances in the control of the Orbital Angular Momentum (OAM) of photons for applications in quantum information and quantum optics. The OAM of light is related with the intensity and phase spatial profiles of the beam, and it can be controlled by different means. Moreover, photons can be entangled in this degree of freedom, which is a particular case of spatial entanglement of photons.

We report the observation of deflection of optical solitons generated in the frequency doubling of light beams containing edgelike topological amplitude and phase dislocations. The angular deflection of the solitons was found to be controllable through the position of the dislocation. The experiments were conducted near phase matching in a bulk potassium titanyl phosphate crystal pumped with picosecond light pulses at 1064 nm.

Our first proposal is to generate four-photon, multidimensional, orbital angular momentum (OAM) entangled states. While the final goal is to make them interact with nanostructures for quantum metrology applications, reaching unprecedent sub-wavelength sensitivity phase measurements.