The possibility to use the tip of a scanning tunneling microscope (STM) for the realization of a highly localized
molecular light source is discussed. Since it is not limited by photodegradation or quenching effects, Second Harmonic
Generation (SHG) appears as a valuable alternative to luminescence. In the case of dipolar approximation however, the
existence of a noncentrosymmetry is mandatory to get a non-vanishing signal. We show that the static electric field
present inside a scanning tunneling microscope (STM) junction can be used towards creating a very local
noncentrosymmetry via molecular orientation under the tip. An experimental set-up was specifically designed consisting
in the integration of a STM head to an inverted optical microscope, coupled to a femtosecond Ti-Saph laser excitation.
The operation of this system has enabled to get the first images with a SHG contrast of a sample structured at the micron
scale. The objective is now to improve resolution. To this respect, electromagnetic field engineering appears as a key
point. One way consists in exploiting optical nano-antenna effects. In a first approach, the possibility to benefit from
local electromagnetic field enhancement effects occurring in the presence of metallic nano-wires was studied.
Extrapolation of these results shows that imaging with about 50 nm resolution should be within reach, which opens new
perspectives in the field of optical local probe microscopy.
Using the electric field present inside a Scanning Tunnelling Microscope (STM) junction, we demonstrate the possibility
to create a very local non-centrosymmetry via molecular orientation under the tip. We show this can be used to get
localized light emission through second harmonic generation (SHG). Experiments were performed by coupling a
femtosecond laser inside a metallic-substrate / metallic-tip junction immersed in concentrated solutions of highly
nonlinear azo-dye molecules. The quadratic dependence of the SHG signal intensity with the tip voltage unambiguously
shows that it comes from an electric field induced molecular polarisation under the tip. The potentialities of such effects
are evoked as an original concept for scanning probe microscopy. Extrapolating the SH intensities that could be
recorded, it is estimated that the minimum volume that can be detected in the present experimental configuration is about
1 μm3. In order to be able to decrease this limit, a new experimental set-up is developed towards better signal collection.
The implementation of sharp metallic tips to induce amplifying nanoantenna effects (electrostatic lightning rod effects or
localized surface plasmon resonances) is also discussed as a complementary way to increase the system lateral
We propose an original technique which takes profit of Second Harmonic Generation (SHG) effects in
molecular solutions. Our technique exploits the specificities of molecular contributions. We show that we can use the
electric field present inside a Scanning Tunneling Microscope (STM) junction towards creating a local non-centrosymmetry
via molecular orientation under the tip. Experiments were performed inside a STM junction immersed
in concentrated solutions of azo-dyes molecules chosen for their highly nonlinear properties and the possibility to
generate a local SHG signal from those molecules was demonstrated. More particularly, the quadratic dependence of the
SHG signal intensity with the voltage applied between the tip and the substrate unambiguously shows that it comes from
an electric field induced molecular polarization under the tip. The dependence of the signal with the tip height or size is
reported and discussed. This approach opens the way to a new and original near field optical microscopy technique.