Lucio Martinelli

Sensor-integrated fluorescent microarray for ultrahigh sensitivity direct-imaging bioassays: Role of a high rejection of excitation light

Fluorescent microarrays exploit fluorescent labeled targets bound to immobilized biomolecular probes. Their signal-to-noise ratio is limited by the collection aperture in common confocal geometries. Taking advantage of a very high rejection filter deposited onto a silicon arrayed detector (coupled-charge device or complementary metal-oxide semiconductor), it is demonstrated that a highly compact lens-free assay with photon collection of order unity operates with a 30-fold improvement over a conventional (substrate + free-space optics) scheme. Through analysis of improvements over the present demonstrator, a single molecule per pixel sensitivity is predicted.

A closer look at the light-induced changes in the mechanical properties of azobenzene-containing polymers by statistical nanoindentation

The mechanical properties of azobenzene-containing polymer films are statistically measured by instrumented nanoindentation experiments in the dark and under illumination in the absorption band of the azobenzene molecules. The material is obtained from a commercial PMMA compound grafted with Disperse Red 1 (DR1) azobenzene derivative. In the dark, DR1 molecules remain in the stable trans isomer state while, under illumination, they undergo photoisomerisation cycling and form a photo-stationary equilibrium between cis and trans isomers. This material is known to exhibit light-induced deformation phenomena related to the photoisomerization cycling of the DR1 units. Statistical loading/unloading tests performed in the tens of μN load range reveal a significant change in the mechanical properties of the film under light excitation. The material hardness and irreversible viscosity are seen to decrease, while the creep coefficient value increases, indicating a significant reinforcement of the viscoplastic response of the film under illumination. Moreover, creep experiments performed at a constant load show striking dissipative effects when light is turned on and also, surprisingly, when light is turned off. These features are supposedly related to the transient changes in the balance between the cis and trans isomer populations.

Identification of Auger effect as the dominant mechanism for efficiency droop of LEDs

We discuss the unambiguous detection of Auger electrons by electron emission (EE) spectroscopy from a cesiated InGaN/GaN light-emitting diode (LED) under electrical injection. Electron emission spectra were measured as a function of the current injected in the device. The appearance of high-energy electron peaks simultaneously with the droop in LED efficiency shows that hot carriers are being generated in the active region (InGaN quantum wells) by an Auger process. A linear correlation was measured between the high energy emitted electron current and the “droop current” - the missing component of the injected current for light emission. We conclude that the droop originates from the onset of Auger processes. We compare such a direct identification of the droop mechanism with other identifications, most of them indirect and based on the many-parameter modeling of the dependence of the external quantum efficiency on the carrier injection.

Quantitative analysis of enhanced light irradiance in waveguide-based fluorescent microarrays

Probing microarray assays in the presence of a hybridization mix retrieves precious information on hybridization kinetics. However, in common detection schemes, useful surface signals compete with the high supernatant background from labelled targets in the mix. A known solution consists in exciting specifically the microarray surface with evanescent fields. Configurations using planar optical waveguides to produce such fields are shown here to present also a dramatic excitation irradiance enhancement at the guide/surrounding matter interface. We compare theoretically and experimentally a guided excitation with a classical external excitation. A full electromagnetic analysis predicts an irradiance increase higher than 10(4) for adequately tailored waveguides. We deposited high-index TiO(2) sol-gel waveguides on glass substrates according to best simulations. Quantitative enhancement analysis exploiting actual biological fluorescent spots perfectly confirms the irradiance amplification effect of a thin waveguide. The impact of amplification on the design of biochip readers is discussed since it leaves ample margin for simple and low-cost light couplers, advantageous in affordable readers and sensor systems.

Absence of carrier separation in ambipolar charge and spin drift in p+-GaAs

The electric field-induced modifications of the spatial distribution of photoelectrons, photoholes, and electronic spins in optically pumped p+ GaAs are investigated using a polarized luminescence imaging microscopy. At low pump intensity, application of an electric field reveals the tail of charge and spin density of drifting electrons. These tails disappear when the pump intensity is increased since a slight differential drift of photoelectrons and photoholes causes the buildup of a strong internal electric field. Spatial separation of photoholes and photoelectrons is very weak so that photoholes drift in the same direction as photoelectrons, thus exhibiting a negative effective mobility. In contrast, for a zero electric field, no significant ambipolar diffusive effects are found in the same sample.

Luminescence imaging of photoelectron spin precession during drift in a p-type GaAs microfabricated Hall bar

Using a microfabricated p-type GaAs Hall bar, it is shown that the combined application of co-planar electric and magnetic fields enables the observation of spatial oscillations of the photoluminescence circular polarization due to the precession of drifting spin-polarized photoelectrons. Observation of these oscillations as a function of electric field gives a direct measurement of the minority carrier drift mobility and reveals that the spin coherence length can be tuned up to more than 10μm with electric fields below 1 kV/cm.

Effect of the Pauli principle on photoelectron spin transport in p+ GaAs

In p+ GaAs thin films, under excitation by a tightly-focused laser, the spatial profile of the spin polarization is monitored as a function of excitation power. It is found that photoelectron diffusion depends on spin, as a direct consequence of the Pauli principle which causes a concentration dependence of the spin stiffness. Thermoelectric currents are also predicted to depend on spin under degeneracy (spin Soret currents), but these currents play a relatively small role in this case. The spin dependence of the mobility is also found weak. Conversely, ambipolar coupling with holes increases the steady-state photo-electron density at the place of excitation and therefore the amplitude of the degeneracy-induced polarization decrease at the place of excitation.

Using scanning near-field microscopy to study photo-induced mass motions in azobenzene containing thin films

Scanning near-field optical microscopy (SNOM) is used to study the photo-induced deformation of layered structures containing azobenzene derivatives. This approach is particularly relevant since it allows detecting in real-time, with the same probe the surface topography and the optical field distribution at the nanoscale. The correlation between the local light pattern and the ongoing photo-induced deformation in azobenzene-containing thin films is directly evidenced for different light polarization configurations. This unveils several fundamental photodeformation mechanisms, depending not only on the light field properties, but also on the nature of the material. Controlling the projected electromagnetic field distribution allows inscription of various patterns with a resolution at the diffraction limit, i.e. of a few hundreds of nm. Surface relief patterns with characteristic sizes beyond the diffraction limit can also be produced by using the nearfield probe to locally control the photo-mechanical process. Finally, the photo-mechanical properties of azo-materials are exploited to optically patterned metal/dielectric hybrid structures. Gratings are inscribed this way on thin gold films. The characteristic features (enhancement and localization) of the surface plasmons supported by these noble metal structures are studied by near-field optical microscopy.