Low-cost and large-size nanoplasmonic sensor based on Fano resonances with fast response and high sensitivity

We have developed a low-cost, fast and sensitive plasmonic sensor with a large-size for easy handling. The sensor is formed by a Au nanobelt grating fabricated by soft lithography with a period of 780 nm and a width of 355 nm in an even and uniform area of ~2 × 2 cm². The sensor uses the Fano-shaped third order mode localized plasmon resonance of the Au nanobelts, which appears in the visible part of the transmission spectrum. We have found a detection resolution of 1.56 × 10^-5 refractive index units with a temporal resolution of 1 s in a sensing area of 0.75 × 0.75 mm². The high uniformity and size of the sensor permit the detection using a simple optical system, which provides the device with the potential to be used as an easy to handle, portable and disposable sensor.

Tailored Height Gradients in Vertical Nanowire Arrays via Mechanical and Electronic Modulation of Metal-Assisted Chemical Etching

In current top-down nanofabrication methodologies the design freedom is generally constrained to the two lateral dimensions, and is only limited by the resolution of the employed nanolithographic technique. However, nanostructure height, which relies on certain mask-dependent material deposition or etching techniques, is usually uniform, and on-chip variation of this parameter is difficult and generally limited to very simple patterns. Herein, a novel nanofabrication methodology is presented, which enables the generation of high aspect-ratio nanostructure arrays with height gradients in arbitrary directions by a single and fast etching process. Based on metal-assisted chemical etching using a catalytic gold layer perforated with nanoholes, it is demonstrated how nanostructure arrays with directional height gradients can be accurately tailored by: (i) the control of the mass transport through the nanohole array, (ii) the mechanical properties of the perforated metal layer, and (iii) the conductive coupling to the surrounding gold film to accelerate the local electrochemical etching process. The proposed technique, enabling 20-fold on-chip variation of nanostructure height in a spatial range of a few micrometers, offers a new tool for the creation of novel types of nano-assemblies and metamaterials with interesting technological applications in fields such as nanophotonics, nanophononics, microfluidics or biomechanics.

Detection of cancer biomarkers in serum using a hybrid mechanical and optoplasmonic nanosensor

Blood contains a range of protein biomarkers that could be used in the early detection of disease. To achieve this, however, requires sensors capable of detecting (with high reproducibility) biomarkers at concentrations one million times lower than the concentration of the other blood proteins. Here, we show that a sandwich assay that combines mechanical and optoplasmonic transduction can detect cancer biomarkers in serum at ultralow concentrations. A biomarker is first recognized by a surface-anchored antibody and then by an antibody in solution that identifies a free region of the captured biomarker. This second antibody is tethered to a gold nanoparticle that acts as a mass and plasmonic label; the two signatures are detected by means of a silicon cantilever that serves as a mechanical resonator for 'weighing' the mass of the captured nanoparticles and as an optical cavity that boosts the plasmonic signal from the nanoparticles. The capabilities of the approach are illustrated with two cancer biomarkers: the carcinoembryonic antigen and the prostate specific antigen, which are currently in clinical use for the diagnosis, monitoring and prognosis of colon and prostate cancer, respectively. A detection limit of 1 × 10(-16) g ml(-1) in serum is achieved with both biomarkers, which is at least seven orders of magnitude lower than that achieved in routine clinical practice. Moreover, the rate of false positives and false negatives at this concentration is extremely low, ∼10(-4).

Mimicking electromagnetically induced transparency in the magneto-optical activity of magnetoplasmonic nanoresonators

We show that the interaction between a plasmonic and a magnetoplasmonic metallic nanodisk leads to the appearance of magneto-optical activity in the purely plasmonic disk induced by the magnetoplasmonic one. Moreover, at specific wavelengths the interaction cancels the net electromagnetic field at the magnetoplasmonic component, strongly reducing the magneto-optical activity of the whole system. The MO activity has a characteristic Fano spectral shape, and the resulting MO inhibition constitutes the magneto-optical counterpart of the electromagnetic induced transparency.

Magnetic modulation of surface plasmon modes in magnetoplasmonic metal-insulator-metal cavities

The magnetic modulation of the surface plasmon-polariton (SPP) wavevector is experimentally and theoretically studied for the plasmonic modes excited in metal-insulator-metal (MIM) magnetoplasmonic cavities. For this purpose, Ag/SiO₂/Ag multilayers with different SiO₂ layer thickness in which a thin Co layer is positioned near the top Ag/SiO₂ interface, near the bottom SiO₂/Ag one, or near both of them, are studied. The magnetoplasmonic MIM cavities present symmetric (SM) and antisymmetric (AM) plasmonic modes, of different wavevector and electromagnetic field profiles inside the MIM cavity. We show that the magnetic SPP wavevector modulation strongly depends on which mode is considered, the cavity thickness, and the number and specific location of Co layers within the structure. With only one ferromagnetic layer, a net modulation is obtained, of higher magnitude as we reduce the SiO₂ layer thickness. The introduction of a second Co layer in the structure reduces the modulation due to the non-reciprocal character of SPP modes under an applied magnetic field. Moreover, we demonstrate that the non-reciprocal nature of the SPP modulation can be experimentally visualized in the magnetic hysteresis loops under plasmon excitation conditions by using two Co layers with different magnetization switching fields.

Self-assembling processes involved in the molecular beam epitaxy growth of stacked InAs/InP quantum wires

The growth mechanism of stacked InAs/InP(001) quantum wires (QWRs) is studied by combining an atomic-scale cross-sectional scanning tunnelling microscopy analysis with in situ and in real-time stress measurements along the [110] direction (sensitive to stress relaxation during QWR formation). QWRs in stacked layers grow by a non-Stranski-Krastanov (SK) process which involves the production of extra InAs by strain-enhanced As/P exchange and a strong strain driven mass transport. Despite the different growth mechanism of the QWR between the first and following layers of the stack, the QWRs maintain on average the same shape and composition in all the layers of the stack, revealing the high stability of this QWR configuration.