Jacques Peretti

Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films

The local optical field enhancement which can occur at the end of a nanometer-size metallic tip has given rise to both increasing interest and numerous theoretical works on near-field optical microscopy. In this article we report direct experimental observation of this effect and present an extensive study of the parameters involved. Our approach consists in making a “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end using photosensitive azobenzene-containing films. This distribution is coded by optically induced surface topography which is characterized in situ by atomic force microscopy using the same probe. We perform an extensive analysis of the influence of several experimental parameters. The results are analyzed as a function of the illumination parameters (features of the incident laser beam, exposure time, illumination geometry) as well as the average tip-to-sample distance and tip geometry. The results obtained provide substantial information about t...

Near-field optical patterning on azo-hybrid sol–gel films

We report on the near-field optical patterning of photochromic sol–gel films with subwavelength resolution. The sample containing functionalized azobenzene species is locally illuminated in the visible absorption band of these photochromes through the aperture of a metallized tapered optical fiber. The surface topography imaged by in situ shear-force microscopy reveals that, due to repeated photoisomerization cycles of the azobenzene molecules, photoinduced matter migration occurs under the tip leading to the formation of a surface relief. The shape of this structure is characteristic of the electromagnetic field distribution and strongly depends on the tip-to-sample distance. In near-field illumination conditions, protrusions of lateral dimension as small as 60 nm (≈λ/10) are currently produced. When repeating this process, compact arrays of nanodots are optically inscribed.

Origin of electrons emitted into vacuum from InGaN light emitting diodes

The mechanism responsible for efficiency droop in InGaN light-emitting diodes (LEDs) has long been elusive due to indirect measurement techniques used for its identification. Auger recombination is unique among proposed efficiency droop mechanisms, in that it is the only mechanism capable of generating hot carriers. In a previous study [J. Iveland et al., Phys. Rev. Lett. 110, 177406 (2013)], we performed electron energy analysis of electrons emitted into vacuum from a forward biased InGaN LED that had been brought into negative electron affinity by cesiation. Three peaks were observed in the energy spectrum of vacuum emitted electrons. In this Letter, we unambiguously identify the origin of the peaks. The two higher energy peaks correspond to accumulation of electrons transported to the surface in the bulk Γ and side L conduction band valleys. The L-valley peak is a direct signature of a hot Auger electron population. The lower energy peak results from surface photoemission induced by the internal LED light emitted from the InGaN quantum wells. Two control experiments were performed. In the first, a simple GaN pn junction generated only a single Γ peak in electroemission. In the second, selective detection of the photoemission from an LED under modulated light excitation and DC electrical injection confirms that only the low energy peak is photogenerated and that LED light is incapable of generating Γ or L-valley peaks, the latter only occurring due to the Auger effect in the LED active region.

Real-time near-field imaging of photoinduced matter motion in thin solid films containing azobenzene derivatives

We present a study of the formation of surface relief gratings in thin solid films containing azobenzene derivatives upon illumination with an interference pattern. This study is based on near-field microscopy techniques that provide real-time imaging of both the photomechanical response of the material and light excitation profile. We demonstrate that the material deformation follows two distinct regimes characterized by different kinetics, a different phase relative to the light intensity pattern, and a specific dependence on light polarization.

Optically Anisotropic Thin Films by Shear‐Oriented Assembly of Colloidal Nanorods

Device-scale thin films of highly oriented (in-plane) colloidal nanorods are made available by using a simple coating process involving thixotropic rod gel suspensions. Application of this process to LaPO₄ nanorods leads to films exhibiting outstanding anisotropic optical properties, such as a remarkably large birefringence (Δn = 0.13) associated with high transparency, and sharply polarized fluorescence spectra when doped with europium.

Remanent photoinduced birefringence in thin photochromic sol–gel films

High remanent optical anisotropy in photochromic sol–gel films is shown to be photoinduced by discoloration with linearly polarized visible light. This anisotropy results in a linear dichroism in the visible absorption band and in an important birefringence in the near-infrared transparency region. The kinetics of the whole process are quantitatively well described by a simple analytical model. Intrinsic characteristics of the individual molecule, such as the photochemical quantum yield and the anisotropy of their optical properties, are determined. The potential application of this effect to optical data storage is discussed.

Near-field magneto-optics with polarization sensitive STOM

Abstract A scanning tunneling optical microscope (STOM) operating with polarized light has been developed to study thin magnetic films. The magnetic film is deposited on the external face of a prism and illuminated in total reflection conditions with linearly polarized laser light. The evanescent mode close to the magnetic film surface is detected with a tip-ending monomode optical fiber connected at its other end to a light-polarization analyzer mounted at the entrance of a photomultiplier tube. The polarization sensitivity of the whole system, which was found to depend on the tip condition, was characterized on the bare prism with s- and p-polarized excitations. The magneto-optical effect in the evanescent mode is measured through a lock-in amplifier by modulating the magnetic field produced by a coil surrounding the tip. With this set-up we have studied two different systems, both exhibiting perpendicular magnetization. The first one is a dielectric garnet film. The images, obtained on this sample by measuring the magneto-optical effect under very low amplitude of the external magnetic field modulation, show up submicronic details due to magnetic domain wall motion. The second system is a metallic 25 nm Au/1 nm Co/4 nm Au sandwich with a large coercive field (≈ 1 kOe). The magneto-optical effect is here measured by modulating the field with an amplitude larger than the coercive field so that the saturated magnetization is periodically flipped. In this system we have taken advantage of the possibility to excite surface plasmon resonances in noble metal thin films with p-polarized light. Near-field measurements performed with our microscope demonstrate that the intensity of the evanescent mode is strongly enhanced (two orders of magnitude) at resonance. Moreover, the interaction of the light electric field with the gold surface plasmon leads to a related amplification of the magneto-optical effects in the evanescent mode.

Imaging of magnetic domains with scanning tunneling optical microscopy

Near field magneto-optical images of magnetic domains in ferromagnetic soft garnet films have been obtained with a scanning tunneling optical microscope working in total reflection geometry with shear-force control of the tip-to-sample distance. In this geometry a magneto-optical contrast is observed for the first time between domains of opposite magnetization without using modulation techniques. When applying a static or alternating external magnetic field, the magneto-optical images provide the location of domain wall pinning points.

Alternating photoinduced mass transport triggered by light polarization in azobenzene containing sol-gel films

Combined shear-force and near-field optical microscopies are used for real-time monitoring of the formation of photoinduced surface relief gratings in photochromic thin films containing azobenzene derivatives. The correlated optical and topographical images provide evidence that the direction of the photoinduced matter migration is defined by the light polarization pattern and that, for a given light intensity pattern, modulating the polarization between two orthogonal states gives rise to alternating mass transport.

Monitoring the orientation of rare-earth-doped nanorods for flow shear tomography

Rare-earth phosphors exhibit unique luminescence polarization features originating from the anisotropic symmetry of the emitter ions chemical environment. However, to take advantage of this peculiar property, it is necessary to control and measure the ensemble orientation of the host particles with a high degree of precision. Here, we show a methodology to obtain the photoluminescence polarization of Eu-doped LaPO4 nanorods assembled in an electrically modulated liquid-crystalline phase. We measure Eu3+ emission spectra for the three main optical configurations (σ, π and α, depending on the direction of observation and the polarization axes) and use them as a reference for the nanorod orientation analysis. Based on the fact that flowing nanorods tend to orient along the shear strain profile, we use this orientation analysis to measure the local shear rate in a flowing liquid. The potential of this approach is then demonstrated through tomographic imaging of the shear rate distribution in a microfluidic system.