A local, low-energy, electrical method for the excitation of localized and propagating surface plasmon polaritons (SPPs) is attractive for both fundamental and applied research. In particular, such a method produces no excitation background light and may be integrated with nanoelectronics. Here we report on the electrical excitation of SPPs through the inelastic tunneling of low-energy electrons from the tip of a scanning tunneling microscope (STM) to the surface of a two-dimensional plasmonic lens. The plasmonic structure is a series of concentric circular slits etched in a thick gold film on a glass substrate. An out-going circular SPP wave is generated from the tip-sample junction and is scattered into light by the slits. We compare the resulting emission pattern to that observed when exciting SPPs on a thin, unstructured gold film. For optimized parameters, the light emitted from the plasmonic lens is radially polarized. We describe the effects of the slit period and number, and lens diameter on the emission pattern and we diskuss how the light beam of low divergence is formed.
Electrical nanosources of surface plasmons will be an integral part of any future plasmonic circuits. Three different types of such nanosources (based on inelastic electron tunneling, high energy electron bombardment, and the electrical injection of a semiconductor device) are briefly described here. An example of a fundamental experiment using an electrical nanosource consisting of the tunnel junction formed between a scanning tunneling microscope (STM) and a metallic sample is given. In this experiment, the temporal coherence of the broadband STM-plasmon source is probed using a variant of Young's double slit experiment, and the coherence time of the broadband source is estimated to be about 5-10 fs.
The highly confined nature of the fields from surface plasmons makes them excellent candidates for future nano-optical devices. Most often, optical excitation is used to excite surface plasmons. However, a local, low energy, electrical method for surface plasmon excitation would be preferable for device applications. The scanning tunneling microscope (STM) is an ideal, low energy, local source of electrons that can excite both localized (LSP) and propagating surface plasmons (SPP). Its local nature, along with the ability to precisely position the excitation source and the absence of any background light from the excitation are essential for our experiments. We have used this technique to locally excite surface plasmons on a variety of metal structures. In our setup, the STM is coupled to an inverted optical microscope and the resulting emitted light is collected through the glass substrate. In such a configuration, both the light emitted from localized plasmons as well as the leakage radiation from propagating surface plasmons may be recorded. Both real plane (spatial information) and Fourier plane (angular information) images may be obtained, as well as emission spectra. In this article we will present the results of STM-SPP excitation on thin Au films on glass and investigate the effect of Au film thickness on the SPP propagation length. These results demonstrate the unique features of STM-excited SPPs: the STM plasmon source may be considered equivalent to a series of oscillating vertical point dipoles, and the resulting plasmons consist of a 2D circular wave with a broadband spectrum. These properties are then exploited to study how SPPs scatter into photons from super and sub-wavelength sized holes. It is found that the larger the hole diameter, the more directional the scattering light. From a type of SPP-Young's experiment we determine that the orientation of the electric field is maintained when SPPs are scattered into photons at holes.
The last decade has witnessed momentous advances in fluorescence microscopy. The introduction of novel fluorescent markers, together with the development of original microscopy techniques, made it possible to study biomolecular interactions in living cells and to examine the structure and function of living tissues. The emergence of these innovative techniques had a remarkable impact on all the life sciences. However, many biological and medical applications involve the detection of minute quantities of biomolecules, and are limited by the signal weakness in common observation conditions. Here, we show that silver and gold-coated microscope
slides can be used as mirror substrates to efficiently improve detection sensitivity when fluorescence microscopy
is applied to micrometer-thick biological samples. We report a fourfold enhancement of the fluorescence signal
and a noticeable strengthening of the image contrast, when mirror substrates are used with standard air microscope
objectives. We demonstrate that metal-coated substrates provide the means to get sensitivity-enhanced fluorescence detection with dry optics, while keeping a wide field observation and a large depth of field. This is a crucial advantage for automated and high-throughput applications to cell and tissue diagnostic analysis.
Fluorescence microscopy has become the method of choice in the majority of life-science applications. We describe development and use of mirror slides to significantly enhance the fluorescence signal using standard air microscope objectives. This technique offers sufficient gain to achieve high-sensitivity imaging, together with wide field of observation and large depth of focus, two major breakthroughs for routine analysis and high-throughput screening applications on cells and tissue samples.
In our attempt to reveal highly localized field enhancements on random metallic films using near-field
scattering probe microscopy we experimentally demonstrated the existence of narrow peaks when using a
monochromatic illumination. In order to get a better understanding of the second harmonic generation taking
place on such films we have undertaken the same kind of near-field experiments using femtosecond lasers
sources with high peak power able to induce the non linear response. These lasers have a spectral bandwidth
associated with the pulse duration, which is in the femtosecond range. With such spectral broadening we have
observed, as expected, a spatial broadening of the peaks at ω, which spread over distances in the 100-500 nm
range. The behavior of the peaks is quite different at 2 ω: they are found to be always very <i>well localized</i> (~10
nm) despite of the polychromatic nature of the light; moreover there is no clear correlation between the peaks
position at ω and those at 2 ω. This observation indicates, as often underlined in non linear processes, that
<i>coherent interactions</i> involving a distribution of available frequencies in the lasers spectra take place. These
frequencies ωn, coherently induce second harmonic generation as long as ω<sub>n</sub> + ω<sub>m</sub> = 2 ω.
Fluorescence is widely used as a spectroscopic tool or for biomedical imaging, in particular for DNA chips. In some cases, detection of very low molecular concentrations and precise localization of biomarkers are limited by the weakness of the fluorescence signal. We present a new method based on sample substrates that improve fluorescence detection sensitivity. These active substrates consist in glass slides covered with metal (gold or silver) and dielectric (alumina) films and can directly be used with common microscope set-up. Fluorescence enhancement affects both excitation and decay rates and is strongly dependant on the distance to the metal surface. Furthermore, fluorescence collection is improved since fluorophore emission lobes are advantageously modified close to a reflective surface. Finally, additional improvements are achieved by structuring the metallic layer. Substrates morphology was mapped by Atomic Force Microscopy (AFM). Substrates optical properties were studied using mono- and bi-photonic fluorescence microscopy with time resolution. An original set-up was implemented for spatial radiation pattern's measurement. Detection improvement was then tested on commercial devices. Several biomedical applications are presented. Enhancement by two orders of magnitude are achieved for DNA chips and signal-to-noise ratio is greatly increased for cells imaging.
We have developed a new detection scheme based on a scanning near-field optical microscope to image both the linear and the non-linear (e.g. second harmonic generation (SHG)) on surfaces with sub-wavelength resolution. The microscope we used here scatters the evanescent waves that contain sub-wavelength information with the apex of a metallic tip. The resolution of this microscope is directly given by the size of the radius of curvature of the metal tip end
(about 5 nm). Our set-up is applied to the optical study of crystalline films and random metal surfaces. Using thin SBN films
(Strontium Barium Niobate) we demonstrate that near-field optics is a surface sensitive measurement. The ability to perform high quality and highly resolved images is transposed to increase the resolution of imaging in the THz domain. It is also used in the visible domain on random metal films. Several studies have demonstrated that random metal surfaces show localization of the electric field on small area called "hot spots" where the electric field can exceed the applied field by several orders of magnitude. Position of the hot spots depends on film structure, on the polarization and wavelength of the illuminating laser beam. In addition, these random metal films are known to be the source of nonlinear optical effects. We are currently working to precisely locate the respective position of the linear and non-linear hot spots on silver.
Fluorescence is widely used as a spectroscopic tool or for biomedical imaging, in particular for
DNA chips. Nanostructured metallic substrates permit to locally enhance the fluorescence signal
which offer the possibility both to detect very small fluorophore concentrations and to trace
precisely the bio-markers. We have developed substrates made of silver or gold nanoparticles
covered with a spacer layer of alumina. Double metallic and dielectric gradients permit to directly
map the fluorescence enhancement factor and to determine the best condition for maximum
enhancement. One and two photons excitations are studied. Fluorescence enhancement reaches two
orders of magnitudes. Lifetime measurements reveal additional information on the decay channels
induced by the nanoparticle presence.
Fluorescence is widely used as a spectroscopic tool or for biomedical imaging. To extend these measurements to small concentrations or to fluorophores with very low quantum yield we have developed nanostructured substrates made of silver nanoparticles covered with a spacerlayer of alumina. Factors of about 200 are obtained for fluorescence enhancement with two photon excitation. Lifetime measurements reveal additional information on the decay channels induced by the nanoparticle presence.