The on-demand tailoring of the properties of light, such as phase, polarization or spatial shape, has completely changed the landscape of photonic-based applications. In this way, complex light fields have become an ubiquitous tool in areas of research such as classical and quantum communications, optical tweezers and super-resolution microscopy, among many others. Here we will present some novel applications to optical metrology. First we will show how appropriate tailoring of the properties of light interacting with chiral molecules can enhance their chiral response by two orders of magnitude compared to circularly polarized light. As a second application, we will present a highly sensitive digital technique capable to measure layer thickness in the nanometer regime. This technique is interferometric in nature and contrary to others based on the same principle does not require the highly engineered construction of holders. Finally, we will describe a novel laser remote sensing technique that enables the direct measurement of the transverse component of velocity, a measure that up to now has relied in complicated techniques based on measurements of the longitudinal component of the velocity. This technique offers the possibility to also measure in a direct way the vorticity in fluids, a measure that is commonly measured through the curl of the fluid velocity.
In this work we will review some of the novel applications, recently proposed, where the use of structured light has played a crucial role. First, in the field of laser remote sensing, we discuss about a technique that allows to measure, in a direct way, the component of velocity perpendicular to the line of sight. This technique has found applications in the field of fluid dynamics, where an effective and simple optical technique capable to provide accurate measurements of ow vorticity, the tendency of a ow to rotate, was recently demonstrated. We then move to the field of profilometry to revise the key ideas behind a highly sensitive interferometric technique for thickness measurement, which is based on mode projection. We finally enter the field of optical activity to explore a novel proposal where an enhanced interaction between the handedness of structured light and chiral molecules was predicted.
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.
It has been shown that a spatial soliton can be created when a CW laser travels through a suspension of dielectric nanoparticles, provided its power is above a critical value [Opt. Lett. Vol. 7: 276 (1982)]. Recently, it was demonstrated that these soliton-like beams can be used as waveguides for controlling an additional low-power laser (probe beam) [Opt. Lett. Vol. 38: 5284 (2013)]. Here we present an experimental study of the interaction between two solitons propagating through a nanocolloid and we analyze their use to create a beam splitter for a probe beam.
We present and discuss a set of experiments based on the application of the nonlinear properties of colloidal nanosuspensions to induce waveguides with a high‐power CW laser beam (wavelength 532nm) and its use for controlling an additional probe beam. The probe is a CW laser of a different wavelength (632nm), whose power is well below the critical value to induce nonlinear effects in the colloidal medium. We also discuss a technique for the characterization of the induced waveguides.
In this paper, we present Bragg reflection waveguides as a novel universal platform for reaching the phasematching
of spontaneous parametric downconversion process in semiconductor materials. We have designed two
different waveguide structures. The first one is based on AlGaN and it is able to produce spectrally uncorrelated
photon pairs. The second one is based on AlGaAs and it allows us to generate entangled photon pairs with
ultra-broad spectra. Spontaneous-parametric-downconversion and second-harmonic-generation experiments are
One of the goals of quantum optics is to implement new sources of quantum light with tunable control of the
relevant photonic properties. Here, we add to the toolkit of available techniques in quantum optics for the full
control of the properties of quantum light, new strategies to manage the spectrum of photons, namely, type
of frequency correlations, bandwidth and waveform. As a source of quantum light, spontaneous parametric
downconversion (SPDC) is considered. Interestingly, the techniques presented might be used in any nonlinear
medium and frequency band of interest. One of the schemes to control the frequency correlations makes use of
light pulses with pulse-front tilt. The method is based on the proper tailoring of the group velocities of all the
waves that interact in the nonlinear process, through the use of beams with angular dispersion. Noncollinear
SPDC is the other strategy that is considered, since it allows mapping the spatial characteristics of the pump
beam into the frequency properties of the downconverted photons.
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 put forward a new scheme to tailor the frequency correlations of paired photons which allows their spectral properties to be tuned from correlation to anticorrelation, including uncorrelation. The method is based on the proper tailoring of the group velocities of all interacting waves through the use of beams with angular dispersion. The method can be implemented in materials and frequency bands where conventional solutions do not hold. This technique makes possible the generation of frequency correlated photons, heralded single photons with a high degree of purity from pairs of uncorrelated photons, and the suppression of distinguishing information contained in the frequency spectrum of polarization entangled photon pairs.
The bandwidth and the frequency correlations of quantum light can be considered as a resource for the implementation of new quantum information algorithms, and it should enable the applicability of quantum techniques not yet implemented. For that purpose, the control of the frequency correlations, and the bandwidth, of single and paired photons is an essential ingredient, since the optimum bandwidth, as well as the most appropriate type of frequency correlations for a specific quantum application, depend on the specific quantum information application under consideration. Here we elucidate and implement new strategies to tailor the frequency properties of quantum light. Such strategies, which are based on the use of non collinear spontaneous parametric down conversion (SPDC) configurations, include the generation of narrow and enhanced bandwidth quantum light, the control of the frequency correlations of paired photon, and the generation of heralded single photons with a high degree of purity from pairs of uncorrelated 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.
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.
The two-photon state generated by spontaneous parametric down-conversion (SPDC) exhibit spatial entanglement embedded in the corresponding mode function. The control of the spatial characteristics of the generated two-photon state is an issue of paramount importance. For example, the spatial entanglement of the two down converted photons forms the basis of quantum imaging, and entanglement in orbital angular momentum has opened a new scenario for implementing multidimensional Hilbert spaces. We put forward several techniques to engineer the spatial structure of entangled two-photon states generated in SPDC. The first strategy we consider for spatial control of the quantum state makes use of the direct manipulation of the pump beam. This technique makes feasible to prepare arbitrary engineered entangled states in any d-dimensional Hilbert space. The second strategy is based on the proper preparation of the down-converting crystal itself, namely quantum state manipulation by quasi-phase-matching (QPM) engineering. We use properly designed transversely varying QPM gratings in nonlinear crystals.