1 May 2012 Near-field microscopy using localized molecular second harmonic generation at a metallic tip
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Abstract
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.
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I. Berline, F. Charra, C. Fiorini-Debuisschert, "Near-field microscopy using localized molecular second harmonic generation at a metallic tip", Proc. SPIE 8424, Nanophotonics IV, 84241H (1 May 2012); doi: 10.1117/12.922294; https://doi.org/10.1117/12.922294
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