Thermo-optic coefficient of porous silicon in the infrared region and oxidation process at low temperatures

[EN] In this work, a porous silicon nanostructure has been fabricated by electrochemical means and used as a thermal sensor. The thermo-optic effect in the near infrared region has been experimentally studied based on spectroscopy measurements. Values of the thermo-optic coefficient between 3.2 and 7.9·10¿5 K¿1 have been obtained, depending on the porosity, reaching a maximum thermal sensitivity of 91 ± 3 pm/°C during the experiments carried out with the fabricated samples. Additionally, the oxidation process of the sensor at temperatures below 500 K has been studied, showing that the growth of the silicon oxide was dependent on the characteristics of the porous layers. Based on the experimental results, a mathematical model was developed to estimate the evolution of the oxidation process as a function of porosity and thickness.The authors acknowledge the funding from the Spanish government through the project TEC2015-63838-C3-1-R-OPTONANOSENS.Martín-Sánchez, D.; Kovylina, M.; Ponce-Alcántara, S.; García-Rupérez, J. (2019). Thermo-optic coefficient of porous silicon in the infrared region and oxidation process at low temperatures. Journal of The Electrochemical Society. 166(6):B355-B359. https://doi.org/10.1149/2.0341906jesSB355B359166

Impact of GST thickness on GST-loaded silicon waveguides for optimal optical switching

[EN] Phase-change integrated photonics has emerged as a new platform for developing photonic integrated circuits by integrating phase-change materials like GeSbTe (GST) onto the silicon photonics platform. The thickness of the GST patch that is usually placed on top of the waveguide is crucial for ensuring high optical performance. In this work, we investigate the impact of the GST thickness in terms of optical performance through numerical simulation and experiment. We show that higher-order modes can be excited in a GST-loaded silicon waveguide with relatively thin GST thicknesses (<100 nm), resulting in a dramatic reduction in the extinction ratio. Our results would be useful for designing high-performance GST/Si-based photonic devices such as non-volatile memories that could find utility in many emerging applications.This work is supported by grants PID2019-111460GB-I00, ICTS-2017-28-UPV-9F, and FPU17/04224 funded by MCIN/AEI/ 10.13039/501100011033, by "ERDF A way of making Europe" and "ESF Investing in your future". Funding from Generalitat Valenciana (PROMETEO/2019/123). Funding for open access charge: Universitat Politecnica de Valencia. The authors would like to thank Helen Urgelles for her help with the experimental measurements.Parra Gómez, J.; Navarro-Arenas, J.; Kovylina-Zabyako, M.; Sanchis Kilders, P. (2022). Impact of GST thickness on GST-loaded silicon waveguides for optimal optical switching. Scientific Reports. 12(1):1-9. https://doi.org/10.1038/s41598-022-13848-01912

Dual refractive index and viscosity sensing using polymeric nanofibers optical structures

Porous materials have demonstrated to be ideal candidates for the creation of optical sensors with very high sensitivities. This is due both to the possibility of infiltrating the target substances into them and to their notable surface-to-volume ratio that provides a larger biosensing area. Among porous structures, polymeric nanofibers (NFs) layers fabricated by electrospinning have emerged as a very promising alternative for the creation of low-cost and easy-to-produce high performance optical sensors, for example, based on Fabry-Perot (FP) interferometers. However, the sensing performance of these polymeric NFs sensors is limited by the low refractive index contrast between the NFs porous structure and the target medium when performing in-liquid sensing experiments, which determines a very low amplitude of the FP interference fringes appearing in the spectrum. This problem has been solved with the deposition of a thin metal layer (∼ 3 nm) over the NFs sensing layer. We have successfully used these metal-coated FP NFs sensors to perform several real-time and in-flow refractive index sensing experiments. From these sensing experiments, we have also determined that the sponge-like structure of the NFs layer suffers an expansion/compression process that is dependent of the viscosity of the analyzed sample, what thus gives the possibility to perform a simultaneous dual sensing of refractive index and viscosity of a fluid

Bottom-Up Synthesis of Mesoporous TiO2 Films for the Development of Optical Sensing Layers

[EN] Many optical sensors exploit the interesting properties of porous materials, as they ensure a stronger interaction between the light and the analyte directly within the optical structure. Most porous optical sensors are mainly based on porous silicon and anodized aluminum oxide, showing high sensitivities. However, the top-down strategies usually employed to produce those materials might offer a limited control over the properties of the porous layer, which could affect the homogeneity, reducing the sensor reproducibility. In this work, we present the bottom-up synthesis of mesoporous TiO2 Fabry-Perot optical sensors displaying high sensitivity, high homogeneity, and low production cost, making this platform a very promising candidate for the development of high-performance optical sensors.This work was supported by the Spanish Ministerio de Ciencia e Innovación (MCIN/AEI/10.13039/501100011033) through the PID2019-106965RB-C21 project, by the Generalitat Valenciana through grant PPC/2021/036, and by the European Union through the operational program of the European Regional Development Fund (FEDER) of the Valencia Regional Government 2014-2020 and of the Ministerio de Ciencia e Innovación-Agencia Estatal de Investigación (Ref.ICTS-2017-28-UPV-9)Ortiz De Zárate-Díaz, D.; Serna, S.; Ponce-Alcántara, S.; Kovylina, M.; García-Rupérez, J. (2021). Bottom-Up Synthesis of Mesoporous TiO2 Films for the Development of Optical Sensing Layers. Chemosensors. 9(12):329.1-329.14. https://doi.org/10.3390/chemosensors9120329S329.1329.1491

All-Silicon On-Chip Optical Nanoantennas as Efficient Interfaces for Plasmonic Devices

[EN] Plasmonic technology promises to unfold new advanced on-chip functionalities with direct applications in photovoltaics, light¿matter interaction, and the miniaturization of optical interconnects at the nanoscale. In this scenario, it is crucial to efficiently drive light to/from plasmonic devices. However, typically used plasmonic wires introduce prohibitive losses, hampering their use for many applications. Recently, plasmonic nanoantennas have been proposed to overcome this drawback, not only providing a notable loss reduction, but also an enhanced on-chip flexibility and reconfigurability. Nevertheless, these devices still perform poorly for long-reach interconnects, owing to their low-directive radiation and low efficiency. Here, we introduce a class of slot-waveguide-based silicon nanoantennas that lift all these limitations and show their feasibility to be connected directly and efficiently to plasmonic devices. To test the performance of these antennae, an on-chip plasmonic-dielectric interconnect is experimentally demonstrated over distances as high as 100 ¿m. In an outstanding manner, our wireless scheme clearly outperforms previous plasmonic approaches in terms of link efficiency and effective gain. This work paves the way for the development of ultrafast on-chip wireless reconfigurable and flexible interconnects and, additionally, opens new avenues in optical manipulation and sensing applications.This work was supported by Project TEC2015-73581-JIN PHUTURE (AEI/FEDER, UE) and Generalitat Valenciana s PROMETEO Grant NANOMET PLUS (PROMETEO II/2014/34).Lechago-Buendia, S.; García Meca, C.; Griol Barres, A.; Kovylina, M.; Bellieres, LC.; Martí Sendra, J. (2019). All-Silicon On-Chip Optical Nanoantennas as Efficient Interfaces for Plasmonic Devices. ACS Photonics. 6(5):1094-1099. https://doi.org/10.1021/acsphotonics.8b01596S109410996

Continuous-wave frequency upconversion with a molecular optomechanical nanocavity

[EN] Coherent upconversion of terahertz and mid-infrared signals into visible light opens new horizons for spectroscopy, imaging, and sensing but represents a challenge for conventional nonlinear optics. Here, we used a plasmonic nanocavity hosting a few hundred molecules to demonstrate optomechanical transduction of submicrowatt continuous-wave signals from the mid-infrared (32 terahertz) onto the visible domain at ambient conditions. The incoming field resonantly drives a collective molecular vibration, which imprints a coherent modulation on a visible pump laser and results in upconverted Raman sidebands with subnatural linewidth. Our dual-band nanocavity offers an estimated 13 orders of magnitude enhancement in upconversion efficiency per molecule. Our results demonstrate that molecular cavity optomechanics is a flexible paradigm for frequency conversion leveraging tailorable molecular and plasmonic properties.This work received funding from the European Union's Horizon 2020 Research and Innovation Program under grant agreement nos. 829067 (FET Open THOR), 820196 (ERC CoG QTONE), and 732894 (HOT). C.G. acknowledges support from the Swiss National Science Foundation (project nos. 170684 and 198898). This work is part of the research program of the Netherlands Organisation for Scientific Research (NWO). A.I.B. acknowledges financial support by the Alexander von Humboldt Foundation.Chen, W.; Roelli, P.; Hu, H.; Verlekar, S.; Amirtharaj, SP.; Barreda, ÁI.; Kippenberg, TJ.... (2021). Continuous-wave frequency upconversion with a molecular optomechanical nanocavity. Science. 374:1264-1267. https://doi.org/10.1126/science.abk31061264126737

Manipulation of competing ferromagnetic and antiferromagnetic domains in exchange-biased nanostructures

Using photoemission electron microscopy combined with x-ray magnetic circular dichroism we show that a progressive spatial confinement of a ferromagnet (FM), either through thickness variation or laterally via patterning, actively controls the domains of uncompensated spins in the antiferromagnet (AF) in exchange-biased systems. Direct observations of the spin structure in both sides of the FM/AF interface in a model system, Ni/FeF2, show that the spin structure is determined by the balance between the competing FM and AF magnetic energies. Coexistence of exchange bias domains, with opposite directions, can be established in Ni/FeF2 bilayers for Ni thicknesses below 10 nm. Patterning the Ni/FeF2 heterostructures with antidots destabilizes the FM state, enhancing the formation of opposite exchange bias domains below a critical antidot separation of the order of a few FeF2 crystal domains. The results suggest that dimensional confinement of the FM may be used to manipulate the AF spin structure in spintronic devices and ultrahigh-density information storage media. The underlying mechanism of the uncompensated AF domain formation in Ni/FeF2 may be generic to other magnetic systems with complex noncollinear FM/AF spin structures

Ac conductance in granular insulating Co-ZrO2 thin films: A universal response

The ac conductance in granular insulating Co-ZrO 2 thin films prepared by pulsed laser deposition is systematically studied as a function of the Co volume content x . An absorption phenomenon at low frequencies that mimics the universal response of disordered dielectric materials is observed in the range of metal content below the Co percolation threshold x p ≈ 0.35 in the so-called dielectric regime. The temperature and frequency dependences of this absorption phenomenon are successfully analyzed in terms of random competing conduction channels between Co particles through thermally assisted tunneling and capacitive conductance. The ac conductance is well correlated with the nanostructure of the samples obtained by the transmission electron microscopy and perfectly matches the calculated ac response for a random resistor-capacitor network. We also show the occurrence of fractional power-law dependences on the frequency of the ac conductance taking place at very low frequencies as compared to the typical ranges at which dispersive behavior is observed in classical-disordered dielectric materials

Ac conductance in granular insulating Co-ZrO2 thin films: A universal response

The ac conductance in granular insulating Co-ZrO 2 thin films prepared by pulsed laser deposition is systematically studied as a function of the Co volume content x . An absorption phenomenon at low frequencies that mimics the universal response of disordered dielectric materials is observed in the range of metal content below the Co percolation threshold x p ≈ 0.35 in the so-called dielectric regime. The temperature and frequency dependences of this absorption phenomenon are successfully analyzed in terms of random competing conduction channels between Co particles through thermally assisted tunneling and capacitive conductance. The ac conductance is well correlated with the nanostructure of the samples obtained by the transmission electron microscopy and perfectly matches the calculated ac response for a random resistor-capacitor network. We also show the occurrence of fractional power-law dependences on the frequency of the ac conductance taking place at very low frequencies as compared to the typical ranges at which dispersive behavior is observed in classical-disordered dielectric materials