Frequency conversion of structured light

We demonstrate the coherent frequency conversion of structured light, optical beams in which the phase varies in each point of the transverse plane, from the near infrared (803nm) to the visible (527nm). The frequency conversion process makes use of sum-frequency generation in a periodically poled lithium niobate (ppLN) crystal with the help of a 1540-nm Gaussian pump beam. We perform far-field intensity measurements of the frequency-converted field, and verify the sought-after transformation of the characteristic intensity and phase profiles for various input modes. The coherence of the frequency-conversion process is confirmed using a mode-projection technique with a phase mask and a single-mode fiber. The presented results could be of great relevance to novel applications in high-resolution microscopy and quantum information processing

Antireflective multilayer surface with self-cleaning subwavelength structures

The suppression of optical reflection from a surface is essential in many applications, ranging from displays with reduced disturbance from ambient light to high-efficiency photovoltaic cells and stable light detection and ranging (LIDAR) systems. Traditionally, antireflection (AR) surfaces are made of multilayer (ML) coatings that produce destructive interference of light beams reflected from each interface. More advanced AR surfaces are based on biomimetic nanostructures (NS) that rely on a gradation of refractive index to suppress reflection. While AR-ML coatings tend to work for restricted light wavelengths and angles of incidence, AR-NS can be broadband and omnidirectional. In addition, AR-NS can provide superhydrophobicity and self-cleaning effects. Unfortunately, AR-NS often suffer from mechanical failure, this being more critical for taller structures required for operation at longer wavelengths. Here we propose to combine ML and shorter NS to achieve an AR surface with several crucial advantages, including greater spectral and angular bandwidth and water repellency compared to only AR-ML, easier fabrication, lower scattering, and higher mechanical durability compared to only AR-NS, which requires taller structures. We present theoretical and experimental studies for combined AR-ML-NS glass surfaces operating in the visible (VIS) between 380 and 780 nm and especially at longer wavelengths in the near-infrared (NIR) at around 900 nm, where applications such as LIDAR for autonomous vehicles are of high interest.Author acknowledges financial support from the Spanish State Research Agency through the “Severo Ochoa” Programme for Centres of Excellence in R&D (CEX2019-000910-S) and Project TUNA-SURF (PID2019-106892RB-I00), Fundació Cellex, Fundació Mir-Puig, and from Generalitat de Catalunya through the CERCA Program, from AGAUR 2017 SGR1634Peer ReviewedPostprint (published version

Ultrasensitive interferometric on-chip microscopy of transparent objects

Light microscopes can detect objects through several physical processes, such as scattering, absorption, and reflection. In transparent objects, these mechanisms are often too weak, and interference effects are more suitable to observe the tiny refractive index variations that produce phase shifts. We propose an on-chip microscope design that exploits birefringence in an unconventional geometry. It makes use of two sheared and quasi-overlapped illuminating beams experiencing relative phase shifts when going through the object, and a complementary metal-oxide-semiconductor image sensor array to record the resulting interference pattern. Unlike conventional microscopes, the beams are unfocused, leading to a very large field of view (20 mm(2)) and detection volume (more than 0.5 cm(3)), at the expense of lateral resolution. The high axial sensitivity (<1 nm) achieved using a novel phase-shifting interferometric operation makes the proposed device ideal for examining transparent substrates and reading microarrays of biomarkers. This is demonstrated by detecting nanometer-thick surface modulations on glass and single and double protein layers.Peer ReviewedPostprint (published version

Mid-infrared Gas Sensing Using Graphene Plasmons Tuned by Reversible Chemical Doping

Highly confined plasmon modes in nanostructured graphene can be used to detect tiny quantities of biological and gas molecules. In biosensing, a specific biomarker can be concentrated close to graphene, where the optical field is enhanced, by using an ad-hoc functional layer (e.g., antibodies). Inspired by this approach, in this paper we exploit the chemical and gas adsorption properties of an ultrathin polymer layer deposited on a nanostructured graphene surface to demonstrate a new gas sensing scheme. A proof-of-concept experiment using polyethylenimine (PEI) that is chemically reactive to CO2 molecules is presented. Upon CO2 adsorption, the sensor optical response changes because of PEI vibrational modes enhancement and shift in plasmon resonance, the latter related to polymer-induced doping of graphene. We show that the change in optical response is reversed during CO2 desorption. The demonstrated limit of detection (LOD) of 390 ppm corresponds to the lowest value detectable in ambient atmosphere, which can be lowered by operating in vacuum. By using specific adsorption polymers, the proposed sensing scheme can be easily extended to other relevant gases, for example, volatile organic compounds.Peer ReviewedPostprint (published version

An antireflection transparent conductor with ultralow optical loss (o2 %) and electrical resistance (o6O 2)

Transparent conductors are essential in many optoelectronic devices, such as displays, smart windows, light-emitting diodes and solar cells. Here we demonstrate a transparent conductor with optical loss of B1.6%, that is, even lower than that of single-layer graphene (2.3%), and transmission higher than 98% over the visible wavelength range. This was possible by an optimized antireflection design consisting in applying Al-doped ZnO and TiO2 layers with precise thicknesses to a highly conductive Ag ultrathin film. The proposed multilayer structure also possesses a low electrical resistance (5.75O 2), a figure of merit four times larger than that of indium tin oxide, the most widely used transparent conductor today, and, contrary to it, is mechanically flexible and room temperature deposited. To assess the application potentials, transparent shielding of radiofrequency and microwave interference signals with B30 dB attenuation up to 18 GHz was achieved.Peer ReviewedPostprint (author's final draft

Technique for generating periodic structured light beams using birefringent elements

We put forward a simple, scalable and robust technique for generating periodically structured light beams with intensity patterns, e.g. of the form cos2n(kxx) cos2m(kyy), where kx and ky are real numbers that can be tailored and n and m are integers. The technique combines the use of Gaussian beams with curved wavefronts, birefringent crystals (Savart plates) and linear polarizers. Applications range from photolithography to fabrication of micro-lens array and fiber Bragg gratings, 3D printing and tailoring of optical lattices for trapping atoms and molecules.Peer ReviewedPostprint (published version

An ultra-compact particle size analyser using a CMOS image sensor and machine learning

Light scattering is a fundamental property that can be exploited to create essential devices such as particle analysers. The most common particle size analyser relies on measuring the angle-dependent diffracted light from a sample illuminated by a laser beam. Compared to other non-light-based counterparts, such a laser diffraction scheme offers precision, but it does so at the expense of size, complexity and cost. In this paper, we introduce the concept of a new particle size analyser in a collimated beam configuration using a consumer electronic camera and machine learning. The key novelty is a small form factor angular spatial filter that allows for the collection of light scattered by the particles up to predefined discrete angles. The filter is combined with a light-emitting diode and a complementary metal-oxide-semiconductor image sensor array to acquire angularly resolved scattering images. From these images, a machine learning model predicts the volume median diameter of the particles. To validate the proposed device, glass beads with diameters ranging from 13 to 125¿µm were measured in suspension at several concentrations. We were able to correct for multiple scattering effects and predict the particle size with mean absolute percentage errors of 5.09% and 2.5% for the cases without and with concentration as an input parameter, respectively. When only spherical particles were analysed, the former error was significantly reduced (0.72%). Given that it is compact (on the order of ten cm) and built with low-cost consumer electronics, the newly designed particle size analyser has significant potential for use outside a standard laboratory, for example, in online and in-line industrial process monitoring.This work is funded by the European Union’s Horizon 2020 research andinnovation programme under Grant Agreement No. 637232 (ProPAT project).R.H. and V.P. acknowledgefinancial support from the Spanish Ministry ofEconomy and Competitiveness through the‘Severo Ochoa’Programme forCentres of Excellence in R&D (SEV-2015-0522), from Fundació Privada Cellex,and from Generalitat de Catalunya through the CERCA programme, fromAGAUR 2017 SGR 1634. V.P. acknowledgesfinancial support from the SpanishMinistry of Economy and Competitiveness through the project OPTO-SCREEN(TEC2016-75080-R). This project has received funding from the EuropeanUnion’s Horizon 2020 research and innovation programme under the MarieSkłodowska-Curie grant agreement No 665884. The authors acknowledge theChemometrics group at the Universitat de Barcelona, especially Adrián GómezSánchez and Rodrigo Rocha de Oliveira, for their contribution in the helpfuldiscussions on measurement optimisation and background correction.Peer ReviewedPostprint (published version

Phase-stable source of polarization-entangled photons in a linear double-pass configuration

We demonstrate a compact, robust, and highly efficient source of polarization-entangled photons, based on linear bi-directional down-conversion in a novel 'folded sandwich' configuration. Bi-directionally pumping a single periodically poled KTiOPO$_4$ (ppKTP) crystal with a 405-nm laser diode, we generate entangled photon pairs at the non-degenerate wavelengths 784 nm (signal) and 839 nm (idler), and achieve an unprecedented detection rate of 11.8 kcps for 10.4 $\mu$W of pump power (1.1 million pairs / mW), in a 2.9-nm bandwidth, while maintaining a very high two-photon entanglement quality, with a Bell-state fidelity of $99.3\pm0.3$%